Mark E. Cinque, MD and Geoffrey D. Abrams, MD
Multidirectional instability (MDI) of the shoulder is described as symptomatic shoulder instability in 2 or more directions.1 The exact incidence of MDI has been difficult to define; however, it is a condition that often affects athletes who participate in overhead sports.2 Athletes with MDI represent a challenging population to treat because their training and competition load often predispose them to microtrauma of the shoulder, which can lead to symptomatic instability and decreased performance.
MULTIDIRECTIONAL INSTABILITY DEFINITION
Defining MDI of the shoulder joint has proven difficult. Neer and Foster were among the earliest surgeons to describe MDI of the shoulder, and they defined MDI as anterior and posterior instability associated with involuntary inferior subluxation or dislocation.3 The early definition of MDI has been greatly expanded by a rapidly growing body of basic science, biomechanics, and clinical literature that have refined the definition. Specifically, distinguishing between instability and laxity has proved to be crucial in defining what is true MDI. Instability is characterized by the presence of symptoms with abnormal laxity2,4 and may be caused by repetitive microtrauma or an episode of macrotrauma. In contrast, laxity is often congenital and may be part of a global hyperlaxity condition such as Ehlers-Danlos or Marfarn syndrome, as well as others.
To address the confusion between the roles of instability and hyperlaxity on glenohumeral motion, Gerber and Nyffeler classified dynamic stability as unidirectional or multidirectional, with the presence or absence of hyperlaxity.4 This classification system is of great utility because it allows patients to be stratified into groups in which different nonsurgical or surgical treatment options may be optimal (Table 26-1).
Although there is not a perfect definition of MDI currently agreed on, for the purposes of this chapter, MDI will be defined as symptomatic shoulder instability in 2 or more directions.
MULTIDIRECTIONAL INSTABILITY: ANATOMY AND ETIOLOGY
A sound understanding of the anatomy of the shoulder is paramount to diagnosing and treating MDI. Components of the static and dynamic stabilizers of the shoulder have been implicated in the pathogenesis of MDI. When either the static or dynamic restraints are injured by repetitive microxstrauma or macrotrauma, the shoulder becomes unstable. Understanding the role of each restraint allows the surgeon to apply the proper rehabilitation or surgical strategy for each stabilizer injured.
The static restraints of the shoulder include the native glenoid concavity and version, labral height, and the glenohumeral ligaments (GHLs). Studies have demonstrated that patients with MDI often have shallower glenoid cavities as compared to age-matched controls.5,6 The degree of contribution of the GHLs to MDI is not well studied; however, it is agreed that the GHLs provide stability at the terminal ends of motion. Results of clinical reconstruction studies have demonstrated the importance of ligamentous restraint to glenohumeral stability and highlight the increased strain placed on other stabilizers when they are deficient.7–10
The dynamic restraints of the shoulder include the rotator cuff (RTC) and scapulothoracic musculature. The RTC provides stability by increasing the compression of the humeral head to the glenoid and by centering the humeral head in the glenoid.11,12 Patients with improperly functioning RTC musculature have been shown to have off-center humeral head articulation, which can lead to the symptomatic shoulder instability seen in MDI.13 In this regard, Lippet et al reported that the concave compression generated by the RTC was able to resist translational forces as great as 60% of applied compressive load.12 Incompetent RTC musculature has also been studied using electromyography, which demonstrated more frequent hand-position error with precision-movement testing and decreased dynamic humeral head centering in those with RTC pathology.11,14 The periscapular muscular groups also play a critical role in positioning the glenoid by dynamically altering the glenoid version and inclination. Abnormal scapular kinematics have been demonstrated in patients with MDI compared to asymptomatic controls.15
The precise etiology of MDI has been less well defined. MDI can be traumatic or atraumatic but is much more frequently the latter. Unfortunately, patients with single-episode macrotrauma have superior surgical outcomes as compared to patients who develop MDI atraumatically or through repetitive microtrauma.16
Traditionally, a patulous inferior capsule was defined as the etiology of MDI. However, the majority of MDI patients are not symptomatic at birth, which implies symptomatic instability likely occurs over time. Thus, a patulous capsule is likely necessary to make the diagnosis of MDI, but in isolation may not be significant enough to produce symptoms. Moreover, athletes who participate in overhead sports with repetitive motions are some of the most common with MDI.
