The Science of Skull Fracture

Everyone has seen dramatized television shows where the crime scene investigation team matches the skull fracture pattern to the murder weapon and cracks the case. While real-world investigations often lack the Hollywood aesthetic, the conclusions drawn in the scripts of our favorite shows highlight an important biomechanical principal: that skull fracture patterns are subject to the laws of physics. By studying and evaluating the skull fracture patterns arising from controlled head impacts, biomechanical engineers are helping to build the science of skull fracture. 

Experts at Explico are contributing to the body of skull fracture mechanics knowledge by teaming up with engineers and forensic anthropologists at Michigan State University to explore skull fractures arising from impacts from different instruments (such as a hammer, baseball bat, or brick), from multiple blows of the same instrument, and at different impact energies. Explico is investigating how exceptionally thick and thin skulls deviate from expected fracture patterns, and how improved modeling may help to incorporate these differences for more predictive results. 

A Brief History of Skull Fracture Biomechanics 

Among the earliest investigations into the mechanics of skull fracture were performed in the 1940s and 1950s by neurosurgeon Dr. Elisha Gurdjian when he dropped dried cranial specimens from varying heights onto a steel slab to observe the relationship between impact energy, stress distribution, and fracture pattern. In these experiments, the skulls were coated with a brittle lacquer that cracked under tension to reveal the stress distribution arising from the blow. More severe blows were administered to demonstrate that the regions where the lacquer cracked were predictive of fracture location. Among the many observations of the landmark study were the regions of tensile stress corresponding to local inbending (bending of the inner surface of the skull near the location of impact) and peripheral outbending (bending of the outer surface of the skull remote to the location of impact), which demonstrated that the location of a skull fracture is not always indicative of the origin of the blow. 

In the 1990s Dr. Narayan Yoganandan applied quasi-static and dynamic loads to unembalmed skulls and measured the force-deflection response, that is, how much force is required to bend or crush the skull. Quasi-static loading is akin to a slow crushing of the head, where dynamic loading is more like a blow to the head from a moving object. It was found that the skull behaves differently to these loading conditions, namely that the dynamically loaded skull fractures at higher force and experiences less bending before fracturing. Stated another way, the dynamically loaded head behaves more stiffly than a quasi-statically loaded head. Despite these differences, however, the energy required to generate skull fracture was similar in both loading conditions. 

Predicting Fracture Patterns 

As early as the 1850s, surgeon Victor von Bruns postulated that exact calculation of the course of skull fracture could be possible if the skull had a perfectly spherical shape and the physical properties of the skull were known. With the continued advancement of medical imaging technology and simulation models, von Bruns’ hypothesis has become more and more a reality. Among the most advanced head models is the Wayne State University model developed by Dr. King Yang and associates. The model incorporates detailed anatomy and material properties to predict skull fracture response from pivotal studies, such as those performed by Yoganandan. However, many models are not adaptive and are validated against studies published by other researchers which may not include the detail required to accurately model specific case studies. For example, Dr. Gurdjian describes how in the course of forty-six cadaver experiments they were able to obtain fracture with as little as 400 in-lb of energy for one specimen, yet unable to achieve fracture with up to 1100 in-lb of energy for another. In other words, there are skulls that deviate from the typical response by fracturing more readily or not fracturing when expected – all dependent on subject-specific skull characteristics. Subject-specific skull thicknesses have been hypothesized as a reason for these differences, but many studies do not provide detailed thickness information about the specimens to explore this hypothesis. 

The Michigan State University and Explico team hopes to bridge the experimenter-validator gap in development of an adaptive model by collecting detailed specimen as well as impact parameter data. Early results of these studies indicate that while there are expected results in many impact scenarios, certain skulls exhibit unexpected results depending on the specific properties of the skull itself. Research is ongoing and is contributing to our understanding of subject-specific skull fractures. 

The Modern Role of the Biomechanical Expert 

Biomechanical experts are often called upon to investigate whether skull fractures are consistent with a specific event. Skull fracture is a particularly challenging subset of biomechanical analysis because it often renders the injured party unable to provide account for the circumstances under which the trauma was sustained due to proximate loss of consciousness, continued cognitive deficit, or death. Explico’s experts are equipped with the tools and knowledge to investigate a wide array of skull fracture scenarios. 

Visit our research projects page to learn more about Explico’s contributions to the science of skull fracture biomechanics. 

Click here to get in contact with one of our experts and find out how Explico can assist you with your lawsuit or investigation. 

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