Atraumatic Shoulder Instability.
Last updated Thursday, February 10, 2005
Figure 1 - Scapula drooping Figure 2 - Age distribution of atraumatic instability patients Figure 3 - SST functional deficits: AMBRII patients About atraumatic instabilitySummary 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.
Contributing factors 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
Types 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.Diagnosis 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.Age distribution The age distribution of 51 patients presenting to our service with the
atraumatic (AMBRII) instability shows that this appears to be a
condition which presents predominantly under the age of 30 (see figure
2).Causes AMBRII instability often begins with some minor event or series of
events which lead to progressive decompensation of the glenohumeral
stability mechanisms.
An awkward lift, reaching over the back seat of the car, or a sneeze
may be all that is necessary to launch the predisposed, but
compensated, shoulder down the path toward instability. The patient
notices that the shoulder has become loose and may feel it slip out and
clunk back in with different activities. These episodes almost never
require manipulative reduction.
The instability may be sufficiently subtle that the patient is
unaware of the humerus translating on the glenoid. The patient may only
be aware of a feeling that the shoulder does something unnatural in
certain positions, or that certain functions cannot be performed, such
as reaching out in front or lifting at the side.
In contrast to the situation in traumatic instability, discomfort
with activities of daily living may be a significant component of the
complaint. A patient may volunteer that he or she can make the shoulder
"pop out" and that at times the shoulder feels as if it "needs to be
popped out" on purpose.
The patient should indicate each and every position in which
problems with instability have been noted. Instability with the arm out
in front of the body and problems lifting or reaching down are
particularly suggestive of the AMBRII condition. It is important to
note how frequently the problem occurs and whether the problem is
"avoidable" if the patient concentrates on how the shoulder is used.
Finally, we record the extent and effectiveness of previous
non-operative and operative treatment and the presence or absence of
instability symptoms in the opposite shoulder or other joints.
Simple shoulder test The Simple Shoulder Test provides a minimal data set for
characterizing some of the functional impairment from atraumatic
multidirectional glenohumeral instability. These patients had greatest
difficulty sleeping, lifting overhead, and throwing (see figure 3).
Particular emphasis is placed on the patient's functional goals with
respect to work and sport. We try to determine whether these goals are
realistic, considering the condition of the shoulder. Summary In summary, patients with atraumatic instability are usually young,
perhaps with a family predisposition to "loose shoulders." The
instability is most prevalent in midrange positions, those commonly
used in activities of daily living, such as lifting at the side or
raising the arm to the front. The contralateral shoulder may also seem
"loose." The patient may have difficulty defining exactly what it is
about the shoulder that is bothersome. The history does not reveal an
injury of sufficient magnitude to tear the capsule or ligaments.Physical examination The physical examination of patients with AMBRII syndrome is usually
started by asking them to demonstrate the positions in which the
shoulder feels unstable. They may demonstrate a spontaneous jerk test
by bringing the elevated arm horizontally across the chest, causing the
humeral head to subluxate posteriorly. Then by returning the elevated
humerus to the coronal plane they produce a "clunk" on reduction of
glenohumeral joint (much like the Ortolani and Barlow signs of the
hip). Using the palpable scapular coordinates, we can estimate the
scapular plane in which the shoulder subluxes and the plane in which it
reduces. Patients may also demonstrate that when they attempt to lift
an object or tie their shoes, the shoulder subluxates inferiorly. They
may demonstrate that when they lie on the affected shoulder it is
pushed forward out of joint. Finally, they may demonstrate by elevating
the arm in a posterior humerothoracic plane that they can produce
anterior subluxation with spontaneous reduction on return to the
coronal plane. By allowing the patient to demonstrate the symptomatic
positions and motions of instability, our hands are free to define the
humeroscapular positions at the moments of interest. These observations
may reveal faulty patterns of scapulohumeral mechanics, such as
allowing the lateral scapula to droop during lifting or retracting the
scapula during anterior elevation of the humerus.
We have described our investigations of classic clinical laxity
tests showing that, in a small group of subjects, the magnitude of
translation for shoulders with atraumatic instability is essentially
the same as that of normal shoulders or shoulders with traumatic
instability. Therefore, we pay particular attention to the patient's
response during laxity testing: we are seeking to reproduce the
translations which duplicate the symptoms that brought the patient in
for treatment. Our best diagnostic confirmation occurs when, during a
laxity test, the patient states "that's it, that's the thing that's
bothering me." We refer to this as recognition of the symptomatic event
when it is reproduced during the examination.
We always make a point of examining the laxity of the contralateral
glenohumeral joint. Occasionally, laxity tests will yield different
results on the symptomatic side. More often, however, examination of
the contralateral shoulder is similar to the symptomatic one. This
allows us the opportunity to demonstrate to the patient and the family
that, while both shoulders demonstrate similar degrees of laxity, the
patient is able to control one of them using good mechanics. This
demonstration helps set the foundation for our discussion of the need
to regain stabilizing neuromuscular control of the symptomatic
shoulder.
Finally, we examine the strength of abduction and rotation to gauge
the power of the muscles contributing to stability through concavity
compression. We also examine the strength of the scapular protractors
and elevators which are necessary to position the scapula securely. Radiographs In atraumatic instability shoulder radiographs characteristically
show no bony pathology. Because these patients characteristically
demonstrate midrange instability, radiographs may show translation of
the humeral head with respect to the glenoid. The axillary view may
show posterior subluxation. Occasionally, radiographs may suggest
factors underlying the atraumatic instability such as a relatively
small or hypoplastic glenoid or a posteriorly inclined or otherwise
dysplastic glenoid. The bony glenoid fossa may appear quite flat;
however, it is difficult to relate the apparent depth of the bony
socket to the effective depth of the fossa formed by cartilage and
labrum covering the bone.
We do not use stress radiographs, arthrography, MRI, or arthroscopy in the diagnosis of atraumatic instability. Surgery is not always an option The goal of treatment for patients with atraumatic instability is the restoration of shoulder function.
Many patients with the AMBRII syndrome have simply become
deconditioned from their normal state of dynamic glenohumeral
stability. They have lost the proper neuromuscular control of
humeroscapular positioning; concavity compression has become
dysfunctional.
Neuromuscular control cannot be restored surgically; rather, it
requires prolonged adherence to a well-constructed reconditioning
program. The patient may need to be convinced that training and exercises
constitute a reasonable therapeutic approach. Many would prefer a
surgical "cure." We have found it useful to demonstrate that often the
contralateral shoulder has substantial laxity on examination yet is
clinically stable. In this way we try to educate the patient and family
that a loose shoulder is not necessarily clinically unstable. We
emphasize that gymnasts usually have very lax, yet very stable
shoulders.
Non-operative management There are two aspects of the non-operative management of atraumatic instability:
- strengthening the compressor muscles, and
- training for humeroscapular balance.
First, it is essential to optimize the strength and endurance of the
muscles compressing the head of the humerus into the glenoid concavity.
Weakness or poor endurance of the rotator cuff muscles can usually be
managed by a regular exercise program. The second component of the
exercise program emphasizes regaining stability through neuromuscular
control of humeroscapular positions. If an major and protracted effort
with the exercise program is not successful in improving shoulder function, a surgical repair may be considered.
|