Cadaveric biomechanical study testing four glenosphere positions in the reverse Delta III shoulder prosthesis. The central question: which placement best prevents the inferior polyethylene impingement responsible for scapular neck notching? Eight fresh-frozen specimens were tested across scapular, frontal, and sagittal planes.
When implanting a reverse shoulder prosthesis, the default manufacturer instruction to center the glenosphere leaves the inferior glenoid rim uncovered and guarantees polyethylene cup contact during adduction — the mechanical source of scapular notching seen in nearly two-thirds of patients at mid-term follow-up.
This paper is the biomechanical foundation for why surgeons place the metaglene flush with the inferior glenoid rim rather than centered on the face of the glenoid. That single adjustment shifts the glenosphere distally so it overhangs the scapular neck, converting a 25° adduction deficit into near-neutral adduction.
In practice: when you set up your metaglene, position the inferior border of the baseplate at the inferior glenoid rim. The glenosphere will then extend beyond the scapular neck, providing the clearance needed for full adduction without bone contact.
Avoid the 15° oblique osteotomy as your primary notch-prevention strategy. It helps, but it sacrifices inferior cortical bone surface area and still underperforms distal placement on the uncut glenoid.
Cadaveric biomechanical study testing four glenosphere positions in the reverse Delta III shoulder prosthesis. The central question: which placement best prevents the inferior polyethylene impingement responsible for scapular neck notching? Eight fresh-frozen specimens were tested across scapular, frontal, and sagittal planes.
When implanting a reverse shoulder prosthesis, the default manufacturer instruction to center the glenosphere leaves the inferior glenoid rim uncovered and guarantees polyethylene cup contact during adduction — the mechanical source of scapular notching seen in nearly two-thirds of patients at mid-term follow-up.
This paper is the biomechanical foundation for why surgeons place the metaglene flush with the inferior glenoid rim rather than centered on the face of the glenoid. That single adjustment shifts the glenosphere distally so it overhangs the scapular neck, converting a 25° adduction deficit into near-neutral adduction.
In practice: when you set up your metaglene, position the inferior border of the baseplate at the inferior glenoid rim. The glenosphere will then extend beyond the scapular neck, providing the clearance needed for full adduction without bone contact.
Avoid the 15° oblique osteotomy as your primary notch-prevention strategy. It helps, but it sacrifices inferior cortical bone surface area and still underperforms distal placement on the uncut glenoid.