It is widely accepted that a bicycle helmet will in many cases prevent or reduce skull fractures and superficial head injuries. Less clear is how effective helmets are at preventing injuries to the brain. Brain injuries can be either focal or diffuse. It is the diffuse brain injuries which are the most serious. A hospital emergency room case-control study by Thompson et al[1] found that bicycle helmets reduce brain injury by 88 percent, but this study has since been widely criticized for it's faulty methodology. Studies of Australian statistics are lacking in evidence of any reduction in the risk of serious brain injury to cyclists after the introduction of compulsory bicycle helmet legislation[2].
It is erroneous to associate skull fracture directly with brain injury. In a study by Baker[3], of all bicyclists with head injuries, 8.5 percent had skull fractures without evidence of injury within the skull. Obviously in an impact severe enough to cause a skull fracture, other forces are likely to be present.
Holbourn[4] and others[5] studied the effect of impacts to the head. While the forces involved in head impacts can be quite complex, there are two main forces involved: linear and rotational. Holbourn used two-dimensional jelly moulds to compare the effects of these two forces. It was possible to see the damage caused to the jelly, and compare that with the autopsies of actual head injury victims. Gennarelli et al[5] followed up on Holbourn's work using live monkeys.
Holbourn first noted the following properties of the brain:
Compression and rarefaction strains are not considered to be a cause of injury. Holbourn noted work by Grundfest which found that nerves continued to conduct when subjected to a compression strain due to a pure hydrostatic pressure of 10,000 lb. per sq. in. This pressure is far greater than anything which can arise in a head injury. Holbourn noted that if the pressure is not purely hydrostatic, ie there are different pressures in different directions, there will be shear strains present and a small pressure of this type may be sufficient to injure a nerve.
Linear impacts were found to cause mainly only localised (focal) injury at the point of impact. These brain injuries were the result of deformation of the skull (with or without fracture) and were found to be mostly superficial. The impact causes shock waves to emanate back and forth within the brain. Holbourn notes that these shock waves are non-injurious as they do not cause permanent displacement of brain matter.
Sudden rotation of the head was found to be the cause of most severe diffuse brain injuries such as contrecoup injuries, intracranial haemorrhages, and concussions. When rotational forces are applied, there is a change in the angular velocity of the brain and the skull. This results in diffuse shearing strains which can cause permanent displacement of matter throughout the entire brain. The irregular shape of the skull means that some parts of the brain fare worse than others.
Holbourn notes as erroneous the common misconception that the brain is loose inside the skull and that it rattles about like "a die in a box" when the head is struck, causing coup (at the point of impact) and contrecoup (remote from the point of impact) injuries. Claims that translational motion can damage brain tissue by bouncing it off the inside of the skull were therefore rejected.
It should be pointed out that Holbourn's work was published in 1943, and Gennarelli et all in 1974, so this is hardly recent research. It is knowledge which has been around since long before bicycle helmets came into popular use, and yet it is almost never mentioned by helmet researchers who perhaps have their own agenda in promoting helmets but disguising any possible adverse consequences.
Bicycle helmets are primarily designed to reduce the effect of linear forces, by providing a soft crushing layer which reduces the peak linear acceleration to the brain during impact. The current Australian helmet standard specifies that the peak forces of acceleration shall not exceed 400g from a drop height of 1.5 metres. As has been noted however, it is doubtful that these linear forces are injurious to the brain, except through deformation of the skull.
Head impacts from bicycle crashes do not generally involve a direct square-on impact. Most commonly there is an angled impact as the head hits the ground with forward momentum; or the windshield of a motor vehicle. Such an impact is likely to impart some degree of rotational force on the head and brain.
The effect of helmets on rotational forces to the brain is not entirely clear. On the one hand, rotational forces may be reduced by virtue of the same crushing effect of the helmet which reduces linear forces; on the other hand, they may be increased due to the increased size and mass of the head. It should be noted that there are presently no helmet performance standards which monitor for the ability to affect angular acceleration.
The (Australian) National Health and Medical Research Council in a 1994 study of football injuries[6] noted that studies of cycling showed that helmets reduce soft tissue injuries but stated: "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. ... The addition of a helmet will increase both the size and mass of the head. This means blows that would have been glancing become more solid and thus transmit increased rotational forces to the brain and may increase diffuse brain injury".
Another consideration is the differences in friction. There is little difference between bare heads and hard shell helmets, both of which are known to slide readily on impact. However, tests of impacts of helmets on asphalt at 34km/h have shown that, unlike hard-shell helmets which slide, soft helmets grab the surface, rotating the head and producing angular accelerations of four to six times the tolerable maximum[7]. This is a matter of concern given the enormous popularity of soft-shelled helmets, although it has been claimed by some that "these concerns have been largely alleviated by the widespread use of smooth coatings on helmet shells".
In conclusion, while it is readily accepted that bicycle helmets may reduce skull fracture and focal brain injuries, it remains highly questionable whether they can prevent serious brain injury, and there is a risk that they may actually cause increased brain injury.
[1] Thompson, R.S., Rivara, F.P. and Thompson, D.C., A case-control study of the effectiveness of bicycle safety helmets, The New England Journal of Medicine, 320: 21, 1989
[2] Robinson, D.L., Head Injuries and Bicycle Helmet Laws, Accident Analysis and Prevention, volume 28, number 4, pages 463-75 (1996).
[3] Baker S P, Li G Fowler C and Dannenberg A L, Injuries to Bicyclists, a National Perspective, Johns Hopkins Injury Prevention Centre, Johns Hopkins School of Public Health. Baltimore, Maryland, 1993.
[4] Holbourn, A.H.S., Mechanics of head injuries, The Lancet, 2, 438- 441, 1943.
[5] Ommaya, A.K. and Gennarelli, T.A., Cerebral concussion and traumatic unconsciousness: correlations of experimental and clinical observations on blunt head injuries, Brain, 97, 633-654, 1974.
[6] National Health and Medical Research Council, Football injuries of the head and neck, AGPS, 1994.
[7] Andersson, T., Larsson, P. and Sandberg, U., Chin strap forces in bicycle helmets, Swedish National Testing and Research Institute, Materials & Mechanics, SP report 1993:42.
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