Development of a Probabilistic Skull Fracture Model for a 50th Percentile Adult Male Motorcyclist ATD Headform (2011)

The ability to measure and quantify the differences in injuries between helmeted and unhelmeted riders of motorcycles, quadricycles and other small open vehicles as well as injuries to other unhelmeted vulnerable road users such as pedestrians has led to the desire to extend motorcyclist injury assessment methods such as those in ISO 13232 (2005) to include the potential for skull fracture due to head contact forces (e.g., direct impact and crushing type injury mechanisms), in addition to the closed-skull brain injury probability calculations based on head accelerations which are currently in the ISO 13232 Standard (2005). A probabilistic injury model was developed for a 50th percentile adult male by correlating human biomechanical data on skull fractures with cranial vault and facial contact mechanical “work.” This injury model was then transformed into an “equivalent work” model applicable to Motorcyclist Anthropometric Test Device (MATD) headform contact forces based on the series of assumptions listed. The biomechanical data comprised 64 cases involving temporoparietal region, maxilla and zygoma fractures as reported in Nahum et al. (1968) and Schneider and Nahum (1972). Laboratory tests were also conducted to measure the MATD contact forces during impact conditions that replicated those of the human cadaver tests reported in the scientific literature. The results demonstrated that the MATD headform contact forces can be measured and used to estimate the probabilities of an AIS 1, 2, 3, and 4 human cadaver face or vault fracture. The AIS probabilities generated from MATD testing were in substantial agreement with the scientific literature. Since the proposed skull fracture criteria are closely related to the forces acting on the skull that can result in bone fracture, rather than indirect measurements of these forces such as head acceleration, it is better suited for predicting skull fractures in some types of injury mechanisms (e.g., crushing type injury mechanisms) that may result in high contact forces but low resultant head accelerations. The resulting injury criteria for a 50th percentile adult male can be used to assess the probabilities of an AIS 2, 3, and 4 vault or AIS 1, 3, and 4 facial fracture resulting from unhelmeted head contact forces, using specialized test sensors and methods or calibrated ISO 13232 type computer simulations. The criteria are well suited for evaluating skull fracture injuries resulting from head contact forces and are complementary to existing head acceleration based injury criteria for closed skull brain injuries that are currently in the Standard. Limitations of the injury criteria are the relatively small number of available biomechanical data and the series of assumptions made. In addition, for potential use in crash tests, an ATD headform with specialized force sensors would also be needed to measure head contact forces in crash tests. The resulting probabilistic injury criteria provide a useful tool to assess the change in injury risks and benefits of potential protective devices for unhelmeted motorcycle and quadricycle riders, as well as pedestrians, using ISO 13232 type computer simulation methods.