Internally bevelled conoidal entry wounds in sandwich bones; their genesis, impact dynamics and macroscopic and microscopic morpholog

Abstract

Projectile trauma to the sandwich bones of the neurocranium is of considerable forensic anthropological importance due to the high lethality associated with gunshot wounds to this anatomical region. The internally bevelled conoidal wound that typically results from perpendicular projectile impact has long been considered diagnostic of bullet involvement and is utilised in both differential diagnosis of gunshot trauma and in making trajectory determinations using the direction of bevel asymmetry as an indicator. However, despite the importance of these activities in forensic anthropological casework the fracture mechanisms underlying bevel formation have remained elusive, with multiple hypotheses presented but few empirical investigations carried out to test them. Further diagnostic complexity has been introduced by the recognition that bevelling also results from low-velocity impacts and that taphonomic agencies can both modify and create internally bevelled fractures. Despite the fact that bone is a complex hierarchical material the majority of analyses of projectile entry wounds in sandwich bones have been conducted at the macroscopic scale, leaving important questions as to whether lower scales of organisation may yield signatures of diagnostic importance. This thesis presents the findings of an experimental investigation into the fracture mechanisms that underlie internal bevelling and presents an analysis of the relationships between the dynamics of impact and quantitative wound morphology. A refined nomenclature for the components of internally bevelled fractures is presented and the layers composing the bevel formally defined. This thesis also presents a Scanning Electron Microscope (SEM) analysis of the effects of projectile impact on compact bone at lower hierarchical scales of bone organisation. Examination of fracture morphology with micro-computerised tomography (µ-CT) coupled with high-speed videography of the impact events revealed a novel cone cracking mechanism that fundamentally changes our understanding of bevel morphology. This mechanism explains how internal bevelling results from both low- and high-velocity impact events and suggests caution should be applied to trajectory determinations made using bevel asymmetry alone. Quantitative analysis of the perforation event revealed, for the first time in sandwich bones, a power relationship between incident velocity and absorbed kinetic energy. SEM analysis of cortical bone around the entry captured the early genesis of conoidal wounds through ring crack formation. Analysis of the periphery of the entry revealed two types of plastic deformation that are suggestive of changes to hydroxyapatite (HA) crystal structure that may be of diagnostic utility. Collectively, the findings presented in this thesis will enhance the diagnosis and interpretation of conoidal wounds in both forensic and archaeological contexts.