Special Feature
Atypical Shoulder Dislocations: Posterior and Inferior Glenohumeral Dislocations
By William J. Brady, MD
The vast majority of acute shoulder dislocations are anterior (85-90%); they usually are diagnosed correctly and managed in the emergency department (ED). Acute traumatic posterior dislocation of the shoulder, despite its infrequent occurrence (2-5% of all shoulder dislocations), is the most commonly missed joint dislocation in the body. In fact, more than 50% of posterior dislocations are diagnosed incorrectly and/or poorly managed during the initial encounter—often by the emergency physician.1,2 Reflecting the magnitude of this initial mismanagement, the orthopedic literature is replete with discussions of the treatment of chronic, unreduced posterior dislocations. The diagnosis often is missed simply because the examining physician does not have a sufficient index of suspicion to look for the classic and frequently present physical findings. These injuries should be suspected in all patients with shoulder injuries and all patients who have suffered a seizure or electric shock.3
Posterior Shoulder Dislocation
Certain mechanisms of injury are highly suggestive of posterior glenohumeral dislocation; they generally occur with the arm flexed forward and slightly internally rotated. The dislocation can occur when an axial load is applied while the arm is in this position. Striking a heavy punching bag or impacting the dashboard with the arm extended to the front are examples of mechanisms linked to this type of dislocation. Posterior dislocation more commonly is the result of indirect forces producing a combination of internal rotation, adduction, and flexion. It most commonly is associated with seizures, alcohol withdrawal, or electrical shock. In these patients, the diagnosis should be entertained and actively ruled out even if the shoulder is not a presenting complaint.
Certain classic physical findings of posterior dislocation may be found on physical examination. The patient will be in tremendous pain; these injuries tend to be more painful than anterior dislocations. The patient usually presents with the arm held tightly across the front of the trunk. The arm is fixed in a position of adduction and internal rotation. External rotation, active or passive, is blocked. Abduction is severely limited. The posterior aspect of the shoulder is rounded and more pronounced than the normal shoulder. The anterior aspect of the shoulder is flattened and the coracoid process is prominent compared to the uninjured side. These contour changes in the shoulder can be best viewed with the patient sitting on a low stool, with his/her back to the examiner and the gown draped beneath the shoulders in the axillae. This allows simultaneous visualization of the front and back of the shoulders bilaterally for comparison. A more subtle finding, which always is present, is the lack of wrist supination on the affected side when the arms are forward flexed.
Radiography. Appropriate radiographs are obtained to confirm the diagnosis and to note associated fractures; the initial diagnosis should be made based on physical examination of the patient. A routine anteroposterior (AP) shoulder film usually is sufficient to diagnose the various types of anterior dislocations. However, it is not adequate to diagnose posterior glenohumeral dislocation. This type of dislocation may look deceptively normal on the AP radiograph. (See figures 1A [normal] and 1B [dislocation].)
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The point should be made that no AP film made in the plane of the chest is diagnostic of posterior dislocation, although such films may be suggestive as is seen in Figure 1B. With that caveat in mind, however, certain radiographic signs seen on the routine AP view may suggest the diagnosis:
Absence of the normal elliptical shadow. On the routine AP view, there usually is an overlap shadow, created by the head of the humerus superimposed on the glenoid fossa. The shadow is a smooth bordered ellipse (see figure 1A). In posterior dislocations, the articular surface of the humeral head is posterior to the glenoid and the elliptical overlap shadow is distorted (see figure 1B).
Vacant glenoid sign. In the normal shoulder, the humeral head occupies the majority of the glenoid cavity. In posterior dislocations, the head rests behind the glenoid; thus, the glenoid fossa appears to be partially vacant. This finding also has been referred to as the "positive rim" sign. If the space between the anterior rim and the humeral head is greater than 6 mm, a posterior dislocation is likely (see figure 1B).
The "trough" line. If a "trough" line is noted on the AP radiograph, it is the result of the impaction fracture of the humeral head caused by the posterior rim of the glenoid; it is analogous to the Hill-Sachs impaction fracture seen in anterior dislocations. Radiographically, two parallel lines of cortical bone are visible on the medial cortex of the humeral head (see figure 1C). One line represents the medial cortex of the humeral head; the other line represents the margin of an impaction fracture.
"Hollowed out" or "cystic" humeral head. In posterior dislocation, the arm is locked in internal rotation. The x-ray beam, therefore, passes through both the greater and lesser tuberosities, creating the image of a hollow or cystic humeral head (see Figure 1D).1,2,4 The scapular lateral (Y view) radiograph is perhaps the most clinically useful and patient-friendly of the shoulder films used to diagnose posterior dislocation—yet it is used infrequently. The axillary view is the most commonly ordered lateral view and is the easiest to interpret.
Management. The use of intravenous sedation and muscle relaxants, which almost always are successful when used for the reduction of anterior dislocations, may be insufficient for posterior dislocations. These patients may require general anesthesia if initial attempts at reduction are unsuccessful. The patient should be placed in the supine position and traction should be applied to the adducted arm in the line of deformity. While applying traction, the humeral head should be lifted gently back into the glenoid fossa. The arm should not be forced into external rotation. If the head is locked on the glenoid rim, forced rotation will fracture the head or shaft of the humerus. If the prereduction radiographs reveal the head to be locked on the glenoid rim, distal traction should be combined with lateral traction on the upper arm. This combined traction approach can be achieved using a folded towel to apply the lateral contribution. The type of postreduction immobilization is dependent on the stability of the shoulder after reduction. If the shoulder is stable, a sling and swathe or standard immobilizer is adequate. However, if the shoulder tends to sublux in the sling and swathe, a shoulder spica cast should be applied with the shoulder in external rotation.