MULTIDIRECTIONAL INSTABILITY: CLINICAL PRESENTATION, PHYSICAL EXAMINATION, AND IMAGING
MDI includes multiple instability patterns, and its diagnosis must be approached using multiple modalities. Unidirectional shoulder instability is often associated with a single, identifiable anatomic lesion. Therefore, MDI should be suspected in patients with a history of unidirectional instability without an identifiable anatomic lesion. Furthermore, MDI should also be suspected in young athletes (age < 40 years) with prior failed instability procedures. The prevalence of MDI is equal in male and female athletes.2,17
The majority of MDI patients present with an insidious onset of shoulder pain and instability, often worse with activities. However, some patients may also report recurrent shoulder dislocations. The classic MDI patient is an overhead-sport athlete in the second to third decade of life. Athletes will often complain of decreased power with overhead motion of their affected shoulder and often can identify the motions that generate instability and/or pain. Because patients with longstanding instability often learn to avoid movements or positions that generate instability, it is important to elicit positions of apprehension or instability when obtaining a history from the patient.
Patients with anterior instability often avoid overhead activity, external rotation, and abduction, whereas patients with posterior instability often experience instability with active forward flexion and internal rotation. Patients with inferior instability may experience pain or instability with carrying heavy objects, especially with the arm adducted.
Patients with a history of generalized ligamentous laxity represent a special subset of patients that require more detailed evaluation. Using the Beighton score to determine the degree of hyperlaxity is vital during patient evaluation. The Beighton score includes the following assessments: hyperextension of the elbow or knee greater than 10 degrees, thumb opposition to the ipsilateral lateral forearm, greater than 90-degree small finger dorsiflexion, and ability to place palms flat on the ground while bending at the trunk.18 The first 4 maneuvers can be performed bilaterally, and one point should be assessed for each positive finding, with a maximum score of 9. Patients with a score greater than or equal to 2 on the Beighton score were more than twice as likely to have experienced glenohumeral instability compared to those with lower Beighton scores.19
After obtaining a thorough history of a patient who is suspected to have MDI, a comprehensive physical examination is critical. Areas of atrophy and/or weakness identified on physical exam must be noted because failure to identify weakness can result in surgical failure. When evaluating for instability, the surgeon must be sure to identify voluntary subluxators and dislocators. It is important to identify these patients because they have been demonstrated to have poor outcomes after surgical treatment.2
The examination of a patient with suspected MDI should begin with a careful inspection looking for asymmetry, atrophy, and previous surgical incisions. Next, the examiner should evaluate the patient’s scapular motion, specifically evaluating for scapular dyskinesia and asymmetry. Patients with MDI have a higher rate of scapula dyskinesia compared to age-matched, asymptomatic controls.15
In patients with possible MDI, 3 specific tests should be performed. The sulcus sign, if present, can indicate shoulder instability. Humeral head displacement great than 2 cm is defined as a positive sulcus sign and is indicative of excessive glenohumeral laxity. The presence of a sulcus sign at 90 degrees of abduction has been shown to be highly correlated with inferior capsular laxity.3,20 However, it must be highlighted that more than 2 cm of humeral head translation is not considered abnormal unless the patient is symptomatic in that shoulder (Figure 26-1). The presence of a sulcus sign has been specifically studied in athletes as well. In a study of 178 asymptomatic athletes, 9% of female athletes and 3% of male athletes demonstrated positive sulcus sign without clinical instability.21 These findings highlight that athletes must be examined using multiple exam maneuver tests because they may have developed compensatory mechanisms allowing them to remain active in their sports.
The second physical exam maneuver that can elicit MDI symptoms is the load-and-shift test. This test is performed with the patient in the supine position, with the arm in 90 degrees of abduction and the elbow flexed to 90 degrees. The examiner applies an axial load through the humerus and then translates the humerus anteriorly and posteriorly along the glenoid to assess the degree of humeral head translation (Figure 26-2). There are 3 degrees of translation based on the load-and-shift test: grade 1 is humeral head translation within the glenoid rim, grade 2 is humeral head subluxation over the glenoid rim with spontaneous reduction, and grade 3 is humeral head subluxation over the glenoid rum requiring manual reduction.2
The third physical exam test for MDI is the hyperabduction test. To perform this test, the examiner passively hyperabducts the patient’s arm while stabilizing the ipsilateral shoulder (Figure 26-3). Gagey and Gagey reported that in 90 patients with instability, 85% of patients had passive abduction of greater than 105 degrees, whereas patients without instability had less than 90 degrees. Furthermore, the authors noted that patients with greater than 105 degrees of abduction had concomitant labral pathology, which highlights the contribution of GHL laxity to clinically significant instability.18