Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses

Acromial fractures after reverse shoulder arthroplasty (RSA) have been reported to occur in up to 7% of patients. Whereas RSA implant parameters can be configured to alter stability, range of motion, and deltoid mechanical advantage, little is known about the effect of these changes on acromial stresses. The purpose of this finite element study, therefore, was to evaluate the effect of RSA humeral and glenoid implant position on acromial stresses.

Solid body models of 10 RSA reconstructed cadaveric shoulders (38-mm glenosphere, 155° neck-shaft angle) were input into custom software that calculated the deltoid force required to achieve an abduction arc of motion (0°-120°). The resulting forces were applied to a finite element study model of the scapula to ascertain the acromial stress distribution. This process was repeated for varying glenoid inferiorizations (0, +2.5, +5.0 mm), lateralizations (0, +5.0, +10.0 mm), and humeral lateralizations (-5.0, 0, +5.0 mm).

Glenosphere inferiorization decreased maximum principal stress in the acromion by 2.6% (0.7 ± 0.2 MPa; P = .007). Glenosphere lateralization produced a greater effect, increasing stress by 17.2% (4.1 ± 0.9 MPa; P = .001). Humeral lateralization caused an insignificant increase in stress by 1.7% (0.5 ± 0.2 MPa; P = .066), and humeral medialization decreased stress by 1.4% (0.8 ± 0.3 MPa; P = .038). The highest acromial stresses occurred in the region where fractures most commonly occur, Levy type II, at 33.7 ± 3.81 MPa (P < .001).

Glenosphere positioning has a significant effect on acromial stress after RSA. Inferior and medial positioning of the glenosphere serves to decrease acromial stress, thought to be primarily due to increased deltoid mechanical advantage. The greatest effect magnitudes are seen at lower abduction angles, where the humerus is more frequently positioned.