Instability After Reverse Total Shoulder Arthroplasty: Which Patients Dislocate?

Risk factors for dislocation after reverse total shoulder arthroplasty (RTSA) are not clearly defined. Prosthetic dislocation can result in poor patient satisfaction, worse functional outcomes, and return to the operating room.^1-3 As a result, identification of modifiable risk factors for complications represents an important research initiative for shoulder surgeons.

There is a paucity of literature devoted to the study of dislocation after RTSA. Chalmers and colleagues⁴ found a 2.9% (11/385) incidence of early dislocation within 3 months after index surgery—an improvement over the 15.8% reported for early instability over the period 2004–2006.⁵ As prosthesis design has improved and surgeons have become more comfortable with the RTSA prosthesis, surgical indications have expanded,^6,7 and dislocation rates appear to have decreased. Although the most common indication for RTSA continues to be cuff tear arthropathy (CTA),⁶ there has been increased use in rheumatoid arthritis^8-10; proximal humerus fractures, especially in cases of poor bone quality and unreliable fixation of tuberosities^11-13; and failed previous shoulder reconstruction.^14,15 As RTSA is performed more often, limiting the complications will become more important for both patient care and economics.

We conducted a study to analyze dislocation rates at our institution and to identify both modifiable and nonmodifiable risk factors for dislocation after RTSA. By identifying risk factors for dislocation, we will be able to implement additional perioperative clinical measures to reduce the incidence of dislocation.

Materials and Methods

This retrospective study of dislocation after RTSA was conducted at the Rothman Institute of Orthopedics and Methodist Hospital (Thomas Jefferson University Hospitals, Philadelphia, PA). After obtaining Institutional Review Board approval for the study, we searched our institution’s electronic database of shoulder arthroplasties to identify all RTSAs performed at our 2 large-volume urban institutions between September 27, 2010 and December 31, 2013. For the record search, International Classification of Diseases, Ninth Revision (ICD-9) codes were used (Table 1).

The medical records of each patient were used to identify independent variables that could be associated with dislocation rate. Demographic variables included sex, age, and race. Preoperative clinical data included body mass index (BMI), etiology of shoulder disease leading to RTSA, individual comorbidities, and Charlson Comorbidity Index (CCI)¹⁶ modified to be used with ICD-9 codes.¹⁷ In addition, prior shoulder surgery history and arthroplasty type (primary or revision) were determined. Postoperative considerations were time to dislocation, mechanism of dislocation, and intervention(s) needed for dislocation. Although the institutional database did not include operative variables such as prosthesis type and surgical approach, all 6 surgeons in this study were using a standard deltopectoral approach in beach-chair position with a Grammont style prosthesis for RTSA cases.

Descriptive statistics for RTSA patients and the dislocation subpopulation were compiled. Bivariate analysis was used to evaluate which of the previously described variables influenced dislocation rates. Last, multivariate logistic regression analysis was performed to evaluate which factors were independent predictors of dislocation. We included demographic variables (age, sex, ethnicity), clinical variables (BMI, individual comorbidities, CCI), and surgical variables (primary vs revision, diagnosis at time of surgery). All statistical analyses were performed with Excel 2013 (Microsoft) and SPSS Statistics Version 20.0 (SPSS Inc.).

Results

From the database, we identified 487 patients who underwent 510 RTSAs during the study period. These surgeries were performed by 6 shoulder and elbow fellowship–trained surgeons. Of the 510 RTSAs, 393 (77.1%) were primary cases, and 117 (22.9%) were revision cases.

Of the 510 shoulders that underwent RTSA, 15 (2.9%; 14 patients) dislocated. Of these 15 cases, 5 were primary (1.3% of all primary cases) and 10 were revision (8.5% of all revision cases). Mean time from index surgery to diagnosis of dislocation was 58.2 days (range, 0-319 days). One dislocation occurred immediately after surgery, 2 after falls, 4 from patient-identified low-energy mechanisms of injury, and 8 without known inciting events. Nine dislocations (60%) did not have a subscapularis repair (7 were irreparable, 2 underwent subscapularis peel without repair), and the other 6 were repaired primarily (Table 2).

Male patients accounted for 32.2% of the study population but 60.0% of the dislocations (P = .019) (Table 3).

Multivariate logistic regression analysis revealed revision arthroplasty (OR = 7.515; P = .042) and increased BMI (OR = 1.09; P = .047) to be independent risk factors for dislocation after RTSA. Analysis also found a diagnosis of primary CTA to be independently associated with lower risk of dislocation after RTSA (OR = 0.025; P = .008). Last, the previously described risk factor of male sex was found not to be a significant independent risk factor, though it did trend positively (OR = 3.011; P = .071).