17.2.1 Patient-Specific Factors

The definitive cause of instability must be determined on a case-by-case basis. A number of patient factors may predispose patients to instability. In an era of cost-containment and minimizing complications, identification of patient factors that may lead to instability is increasingly important. Patient’s age remains a factor when counseling patients regarding RTSA. ¹ , ¹⁰ , ¹⁸ , ²⁴ , ²⁵ Though an area of controversy, several studies to date have demonstrated increased instability and subsequent revision rates in patients younger than 65 years, ²³ , ²⁶ while others report satisfactory early results. ²⁷

Male patients undergoing RTSA may be at a higher risk for dislocation. In one recent series, 82% of dislocations occurred in male patients. ¹⁰ A recent database study of a large Medicare population demonstrated that males were at twice the risk for early revision, which is similar to a Norwegian database study. ²³ , ²⁸ In this study, instability was the most common reason for need for early revision. ²³

Obesity has become a significant public health concern and is associated with many perioperative and postoperative complications most commonly noted in the hip and knee literature. ²⁹ , ³⁰ , ³¹ , ³² The exact nature of obesity as a risk factor for RTSA dislocation is unclear. In our experience, obesity may contribute to RTSA instability in several ways. Obesity may prevent the surgeon from accurately assessing the tension of the myofascial sleeve intraoperatively. Exposure and soft-tissue handling is more difficult in obese patients. Additionally, the amount of soft-tissue bulk created medial to the patient’s humerus may create a lever effect with adduction of the arm de-coaptating the humerus and glenoid leading to dislocation. Finally, metabolic syndrome can contribute to overall poor health, poor soft-tissue healing, and poor bony healing. ³³ Gupta et al demonstrated an increased rate of complication in patients undergoing RTSA with increased body mass index (BMI). ³⁴ Chalmers et al reported an average BMI of 32 for patients who experienced a dislocation after reverse TSA with 82% of these patients being obesity class I or greater by the World Health Organization standards. ¹⁰ Several other recent studies have demonstrated a relationship between increasing rate of dislocation and increasing degree of obesity (► Fig. 17.1). ²³ , ³⁵ , ³⁶ , ³⁷

17.2.2 Management of Subscapularis

Management of the subscapularis tendon remains controversial. It is our preference to repair the subscapularis and retain the posterior rotator cuff whenever possible, because we feel it may prevent instability. Absence or poor quality of the subscapularis has been proposed as a potential factor in the unstable RTSA. ¹⁰ , ³⁸ Though several authors advocate for not taking down the subscapularis during RTSA, this may lead to difficulty with component positioning due to decreased visualization. ¹⁷

In some challenging cases, extreme lateralization of the center of rotation may lead to inability to repair the subscapularis tendon. Other patients requiring an RTSA may have a chronic or retracted tear, which cannot be repaired. Revision cases with severe bone loss involving the lesser tuberosity may preclude repair of the subscapularis tendon also, thereby potentially increasing dislocation rate. Many series to date do not report subscapularis status, making comparison of outcomes with regard to instability difficult. Edwards et al prospectively evaluated 138 RTSAs in which only 62 subscapularis tendons were deemed repairable. ¹⁴ In their series, all seven patients with postoperative dislocations had irreparable subscapularis tendons. This evidence adds to a body of compelling preliminary data in favor of subscapularis repair whenever possible. Certain RTSA designs, which do not medialize the glenoid component, may not benefit from subscapularis repair.

17.2.3 Fractures and Instability

Complex proximal humerus fractures have emerged as a compelling indication for RTSA in elderly patients. ³⁹ Recent literature reports a variable dislocation rate after RTSA in the treatment of proximal humerus fractures. ⁴⁰ Postoperative acromial fractures have been reported to occur in 3 to 7% of patients and have been reported to lead to instability. ⁴¹ Careful tuberosity management and soft-tissue repair may decrease the higher complication rates associated with patients undergoing RTSA for fracture.

17.2.4 The Influence of Underlying Diagnosis

The rise in utilization of RTSA has paralleled the expanding indications (► Fig. 17.1). Wall et al evaluated outcomes after RTSA for patients for varied etiologies. Although substantial clinical and functional improvements are broadly observed, patients with primary rotator cuff tear arthropathy, primary osteoarthritis with a rotator cuff tear, and a massive rotator cuff tear with chronic pseudoparalysis had better outcomes on average than patients with posttraumatic arthritis and patients undergoing revision arthroplasty. ⁴² However, instability remained the most common complication regardless of etiology, occurring 15 times in 168 patients. ⁴²

17.2.5 Revision Reverse Total Shoulder Arthroplasty

Revision to RTSA or revision of RTSA has a significantly higher complication rate than primary RTSA ⁶ , ¹¹ , ¹² (► Fig. 17.2). In one series of 230 cases of RTSA, there were 37 cases of instability. ⁸ Instability occurred more commonly in the revision group (9.4%) compared with the primary arthroplasty group (4.1%), with 87.5% of these cases requiring revision to achieve stability. However, there appears to be little long-term effect on final outcome. ⁸ , ⁹ , ¹⁰ Streubel et al reported a higher incidence of reoperation within 30 and 90 days after revision arthroplasty, as compared with primary arthroplasty, with instability being the most frequent cause. ⁴³ These data suggest that when approaching revision shoulder arthroplasty with RTSA, one must be particularly vigilant to ensure stability.