The hip joint is located where the thigh bone (femur) meets
the pelvic bone. It is a “ball and socket” joint. The upper
end of the femur is formed into a round ball (the “head”
of the femur). A cavity in the pelvic bone forms the
socket (acetabulum). The ball is normally held in
the socket by very powerful ligaments that form a complete
sleeve around the joint (the joint capsule). The
capsule has a delicate lining (the synovium). The
head of the femur is covered with a layer of smooth cartilage
which is a fairly soft, white substance about 1/8 inch thick.
The socket is also lined with cartilage (also about 1/8
inch thick). The cartilage cushions the joint, and allows
the bones to move on each other with very little friction.
An x-ray of the hip joint usually shows a “space” between
the ball and the socket because the cartilage does not show
up on x-rays. In the normal hip this “joint space” is
approximately 1/4 inch wide and fairly even in outline.
