The effect of glenoid lateralization and glenosphere size in reverse shoulder arthroplasty on deltoid load: A biomechanical cadaveric study

Purpose: Use of a relatively larger glenosphere with some larger overhang helps to minimize posterior impingement and some degree of glenoid lateralization is also beneficial in reverse shoulder arthroplasty (RSA). The optimal amounts of inferior overhang and lateralization are not agreed upon. The purpose of this in-vitro biomechanical study is to analyze the effect of glenosphere size and glenoid lateralization on deltoid load, focusing on differences in its three distinct heads. Methods: Reverse shoulder arthroplasty (DeltaXtend, Fa. Depuy/Synthes) was performed on six cadaveric specimens. The load on each head of the deltoid muscle (spinalis, acromialis, and clavicularis) was measured indirectly by transferring deformation (mu m/m) via strain gauges (Fa. Vishay) with stepwise lateralization: +0 mm (subgroup I), +5 mm (subgroup II), +10 mm (subgroup III). Each scenario was done with a 38 mm (group A) and a 42 mm (group B) glenosphere. Results: In group A as well in group B, the mean measured deformation in the respective titanium omega increased with lateralization in the clavicular (AI: 119.6 mu m/m, AIII: 307.3 mu m/m; BI: 173.3 mu m/m, BIII: 358.5 mu m/m), in the spinal (AI: 85.3 mu m/m, AIII: 188.5 mu m/m; BI: 138.8 mu m/m, BIII 261.2 mu m/m) and in the acromial head (AI: 340.5 mu m/mu; AIII: 454.2 mu m/m; BI: 388.5 mu m/m, BIII: 538.8 mu m/m). A significant difference between the subgroups in the spinal (p = .048) and clavicular heads (p = .028) was found. The use of a 42 mm glenosphere increased significantly the load in each segment. Conclusion: Lateralization and glenosphere size increased significantly deltoid muscle loading, especially in the clavicular head. According to these in-vitro data, the high variability in the amount of lateralization influences the soft-tissue balance in reverse shoulder arthroplasty. Level of evidence: Basic science study