Peter J. Millett, M.D., M.Sc., Philip C. Nolte, M.D., M.A., Jon W. Miles, M.Sc., Kira K. Tanghe, B.S., Alex W.Brady, M.Sc., Kaare S. Midtgaard, M.D., Joe D. Cooper, M.D., Lucca Lacheta, M.D., and Matthew T. Provencher, M.D.(CAPT)
A medialized center of rotation (COR) in reverse total shoulder arthroplasty (RTSA) comes with limitations such as scapular notching and reduced range of motion. To mitigate these effects, lateralization and inferiorization of the COR are performed, but may adversely affect deltoid muscle force. The study’s purposes were to measure the effect of RTSA with varying glenosphere configurations on 1) the COR and 2) deltoid force compared to intact shoulders and shoulders with massive posterosuperior rotator cuff tears (PS-RCT). We hypothesized that the highest deltoid forces would occur in shoulders with PS-RCT, and that RTSA would lead to a decrease in required forces that is further minimized with lateralization and inferiorization of the COR but still higher compared to native shoulders with an intact rotator cuff.
In this study, eight cadaveric shoulders were dissected leaving only the rotator cuff muscles and capsule intact. A custom apparatus incorporating motion capture and a dynamic tensile testing machine to measure the changes in COR and deltoid forces while simultaneously recording glenohumeral abduction was designed. Five consecutive testing states were tested: 1) intact shoulder, 2) PS-RCT, 3) RTSA with standard glenosphere, 4) RTSA with 4mm lateralized glenosphere, 5) RTSA with 2.5mm inferiorized glenosphere. Statistical Parametric Mapping (SPM) was used to analyze the deltoid force as a function of the abduction angle. One-way repeated-measures within-specimens analysis of variance (ANOVA) was conducted, followed by post-hoc t-tests for pairwise comparisons between the states.
All RTSA configurations shifted the COR medially and inferiorly with respect to native (Standard: 4.2±2.1mm, 19.7±3.6mm; 4mm lateralized: 3.9±1.2mm, 16.0±1.8; 2.5mm inferiorized: 6.9±0.9mm, 18.9±1.7mm). ANOVA showed a significant effect of specimen state on deltoid force across all abduction angles. Of the ten paired t-test comparisons made between states, only three showed significant differences: 1) Intact shoulders necessitated significantly lower deltoid force than specimens with PS-RCT below 42° abduction, 2) RTSAs with standard glenospheres required significantly lower deltoid force than RTSA with 4mm lateralized glenospheres above 34° abduction, and 3) RTSAs with 2.5mm inferiorized glenospheres had significantly lower deltoid force than RTSA with 4mm of glenosphere lateralization at higher abduction angles.
RTSA with a 2.5mm inferiorized glenosphere and no additional lateralization resulted in less deltoid force to abduct the arm compared to
standard or 4mm lateralized glenospheres. Therefore, when aiming to mitigate downsides of a medialized COR, an inferiorized glenosphere may be preferable in terms of its effect on deltoid force.
Level of Evidence
Basic Science Study; Biomechanics