This study aimed to define the precise body surface puncture points and depths for the central region of highest muscle spindle abundance (CRHMSA) in posterior forearm muscles to guide botulinum toxin injections. In a study using 24 cadavers, we employed Sihler's staining for intramuscular nerves, histological quantification for muscle spindles, and computed tomography scanning for CRHMSA localization. Reference lines included the horizontal (H-line, between the humeral epicondyles) and longitudinal (L-line, from the lateral epicondyle to the radial styloid). The PP' line, representing puncture depth, was defined as the distance between the CRHMSA's anterior (P) and posterior (P') skin projections. The CRHMSA was located in the middle of the nerve-dense region (INDR) for the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, abductor pollicis longus, extensor indicis, and supinator, and in the upper portion of the INDR for the extensor carpi ulnaris, extensor pollicis brevis, and extensor pollicis longus. The H-line coordinates were 14.67 %, 19.61 %, 10.66 %, 37.56 %, 25.75 %, 48.48 %, 39.51 %, 45.86 %, 33.40 %, and 50.80 %, respectively. L-line coordinates were 3.45 %, 39.50 %, 21.61 %, 44.20 %, 53.88 %, 86.39 %, 25.74 %, 37.67 %, 77.51 %, and 69.46 %, respectively. Puncture depths were 9.67 %, 6.52 %, 13.62 %, 10.35 %, 30.48 %, 14.99 %, 22.45 %, 12.65 %, 16.21 %, and 28.44 % of the PP' line, respectively. These findings provide anatomical guidance to enhance the accuracy of botulinum toxin injections for spasticity treatment.
KEY WORDS: Muscles of the compartment of forearm; Intramuscular nerve; Muscle spindle abundance; Botulinum toxin; Target localization.
