Ultrasonographic Evaluation of the Needle Insertion Site for the Flexor Pollicis Longus Using the Flexor Carpi Radialis Tendon

Objective: In needle electromyography (EMG), the flexor pollicis longus (FPL) muscle is important for diagnosing anterior interosseous nerve syndrome and proximal median neuropathy. However, different methods have been proposed for needle EMG placement in the FPL. This study aimed to determine the optimal needle insertion position for the examination of the FPL via needle EMG under ultrasound guidance.Methods: This study included 15 men (30 upper limbs) and 13 women (26 upper limbs). At the mid-forearm level, the angles between the vertical line descending from the radial side of the flexor carpi radialis (FCR) tendon and radial artery (angle A), flexor digitorum profundus 1 (angle B), and median nerve (MN) (angle C) were measured. Angle D was defined as the average of angles A and B.Results: The median value of angle D was –19.8°. The probability of radial artery injury at an angle D of 10° was 10.7%. The likelihood of MN injury was 7.1%.Conclusion: Needle insertion through the FCR tendon is relatively safe, although the risk of neurovascular injury remains. Therefore, an ultrasound examination could be considered as an assistive method.