Hip & Pelvis Anatomy

The pelvis has a complex structure made up of three bones - the lower part of the spine, the sacrum, the right innominate bone, and the left innominate bone. 

In the back part of the pelvis, the innominate bones are joined to the sacrum through the sacroiliac joint. Very strong sacroiliac ligaments hold the bones together. In the front part of the pelvis, the innominate bones are joined together by a cartilaginous disc called the symphysis pubis. There are also very strong ligaments in the front of the pelvis at the symphysis pubis. 

Within each of the innominate bones is the hip socket or acetabulum. The acetabulum is a semi-circular structure with a roof, a front wall, and a back wall. The inside of the acetabulum has a crescent shape which is lined by a specialized tissue, the articular cartilage. Around the rim of the acetabulum is a structure called the labrum. The labrum functions as a seal around the femoral head. 

The femoral head is the other bony component of the hip joint. The femoral head is a sphere. The covering of the femoral head is articular cartilage. 

The articular cartilage of the acetabulum and the femoral head provide very low friction surfaces which allows the two parts to move smoothly against one another. The femoral head is connected to the acetabulum by a very strong ligament called the ligamentum teres. The hip joint is enclosed by a tough fibrous tissue called the hip joint capsule. The lining of the hip joint capsule is a specialized tissue called synovium. The synovium produces a fluid which lubricates the joint and also contributes to the very low friction surfaces of the acetabulum and the femoral head.