This cadaver study used a biorobotic simulator to compare inlay versus onlay humeral components in reverse total shoulder arthroplasty. Twenty fresh-frozen shoulders performed scapular plane abduction driven by real patient-derived kinematics. The study asked how the two humeral designs affect deltoid and rotator cuff forces, passive range of motion, and muscle excursion.
Choosing an inlay versus onlay humeral component is a trade-off between range and muscle efficiency. Onlay reaches greater passive elevation but demands far higher rotator cuff force to get there actively, raising concern for muscle fatigue and, over time, failure.
Inlay keeps the cuff insertions closer to the center of rotation, so it needs less force and was the only design that lowered deltoid force below the native shoulder. A key board point: after rTSA the residual cuff sits below the new center of rotation and acts as an adductor antagonizing the deltoid, which is why cuff force demand jumps 4 to 9 times above native.
Remember that neck shaft angle (135° vs 145°) and glenosphere diameter had little effect compared with the inlay-versus-onlay decision itself. Do not treat preoperative planning ROM numbers as absolute targets, since they diverged sharply from measured motion.
This cadaver study used a biorobotic simulator to compare inlay versus onlay humeral components in reverse total shoulder arthroplasty. Twenty fresh-frozen shoulders performed scapular plane abduction driven by real patient-derived kinematics. The study asked how the two humeral designs affect deltoid and rotator cuff forces, passive range of motion, and muscle excursion.
Choosing an inlay versus onlay humeral component is a trade-off between range and muscle efficiency. Onlay reaches greater passive elevation but demands far higher rotator cuff force to get there actively, raising concern for muscle fatigue and, over time, failure.
Inlay keeps the cuff insertions closer to the center of rotation, so it needs less force and was the only design that lowered deltoid force below the native shoulder. A key board point: after rTSA the residual cuff sits below the new center of rotation and acts as an adductor antagonizing the deltoid, which is why cuff force demand jumps 4 to 9 times above native.
Remember that neck shaft angle (135° vs 145°) and glenosphere diameter had little effect compared with the inlay-versus-onlay decision itself. Do not treat preoperative planning ROM numbers as absolute targets, since they diverged sharply from measured motion.