The most common instability pattern encountered in the overhead athlete is subtle anterior instability secondary to repetitive microtrauma to the anterior capsule. Since Jobe’s initial description of this entity in baseball pitchers, the concept has been carried over to other sports in which the shoulder is repetitively challenged by an anterior joint reactive force (1). One must, however, maintain a high index of suspicion for all instability patterns in overhead athletes, in that any busy shoulder or sports medicine specialist will encounter patients with multidirectional or unidirectional posterior instability. Table 13-1 lists athletic activities that may repetitively stress the posterior capsule. Because isolated posterior instability is rare in the overhead athlete, little is written pertinent to this group of patients. This chapter integrates basic science, historical treatment approaches, and modern rehabilitative and arthroscopic experience in managing overhead athletes with posterior instability. In addition, the importance of maintaining a balance of mobility and stability is emphasized in the treatment approaches discussed for overhead athletes with posterior instability.

A thorough understanding of shoulder anatomy and sport-specific biomechanics is necessary when attempting to diagnose and treat this group of patients with precision. Several basic science studies have helped to define the roles of various anatomic structures in maintaining glenohumeral stability. In addition, throwing mechanics have been studied extensively through clinical observation, the use of video cinematography, and electromyography (EMG). Other overhead activities that have been biomechanically analyzed include throwing a football, various swimming strokes, the golf swing, tennis swings, and volleyball serves (2, 3, 4, 5, 6, 7, 8, 9, 10, 11).

A comprehensive knowledge of the basic science of instability and the biomechanics of sport assists diagnostically in targeting the offending phase of physical activity and therapeutically in precisely designing a rehabilitative or surgical plan. Both compromised soft tissue restraints and abnormal scapulohumeral anatomy may contribute to posterior instability. Osseous architectural variants that have been described in association with posterior instability include excessive glenoid or humeral retroversion, glenoid hypoplasia, and diminished tilting angles and concavity of the inferior glenoid (12, 13, 14, 15, 16, 17). These abnormalities are extremely rare in the athletic population. By far, most symptomatic overhead athletes demonstrate occult posterior subluxation, most commonly due to posterior capsular laxity. There have only been a select few studies defining the primary soft tissue restraints to posterior translation (18, 19, 20, 21, 22, 23, 24). In a cadaveric sectioning study, Ovesen and Nielsen showed that the infraspinatus and teres minor muscle-tendons stabilized the glenohumeral joint for internal rotation in the first half of abduction, whereas the lower half of the posterior capsule became more important beyond 40 degrees of abduction (18). The work of Debski and co-workers suggest that passive tension in the rotator cuff plays a more significant role than other soft tissue structures in resisting posterior loads at 30, 60, and 90 degrees of abduction (19). More specifically, Blasier and colleagues found the subscapularis to be
the most significant active restraint to posterior humeral subluxation at 90 degrees of abduction (20). Of the passive restraints, the coracohumeral ligament contributed most with the arm in neutral rotation, whereas the inferior glenohumeral ligament (IGHL) complex contributed most in internal rotation. The contribution of the posterior capsule superior to the posterior band of the IGHL was less significant and decreased with greater displacement of the humeral head. O’Brien found the IGHL complex (with associated posteroinferior capsule) to be the primary static stabilizer against posterior instability with the arm in 90 degrees of abduction (21). Several other studies have shown that the rotator interval structures (superior glenohumeral ligament and coracohumeral ligament) limit inferior translation and serve as secondary restraints to posterior translation (25, 26, 27, 28). Although the importance of the anteroinferior labrum as an anchor for the anterior band of the inferior glenohumeral ligament is well recognized, damage to the posterior labrum may also contribute to instability. From a quantitative standpoint, the labrum has been shown to deepen the glenoid socket by 50% and contribute up to 20% to the stability ratio in the inferior and posteroinferior directions (29,30). Weber and Caspari created posterior dislocations in nine cadaver shoulders and found posterior Bankart or capsular tears in all specimens. No anterior pathology was seen (24). This is in contrast to Warren’s work, which noted that damage to both the anterior and posterior capsule was necessary to experimentally produce a posterior dislocation (23).

In addition to describing the primary and secondary stabilizing effects of various regions of the glenohumeral capsule, several studies have sought to determine the effects of surgical alteration of these same regions. A recent study by Gerber and colleagues quantified the effect of selective capsulorrhaphy on passive range of motion of the glenohumeral joint (31). This information may be used to more specifically target pathologic zones of capsule in the form of surgical plication or releases. Tibone and co-workers studied the effects of arthroscopic thermal capsuloplasty using a radiofrequency probe on anterior and posterior glenohumeral translation. They found that both laser and thermal shrinkage of the anteroinferior capsular structures resulted in a significant reduction in anterior and posterior translation (32,33). Conversely, thermal shrinkage of the posterior capsule using a radiofrequency probe failed to result in a significant decrease in either posterior or anterior translation using the same model (34). This may be due to the fact that the posterior capsule is generally thinner and stiffer, and has a lower strain to failure than its anterior counterpart (35,36). Selecky and co-workers used the same model to demonstrate a significant reduction in both anterior and posterior glenohumeral translation following rotator interval thermal capsuloplasty (37). The effects were slightly more significant with respect to posterior translation. These studies lend support to the surgical philosophy of addressing the thicker, more robust anterior capsular tissues to achieve a posterior tensioning effect via the “circle concept.”

