Bicycle helmets provide a 63-88% reduction in the risk of head, brain and severe brain injury for all ages of bicyclists. Helmets provide equal levels of protection for crashes involving motor vehicles 69% and crashes from all other causes 68%. Injuries to the upper and mid-facial areas are reduced 65%.(1) These five studies, which are part of our Cochrane review, compare helmet use to bare heads. The helmets in all studies were as worn by the riders (some fit better than others). Loosing a helmet during a crash is a RARE occurrence. Your arguments are hypothetical and happen to be wrong based on the published peer-reviewed empirical data. If you want to advance your argument conduct a well-designed study in the field and publish your results in a peer-reviewed journal.

The remainder of your latest comment does not meet the 300-word limit established in the Cochrane House Rules. All but the first two paragraphs are nearly verbatim from your anti-helmet website. Many of the statements on your website have not been subjected to scientific peer review. As always, it is good to hear from you.

Reference

1) Thompson DC, Rivara FP, Thompson R. Helmets for preventing head and facial injuries in bicyclists (Cochrane Review). In: The Cochrane Library, Issue 1,2004. Chichester, UK: John Wiley & Sons, Ltd.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

The halving of helmet effectiveness due to ill-fitting is important when examining the legislative failure in mandatory bicycle helmet jurisdictions. If the 1999 study findings are correct, cyclists who choose to wear a helmet have up to three times less chance of head injury than those who choose to cycle with a bare head - assuming the loss of helmet during an accident leaves the victim bare headed and it is thus not possible for the head to be less protected. This comparison relies on the helmet being an excellent fit. However, research suggests most helmets (96%) are in inadequate condition or with inadequate fit (1). By implication, most cyclists have poorly fitting helmets and considerably less than a x3 reduced risk of head injury in an accident by comparison with a bare head cyclist.

It is relevant to know that most helmets are worn and/or fitted in a manner that provides reduced or the weakest protection as this may be one of various reasons why helmets provide inadequate protection against the increased risks they create. This is particularly so in mandatory environments where helmet wearing is resented by a proportion of cyclists less likely to wear them correctly. The reality of ill-fitting also throws into question the relevance of helmet design standards such as Snell, as these drop-tests are based on an assumption of correct fit.

Cycling discouragement

The claimed reductions of head injury in mandatory helmet jurisdictions are based upon a dismissal of cycling survey results. The 2000 analysis of New Zealand results by Scuffham has already been dissected from a time analysis perspective (2 & 3). Scuffham references a 19% reduction in cyclist head injuries in the three years after 1994 helmet law enforcement in New Zealand. Data from the Land Transport Safety Authority in New Zealand (4) shows a 19% reduction in cycling hours from 89/90 to 97/98 and a 26% reduction in cycling trips from 89/98 to 97/98, during which time the New Zealand population increased by 11%. It would be wrong to claim the reduction in cycling popularity began prior to 1994 helmet law enforcement and there is data available suggesting the New Zealand decline in public cycling participation has worsened further between 1998 and 2004 (5).

Data from Australian and New Zealand jurisdictions shows a substantial fall in recreational and commuter cycling numbers throughout the 1990s, resulting in negative public health consequences. Australia and America are the two most obese countries in the world and New Zealand also is suffering an obesity crisis among the worst in the world (6). Australian Bureau of Statistics data shows the percentage of commuter cycling remained well down on pre-law figures for all Australian states through to 2001, despite rising consistently during the 15 years before law enforcement.

Cyclist hospital admissions

Mandatory bicycle helmet legislation was enforced in Western Australia in 1992. As a result of the law, the estimated percentage of West Australian cyclists wearing helmets increased from 39% in 1991 to 77% in 1995.

West Australian Health Department data (7) shows the percentage of cyclist hospitalisations for head injuries (including fractures, intracranial, facial and other) fell from 38.7% in 90/92 to 35% in 93/95, continuing a decline evident from at least 1981 - ten years before helmet law enactment. The actual number of cyclist skull fractures in Western Australia was 64 in 1990-1992 and 44 in 1993-1995 (-31.25%). The actual number of intracranial injuries was 423 in 1990-1992 and 403 in 1993-1995 (-4.8%). A full breakdown of the 90/92 93/95 comparison shows there were an average 14 less head injuries of all types per year. Similarly, Hendrie et al estimated an average reduction of just 18 cyclist head injuries per year in Western Australia between 1992 and 1995 under the individual pooled model and based upon the actual results. (8)

The data is too comprehensive to doubt a 30% or similar increase in either West Australian bicycle accidents or injuries per accident following 1991 helmet law enactment. The average annual cyclist hospitalisation number in Western Australia before 1991 was 641. This average showed no downward trend after law enforcement in 1992 and was exceeded by 1995, by which time government bicycle survey data indicates cycling in the state had declined by more than 30% (9). Cycling data from the central survey points in the capital of Perth show cyclist numbers had not recovered to comparable 91/92 levels until 2000/01.

Annual cyclist hospitalisations in Western Australia:

1985 - 623

1986 - 660

1987 - 630

1988 - 698

1989 - 596

1990 - 638

1991 - 730

1992 - 574

1993 - 633

1994 - 644

1995 - 660

1996 - 715

1997 - 754

1998 - 850

1999 - 862

2000 - 913

Other "multivariate" factors that could influence these trends are a West Australian population increase of more than 10% from 1991 (helmet law enactment) to 2001, a 33% increase in residential population near the cycling survey points, a 50% increase in petrol prices from 1991 to 2001, an expensive government cycling promotion campaign to counter the decline, and a new or increased use of other transport modes such as roller blades (helmets not required) which can trigger the electronic bicycle survey monitors. These factors should and by 2000 did eventually counter the discouragement of cycling caused by the law, although there is solid anecdotal evidence that a substantial number of people still either don't cycle or cycle less in Western Australia because of their dislike of helmets and/or the law.

Despite the public cycling reductions throughout Australia, available evidence for different Australian jurisdictions shows total cyclist hospitalisations increased during the 1990s. Head injuries supposedly represent about a third of bare-head cyclist emergency department visits and two thirds of hospital admissions, and helmets supposedly reduce the risk of that injury by 85% (between 63% and 88% for head, brain and severe brain injury, according to the Cochrane Review (10)). If these estimates were accurate, Australia and New Zealand should both have experienced major reductions in cyclist hospital admissions, even if cycling numbers hadn't fallen.

Similar weak results can be found for all Australian states and for New Zealand. Cyclists represented 17% of road users admitted to West Australian hospitals in 1992, the year mandatory bike helmet legislation was enforced. This had steadily risen to 25.9% by the year 2000. In 1980, cyclists represented about 12% of road users admitted to hospitals. Mandatory helmets have done nothing to reduce cyclists as a proportion of road crash admissions to West Australian hospitals, despite their claimed ability to substantially reduce head injury, despite the vast majority of cyclists wearing helmets, and despite ongoing proportionate and actual reductions for other road user groups.

Examples abound within mandatory bicycle helmet jurisdictions to discredit any notion that helmets confer a 3x protection against head injury. Surveys in the Victorian capital of Melbourne during the first 12 months after 1990 helmet law enforcement found a decline in child and adult cyclist numbers of 42% and 29% respectively (11). The reduction in cycling among 5-18 year olds was reported at 36% in the first year. There was a 36% reduction in cyclist head injuries during the first year - similar to the reduction in cyclist numbers. However, there was only a 4% reduction in other injuries (12), reinforcing the evidence that helmets have little effect on head injury but can increase the overall risk.

Perusal of recent draft data published by the Federal Government's Australian Bicycle Council (13) shows that from 1998-2002, the Northern Territory jurisdiction had Australia's highest proportion of people who cycle for recreation, the highest proportion of trips to work by bicycle, the highest proportion of females who cycle to work, the third highest rate of female recreational cycling at 3.4% above the national average, and the lowest ratio of cyclist injuries per capita in Australia. The NT averaged one cyclist death per year in a population around 210,000. In 1994, the Northern Territory abolished mandatory bicycle helmets for cyclists aged 17 and over [on bicycle paths only] and is the only jurisdiction in Australia where adult bare-head cycling is legal.

It is time to discard the 15 year-old Seattle claim that helmets reduce the risk of head injury by 85%. The Cochrane Review found the risk of head, brain or severe brain injury is reduced from 63% to 88%, but 85% is nevertheless the benchmark figure used by governments and helmet law advocates to justify their belief. Medical researchers should examine the actual results in mandatory helmet jurisdictions to determine why total accident/injury numbers have risen despite decreased public cycling, and to establish what has been the real reduction or increase in the risk of head injury, body injury and cycling accidents. We might then have a better understanding why the 2x or 3x decrease in helmet effectiveness is a contributor to the cumulative failure of mandatory helmet laws.

In light of the unknown difference in risk between helmet and bare head cycling, it's not possible to determine the effect of a 2x (or 3x) increase in head injury risk from ill-fitting helmets, but it may cause them to exceed the risk of a bare head (choking via helmet strap, increased radius, neck injury, etc), dependent upon the particular circumstances of each crash. This ignores the fact that even an excellent fitting helmet can make angular contact with a hard surface when a bare head would have avoided any contact (and further ignoring the possibility of worsened rotational brain injury).

Based upon these realities, I challenge the correction to my original point and reject any suggestion that I have bent the facts to support an argument against helmet legislation. The facts don't need bending. They speak for themselves.

When there is such strong evidence that mandatory helmet wearing reduces cycling participation yet increases hospital admissions with a negligible proportionate reduction in head injuries, I think it is bending the facts to claim helmets (and by implication the helmet law) reduce the risk of head injury by 63%, 85% or anything similar.

1) Gregory W. Parkinson, MD, FAAP and Kelly E. Hike, BA / - Bicycle Helmet Assessment During Well Visits Reveals Severe Shortcomings in Condition and Fit

http://pediatrics.aappublications.org/cgi/content/abstract/112/2/320

2) Dr D.L. Robinson - Costs and Benefits of the NZ Helmet Law /

http://www.cyclehelmets.org/papers/c2019.pdf

3) Dr N. Perry / Head Injuries to Bicyclists and the New Zealand Bicycle Helmet Law / http://www.cyclehelmets.org/papers/c2007.pdf

4) The New Zealand Travel Survey 1997/98 /

http://www.ltsa.govt.nz/research/travel-survey/

5) http://www.cycle-helmets.com/nzhelmets-ltsa.doc

6) http://onenews.nzoom.com/onenews_detail/0,1227,262934-1-6,00.html /

http://www.cycle-helmets.com/hawkes_nz_study.pdf

7) http://www.cycle-helmets.com/results.html

8) An economic evaluation of the mandatory bicycle helmet legislation in Western Australia; Delia Hendrie, Matthew Legge, Diana Rosman and Carol Kirov / http://www.officeofroadsafety.wa.gov.au/Facts/papers/bicycle_helmet_legislation.html

9) http://www.cycle-helmets.com/bicycle_numbers.html

10) http://www.cochrane.org/cochrane/revabstr/ab001855.htm

11) Dr D.L Robinson - Helmet Laws and Health /

http://agbu.une.edu.au/~drobinso/ozdoc.html

12) Cameron M, Newstead S, Vulcan P, Finch C, 1994, 'Effects of

compulsory bicycle helmet wearing law in Victoria during the first three years', Table II.

