A quantitative analysis of the effect of baseplate and glenosphere position on deltoid lengthening in reverse total shoulder arthroplasty

CONTEXT: Optimizing deltoid tension is important to achieve maximal function after reverse total shoulder arthroplasty (RTSA), but the effects of baseplate and glenosphere positions on deltoid tension have not been quantified. AIMS: To quantify deltoid elongation and elongation to failure under physiologic loads with three baseplate-glenosphere configurations with increasing inferior offset. SETTINGS AND DESIGN: Cadaver biomechanical study. MATERIALS AND METHODS: Twenty-four cadaver shoulders were divided into three groups. The starting point for baseplate insertion in Group 1 was the center of the glenoid, with glenospheres placed in minimal inferior offset (0.5 mm). Groups 2 and 3 baseplates were placed 2 mm inferior to the center point and glenospheres in minimal (2.5 mm) offset (Group 2) or maximal (4.5 mm) offset (Group 3). Tensile testing was done to quantify deltoid elongation and evaluate failure. STATISTICAL ANALYSIS USED: A one-way analysis of variance was performed to detect statistically significant differences among treatment groups. A post-hoc Neuman-Keul's comparison was conducted to perform discrete comparisons among treatment groups. RESULTS: Deltoid elongation after loading decreased with increasing inferior offset of >2.5 mm. No significant difference in deltoid yield load was found among groups. The percent of elongation was decreased significantly between groups 2 and 3. Deltoid displacement at failure decreased from 33.3 mm for Group 2-17.3 mm for Group 3. 16 of the 24 specimens (67%) failed by anterior deltoid detachment from the acromion. CONCLUSIONS: Increasing inferior offset in RTSA constructs appears to increase stretch forces on the deltoid, resulting in a diminished ability of the deltoid to further elongate under physiologic loads, (most pronounced when the inferior offset exceeds 2.5 mm) and significantly decreasing the yield displacement of the construct.