Complications of Posterior Shoulder Dislocation
Posterior shoulder dislocations carry a 65% rate of associated injuries, with reverse Hill-Sachs lesions (86%), posterior labrocapsular tears (58%), rotator cuff tears (19%), and reverse Bankart lesions (31%) being the most common complications 1, 2.
Acute Osseous Complications
The most frequent bony injury is the reverse Hill-Sachs lesion (anteromedial humeral head impaction fracture), occurring in 86% of cases 2. This represents the humeral head impacting against the posterior glenoid rim during dislocation. Additionally:
- Reverse osseous Bankart lesions (posterior glenoid rim fractures) occur in 31% of patients 2
- Locked dislocation develops when the humeral head becomes incarcerated behind the glenoid, occurring in 40-90% of cases with impression fractures 3
- Proximal humerus fractures can range from simple to significantly comminuted four-part fractures 4
Soft Tissue Injuries
Full-thickness rotator cuff tears occur in approximately 19% of posterior dislocations, affecting the supraspinatus (most common), infraspinatus, and subscapularis tendons 2. A critical finding: when reverse Hill-Sachs lesions or fractures are absent, the risk of rotator cuff tears increases nearly fivefold (odds ratio 4.6) 1.
Posterior labrocapsular complex injuries affect 58% of patients, presenting as either posterior labral sleeve avulsions (48%) or reverse Bankart lesions (52%) 2. Biceps tendon abnormalities complicate 17% of cases 2.
Irreducible Dislocations
Certain injuries create mechanical blocks preventing closed reduction 4:
- Osteochondral humeral defects
- Soft tissue interposition
- Long head of biceps tendon incarceration
- Scar tissue formation under the coracoid and subscapularis tendon 3
Delayed Diagnosis Complications
The most devastating complication is missed or delayed diagnosis, which occurs frequently due to subtle clinical presentation and radiographic findings 5, 6. When diagnosis is delayed beyond 6 weeks (chronic locked dislocation):
- Avascular necrosis of the humeral head develops from compromised vascularity 4, 7
- Chronic instability and recurrent dislocations 5
- Degenerative osteoarthritis of the glenohumeral joint 5
- Severe functional deficits with permanent limitation of motion 6
Duration of dislocation is the single most important prognostic factor for clinical outcomes, showing strong to excellent correlations with all functional scores regardless of age, gender, or defect size 8. Longer dislocation duration correlates with increased necessity for revision surgery following both hemiarthroplasty and total shoulder arthroplasty 6.
Anatomic Predisposition
75% of patients with posterior dislocation demonstrate glenoid retroversion (mean scaphoglenoid angle of 4.5 degrees), and 92% show posterior humeral head translation relative to the glenoid fossa (mean -4.8 mm) 2. This anatomic configuration may predispose to injury or represent post-traumatic changes.
Common Pitfall
The diagnosis is frequently missed on initial presentation because symptoms mimic shoulder contusions or rotator cuff injuries 5. Maintain high clinical suspicion in any patient with shoulder trauma following seizures, electrical shock, or high-impact trauma who presents with limitations in forward flexion and abduction 8. Standard AP radiographs may appear deceptively normal; axillary or scapular Y views are essential, and CT imaging should be obtained when posterior dislocation is suspected 9.