13) http://www.abc.dotars.gov.au/Cycling_Data_Indicators.htm

* I am a self-employed freelance journalist/editor and graphics/web

designer. My primary interest in the helmet issue is a desire for the broader public to know via my web site the results of bicycle helmet legislation in mandatory jurisdictions ... http://www.cycle-helmets.com. I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

Correcting Chris Gilham

It is always good to hear from cycling advocates. I am replying to the two major points you raised: 1) the issue of improperly fitted helmets and 2) the Western Australia statistics.

First on the issue of improperly fitted helmets and increased risk of head injury, you said "while Rivera, Thompson et al find improper helmet use could increase the risk of head injury by a factor of 3. Is this considered satisfactory?" Your argument is erroneous, and here's why. In our study 1 we compared children wearing poorly fitting helmets to children wearing excellent fitting helmets. We found that children whose helmets fitted poorly had double the risk of head injury compared to those with excellent helmet fit (odds ratio 1.96 (95%CI 1.0 to 3.75) If the helmet came off during the crash the head injury risk increased 3 times compared to children with excellent helmet fit. (odds ratio 3.25 (95%CI 1.82 to 5.75)(please see table 1) The sentence you quoted from the 2001 Pediatric article was taken from the discussion section where the authors were emphasizing the importance of excellent fit compared to poorly fitting helmets. The comparison of poorly fitting helmets to no helmets was never made and could not be made from our data. The premise of your argument is not valid and may lead some readers to draw the conclusion that you are bending the facts to fit your argument against helmet legislation.

The Western Australian statistics that you cite in your comment are numbers (i.e., 30% increase in the number of bicycle accidents). Arguments based on numerator data instead of rates are seldom convincing. We briefly discussed the use of multivariate time series analyses to evaluate the population effects of helmet legislation in our reply to Dorothy Robinson in 2000. Please see comment 2, replies # 5 and #6 at www.cochrane-injuries.lshtm.ac.uk/helmetcomment.pdf.

I would like to suggest that for further communications to the Cochrane collaboration web site you identify your affiliations (journalist and owner of what appears to be an anti-helmet legislation web site) in addition to the declaration of no conflicts of financial interests. This would provide readers with a reasonable reference framework for evaluating your comments.

References:

1. Rivara, FP, Astley, SJ, Clarren, SK, Thompson, DC, Thompson, RS. Fit of bicycle safety helmets and risk of head injuries in children. Injury Prevention 1999:5:194-197.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

"Furthermore, when this knowledge is applied at the population level by campaign or policy, head and brain injuries in cyclist decrease while other injuries remain stable. The latter finding indicates that helmets do not decrease cycling or increase crashes, two of the hypotheses you and others have consistently espoused."

The hypotheses re decreased cycling or increased crashes following helmet law-enforcement are evidence-based. In the few jurisdictions where all-age mandatory helmet laws have been introduced, surveys consistently point to overall declines in public cycling participation around 30%. The data at http://www.cycle-helmets.com/bicycle_numbers.html is comprehensive and drawn from detailed road surveys conducted by the government of Western Australia pre and post 1991 law enforcement. Furthermore, the government statement at http://www.cycle-helmets.com/bikewest.html indicates a 50%+ decline in children cycling to school in the five years after law enforcement, and media reports in Western Australia indicate a continuing low level of cycling among schoolchildren in 2004. The data indicates overall public cycling participation in Western Australia had recovered to pre-law levels by 2000 - nine years after law enforcement - but hospital data shows that by this time cyclist admissions had increased to record levels averaging 30% greater than their pre-law average. It is true that the proportion of head injuries among hospitalised cyclists decreased marginally in Western Australia, but the number of head injuries exceeded pre-law numbers by 1995 as a result of the 30% increase in total injuries. The West Australian data points to either a 30% increase in the number of bicycle accidents OR a 30% increase in overall injuries per accident after helmets became mandatory. This is accurate government data from a compulsory rather than voluntary environment and over a long timespan providing reliable trends, and is the most detailed in the world concerning an actual helmet law jurisdiction rather than a hypothetical case study. This evidence might not have been published in a reputable scientific journal, but it has been published in government departmental reports and it is fact. Research by Thompson et al actually helps explain why the official data from Western Australia shows a 30% increase in hospital admission risk among cyclists following mandatory helmet law enforcement. A likely partial explanation for the increase in injury numbers (apart from risk compensation, rotational force, greater impact area, etc.) is the ill-fitting of helmets. A 2001 study funded by the American Academy of Paediatrics (http://www.cycle-helmets.com/helmet_fit.pdf) found that 96% of survey participants failed criteria for proper helmet condition and/or fit. To quote that study, "... Rivera et al reported that improper helmet use is estimated to increase the risk of head injury by a factor of 3. In this light, our data suggests that most children and adolescents are receiving sub-optimal head protection." Diane Thompson was a collaborator in the Rivera et al study and it is assumed she concurs with the study findings re increased risk from improper helmet use. The peer-reviewed Academy of Paediatrics study suggests 96% of cyclists have inadequate or poorly fitted helmets, while Rivera, Thompson et al find improper helmet use could increase the risk of head injury by a factor of 3. Is this considered satisfactory? The Academy of Paediatrics study strongly indicates that only a minority of cyclists wear a helmet in a way that does NOT increase the risk of head injury. Thompson et al should accept and publicly state that according to their own research and supported by the proved results in helmet law jurisdictions, the reality of poor helmet use is a reason why compulsory helmet wearing poses a risk to the safety of cyclists.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

Reply to Bill Curnow

Re: "Helmets for preventing head and facial injuries in bicyclists" (Cochrane Review)

We are grateful for the brevity of your latest comments; however, we are a bit disappointed that you are rehashing the issues we answered 4 years ago. Our detailed replies are available at http:///www.cochrane-injuries.lshtm.ac.uk/helmetcomment.pdf and at the end of our review in the Cochrane Library.(1,2)

Our specific reply to your latest critique follows:

1. Our review followed the protocol of the Cochrane collaboration. The protocol was peer reviewed and approved by the Cochrane Injuries Group. A Cochrane review looks at the impact of certain pre-defined interventions (for example helmet use) in terms of pre-defined outcomes (effect on head, brain and facial injury). It is incorrect to assume that any systematic review would tell its readers everything there is to know on a particular subject. We stand by our summary statement that the five case-control studies establish scientific evidence that all standard types of bicycle helmets protect against head, brain and facial injury.

2. We have explained how case control studies are an appropriate study design for evaluating helmet effectiveness in the absence of randomized controlled trials.(1,2) We agree that RCT's would be interesting to contemplate, but believe RCT trials to evaluate helmet effectiveness can not be realistically carried out. Reputable independent scientific groups have established criteria for grading evidence.(3) They conclude that other experimental and non-experimental designs, for example, ecological designs, can, in combination, provide information of sufficient fidelity to reach actionable conclusions.

Although complete understanding of causal mechanisms of head injury in cyclists is potentially useful for generating hypotheses to prevent injuries, it is not, in fact, necessary to make policy decisions. At present there exists an increasingly strong and coherent body of empirical evidence that bicycle helmets work in decreasing head and brain injuries in cyclists. Furthermore, when this knowledge is applied at the population level by campaign or policy, head and brain injuries in cyclist decrease while other injuries remain stable. The latter finding indicates that helmets do not decrease cycling or increase crashes, two of the hypotheses you and others have consistently espoused. A number of ecological studies were referenced in our reply to critics in 2000.(1,2) More recently national studies from Canada provide evidence that mandatory helmet legislation increases helmet use and reduces head and brain injuries. These studies also provide evidence that cycling rates did NOT decrease as helmet use was increased by campaigns or through policy initiatives.(4-7) Additionally there is independent information from the US Consumer Product Safety Commission, which indicates that there were approximately 67 million cyclists in the USA in 1991, and that by 1998 the number was in excess of 81 million. Helmet use increased substantially in that time period.(8)

In summary we are convinced that there is a persuasive body of evidence from case control and ecological studies indicating that the relationship between increased helmet use and decreased head and brain injury is a causal one. Since helmets work their use is to be encouraged. In the meantime, we encourage you to do some research on biomechanical hypotheses which are of great interest to you and to publish your results in peer reviewed journals.

3. We provided a detailed reply to your statements about helmet effectiveness against brain and severe brain injury in 2000.(1,2) At present, there is NO scientific evidence that helmets increase axonal shear injuries.

4. One of the 5 studies in our Cochrane review (9) was large enough to evaluate the protective effect of 3 helmet types, hard, thin-shell and soft shell (table 4). All three types provided EQUAL protection from head, brain and severe brain injury. The thin-shell helmets that predominate in the current market place provide protection equal to that of hard shell helmets. Speculation on your part is not corroborated by scientific research.

In conclusion, your opinions and commentary are not based on scientific evidence. You have not provided evidence subjected to peer review and published in reputable scientific journals.

References

1. Cochrane Injuries Group http:///www.cochrane-injuries.lshtm.ac.uk/helmetcomment.pdf

2. Thompson DC, Rivara FP, Thompson R. Helmets for preventing head and facial injuries in bicyclists (Cochrane Review). In: The Cochrane Library, Issue 1, 2004. Chichester, UK: John Wiley & Sons, Ltd.

3. Briss PA, Zaza S, Pappaioanou M, Fielding J, Wright-DeAguero, Truman BI, Hopkins, DP, Mullen PD, Thompson, RS, Woolf SH, Carande-Kulis VG, Anderson L, Hinman AR, McQueen DV, Teutsch SM, Harrie JR, & The Task Force on Community Preventive Services. Developing an evidence-based Guide to Community Preventive Services-Methods. Am J Prev Med 2000;18(1S):35-43.

4. Macpherson AK, To TM, Macarthur C, et al. Impact of mandatory helmet legislation on bicycle-related head injuries in children: a population-based study. Pediatrics, Nov 2002; 110 (5) http://www.pediatrics.org/cgi/content/full/110/5/e60

5. LeBlanc JC, Beattie TL, Culligan C. Effect of legislation on the use of bicycle helmets. Canadian Med Assn J 2003; 166(5):592-595.

6. Macpherson AK, Parkin PC, To TM. Mandatory helmet legislation and children's exposure to cycling. Injury Prevention 2001;7:228-230.

7. Le Blanc JC. Butting heads over bicycle helmets Can. Med. Assoc. J, Aug 2002; 167:338-339.

8. United States Consumer Product Safety Commission, Helmet Usage Survey, April 1999, release # 99-099.

9. Thompson DC, Rivara FP, Thompson RS. Effectiveness of bicycle safety helmets in preventing head injury: A case control study. JAMA 1996; 276:24:1968-73.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

COMMENT BY W.J. CURNOW, ON REVIEW PAPER

"Helmets for preventing head and facial injuries in bicyclists" (Cochrane review)

The review concludes that its five case-control studies establish scientific evidence that all standard types of bicycle helmet protect against head, brain and facial injury.

