The main cause of instability after unconstrained shoulder prosthesis is soft tissue deficiency.

BACKGROUND:
Instability is one of the major causes of failures in unconstrained anatomic total shoulder arthroplasty (TSA). This study reviewed the instabilities that may occur in an anatomic shoulder platform system to identify its potential predictors. We hypothesized that soft tissue deficiency was the main cause of instability and that the best treatment option would be conversion to a reverse shoulder arthroplasty (RSA).

MATERIALS AND METHODS:
Between 2003 and 2013, we reviewed 27 patients who experienced postoperative instability, and the overall incidence was 5.07%. There were 8 hemiarthroplasties (HAs), 14 TSAs with metal-backed glenoid components, and 5 TSAs with cemented glenoid components.

RESULTS:
We reported 10 isolated subscapularis tears, 6 massive rotator cuff tears, 8 component malpositions, 2 component dissociations or loosening, and 1 humeral shortening. These dislocations occurred early, within the first 6 months postoperatively, in 20 patients and later in 7. Specific procedures were performed in 8 patients, 17 were converted successfully to a RSA, and no surgery was done in 2 patients. At the last follow-up (mean, 36.96 months) Constant scores, Subjective Shoulder Value, and Simple Shoulder Test scores improved significantly to 49.9, 56.4%, and 6.9 of 12, respectively (P <  .05). None of the 25 patients who were revised were categorized as failures. Patients who underwent conversion had a better outcome than those who had other specific procedures (P = .001).

CONCLUSION:
The major cause of instability in our series was soft tissue deficiency. Most of the patients required conversion, and the platform system we used made conversions easier.