Riding helmets do not appear strong enough to prevent skull fractures in adults when crushed from the side, as may occur if a horse falls on their rider, research suggests.
Thomas Connor and his colleagues, writing in the journal Applied Sciences, created a special instrumented headform, modelled on an average male, to occupy a helmet. They then carried out simulations that mimicked the forces that might occur in a fall.
The headform, which contained sensors, was placed in a commonly available 57cm jockey-style equestrian helmet for the study. The researchers said the helmet was a good representative of commercially available helmets.
For the study, the cadavers of two horses of different weights were dropped on the helmeted and unhelmeted headform so the sensors could measure the forces involved. The horses, a 343kg mare and a 370kg male, had been euthanised for reasons unrelated to the study.
The drop height of 1.2 metres gave a theoretical impact velocity of 4.43 metres per second.
While the helmet significantly reduced the forces applied to the headform, the levels remained high enough in all cases to fracture an adult skull.
The researchers set up the drops to allow four different parts of the cadavers to land on the helmet/headform: the lumbosacral vertebrae, the sacral vertebrae, and the fleshier left and right hind quarters.
In total, 30 drop tests were carried out, comprising 24 unhelmeted tests and 6 helmeted tests.
The highest peak loads applied to the headform by the lighter horse were measured at the bony sacral impact location, with an average of 15.57 kilonewtons (kN). The lowest average peak loads were measured at the right hind quarter (7.91 kN).
For the heavier horse, the highest average peak loads applied to the headform were measured at the same bony sacral impact location at 16.02 kN, whilst lowest mean peak loads were measured at the more compliant left hind quarter (10.47 kN).
When compared with the unhelmeted averages, a reduction of 29.7% was recorded for the sacral impact location and a reduction of 43.3% for the lumbosacral junction location for helmeted tests.
“Notably, all measured loads were within or exceeded the range of published data for the fracture of the adult lateral skull bone.”
Force levels required for lateral skull fractures vary significantly. Average values reported in the scientific literature range from 3.5 kN to 12.4 kN. However, these forces vary with the surface area of the impacting object and the impact speed.
“It should also be noted that skull fracture occurs at much lower loads for children.”
The researchers note that equestrian helmet certification tests are designed to ensure that a minimum performance and quality level is achieved in terms of helmet crashworthiness and structural integrity.
They said it makes good sense that the main helmet functional test in the standards involves recreating some simplified impact conditions.
Helmet testing includes a lateral crush test, also referred to as the lateral deformation test or rigidity test.
“However, unlike impact tests, the origins of which are well documented in the literature, the rationale and evidence basis for the crush test are unclear.”
The test, they say, is relatively simple. A helmet is placed between two metal plates and is crushed until a peak force is reached (800N for four recognised standards) at a specified loading rate, with no headform in the helmet.
In all cases, the maximum permitted crush is 30mm and the residual crush may not exceed 10mm.
“In discussions with engineers working within the standards industry and with standards committee members, it is understood that the lateral crush tests are used to ensure that the helmet is ‘not too soft’ and that the structure of the helmet has some ‘stabilizing effect’.
“It is not intended to simulate a real-world accident.”
However, in discussions with the equestrian community, it is clear that the lateral crush test is believed to represent a horse falling onto a helmet, according to the researchers.
Indeed, one standard was recently revised to increase the peak force that could be sustained from 630 N to 800 N.
“That decision was taken on the basis that it should improve helmet performance in the event of a horse falling onto a rider’s head. However, there is no evidence that this change would have any influence on helmet performance.”
Discussing their findings, the researchers say horse mass appears to be a factor in the impacts. A 7.3% increase in the mass of the falling horse resulted in, on average, an 11.8% increase in the peak load applied to the headform.
“The horses used in this study were not large and would typically fall into the pony category.
“Much heavier animals are ridden by equestrians and most sport horses weigh around 500 to 600 kg. A similar drop test with an animal of such a larger mass would result in a significant increase in loads applied to the headform.”
The authors say current tests do not come close to simulating the loading conditions of real-world accidents. “Any future changes to a standard test method should have a firm evidence basis to ensure the test is both useful and can lead to the desired safety outcome.
“The current standard lateral crush test method should be evaluated to see if there is any relationship between this quasi-static test and dynamic crush.
“It may,” they say, “be possible to adapt current tests as a cost-effective measure to ensure better helmet performance.”
In conclusion, they say current helmet certification tests are not biofidelic — that is, they are not faithfully modelled on a biological system — and do not adequately represent the loading conditions of real-world “lateral crush” accidents in equestrian sports.
“This work presents the first-ever evidence basis upon which any future changes to a certification standards test method might be established, thereby ensuring that such a test would be both useful, biofidelic, and could ensure the desired safety outcome.”
The study team comprised Connor, Michio Clark, Pieter Brama, Matt Stewart, Aisling Ní Annaidh and Michael Gilchrist, variously affiliated with University College Dublin, COMFG Ltd, R&D Consulting Engineers Ltd, and Vector Scientific Inc.
An Evidence Basis for Future Equestrian Helmet Lateral Crush Certification Tests
Thomas A. Connor, J. Michio Clark, Pieter Brama, Matt Stewart, Aisling Ní Annaidh and Michael D. Gilchrist.
Appl. Sci. 2020, 10(7), 2623; https://doi.org/10.3390/app10072623