BACKGROUND:
Irreparable subscapularis (SS) tears alter the dynamic force coupling of the shoulder, resulting in pain, weakness, and impaired shoulder function. Pectoralis major (Pma), Pectoralis minor (Pmi) and Latissimus dorsi (LD) transfers are treatment options for irreparable SS tears, but clinical outcomes vary. The purpose of this study is to compare the biomechanical properties of Pma, Pmi, and LD transfers in a subscapularis deficient shoulder using a computational model.

METHODS:
A computer shoulder model was used to investigate the moment arms of Pma, Pmi, and LD tendon transfers compared to an intact SS. Nine computerized tomography scans from non-osteoarthritic (OA) subjects were utilized. Virtual Pma, Pmi, and LD transfers were performed to the upper border of the SS insertion site on the lesser tuberosity of the humerus. Muscle moment arms were computed for functional motions of 00 - 800 of internal rotation with the arm in 200 and 900 of shoulder abduction and 00 - 1500 of shoulder abduction. The results were compared to those of the native SS moment arms. A repeated measures ANOVA was then performed to determine significant differences.

RESULTS:
Internal rotation moment arms of the transferred Pma and Pmi decreased significantly after 30o and 40o of internal rotation compared to the SS moment arm of the intact shoulder, while the moment arm of the LD transfer more closely mimicked that of the native SS through 0 - 80o of internal rotation. All three tendon transfer configurations demonstrated weak abduction moment arms (7.6 - 8.0mm), comparable to the intact SS (7.8mm), but significantly lower than the intact adductive moment arms of the native Pma and LD (26.8mm and 28.2mm, respectively).

CONCLUSION:
LD transfer most closely approximates the native subscapularis with regard to internal rotation moment arm. However, the LD transfer also showed a reduction in adducting moment arms.





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