Posterior Shoulder Instability

Overview

Classification: ABC System (Moroder & Scheibel)

• Rockwood and Green's Fractures in Adults 10th Ed 2025, p. 1268

Etiology and Mechanisms

Traumatic

• Direct blow to the anterior shoulder
• Fall on a forward-flexed, adducted, internally rotated extremity
• High-energy events associated with altered consciousness (seizures, electrical shock, intoxication) - classically associated with acute locked posterior dislocation

Microtraumatic / Atraumatic

• Repetitive posterior capsule loading from:
  • Pitching, tennis, swimming (backstroke/breaststroke)
  • Weight lifting - especially bench press
  • Offensive linemen blocking
  • Baseball batters (check swing or pulling an outside pitch - alters mechanics and increases posterior shear forces)

Voluntary Instability

• Patients with microtraumatic or atraumatic origin may learn to sublux with horizontal adduction + internal rotation
• Patients who sublux using muscular contraction alone (bland affect) may have psychologic overlay and secondary gain
• These patients should rarely, if ever, be surgically treated
• Campbell's Operative Orthopaedics 15th Ed 2026, p. 2908

Pathoanatomy and Bony Lesions

Reverse Hill-Sachs Lesion

• Impaction fracture of the anterosuperior humeral head (the "trough sign" on AP radiograph)
• Created as the posterior glenoid rim engages the front of the humeral head during dislocation
• When >20-25% of articular surface involved: McLaughlin procedure (subscapularis transfer) or Neer modification (lesser tuberosity transfer)
• When >35-40% articular loss: hemiarthroplasty or TSA

Reverse Bankart Lesion

• Posterior labrocapsular avulsion from the posterior glenoid rim
• Most common structural lesion in traumatic recurrent posterior instability

Kim Lesion

• Incomplete and concealed avulsion of the posteroinferior labrum (Type II posteroinferior labral lesion in Kim's classification)
• Associated with posterior and multidirectional instability
• May be subtle or falsely negative on MR arthrography

Glenoid Retroversion

• Contributes significantly only in severe congenital dysplasia or traumatic disruption
• Glenoid osteotomy indicated when retroversion exceeds 10 degrees (Fuchs, Jost, Gerber)
• High complication rates have led to these falling out of favor

Clinical Diagnosis

History

• Pain and apprehension with forward flexion + adduction + internal rotation
• Athletes in overhead sports with posterior-dominant pain
• Clicking or subluxation during specific activities

Key Physical Examination Tests

Imaging

• AP radiograph: Lightbulb sign (humeral head appears rounded/symmetric on AP due to internal rotation in locked dislocation); trough sign
• Axillary lateral: Best view for confirming posterior dislocation - diagnosis is easy with axillary view but missed in ~50% of cases on AP alone
• MRI / MR arthrography: T2-weighted axial with contrast - evaluates capsular deficiency, chondrolabral containment, Kim lesion (may be subtle), rotator interval, and glenoid version
• Kim et al. and Antoniou & Harryman found chondrolabral cavity lesions in >80% of patients with posterior instability

Treatment

Nonoperative (First Line)

• Avoid provocative activities and voluntary subluxation maneuvers
• Strengthening of external rotators and posterior deltoid
• Restore normal range of motion
• In overhead athletes: coaching correction of mechanics
• Most patients with microtraumatic/atraumatic origin respond well, especially those with generalized ligamentous laxity

Surgical Indications

1. Appropriate rehabilitation has failed
2. Habitual/voluntary dislocation has been ruled out
3. Patient is emotionally stable
4. Pain and instability preclude adequate function
5. Structural lesion confirmed (positive jerk test, labral pathology, capsular deficiency on MRI)

Surgical Options

Postoperative Protocol (Open Posterior Capsular Shift)

• Arm immobilized in neutral rotation, slight abduction x 6 weeks
• ROM with elevation in scapular plane + external rotation begun at 6 weeks
• Avoid elevation >150° and internal rotation exercises x 3 months
• Sports (swimming, throwing): 9 months to 1 year

Causes of Surgical Failure

• Inadequate capsular shift / under-tensioned repair
• Failure to identify and address all labral lesions (especially Kim lesion)
• Unrecognized significant glenoid bone loss (>11-15% increases failure risk)
• Voluntary instability / psychologic overlay not identified preoperatively
• Female sex, reduced glenoid bone width - identified as independent risk factors for failure after arthroscopic posterior capsulolabral repair

Recent Evidence (2024-2026)

• A 2024 systematic review (Ralph et al., PMID 38825224) confirmed generally good outcomes for arthroscopic stabilization of posterior instability
• A 2025 systematic review (Afetse et al., PMID 40738256) found that female sex, reduced glenoid bone width, and glenoid bone loss >11-15% increase failure risk after arthroscopic posterior capsulolabral repair - important for pre-surgical risk stratification
• A 2026 systematic review (Wollenman et al., PMID 40426285) noted significant variability in return-to-play criteria after posterior shoulder soft tissue stabilization - lack of standardized criteria remains a gap