Severe brain injury is caused by rotational acceleration (the head turning quickly). Bicycle helmets cannot protect against it, and are known to amplify rotational acceleration to levels causing permanent disability.
Helmets should not be misrepresented as preventing permanent disability when they are more likely to cause it.
What surgeons say
There is a common belief that bicycle helmets protect from permanent brain injury. This article reports from a surgeon who operates on cyclists:
” “The ones with brain swelling, that’s diffuse axonal injury, and that’s bad news” …
the whole brain is shaken up, creating many little tears in its inner structure …
Such patients undergo personality change, can contract epilepsy and have difficulty controlling their anger. They might become unemployable. Depression is a common accompaniment to brain injury. Rosenfeld sees patients’ families shattered, too. “They’re never the same. It often leads to marriage disharmony and family breakdown.” …
Rosenfeld’s opinion is candid. “I don’t know if [helmets] do much to protect the inner part of the brain,” “
According to another Australian surgeon who operates on cyclists:
“The best evidence is that [a helmet] doesn’t make any difference to serious head injury when riding a bicycle …
initial research used to back the mandatory laws was “deeply flawed and criticised”. Some newer findings, he said, showed that these laws could increase the chance of serious injury. “
Distinguishing between focal brain injury and diffuse brain injury
What’s going on? Why do people believe that bicycle helmets protect against brain injury while surgeons say they don’t? Much of the confusion comes from a lack of distinction between focal brain injury and diffuse brain injury. The old popular belief is that brain injury is caused by a direct hit the head, like a head hitting a wall, causing linear acceleration. This is called focal brain injury, as it is focused on a particular area where the skull has been hit. This what the polystyrene in helmets is designed to mitigate: it reduces linear acceleration by compressing on impact.
However, scientific research done in the 1970′s has revealed that the main cause of severe brain injury is not focal injury but diffuse axonal injury:
“The experiments gave special attention to direct comparison of the contributions of linear and angular acceleration because the widely accepted Head Injury Criterion for head injury is based on measurements of the former – see below. The essential role of angular acceleration in producing cerebral concussion was shown, the threshold being estimated as 2000-3000 rad/sec². Translation was also responsible for brain injuries, albeit only focal, and did not produce concussion. …
Along with the support which research has provided for the new theory, it has discredited the notions of coup and contre-coup and of linear acceleration of the brain being a major factor in injury to it.“
Diffuse axonal injury, also called rotational injury, is caused by angular acceleration, for example when the head rotates quickly. The skull may not be damaged, but there can be severe internal brain damage. This is what the surgeon Dr. Rosenfield was describing in the article mentioned above.
When bicycle helmets were first introduced, many people pushed for helmets claiming that they protected against serious brain injury. Ironically, not only is this not true, but the opposite is more likely to be the case:
“Protecting the brain from injury that results in death or chronic disablement provides the main motivation for wearing helmets. Their design has been driven by the development of synthetic polystyrene foams which can reduce the linear acceleration resulting from direct impact to the head, but scientific research shows that angular acceleration from oblique impulse is a more important cause of brain injury. Helmets are not tested for capacity to reduce it and, as Australian research first showed, they may increase it.“
The popular belief that bicycle helmets protect against brain injury has been reinforced by misleading helmet promotion campaigns. Powerful emotional testimonies claiming “my helmet saved my life” were used, as well as other deceitful techniques suggesting that cyclists would suffer serious brain injury unless they wore a helmet.
Even though helmets do not provide a solution to serious brain injury, fear of chronic disability has been used as a scare tactic to promote helmets. This deceitful practice has lead to an exaggerated opinion of the protection provided by helmets. A false sense of safety can lead to increase risk taking, as reported in the New York Times:
“the increased use of bike helmets may have had an unintended consequence: riders may feel an inflated sense of security and take more risks. …
The helmet he was wearing did not protect his neck; he was paralyzed from the neck down. …
”I definitely felt safe. I wouldn’t do something like that without a helmet.” ”
Although helmets have been portrayed as preventing permanent disability, they can actually cause it, for example through a combination of increased risk taking and a tendency for helmets to increase neck injuries.
