title: A comparison between rib fracture patterns in peri- and post-mortem compressive injury in a piglet model.
abstract: OBJECTIVE
  Forensic biomechanics is increasingly being used to explain how observed injuries occur. We studied infant rib fractures from a biomechanical and morphological perspective using a porcine model.
  
  
  METHODS
  We used 24, 6th ribs of one day old domestic pigs Sus scrofa, divided into three groups, desiccated (representing post-mortem trauma), fresh ribs with intact periosteum (representing peri-mortem trauma) and those stored at -20°C. Two experiments were designed to study their biomechanical behaviour fracture morphology: ribs were axially compressed and subjected to four-point bending in an Instron 3339 fitted with custom jigs. Morphoscopic analysis of resultant fractures consisted of standard optical methods, micro-CT (μCT) and Scanning Electron Microscopy (SEM).
  
  
  RESULTS
  During axial compression fresh ribs did not fracture because of energy absorption capabilities of their soft and fluidic components. In flexure tests, dry ribs showed typical elastic-brittle behaviour with long linear load-extension curves, followed by short non-linear elastic (hyperelastic) behaviour and brittle fracture. Fresh ribs showed initial linear-elastic behaviour, followed by strain softening and visco-plastic responses. During the course of loading, dry bone showed minimal observable damage prior to the onset of unstable fracture. Frozen then thawed bone showed similar patterns to fresh bone. Morphologically, fresh ribs showed extensive periosteal damage to the tensile surface with areas of collagen fibre pull-out along the tensile surface. While all dry ribs fractured precipitously, with associated fibre pull-out, the latter feature was absent in thawed ribs.
  
  
  CONCLUSIONS
  Our study highlights the fact that under controlled loading, fresh piglet ribs (representing perimortem trauma) did not fracture through bone, but was associated with periosteal tearing. These results suggest firstly, that complete lateral rib fracture in infants may in fact not result from pure compression as has been previously assumed; and secondly, that freezing of bone during storage may affect its fracture behaviour.
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Bradley
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: Michael V
authors.initials: MV
authors.lastname: Swain
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: J
authors.initials: J
authors.lastname: Neil Waddell
authors.affiliation: Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand.
authors.forename: Raj
authors.initials: R
authors.lastname: Das
authors.affiliation: Department of Preventive & Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand.
authors.forename: Josie
authors.initials: J
authors.lastname: Athens
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. Electronic address: jules.kieser@otago.ac.nz.
authors.forename: Jules A
authors.initials: JA
authors.lastname: Kieser
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D019245:Compressive Strength; D044707:Forensic Sciences; D008422:Materials Testing; D012253:Rib Fractures / Q000503:physiopathology; D012272:Ribs / Q000293:injuries / Q000502:physiology; D013552:Swine; D016474:Weight-Bearing
keywords: Forensic biomechanics; Four-point bending; Rib fracture; SEM; Shaken baby syndrome