The shoulder girdle is a complex articulation connecting the upper extremity to the axial skeleton. The glenohumeral joint of the shoulder is the most commonly dislocated joint in the body and accounts for approximately 50% of all major dislocations seen in the emergency department. Posterior shoulder dislocations account for about 2% to 5% of all shoulder dislocations. Recurrent posterior dislocations occur in 30% of patients and predispose the joint to degenerative changes. Dislocations in a posterior direction can be difficult to diagnose, so a high index of suspicion should be maintained when the typical history and findings on the physical exam are present.
This shoulder girdle contains the glenohumeral joint which is a ball-and-socket joint. The humeral head and the glenoid fossa of the scapula form an articulation that is extremely shallow; the glenoid is approximately 1/3 the size of the humeral head. The shoulder is the most mobile joint in the body allowing for the largest amount of range of motion. The shallowness of the joint, while allowing for great mobility, also contributes to the joints' instability. The cartilaginous labrum that runs around the rim of the glenoid helps to deepen the socket and improve stability along with the joint capsule, ligaments and muscular attachments. The nerves of the brachial plexus and subclavian vessels run along the anterior shoulder girdle between the first rib and clavicle and can be prone to injury, but this is much less common in posterior dislocations. Dislocations can occur with any injury to the shoulder, whether it be traumatic, blunt, or a twisting type of injury. The most common mechanism is a forceful adduction with internal rotation, but a direct, blunt blow to the anterior shoulder may also result in dislocation. Any unexplained nocturnal posterior dislocation should prompt one to consider a seizure.
Shoulder girdle pathology and dislocation can affect all adult patients regardless of age or sex. Elderly patients tend to have more associated soft tissue injury and instability post-dislocation, so care should be taken to give proper referral and follow-up.
The classic mechanisms of posterior dislocation involve tonic-clonic seizures, electrical shock, or anterior-directed shoulder trauma (such as a grabbing the dashboard in a motor vehicle collision or falling on an outstretched hand). These injuries create forceful internal rotation, adduction, and flexion of the shoulder. Posterior dislocations can be subclassified into three anatomic types based on the final resting position of the humeral head: (1) subacromial, the most common, (2) subglenoid, and (3) subspinous. Posterior dislocations usually result in the humeral head being posterior to the glenoid and inferior to the acromion.
A pertinent history must be acquired when dealing with shoulder complaints. This includes age, arm dominance, location and intensity of pain, when the injury occurred, aggravating and alleviating factors, radiation of pain, and the mechanism of the injury. Patients should be fully undressed to expose both shoulders and allow for a complete inspection. Assessment of the range of motion should be performed to evaluate the limits of internal and external rotation along with flexion and extension of the shoulder. Up to 50% of posterior dislocations are missed on the initial evaluation. On exam, assess for any associated neurovascular injuries. Inspection and palpation of the anterior, lateral, and posterior shoulder may show the posterior prominence of the shoulder with the loss of the normal anterior contour along with a prominent coracoid and acromion. The arm is usually adducted with slight internal rotation. Patients will not allow external rotation or abduction secondary to severe pain.
Pre- and post-reduction radiographs are standard when evaluating a shoulder dislocation. When there has been significant trauma, dislocation and fracture-dislocation can have similar findings on physical exam. Postreduction radiographs are necessary to confirm reduction and rule out any injury or fracture during the reduction procedure. In very select cases of atraumatic chronic dislocation, radiographs may be omitted. A three view shoulder series is standard when evaluating a painful shoulder. Anterior-posterior (AP) and scapular "Y" view radiographs should be obtained to assess for posterior shoulder dislocations; however, approximately 50% are initially missed on standard AP views. Therefore, the "Y" view can confirm the diagnosis with the humeral head displaced posterior to the glenoid which is at the center of the "Y" of the scapula. An axillary view may also be acquired to better assess the glenoid-to-humeral head relationship if the standard views do not suffice. CT scan can be considered when suspected posterior dislocations are not seen on radiographs. More recently bedside ultrasound has also successfully been used to diagnose dislocations with high accuracy, but fracture diagnosis is limited. On AP views, the normally visualized humerus is internally rotated, this causes the humeral head to appear symmetrical, and it has been compared to the shape of a light bulb or drumstick. A "rim sign" can occur when the space between the articular surface of the humeral head and the anterior glenoid rim exceeds 6 mm; also occasionally seen is a "trough sign," which may appear as a dense line on the medial aspect of the humeral head that represents a compression fracture. Further, it is important to identify if there is a reverse Hill-Sachs deformity which may lead to chronic pain and ultimately to avascular necrosis. These fractures are common with posterior shoulder injuries and represent an impaction fracture of the anteromedial humeral head as it abuts against the glenoid rim during the dislocation injury.
Posterior dislocations commonly are associated with severe pain and muscle spasm; therefore, procedural sedation is frequently administered. Intra-articular injections of Lidocaine or Bupivacaine have also been used as an adjunct to pain management to aid in the reduction. Closed reduction is accomplished with in-line traction on the affected arm, which lies internally rotated and adducted. The traction along with gentle, anteriorly-directed manipulation of the humeral head, will help guide it into the glenoid sulcus. Counter-traction can be applied with a sheet around the affected axilla. Forceful external rotation should be avoided as this can put undue stress on the proximal humerus, leading to fracture. The successful reduction is evident when a "clunk" is felt as the joint is reduced, pain is relieved, normal anatomy returns, and range of motion allows the patient to place the palm of the injured arm on the opposite shoulder. If unsuccessful, orthopedic consultation is indicated for closed versus open reduction. Once reduction is accomplished, reassess the neurovascular status. Postreduction radiographs are useful for confirmation and documentation of successful reduction and diagnosis of any injuries occurring during the reduction procedure. Patients should have their shoulder immobilized with a sling or shoulder immobilizer in external rotation and slight abduction. Urgent orthopedic follow-up is necessary to ensure proper healing, early range of motion, and rehabilitation. In cases that are diagnosed late or are more chronic, orthopedic consultation is indicated because these patients often require semi-elective open reduction and internal fixation or arthroplasty. Early operative repair may decrease the incidence of recurrence as well.
Complications of any shoulder dislocation include fracture and neurovascular injury, but in contrast to anterior dislocations, neurovascular injuries are less common due to the anterior location of the neurovascular structures and the posterior location of the humeral head in this type of dislocation. Soft tissue injuries such as rotator cuff tears are also common, especially in the elderly. Reverse Hill-Sachs deformities, Bankart lesions, and glenoid rim fractures are associated with an increased incidence of recurrence and prolonged recovery. Proper referral to an orthopedist or specialist in shoulder injury should be ensured to avoid the long-term complications that can be associated with posterior shoulder dislocations.