Superior Baseplate Inclination Is Associated With Instability After Reverse Total Shoulder Arthroplasty
Authors: Tashjian Robert Z. , Martin Brook I. , Ricketts Cassandra A. , Henninger Heath B. , Granger Erin K. , Chalmers Peter N. 
Published: May 2018

Abstract

Background
Instability is the most common complication after reverse total shoulder arthroplasty (rTSA). In the native glenohumeral joint, in addition to full dislocations, more subtle forms of instability exist. However, the incidence of more subtle forms of instability, the factors associated with instability, and the effect of instability on validated outcome scores after rTSA remain poorly understood.

Questions/purposes
(1) After rTSA, what is the risk of instability, including more subtle forms of subjective instability? (2) What are the factors associated with instability? (3) Are more subtle forms of instability associated with lower American Shoulder and Elbow Surgeons (ASES) functional outcome scores than those patients without instability?

Methods
A total of 168 rTSAs were performed during the study period. Six patients had died at the time of study initiation. Thirty patients were excluded, nine because rTSA was performed for an acute proximal humeral fracture, one because a lateralized humeral component was used, 17 because a retaining liner was used, and three because a lateralized glenosphere was used. One hundred thirty-two patients met inclusion and exclusion criteria. Thirty-five patients were lost to followup. Thus, 97 patients with a minimum of 2 years followup were included in the final cohort (74% of included patients). Followup was 47 ± 22 months (mean ± SD). The cohort included 23 men and 74 women with an age of 70 ± 9 years who underwent 78 primary and 19 revision rTSAs. Primary and revision patients were combined for subsequent analyses. A postoperative questionnaire was used to assess instability symptoms. Although it has not been validated, it is simple and we believe has high face validity. Briefly, it scored instability as (1) none; (2) feelings of instability; (3) probable dislocation/subluxation–self-reduced; and (4) dislocation with surgical reduction or dislocation with closed reduction (such as in the emergency department or the doctor’s office). ASES scores were collected specifically for this study. The preoperative and postoperative β angle was measured to determine glenoid inclination. Larger β angles denote more superior inclination, whereas smaller β angles denote more inferior inclination. Thus, a positive change in β angle from preoperatively to postoperatively denotes a change into more superior inclination, whereas a negative change in β angle from preoperatively to postoperatively denotes a change into more inferior inclination. Associations between instability symptoms and patient, implant, and surgical factors were evaluated in a multivariate model that considered age, sex, body mass index, and whether it was a primary or a revision procedure.

Results
A total of 13 of 97 (13%) patients reported some instability (Grades 2-4); four of 97 patients (4%) had full dislocations with reduction (Grade 4), four of 97 patients (4%) reported subluxations (Grade 3), and five of 97 patients (5%) reported feelings of instability or apprehension (Grade 2). After controlling for potential confounding variables like age, sex, body mass index, and revision versus primary procedure, the only factors associated with instability were greater superior baseplate inclination (larger β angle; odds ratio [OR], 1.15 [95% confidence interval {CI}, 1.042-1.258]; p = 0.005) and a greater change into superior inclination from preoperative to postoperative (greater positive change in ß angle; OR, 1.08 [1.009-1.165]; p = 0.027). Patients with any instability (Grades 2-4) reported lower final ASES scores than did patients without instability (Grade 1) (61 ± 16 versus 72 ± 19 mean difference 11 [95% CI, 0-22]; p = 0.032).

Conclusions
When more subtle instability after rTSA is included, instability may occur in up to 13% of patients. Instability is associated with greater superior baseplate inclination and less inferior correction of the β angle and thus surgeons should consider inferiorly inclining the baseplate to avoid postoperative instability. Although our study only demonstrates an association and not causation, the authors hypothesize that superior baseplate inclination increases inferior impingement, which leads to instability. Instability negatively influences final ASES score.

Level of Evidence
Level III, therapeutic study.