Because the impetus for use of helmets is fear of death and chronic disability, the critical question is their efficacy against severe injury to the brain and its main causes: a blow that damages the skull and angular acceleration of the head (see Curnow 2003). By the first cause, skull bone or an external object strikes the brain, leading to focal injury such as laceration and contusion. Fatal sub-dural haematoma may follow. By the second, an oblique impulse makes the head rotate. Bone within it may then strike the brain, causing focal injury, including the so-called coup and contre-coup. Even if the brain is not struck, diffuse injury to its small blood vessels and neurons may occur, including diffuse axonal injury, which is often fatal and is the commonest cause of severe disability.

It has long been known that helmets with hard shells can protect the skull, but whether soft shells can is problematic. No helmet has proven efficacy against angular acceleration; indeed, experiments have shown that the mass added by wearing one can increase it (Corner et al., 1987). Therefore, any scientific study of effects of helmets on brain injury should take account of these realities and examine, according to laws of physics, the causal links between each type of helmet, hard/soft, and injury, focal/diffuse. None of the five studies attempts to do this, nor are their data precise enough for it. Further, none of the studies is a randomised controlled trial or uses random samples of the population as controls, and when their data were obtained hard-shell helmets predominated. Now, soft helmets do.

In conclusion, lower rates of brain injury observed when helmets are worn, as per some studies of the review, may reflect nothing more than hard shells protecting against trauma consequent upon damage to the skull. The review provides no evidence that the soft helmets predominant since it was published protect the brain at all. Its claim to establish scientific evidence that all standard types of bicycle helmet protect against brain injury is therefore not supported.

References

Corner, J.P., Whitney, C.W, O'Rourke, N., Morgan, D.E., Motorcycle and bicycle protective helmets: requirements resulting from a post crash study and experimental research, Federal office of Road Safety report CR 55, Canberra. 1987, pages 34, 105.

Curnow, W.J. The efficacy of bicycle helmets against brain injury, Accident Analysis & Prevention, 35, 287-292. 2003.

More detailed comments are available from bilcurno@pcug.org.au.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

RE: "Helmets for preventing head and facial injuries in bicyclists", by Diane Thompson, Fred Rivara and Robert S. Thompson

The comments listed on your website are incomplete. You have listed comments from three people(Richard Keatinge, Dorothy Robinson, Bill Curnow) that are very critical of our review without listing our replies. Comments from Richard Keatinge, Dorothy Robinson, Mayer Hillman and Bill Curnow along with our replies are available at http://www.cochrane-injuries.lshtm.ac.uk/helmetcomment.pdf. This document was also published with our review in the Cochrane Library. We were not informed that Richard Keatinge had published a second set of comments (13 pages worth) and thus have not had the opportunity to reply. Would you please explain why you have given space to critics of our review without allowing us the opportunity to reply. Please reply to each of the authors dianecthomp@msn.com, thompson.rs@ghc.org, fpr@u.washington.edu. Thank you very much.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

Helmets for preventing head and facial injuries

Summary of response

A detailed reply to the first reviewers reply

Summary

It is not easy to respond briefly to a long, muddled, repetitive, and inadequately referenced set of comments. I therefore summarise my main points as briefly as possible:

I have no serious argument with the arithmetical aspects of Thompson, Thompson, and Rivara's review. However, they still include the conclusion that "Bicycle riders of all ages should be encouraged to wear helmets." There are three main remaining hypotheses that need to be disproved before this recommendation of Thompson, Thompson, and Rivara can be considered worthy of serious discussion. Primary research by unbiased groups should test them. These hypotheses are:

Firstly, that helmet use deters cycling and reduces the amount of healthy exercise. Helmet promotion is therefore likely to result in serious net loss of health, even given the dangerous state of the roads.

Secondly, that helmet use may result in a slight increase of risk-taking behaviour, and a slight decrease of riding stability, resulting in slightly more accidents per million kilometres cycled. Helmet promotion may therefore result in an unchanged or increased net toll of death and morbidity per million kilometres cycled.

Thirdly, that helmets may protect against minor injuries, but have much less value against any trauma likely to have a long-term effect on the victim.

There are other issues, some ably raised by Robinson and by Curnow and inadequately answered by Thompson, Thompson, and Rivara. But the above aggregate the main points of concern. Until these points are thoroughly answered, the main recommendation of Thompson, Thompson, and Rivara lacks solid foundation and should be removed from their Cochrane Review.

Summary of my initial comments

Each comment has a number with a corresponding response from the reviewers in the reply section below.

1. The quantitative aspects of this review seem reasonable to me.

2. The analysis and discussion are so badly flawed as to make the main practical conclusion "Bicycle riders of all ages should be encouraged to wear helmets" entirely mistaken. In fact, it seems likely that encouraging helmet use would have serious adverse consequences on the public health, without making any significant difference to the dangers of riding.

3. There are conceptual confusions in the review-Risk compensation

Quoting from the review: "Some bicycling advocates have argued that helmeted cyclists will feel safe and, therefore, ride in a less cautious manner and be more subject to crashes (Hillman 1993). The converse argument has also been made that helmeted cyclists may ride more carefully and that these behaviours account for the reduction in head injury, not helmet use (Spaite 1991). We believe these arguments to be specious."

These arguments are not specious, nor are they the converse of each other. They refer, first, to the fundamental practical reason why helmets may not be effective in reducing head injury rates, and, second, to the main scientific reason why case-control studies are not a valid way to estimate the effect of helmet use on injuries.

Firstly, people who find themselves compulsorily protected from a customary hazard may indeed slacken their vigilance - this idea is known as "risk compensation" and is well-evidenced at least as far as motor vehicle accidents are concerned (Adams, 1995). Secondly, the people who voluntarily choose to take "safety" precautions such as wearing a helmet are likely to be more cautious in many other aspects of their activities on the roads. This means that comparisons of voluntary helmet-wearers with non-wearers are likely to show a lesser rate of injury among wearers.

4. Confusion with outcome measures

From the review: "The fundamental issue is whether or not when bicycle riders crash and hit their heads they are benefited by wearing a helmet." That is one relevant issue, and the review does competently confirm that indeed, this probably is the case. There are other issues.

5. Confusion with outcome measures -Not all injuries are identical

From the review: "Cyclists would have to increase their risk taking four-fold to overcome the protective effect of helmets. This seems unlikely."

If head injuries were the only threat to cyclists, and if all head injuries were of similar severity, this would be correct. However, as Gilbert and McCarthy describe; "Almost all cyclists' deaths in London are due to collision with motor vehicles, especially heavy goods vehicles" (Gilbert 1994). Precise analysis is lacking, but one may reasonably doubt if any practical helmet would help most of those severely injured by a motor vehicle. Therefore, only small changes in risk-taking would be needed to overcome any slight protective effect that helmets might have in saving lives.

6. Cycling is healthy and the review fails even to mention that helmets discourage cycling

The main relevance of cycling to health is probably mediated through its beneficial effect on cardiovascular risk factors. The BMA's report Cycling: Towards Health and Safety used actuarial data to determine the life years lost by cyclists killed in road crashes, which were then compared with the life years gained by people engaging in exercise programmes such as cycling several times a week. (Hillman 1992.) This suggested that cyclists who cover at least 40 kilometres each week may halve their risk of heart disease when compared with those who do not cycle.

7. I can find no reference in the review to the serious adverse effect of helmet use on the use of cycling, but, for example, in Australian states which have enacted laws compelling cyclists to wear helmets, Robinson points out "The deterrent effect on cycling was substantial. "

8. Failure to search or understand the literature. The authors say "There are no objective data to support this risk homeostasis theory"

This is straightforwardly wrong. The idea of risk compensation is well-evidenced, at least in the case of motor vehicle accidents. (Adams 1995)

9. I am not aware of any good-quality studies which have aimed to study its relevance to bike helmets, but "it seems extraordinary that a change in behaviour after the reduction in perceived risk would be invalid only in this instance." (Hillman 1997)

10. Inadequate analysis of time-trend data

From the review: "Additional evidence of helmet effectiveness has been provided from time series studies in Australia, Europe and the US (Vulcan 1992, Carr 1995, Pitt 1994, Ekman 1997, Rivara 1998). These study results indicate that increased rates of helmet use resulting from multifaceted educational campaigns and/or legislation is linked to significant decreases in bicycle related head injuries." Actually, it is rather difficult to get the stated results from the figures quoted. For example, W Robert Pitt and colleagues' graph, referenced by the review, (Pitt 1994) suggests an increase of non-head injuries due to cycle helmets.

11. I suggest that human behaviour is too confusing and complex for valid analysis in the face of insufficient numbers, inadequate information, inconstant underlying trends, and a poor scientific approach to data that are selectively quoted and potentially biased from the point of collection. Much of the published work on cycle helmets displays all of these problems. It is unfortunate that the Cochrane review dismisses or ignores the main issues. It is still more unfortunate that the reputation of the Cochrane Collaboration is then used to promote a damaging misunderstanding in a widely-respected popular scientific journal (Mullins 2000). It is greatly to the credit of New Scientist that they nevertheless come to a sensible conclusion (anonymous editorial 2000).

The main question for the Cochrane Collaboration is the issue of including non-randomised comparisons. Few issues are so conceptually complex as road safety measures, and few are so beset by simplistic preconceptions. This may therefore be a particularly interesting matter for methodological debate.

Reply to my initial comments

These are interleaved with my second set of comments, which are in italic script.

Reviewer's reply

The authors welcome the opportunity to respond to Dr Richard Keatinge's comments on our Cochrane review "Helmets for preventing head and facial injuries in bicyclists".

Some of his points have been discussed in Comments 1 and 2. Please read our replies to Bill Curnow and Dorothy Robinson for a discussion of epidemiological methods, particularly case control methods.

1. Answer: We agree.

2. Answer: We disagree. Dr Keatinge has already agreed the quantitative aspects of the review (the analysis) are reasonable. The results of five case control studies from three different countries have shown large protective effects of bicycle helmets. Wearing a helmet decreases the risk of head or brain injury among cyclists who fall or crash. Dr Keatinge disagrees with our overall conclusion that riders of all ages should be encouraged to wear helmets. He bases this on his opinion.

Helmets deter people from using a form of transport that is safe for bystanders and health-promoting for themselves. Helmets may encourage cyclists, or motorists, to take slight extra risks; they may lessen the stability of cyclists who wear them. Since helmets provide, at the most optimistic possible estimate, a very small lessening of absolute risk per journey, even a tiny increase in risk-taking could easily cancel out any health benefits of helmet use.

We do not think there is valid scientific evidence to support the contention that encouraging helmet use has detrimental effects on public health.

There is a very strong link between more exercise and a longer and better-quality life. Helmets are unpopular, uncomfortable, expensive, and their increasing use is associated with a decrease in cycling.

3. Answer: We disagree. We stick by our prior statements. Furthermore, we think that the conceptual confusion is with Dr Keatinge, because he clearly did not understand that in our case control studies, we, in fact, controlled for crash severity by a number of measures such as involvement with motor vehicles, degree of damage to the bicycle, surface impact, etc.

I am happy to reassure Thompson, Thompson, and Rivara that I understand this point. They describe the main reason why their review, arithmetically competent though it seems, cannot address at least two of the main points at issue. I will try to make these problems even clearer. First, a case-control study cannot test the likelihood that people will be deterred from cycling by helmet legislation or helmet propaganda. The evidence from areas that have introduced compulsory helmet use (e.g. LeBlanc et al) supports this likelihood. Second, it cannot test the hypothesis, so well supported in other areas of road safety, that any perceived improvement in safety will be consumed by greater risks of accident.

