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.