Original Research

Instability After Reverse Total Shoulder Arthroplasty: Which Patients Dislocate?

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Given the increasing use of reverse total shoulder arthroplasty (RTSA), it is important to study the complications associated with this procedure. We conducted a study of the incidence, predisposing factors, and treatment of RTSA dislocations. Using our institutional database, we retrospectively searched for RTSAs performed between September 27, 2010 and December 31, 2013 and identified postoperative dislocations. Four hundred eighty-seven patients underwent 510 RTSAs (393 primary, 117 revision). Fourteen patients had 15 dislocations (5 in primary RTSAs, 10 in revision RTSAs). Mean time from surgery to diagnosis was 58.2 days (range, 0-319 days). One dislocation occurred immediately after surgery, 2 after falls, 4 from low-energy mechanisms of injury, and 8 without known inciting events. Logistic regression analysis revealed revision RTSA (odds ratio [OR] = 7.515; P = .042) and higher body mass index (BMI) (OR = 1.09; P = .047) to be independent risk factors. The diagnosis of primary cuff tear arthropathy (CTA) was independently associated with a lower rate of dislocation (OR = 0.025; P = .008); dislocation occurred in only 1 (0.35%) of 285 patients with CTA. All dislocations were treated in the operating room; no dislocation was successfully treated with simple closed reduction in the clinic. Although dislocation after RTSA is uncommon, the risk is higher for patients with higher BMI and for revision patients. Patients may benefit from lifestyle modifications, preoperative counseling, intraoperative considerations, and rehabilitation modifications. Patients who undergo RTSA for primary CTA can be reassured that the likelihood of dislocation is low.


 

References

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 arthritis8-10; proximal humerus fractures, especially in cases of poor bone quality and unreliable fixation of tuberosities11-13; 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).

Table 1.
These unique procedure codes are used by the hospital system for billing, but are not always specific to assigned procedures. Therefore, the individual operative reports identified were reviewed to identify the patients who actually underwent RTSA. From this database, all patients who underwent RTSA were selected. Using the subpopulation of patients who underwent RTSA, we searched individual medical records to identify patients who had a dislocation after RTSA. This information was cross-referenced with ICD-9 codes for shoulder dislocation (831.0, 831.01, 831.02, 831.03) to ensure that all patients were identified.

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).

Table 2.
In addition, 11 dislocations (73.3%) previously underwent open or arthroscopic shoulder surgery. All patients who had a dislocation after RTSA returned to the operating room at least once; no dislocation was successfully treated with closed reduction in the clinic. The 15 dislocations underwent 17 surgeries: 7 isolated polyethylene exchanges, 2 isolated closed reductions, 1 hematoma aspiration with closed reduction, 1 open reduction, 2 humeral component revisions with polyethylene exchange, 1 humeral augmentation with polyethylene exchange, 2 glenosphere exchanges with polyethylene exchange, and 1 polyethylene exchange with concurrent subscapularis repair.

Male patients accounted for 32.2% of the study population but 60.0% of the dislocations (P = .019) (Table 3).
Table 3.
In addition, mean BMI was 33.2 for patients with dislocation and 29.5 for patients without dislocation (P = .039) (Table 3). Revision arthroplasty was found to be a risk factor for dislocation in univariate analysis: 66.7% of the dislocations occurred after revision RTSA, and only 21.6% of nondislocated shoulders were revision cases (P < .001) (Table 4).
Table 4.
Patients who underwent RTSA for CTA had a very low incidence of dislocation (0.35%, 1/285), accounting for 6.7% of the dislocated group and 57.6% of the nondislocated group (P < .001) (Table 4). The 1 patient with a dislocation after primary RTSA for CTA had an indolent infection at time of surgery after dislocation.

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).

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