POSTERIOR DISLOCATION OF
GLENOHUMERAL JOINT
Posterior dislocations
account for approximately 5% of shoulder dislocations. In most cases, posterior
instability is traumatic. Like anterior dislocation, posterior dislocation can
also be atraumatic. An atraumatic cause is more common in posterior
instability. This type of atraumatic instability is often recurrent subluxation
or partial dislocation and is associated with generalized ligamentous laxity,
developmental glenoid hypoplasia resulting in posterior inferior glenoid bone
deficiency as shown in Plate 1-28,
or muscle imbalance often seen with scapula winging or abnormal scapula motion.
Atraumatic posterior instability subluxation is not usually associated with a
defect or injury to the posterior capsule or labrum. Complete dislocation of
the shoulder with the humeral head posterior to the glenoid rim is most
commonly associated with posterior ligament and posterior labrum tears as one
would see with anterior instability (e.g., a Bankart lesion). Traumatic
posterior dislocation can be a fixed deformity requiring physician-assisted
reduction. In these cases, a reverse Hill-Sachs lesion on the anterior aspect
of the humeral head can been seen and occurs by the same mechanism as the more
common posterior Hill-Sachs lesions. Unlike anterior dislocation, posterior
dislocation is more often missed on routine shoulder anteroposterior
radiographs, as seen in Plate
1-28. The posterior displacement of the humeral head is much more
easily seen on the transscapular lateral or axillary view. It is for this reason
that when evaluating a patient who has sustained a traumatic injury it is
essential that at least two if not all three of these radiographic views be
included.
Traumatic posterior dislocation is
more common in patients with major motor seizure disorders. Underde- velopment
of the glenoid (hypoplasia) occurs in patients with growth plate abnormalities
of the glenoid; the posterior and inferior portions of the glenoid are
underdeveloped, resulting in a hypoplastic glenoid.
Closed reduction of posterior dislocation
follows the same principles of closed reduction of anterior dislocation. Axial
longitudinal traction of the arm and muscle relaxation are important for a
gentle nontraumatic reduction. Direct pressure over the posterior aspect of the
humeral head can help reduce the dislocation, assuming that the treating
physician performs the first two parts of the procedure without initial
success.
