A 53-year-old male presents to the emergency department with severe right shoulder pain after having a witnessed seizure while at home. The patient is unable to forward flex or abduct his right arm after the injury. Anteroposterior and scapula view x-rays of the right shoulder taken in the emergency department (see images) are read by the radiologist as normal.
Submit your diagnosis to see full explanation.
The patient has likely sustained a posterior shoulder dislocation, which can be confirmed on a Velpeau axillary x-ray.
Posterior shoulder dislocations are rarely seen and account for an estimated 3% of all shoulder dislocations. Posterior shoulder dislocations are commonly associated with a seizure that causes a sudden internal rotation, adduction, and axial loading of the humeral head. Bilateral shoulders are involved in an estimated 11% of posterior dislocations after a seizure. Because the humeral head internal rotators are twice as strong as the external rotators, a sudden forceful contraction can cause a dislocation. X-rays are often read as normal, and as many as 50% of these injuries that present in the emergency department are not identified.
Physical examination findings may include a prominent posterior shoulder and coracoid. The humeral head may be engaged on the posterior glenoid, causing an external rotation block. The arm is held in adduction and internal rotation for comfort, and attempts to abduct and externally rotate the arm will cause pain. Standard radiographs in the emergency department usually include an anteroposterior and a scapula outlet view. An axillary view is often not performed, as shoulder abduction is too painful for the patient to tolerate. However, obtaining an axillary view is critical to diagnosing shoulder dislocations. A Velpeau axillary or modified axillary can be performed in patients in a sling who cannot abduct their arm. During a Velpeau axillary, the patient leans back 30 degrees against a table with a cassette on top. The x-ray tube is directed down vertically through the shoulder and onto the cassette. A CT scan can diagnose a posterior dislocation and also helps determine the extent of the cortication on the anterior medial portion of the humeral head from the impacted posterior glenoid. This compression fracture that occurs after a posterior dislocation is called a reverse Hill-Sachs lesion. MRI is useful to evaluate the rotator cuff and other soft tissues.
Posterior dislocations occasionally reduce spontaneously, but most require closed reduction. Open reduction may be necessary if the posterior dislocation is chronic or is more than six weeks old or if the reverse Hill-Sachs lesion cannot not be disengaged. Closed reduction using traction and external rotation should not be performed if the reverse Hill-Sachs lesion is still engaged, as this may cause a fracture of the remaining humeral surface. After reduction, functional range of motion should be assessed. If the shoulder is stable through range of motion and the reverse Hill-Sachs lesion involves less than 25% of the humeral surface, then the patient can be treated nonoperatively. Nonoperative treatment involves sling immobilization in a neutral or externally rotated position for four to six weeks. Shoulder stability is reassessed, and physical therapy is initiated if the joint is stable. Patients who sustain a dislocation as a result of a seizure and who present with a large reverse Hill-Sachs lesion have a higher risk of developing recurrent instability.
If the shoulder remains unstable after closed reduction, then a stabilization procedure is recommended. Techniques involve transposition of the subscapularis tendon into the humeral defect with or without an osteomized lesser tuberosity, as well as disimpaction and bone grafting for lesions less than 25% of the humeral surface. Lesions up to 40% to 50% of humeral head involvement may require a structural allograft to fill the defect. Injury to the posterior capsule and fracture of the glenoid may cause residual instability after these stabilization procedures, and both should be addressed surgically as well.
Posterior dislocations with a large humeral defect greater than 50% of the articular surface are most commonly treated with hemiarthroplasty. Hemiarthroplasty is also indicated for chronic dislocations, avascular necrosis, humeral head collapse, and humeral head arthritis.
- Robinson CM, Aderinto J. Posterior shoulder dislocations and fracture-dislocations. J Bone Joint Surg Am. 2005;87(3); 639-650.
- Posterior shoulder dislocations. Available at: www.orthobullets.com. Accessed August 15, 2015.