Posterior Shoulder Instability

With little bony constraint, the glenohumeral joint is the most unstable in the human body. Cases of anterior shoulder instability can be found in literature dating back to the time of Hippocrates. However, posterior shoulder instability was not reported in the literature until 1741 by White et al. In 1952, Mclaughlin noted the wide clinical spectrum of posterior shoulder instability ranging from recurrent posterior subluxation to locked posterior dislocations. Confusion in the literature surrounding the terms ensued until 1984 when Hawkins et al. clarified a distinction between fixed dislocations and recurrent subluxations, noting that compared to subluxations, recurrent posterior dislocations are extremely rare. Posterior instability is less common than anterior instability but is increasingly recognized in the athletic population due to a better understanding of the underlying pathophysiology and the ability to treat with arthroscopic procedures. A patient may present with posterior instability after sustaining a traumatic dislocation or with posterior shoulder pain secondary to blunt trauma to the shoulder. However, more commonly, patients present with vague symptoms of shoulder pain, making the diagnosis difficult. The diagnosis is largely centered around history and physical examination findings, and clinicians must maintain a high index of suspicion. Depending on the underlying etiology and pathology, treatment of posterior shoulder instability ranges from physical therapy to operative intervention. Historically, surgical treatment was done via open procedures; however, arthroscopic management is quickly becoming the treatment of choice. The shoulder joint is the least congruent joint in the body with the joint commonly described as resembling a golf ball on a tee. In fact, only about one-third of the humeral head articulates with the glenoid at any given time. This lack of bony constraint provides the shoulder with a great range of motion for everyday activities. The stability of the shoulder thus relies upon a dynamic interplay of static and dynamic stabilizers. Static stabilizers of the shoulder include the glenoid labrum, which attaches to the periphery of the glenoid and increases the depth of the socket. Other static stabilizers include the articular congruity, glenohumeral ligaments, joint capsule, and negative intra-articular pressure. The most important static stabilizers against posterior translation are the posterior labrum, capsule, and the posterior inferior glenohumeral ligament (PIGHL). The PIGHL plays a primary role in stabilizing the joint when the shoulder is loaded in a position of flexion and internal rotation. When the shoulder is in this position, as seen in football linemen while blocking, the PIGHL is tensioned in an anteroposterior direction. Controversy exists as to the role of the rotator interval in preventing posterior instability. While this structure has shown to be a static stabilizer against inferior and posterior translation while the arm is adducted, other cadaveric studies have suggested the rotator interval plays little role in the posterior stability of the shoulder.  The rotator cuff muscles are the most important dynamic stabilizers of the shoulder. Contraction of the rotator cuff provides a concavity-compression effect of the humeral head against the glenoid aiding stability and increasing the load needed to translate the humeral head. The subscapularis is of particular importance as studies have shown it to be the primary dynamic restraint to posterior translation. Although their contributions vary depending on shoulder position, other dynamic stabilizers include the long head of the biceps tendon and deltoid muscle.