This in vivo fluoroscopic study measured 3D shoulder kinematics after lateralized onlay reverse shoulder arthroplasty. It compared scaption motion and scapular neck-insert distance against previously published data for Grammont-type RSA using identical model-image registration methods. The question: does implant design change how the shoulder actually moves in living patients?
When counseling a patient about RSA implant choice, this paper supplies the in vivo mechanism behind a fact you already know from boards: lateralized designs notch less. The reason is geometric. A smaller neck-shaft angle moves the polyethylene insert away from the inferior scapular neck. This study directly measured that gap in living patients at 3.2-5 mm versus about 1 mm for Grammont.
The counterintuitive teaching point: lateralized shoulders were more adducted during external rotation yet still had more clearance. That confirms the clearance comes from the implant geometry, not from how the arm is positioned.
Appreciate the design trade-off framework. Grammont maximizes deltoid tension and elevation but risks notching and limited rotation. Lateralized designs restore rotator cuff tension, improve external rotation (47° vs 26-29° here), and reduce notching.
One caveat for critical appraisal: this is a small basic-science kinematic study comparing to historical control cohorts, and glenosphere size varied in the lateralized group.
This in vivo fluoroscopic study measured 3D shoulder kinematics after lateralized onlay reverse shoulder arthroplasty. It compared scaption motion and scapular neck-insert distance against previously published data for Grammont-type RSA using identical model-image registration methods. The question: does implant design change how the shoulder actually moves in living patients?
When counseling a patient about RSA implant choice, this paper supplies the in vivo mechanism behind a fact you already know from boards: lateralized designs notch less. The reason is geometric. A smaller neck-shaft angle moves the polyethylene insert away from the inferior scapular neck. This study directly measured that gap in living patients at 3.2-5 mm versus about 1 mm for Grammont.
The counterintuitive teaching point: lateralized shoulders were more adducted during external rotation yet still had more clearance. That confirms the clearance comes from the implant geometry, not from how the arm is positioned.
Appreciate the design trade-off framework. Grammont maximizes deltoid tension and elevation but risks notching and limited rotation. Lateralized designs restore rotator cuff tension, improve external rotation (47° vs 26-29° here), and reduce notching.
One caveat for critical appraisal: this is a small basic-science kinematic study comparing to historical control cohorts, and glenosphere size varied in the lateralized group.