An intervention which provides minimally improved prognosis in the event of a crash, but which also makes crashing more likely and deters people from the exercise that so many of us need, is a danger to the public health. The case-control studies summarised here cannot test this hypothesis, and the recommendation of Thompson, Thompson, and Rivara is therefore without adequate support.

When this is done the focus is on comparing outcomes of cyclists with similar crash forces, which is the issue we addressed. The issue of risk compensation, while of interest to the scientific community, is irrelevant in this situation. It does not mean that risk compensation could not play a role in altering the risks of crashing in the first place. However, there is no evidence for this.

Thompson, Thompson, and Rivara allude to papers that assert the irrelevance of risk compensation. But they do not provide any references to evidence that actually backs their opinion. Repeated assertion is not evidence.

Contrary to Dr. Keatinge's opinion, case control studies which are well executed, can provide a solid basis for causal inference such as the protective effect of helmets on head injury. Please see our reply #2 to Bill Curnow.

Dr. Keatinge states that "risk compensation is well-evidenced at least as far as motor vehicle accidents are concerned "(Adams 1995). While this reference is widely quoted by the advocates of the risk compensation theory, there is ample evidence to the contrary. (NHTSA 1999, 1999a, MacKay 1945, MMWR 2001 in press).

As I comment in more detail later, I do not find that these references and allusions actually provide any such evidence.

Whether wearing a helmet contributes to risk-taking behaviour can be illuminated by looking at evidence from seat belt legislation for motor vehicles, and motorcycle helmet legislation. The evidence from laws requiring seat belt use in cars indicates that any change in risk taking by drivers is small and there is more likely to be a reduction in injury risk if wearing seat belts is mandatory. (Evans 1991.) In the United States nearly all 50 states passed laws requiring motorcycle helmets in the mid 1960's. In 1976 almost half of the states repealed their laws. This provided an opportunity for a natural experiment. Fewer motorcyclists wore helmets following the repeal of motorcycle helmet laws resulting in a 25% to 40% increase in motorcycle deaths. (Evans 1991, GAO 1991, Fleming 1992, Kraus 1994). This effect rules out the possibility that wearing helmets leads to any large increase in risk taking. (Evans 1994).

Thompson, Thompson, and Rivara go on to quote selectively from the literature on risk compensation relating to seat belts and motorcycle helmets. They refer elsewhere to Adams' classic work (1995). They do not seem to realise how thoroughly it discredits their assertion that " This effect rules out the possibility that wearing helmets leads to any large increase in risk taking." I would be interested to read their detailed response to Adam's work.

As we pointed out in the discussion section (last paragraph) of our Cochrane Review, there are no objective data to support a risk compensation/homeostasis theory for bicycle helmet effectiveness. To quote from our review-"The fundamental issue is whether or not when bicycle riders crash and hit their heads they are benefited by wearing a helmet." Adjusting for crash severity enables us to address this question regardless of any increase or decrease in risk behaviour by the cyclists. Change in risk behaviour may contribute to the risk of crashing or falling. This is a different issue and has not been studied.

Work is in progress, which strongly supports the idea of increased risk after helmet laws, see, for example, Burdett, Wardlaw, and Gillham. Thompson, Thompson, and Rivara need to assume the wrong answer in order to come to their main conclusion.

Risk compensation behaviour has been widely debated in the literature. We recommend that interested readers consult a comprehensive overview of the risk compensation debate presented by James Hedlund at the Fifth World Conference on Injury Prevention and Control (Hedlund 2000).

I agree. Hedlund's paper is a mild and balanced non-systematic review of much of the relevant literature. I am not clear how, if at all, it supports the ideas of Thompson, Thompson, and Rivara.

See also author's reply to Dorothy Robinson's comments, (Comment 2, reply #8). An extensive systematic review of automobile safety interventions by a non-federal national Task Force on Community Preventive Services has been spon ored by the Centers for Disease Control since 1996. The Task Force found that child safety seats, seatbelts and alcohol laws all contributed to a substantial reduction in motor vehicle injuries and deaths. The Task Force recommended a number of community-wide information and enforcement campaigns for these areas. Based on results of systematic reviews, the Task Force makes recommendations on population-based interventions to promote health and prevent disease, injury, disability and premature death, and to reduce environmental hazards. (MMWR 2001). These reviews took special care to examine both beneficial and adverse effects, such as would be produced through risk compensation. The Task Force conclusions contrast with views proposed by John Adams. (Adams, 1995, 1999)

I assume that the cryptic references to MMWR has something to do with the Morbidity and Mortality Weekly Report of the Center for Disease Control, vol. 50, May 18th 2001, file RR5007.pdf on the CDC website, http://www.cdc.gov. I note that Robert S. Thompson is listed as a member of the relevant task force and as one of the group that prepared the report. Their conclusions (assumptions) are clear - but I can find no hint that risk compensation, or riding instability due to helmet use, has been considered at any point, let alone systematically reviewed. Perhaps Thompson, Thompson, and Rivara could make explicit any methods that have been used.

The file RR5007.pdf refers to work to be published in the American Journal of Preventive Medicine in 2001. It also mentions that "additional information about the Task Force and Community Guide is available on the Internet at <http://www.thecommunityguide.org>. I have checked this site and have followed Thompson, Thompson, and Rivara's chain of allusions to:

The evolving role of prevention in health care: Contributions of the U.S. Preventive Services Task Force

Steven H. Woolf, David Atkins. American Journal of Preventive Medicine, 20:3 (Supplement 1) : 13-20

This does not mention risk compensation. I would recommend especially to Thompson, Thompson, and Rivara the following comment: "Over time, the principal question of interest to the USPSTF in the past-Does the preventive service work?-has matured into more sophisticated questions about the magnitude of benefit, the trade-off between benefits and harms, and the influence of individual preferences on those trade-offs." It seems that Thompson, Thompson, and Rivara have failed to keep up with this admirable change.

Current methods of the U.S. Preventive Services Task Force: A review of the process

Russell P. Harris, Mark Helfand, Steven H. Woolf, Kathleen N. Lohr, Cynthia D. Mulrow, Steven M. Teutsch, David Atkins Methods Work Group Third U.S. Preventive Services Task Force

American Journal of Preventive Medicine, 20:3 (Supplement 1) : 21-35

This paper includes the salutary statement that: "Poor evidence has a formidable break in the evidence chain such that information is inadequate to connect the preventive service and health outcomes."

The recommendation of Thompson, Thompson, and Rivara is based on just such poor evidence, for the reasons given in my summary.

Harris et al proceed: "To make its reasoning explicit, the Task Force includes an explanatory narrative about its overall rating of the evidence in the recommendation statement." I can find no such narrative relating to either risk compensation or to the abandonment of healthy exercise in the file RR5007.pdf, nor in their Cochrane Review, and the responses of Thompson, Thompson, and Rivara have not so far included any adequate attempt to address the evidence.

The next paper is also from the U.S. Preventive Services Task Force:

The art and science of incorporating cost effectiveness into evidence-based recommendations for clinical preventive services

Somnath Saha, Thomas J. Hoerger, Michael P. Pignone, Steven M. Teutsch, Mark Helfand, Jeanne S. Mandelblatt Cost Work Group of the Third U.S. Preventive Services Task Force

American Journal of Preventive Medicine, 20:3 (Supplement 1) : 36-43

In this paper I can find no reference whatever to either risk compensation or to the deterrent effect of helmets on cycling. It does however include the statements "Table 2: Sample quality rating items for cost effectiveness analyses):

Are all important drivers of effectiveness included?

Are key harms included?

Is the best available evidence used to estimate effectiveness?"

The work of Thompson, Thompson, and Rivara clearly fails against what seems to be their own criteria. They simply do not include essential drivers of effectiveness, namely risk compensation and the degree to which helmets deter people from the healthy effects of cycling.

Dr. Keatinge states that those who choose to wear helmets may be more cautious in other aspects of their behaviour. This means that they will have lesser injury rates than those who don't wear helmets. We disagree.

Do Thompson, Thompson, and Rivara have any evidence at all? If so, what?

When examining the issue of helmet effectiveness injury severity will depend on the crash forces and helmet use status. Helmeted cyclists have fewer or less severe head injuries than non-wearers due to the protection provided by helmets. As we pointed out above, when crash severity is taken into account, the issues raised about either increased risk taking or more cautious behaviour on the part of helmeted cyclists becomes a non-issue. We submit that any conceptual confusion on this issue is with Dr. Keatinge.

I can only try to explain more clearly. The case-control studies aggregated by Thompson, Thompson, and Rivara as part of this Cochrane Review appear arithmetically competent and may well describe something useful about the effect of helmets in a crash. (There is much more to be said about this statement, but I accept it here for the sake of the present argument.) However, the studies aggregated by Thompson, Thompson, and Rivara cannot address the effects of helmets on the likelihood of having crashes, nor the deterrent effect of helmets on cycling. They do not, as reported, address the issue that cycle helmets may be tolerably effective against trivial trauma, but much less effective against serious trauma. All of these points are critical to their recommendation, which is therefore without adequate support. The U.S. Preventive Services Task Force would seem to have guides to methodology which Thompson, Thompson, and Rivara could usefully follow in future.

4.Answer: We agree.

5. Answer: We agree that not all injuries are identical. We disagree with his statement that helmets fail to protect cyclists from head injury in a motor vehicle crash.

I made no such statement. I do doubt if helmets are likely to protect against most fatal accidents involving a heavy goods vehicle.

In our study in 1996, which had 3390 dead and injured cyclists identified from medical examiner records, emergency department and hospital records we had sufficient numbers to look at specific strata. One stratum was comprised of cyclists who were, in fact, in collisions involving motor vehicles. We found that the protective effect of helmets controlling for crash severity and other factors (age, gender, cycle speed, crash surface, motor vehicle involvement) was the same as individuals who crashed on their cycles, but did not impact motor vehicles. (Thompson 1996) The Gilbert and McCarthy studied included only cyclists who died in a motor vehicle/heavy goods "vehicles" collision. Their study is sound. Obviously motor vehicles are a hazard to cyclists. However many cyclists involved in a motor vehicle crash are not killed. In order to evaluate helmet effects, it is necessary to include cyclists treated in the emergency departments and hospitals as well as those who died. We think that while it is not unreasonable to suppose what Dr. Keatinge suggests, the scientific data indicate that he is wrong.

My earlier comment may bear repeating: "Precise analysis is lacking, but one may reasonably doubt if any practical helmet would help most of those severely injured by a motor vehicle. Therefore, only small changes in risk-taking would be needed to overcome any slight protective effect that helmets might have in saving lives."

To repeat the point in even simpler language, cyclists who are killed by a motor vehicle are likely to have multiple injuries to most of their bodies. Even a helmet that protected from all head injuries would probably not save the majority. The studies reviewed by Thompson, Thompson, and Rivara include in the large majority nonfatal injuries, most of them probably without long-term significant consequences. To the extent that their results are valid, they may greatly overstate any protection offered by helmets against death or serious trauma. This conclusion is compatible with the evidence lumped together by Thompson, Thompson, and Rivara; the studies showing an effect of helmets came from conventional accident and emergency departments, where the enormous majority of head trauma seen or admitted is trivial. The study which showed no effect was from a regional paediatric trauma unit, where only genuinely severe cases would ordinarily be seen. The evidence is that cycle helmets have limited effect in protecting against most severe trauma.

