The effect of reverse shoulder arthroplasty design and surgical indications on deltoid and rotator cuff muscle length

Abstract

Background: Advancements in surgical planning, technique, and prosthesis design have improved adaptation to patient anatomy in reverse total shoulder arthroplasty (rTSA). Postoperative changes in deltoid and rotator cuff muscle length are important and may vary based on preoperative indications and prosthesis selection. The purpose of this study is to demonstrate the changes in deltoid and rotator cuff muscle length for planned rTSA using the spectrum of prosthesis configurations in both glenohumeral arthritis (GHOA) and rotator cuff tear arthropathy (RCA).

Methods: Ten shoulder arthroplasty surgeons used preoperative planning software to plan rTSA cases for 20 subjects (10 GHOA, 10 RCA) following surgical guidelines. Each surgeon planned each case using 3 prosthesis configurations: (1) 8-mm lateralized glenosphere and 135° neck-shaft angle (135 + 8), (2) 4-mm lateralized glenosphere and 145° neck-shaft angle (145 + 4), and (3) 0-mm lateralized glenosphere and 155° neck-shaft angle (155 + 0). Pre- and postoperative deltoid and rotator cuff muscle lengths and percentage-change were calculated and compared between prosthesis configurations within each indication. Different muscle lines of action were included representing the deltoid, subscapularis, infraspinatus, and teres minor.

Results: Preoperatively, the RCA cohort had significantly shorter muscle lines of action in the posterior, lateral, and anterior deltoid (P < .001), a longer inferior subscapularis (P = .022), and a longer teres minor (P = .001) than the GHOA cohort. ANOVA and post-hoc analysis showed that postplanning lengths of each deltoid action line were greater in the 155 + 0 configuration compared to the 135 + 8 configuration in the RCA cohort (P < .001, P = .003, P = .032, respectively), and postplanning lengths of the anterior and middle deltoid action lines were also greater for the same comparison in the GHOA cohort (P = .004 and P = .017, respectively). There were no significant differences in postplanning deltoid lengths between the 135 + 8 and 145 + 4 configurations in either diagnosis cohort (P > .05). All postplanning rotator cuff muscle lengths (subscapularis, infraspinatus, and teres minor) differed significantly (P < .001) between all prosthesis configurations in both diagnosis cohorts, with the 135 + 8 configuration resulting in the longest lengths and the 155 + 0 configuration resulting in the shortest lengths.

Conclusion: Automated preoperative planning software calculates the lengths of muscle action lines, which vary between GHOA and RCA diagnoses. Varying rTSA implant geometries result in predictable differences in deltoid lengthening and rotator cuff shortening. Shoulder prostheses with a more lateralized center of rotation show greater rotator cuff muscle length and similar deltoid muscle length when compared to medialized designs with similar deltoid lengthening. Surgeons can use this software to understand the impact of implant geometry on muscle length.

Keywords: Reverse total shoulder arthroplasty; deltoid length; glenohumeral osteoarthritis; prosthesis design; rotator cuff; rotator cuff arthropathy; surgical planning.