Evaluation of Recurrent Instability.

Last updated Friday, November 16, 2007

Figure 1

Recurrent atraumatic instability

AMBRII syndrome

Certain shoulders may be more susceptible to atraumatic instability. A small or functionally flat glenoid fossa may jeopardize the concavity compression, adhesion-cohesion, and glenoid suction cup stability mechanisms. Thin, excessively compliant capsular tissue may invaginate into the joint when traction is applied, limiting the effectiveness of stabilization from limited joint volume. A large, potentially capacious capsule may allow humeroscapular positions outside the range of balance stability. Weak rotator cuff muscles may provide insufficient compression for the concavity compression stabilizing mechanism. Poor neuromuscular control may fail to position the scapula to balance the net humeral joint reaction force. Voluntary or inadvertent malpositioning of the humerus in excessive anterior or posterior scapular planes may cause the net humeral joint reaction force to lie outside balance stability angles. Once initiated, the instability may be perpetuated by compression of the glenoid rim resulting from chronically poor humeral head centering. Excessive labral compliance may predispose to this loss of effective glenoid depth.

Any of these factors, individually or in combination, could contribute to instability of the glenohumeral joint. For example, posterior glenohumeral subluxation may result from the combination of a relatively flat posterior glenoid and the tendency to retract the scapula during anterior elevation of the arm, resulting in use of the elevated humerus in excessively anterior scapular planes. Excessively compliant capsular tissue in combination with relatively weak rotator cuff muscles could contribute to inferior subluxation on attempted lifting of objects with the arm at the side. If the lateral scapula is allowed to droop (whether voluntarily or involuntarily) the superior capsular structures are relaxed, permitting inferior translation of the humerus with respect to the glenoid (see figure 1). (Itoi et al, 1993)

Because they usually result from loss of midrange stability, atraumatic instabilities are more likely to be multidirectional. Pathogenic factors such as a flat glenoid, weak muscles, and a compliant capsule may produce instability anteriorly, inferiorly, posteriorly, or a combination. Although the onset of atraumatic instability may be provoked by a period of disuse or a minor injury, many of the underlying contributing factors may be developmental. As a result, the tendency for atraumatic instability is likely to be bilateral and familial as well.

It is apparent that atraumatic instability is not a simple diagnosis, but rather a syndrome that may arise from a multiplicity of factors. To help recall the various aspects of this syndrome, we use the acronym AMBRII. The instability is Atraumatic, usually associated with Multidirectional laxity and with Bilateral findings. Treatment is predominantly by Rehabilitation, directed at restoring optimal neuromuscular control. If surgery is necessary, it needs to include reconstruction of the rotator Interval capsule-coracohumeral ligament mechanism and tightening of the Inferior capsule. The diagnosis and management of this condition has been presented in detail. (Cofield, 1993; Lippitt et al, 1991; Matsen, Lippitt, Sidles et al, 1994; O'Driscoll, 1993)

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