Bicycle Helmets and Diffuse Axonal Injury

In an examination of a meta-analysis demonstrating bicycle helmets are effective in mitigating head injury[1], Bill Curnow suggested helmets exacerbate rotational injuries, the more serious being diffuse axonal injury (DAI) [2]. The implication of this hypothesis is clear — bicycle helmets have the potential to increase injury risk. However, no supportive cycling injury data is presented and many have taken this hypothesis as fact[3-8]. Note that these groups openly advocate against helmet use and/or helmet laws and Curnow runs an anti-helmet website (crag.asn.au). The notable exception is a website maintained by the University of Michigan School of Public Health which, given the lack of evidence discussed below, is disappointing.

There are now several studies, with varying degrees of experimental control and real-world comparability, that have investigated the proposed bicycle helmet/DAI link. None of these studies has found evidence supportive of Curnow’s hypothesis.

  • A series of computer simulations found helmets did not increase the likelihood of neck injury for children or adults[9,10],
  • a series of oblique impact dummy tests found helmets did not increase angular acceleration[11],
  • there were no DAI cases found among 110 cyclists in a trauma registry[12] and
  • only 12 potential DAI cases were identified among 6745 cyclists involved in a collision with a motor vehicle (7 were unhelmeted)[13].

In summary, there is no evidence supportive of Curnow’s DAI hypothesis for bicycle helmets. In fact, given available injury data, diffuse axonal injury is uncommon for cyclists presenting to a hospital or trauma center. Given the lack of supportive evidence, Curnow’s DAI hypothesis should not be used to influence cycling safety policy.

A more detailed analysis (and other cycling-related analyses) can be found in our peer-reviewed paper[14].

  1. Attewell, R.G., Glase, K. & McFadden, M. (2001). Bicycle helmet efficacy: a meta-analysis. Accident Analysis and Prevention, 33, 345–352.
  2. Curnow, W.J. (2003). The efficacy of bicycle helmets against brain injury. Accident Analysis and Prevention, 35, 287-292.
  3. BHRF. (2003). Cycle helmets and rotational injuries. Bicycle Helmet Research Foundation. Available at: http://www.cyclehelmets.org/1039.html. (accessed 19.07.13)
  4. Bicycle Australia. (2010). Bicycle Helmets. Available at: http://bicycleaustralia.org/helmets.php. (accessed 19.07.13)
  5. Bicycle NSW. (2013). Bicycle Helmets. Available at: http://bicyclensw.org.au/advocacy/positions/legal/helmets/. (accessed 19.07.13)
  6. Gillham, C. (2011). Mandatory bicycle helmet law in Western Australia. Available at: http://www.cycle-helmets.com/. (accessed 19.07.13)
  7. Stewart, M. (2012). The Myth of the Bicycle Helmet. Available at: http://www.risksense.org/2012/06/14/the-myth-of-the-bicycle-helmet/. (accessed 19.07.13)
  8. Rissel, C. (2012). The impact of compulsory cycle helmet legislation on cyclist head injuries in New South Wales, Australia: A rejoinder. Accident Analysis and Prevention, 45, 107-109.
  9. McNally, D.S. & Rosenberg, N.M. (2013). MADYMO simulation of children in cycle accidents: A novel approach in risk assessment. Accident Analysis and Prevention, 59, 469–478.
  10. McNally, D.S. & Whitehead, S. (2013). A computational simulation study of helmet wearing on head injury risk in adult cyclists. Accident Analysis and Prevention, 60, 15–23.
  11. McIntosh, A.S., Lai, A. & Schilter, E. (2013). Bicycle Helmets: Head Impact Dynamics in Helmeted and Unhelmeted Oblique Impact Tests. Traffic Injury Prevention, 14, 501-508.
  12. Dinh, M.M., Curtis, K. & Ivers, R. (2013). The effectiveness of helmets in reducing head injuries and hospital treatment costs: a multicentre study. MJA, 198, 416-417.
  13. Walter, S.R., Olivier, J., Churches, T. & Grzebieta, R. (2013). The impact of compulsory helmet legislation on cyclist head injuries in New South Wales, Australia: A response. 
  14. Olivier, J., Grzebieta, R., Wang, J.J.J. & Walter, S. (2013). Statistical Errors in Anti-Helmet Arguments. Australasian College of Road Safety Conference.

Safety in Numbers Hypothesis for Cycling

Safety in Numbers is a well known hypothesis in cycling safety. Essentially, the argument is the number of injuries per cyclist decreases as the amount of cycling increases [1,2]. Or put another way, the risk of cycling injury increases when cycling amounts decrease.

The mathematical expression for safety in numbers can be written as

I=I_0\left(\dfrac{C}{C_0}\right)^{0.4}

where I and C represent number of injuries and amount of cycling respectively. The exponent 0.4 has been suggested by Robinson[2].

Is there evidence supporting this phenomena using NSW hospitalization and cycling participation surveys? The data used can be found here [3,4].

Here are plots of the expected (red dashed line) number of head and arm injuries (left panel) and head injuries only (right panel) if the Safety in Numbers hypothesis is true. The black line represents the observed number of injuries by year.

SiN Effect

There is a clear divergence between what was observed and what was expected. Therefore, the evidence does not support Robinson’s safety in numbers hypothesis. In fact, the estimated exponent is 0.94 (95% CI: 0.59-1.30) and suggests increases in cycling is associated with a roughly equal increase in injury.

A more detailed analysis (and other cycling-related analyses) can be found in our peer-reviewed paper[5].

  1. Jacobsen, P.L. (2003). Safety in numbers: more walkers and bicyclists, safer walking and bicycling. Injury Prevention, 9, 205-209.
  2. Robinson, D.L. (2005). Safety in numbers in Australia: more walkers and bicyclists, safer walking and bicycling. Health Promotion Journal of Australia, 16, 47-51.
  3. Olivier, J., Walter, S.R., & Grzebieta, R.H. (2013). Long-term bicycle related head injury trends for New South Wales, Australia following mandatory helmet legislation. Accident Analysis and Prevention, 50, 1128–1134.
  4. Australian Bureau of Statistics, 2001. Participation in Exercise, Recreation and Sport 2001. ABS, Canberra.
  5. Olivier, J., Grzebieta, R., Wang, J.J.J. & Walter, S. (2013). Statistical Errors in Anti-Helmet Arguments. Australasian College of Road Safety Conference.