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Functional Anatomy
The hip joint, an enarthrodial joint, can be described as a ball and socket joint. The femur connects at the acetabulum of the pelvis and projects laterally before angling medially and inferiorly to form the knee. Although this joint has three degrees of freedom, it is still stable due to the interaction of ligaments and cartilage. The labrum lines the circumference of the acetabulum to provide stability and shock absorption. Articular cartilage covers the concave area of acetabulum, providing more stability and shock absorption. Surrounding the entire joint itself is a capsule secured by the tendon of the psoas muscle and three ligaments. The iliofemoral, or Y, ligament is located anteriorly and serves to prevent hip hyperextension. The pubofemoral ligament is located anteriorly just underneath the iliofemoral ligament and serves primarily to resist abduction, extension, and some external rotation. Finally the ischiofemoral ligament on the posterior side of the capsule resists extension, adduction, and internal rotation. When considering the biomechanics of hip fractures, it is important to examine the mechanical loads the hip experiences during low energy falls.

Biomechanics
The hip joint is unique in that it experiences combined mechanical loads. An axial load along the shaft of the femur results in compressive stress. Bending load at the neck of the femur causes tensile stress along the upper part of the neck and compressive stress along the lower part of the neck. While osteoarthritis and osteoporosis are associated with bone fracture as we age, these diseases are not the cause of the fracture alone. In a study conducted in Umea, Sweden, Bergsten et al. discovered that low energy falls from heights of one meter or less were the leading cause of hip fracture in the elderly adult population (Bergström, Björnstig, Stenlund, Jonsson, & Svensson, 2008). Taking into account that falls were the leading cause of hip fracture, Hwang et al studied how the manner in which a fall occurs affects the chances of hip fracture. In their study, they found three contributing factors, with fall direction being the strongest predictor (Hwang, Lee, Huang, Chen, & Lin, 2011). During a sideways fall, the chances of hip fracture see a 15-fold and 12-fold increase in elderly males and females, respectively. This is likely due to a mechanical load experienced by bones weakened by osteoporosis.

Case Studies


For example, in case studies by Kelly and Kelly, two elderly females experienced a low energy fall with impact occurring to the knee in a flexed position. Compression along the shaft of the femur caused the neck of the femur at the hip to experience a bending load. The stresses experienced at the neck of the femur resulted in a fracture in both cases. Figure 1 shows the forces acting at the hip visually, while Figure 2 displays the kind of hip fracture discussed. Neither one of the women had osteoporosis, but the manner in which the hip joint was mechanically loaded resulted in a hip fracture. For hip fracture prevention, it is important to consider both onset of osteoporosis and mechanical loading during a fall.