Geometric considerations for designing the hip joint and head of the femur: (a) The quadriceps angle (q-angle) is the angle the quadriceps muscle attaches to the patella and is measured using the anterior superior iliac spine, midpoint of the patella, and the tibial tubercle and (b) Narrow neck width is measured at the narrow point of the femoral neck. The femoral neck length is determined by the center of the femoral head to the intersection of the neck-shaft axis and the femoral shaft axis. Finally, the neck-shaft angle is the angle between the neck-shaft axis and the femoral shaft axis. These variables are important for design consideration because in humans, they are markers for fracture.