There are numerous examples of cyclists hit by motor vehicles that had their helmets destroyed but were saved from head injuries. A recent example is Tour de France winner Lance Armstrong.

This is an interesting example of a non-argument, but one that has a superficial plausibility. Thompson, Thompson, and Rivara fail to give any reference for this story, but it seems a clear example of helmet enthusiasm, rather than evidence. From the Calgary Herald, June 11, 2001, I quote an analogous story:

Doctor says helmet may have saved teen's life / Robert Walker (Reporter)

A 16-year-old cyclist's life was saved by his helmet Sunday morning after a van drove over his head on Lower Springbank Road west of Calgary, says a city brain surgeon.

Dr. John Hurlbert, who has campaigned for a compulsory bicycle helmet law in Alberta, said the teen, who is in critical but the stable condition at Foothills Hospital intensive care unit, could pull through as a result of having some protection for his head.

"This just underlines the need for mandatory helmet use," said Hurlbert.

(snip)

"We are going to be doing more tests later on," she said. "It looked to emergency medical services like the van drove over his head. There was a crack in the helmet." Hurlbert said.

"We don't know the prognosis at this point, but there is no doubt the helmet saved his life," the doctor said.

"Without wearing the helmet his head would have been squashed."

I am interested in the story. I have a spare cycle helmet. As soon as I can easily mobilize a friend with a video camera, I propose to drive a small car over the helmet. Do Thompson, Thompson, and Rivara wish to make any predictions about the state of the helmet afterwards? If they have any specific hypothesis about it, or the melon that the helmet will contain, I will be interested to test it.

This story illustrates an alarmingly common mindset among helmet enthusiasts. Namely, that helmet use is obviously worthwhile, distorted anecdotes are evidence, and no real research, or thought, is needed.

6. Answer. We agree that cycling is healthy, particularly if cyclists wear helmets to protect their brains in the event of a fall. There is no long-term evidence that helmets discourage cycling. This posited effect of helmets on decreasing cycling (Does helmet promotion or mandated use decrease cycling?) is an appropriate topic for research. This topic was not part of our review.

This omission is one of the main reasons why the recommendations of the review is invalid. Without such a review, Thompson, Thompson, and Rivara have no foundation for their recommendations, which therefore do not belong in a Cochrane Review.

The opinions expressed by Hillman in "Cycling and Health" do not scientifically prove discouraging cycling leads to increase rates of heart disease. Those who stop cycling could well take up other activities such as walking.

This suggestion does not accord with everyday observation. It too should be tested by rigorous research before it is made the basis of public recommendations in a Cochrane Review.

7. Answer We have replied to Dorothy Robinson's comments on this topic. Please see our replies (Comment 2, numbers 5, 6, and 7). We disagree with Dorothy Robinson's selective interpretation of the Australian data. In our review, we pointed out that the Australian and New Zealand experiences after passage of helmet legislation may have suggested some diminution in cycling initially. These reports were subsequently corrected for various factors, such as change in hospital admission policies (Australia) and a longer period of follow-up (New Zealand). The follow-up reports indicated no effects on actual cycling rates. This issue still remains an open question, but it is far from clear that helmet usage is associated with decreased cycling, given the presently available published data. In the meanwhile, we stick by our conclusion, which is that there are no published studies that can convincingly make the case that wearing bicycle helmets significantly decreases cycling behaviour.

Both the published literature, and careful work available on the Internet, indicate a large effect. A recent example is John C. LeBlanc, Tricia L. Beattie, and Christopher Culligan. Effect of legislation on the use of bicycle helmets. CMAJ 2002; 166: 592-595, see also letters by Burdett, Wardlaw, and Gillham at http://www.cmaj.ca/cgi/eletters/166/5/592. This showed that after a helmet law in Nova Scotia, the number of cyclists approximately halved. A review of this topic is in preparation.

.

I note that in most countries cycle use is not subject to routine statistics, but I would expect fluctuations from time to time in this dimension of transport use, as in so many others. This provides almost unlimited scope for wishful thinking to inform "correction for various factors". A "longer period of follow-up" can also mean "we waited until ordinary fluctuations give us the results that we wanted in the first place".

8. Answer We disagree. Our literature search was extensive. The topic of our Cochrane Review was the effectiveness of bicycle helmets in preventing head and facial injury, not risk compensation. We feel that the proponents of risk compensation/homeostasis have shown that they select articles and anecdotal evidence that support their point of view. There are alternative explanations and selection biases that they do not mention. There are a number of studies in the traffic literature that point out problems or show data at odds with the RC/RH Theory.

I would be grateful for any such references that could be supplied. I would prefer if, in future, all references were complete, available, and used recognisable scientific methods to address the points at issue. It appears so far that the supporters of cycle helmets have failed to supply evidence to support their claims. Often they seem not even to understand what arguments they are opposing. Thompson, Thompson, and Rivara admit to a fundamental omission, which renders their recommendation valueless.

This is a varied literature and there hasn't been a systematic review. See our reply # 3 to Dr. Keatinge.

I refer Thompson, Thompson, and Riv ra to the summary at the head of this reply. Such a review is essential if their recommendations are to have any validity. One is in preparation. Until this is done, their recommendations can only be an embarrassment to the Cochrane Collaboration.

9. Answer. We agree that there are no quality studies supporting risk compensation behaviour among bicyclists. This is an appropriate area for a systematic review using pre-established criteria to judge the quality of the articles included in the review. Some well recognised groups using these methods are: The world-wide Cochrane Collaboration (hiru.mcmaster.ca/COCHRANE);Centre for Evidence-Based Medicine (CEBM) (http://www.cebm.jr2.ox.ac.uk; Agency for Health Research and Quality (AHRQ) Clinical Practice guidelines, http://www.text.nlm.nih.gov/fts/dbacess/ahcpr; the Canadian Preventive Services Task Force, US Community Preventive Services Task Force, and other groups. A well-conducted systematic review can take all the literature (peer reviewed, government reports and unpublished papers) and rate the study quality. Appropriately this gives more weight to better-designed and conducted studies. The evidence is then summarised across all the studies.

Since almost all funding in this area comes to people who are convinced that risk compensation is heretical, the lack of evidence is hardly surprising. The extensive evidence for the operation of risk compensation in other areas of transport has mostly, perforce, come from unfunded researchers using routine statistics. Given the very small rate of accidents per million kilometres cycled, definitive studies may be long and expensive. Work is nevertheless under way.

10. We disagree. Please see the discussion of time series studies in our reply to Dorothy Robinson, (Comment 2, replies #5 and #6). Note also that a new study from New Zealand reporting on the results of 3 years of helmet legislation found that "the helmet law has been an effective road safety intervention" (Scuffman 2000). Regarding the Pitt study, they found a reduction in head injuries following a sharp increase in helmet use. The authors concluded other bicycle injuries showed no change (Pitt 1994).

Helmets offer protection to the head and face, but not to other portions of the body. An increase in non-head injuries could be interpreted to indicate more people were cycling.

The evidence used by Pitt et al was unsatisfactory and so was their analysis. They took statistics on injuries to cyclists over a period during which cycle helmets became widely used, and indeed compulsory at the end. They included a three-year period before the law, during which head injuries and non-head injuries to cyclists fell in parallel until 1988/9. Thereafter, head injuries continued to fall, by 20.

(692207.jpg from http://www.bmj.com)

They claim this as evidence of the effectiveness of helmets, without seriously considering any underlying trends or the ordinary fluctuations in such data. The text fails even to mention that non-head injuries to cyclists increased by 20 over the last two-year period. The data in Pitt et al's paper is more obviously compatible with risk compensation, which Pitt et al completely fail to mention. However, my only real conclusion is that the data presented are insufficient to bear any relevant conclusions. Their paper is a prima facie case of absurdly biased interpretation, and, possibly, biased data selection. This paper displays no rational desire to test any relevant hypotheses, and its data is entirely inadequate to do so.

Much of the literature purporting to demonstrate the utility of cycle helmets is of a similar or lower standard. I also note that, to whatever extent Curnow's points about the mechanism of head injury may be correct, cycle helmets seem much more likely to protect against superficial injury than against serious brain trauma. Their effectiveness against serious injury will consequently be overestimated by the routine admission statistics that have commonly been used. As Robinson has correctly pointed out, the more sophisticated studies incorrectly identify underlying trends in accident rates with the results of helmet laws. Aggregating such studies could not improve their quality; garbage in, garbage out.

11. Answer: We agree that human behaviour is complex. We disagree with Dr. Keatinge's criticism of our Cochrane Review. Well-conducted observational and non-experimental study designs can yield information of sufficient quality for action. Please see Causal Relations in Medicine: A practical system for critical appraisal. (Elwood 1988).(see also Comment 1, reply #2) He has offered no scientific evidence to support his opinion. Systematic reviews of the evidence for the effectiveness of a wide range of road safety measures would be an appropriate topic for several Cochrane Reviews. We invite him to undertake systematic review of a topic in this area.

It will be interesting to consider what methods, if any, could give such a review validity. In the case of changes in cycle use after helmet laws, aggregating the results of invalid, often grotesquely biased studies does not appear to be a good investment of time. Possibly some helmet enthusiasts will proceed regardless. I suggest instead that research should test the main remaining hypotheses that need to be disproved before the recommendations of Thompson, Thompson, and Rivara can be considered worthy of serious discussion. To repeat, these hypotheses are:

Firstly, that helmet use deters cycling and reduces the amount of healthy exercise, therefore resulting in serious net loss of health.

Secondly, that helmet use results in physical instability and a slight increase of risk-taking behaviour, either by cyclists or motorists (the latter being the main source of danger on the roads). This results in slightly more accidents per million kilometres cycled. Helmet promotion may therefore result in an unchanged or increased net toll of death and morbidity per million kilometres cycled.

Thirdly, that helmets may protect against minor injuries, but have very limited value against any trauma likely to have a long-term effect on the victim.

There are other issues, many ably raised by Robinson and by Curnow and inadequately answered by Thompson, Thompson, and Rivara. But the above aggregate my main points of concern. A systematic review is under way.

Thompson, Thompson, and Rivara have inserted into a Cochrane Review a conclusion that requires reviews of four points, not the single point that their evidence addresses. Their main conclusion reflects no credit upon the Cochrane Collaboration. It should be withdrawn, with a note remaining to that effect.

References

Adams J. Risk. London: University College London, 1995.

Burdett, Wardlaw, and Gillham at http://www.cmaj.ca/cgi/eletters/166/5/592

Elwood MJ. Causal Relationships in Medicine: A practical system for critical appraisal. Oxford University press, 1988. See chapter 8, pages 164-165 on the assessment of causation, as well as other chapters in the text.

Evans L. Cycle helmets and the law: even when the science is clear policy decisions may still be difficult. BMJ 1994;1521-1522.1994).

Evans L. Traffic safety and the driver. New York: Van Nostrand Reinhold, 1991.

Fleming HS, Becker ER. The impact of the Texas 1989 motorcycle helmet law on total and head-related fatalities, severe injuries, and overall injuries. Medical Care 1992;30:832-45.

