A 3D computer model of reverse shoulder arthroplasty isolated humeral-side design from the glenoid. It compared a traditional inlay Grammont stem to a curved onlay stem at three inclinations (155°, 145°, 135°) and varied tray eccentricity. Outcomes were humeral offset, acromiohumeral distance, and impingement-free range of motion.
When you choose a reverse shoulder implant, humeral inclination is a lever that trades abduction for rotation and adduction. Moving from the classic 155° Grammont toward a varus 135° angle rotates the polyethylene off the scapular pillar. This improves adduction, extension, and external rotation at the side, and reduces scapular notching.
The catch: 135° sacrifices external rotation with the arm at 90° abduction because the tuberosity abuts the acromion. That motion drives hair care and grooming, so the deficit matters functionally. The practical takeaway is that a 145° onlay stem preserves rotational gains while keeping acromiohumeral distance close to a Grammont, avoiding acromial abutment.
Remember also that the curved onlay stem itself, not the inclination, is the dominant driver of humeral offset. This is a modeling study with no soft tissue tension, so treat these as geometric tendencies, not clinical outcomes.
A 3D computer model of reverse shoulder arthroplasty isolated humeral-side design from the glenoid. It compared a traditional inlay Grammont stem to a curved onlay stem at three inclinations (155°, 145°, 135°) and varied tray eccentricity. Outcomes were humeral offset, acromiohumeral distance, and impingement-free range of motion.
When you choose a reverse shoulder implant, humeral inclination is a lever that trades abduction for rotation and adduction. Moving from the classic 155° Grammont toward a varus 135° angle rotates the polyethylene off the scapular pillar. This improves adduction, extension, and external rotation at the side, and reduces scapular notching.
The catch: 135° sacrifices external rotation with the arm at 90° abduction because the tuberosity abuts the acromion. That motion drives hair care and grooming, so the deficit matters functionally. The practical takeaway is that a 145° onlay stem preserves rotational gains while keeping acromiohumeral distance close to a Grammont, avoiding acromial abutment.
Remember also that the curved onlay stem itself, not the inclination, is the dominant driver of humeral offset. This is a modeling study with no soft tissue tension, so treat these as geometric tendencies, not clinical outcomes.