Finite element study of 10 RSA-reconstructed cadaveric shoulders examining how glenosphere and humeral implant position affect acromial stress. The question: which implant positioning choices raise the risk of acromial stress fracture after reverse shoulder arthroplasty?
Acromial stress fractures complicate up to 7% of reverse shoulder arthroplasties and carry worse function and higher revision risk, yet their cause was poorly understood. This model links the fracture to implant positioning: glenosphere lateralization drives stress up, while inferiorization and medialization drive it down by lengthening the deltoid moment arm.
The practical mental model is a tradeoff. Lateralizing the glenosphere buys stability and range of motion but loads the acromion. Inferiorizing achieves similar range-of-motion goals with the added benefit of lower stress.
The model's peak-stress zone matched the Levy type II region where fractures actually occur, and modeled stresses (~40 MPa) stayed well below the 120–130 MPa traumatic threshold. This supports a fatigue mechanism, consistent with fractures appearing 3 to 10 months postoperatively.
Remember this is a computational, relative-stress study, so it guides positioning philosophy rather than dictating exact millimeter targets.
Finite element study of 10 RSA-reconstructed cadaveric shoulders examining how glenosphere and humeral implant position affect acromial stress. The question: which implant positioning choices raise the risk of acromial stress fracture after reverse shoulder arthroplasty?
Acromial stress fractures complicate up to 7% of reverse shoulder arthroplasties and carry worse function and higher revision risk, yet their cause was poorly understood. This model links the fracture to implant positioning: glenosphere lateralization drives stress up, while inferiorization and medialization drive it down by lengthening the deltoid moment arm.
The practical mental model is a tradeoff. Lateralizing the glenosphere buys stability and range of motion but loads the acromion. Inferiorizing achieves similar range-of-motion goals with the added benefit of lower stress.
The model's peak-stress zone matched the Levy type II region where fractures actually occur, and modeled stresses (~40 MPa) stayed well below the 120–130 MPa traumatic threshold. This supports a fatigue mechanism, consistent with fractures appearing 3 to 10 months postoperatively.
Remember this is a computational, relative-stress study, so it guides positioning philosophy rather than dictating exact millimeter targets.