Relating the micro- and nano-structure of bone to rib fracture propagation

• Main Author: Beadle, Nicola
• Published: University of Manchester 2016
• Subjects: 616.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.680018

Physical abuse of infants at the hand of a parent or carer is a significant and growing issue worldwide. In particular, rib fractures in the absence of major accidental trauma are highly specific for physical abuse. Despite the serious implications of physical abuse in infants, definitive knowledge regarding the hierarchical structure of the immature rib and how this influences the mechanical failure is lacking. Moreover, the diagnosis of physical abuse is often disputed in a legal setting, and as a consequence there is a need to model such injuries ex vivo in order to characterise the forces required to inflict rib fractures. In this study, a novel method of producing fractures in immature ribs, having established porcine ribs as the most suitable alternative to infant human ribs, was developed and utilised for assessing the mechanical behaviour and fracture properties of immature ribs. Multiscale, correlative imaging using microscale X-ray-CT (micro-CT), histology and atomic force microscopy (AFM) was performed to investigate the hierarchical structure and the toughening mechanisms that occur during the fracture of immature ribs at multiple length scales. Porcine and human infant ribs were shown to share a number of macro- and micro-structural features, including comparable amounts of woven and lamellar bone. A method of inducing fractures in porcine ribs was developed. Individual ribs (n=96) were loaded in axial compression at a range of loading rates (1, 30, 60 and 90 mm/s) in order to assess the mechanical performance of the ribs. Incomplete fractures around the midpoint of the rib were typically produced (87%), with higher loads and less deformation required for ribs to fracture completely. Loading rate, within this range, did not have a significant influence on the mechanical performance of the ribs. Load–displacement curves displaying characteristic and quantifiable features were produced for 90% of the ribs tested, and multiple regression analyses indicated that, in addition to the geometrical variables, there were other factors such as the micro-and nano-structure of the bone that influenced the measured mechanical data. Following this, correlative, multi-scale imaging techniques were employed in combination with mechanical loading of individual ribs in order to identify structural mechanisms that occur during the process of failure through the length scales. Micro-CT demonstrated the general appearance and location of the fracture in three-dimensions, highlighting the difference in porosity of the outer and inner rib cortices. Histology provided evidence of a number of fracture toughening mechanisms such as osteon crack deflection and crack bridging of microcracks, in addition to the influence of the heterogeneous distribution of woven and lamellar bone on the fracture path. AFM of demineralised, paraffin-embedded sections revealed no significant difference in the periodicity of woven and lamellar regions, nor in regions adjacent to microcracks. The results of the large-scale loading study indicated that the histological appearance and the type of the fracture may be indicative of the amount of load and deformation required to induce fractures. Moreover, the identification of fracture toughening mechanisms suggests that the intrinsic structure of the developing bone will influence the amount of load required to cause the propagation of a catastrophic fracture. Addressing the clinical context, the study has shed light on the link between structure and mechanical failure of immature ribs, suggesting a vital role for the multi-scale structural elements in determining the amount of force required to fracture.