Relationships between human tibia diaphysis shape and experimental injury outcomes.

Interpretation is central to skeletal trauma analysis; however, the influence of intrinsic skeletal variation, such as long bone shape, on injury is not fully understood. While previous studies have emphasized extrinsic variables such as loading rate, direction, and number of impacts, this study investigates whether diaphyseal shape variation of the tibia contributes to differential injury outcomes sustained under standardized blunt trauma experiments. Sixty adult human tibiae were subjected to controlled four-point bending tests in a lateral-medial direction at 6 m/s, and resulting fractures were analyzed by count and overall injury complexity. Diaphyseal surface morphology was quantified as continuous shape traits represented by unscaled and scaled principal components to capture size-dependent and shape-only variation, respectively. Multinomial logistic regression models identified shape-injury associations and examined the moderating effects of age and sex. Multiple shape traits, particularly those involving anterior crest morphology and diaphyseal curvature, were significantly associated with fracture count and injury complexity. Notably, low-variance traits also influenced outcomes, suggesting individual morphology may modulate injury outcomes. Sex- and age-based interactions further indicate that similar morphologies may result in different outcomes between males or females and across adult age groups. Compared to fracture count, injury complexity produced more interpretable and biologically meaningful results. Collectively, these results show injury outcomes vary with tibial morphology, even under identical loading conditions, suggesting shape may warrant consideration when interpreting trauma in forensic casework. Accounting for shape variation may improve interpretation reliability, avoid misinterpretation of extrinsic factors, and support the development of probabilistic frameworks in forensic reconstructions.