Sex and Age Effects on Tibia Biomechanical Properties in Dynamic Four-Point Bending

Globally, approximately 11 million pedestrian impacts each year result in injury or death. The lower extremity is the most frequently injured body region in pedestrian impacts, and these injuries often lead to long-term disability. To better understand lower-extremity injury risk, biomechanical testing of the leg and tibia has previously been conducted. However, this work has mainly focused on males, despite females having a higher risk of sustaining lower-extremity injuries in these loading events. This study aimed to evaluate the effects of sex and age on biomechanical properties of the isolated human tibia in a diverse sample. Sixty-six tibiae from 35 females and 31 males with similar age distributions (p > 0.05; females = 68.8 ± 3.4 years; males = 59.8 ± 4.0 years) were utilized. Tibial diaphyses were loaded to failure at 6 m/s in lateral–medial four-point bending to replicate vehicle bumper interaction in pedestrian impacts. Reaction forces were measured at each end of the bone. Proximal and distal moments were calculated as the product of reaction force and the distance from the axis of rotation to the nearest impact site. Peak bending moment at midshaft was further calculated as the maximum of the average proximal and distal moments. Strain at time of fracture was determined from axial strain gages on the lateral and medial bone surface between impactor locations. Maximum deflection was normalized by specimen length and area moment of inertia. Males exhibited significantly greater peak moments than females (p = 0.001). In the full sample, age demonstrated a weak but significant negative relationship with peak moment (p = 0.013, R2 = 9.3%); however, no significant age relationship was observed when males or females were analyzed separately. Male tibiae had a significantly greater deflection than female tibiae (p = 0.001), but no significant sex difference was observed with normalized deflection. Age was strongly and negatively associated with deflection in the full sample (p < 0.0001, R2 = 39.7%), and for males (p < 0.0001, R2 = 44.8%) and females (p < 0.0001, R2 = 25.4%) separately. No significant sex differences were observed for tensile strain at fracture. However, age had a significant negative relationship with tensile strain in the full sample and within each sex (p < 0.0001). Further investigation with larger sample sizes is needed to characterize the interacting effects of sex, age, and size. Overall, this work advances understanding of tibial biomechanics and provides insight for improving pedestrian safety tools including anthropomorphic test devices and human body models.