Structural Analysis of Porcine Rib Fractures

Abstract

Accurate interpretation of bony trauma is vitally important to forensic, anthropological and archaeological investigations. Perimortem bony trauma, defined as injury at the time of death, is of particular importance and often involves blunt force trauma to the chest. Bone is a composite structure in which an inorganic component consisting of hydroxyapatite confers stiffness and an organic matrix of collagen provides toughness. Yet bone is fragile and will fracture when it cannot absorb traumatic energy. While fracture patterns are a function of the type of force applied, these also depend on other factors including the microstructure of the bone, its micro-architecture and its degree of mineralisation. Moreover, it is known that there is a non-linear relationship between bone strength and water loss. Dry bones have an increase in their elastic modulus and strength but decrease in their toughness compared to fresh bones.

Because the forensic reconstruction of traumatic events to the chest requires a detailed understanding of how individual ribs fracture, I decided to study such patterns under strictly controlled conditions. In the first part of this thesis, I chose an individual type of rib (the 5th) and investigated fracture patterns in fresh and desiccated pig ribs under controlled anteroposterior loading. In the second part, I studied the fracture location under controlled anteroposterior bending. Finally, I subjected this known fracture location on fresh and desiccated 5th ribs to 4-point bending.

My results showed that fracture patterns were strongly influenced by water content with the desiccated ribs displaying catastrophic brittle fractures while the fresh ribs all exhibited non-precipitous incomplete fractures. When subjected to anteroposterior loading, ribs failed in the anterior region. Finally, fresh and desiccated pork ribs subjected to 4-point bending tests showed there are significant differences between the biomechanical properties in the two rib conditions.

My results underline a number of generalisations that may be made for the behaviour of pork ribs subjected to controlled loading. For instance, I noticed a prevalence of buckle fractures, which occur due to compressive instability, while previous research indicates that ribs are expected to fail in tension before compression.

Perimortem (fresh) bone fractures may be distinguished from post-mortem (desiccated) fractures by morphological features such as sharp angled, butterfly fractures and right angle fractures in wet and dry bone specimens. Four-point bending is important because of its ability to produce pure bending between the two upper loading arms while avoiding any shear forces that may be present with 3-point bending tests. My results suggest that dehydration has a significant effect on the biomechanical properties of bone, which emphasises the fundamental role viscoelastic properties have in the protective mechanism in bone.

In conclusion, this thesis offers additional information on the fracture morphology, fracture location and the biomechanical behaviour of pork ribs to add to previous research and in future, aid experts in their understanding and interpretation of rib fractures.