Localization of the motor entry point of the adductor pollicis muscle using surface landmarks for safe and effective...

Hyperactivity of the adductor pollicis muscle in upper motor neuron lesions can cause a thumb-in-palm deformity, which can be treated with botulinum toxin injection to reduce spasticity. This study aimed to identify reliable surface landmarks for localizing the motor entry point to administer botulinum toxin.

A total of 21 formalin-fixed adult cadaveric hands were dissected. The anatomical landmarks of the hand, such as the styloid process of the radius, first metacarpophalangeal joint, and the longitudinal axis of the shaft of the third metacarpal, were used to create a grid divided into four zones. Morphometric measurements of the hand and adductor pollicis muscle, and distances from the anatomical landmarks to the motor entry point, were recorded using a digital caliper.

In 13 hand specimens, the motor entry point was located between zones 1 and 2 along the longitudinal axis of the shaft of the third metacarpal; in 8 specimens, it was in zone 2, medial to the third metacarpal. The mean distances of the motor entry point from the first metacarpophalangeal joint, styloid process of the radius, midpoint of the two, and the third metacarpal were 13.74 ± 6.56 mm, 48.51 ± 8.23 mm, 17.59 ± 7.18 mm, and 2.61 ± 3.63 mm, respectively. Both the first metacarpophalangeal joint and the midpoint between it and the styloid process of the radius showed comparable proximity to the motor entry point. They remained consistent despite variations in the length of the hand and the muscle.

These observations provide reference anatomical points that may help guide botulinum toxin injections, nerve blocks, and reconstructive procedures; however, their applicability to broader populations requires further validation.