In-silico biomechanical simulation of tendon transfers for finger extension in radial nerve palsy

Tendon transfers are commonly used to restore finger extension in patients with radial nerve palsy, but quantitative comparisons of different techniques remain limited. Achieving the correct tension continues to be a surgical art, guided more by experience than by objective standards. In-silico biomechanical simulation may serve as a valuable tool to evaluate and optimize tendon transfer strategies in a more reproducible and quantitative manner.

We used OpenSim software to simulate four tendon transfers targeting the extensor digitorum communis (EDC): flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), 4th flexor digitorum superficialis (FDS), and combined 3rd/4th FDS. Tendon paths were modeled to reflect surgical anatomy, and post-transfer tension was adjusted to maintain each donor tendon's native tension. We also proposed optimal tensioning, defined as the overlap length allowing maximal actin-myosin contraction. Metacarpophalangeal (MCP) joint extension moments were compared before and after applying the calculated optimal overlap lengths.

At maximum MCP extension, FCU, FCR, and 3rd/4th FDS restored 77.1%, 75.0%, and 75.8% of the original EDC moment, respectively (4th FDS alone: 30.7%). After applying optimal overlap lengths (4.3 mm for FCR, 14.1 mm for FCU, and 9.8 mm for FDS), extension moments increased to 88.0% for FCU, 79.9% for 3rd/4th FDS, 76.6% for FCR, and 32.1% for 4th FDS.

FCU was the most effective donor tendon, especially after tension optimization. In-silico simulation enables objective comparisons and may guide intraoperative tensioning. Further validation in cadaveric or clinical settings is warranted.

Level of Evidence: V, Biomechanical Study.