Associated injuries. Fractures of the glenoid rim, humeral head, tuberosities, and upper humeral shaft commonly are associated with posterior shoulder dislocation. The incidence of coexistent fractures is approximately 50%.5 These fractures actually contribute to the frequency of misdiagnosis, as they lead physicians to believe they have identified a source of the pain and motion loss. Posterior dislocation should be considered in all proximal humerus or glenoid fractures; a lateral film must be obtained. Neurologic injuries are far more common with anterior dislocation, yet they still must be ruled out with posterior dislocation. Generally, these are injuries of neuropraxia that recover spontaneously. Fortunately, vascular injuries also are rare in posterior glenohumeral dislocations but must be considered. This consideration is particularly true in the elderly patient who has atherosclerotic vascular disease with loss of vessel elasticity. Often, the only physical finding in these cases is asymmetry of upper extremity pulses.
Inferior Shoulder Dislocation (Luxatio Erecta)
Inferior shoulder dislocation, also known as luxatio erecta, is a rare form of shoulder dislocation.6 The incidence of luxatio erecta is reported to be 0.5% of all shoulder dislocations. Luxatio erecta can occur at any age. The injury most often is unilateral, although simultaneous bilateral luxatio erecta has been reported. The mechanism of injury involves hyperabduction of the arm at the shoulder with extension at the elbow. The forearm is pronated. Direct or violent force is applied to the shoulder from a superior direction, causing inferior movement of the humeral head relative to the glenoid fossa. The inferior portion of the glenohumeral capsule then is disrupted (see figure 2A), and glenohumeral dislocation occurs. Alternatively, leverage of the humeral head across the acromion by a hyperabduction force also can result in inferior dislocation of the humeral head.
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The presentation of inferior dislocation of the shoulder is unique. The patient usually is in distress, with the involved arm hyperabducted at the shoulder and flexed at the elbow. The forearm frequently rests behind the head. Patient or physician attempts at adduction are extremely painful. The glenoid fossa is empty and the humeral head is palpitated in the axilla adjacent to the lateral chest wall. Skin creases are noted on the superior aspect of the shoulder, indicating the acute angle formed by the acromion and humerus. Neurovascular compromise may be found.
Radiography. Radiographic examination in cases of suspected luxatio erecta includes the following views: AP views in both internal and external rotation and lateral views, including the "Y" or axillary approaches. The AP view often will demonstrate an inferior displacement of the humeral head relative to the glenoid fossa (see figure 2B); furthermore, the arm is hyperabducted at the shoulder. The lateral views, particularly the axillary view, closely define the relationship of the humeral head and glenoid fossa and consequently are of significant use in the difficult case.
Management. Treatment in the ED most frequently involves closed reduction, with adequate muscle relaxation and anesthesia. In-line traction is applied to the fully abducted arm while firm, cephalad pressure is maintained on the humeral head.7 Counter-traction is used with a rolled sheet placed superior to the shoulder. When the humeral head is reduced into the glenoid fossa, the arm is adducted toward the body and the forearm supinated. Post-reduction, the shoulder is immobilized in the typical fashion (either a sling and swathe or shoulder immobilization) and an outpatient orthopedic referral is made.
Associated injuries. A variety of injuries related to inferior dislocation of the shoulder have been described: disruptions of various shoulder muscles (supraspinatus, infraspinatus, subscapularis, and pectoralis major); fractures of the clavicle, coracoid, acromion, inferior glenoid, and greater tuberosity of the humerus; and neurovascular compromise involving the brachial plexus and the axillary artery. Concomitant fracture or rotator cuff injury is reported in 80% of cases. Additionally, 60% of these patients manifest neurologic injury on presentation, most commonly to the axillary nerve. The neurologic deficits usually resolve rapidly. A small percentage of cases are complicated by vascular injury. Fracture of the greater tuberosity of the humerus reportedly spares injury to the rotator cuff.8 The prognosis for normal shoulder function in complicated cases usually is favorable. Recurrent inferior dislocation of the shoulder is very unusual. Particularly, violent forces have resulted in open fractures complicating the inferior dislocation.
References
1. Paton DF. Posterior dislocation of the shoulder. A diagnostic pitfall for physicians. Practitioner 1979;223: 111-112.
2. Bloom MH, et al. Diagnosis of posterior dislocation of the shoulder with use of Velpeau axillary and angle-up roentgenographic views. J Bone Joint Surg Am 1967; 49:943-949.
3. Ahlgren O, et al. Posterior dislocation of the shoulder associated with general seizures. Acta Orthop Scand 1981;52:694.
4. Cisternino SJ, et al. The trough line: A radiographic sign of posterior shoulder dislocation. AJR Am J Roentgenol 1978;130:951-954.
5. Messner DG. Posterior dislocation of the shoulder: With or without associated fracture. J Bone Joint Surg 1966;48A:1220.
6. Brady WJ, et al. Bilateral inferior glenohumeral dislocation: Luxatio erecta, an unusual presentation of a rare disorder. J Emerg Med 1995;13:37-42.
7. Murrard J. Un cas de luxatio erecta de l’épaule, double et symétrique. Revue D’Orthopédie 1920;7:423.
8. Neer CS, Rockwood CA. Fractures and dislocations of the shoulder. In: Rockwood CA, Green DP, eds. Fractures in Adults. Philadelphia: Lippincott; 1984:819-860.
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