The hip is a ball and socket joint, formed by the articulation between the top of the thigh bone (femoral head) and the pelvis (acetabulum).
It connects the lower limb to the pelvic girdle, and is designed for stability and weight bearing. The acetabulum forms a cup-shaped socket with which the spherical femoral head articulates. The acetabular labrum, formed by cartilage, attaches to the periphery of the acetabulum, deepening the socket and providing added stability.
The outer capsule of the hip joint attaches to both the pelvis and femur. There are strong extracapsular ligaments continuous with the outer surface of the hip joint capsule which tighten on extension to further improve stability. A number of muscles cross the hip joint, facilitating movement of the hip and upper leg.
The articular surfaces of the hip joint are lined with hyaline cartilage, which is smooth and allows for frictionless, painless movement. Degeneration of this hyaline cartilage results in osteoarthritis of the hip joint, a very common problem which is characterised by pain and stiffness.
Dislocation of the hip joint is rare owing to shape of the bony structures and the strength of the stabilising ligaments of the joint. Severe trauma or accidents can cause fracture of the hip joint or pelvis, and occasionally dislocation. In the elderly or those with osteoporosis, simple falls onto the side can cause femoral neck fractures, which reduce stability and function of the hip joint.