Atraumatic Shoulder Instability.
Last updated Thursday, February 10, 2005
Figure 1 - Scapula drooping About atraumatic instabilityWhat is atraumatic glenohumeral instability? Atraumatic instability is a condition in which the shoulder starts
to slip part way out of joint without having had a significant injury.
Atraumatic instability may arise from a variety of causes. A flat or
small socket, weak muscles, stretchy ligaments, periods of disuse, and
loss of normal coordination may contribute to atraumatic instability.
A persistent program of stabilizing exercises is the best place to start with managing atraumatic instability.
Uncommonly, if a prolonged, dedicated exercise program is not successful, capsular tightening by surgery may be considered.
The postoperative rehabilitation after this surgery is particularly important.
What factors may contribute to atraumatic glenohumeral instability? A shoulder that has been stable may become unstable after a minor
injury or a period of disuse. Certain shoulders may be more susceptible
to atraumatic instability.
- A flat or small glenoid fossa may jeopardize the balance, concavity compression, adhesion-cohesion, and glenoid suction cup stability mechanisms. Attenuation of the glenoid labrum may further compromise these stabilizing mechanisms.
- Thin, excessively compliant capsular tissue may invaginate into
the joint when traction is applied, limiting the effectiveness of
stabilization from limited joint volume.
- An extensive glenohumeral joint capsule may allow humeroscapular positions outside the range of balance stability.
- Weak 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 reaction force
to lie outside the confines of the glenoid fossa.
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 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).
Movies
What are the types of atraumatic glenohumeral instability? Because they usually result from loss of midrange stability, atraumatic
instabilities are more likely to be multidirectional. Pathogenetic
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.How is atraumatic glenohumeral instability diagnosed? It is now 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. We have established
some diagnostic criteria for making this diagnosis.
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