Instability and dislocation after reverse total shoulder arthroplasty - are we any smarter about preventing them?

Instability is one of the most common complications after a reverse total shoulder.

The 56 authors of Predictors of Dislocations after Reverse Shoulder Arthroplasty: A study by the ASES Complications of RSA Multicenter Research Group identified 6,621 patients from a multicenter database with minimum 3 month (mean 19.4, range 3-84) followup after a primary or revision reverse total shoulder (RSA) performed by one of 24 experienced surgeons. The study population was 40% male with an average age of 71.0 years. The rate of dislocation was 2.1% (n=138) for the whole cohort, 1.6% (n=99) for primary RSAs, and 6.5% for revision RSAs.

Dislocations occurred at a median of 7.0 weeks after surgery; 23.0% followed a trauma.

The risk factors for dislocation identified in this study were non-modifiable:
(1) diagnosis other than glenohumeral osteoarthritis with an intact rotator cuff (e.g. fracture non-union, rotator cuff disease, failed prior arthroplasty).
(2) history of postoperative subluxations prior to radiographically confirmed dislocation,
(3) male sex,
(4) trauma
and
(5) no subscapularis repair.


Comment:
These authors did not identify modifiable risk factors for dislocation, such as implant type, implant size, implant position, distalization, lateralization, unwanted contact between the humeral component and scapula, or rehabilitation.

By contrast, the authors of Dislocation following reverse total shoulder arthroplasty found two modifiable risk factors - inadequate soft-tissue tensioning and bony impingement (especially in adduction) -  among 14 early (less than three months after surgery) and 5 late (more than 3 months after surgery) dislocations. Non-modifiable risk factors included male sex and prior surgery on the shoulder. Other findings associated with dislocation included asymmetric liner wear and mechanical liner failure; these factors may be modified by prosthesis design and surgical technique.

The authors of Classification of instability after reverse shoulder arthroplasty guides surgical management and outcomes and Revision for instability following reverse total shoulder arthroplasty: outcomes and risk factors for failure identified four categories of factors that contributed to instability in 36 patients having revision of a reverse total shoulder for glenohumeral instability. Many of these are related to implant design and surgical technique and are, therefore, modifiable. The most common mechanism leading to persistent instability was loss of compression.

 (D/R ratio is the ratio of the depth of the polyethylene cup divided by the radius of the cup's concavity).

As emphasized in The normal shoulder, aTSA, and RSA are stabilized by concavity compression and in the articles referenced above, stability of the reverse requires a compressive force aligned with a competent concavity = concavity compression.

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).