Types of Glenohumeral Instability.

Last updated Thursday, February 10, 2005

Glenohumeral instability

What is glenohumeral instability?

What are circumstances of glenohumeral instability?

Congenital instability may result from local anomalies, such as glenoid dysplasia (Wirth, Lyons and Rockwood, 1993) or systemic conditions such as Ehlers-Danlos syndrome. Instability is acute if seen within the first days after its onset; otherwise, it is chronic. A dislocation is locked (or fixed) if the humeral head has been impaled on the edge of the glenoid, making reduction of the dislocation difficult. If a glenohumeral joint has been unstable on multiple occasions, the instability is recurrent. Recurrent instability may consist of repeated glenohumeral dislocations, subluxations, or both.

Instability may arise from a traumatic episode in which an injury occurs to the bone, rotator cuff, labrum, capsule, and/or a combination of ligaments. Recurrent traumatic instability typically produces symptoms when the arm is placed in positions near that of the original injury. Conversely, instability may arise from the atraumatic decompensation of the stabilizing mechanisms. The degree to which the shoulder was "torn loose" as opposed to "born loose" or just "got loose" is critical in determining the best management strategy.

We have found that most patients with recurrent instability fall into one of two groups. On one hand, patients with a traumatic etiology usually have unidirectional instability; often have obvious pathology, such as a Bankart lesion; and often require surgery when instability is recurrent, thus the acronym: TUBS. On the other hand, patients with atraumatic instability often have multidirectional laxity; which is frequently bilateral; usually responding to a rehabilitation program. However, should surgery be performed, the surgeon must pay particular attention to performing an inferior capsular shift and closing the rotator interval, thus the acronym: AMBRII. Rowe (Rowe, 1956) carefully analyzed 500 dislocations of the glenohumeral joint and determined that 96 per cent were traumatic (caused by a major injury) and the remaining 4 per cent were atraumatic. DePalma, (DePalma, 1983) Rockwood, (Rockwood, 1979) and Collins and Wilde (Collins and Wilde, 1973) also recognized the importance of distinguishing between traumatic and atraumatic instability of the shoulder.

Patients with atraumatic instability may have generalized joint laxity. Imazato (Imazato, 1992) and Hirakawa (Hirakawa, 1991) demonstrated that patients with loose shoulders have relatively immature, more soluble and less cross-linked collagen fibers in their capsule, muscles and skin than controls; presumably tissues like the glenoid labrum would contain immature collagen as well, making them more deformable under load. Further evidence of constitutional factors is gained from a number of reports of positive family histories and bilateral involvement among those individuals with shoulder dislocations. O'Driscoll and Evans (O'Driscoll and Evans, 1988) and Dowdy and O'Driscoll (Dowdy and O'Driscoll, 1993) found a family history of shoulder instability in 24% patients requiring surgery for anterior glenohumeral instability. Morrey and Janes (Morrey and Janes, 1976) reported a positive family history in approximately 15% of patients who were operated on for recurrent anterior shoulder instability. A positive family history was also noted twice as frequently in patients whose postoperative course was complicated by recurrent instability compared with patients with successful surgery. Rowe and colleagues (Rowe, Patel and Southmayd, 1978) reported a positive family in 27% of 55 patients with anterior shoulder instability who were treated with a Bankart procedure. Bilateral instability was noted in 50% of patients with a positive family history compared with 26% of patients with negative family history, which suggested the possibility of a genetic predisposition.

When instability develops with no or minimal injury (Garth et al, 1987; Protzman, 1980; Rowe and Zarins, 1981), the initial reason for the loss of stability is often unclear. However, it appears that once lost, the factors maintaining stability may be difficult to regain. Certain phenomena may be self perpetuating: when the humeral head rides up on the glenoid rim, the rim becomes flattened and less effective, allowing easier translation. Furthermore, when normal neuromuscular control is compromised, the feedback systems which maintain head centering fail to provide effective input. Thus the joint becomes launched on a cycle of instability leading to loss of the effective glenoid concavity and loss of neuromuscular control leading to more instability.

If a patient intentionally subluxates or dislocates his or her shoulder, instability is described as voluntary. If the instability occurs unintentionally, it is involuntary. Voluntary and involuntary instability may coexist. Voluntary anterior dislocation may occur with the arm at the side or in abduction/external rotation. Voluntary posterior dislocation may occur with the arm in flexion, adduction and internal rotation, or with the arm at the side. The association of voluntary dislocations of the shoulder with emotional instability and psychiatric problems has been noted by several authors. (Carew-McColl, 1980; Rowe et al, 1973) The desire to voluntarily dislocate the shoulder cannot be treated surgically. However, the fact that patients can voluntarily demonstrate their instability does not necessarily mean they are emotionally impaired.

Neuromuscular causes of shoulder instability have been reported as well. Percy (Percy, 1960) described a woman who, following an episode of encephalitis, developed a posterior dislocation. Kretzler and Blue (Kretzler and Blue, 1966) have discussed the management of posterior dislocations of the shoulder in children with cerebral palsy. Sever, (Sever, 1927) Fairbank, (Fairbank, 1913) L'Episcopo, (L'Episcopo, 1939) Zachary, (Zachary, 1947) and Wickstrom (Wickstrom, 1962) have reported techniques for the management of neurological dislocation of the shoulder caused by upper brachial plexus birth injuries. Stroke is another important neurological cause of instability. (Zorowitz et al, 1995)

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