Biomechanics of Hip Arthroplasty

The biomechanics of the hip joint has been of great interest to researchers and clinicians since the early days of anatomical studies. Julius Wolff addressed the relation between the inner architecture of the bone and the functional loading already in the nineteenth century [31] and Friedrich Pauwels built the foundation for a mechanical approach to understand joint loading 65 years later [24]. Both researchers, despite dealing with very different questions (bone remodeling vs. fracture mechanics), are good examples for the spread of the biomechanics field. It can be defined as the science concerned with the internal and external forces acting on the human body and the effects produced by these forces [13]. Pauwels elaborated on the influence of valgus (steep) and varus (flat) anatomical position of the femoral neck, demonstrating for a given joint position, that a valgus neck is associated with a smaller lever arm of the abductor muscles and larger abductor muscle forces. This increases the magnitude of the resultant hip joint force and also changes its point of action in the pelvis to a more lateral position. His findings influenced the treatment of femoral neck fractures and femoral osteotomies in a major way. The biomechanical situation is more complicated when applied to Total Hip Replacement (THA) since all joint parameters are influenced by the operation: joint center, neck angle, offset, lever arms, and the range of motion until impingement. Range of motion and joint stability are decisive issues, especially in younger patients with high expectations on their quality of life after THA. The varus and valgus situation as well as the hip joint center are determined by the position of the implant in the pelvis and femur. This positioning also influences the local loading situation at the implant component – bone interface. For example, a slightly superior, posterior, and medial hip joint center after replacement can be associated with markedly higher joint forces (Fig. 2.1).