Anatomic localization of motor entry points and accurate regions for botulinum toxin injection in the flexor digitorum superficialis

Purpose: The aim of this study was to determine the motor entry points (MEPs) and the precise intramuscular nerve distribution of the flexor digitorum superficialis (FDS) and to provide accurate injection regions for botulinum toxin. Methods: This study was performed on 46 fresh cadaveric arms with exposed intramuscular innervation of the FDS. For each main motor branch of the FDS, MEPs, where the nerve branch first pierced the muscle belly, and the proximal and distal limit points (PLPs and DLPs, respectively) of the terminal intramuscular nerve endings were located. These data were expressed as relative percentages and absolute distances in relation to the coordinate system, which defined the line between medial and lateral epicondyle of the humerus (ME and LE, respectively) as y-axis and the midpoint of ME and LE as origin. MEP distributions were analyzed using distances measured in tenths of the x and y axes. Results: Two main branches innervated the FDS in 27 cases, the distal main branches of the FDS were classified into three types by origin. For proximal main branches, MEPs were located at x = 19.7% and y = 18.5%, whereas PLPs were located at x = 16.4%, and DLPs were located at x = 37.7%. For distal main branches, corresponding values were 31.6, 5.5, 50.9, and 73.1%. Conclusions: The parameters provided by this study should increase the efficacy and precision of neuromuscular botulinum toxin blockades administered to treat finger spasticity.