The rotator cuff muscles are antagonists after reverse total shoulder arthroplasty.

There is disagreement regarding whether, when possible, the rotator cuff should be repaired in conjunction with reverse total shoulder arthroplasty (RTSA). Therefore, we investigated the effects of rotator cuff repair in RTSA models with varying magnitudes of humeral and glenosphere lateralization. Six fresh frozen cadaveric shoulders were tested on a validated in vitro muscle-driven motion simulator. Each specimen was implanted with a custom adjustable, load-sensing RTSA after creation of a simulated rotator cuff tear. The effects of 4 RTSA configurations (0 and 10 mm of humeral lateralization and glenosphere lateralization) on deltoid force and joint load during abduction with and without rotator cuff repair were assessed. Deltoid force was significantly affected by increasing humeral lateralization (-2.5% ± 1.7% body weight [BW], P = .016) and glenosphere lateralization (+7.7% ± 5.6% BW, P = .016). Rotator cuff repair interacted with humeral and glenosphere lateralization (P = .005), such that with no humeral lateralization, glenosphere lateralization increased deltoid force without cuff repair (8.1% ± 5.1% BW, P = .012). This effect was increased with cuff repair (12.8% ± 7.8% BW, P = .010), but the addition of humeral lateralization mitigated this effect. Rotator cuff repair increased joint load (+11.9% ± 5.1% BW, P = .002), as did glenosphere lateralization (+13.3% ± 3.7% BW, P < .001). These interacted, such that increasing glenosphere lateralization markedly increased the negative effects of cuff repair (9.4% ± 3.2% BW [P = .001] vs. 14.4% ± 7.4% BW [P = .005]). Rotator cuff repair, especially in conjunction with glenosphere lateralization, produces an antagonistic effect that increases deltoid and joint loading. The long-term effects of this remain unknown; however, combining these factors may prove undesirable. Humeral lateralization improves joint compression through deltoid wrapping and increases the deltoid's mechanical advantage, and therefore, could be used in place of rotator cuff repair, thus avoiding its complications.

Giles JW ，Langohr GD ，Johnson JA ，Athwal GS ... -  《-》

Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load.

Giles JW ，Langohr GD ，Johnson JA ，Athwal GS ... -  《-》

The effect of glenosphere lateralization and inferiorization on deltoid force in reverse total shoulder arthroplasty.

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 purposes were to measure the effect of RTSA with varying glenosphere configurations on (1) the COR and (2) deltoid force compared with 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 with native shoulders with an intact rotator cuff. In this study, 8 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 4 mm lateralized glenosphere, and (5) RTSA with 2.5 mm inferiorized glenosphere. Statistical Parametric Mapping was used to analyze the deltoid force as a function of the abduction angle. One-way repeated-measures within-specimens analysis of variance 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 ± 2.1 mm, 19.7 ± 3.6 mm; 4 mm lateralized: 3.9 ± 1.2 mm, 16.0 ± 1.8; 2.5 mm inferiorized: 6.9 ± 0.9 mm, 18.9 ± 1.7 mm). Analysis of variance showed a significant effect of specimen state on deltoid force across all abduction angles. Of the 10 paired t-test comparisons made between states, only 3 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 4 mm lateralized glenospheres above 34° abduction, and (3) RTSAs with 2.5 mm inferiorized glenospheres had significantly lower deltoid force than RTSA with 4 mm of glenosphere lateralization at higher abduction angles. RTSA with a 2.5 mm inferiorized glenosphere and no additional lateralization resulted in less deltoid force to abduct the arm compared with 4 mm 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.

Nolte PC ，Miles JW ，Tanghe KK ，Brady AW ，Midtgaard KS ，Cooper JD ，Lacheta L ，Provencher MT ，Millett PJ ... -  《-》

Does Humeral Component Lateralization in Reverse Shoulder Arthroplasty Affect Rotator Cuff Torque? Evaluation in a Cadaver Model.

Chan K ，Langohr GDG ，Mahaffy M ，Johnson JA ，Athwal GS ... -  《-》

Inlay vs. onlay humeral components in reverse total shoulder arthroplasty: a biorobotic shoulder simulator study.

Zitnay JL ，Tashjian RZ ，Walch G ，Chalmers PN ，Joyce CD ，Henninger HB ... -  《-》