Straightness of Central Anatomical Axes of Human Tibia–Fibula System: Role of Fibula and Its Importance in Biomechanics

Fracture may occur in human lower leg bones considering varying loads acting on them while performing various functions. Implants, commonly used in healing the fractures, must be properly aligned with the bones' anatomical axes for their proper functioning. While attempts to establish tibial anatomical axis have been made previously, combined central anatomical axis of human tibia and fibula remains unexplored. In this study, profiles of individual and combined central anatomical axes of tibia and fibula have been obtained using computed tomography (CT). These profiles are found to be approximately straight with the deviation from straight line for the centroidal axis of the combined system being approximately half of that for the tibia. It is further utilized in assessing the role of fibula in determining the critical stresses in the tibia with the help of static finite element analysis (FEA) of a tibia–fibula model. Even though fibula takes a smaller proportion of the total axial load, its absence increases the compressive stresses in the tibia due to increased bending moments at the critical cross section. Furthermore, FEA has been carried out to understand the clinical significance of the mismatch in the centroidal axis of an implant and its parent bone taking the example of the human tibia alone. FEA suggests that these central anatomical axes are in fact the load bearing axes of the bones and any misalignment of implant with the central anatomical axis may lead to patient discomfort and premature failure of the implant.