This 3D computer model simulated reverse shoulder arthroplasty using a 145° onlay humeral stem paired with five different glenosphere configurations. It asks which glenoid design best optimizes offset, acromio-humeral distance, range of motion, and rotator cuff muscle length.
Most classic RSA data come from the 155° Grammont design, but many modern implants use 145° or 135° inclinations with onlay humeral trays. This study isolates the glenoid variable in that newer context.
The practical takeaway: with a 145° onlay stem, a 36 mm inferiorly eccentric glenosphere maximizes extension and external rotation at the side while avoiding the adduction deficit that drives scapular notching.
Going bigger (42 mm) or adding a bone graft (BIO-RSA) does not buy you motion. BIO-RSA maximizes offset and supraspinatus stretch (18.6 mm) but still impinges against the graft and yields a 13° adduction deficit.
Remember the lever rule: the glenoid changes arm length by only ~3 mm, so lengthening is set on the humeral side. This is a single-specimen computer model with no soft-tissue tensioning, and clinical RCTs comparing these designs have shown mixed results, so treat these as theoretical optima.
This 3D computer model simulated reverse shoulder arthroplasty using a 145° onlay humeral stem paired with five different glenosphere configurations. It asks which glenoid design best optimizes offset, acromio-humeral distance, range of motion, and rotator cuff muscle length.
Most classic RSA data come from the 155° Grammont design, but many modern implants use 145° or 135° inclinations with onlay humeral trays. This study isolates the glenoid variable in that newer context.
The practical takeaway: with a 145° onlay stem, a 36 mm inferiorly eccentric glenosphere maximizes extension and external rotation at the side while avoiding the adduction deficit that drives scapular notching.
Going bigger (42 mm) or adding a bone graft (BIO-RSA) does not buy you motion. BIO-RSA maximizes offset and supraspinatus stretch (18.6 mm) but still impinges against the graft and yields a 13° adduction deficit.
Remember the lever rule: the glenoid changes arm length by only ~3 mm, so lengthening is set on the humeral side. This is a single-specimen computer model with no soft-tissue tensioning, and clinical RCTs comparing these designs have shown mixed results, so treat these as theoretical optima.