A Dual-Cup Reverse Shoulder Replacement Improves Impingement-Free Motion

Abstract

Background <br> Advances in reverse shoulder arthroplasty (RSA) design have improved functional outcomes, which approach those of the anatomic total shoulder arthroplasty (aTSA) in certain patients. However, restoration of motion, especially functional internal rotation, remains a concern following RSA. A novel dual-cup RSA design1 was developed to improve impingement-free range of motion. The passive motion of the dual-cup RSA is compared to that of a commercially-available RSA design in a cadaveric model. <br> Methods ,<br> Five fresh-frozen cadaveric upper extremity specimens were included in the study. The scapula was rigidly fixed, allowing for isolation of glenohumeral joint motion. The novel dual-cup RSA design was tested with and without a flanged component. The dual-cup has an outer diameter of 36 mm, articulates with a 32+4 mm glenosphere, and provides 6 mm of glenoid lateralization. A 32+4 mm lateralized glenosphere and a 36+6 mm lateralized glenosphere served as controls. Measurements of motion were performed using an electromagnetic tracking device and modes of impingement were recorded. An Analysis of Variance (ANOVA) test was performed with a post-hoc Tukey test to compare the relative changes in range of motion among groups. Results The dual-cup (without flange) design improved external rotation (ER) at 0° abduction by 20.9° and improved IR by 11.5° compared to the 32+4 mm control (p<0.01). In 90° abduction, the dual-cup improved ER by 11.9° compared to the 32+4 mm control, a significant improvement relative to the 36+6 mm control (p<0.05). The dual-cup improved functional IR by 12.4° relative to the 32+4 mm control, a significant improvement relative to the 36+6 mm control (p<0.05). The dual-cup did not significantly improve IR at 45°or 90° abduction compared to control models. Humeral component impingement on the glenoid was not observed at any end-range of motion in the implant with the flanged design. <br> Discussion <br> The dual-cup design improves all rotational motions relative to control models except internal rotation with an abduction angle reaching or exceeding 45°. The dual-cup design improves impingement free motion, including functional internal rotation, without additional lateralization in this study. The dual-dup with flange design eliminates impingement of the humeral cup on the glenoid neck without significantly affecting most motions relative to control models.