# Radial and Oblique Impact Testing of Alpine Helnets onto Snow Surfaces
## 4 Discussion  
Head injuries are common in alpine sports despite increases in helmet use over recent decades. Recent studies have found that alpine helmets reduce the risk of focal injuries associated with radial impacts, such as skull fracture, but are less effective in reducing the risk of diffuse injuries associated with rotation, such as intracranial haemorrhage. Current helmet standards involve drop tests onto rigid anvils, but there is a need to evaluate the performance of alpine helmets with more realistic impacts. Therefore, the current study developed a method to assess the performance of alpine helmets for radial and oblique impacts onto snow surfaces in a laboratory setting and compared peak headform kinematics across headform conditions. For oblique impacts, the recreational alpine sports helmet with RDS significantly (\(p\) = 0.002) reduced mean peak angular acceleration compared to the same helmets with no RDS by approximately 44%, which supports the findings of previous studies that compared helmet models during oblique impacts to rigid anvils [30; 31]. Typical helmets are optimised to reduce the linear acceleration of the head [18], which is associated with focal head injuries, such as skull fractures. This is likely due to alpine helmet standards requiring helmets to be drop tested on flat anvils with only linear acceleration pass criteria. Currently, no alpine helmet standard includes an oblique impact test with rotational criteria. Figure 4: Time-histories of angular headform acceleration for helmeted oblique impacts tests onto frozen snow samples. Traces in grey depict time histories of repeat tests. Traces were time aligned for comparison. However, there are efforts to revise helmet standards [34; 35; 36], including alpine helmet standards [30; 33], to include rotational testing components. For example, several current bicycle and motorcycle helmet standards involve rotational testing. The American Society for Testing and Materials (ASTM) has developed a standard test method for measuring impact attenuation characteristics of helmets induced under rotational loading using an incline anvil; however, this has yet to be incorporated into the ASTM standard specification for helmets used for recreational snow sports. Such rotational testing is required to be representative of sports-specific impacts; therefore, developing a foam that behaves similarly to snow during dynamic impacts would allow for a repeatable test method suitable for use in alpine helmet standards. In addition, Virginia Tech recently published the Summation of Tests for the Analysis of Risk (STAR) protocol for snow sport helmets, which comprises a rotational testing component [37]. The ski racing helmet also significantly (\(p\) = 0.006) reduced mean peak angular acceleration compared to the recreational alpine sports helmet with no RDS by approximately 33%. This finding may be attributed to the difference in vent location on the helmet shells. The crown of the ski racing helmet shell was smooth with only small circular vent holes at the rear of the helmet, whereas the recreational alpine sports helmets, both with and without RDS, had large vent holes at the crown and rear of the helmet. The crown of the helmet was the target impact site for the tests in the current study; however, impacting a different site (e.g., rear) may have resulted in different peak angular accelerations. Helmets are typically designed with smooth outer shells, which lowers the friction between the helmet and the impacted surface and, therefore, decreases the tangential forces and angular acceleration of the head [34; 38]. Substantial variations in peak angular headform acceleration have been previously reported in oblique impact testing of motorcycle helmets, which was attributed to interactions between the helmet vent and the high friction surface of the angled anvil [39].