Acetabulum

The acetabulum is the cup-shaped socket on the lateral aspect of the hip bone that receives the head of the femur, or thigh bone. Together, the acetabulum and femoral head form the hip joint, a ball-and-socket synovial joint. The socket’s depth and shape contribute to the hip’s substantial inherent stability while still allowing movement in several directions .^1,2

The acetabulum is not a separate bone. It is formed where three parts of the hip bone—the ilium, ischium, and pubis—meet and fuse during development. In the mature skeleton, these contributions form a continuous bony socket .¹

Not all of the acetabulum directly contacts the femoral head. Its cartilage-covered, horseshoe-shaped articular region is called the lunate surface. This surface bears and distributes forces transmitted through the hip. The deeper central region, called the acetabular fossa, is non-articular and is not covered by the same articular cartilage .^1,2

Around the rim of the acetabulum is the acetabular labrum, a ring of fibrocartilaginous tissue. The labrum is distinct from the bony acetabulum itself. It deepens the socket and contributes to joint stability, load transmission, and maintenance of a fluid seal between the articular surfaces .²