Comparison of femoral head loading in Chimpanzees and Humans

Chimpanzees are the closest living relatives of humans, despite this, the species do have very different loading on the epiphysis of the proximal head of the femur. The difference in loading is primarily due to a difference in locomotion. These differences affect the properties of subchondral and trabecular bone properties in the femoral head. Previous studies have shown that chimpanzees have thick and concave trabeculae that create a dense bone structure (Georgiou et al., 2018). These are found mostly on the inferior and central part of the femoral head. In humans, the structure of the trabecular bone volume is less than in chimpanzees, but there is high bone density in the inferior part of the femoral head (Georgiou et al., 2019). Differences in bone volume can be expected due to more loading. This is supported by Wolff’s Law, which states that bone responds to the stress that it experiences, meaning that in areas of higher loading, there can be an expectation to find more trabecular bone volume. Chimpanzees are facultative bipeds (Johnson et al., 2022), meaning they can walk bipedally but are primarily quadrupeds. Bipedalism in chimpanzees is characterized by their posture, which is “abducted, flexed hind limbs with greater rotations in the pelvis” (Johnson et al., 2022). When walking, chimpanzees will exhibit elevation in their pelvis on the side of the limb that is in the swing phase. On the contrary, humans will drop their pelvis during the same type of motion. The differences in pelvis elevation during the swing phase can potentially affect the loading on the femoral head. Thus, if differences in gait and posture are distinct between humans and chimpanzees, there can be an expected bone volume and trabecular volume difference in different regions of the femoral epiphysis in the two species.