Maxime Llari

Effects of fall conditions and biological variability on the mechanism of skull fractures caused by falls

In a forensic investigation, there is considerable difficulty in distinguishing between different mechanisms that could explain the head injury sustained. The key question is often whether the injury was the consequence of a fall, a blow, or a fall caused by a blow. Better understanding of the parameters influencing the mechanism of skull fracture could be of use when attempting to distinguish between different causes of injury. Numerous parameters concerning fall conditions and biological variability are reported in the literature to influence the mechanism of skull fracture. At the current time, there are no studies that investigate both the effect of a fall and biological parameters. The aim of this paper is to study the influence of these parameters on the mechanism of skull fracture using a numerical approach. We focused on accidental falls from a standing height. A multibody model was used to estimate head impact velocities and a finite element model was used to investigate the effect of the fall conditions and of biological variability on skull fracture. The results show that the mechanism of skull fractures is influenced by a combination of at least four parameters: impact velocity, impact surface, cortical thickness and cortical density.

Head impact in a snowboarding accident.

To effectively prevent sport traumatic brain injury (TBI), means of protection need to be designed and tested in relation to the reality of head impact. This study quantifies head impacts during a typical snowboarding accident to evaluate helmet standards. A snowboarder numerical model was proposed, validated against experimental data, and used to quantify the influence of accident conditions (speed, snow stiffness, morphology, and position) on head impacts (locations, velocities, and accelerations) and injury risk during snowboarding backward falls. Three hundred twenty‐four scenarios were simulated: 70% presented a high risk of mild TBI (head peak acceleration >80 g) and 15% presented a high risk of severe TBI (head injury criterion >1000). Snow stiffness, speed, and snowboarder morphology were the main factors influencing head impact metrics. Mean normal head impact speed (28 ± 6 km/h) was higher than equivalent impact speed used in American standard helmet test (ASTM F2040), and mean tangential impact speed, not included in standard tests, was 13.8 (±7 km/h). In 97% of simulated impacts, the peak head acceleration was below 300 g, which is the pass/fail criteria used in standard tests. Results suggest that initial speed, impacted surface, and pass/fail criteria used in helmet standard performance tests do not fully reflect magnitude and variability of snowboarding backward‐fall impacts.

Biomechanical analysis of skull fractures after uncontrolled hanging release

In forensic research, biomechanical analyses of falls are widely reported. However, no study on falls consecutive to uncontrolled hanging release, when a hanging body is cut down, has ever been published. In such cases, the presence of cranial trauma can raise interpretation issues, and there may be doubt as to whether the fall was an accident or a crime disguised as suicide. The problem remains as to whether or not a fall after a free hanging release can lead to a skull fracture. To address this question, numerical simulations, post-mortem human subject tests and parametric studies were performed. We first recreated the kinematics and velocity of this atypical fall with post-mortem human subject tests and multibody simulations. We then tested the influence of biological variability on fracture production using a finite element model of the head. Our results show that fall severity depends largely on the direction of the fall. The risk of fracture is highest in the occipital region and with a backward fall. Our study also highlights the frequent occurrence of lower limb trauma in a free hanging release. Most importantly, we show that a fracture is produced in only 3.4% of falls that occur in a 10-90 cm height range. The overall findings of this study provide tools for pathologists and magistrates to decide on the most likely scenario and to justify further forensic investigations if required.