What causes the shoulder to subluxate or dislocate?

Causes of Shoulder Subluxation and Dislocation

The shoulder is prone to subluxation or dislocation due to its inherently unstable ball-and-socket design, with the primary causes being anatomical factors (shallow glenoid, labral tears, bony defects), ligamentous laxity, and traumatic injury to the stabilizing structures. 1

Anatomical Factors Contributing to Shoulder Instability

Bony Architecture

• Shallow glenoid fossa: The glenoid provides minimal bony constraint to the humeral head
• Bony defects:
  • Hill-Sachs lesions: Compression fractures on the posterolateral humeral head from anterior dislocations
  • Bony Bankart lesions: Fractures of the anterior-inferior glenoid rim
  • These bony defects significantly increase risk of recurrent instability 2

Soft Tissue Factors

• Labral tears: Detachment of the fibrocartilaginous labrum that normally deepens the glenoid socket
• Capsular laxity: Excessive elasticity or stretching of the joint capsule
• Rotator interval defects: Open Foramen Weitbrecht (seen in 46% of recurrent instability cases) 3
• Rotator cuff deficiency: Compromises dynamic stabilization of the humeral head

Types of Instability

Traumatic Instability

• Results from significant injury causing damage to:
  • Labrum (Bankart lesion)
  • Capsular ligaments
  • Bony structures (glenoid rim fractures, Hill-Sachs lesions)
• Most common direction is anterior (approximately 95% of traumatic dislocations) 1

Atraumatic/Congenital Instability

• Often presents as multidirectional instability
• Associated with:
  • Inherent ligamentous laxity
  • Congenitally shallow glenoid
  • Abnormal collagen composition (higher type III collagen in adolescents) 4
• May develop from repetitive microtrauma rather than a single traumatic event 5

Biomechanical Factors

• In throwing athletes: Excessive external rotation forces during arm acceleration can reach up to 7000° per second, creating shear forces at the shoulder 4
• Distraction forces: During arm deceleration, forces equal to body weight can cause eccentric loading of the rotator cuff 4
• Kinetic chain disruption: Any compromise in the kinetic chain leads to overload of the glenohumeral joint 4

Age-Specific Considerations

Adolescent/Skeletally Immature Patients

• Epiphyseal plates don't fully ossify until late teens:
  • Proximal humerus: 17-18 years
  • Glenoid: 16-18 years
  • Clavicle: 18-20 years 4
• Epiphyseal plates are weaker than surrounding joint capsule/ligaments
• Higher proportion of type III collagen makes ligaments more elastic 4

Diagnostic Considerations

The American College of Radiology recommends:

• Standard radiographs (anteroposterior views in internal/external rotation, axillary or scapula-Y view)
• MRI (preferred for soft tissue injuries) or CT (for bone loss assessment) for further evaluation
• MR arthrography as the gold standard for labral tears (86-100% sensitivity) 1, 4

Common Pitfalls in Management

• Failure to recognize bony defects (a major cause of recurrent instability after repair)
• Inappropriate surgical technique selection (25% of revision cases) 3
• Inadequate capsular shift with worn labral tissue (40% of revision cases) 3
• Overlooking multidirectional instability when only addressing one plane of instability
• Assuming all instability is traumatic rather than considering congenital factors

Remember that successful management requires identifying all contributing factors to instability, as multiple pathological changes often coexist in the unstable shoulder joint.