In this study, the reference line was established considering the direction of the FDS and MBP locations of the first and second main branches were identified. The MBPs of the first and second main branch are located at approximately 40% and 70% of the reference line, respectively.
Most previous studies presenting injection points for FDS used forearm length as a reference line. Bickerton et al. identified individual digastric muscle bellies of the FDS and recommended these bellies as optimal sites for botulinum toxin injections [6]. This study found two injection points by focusing on the digastric muscle belly of FDS but could not provide an accurate injection location based only on one-dimensional information from the forearm length as a reference line. In addition, the muscle belly was suggested as the injection point, which was not based on the location of the intramuscular nerve ending,
To increase efficacy [7–9] and minimize side effects, such as paralysis of the adjacent muscles [10], the best place for botulinum toxin injections is considered to be close to the motor end plate, where the neuromuscular junction is located. In addition, targeting not only motor points but also muscle innervation areas may optimize the effectiveness of botulinum toxin injection [11, 12]. To that end, a precise knowledge on the anatomical location of intramuscular nerve endings is needed.
Some studies have recommended injection sites based on the location of intramuscular nerve endings in the FDS, but still only suggested one-dimensional locations based on the forearm length. Won et al. identified intramuscular the nerve distribution patterns of FDS using Sihler’s staining, finding two main nerve branches [13]. The entry points of each branch were 18.1% and 35.9% from the interepicondylar line. Compared to the present results, the location of each branch resulted from the proximal branch whereas that of the distal branch did not. Furthermore, Lepage et al. identified that FDS is innervated with two or three branches of the median nerve [14]. They demonstrated that end twigs of the proximal branch were located between 27.4% and 47.8% along the forearm length, those of the middle branch 44.8% and 63%, and those of the distal branch between 63.5% and 72%. The proximal and distal branches were located similarly to those in this study. However, they suggested that the penetration points of the nerve branches spread along the lateral edge of the muscle and that these sites were appropriate as the injection points. These locations are difficult to determine from the surface anatomy.
Only one previous study used the FDS direction as a reference line. Huber & Heck’s atlas suggested the injection into half of the distance between the medial epicondyle and the line connecting the radial styloid process to the ulnar one [15]. They recommended a reference line considering the origin and insertion of the FDS muscle, similar to this research. However, only one point was proposed for botulinum toxin injection; thus, the digastric muscle bellies of FDS were not considered.
A recent study revealed that the muscle belly of FDS for digit 2 is divided into proximal and distal belly and connected via the intermediate tendon [16]. In addition, most of the muscle belly of FDS for digit 5 was present, and it was suggested that the location was on the distal and ulnar side. In this study, we confirmed that the distal bellies for digits 2 and 5 are more distal than the muscle bellies for digits 3 and 4, which maybe the reason why injections are necessary for both, the distal and proximal part of FDS. In particular, given that the index finger is important for fine motor functions such as lateral and tip pinching, the spasticity of this finger must be treated. Therefore, clinicians need to inject botulinum toxin at the site of the distal nerve branch when treating spasticity of FDS.
This study has some limitations. First, the location of the intramuscular nerve ending was visually confirmed, and the pattern and density of this nerve ending could not be confirmed microscopically. However, considering the volume of botulinum toxin typically used in clinical settings, the injection should be sufficiently close to the target point—the motor end plate—even if injected into the visually confirmed intramuscular nerve ending. Second, although two-dimensional information on the injection site was obtained, the depth was not measured. Since the FDS is located in the intermediate layer of the anterior forearm muscle, the appropriate injection depth can be estimated based on the results of a study that determined the location of the FDS muscle based on magnetic resonance imaging and ultrasonography [17]. Thus, for more accurate injections, using electrical stimulation or ultrasonography as guide to determine the depth closest to the motor point is highly recommended. Third, the study has a small sample size, analyzing only 24 arms from 12 fresh adult cadavers. Thus, anatomical variants in FDS muscle bellies and location of main nerve branches may not have been fully elucidated.
Conversely, the strength of this study is that the proximal and distal intramuscular nerve branches were considered when proposing the injection site for botulinum toxin in the FDS. In addition, two-dimensional location information was proposed, with the FDS driving line used as a reference, and the points involved did not deviate greatly from the FDS line; thus, the clinicians can easily identify the points.