The shoulder joint has been described as a ball-and-saucer joint. Consequently, there is little stability imparted by the bony architecture. In order to maintain that consistent ball-and-saucer relationship (keep the ball from rolling off the saucer), ligaments are present that connect the ball (head of the humerus, or arm bone) directly to the saucer (glenoid process of the scapula, or shoulder blade). These ligaments allow some motion between these two structures, but limit the motion in order to prevent dislocation. Importantly, the muscles around the shoulder help to control this ball-and-saucer movement. These act in concert with the ligaments in order to allow smooth, pain-free, and stable motion of this important joint.
Traumatic dislocations, such as that during a tackle in football, invariably tear some of the stabilizing ligaments. Although the ligaments heal, they often heal looser than what they originally were. These loose ligaments do not provide as much stability as they did prior to injury. This increases the risk of having a further dislocations. With each subsequent re-dislocation, the ligaments are again injured and progressively get even looser. This can progress to the point where the dislocation happens from very trivial activities. At this stage, the only effective treatment is to surgically repair the torn ligaments in order to tighten them and prevent abnormal motion of this ball-and-saucer joint.
Sometimes the instability is not severe enough to allow a dislocation, but enough to allow the ball to move part way off the saucer. This is referred to as a subluxation. This can be thought of as a partial dislocation in that the ball and saucer are still touching, but the ball has moved off the center of the saucer. This abnormal position can cause discomfort. Because the amount that the ball has to move is less than in a complete dislocation, the amount that the ligaments have to be stretched is also less. This lesser degree of instability can therefore often be controlled by strengthening the muscles around the shoulder to help reduce some of the stress of stabilizing the joint from the ligaments. Therefore, these shoulders, although somewhat loose, can often be treated successfully without having to surgically tighten the ligaments.