300 Section - Three-Dimensional Innervation of the Flexor Digitorum Superficialis: Anatomical Implications for Targeted Botulinum Toxin Injection

Botulinum toxin (BoNT-A) injection into flexor digitorum superficialis (FDS) improves wrist and digital function in patients with post-stroke spasticity. Knowledge of the three-dimensional (3D) intramuscular innervation of the proximal and digital bellies of FDS is necessary to identify anatomically optimal sites for BoNT-A injection. The purpose is to: 1) model and map in 3D the intramuscular innervation of the bellies of FDS, 2) identify innervation patterns, and 3) propose sites for BoNT-A injection based on intramuscular innervation patterns.


Design:

Twelve formalin-embalmed forearms (mean age 81.8±9.8years) were dissected, digitized and laser scanned to construct computer 3D models of the volumetric muscle morphology and intramuscular innervation of FDS. 3D models were used to determine the course of the intramuscular branches of the median nerve within the proximal and digital bellies. Innervation patterns were identified and compared between specimens to propose anatomically based BoNT-A injection sites.


Results: The FDS was found to have a two branch (proximal and distal; n=7) or three branch (proximal, middle and distal; n=5) innervation pattern. In the two-branch pattern, the proximal branch directly supplied the proximal belly, and distal branch the 3rd and 4th digital bellies. The 2nd and 5th digital bellies were supplied via intramuscular branches from the 3rd and 4th bellies, respectively. In the three-branch pattern, the proximal branch supplied the proximal belly, middle branch the 3rd digital belly and distal branch the 2nd digital belly. The 4th and 5th bellies were supplied via intramuscular branches from the 3rd and 2nd bellies respectively.

Conclusion:

The intramuscular innervation patterns suggested that anatomically, three injection sites could capture all of the bellies: a proximal site targeting the proximal belly, middle site targeting the 2nd-4th digital bellies, and distal site targeting the 5th digital belly. Clinical study is needed to determine the efficacy of these anatomical findings.