The relationship between fracture morphology and bone biomechanics: A study of changes occurring in juvenile porcine ribs over the early postmortem interval

Forensic anthropologists are often faced with the challenge of determining fracture timing based on bone features, usually discussing it in terms of a “fresh” versus “dry” bone response. Yet it is still unclear how long into the postmortem period bone can retain its fresh characteristics, particularly juvenile rib bone. Juvenile porcine ribs were used to examine 1) changes in the biomechanical properties of bone in response to localized load, and 2) changes in the morphology of the resulting fractures over the early postmortem interval (PMI). Two macroenvironments (subaerial and burial) were recreated in a greenhouse. Samples were placed on the surface of soil filled containers and distinct samples were buried in the same containers, with a total of 16 containers being studied over 12 months. Samples were collected weekly for the first 4 weeks, the subsequent two were collected 2-weeks apart, and the remaining 4-weeks apart. Individual ribs 8-11 were selected from the subaerial (n=146) and the burial (n=134) environments and fractured experimentally. Six biomechanical parameters were collected from each test and median values were obtained for each sample. Each fracture was then examined for eight morphology characteristics and frequencies were calculated for each sample. In the subaerial samples, multiple regression analysis showed that displacement at peak force, displacement at failure and failure stiffness were significantly associated with the PMI. Type of fracture, presence of plastic deformation and presence of cortical peeling were also significantly associated with the PMI. In the buried samples, multiple regression analysis showed no significant association between bone biomechanics and the PMI, and only a moderate association was found between the PMI and fracture morphology, specifically in the type of fracture, fracture surface and presence fiber pull-out. Although a transition from “fresh” to “drier” bone was apparent in the subaerial samples, a persistence of typical “fresh” bone response over the year-long PMI was evident in the buried environment. Accurate timing assessment of juvenile rib fractures is thus likely to be compromised from the analysis of bone features alone and further investigations are necessary for more confident and accurate rib trauma analysis, specifically when involving child remains.