After reverse total shoulder arthroplasty, what is the most common orientation of dislocation (anterior versus posterior)?

Anterior Dislocation is the Most Common Orientation After Reverse Total Shoulder Arthroplasty

After reverse total shoulder arthroplasty, anterior dislocation is the predominant orientation when instability occurs.

Understanding the Mechanism

The unique biomechanics of reverse shoulder arthroplasty create specific vulnerability patterns:

• The reversed ball-and-socket design (glenosphere on glenoid, cup on humerus) fundamentally alters the center of rotation, moving it medially and distally 1
• This configuration relies heavily on deltoid muscle stabilization rather than rotator cuff integrity 1
• The anterior soft tissue envelope, particularly the subscapularis, becomes critical for preventing anterior translation

Evidence for Anterior Predominance

While the ACR guidelines identify dislocation as one of the most common complications of reverse total shoulder arthroplasty 1, biomechanical studies directly demonstrate anterior instability patterns:

Cadaveric biomechanical testing showed that:

• External rotation was the most stable position
• Detachment of the subscapularis significantly increased anterior instability
• Combined rotator cuff deficiency further decreased anterior stability 2

Clinical series confirm this pattern:

• Early dislocations (within 3 months) occurred at a mean of 3.4 weeks postoperatively 3
• Subscapularis deficiency was present in 64% of early dislocators 3
• Anterior dislocations specifically occurred in documented cases requiring revision 4

Key Risk Factors for Anterior Dislocation

The highest-quality multicenter study identified specific predictors 5:

• History of postoperative subluxations (OR: 19.52) - the strongest predictor
• Fracture nonunion as primary diagnosis (OR: 6.53)
• Revision arthroplasty (OR: 5.61)
• Male sex (OR: 2.21)
• No subscapularis repair at surgery (OR: 1.95)

Clinical Recognition

Anterior dislocations typically present with:

• Loss of articulation between humeral component and glenosphere on radiographs
• Median occurrence at 7 weeks postoperatively (though range is 3-36 weeks) 5
• 23% occur after trauma, but 77% are atraumatic 5
• Activity-related pain, decreased range of motion, and apprehension 1

Important Caveats

Two distinct etiologic patterns exist:

1. Early instability (inadequate soft-tissue tensioning and/or axillary nerve palsy) - predominantly anterior
2. Late instability (impingement or liner failure) - 80% had adduction impingement via heterotopic ossification or asymmetric polyethylene wear 6

The direction remains predominantly anterior even in late dislocations, though the mechanism shifts from soft tissue failure to mechanical impingement.

Posterior dislocations are distinctly uncommon in the literature reviewed, with no specific studies documenting posterior as the primary direction of instability after reverse total shoulder arthroplasty.