The evaluation of a patient with shoulder instability should seek to identify any contributing major traumatic event, repetitive microtrauma, or voluntary history. In patients with an atraumatic history, the role of repetitive microtrauma and generalized ligamentous laxity must each be considered. Although congenital posterior instability has been described in patients with abnormal osseous architecture such as glenoid dysplasia and excessive humeral retroversion, the majority of competitive overhead athletes presenting with an atraumatic picture either have generalized ligamentous laxity that places them at increased risk for developing symptoms or have stretched out their ligamentous restraints due to the cumulative microtrauma of sport. Some sport-specific movements exert a direct posterior joint reactive force whereas others impart tractional stress due to muscular pull and momentum. The shoulders of football linemen and boxers, for example, are repetitively subjected to forceful posterior loads. Mair and co-workers recently reported on nine contact athletes with posterior labral detachment resulting in pain without instability (38). It is during the follow-through phase of overhead activity (pitching, swimming, tennis, volleyball) that the integrity of the posterior capsule is challenged by distraction and stretch. A similar mechanism has recently been described in the lead arm of golfers at the top of the backswing (39). The same phenomenon may occur during backhand movements while batting or participating in racquet sports.

Although many instability classification systems have been described, none are comprehensive or absolute. Joseph and colleagues recently described an etiology based classification system for multidirectional instability, which may also be used to better characterize posterior instability (40). Patients with congenital or type I instability exhibit inherited generalized ligamentous laxity. They tend to first present at an early age and frequently become symptomatic with normal activities of daily living or routine light athletic activity. Structurally, their soft tissue restraints are intact but lax. Patients with type II or acquired instability become symptomatic as a direct result of repetitive microtrauma from their offending sport. These patients usually lie within the middle of the connective tissue spectrum, with collagen that undergoes plastic deformation and stretch in response to cumulative loading. Common offending activities include gymnastics, weightlifting, swimming, and throwing sports. This group of patients differs from those with congenital laxity in that they do not possess gross generalized ligamentous laxity. Laxity is primarily found in the joints
subjected to repetitive stretch. Within the group of patients with acquired instability due to repetitive microtrauma exists a subset of patients with bidirectional (anterior and posterior) instability without inferior instability. Type III (posttraumatic) instability follows a specific traumatic event. There should be a higher index of suspicion for structural abnormalities such as labral disruption in these patients. In our experience, pain was also a more consistent component of symptoms in these patients (40). Antoniou and colleagues described four distinct anatomic lesions discovered arthroscopically in patients with posteroinferior instability. Seventy-eight percent of their patients had a traumatic etiology. Arthroscopic findings included labral detachment (12%), chondral or labral erosion (17%), synovial or capsular stripping (22%), and labral split or flap tears (32%) (41).

In addition to subclassifying based on etiology, the presence of voluntary activity should be sought in all cases, because this influences one’s treatment approach, both diagnostically and therapeutically. We routinely ask patients whether or not they are able to demonstrate what happens with their shoulder. This reinforces diagnostic accuracy and often defines the “position at risk.” It is important to then discourage this behavior. Although voluntary instability has been traditionally regarded as a “red flag,” it has come to be well accepted that different voluntary patterns exist (habitual, positional, neuromuscular) and that some of these patients may still benefit from surgical treatment (42).

Because many patients with posterior instability may present with pain as their chief complaint, responsibility often rests with the physician to make an accurate diagnosis based on history and physical examination findings. Most overhead athletes can specifically describe the offending position or action that reproduces their symptoms. In addition, some may be able to voluntarily demonstrate subluxation. Although this behavior should be discouraged, seeing it at least once in the clinical setting can be extremely helpful diagnostically (Fig. 13-1). An opportunity to examine any patient with instability on multiple occasions before surgical intervention is paramount.

Regardless of the suspected instability pattern, the instability examination includes the same inventory in all patients. First the patient is examined for signs of generalized ligamentous laxity. After a cursory cervical spine examination, the patient is asked to actively elevate and lower both arms. Inspection is performed from the front and the back, specifically looking for any asymmetry, dyskinesia, or dynamic scapular winging. Bilateral active and passive range of motion is recorded. Sulcus testing is performed with the patient seated and the arm in neutral and external rotation. For load-and-shift testing, it is helpful to examine the contralateral shoulder first, with the patient supine to facilitate relaxation. While supine, apprehension and relocation maneuvers and posterior stress tests may be performed. The patient is then seated and confirmatory tests such as “the jerk test” for posterior instability, the active compression test, and other labral tests may be performed. It is important in performing the jerk test to start with a light posterior directed force, because patients with significant posterior laxity may forcefully relocate with this maneuver, causing discomfort and subsequent guarding. The examination then proceeds through the usual areas of palpation, other specific shoulder tests, and neurovascular assessment. In any patient who comes to require surgical treatment, the load-and-shift examination is repeated with the patient under anesthesia and compared with the contralateral extremity.