Supraspinous muscle may be injured.
- Stability of the shoulder joint is mostly afforded anteriorly, superiorly, and posteriorly by the rotator cuff.  Inferiorly, however, the shoulder is unsupported and strong abduction, coupled with external rotation, can force the head of the humerus downwards and forwards (sometimes damaging the axillary nerve) to the point that the joint dislocates.  This is termed the anterior shoulder dislocation as the head usually comes to lie anteriorly in the subcoracoid position.  Sometimes the force of the injury is sufficient to tear the glenoid labrum anteriorly thereby facilitating recurrence.  A surgical procedure is always required when the latter has led to repeated dislocations.

Reference:
1. Anatomy at a Glance. Omar Faiz and David Moffat.  Blackwell Science. 2002.