Anatomical location of motor branch points in the flexor digitorum superficialis for the injection site of botulinum toxin

Objective: To locate the intramuscular nerve branches of the flexor digitorum superficialis (FDS) and determine the accurate site for botulinum toxin injection. Design This study anatomically dissected 24 arms of 12 fresh adult cadavers to find intramuscular nerve endings in the FDS. The motor branch points (MBPs), proximal limit points (PLPs), and distal limit points (DLPs) of the terminal intramuscular nerve endings were identified. These three parameters were expressed in longitudinal and transverse coordinates in relation to the FDS driving as a reference line. Results The mean length of the reference line was 234.6 ± 11.2 mm. In the longitudinal coordinate, the MBPs, PLPs, and DLPs were located at 41.6% ± 2.6%, 35.1% ± 4.1%, and 53.4% ± 4.6% of the reference line in the first main branch and 72.4% ± 4.5%, 67.5% ± 1.5%, and 82.0% ± 5.7% in the second main branch, respectively. The mean value of the transverse coordinate was not greatly deviated from the reference line. Conclusion The MBPs of the first and second main branches are located approximately 41.6% and 72.4% of the reference line, which considers the FDS direction, respectively. This finding helps determine the optimal injection site for botulinum toxin in the FDS.