The two bones that make up the shoulder joint are the scapula and the head of the humerus. The head of the humerus has a spherical articular surface that falls right into the glenoid of the scapula. This glenoid is very much like a saucer, with a very shallow lower concave part. However, the shoulder joint also has some weaknesses. In addition to the shallow glenoid, the ligaments around the joint are also weak, especially the ligaments in front of the joint. These two structural features make the shoulder joint particularly susceptible to dislocation.