Bicycle helmets are not as “safe” as we have been led to believe.
Research on soft-shell helmets
Soft-shell helmets, the most common type of bicycle helmet, are helmets without a hard shell. They have a thin layer of plastic on top, less than 1 mm.
In 1987, an Australian government agency released research that highlighted deficiencies with bicycle helmets:
“The substantial elastic deformation of the child head that can occur during impact can result in quite extensive diffuse brain damage. It is quite apparent that the liner material in children’s bicycle helmets is far too stiff …
rotational accelerations were found to be 30% higher than those found in similar tests using a full face polymer motorcycle helmet. More work needs to be done in this area as there would seem to be a deficiency in rotational acceleration attenuation that may be the result of insufficient shell stiffness“
Warnings that soft-shell helmets can increase rotational acceleration have been reported in other studies, like this research done in Sweden:
“The non-shell helmet did in all trials grab the asphalt surface, which rotated the head together with
the helmet. The consequences were in addition to the rotating of the head, a heavily bent and compressed neck, transmitted on through the whole test dummy body after the impact. …
This gives an average angular acceleration of 20800 rad/s² for rotating the head from 0 to 0.26 rad during the 5 ms. Löwenhielm proposes 4500 rad/s² to be the maximum angular acceleration that can be tolerated for a limited time period, which also is suggested by Gilchrist and Mills.”
Soft-shell helmets grabbed the road surface, and then amplified rotational acceleration to four times higher than the tolerable maximum.
On impact, the larger head volume amplifies rotational acceleration. A study done in the UK reports that a difference of just 3cm in helmet circumference increases rotational acceleration by 150%:
“the 3000rad/s² to 8500rad/s² measured during abrasive and projection oblique tests with size 54cm (E) helmeted headforms. However, for the most severe cases using a size 57cm (J) headform, rotational acceleration was typically greater than 10,000rad/s² and increased to levels of 20,000rad/s², a level at which a 35% – 50% risk of serious AIS3+ injuries is anticipated.”
The difference between a helmeted and non-helmeted head is about 20cm.
The mechanics of bicycle helmets
Helmets increase the volume of the head. In the event of an accident, this increases the risk of the head hitting the road. This study reports that helmeted riders are more than twice as likely to hit their head in an accident, with the additional impacts being to the sides. A 1988 study found that helmeted riders report hitting their heads seven times more often than un-helmeted riders. A bare head, often protected by the shoulders, provides the lowest risk of head contact in the event of an accident. Post-crash studies found that most helmets show impacts to the side, areas where a bare head may not have been hit.
If the helmet hits the ground, the polystyrene tends to grab the road, then force the head to rotate more quickly. Those additional impacts when helmets are worn tend to generate rotational acceleration, amplified by the large volume of the head+helmet. This amplified rotational acceleration can cause severe brain injury.
According to a doctor from New Zealand, bicycle helmets do cause severe brain injuries:
“cycle helmets were turning what would have been focal head injuries, perhaps with an associated skull fracture, into much more debilitating global head injuries”
It is easy to take things for granted, and to assume that a device labeled a “helmet” can only protect, and cannot make things worse. It may be preferable to ride carefully though rather than to rely on a flawed device. The tendency of helmets to increase rotational acceleration and serious brain injury is a serious flaw in their design.
The dilemma: minor skull injuries vs serious brain injury
Bicycle helmets are designed to protect cyclists if they fall on top of their heads at speeds below 20 km/h. They are not tested for side impacts. They mitigate against minor skull injuries like bruises and lacerations.
Is it worth to increase the risk of severe brain injury for the benefit of mitigating minor skull injuries? This dilemma about the trade-off between protection in minor accidents vs. risk of serious brain injury enabled an Australian cyclist to successfully challenge a helmet fine on the basis that wearing a helmet could be dangerous. After reviewing evidence in a court of law, NSW District Court Judge Roy Ellis concluded:
“”Having read all the material, I think I would fall down on your side of the ledger …
I frankly don’t think there is anything advantageous and there may well be a disadvantage in situations to have a helmet and it seems to me that it’s one of those areas where it ought to be a matter of choice.”