General Accounting Office. Highway Safety: motorcycle helmet laws save lives and reduce costs to society. Washington, DC: US General Accounting Office, 1991.

Gilbert and McCarthy. Deaths of cyclists in London 1985-92: the hazards of road traffic. BMJ 1994;308:1534-1537 (11 June);

Hedlund J. risky business: safety regulations, risk compensation, and individual behavior. Injury Prevention 2000;6:82-90.

Hillman M, Cycling offers important health benefits and should be encouraged. BMJ 1997;315:490 (23 August)

Hillman M. BMA Professional, Scientific, and International Affairs Division. Cycling: towards health and safety. Oxford: Oxford University Press, 1992

Hillman M. Cycle helmets: the case for and against. London: Policy Studies Institute,1993.

MMWR 2000 in press

Kraus JR, Peek C, McArthur DL, et al. The effect of the 1992 California motorcycle helmet usage law on motorcycle crash fatalities and injuries. JAMA 1994; 272:1506-11.

LeBlanc JC, Beattie TL, and Culligan C. Effect of legislation on the use of bicycle helmets. CMAJ 2002; 166: 592-595

MMWR US Community Preventive Services Task Force recommendations. in press 2001.

Morbidity and Mortality Weekly Report of the Center for Disease Control, vol. 50, May 18th 2001, file RR5007.pdf

Mullins J. Hard-headed choice. New Scientist 22 July 2000, no. 2248, pp. 16-17.

New Scientist, anonymous editorial 22 July 2000, no. 2248, p3.

NHTSA Fourth report to congress: Effectiveness of occupant protection systems and their use. May 1999.

NHTSA. Effectiveness of lap/shoulder belts in the back outboard seating positions. DOT HS 808 945 NHTSA Technical Report. June 1999.

Pitt WR, Thomas S, Nixon J et al. Trends in head injuries among child bicyclists. BMJ 1994:309:877.

Scuffman P, Alsop J, Cryer C, Langley J. Head injuries to bicyclists and the New Zealand bicycle helmet law. Accid Anal Prev 2000; 32,565-573.

Spaite DW, Murphy M, Criss EA, Valenzuela TD, Meislin HW. A prospective analysis of injury severity among helmeted and nonhelmeted bicyclists involved in collision with motor vehicles. Journal of Trauma, 1991; 31(11): 1510-1516.

Thompson DC, Rivara FP, Thompson RS. Effectiveness of bicycle safety helmets in preventing head injuries: a case-control study. JAMA, 1996:276(24): 1968-1973

Contributors to comment

Author of comments: Richard Keatinge

Authors responding: Diane C. Thompson, Robert S. Thompson, Frederick P. Rivara

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

Evidence from Cohort vs Helmet Law Studies

The reason for studying helmet efficacy is to predict changes in injury rates when a population of cyclists all start to wear helmets. Helmets can only be considered effective if the increased helmet wearing actually reduces injuries.

The authors of this Review discuss ways of evaluating helmet efficacy under sub heading "Background", stating that "Randomized control trials are neither feasible nor ethical towards this end. Cohort studies are not practical because of the large population required for follow-up"

Though cohort studies are not practicable, consequences of helmet laws have been studied in countries such as Australia where helmet wearing rates increased from 26% - 38% pre-law to 75% - 85% post-law. Despite these extremely large increases in helmet wearing, virtually no change was detected in head injury rates over and above the prevailing trend for other road users.

As evidence for this, a paper is enclosed containing tables and graphs of head injury rates before and after helmet laws in most Australian States. The laws in Australia were introduced at different times in different States over a two year period. In some States, such as Victoria, bicycle helmet laws coincided with the introduction of other road safety measures including speed cameras and increased random breath testing. In others, eg Western Australia (WA), there were no coincident road safety initiatives. Such data enable comparison of the efficacy of helmet laws with other road safety measures.

It is well known that most case-control studies of bicycle helmets have found substantial differences between those who chose to wear helmets and those who do not. However, the differences relate not just to head injury rates, but also many other attributes of the two groups. In Seattle, an observational study showed children wearing helmets were much more often white than black or other races, and riding in parks or bicycle paths than on city streets (DiGuisseppi et al. 1989). Two of the case-control studies used for the Cochrane Review found helmeted riders were less likely to be involved in vehicle/bike collisions - 12.7% of helmeted riders vs 18.0% of non helmeted riders in the study by Thompson et al. (1996); 31.6% of helmet wearers in the study by Maimaris et al. (1994) compared with 43.4% for non-wearers. McDermott et al. (1993) did not report motor vehicle involvement, but found little difference in percentages with heads or helmets hitting a moving motor vehicle (16.9% vs 14.8%). The remaining two studies in the Cochrane Review do not provide enough information to compare motor vehicle involvement in wearers and non-wearers.

Several studies, eg Maimaris et al. (1994), Thomas et al. (1994) show head injury is 3 to 5 times more likely in cyclists involved in accidents with motor vehicles. The risk of head injury also varies considerably with age (Robinson 1996), as do helmet wearing rates. Fundamental differences between populations choosing to wear or not to wear helmets make it difficult for any case-control study to separate these effects.

In contrast, a study comparing hospital admissions for all cyclists before and after a helmet law, compares the pre-law population of non-wearers with the same population of wearers post-law. Apart from those who give up, essentially the same group of cyclists is being compared. This means there should be less difficulty adjusting for bias, making the results more reliable.

Yet these latter studies (see enclosed paper) show little or no benefit of increased helmet wearing from the laws, contradicting the findings of the case-control studies, perhaps due to difficulties in adjusting for fundamental differences between those choosing to wear helmets and those choosing otherwise. Another potential problems is risk compensation. Cyclists choosing to wear helmets may feel protected and so take more risks. If increased risk taking leads to more accidents, helmet wearers may have increased, rather than decreased total risk relative to the amount of cycling, even if the risk of head injury in any particular accident is decreased. Should helmet wearing be recommended if this happens?

These facts are relevant, interesting and should be an essential, if not the most important part of any Review by the Cochrane Collaboration. What would be the point in promoting helmets, if increased helmet wearing does not result in reduced injuries? Unfortunately, this Review fails to discuss this.

Several references are cited by this Review (Vulcan et al. 1992, Carr et al. 1995, Pitt et al. 1994, Ekman et al. 1997, Rivara et al. 1998) as providing evidence of helmet effectiveness from time series analysis before and after helmet laws. However, examination of these references shows that the evidence for this comment is very shaky, if non existent. When analysing the first three years of post helmet law data in the State of Victoria, Cameron et al. (1994) reported that results showed "the observed proportion of head injured cases to be no different from the downward trend predicted by the model using pre-law wearing rates." A year later, Carr et al. (1995), found that numbers of head injuries had been significantly affected by the law in Victoria, but that an analysis of head injuries alone could not determine whether this was due to reductions in numbers of cyclists, improvements in road safety, or helmets. In fact, if numbers of head and non-head injuries are shown together on the same graph (Attached paper, Figure 4) it is clear that non-head injuries show a similar decline to head injuries, therefore the main effect was not helmets, but other factors, such as safer roads or reduced cycling activity. These data on hospital admissions for head and other injuries in Victoria (and WA and NSW and SA and Qld) following laws which forced literally millions of cyclists to wear helmets directly contradict the case-control studies. It would be difficult if not impossible for helmets to be as effective as claimed by the Review report, yet evaluations of helmet laws be so inconclusive. The contradictions can only arise from problems in the scientific methodology such as difficulties in adjusting correctly for differences between groups choosing whether or not to wear helmets.

For Queensland, Pitt et al. (1994) reported that: "The decrease in bicycle related head injuries started before helmets became widely used and occurred against a background of unchanged admission rates for other bicycle related injuries and a decrease in head injuries due to other causesà. Our findings indicate that the reason for the decrease in bicycle related head injuries is more complex than just increased wearing of helmets."

Ekman et al. (1997) found, in Skaraborg County, Sweden, where helmet wearing programs resulted in a substantial increase in wearing rates, head injuries in children under 15 fell by 59% and non-head injuries 48%. For Sweden as a whole, child cyclist head injuries fell by 43% vs 32% for non-head injuries. In Sweden as a whole, head injuries fell to 84% of non-head injuries (100-43)/(100-32). For Skaraborg, the same calculation gives (100-59)/(100-48) ie 79%. Thus as a proportion of total injuries, the fall for head injuries was little different to that for Sweden as a whole. (79% vs 84%, a difference of only 5 percentage points). A much bigger difference is seen in the fall in non-head injuries in Skaraborg vs Sweden (100-48)/(100-32) = 76%, a difference of 24%, suggesting that the helmet wearing programs had other, larger effects which reduced numbers of non-head injuries, such safer conditions or reduced cycling.

For Seattle, Rivara et al. (1994) reported that an increase in helmet use among school age children from 5.5% in 1987 to 40.2% in 1992 was accompanied by a 66.6% decrease in bicycle related head injuries in 5 to 9 year olds and a 67.6% decrease in 10 to 14 year old members of a health maintenance organisation. However, unless those wearing helmets are many times more likely to seek hospital treatment, a relatively small increase in helmet wearing (5.5% to 40%) cannot plausibly produce a two thirds reduction in head injuries. This would not be possible, even in helmets prevented all head injuries, let alone given that, for helmet wearers seeking emergency room treatment in Seattle, 20% of helmet wearers under 6 years, 12% of helmet wearers aged 6-12, and 13% of helmet wearer aged 13-19 were treated for head injury (Thompson et al. 1996). Other factors such as trends or increased accident risk for helmet wearers must therefore be present in the data studied by Thomson et al. (1996) potentially invalidating their conclusions.

Closer consideration of the five cited reports shows, therefore, that they provide no plausible evidence for the effect of helmets from time series analysis as claimed by the authors of this Review Report. Another major time series analysis, Scuffham and Langley, (1997), not mentioned by this Review, observed a downward trend in the percentage of cyclists suffering head injuries "present before, and independent of, helmet wearing." After accounting for this trend, increased helmet wearing had "little association with serious head injuries as a percentage of all serious injuries to cyclists." Such trends are a confounding factor which the reviewed case-control studies appear not to have estimated or adjusted for. This leaves the results of such case-control studies open to question. After correctly adjusting for such trends, no published time series analysis has been able to show any noticeable decrease in head injury rates, despite the large increases in helmet wearing because of helmet laws or substantial promotion of helmets.

If, in reality, no benefit can be shown from such measures, but other road safety campaigns such as speed cameras and targeting drink-driving produce large and highly noticeable reductions in the death and injury toll for all road users (Powles and Gifford 1993) why not spend the generally limited funds for road safety on the measures shown to be highly effective?

