Clinical History
A 47-year-old male amateur cyclist presented at the emergency department after a fall from a bike. He complained of pain in his left shoulder and restricted mobility. Clinical examination was limited because of antalgic posture of his right arm.
Imaging Findings
The setting and clinical presentation were very suggestive for a fracture or dislocation. Plain radiograph (basic AP view in endo- and exorotation) showed no fracture, but was strongly suggestive for posterior shoulder dislocation. Additional scapular Y view confirmed the diagnosis. CT (with reconstructions) was performed to exclude associated subtle fractures. A reversed Hill-Sachs lesion and bony Bankart lesion were both present on CT.
Subsequent a closed reduction under general anesthetics was performed.
Discussion
Posterior shoulder dislocation (PSD) is very uncommon, accounting for merely 2-4% of all shoulder dislocations. PSD is caused by a forceful blow on the anterior humeral head in internal rotation. The most frequent causes are direct trauma and convulsive seizures. A less frequent cause is spontaneous PSD in patients with predisposing lax joints.
Clinical examination is limited due to pain, swelling and antalgic posture. If external rotation of the humeral head is still possible, PSD is unlikely.
Patients presenting in the typical setting of trauma or convulsive seizures, should raise the suspicion of PSD. Y-view or axillary projections are diagnostic in most cases. Additional CT can exclude associated lesions. A study of Rouleau showed a percentage of 65% of associated injuries, most frequently a fracture, reverse Hill-Sachs lesion or cuff tear.
Cisternino et al. described five typical radiographic signs of PSD: (1) A fixed internal rotated humeral head on frontal X-rays, giving the humeral head a "light bulb” appearance. (2) The “rim sign” is an increased distance between medial border of the humeral head and the anterior glenoid rim, caused by a lateral displacement of the humeral head with respect to the posterior glenoid rim. This sign is not specific and can also occur in haemarthrosis. (3) The same applies to an absent “half moon” overlap of the humeral head and glenoid. (4) When PSD is associated with a reverse Hill-Sachs lesion, the humeral head is displaced medially, showing an abnormal overlap on tangential view of the glenoid. (5) The “through line” on frontal X-ray is made up by two parallel lines of cortical bone on the superomedial aspect of the humeral head. The medial line is the articular cortex of the humeral head and the lateral is the border of the impaction fracture.
Treatment depends on time interval between dislocation and diagnosis. If a humeral head defect is present, the size of the defect also influences treatment strategy. Closed reduction under general anesthesia is the treatment of choice for acute dislocations(< 3 weeks) with small humeral head defects (<20% of articular surface). If closed reduction is unsuccessful (or if there is a large humeral head defect), the following step is open or arthroscopic reduction. For intermediate/large size defects, one can opt for implantation of bone grafts or allografts, whether or not combined with a standard or modified McLaughlin procedure. Without surgery, recurrent posterior instability can occur, which can be fixed with a labrum repair or posterior stabilization procedure.
Differential Diagnosis List
Posterior shoulder dislocation
Fracture
Anterior shoulder dislocation
Final Diagnosis
Posterior shoulder dislocation
