Characterizing moment arms of the coracobrachialis and short head of biceps in native shoulders and after reverse total shoulder arthroplasty during elevation and rotation: a modeling study

Background: Reverse total shoulder arthroplasty (RTSA) alters the line of action of muscles around the glenohumeral joint. The effects of these changes have been well characterized for the deltoid, but there is limited information regarding the biomechanical changes to the coracobrachialis (CBR) and short head of biceps (SHB). In this biomechanical study, we investigated the changes to the moment arms of the CBR and SHB due to RTSA using a computational model of the shoulder. Methods: The Newcastle Shoulder Model, a pre-validated upper-extremity musculoskeletal model, was used for this study. The New-castle Shoulder Model was modified with bone geometries obtained from 3-dimensional reconstructions of 15 nondiseased shoulders, constituting the native shoulder group. The Delta XTEND prosthesis, with a glenosphere diameter of 38 mm and polyethylene thickness of 6 mm, was virtually implanted in all the models, creating the RTSA group. Moment arms were measured using the tendon excursion method, and muscle length was calculated as the distance between the muscle's origin and insertion points. These values were measured during 0 & DEG;-150 & DEG; of abduction, forward flexion, scapular-plane elevation, and -90 & DEG; to 60 & DEG; of external rotation-internal rotation with the arm at 20 & DEG; and 90 & DEG; of abduction. Statistical comparisons between the native and RTSA groups were analyzed using 1-dimensional sta-tistical parametric mapping (spm1D). Results: Forward flexion moment arms showed the greatest increase between the RTSA group (CBR, 25.3 & PLUSMN; 4.7 mm; SHB, 24.7 & PLUSMN; 4.5 mm) and native group (CBR, 9.6 & PLUSMN; 5.2 mm; SHB, 10.2 & PLUSMN; 5.2 mm). The CBR and SHB were longer in the RTSA group by maximum values of 15% and 7%, respectively. Both muscles had larger abduction moment arms in the RTSA group (CBR, 20.9 & PLUSMN; 4.3 mm; SHB, 21.9 & PLUSMN; 4.3 mm) compared with the native group (CBR, 19.6 & PLUSMN; 6.6 mm; SHB, 20.0 & PLUSMN; 5.7 mm). Abduction moment arms occurred at lower abduction angles in the RTSA group (CBR, 50 & DEG;; SHB, 45 & DEG;) than in the native group (CBR, 90 & DEG;; SHB, 85 & DEG;). In the RTSA group, both muscles had elevation moment arms until 25 & DEG; of scapular-plane elevation motion, whereas in the native group, the muscles only had depression moment arms. Both muscles had small rotational moment arms that were significantly different between RTSA and native shoulders during different ranges of motion. Conclusion: Significant increases in elevation moment arms for the CBR and SHB were observed in RTSA shoulders; these increases were most pronounced during abduction and forward elevation motions. RTSA also increased the lengths of these muscles. Level of evidence: Basic Science Study; Computer Modeling & COPY; 2023 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.