Arthroscopic Management of Multidirectional Instability

The most critical step in successful treatment of shoulder instability does not lie in surgical technique, but in accurate assessment of factors contributing to instability. Multidirectional instability (MDI) is initially treated with rehabilitation. The primary goal of rehabilitation is strengthening of the dynamic stabilizers, including the rotator cuff and scapular stabilizers. There are several surgical techniques described to manage MDI, ranging from the classic Neer inferior capsular shift to a variety of arthroscopic procedures. This article focuses on the arthroscopic management of MDI.

In 1980, Charles S. Neer provided the first description of multidirectional instability (MDI) and the operation, the open inferior capsular shift, which was established as the gold standard. He described it as “involuntary inferior and multidirectional subluxation and dislocation.” This original article outlined the basis for much of our modern thinking about this condition. Although it is surprising how little has changed during the past 30 years, one aspect of MDI that has altered our thinking significantly is the advent of arthroscopic surgery and the innovation of arthroscopic techniques to restore stability and minimize morbidity.

Shoulder instability is best understood as a spectrum of disease. The most common type of instability, traumatic unidirectional instability, lies at one end of the spectrum, while multidirectional instability lies at the other extreme. Many cases fall somewhere in between these two classic examples. The most critical step in successful treatment of shoulder instability does not lie in surgical technique, but in accurate assessment of factors contributing to instability. Diagnostic precision is confounded further by variations in the definition of multidirectional instability. In general, MDI consists of symptomatic, involuntary instability of the glenohumeral joint in more than one direction. It is often bilateral and usually atraumatic. One of the frequent findings in MDI compared with traumatic unidirectional instability is pathologically increased capsular volume. MDI should not be confused with asymptomatic hyperlaxity or voluntary instability.

MDI is initially treated with rehabilitation, which requires patience and an extended period of time and effort on the part of the patient. The primary goal of rehabilitation is strengthening of the dynamic stabilizers, including the rotator cuff and scapular stabilizers. Proprioceptive training is also important. Nonoperative management is successful in approximately 80% of compliant patients with MDI. In the recalcitrant case in which nonoperative treatment fails, surgical management is appropriate. There are several surgical techniques described to manage MDI, ranging from the classic Neer inferior capsular shift to a multitude of arthroscopic procedures. This article focuses on the arthroscopic management of MDI.

Anatomic considerations

Shoulder stability is imparted by a combination of static and dynamic stabilizers. The most important static stabilizers include the glenohumeral ligaments, which are thickenings in the glenohumeral joint capsule that tighten and relax depending on the position of the shoulder. As a group, they render the shoulder stable through the full range of motion. The inferior glenohumeral ligaments (anterior and posterior) form a sling beneath the glenohumeral joint that stabilizes the joint in positions of abduction, preventing anterior, posterior, and inferior translation. The middle glenohumeral ligament provides anterior stability in the midrange of abduction, limiting external rotation and inferior translation. The superior glenohumeral ligament and coracohumeral ligament stabilize the shoulder in the adducted position, primarily limiting inferior translation and external rotation. These ligaments also comprise the structural portion of the rotator interval, the area of capsule between the superior edge of the subscapularis tendon, and the anterior edge of the supraspinatus tendon. Incompetence of the rotator interval has been shown to result in a 50% increase in posterior translation and a 100% increase in inferior translation.

The most important dynamic stabilizers include the rotator cuff muscles, scapular stabilizers, and the long head of the biceps. The rotator cuff is able to resist humeral head translation through the mechanism of concavity-compression, in which the humeral head is centered into the glenoid and the rotator cuff imparts a balanced contact pressure to the articulation. The labrum increases the concavity of the socket by up to 50%, but only provides 20% of the concavity-compression stability of the glenohumeral joint. Depth of the socket (including sufficient glenoid bone stock and labral integrity) and adequate compressive force (including intact rotator cuff and sufficient rotator cuff strength) are important for shoulder stability.

Scapular stabilizers are shown to be important for shoulder stability, and abnormal scapular kinematics and periscapular muscle function have been identified in patients with MDI. Decreased upward rotation and increased internal rotation in scapular plane abduction were found in patients with MDI in comparison with asymptomatic control patients. Stabilizing the scapula results in stabilization of the glenoid, the platform on which the humeral head must function.

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