This finite element study models how reverse shoulder arthroplasty component choices affect deltoid and acromial stress. It compares glenoid lateralization from 3 to 9 mm against inlay versus onlay humeral stems using a 135° neck-shaft angle system. Deltoid and acromial stresses were measured after virtual implantation and during 50° of external rotation.
When you select components for a reverse shoulder, glenoid and humeral lateralization are not independent knobs. They stack. This model shows the humeral side dominates acromial loading: an onlay stem at high glenoid lateralization pushes acromial stress up 117%, versus 37% from glenoid lateralization alone.
That matters because acromial stress fractures occur in 1 to 7% of RSA and disproportionately affect the exact patients you often operate on: older women with osteoporosis, rheumatoid arthritis, or cuff tear arthropathy. The peak stress location the model predicts, the mid-scapular spine, is the same place Levy Type II fractures actually occur clinically, which lends the findings credibility.
The decision framework: in a high-risk patient, favor less humeral distalization and an inlay stem to protect the acromion. In a patient where instability is the bigger concern, an onlay stem buttresses against post-impingement subluxation, accepting higher torque and possibly less motion.
This finite element study models how reverse shoulder arthroplasty component choices affect deltoid and acromial stress. It compares glenoid lateralization from 3 to 9 mm against inlay versus onlay humeral stems using a 135° neck-shaft angle system. Deltoid and acromial stresses were measured after virtual implantation and during 50° of external rotation.
When you select components for a reverse shoulder, glenoid and humeral lateralization are not independent knobs. They stack. This model shows the humeral side dominates acromial loading: an onlay stem at high glenoid lateralization pushes acromial stress up 117%, versus 37% from glenoid lateralization alone.
That matters because acromial stress fractures occur in 1 to 7% of RSA and disproportionately affect the exact patients you often operate on: older women with osteoporosis, rheumatoid arthritis, or cuff tear arthropathy. The peak stress location the model predicts, the mid-scapular spine, is the same place Levy Type II fractures actually occur clinically, which lends the findings credibility.
The decision framework: in a high-risk patient, favor less humeral distalization and an inlay stem to protect the acromion. In a patient where instability is the bigger concern, an onlay stem buttresses against post-impingement subluxation, accepting higher torque and possibly less motion.