The hip-joint, like the shoulder, is a ball-and-socket joint, and, like it, moves in all directions. The main function of the shoulder is mobility, but the functions of the hip are mobility and support. To give the necessary support and security, the band-like ligaments uniting the bones are strong and the extent of the movements is restricted. Macalister ("Text Book of Human Anatomy," p. 179) points out that while the shoulder has 118 degrees of motion around a sagittal axis, abduction and adduction, the hip has only 90 degrees; around a coronal axis, flexion and extension, the shoulder has 170 degrees and the hip only 140 degrees. In the vertical axis the shoulder rotates 90 degrees, while the hip rotates only 45 degrees. In the upright position the centre of gravity falls in front of the axis of rotation of the hip-joint.

The head of the femur is 5 cm. (2 in.) in diameter and forms 5/7 of a sphere. Below and behind its centre is the depression for the attachment of the ligamentum teres. The acetabulum is much deeper than the glenoid cavity of the shoulder-joint and its depth is increased by the cotyloid ligament around its edge. This makes the joint air-tight and holds the femur in place by suction, hence it is called by Allis ("An inquiry into the difficulties encountered in the reduction of dislocations of the hip," Philadelphia, 1896) the sucker ligament. The acetabulum is incomplete at its lower anterior edge, forming the cotyloid notch. The cotyloid ligament bridges over this notch, and its deeper part loses its cartilaginous cells, becomes fibrous, and is called the transverse ligament.

Beneath the transverse ligament pass vessels, nerves, fatty tissue, and the extremity of the ligamentum teres, which is attached to the ischium just outside.

Running up in the floor of the acetabulum from the cotyloid notch is a depression in which is lodged the ligamentum teres and a pad of fat called the Haversian gland. The ligamentum teres is composed of synovial and connective tissue. It is not strong and ruptures at about 14 kilos; the small artery it contains affords nourishment for itself alone, only a very small amount of blood going to the head of the femur. Bland Sutton regards it as a vestigial structure and a regression of the pectineus muscle. It is too weak to add much to the strength of the joint, and the view of Allis that its function is to distribute the synovial fluid and act as a lubrieating agent is probably correct. The great pressure to which the articulating surfaces of the hip-joint are subjected requires special lubrication and this is furnished by the ligamentum teres and Haversian gland.

Fig. 509.   Anterior view of the ligaments of the hip joint.

Fig. 509. - Anterior view of the ligaments of the hip-joint.

Like other joints, the hip has a capsular ligament which is strengthened by bands or ligaments. These ligaments are the iliofemoral, pubofemoral, and ischiofemoral.