Why Anterior Shoulder Dislocations Are More Common Than Posterior Dislocations
Anterior shoulder dislocations account for the vast majority of glenohumeral dislocations because the shoulder joint's anatomy provides significantly less bony and soft-tissue support anteriorly, making it inherently vulnerable to anterior displacement during the common mechanism of arm abduction with external rotation. 1, 2
Anatomical Vulnerability
The glenohumeral joint is the most commonly dislocated joint in the body due to its inherent structural design that prioritizes mobility over stability 1, 3:
- The humeral head is relatively large compared to the shallow glenoid cavity, creating minimal bony constraint and allowing remarkable range of motion at the expense of stability 3
- The anterior capsule and ligamentous structures are weaker than posterior stabilizers, particularly when the arm is positioned in abduction and external rotation 1
- The rotator cuff provides less muscular support anteriorly, with the subscapularis being the only anterior rotator cuff muscle compared to multiple posterior stabilizers 4
Common Mechanism of Injury
Anterior dislocations occur through a predictable and frequently encountered mechanism 1, 2:
- Forced abduction with external rotation is the classic position that levers the humeral head out anteriorly, a position commonly encountered in falls, sports injuries, and direct trauma 1, 5
- This mechanism is far more common in daily activities and sports than the mechanisms causing posterior dislocation 2
Rarity of Posterior Dislocations
Posterior dislocations are uncommon and require specific circumstances 3, 5, 6:
- Posterior dislocations typically require violent muscle contractions from seizures, electrocution, or electroconvulsive therapy rather than simple trauma 3, 5
- Bilateral shoulder dislocations, when they occur, are almost always posterior due to these violent symmetric muscle contractions 3, 5
- Posterior dislocations are frequently missed on initial evaluation (over 60% misdiagnosed) because they can appear normal on standard AP radiographs alone, requiring axillary or scapula-Y views for proper diagnosis 7, 8, 6
Clinical Implications
Understanding this anatomical predisposition has important diagnostic consequences 7, 8:
- Always obtain axillary or scapula-Y views in addition to AP views, as glenohumeral dislocations can be misclassified on AP views alone 7, 8
- Anterior dislocations present with the arm held in abduction and external rotation with loss of the normal shoulder contour 5
- Associated injuries differ by direction: anterior dislocations typically cause Hill-Sachs deformities (posterolateral humeral head compression fractures) and bony Bankart lesions (anterior glenoid rim fractures), while posterior dislocations have different fracture patterns 7, 8
Age and Recurrence Patterns
The predominance of anterior dislocations creates specific clinical patterns 1, 2:
- Younger patients (<35 years) with anterior dislocations have higher recurrence rates due to labroligamentous injury and persistent instability 8
- Older patients (>60 years) are more likely to have associated rotator cuff tears with anterior dislocations 8
- Up to one-third of patients may develop long-term shoulder arthritis even after a single anterior dislocation episode 2