Anterior and Posterior Scapular Impingement Associated with Two Different Reverse Shoulder Designs

Introduction Reverse shoulder arthroplasty is an increasingly popular treatment option for cuff tear arthropathy and other degenerative conditions of the glenohumeral joint that have been previously treated with limited success. Concerns about scapular notching have driven design and surgical technique modifications by manufacturers and surgeons in order to improve range of motion (ROM) and stability: most commonly, inferiorly offsetting the glenosphere from 2-4mm. Recent work conducted a ROM assessment (humeral abduction/adduction in the scapular plane) to elucidate additional relationships between reverse shoulder design parameters and functional measurements. Specifically, inferior impingement can be minimized by decreasing humeral neck angle, decreasing humeral liner constraint, increasing glenosphere thickness, inferiorly offsetting the glenosphere, or any combination of the four; ROM can be increased by decreasing humeral liner constraint, increasing the glenosphere thickness, distally offsetting the glenosphere, or any combination of the three; and jump distance (i.e. a measure of prosthesis stability) can be increased by increasing humeral liner constraint, increasing glenosphere diameter, or any combination of the two. The purpose of this study is to expand on this work by quantifying the efficacy of inferiorly offsetting the glenosphere alone to prevent anterior/ posterior scapular impingement (in addition to inferior scapular impingement) for the Grammont reverse shoulder design via a ROM assessment in internal/external rotation at varying levels of humeral abduction/adduction as the Grammont is inferiorly offset from 2-4mm. The Equinoxe reverse shoulder is provided as a control because it is a Grammont-style prosthesis that utilizes several of the aforementioned design modifications to minimize impingement and maximize ROM.