The effect of component offset on acromion and scapular spine strains following reverse total shoulder arthroplasty: An ex-vivo shoulder motion study

The effects of component offset in reverse total shoulder arthroplasty (rTSA) on acromion and scapular spine strains are not well understood, and no comparison of strain patterns between the native and implanted shoulder exists. This study investigated the effects of humerus distalization, humerus lateralization, glenosphere lateralization, and humerus-to-glenosphere lateralization ratio in rTSA on acromion and scapular spine strains during simulated unconstrained shoulder motion and compared the observed strain patterns to the native condition. Seven cadaveric specimens were tested on a shoulder simulator which performed scapular plane abduction and internal-external rotation. Maximum principal strain was measured in four clinically relevant regions according to the Levy classification system using strain gauge rosettes. Overall, rTSA conditions had higher strains than the native condition at low elevation (p < 0.05) but not at high elevation. During scapular plane abduction, strain peaked at low elevation for all reverse conditions, but at high elevation angles in the native condition. Strain increased significantly with humerus distalization in Levy 3A (p = 0.038), while strain decreased significantly in all regions with humerus lateralization (p ≤ 0.049). Isolated glenosphere lateralization had no effect on scapula strains (p ≥ 0.162) while humerus-sided lateralization resulted in significantly less strain than combined humerus and glenosphere lateralization (p ≤ 0.015). RTSA alters the strain patterns on the acromion and scapular spine from those observed in the native shoulder. In addition, component offset had significant effects on scapula strains demonstrating that implant configuration likely has an influence on the occurrence of acromion and scapular spine fractures.