The bones of the hip joint are the femur (the thighbone) and the acetabular part of each side of the pelvis. The top end of the femur is shaped like a ball called femoral head. The femoral head fits into a round socket, called the acetabulum, on the side of the pelvis. On the femur below the femoral head is the femoral neck, the greater and lesser trochanter for muscle attacments and the shaft of the femur or a diaphysis. The hip joint has a very tight capsule around it, providing basic stability to the joint which is moved by different groups of flexion, extension and rotation muscles.