The modified reverse shoulder arthroplasty angle: the effect of both glenoid fossa size and implant size on the relevant inclination of the glenoid

Abstract

BackgroundAvoiding superior tilt of the glenoid baseplate is essential in reverse shoulder arthroplasty (RSA). The purpose of this study was to evaluate the relevance of a new measure, the modified RSA (mRSA) angle, which accounts for the size of the chosen baseplate in a given patient's glenoid fossa. MethodsWe simulated placement of 3 baseplate sizes in 20 shoulders. Measurements included the RSA angle, mRSA angle, β/total shoulder arthroplasty angle, glenoid height, and the percentage of the fossa occupied by the chosen implant. ResultsThe mean difference between RSA and mRSA angles was 4° (P < .001), ranging from −6° to 15°. Smaller baseplates in larger glenoid fossae had the highest mRSA angles and were thus at the most risk of implantation in superior inclination. The amount of the fossa occupied by the implant was strongly correlated with the difference between RSA and mRSA angles (R = 0.729, P < .001). DiscussionThe mRSA angle accounts for inclination of a chosen glenoid implant in a specific patient's glenoid fossa. Smaller baseplates in larger patients that occupy less of the glenoid fossa sit on the most inclined part of the glenoid and are at greatest risk of residual superior inclination of the final implant.