Risk Compensation

The differing helmet wearing rates in one of the studies in this Review could be interpreted as evidence that helmet wearing increases injury rates. In Seattle, in May and September 1987, street surveys found helmet wearing rates of only 3.1% and 3.3% in samples of 1957 and 2544 child cyclists aged 5-14 (DiGuisseppi et al. 1989). Survey sites were stratified according to census data on income and number of children residing in each area, in an attempt to provide an unbiased estimate of the helmet wearing rate. Coincidentally, the first study of helmet efficacy (Thompson et al. 1989) in the Cochrane Review took place around the same time (1 December 1996 to November 30 1987) at the five major teaching hospitals in Seattle. 15 out of 345, ie 4.3% of cyclists 14 years and younger who required treatment at the emergency department were wearing helmets (3 out of 145 = 2.1% of those with head injury). For this age group, helmet wearing in the community control of Thompson et al. (1989), consisting of cyclists who reported falling off their bikes, was 21.1%. Thus there was no significant difference between helmet wearing rates in the survey of child cyclists riding round the streets of Seattle and those requiring emergency room treatment for head injury. The largest and most significant difference was, in fact, between helmet wearing in children seen riding around Seattle and those in the community control group who fell off their bikes.

Thus one might conclude that helmet wearing in children is not significantly associated with reducing head injury, but increased risk of falling off a bike. An alternative possibility is that the community controls differed in their helmet wearing because they were members of a Group Health Cooperative, which may have promoted helmet wearing to its members. Either way, the 7-fold difference in helmet wearing rates between the community controls in this study and in cyclists of the same age group riding on Seattle streets invalidates the conclusions of this study making it ineligible for inclusion in the Cochrane Review. Whilst use of emergency room controls gives a less biased result, helmet wearing in child cyclists requiring emergency room treatment was also higher than in the street survey, suggesting that, if case-control studies are to provide a valid estimate of the total effect of helmets (including any risk compensation), helmet wearing in the 'case' group of head injured cyclists should be compared not only to controls requiring emergency room treatment for other injuries, but to population wearing rates of non-injured cyclists, or controls chosen from street surveys of cyclists not involved in accidents.

Another interesting finding of some other surveys (eg Farris et al. 1997) is that helmet wearers often appear more likely to obey traffic laws and, at least in two of the studies used in the Cochrane Review, where helmet wearing was published by motor vehicle involvement, may also be less likely to be involved in bike/motor vehicle accidents. At first sight, this appears contradictory to the risk compensation hypothesis. However, careful or risk-averse cyclists may be more likely to wear helmets than the general population. Even if helmet wearing encourages such risk-averse cyclists to take more risks, this does not necessarily mean their overall risk will increase beyond the average of those whose chose not to wear helmets. Nonetheless, any change in risk will negate the benefits of helmets for that particular group. The same applies to cyclists required by law to wear helmets, where again increased risk taking may negate any potential benefits of helmets.

Studies in which a number of different treatments are applied to the same person usually have substantially increased sensitivity compared with studies which apply different treatments to different groups of people, because the within-subject variance is generally much lower than that between subjects. For this reason, monitoring changes in accident and head injury rates after helmet laws (where non-helmeted cyclists pre-law are effectively compared with the same group of cyclists now wearing helmets because of the law) should provide a better and more realistic effect of the true effect helmet wearing has on accident and head injury rates. A discussion of helmet wearing and accident rates is available at: http://lash.une.edu.au/~drobinso/bhacc.html.

Misleading Interpretation of Odds Ratios

As well as inadequate discussion of time series analyses and why little or no benefits have been found following helmet laws which required millions of cyclists in Australia to wear helmets, this Review appears to confuse odds ratios with the percentages of head injuries which might be prevented by helmets. This is a serious problem, potentially leading to significant errors of interpretation.

For example, imagine a hypothetical helmet which could prevent 50% of head injuries and a situation where 100 bareheaded cyclists had accidents, resulting in 40% of them suffering head injuries. Imagine a case-control study where 100 cyclists wearing this helmet had identical accidents to the 100 bareheaded cyclists. We would therefore expect on average 50% of 40 = 20 of these cyclists to suffer head injuries.

To calculate the odds ratio, we compute Odds(helmeted) = (No of helmeted with HI)/(No of helmeted without HI) and Odds(bareheaded) = (No of bareheaded with HI)/(No of bareheaded without HI)

The odds ratio is Odds(helmeted)/Odds(bareheaded) = 20/80 divided by 40/60 ie 0.375.

The authors of this Review have used this odds ratio interchangeably with the term "percentage reduction in head injuries" as if suggesting that odds of .375 means a 62.5% reduction in head injuries, not the 50% reduction it represented in this hypothetical example. Page 6 of this Review gives combined adjusted odds ratios from four of the studies of 0.31 for cyclists in bike/motor vehicle accidents and 0.32 for other crashes, which is translated, under the heading "Main results" into protection levels of 69% and 68%.

The fact that this can be confusing to other researchers is well illustrated by first of the studies in the Cochrane Review. Thompson et al. (1989), found 17 out of 120 helmet wearers had head injury (14.2%), compared to 218/548 (39.8%) of non wearers. This leads to an estimate that helmets would prevent (1 - 14.2/39.8) = 64% of head injuries. The crude odds ratio was 0.25 and the adjusted odds ratio was 0.26. The summary of Thompson et al. (1989) did not present this figure, only the value obtained by comparison with the community control group of predominantly child cyclists who fell off their bikes. As already discussed, these children had an extraordinarily high helmet wearing rate of 21.1%, compared to 3.2% in a stratified sample of 4501 children riding around Seattle but not falling off their bikes. The estimate, derived from the comparison with this exceptionally high helmet wearing group, was the only estimate of helmet efficacy presented in the summary of Thompson et al. (1989) "helmeted riders had an 85% reduction in their risk of head injury (odds ratio 0.15; CI 0.0-7 to 0.29)"

Sacks et al. (1991), interpreted the latter figures to mean that, out of 2985 head injury deaths to cyclists in the US over a 5 year period, up to 84% (2500) could be prevented if all cyclists wore helmets. An alternative estimate, based on the adjusted odds ratio of 0.26 was that 2148 (72% of total) could be prevented based. Again, Sacks et al. (1991) mistakenly believed that odds of 0.26 means that 74% of head injuries would be prevented, instead of the actual value of 63%, obtained by comparing injury rates in (potentially disparate) groups of helmeted and non-helmeted cyclists treated in the emergency room. In the Main Results section of their paper, Sacks et al. (1991) translated these figures into the claim that universal use of helmets by all bicyclists "could have prevented as many as 2500 deaths and 757 head injuries, ie one death every day and one head injury every 4 minutes." The latter part of the above comment is currently (May 30, 1999) reproduced on the CDC's web site: http://www.cdc.gov/ncipc/bike/problem.htm

The authors of this Review for the Cochrane Collaboration refer to the paper by Sacks et al., so obviously know of this misinterpretation of their results, but never appear to have attempted to correct it, or explained to the CDC why this is invalid. And they still continue to mislead other scientists by referring to odds ratios as percentage reductions in head injuries.

A final problem with this Review is that the results of McDermott et al. (1993), which happen to show the lowest benefit for helmets, are not used in the computation of odds ratios. This was claimed to be because no adjusted odds ratios were available. The authors of the Review state (p5) that "if the authors (McDermott et al.) had adjusted for motor vehicle involvement the results would have indicated an even greater protective effect of helmet use." Yet there is little evidence in the paper to back up this claim. Table 2 from McDermott et al. (1993) shows 14.8% of non wearers hit their heads or faces on a moving motor vehicle, not significantly different from the 16.9% of approved helmet wearers. Any adjustment for motor vehicle involvement would be very minor. More interestingly, 50% of helmet wearers who hit their heads after collision with a motor vehicle had head injuries, compared to 71% of non helmet wearers. Clearly, if 50% of helmet wearers in who hit their heads after collision with a moving vehicle were head injured, it is implausible to suggest, as this Review does, that helmets normally prevent 69% of such head injuries. McDermott's study is a real live counter example which clearly contradicts this claim.

The post helmet-law results for Australia are also a counter-example to this claim. If helmets prevented 70% of head injuries, then in Western Australia, when helmet wearing increased from less than 39% to more than 80%, it is highly implausible there would be so little evidence for any real effect. (Figure 2, attached paper).

Conclusions and Recommendations

The Cochrane Collaboration has carried out some excellent and necessary work, addressing such questions as: "What evidence is there for the efficacy of spinal fusion?" or "After cardiac bypass surgery, is it beneficial to administer glutamate to reduce the risk of neurological complications such as stroke or seizure?" These questions can and should be answered by meta analysis of available randomised controlled trials. Results rely on the randomisation process in each trial to ensure unbiasedness. The overall findings are valid provided the evidence from the different trials is not contradictory. In data from different sources are contradictory, explanations are necessary for the different circumstances pertaining in each of the trials and the conclusions would most likely be, "In circumstances such as A, administering D is generally effective, but this is not the case for circumstances such as B"

For bicycle helmets, the main questions are likely to be, are they effective (and cost effective), and should we recommend or mandate that cyclists wear them? In this case, the act of recommending or mandating that helmets be worn may change the circumstances, eg by making cyclists feel safer and take more risks. The act of recommending or mandating helmets may also discourage people from cycling. These factors cannot be considered in isolation from the effect of helmets. They are part of the package that comes with mandating or recommending helmets.

For this reason, changes in head injury rates following helmet laws, or the results described by Scuffham et al. (1997) for increased helmet wearing following pre-law promotion are much more relevant than the case-control studies discussed in this Review, especially since the two sets of results contradict each other. The most likely explanations for this are the inherent problems of using self selected samples such as those choosing whether or not to wear a helmet with all that entails for differences in riding behaviour, riding styles and attitudes to risk, as well as potential for helmets to increase rotational injuries, the limited range of impact speeds for which helmets are designed, and failure to adjust for trends in the head injury rates evident in Scuffham's study, the Australian data and, it appears, also in Seattle (Rivara et al. 1994).

This Review completely fails to address these problems, nor even discusses why, in the first study in this Review (Thompson et al. 1989) helmet wearing by children in the community control sample of children was 21.1%, compared with 3.2% in a survey of child cyclists riding round the catchment area for these hospitals, nor whether this discrepancy means we should conclude that helmet wearers have a similar risk of head injury to the general population, but increased risk of falling off their bikes!

This Review also fails to discuss the effects of helmet laws and increased helmet wearing with adequate depth and apparently misquotes or misinterprets the papers cited on changes in head injuries rates after helmet laws or intensive promotion of helmets.

This Review also misleads the general reader by quoting odds ratios as percentage reductions in head injuries.

In short, this Review cannot be recommended as a valid interpretation of the existing published information on helmets.

An evaluation of helmet laws in Australia, drawing on the data from Victoria, New South Wales, South Australia, Queensland and Western Australia, where helmet laws were enacted between 1990 and 1992 is included as a separate appendix to this document. If the subject matter is considered appropriate for the Cochrane Collaboration, a new Review should be commissioned. The new review should incorporate results on head injuries following helmet laws, and discuss all aspects of helmets and helmet promotion, including possible effects of risk compensation resulting in increased accident rates (See http://lash.une.edu.au/~drobinso/bhacc.html), the effect of discouraging of cycling because of laws or increases in perceived danger following helmet promotion, and other effects such as potential increases in rotational injuries of the brain. It may be appropriate to invite comments from cycling groups and organisations, especially in Australia, because of their direct experience of the efficacy of helmet laws compared with other road safety initiatives. Costs and benefits of helmets in cars compared with bicycles (see http://lash.une.edu.au/~drobinso/carhel.htm) should also be included in the review.

Dorothy L Robinson,

Snr Statistician.

