PT Tip of the Month Archive

Shoulder Instability


"Shoulder instability" is a term used to refer to a variety of shoulder conditions including dislocation, subluxation, and laxity. Shoulder dislocations and subluxations occur frequently in athletes, with 95% of traumatic dislocations occurring in the anterior direction. However, posterior and multidirectional instability are also seen.


The glenohumeral joint (aka, GHJ or ball and socket joint of the shoulder) is inherently unstable, as it is designed for mobility. Thus, it is of no surprise that it is the most commonly dislocated major joint of the human body. The GHJ relies on both passive and active stabilizing structures to achieve functional stability.

Passive constraints include bony anatomy, the glenoid labrum, and the various capsuloligamentous structures. The articulating surface of the glenoid fossa (socket) is much smaller than that of the humeral head (ball), which is about 3-4x the size of the glenoid fossa. During normal GHJ motion, only 25-30% of the humeral head is in contact with the glenoid fossa, contributing to its inherent instability. The glenoid labrum's function is to deepen the glenoid and assist in stabilizing the GHJ during mid-range GHJ motions, when the capsuloligamentous structures are lax. It also acts as a load-bearing structure for the humeral head. The GHJ capsule allows for the large range of shoulder motion, as it is large and loose. Three GHJ ligaments reinforce the capsule and contribute to joint stability by becoming taut at extremes of motion.

Active constraints include shoulder girdle musculature, the interaction between muscles and ligaments, and shoulder proprioceptors. Primary active stabilizers include the rotator cuff muscles, the deltoid, and the long head of the biceps brachii muscle. Secondary stabilizers include: teres major, Latissimus dorsi, and the pectoralis major muscles. Synergistic rotator cuff/deltoid interaction creates compression of the humeral head within the glenoid fossa, contributing to joint stability by keeping the humeral head centered within the fossa. As the rotator muscles contract, tension is produced within the capsular ligaments, and again reinforcing the centralized position of the humeral head within the fossa. Scapulothoracic musculature is equally as important in shoulder stability by producing a stable base to support the GHJ musculature. Last but not least, the individual's awareness of joint position, called proprioception, and his/her ability to recruit the proper muscular contraction to stabilize the joint is important in preventing excessive GHJ displacement.

Risk Factors

The link between the angulation of the glenoid fossa and shoulder stability has been investigated by various researchers. One researcher reported that excessive retroversion of the glenoid predisposed patients to posterior GHJ instability. Another researcher noted that increased glenoid anteversion was found in patients who had recurrent dislocations. Patients who have shallower glenoid fossas may also be at greater risk for shoulder instability. Bony defects, such as a Hill-Sachs lesion (osseous defect on the posterolateral portion of the humeral head), can result from the underlying instability. Compromise to the passive constraints (i.e. Bankart lesion or asymmetrical capsular tightness) can result in abnormal translation of the humeral head within the glenoid fossa, resulting in instability. Posterior shoulder weakness, or more often an imbalance between anterior and posterior rotator cuff strength, can result in abnormal GHJ mechanics and a loss of stability.

How? Who?

The most common mechanism of injury with anterior shoulder dislocations is when the shoulder is in a forced externally rotated and abducted position, as seen with basketball players attempting to block an overhead pass. Repetitive overhead sports, such as throwing, volleyball, and tennis may overload the anteroinferior GHJ restraints, increasing GHJ translation, and increasing the likelihood of developing shoulder instability. About 95% of first-time shoulder dislocations occur as a result of a forceful collision/movement or falling on an outstretched arm. The other 5% of dislocations are non-traumatic in nature, and these individuals may already have a natural capsular laxity or altered neuromuscular control of the shoulder complex. Incidence of recurrent dislocations in persons under 20 range from 66%-100%, in persons 20-40 years old, the range is 13%-63%, and in persons 40+ years old, the range is 0%-16%.


Your skilled physical therapist will perform a comprehensive exam of your range of motion and strength of your shoulder and surrounding joints. They may also utilize specific tests to determine if and what type of shoulder instability you may have and prescribe various exercises to strengthen the shoulder, scapular, and core muscles. If you think you may be experiencing pain due to shoulder instability and would like to be scheduled for a physical therapy evaluation, please contact 617-232-PAIN (7246) for our Brookline office and 617-325-PAIN (7246) for our West Roxbury office.


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  3. Hayes K, Callanan M, Walton J, Paxinos A, Murrell GAC. Shoulder instability: management and rehabilitation. J Orthop Sports Phys Ther.2002;32:497-509.
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