This biomechanical study used a Sawbones scapula model with custom reverse shoulder implants to isolate how four design and positioning variables affect motion. It tested glenosphere diameter, center of rotation offset, glenoid position, and humeral neck-shaft angle against abduction ROM and adduction deficit. The goal was to rank which factors matter most for maximizing motion and avoiding inferior scapular impingement.
Scapular notching is the most common radiographic complication after reverse shoulder arthroplasty, reported in 56% to 96% of cases and implicated in some failures. This study explains the two levers you control to fight it: implant design and glenosphere position.
The mental model is simple. To maximize abduction, lateralize the center of rotation and place the glenosphere inferiorly, both of which delay superior impingement on the acromion. To minimize the adduction deficit that drives notching, choose a varus (lower) neck-shaft angle plus inferior placement, which move the humeral socket away from the inferior scapular neck at rest.
These are separable goals: COR offset dominates motion, while neck-shaft angle dominates notching. Remember the caveat: this model had no soft tissue tension, so combinations that look ideal on the bench may overstuff the joint or risk instability clinically.
This biomechanical study used a Sawbones scapula model with custom reverse shoulder implants to isolate how four design and positioning variables affect motion. It tested glenosphere diameter, center of rotation offset, glenoid position, and humeral neck-shaft angle against abduction ROM and adduction deficit. The goal was to rank which factors matter most for maximizing motion and avoiding inferior scapular impingement.
Scapular notching is the most common radiographic complication after reverse shoulder arthroplasty, reported in 56% to 96% of cases and implicated in some failures. This study explains the two levers you control to fight it: implant design and glenosphere position.
The mental model is simple. To maximize abduction, lateralize the center of rotation and place the glenosphere inferiorly, both of which delay superior impingement on the acromion. To minimize the adduction deficit that drives notching, choose a varus (lower) neck-shaft angle plus inferior placement, which move the humeral socket away from the inferior scapular neck at rest.
These are separable goals: COR offset dominates motion, while neck-shaft angle dominates notching. Remember the caveat: this model had no soft tissue tension, so combinations that look ideal on the bench may overstuff the joint or risk instability clinically.