Demes, B.*; Chiu, J.; Truong, C.; Qin, Y.; Larson, S.: The role of the primate fibula in weight-bearing of the leg

Among mammals, primates have relatively robust fibulae. We investigated the role of the primate fibula in weight-bearing in an in vitro study of the deformation of the two leg bones in a chimpanzee, baboon and human. Three rosette strain gauges were attached around the midshaft of the tibia and fibula in a leg preparation with muscles removed, but ligaments and joints intact. The legs were loaded in compression through the knee and ankle joints. The proximal femur was held upside down in a clamp, and the foot was tied to a plate that was subsequently loaded with weights. Loading cycles were repeated after cutting of the interosseous membrane, the proximal tibiofibular ligaments, and, ultimately, the fibula. In all three species, the predominant loading regimes for both bones is bending. In general, strains increase in the tibia and decrease in the fibula after cutting of soft tissues connecting the two bones. The changes in strain following cutting of the interosseous membrane are insignificant. Cutting of the ligaments has more of an effect. Strains increase incrementally and significantly in the tibia when the fibula is cut. In addition, the neutral axis of bending rotates from a mediolateral to a more oblique position. The described changes are more dramatic in the nonhuman primates. In comparison to humans, these species have more robust fibulae that are sandwiched beneath a flange of the tibial plateau rather than being attached to the lateral side of the tibia as in humans. These results indicate that the fibula has a significant role in weight-bearing of the leg in primates, that the interosseous membrane is probably unimportant in weight transfer between the two leg bones, and that interspecific differences in bone size and positioning correlate with the observed strain pattern.