Dislocation of the reverse total shoulder

When unwanted contact occurs between the scapula and humeral component, displacing forces can misalign the compressive force required for stability.

To start, I’d like to direct the reader to several classic articles on this topic: 

I’ll pick up the story from 2024 to the time of this writing (April 2025).

*Dislocation of the reverse total shoulder continues to be a major and prevalent issue for patients and surgeons

Mitigating the Risk of Instability After Reverse Shoulder Arthroplasty: A Critical Analysis Review of Patient and Surgical Factors  Instability and dislocation after reverse shoulder arthroplasty may occur in up to 31% of patients. Clinical risk factors for instability include younger age, male sex, increased body mass index, preoperative diagnosis of proximal humerus fracture or rotator cuff pathology, history of instability of the native shoulder or after surgery, and a medical history of Parkinson’s disease. In patients at a high risk of instability, surgeons should consider a more lateralized prosthesis (particularly in patients with an incompetent rotator cuff), repairing the subscapularis (particularly when using a medialized prosthesis), and upsizing the glenosphere (>40 mm in male and 38-40 mm in female patients). While potentially useful, less evidence exists for the use of a constrained liner.

Midterm outcomes of primary reverse shoulder arthroplasty: a systematic review of studies with minimum 5-year follow-up The rate of shoulder dislocation was 3.7% (0%-20.4%),

Instability after reverse shoulder arthroplasty: a retrospective review of thirty one cases The most frequent etiology for RSA instability was loss of compression, followed by impingement and loss containment.

Revision of reverse total shoulder arthroplasty: A scoping review of indications for revision, and revision outcomes, complications, and re-revisions 22% of the complications were dislocations or instability. 30% of the revisions were for dislocation or instability.

Intraoperative repair of functional subscapularis during RSA by deltopectoral approach could improve internal rotation but does not prevent anterior dislocation. In the functional repair group, three shoulders (1.2%) reported subjective instability and 1 (0.4%) dislocated.None occurred in in either the non-functional repair or non-repair groups. 

Reverse shoulder arthroplasty with a 155 degrees neck-shaft angle inlay implant design without reattachment of the subscapularis tendon results in satisfactory functional internal rotation and no instability: a cohort study. One out of 210 prostheses was revised for dislocation within the first month after primary surgery.

Impact of morbid obesity on postoperative outcomes in reverse total shoulder arthroplasty: A national inpatient sample analysis Morbid obesity (BMI >/=40 kg/m(2)) was associated with a periprosthetic dislocation rate of 2.60 % in comparison to 1.59 % in controls

Impact of accumulating risk factors on the incidence of dislocation after primary reverse total shoulder arthroplasty using a medial glenoid-lateral humerus onlay prosthesis. 1.4% of the patients experienced dislocation with a medialized glenoid-lateralized humerus onlay rTSA prosthesis. The greatest risk factors for dislocation were male sex, age <68 years at the time of surgery, patients with body mass index >30, patients who received glenospheres having a diameter >40 mm, and patients who received expanded or laterally offset glenospheres.

Low success rate of closed reductions when treating dislocations after reverse shoulder arthroplasty: a study by the ASES Complications of RSA Multicenter Research Group.  a closed reduction was initially attempted in the majority of patients, but only about one-third were successful and required no further intervention. Unsuccessful closed reductions were associated with higher patient BMI. Revision surgery for dislocations was complicated by a high rate of recurrent dislocations and rerevision surgery.

*The diameter, depth and orientation of the humeral cup affect stability of the reverse total shoulder. However, it must be remembered that the ability of the RSA to resist dislocation depends not only on the shape and orientation of the cup, but also on the direction and magnitude of the net force as shown by the red arrows in the first two diagrams at the start of this post.

From Grammont to a New 135 degrees Short-Stem Design: Two-Hand Lever Test and Early Superior-Lateral Dislocations Reveal Critical Role of Liner Stability Ratio and Stem Alignment

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Patients having receiving RSA had an 8% dislocation rate for standard liners and a 0% dislocation rate for retentive liners. The authors attribute this difference to the jump height for the 36 mm standard implant of 8.1 and a linear stability ratio (LSR) of 152%; whereas the 36 mm retentive liner had a jump height of 10.1 and linear stability ratio of 195 to 202%

For this design, the most stable liner type was the 36 retentive:

They also found that the mean effective neck-shaft angle was 133 degrees (127-144 degrees) for short stems and 135 degrees (129-143 degrees) for long stems. Long stems significantly reduced varus outliers

which may have an increased risk for instability.