Timing of blunt force injuries in long bones: the effects of the environment, PMI length and human surrogate model.

Timing of blunt force trauma in human bone is a critical forensic issue, but there is limited knowledge on how different environmental conditions, the duration of postmortem interval (PMI), different bone types and different animal models influence fracture morphology. This study aims at evaluating the influence of the type of postmortem environment and the duration of the postmortem period on fracture morphology, for distinguishing perimortem from postmortem fractures on different types of long bones from different species. Fresh limb segments from pig and goat were sequentially left to decompose, under 3 different environmental circumstances (surface, buried and submerged), resulting in sets with different PMI lengths (0, 28, 56, 84, 112, 140, 168 and 196 days), which were then fractured. Fractured bones (total=325; pig tibia=110; pig fibula=110; goat metatarsals=105) were classified according to the Fracture Freshness Index (FFI). Climatic data for the experiment location was collected. Statistical analysis included descriptive statistics, correlation analysis between FFI and PMI, Mann-Whitney U tests comparing FFI medians for different PMI's and linear regression analysis using PMI, pluviosity and temperature as predictors for FFI. Surface samples presented increases in FFI with increasing PMI, with positive correlations for all bone types. The same results were observed in submerged samples, except for pig tibia. Median FFI values for surface samples could distinguish bones with PMI=0 days from PMI≥56 days. Buried samples presented no significant correlation between FFI and PMI, and nonsignificant regression models. Regression analysis of surface and submerged samples suggested differences in FFI variation with PMI between bone types, although without statistical significance. Adding climatic data to surface regression models resulted in PMI no longer predicting FFI. When comparing different animal models, linear regressions suggested greater increases in FFI with increasing PMI in pig, compared to goat, in both surface and submerged environments, although statistically not significant. No differences were found between environments except for buried vs. submerged goat samples and surface vs. buried/submerged tibia pig samples. FFI showed a weak association with PMI and it seems to be affected by various factors, such as different bone types, decomposition environments and climatic factors. Nonetheless, the FFI shows promising discriminating power during the early postmortem period. The apparent variation between bone types from different species suggests that extrapolations to humans can be challenging.