Structural Adaptations of the Primate Flexor Carpi Ulnaris After Tendon Transfer

Muscle remodeling occurs after tendon transfer. However, it is not known whether these adaptations are permanent and clinically significant. This study examined the early and late structural adaptations following a standard tendon transfer in a primate model. A flexor carpi ulnaris (FCU) to extensor digitorum communis (EDC) transfer was performed in 8 adult monkeys. A sham operation was performed in the contralateral forearm. Four animals were sacrificed at 5 months (early cohort) and 4 at 16 months (late cohort). The transferred FCU, contralateral FCU, and EDC muscles were removed for analysis. Fiber length (FL), physiological cross-sectional area (PCSA), and gross morphology of the transferred FCU were compared with the contralateral EDC and FCU. In the early cohort, the FL of the transferred FCU was longer than the control FCU and similar to the contralateral EDC. The PCSA of the transferred FCU was lower than that of the control FCU but greater than the control EDC. In the late cohort, the difference in FL and PCSA between the transferred FCU and the control FCU persisted. The PCSA of the transferred FCU was similar to that of the control EDC. The bipennate transferred FCU had also undergone gross morphological changes to resemble the multipennate EDC. This study demonstrates, in a primate model, that the FCU undergoes structural adaptations to resemble the EDC following an FCU-to-EDC transfer. However, these adaptations are incomplete and not sustained over time. This study demonstrates that there is muscle plasticity in tendon transfers in a primate model. However, it is important to match potential donor muscles to the recipient during tendon transfer.