Kinematic impact of size on the existing glenohumeral joint in patients undergoing reverse shoulder arthroplasty

Background: Glenohumeral relationships in reverse shoulder arthroplasty patients have not been previously reported. The purpose of this study was to quantify and compare the shoulder spatial relationships and moment arms. Measurements were used to define general size categories and determine if sizes scale linearly for all metrics. Methods: Ninety-two shoulders of patients undergoing primary reverse shoulder arthroplasty for functionally-deficient massive rotator cuff tear without bony deformity or deficiency were evaluated using three-dimensional CT reconstructions and computer-aided design software. Multiple glenohumeral relationships (including moment arm) were measured and evaluated for size stratification and linearity. Generalized linear modeling was used to investigate how predictive glenoid height, coronal humeral head diameter, and gender were of greater tuberosity positions. Findings: The 92 shoulders were grouped based on glenoid height: small (<33.4 mm), medium (33.4-38.0 mm), and large (>38.0 mm). All relationships varied between groups. The humeral head size, glenoid width, lateral offset, and moment arm all independently increased linearly (r(2) >= 0.92) but the rate of increase varied (slope range: 0.59-1.92). Glenoid height, coronal humeral head diameter and gender predicted the greater tuberosity position within mean 1.09 mm (standard deviation (SD) 0.84 mm) of actual position in 90% of the population. Interpretation: Distinct groups exist based on the size of the glenoid in shoulder arthroplasty patients. Shoulder modeling should account for size groups, sex, and non-uniform linear scaling of morphometric parameters. Prediction of the greater tuberosity offset can be made using sex and size parameters. Clinical implications include appropriate prosthetic size selection and avoiding large deviations in non-anatomic reconstructions. (C) 2014 Elsevier Ltd. All rights reserved.