Literature Cited

Cameron M, Newstead S, Vulcan P. Finch C; Effects of the compulsory bicycle helmet wearing law in Victoria during its first three years. Proc. 1994 Australian pedestrian and bicyclist safety and travel workshop.

DiGuisseppi, CG.; Rivara, FP.; Koepsell, TD.; Polissar, L. Bicycle helmet use by children. Evaluation of a community-wide helmet campaign. J. Amer. Med. Assoc. 262:2256-61; 1989.

Farris C, Spaite DW, Criss EA, Valenzuela TD, Meislin HW Observational evaluation of compliance with traffic regulations among helmeted and nonhelmeted bicyclists. Ann Emerg Med 199;29(5):625-9

Maimaris C, Summers CL, Browning C, Palmer CR. Injury patters in cyclists attending an accident and emergency department: a comparison of helmet wearers and non-wearers. BMJ 1994; 308: 1537-40.

McDermott FT, Lane JC, Brazenor GA, Debney EA. The effectiveness of bicyclist helmets: a study of 1710 casualties. J Trauma 1993;34:834-45.

Powles JW, Gifford S. Health of nations: lessons from Victoria, Australia. BMJ 1993;306:125-7.

Rivara, FP, Thompson DC, Thompson RS, Rogers LW, Alexander B, Felix D, Bergman AB. The Seattle children's bicycle helmet campaign" changes in helmet use and head injury admissions. Pediatrics 1994 93: 567-9

Rivara, FP, Thompson DC, Patterson MQ, Thompson RS. Prevention of bicycle-related injuries: Helmets, education and legislation. Annu. Rev. Public Health, 1998; 19:293-318.

Robinson DL. Head injuries and bicycle helmet laws. Accident Analysis & Prevention 1996;28(4):463-475.

Scuffham, PA, Langley, JD. Trends in cycle injury in New Zealand under voluntary helmet uses. Accid Anal Prev 1997;29:1-9.

Thomas S, Acton C, Nixon J, Battistutta D, Pitt WR, Clark R. Effectiveness of bicycle helmets in preventing head injury in children: case-control study. BMJ 1994;308:173-6.

Thompson RS, Rivara FP, Thompson DC. A case-control study of the effectiveness of bicycle safety helmets. N Engl J Med 1989;320:1361-7.

Thompson DC, Rivara FP, Thompson RS. Effectiveness of bicycle safety helmets in preventing head injury: a case-control study. JAMA 1996;276:1968-73.

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.

1. Introduction

As the wearing of bicycle helmets is a preventive medical treatment, the evidence for its efficacy is a suitable subject for scientific review within the Cochrane Collaboration. This review is of particular importance because the findings of the studies included in it, hereinafter called review studies, and others like them underpin public policies of advocating and compelling helmet wearing. For example, a pamphlet of the US Department of Transportation that advocates helmet wearing states: "FACT: A bicycle helmet reduces the risk of serious head and brain injury by 85-88 per cent." Upon inquiry to the Department, it verified that the basis of this statement is the 1990 review study by Thompson et al.

The review should be of scientific evidence. This requires, first, that there is a hypothesis to explain how helmet wearing and changes in head injury are linked by proximate cause and effect obeying scientific laws. Second, the hypothesis should be tested by experiment or, with human subjects, randomised controlled trial.

Injury to the brain is the head injury most important for public health, and has been the focus of campaigns in Australia to promote the wearing of helmets. It, and not superficial injury to soft tissue and fracture of the skull, would, if helmets were efficacious, make the wearing of them a more important medical treatment than say pads to protect knees or other parts of the body. The discussion in section 2 below comprises a summary of some scientific principles concerning mechanisms of injury to the brain resulting from a sudden change of velocity, and how thought about bicycle helmets and standards for them developed.

2. Head injury and helmets

Holbourn formulated a mechanics of brain injury according to physical properties of the brain and laws of physics. He pointed out the extreme incompressibility of brain substance and its low rigidity. Thus, the brain does not appreciably change in size when subjected to uniform pressure in all directions, and strains of compression and rarefaction produced by linear forces of acceleration/deceleration from a direct blow do not injure it. But brain substance offers little resistance to changes in shape and is easily injured by shear-strain, the type of deformation which occurs in a pack of cards when it is deformed from a neat rectangular pile into an oblique-angled pile. The predominant cause of shear-strain and brain injury is sudden rotation of the head. After Holbourn, experiments with monkeys showed that linear forces are a minor cause only. ,

Development of bicycle helmets began empirically and has had insufficient regard to the mechanics of brain injury. By analogy with helmets worn by soldiers, miners and the like, it might "make inherent sense" that helmets can protect cyclists against brain injury, but the analogy is false. The dominant mechanism that results in rotation of the head when a cyclist weighing say 50 kg is in a collision is a large change in momentum. By contrast, when the head is struck by a 10 gram bullet travelling at 400 metres per second, per Holbourn's example, the dissipation of its high kinetic energy is dominant but its low mass and momentum produce little rotation - according to Holbourn about the same as that produced when one walks into a wall at 2 miles per hour. Another comparison would be a 100-gram stone that had fallen 80 metres.

The mode of development of helmets is reflected in the standards by which they are tested and designed. The concern should be with the problem of reducing rotation of the head due to change of momentum, but the standards reflect no cognisance of that. Instead, their concern is with reducing the linear deceleration of the head by absorbing energy upon impact. To measure these, a helmeted head form is dropped onto a hard surface. This preoccupation with absorbing energy apparently is a result of plastic foams becoming readily available. Rather than a rational response to the main cause of brain injury, the development of helmets has been driven by the technology of mass-producing new styles in which energy-absorbing foam is the constant component, while hard shells have largely been abandoned in favour of thin shells or no shells.

There is good reason to expect that helmet wearing may actually increase the risk of brain injury. In Australia, Corner et al showed experimentally that the added mass of a helmet has the effect of increasing the rotation which a glancing blow may impart to the head. Corner et al also recommended against soft-shell helmets being allowed, saying that shells should be very stiff with a low impact sliding reaction. Instead, the Australian standard was amended to allow soft-shell helmets. Tests of impacts of helmets on asphalt at 34 km/h since conducted in Sweden have shown that, unlike hard-shell helmets which slide, soft helmets grab the surface, rotating the head.

Australia's National Health and Medical Research Council independently assessed the role of helmets in head protection in a 1994 study of football injuries, copy of page 58 subjoined as Attachment 1. In answering the question "Do helmets reduce injury?", the NHMRC assessed four studies examining the effectiveness of cycling helmets, among them the 1989 review study of Thompson et al, as providing evidence that helmets reduce soft tissue injuries only. The NHMRC's assessment was that helmets may possibly reduce scalp lacerations and other soft tissue injury, but may increase cerebral and non-cerebral injuries including diffuse brain injury. On this assessment, protection from superficial injury would be at the cost of increased risk of brain injury.

The research of the review

The foregoing suggests that the wearing of bicycle helmets of current design is not a medical treatment conducive to public health. Strong scientific evidence would be needed to show the contrary, but none of the five review studies provides it. First, none of them is a randomised controlled trial, the normal standard for Cochrane reviews. Case-control studies are well known to be less reliable, mainly due to deficiencies in the study base, which should be a large random sample.

Second, none of the review studies contains a hypothesis explaining, in accord with scientific knowledge, the mechanism by which helmet wearing might bring about reductions in head and facial injury. The hypothesis implicit in them, and expressed on pages 3 and 6 of the review, is simply that the causal mechanism is that helmets protect cyclists from head injury - which is merely a statement of the problem to be solved. The studies, however detailed their data or refined their statistical methods, are therefore mere empirical correlations of helmet wearing with head injury. They do not establish mechanisms of cause and effect applicable to other circumstances. The Thomas study frankly acknowledges this. Referring to legislation by then operative in Australia, it suggested that studies in populations with high compliance with compulsory helmet wearing will give insight into these issues Indeed, the legislation provided the world's first opportunity to test the efficacy of helmets at a high level of use, but governments did not establish monitoring systems to take advantage of this. Such data as are available suggest that, despite official claims to the contrary, compulsory helmet wearing has not reduced the rate of head injury. Its main effect has been to discourage cycling.

A hypothesis that a helmet protects against lacerations to the scalp and injuries to other soft tissues that it covers is plausible but, prima facie, the suggestion that helmets protect against injuries to the face including orbit/eye, nose and middle face is not. The studies of Maimaris and Thomas appear to acknowledge this. That three of the review studies did find reductions in facial injuries suggests that some important factor not allowed for may have affected their findings, and perhaps similar findings for head injuries too.

Conclusion

The studies included in the review do not provide scientific evidence of the efficacy of helmet wearing for cyclists that is of a standard appropriate for a Cochrane review.

ATTACHMENT 1

NHMRC REPORT ON FOOTBALL INJURIES OF THE HEAD AND NECK, P. 58

6.1.8 Do helmets reduce injury?

The evidence that helmets reduce soft tissue injuries is shown by studies examining the effectiveness of cycling helmets (Dorsh, Woodward & Somers 1987; Thompson, R., Rivara & Thompson, D 1989; Weiss 1987; Wasserman 1990).

In American football, the National Head and Neck Injury Registry (Torg 1982) prospectively monitors catastrophic injury. When the data are compared both before and after the introduction of NOCSAE-certified helmets, there is a 54 per cent drop in head injury rates. However, this must be interpreted with caution, since there were coexistent rule changes related to preventing head injury and it appeared that the incidence of head injury was dropping prior to helmet introduction anyway.

In Australian football and rugby codes, prospective injury surveys (Seward et al. 1993) have found too few players wearing helmets for adequate statistical interpretation.

Following a recent death in Rugby union (1994), media reports questioned the role of helmets in football. In this instance, the player's head impacted on hard ground after a legal tackle, causing an acute subdural haematoma. It would appear that' injuries in the 17-21 age group are more common than in younger age groups. Head protectors worn by players in the 17-21 age group, therefore, may help to reduce the number of injuries; however, this is scientifically unproved. In addition, there appears to be a general reluctance by players of this age group to wear protective equipment. Psychological acceptance of headgear, if head protectors are found to be appropriate after further studies, would be more easily obtained if it was introduced at an earlier age.

6.1.9 Potential risks of helmet use

Whilst helmets may possibly reduce the incidence of scalp lacerations and other soft tissue injury, there is the risk that helmets may actually increase both the cerebral and non-cerebral injury rates through a number of mechanisms.

(a) Sport-specific helmet design has not been established for Australian football or the rugby codes;

The need to use the correct helmet for a specific activity has been supported by research into helmet performance (Bishop 1984).

(b) The addition of a helmet to the head will increase both the size and mass of the head. This means that blows that would have been glancing become more solid and thus transmit increased rotational forces to the brain. The leverage factor means that any head protectors should be close-fitting;

(c) Because helmets distribute the force from focal impacts across a larger area, this may result in reduced fracture/ laceration injuries but may increase diffuse brain injury;

(d) Misplaced faith in an ineffective helmet may create a false sense of security and encourage players to place themselves in dangerous situations and ignore the usual precautionary tactics used in these situations, thereby increasing their injury risk;

58

I certify that I have no affiliations with or involvement in any organisation or entity with a direct financial interest in the subject matter of my criticisms.