Scapula anatomy influences simulated impingement-free range of motion in reverse shoulder arthroplasty

Impingement-free range of motion (ROM) after reverse shoulder arthroplasty (rTSA) may depend on implant position and scapula anatomic parameters. The critical shoulder angle (CSA) is influenced by a combination of scapula parameters. The aim of this study was to evaluate whether the CSA has an influence on impingement-free ROM after rTSA in a virtual simulation using a Statistical Shape Model.

100 scapulae chosen from a database of 10,000 scapulae were used to generate a Statistical Shape Model. Modes corresponding to anatomical characteristics (size, CSA etc.) were defined. Five CSA models were obtained including a mean and 2 standard deviations (SDs) (CSA 32° [-2 SD], CSA 30° [-1 SD], CSA 27° [mean], CSA 25° [+1 SD], and CSA 23° [+2 SD]). A 39-mm glenosphere was virtually implanted in each model. The humeral side was kept consistent with the simulation of a 135° neck-shaft-angle component (Univers Revers, Arthrex Inc., Naples, FL, USA). Glenoid positioning parameters included (1) lateral offset (0-10 mm in 2-mm increments), (2) inferior offset (0, 2.5, 5, 7.5 mm), and (3) posterior offset (0, 2.5, 5 mm). External rotation (ER) at 0° and 60° of abduction and internal rotation (IR) at 60° of abduction were then analyzed for the different positioning parameters (inferior, posterior, and lateral offset) and the combination of 0 mm inferior and 2.5 posterior offset and lateralization from 0-10 mm, 2.5 mm inferior and 0 mm of posterior offset and lateralization (0-10 mm), and the combination of 2.5 mm inferior and 2.5 mm posterior offset and lateralization (0-10 mm).

Lower CSA models showed higher ER 0° values (eg, 435% increase from CSA 32° to CSA 23° at 0 mm lateral, inferior, and posterior offset), while models with greater CSAs showed higher IR 60° values (eg, 505% increase from CSA SD 23° to CSA SD 32° at 0 mm lateral, inferior, and posterior offset). By lateralizing, ROM increased in all CSA models (eg, 884% increase from 0 mm to 10 mm lateralization for CSA 32° for ER 0°). Posterior positioning of 2.5 and 5 mm improved ER not IR. Maximal IR at 60° was achieved with no posterior, 2.5 mm of inferior offset, and lateralization between 2-6 mm according to the evaluated CSA.

Specific CSA ranges require particular implant positioning strategies to optimize impingement-free ROM in rTSA. To achieve the maximal ROM combination of IR and ER in this simulation, 2.5 mm of inferior offset with no posterior offset and lateralization of 4 mm for CSA ≥30° and 6 mm for CSA SD ≤25° was required.