**2. Hip fracture anatomy**

Hip fracture is a medical condition in which there is a break in the continuity of the femoral bone. Hip fracture is generally affected by hip anatomy [31], the applied forces to the hip [32], and bone mechanical properties [33]. In this section, hip anatomy is explained to show why the hip is likely to experience fracture in a fall.

The hip joint is one of the most important joints in the human body. It is also one of the most flexible joints allowing a great range of motions. To better understand hip fracture, it helps to know the anatomy of the hip joint. The hip is a joint formed by the ball-shaped head of the femur and the socket of the pelvis. The femurs are the longest and the strongest bones in the human body, extending from the hip to the knee. Important geometric features of femur bones include the head, neck, and greater and lesser trochanters, as shown in **Figure 1(a)**. A femur is composed of two types of bones, cortical and cancellous. The cortical bone forms the outer layer of the femur and withstands most of the forces and moments. Cancellous bone is mostly enclosed by the cortical bone and mainly absorbs the shock energy produced in walking and running [34]. The hip joint is a stable ball-and-socket joint, much more stable than the shoulder joint. The stability in the hip mainly attributes to the deep socket, i.e., the acetabulum. Additional stability is provided by the strong joint capsule and its surrounding muscles and ligaments. The high level of stability of the hip joint is required to support the upper body [34].

**Figure 1.** (a) Anatomic structure of the hip [35]. (b) Concentration of applied forces on the proximal femur in a lateral fall which increases the risk of fracture.

More than 90% of all hip fractures occur in falls [36] as the femur is subjected to a high-level impact force. As shown in **Figure 1(b)**, in a sideways fall, the greater trochanter and the femoral head are subjected to the impact and the joint force, respectively, from the ground and the acetabulum. The forces produce a moment at the intersection of the neck-shaft axes. Muscles that are attached to the femur also produce forces during the fall. As it is shown in **Figure 1(b)**, the applied forces in a fall are mainly on the proximal femur, and it may explain why the majority of fall-induced hip fractures occur at the proximal femur [37]. A hip fracture refers to any fracture of the proximal femur down to a level of approximately 5 cm below the lower border of the lesser trochanter [38]. The extent of the break depends on the forces that are involved.
