Procedure-Oriented Torsional Anatomy of the Carpal Tunnel

In Brief This is the fourth in a series of papers related to the concept of torsional anatomy. The objective of this paper is to provide musculoskeletal ultrasound anatomy of the carpal tunnel region in the position of hemispastic flexion as a reference for diagnostic and relevant needle procedures.Methods Musculoskeletal ultrasound (MSKUS) images were obtained in a healthy 51-year-old female subject. The MSKUS probe was centered over the carpal tunnel at the level of the proximal and distal flexor retinaculum. A pair of MSKUS images was recorded for each site: (1) elbow flexed, supinated neutral, and open hand; and (2) in hemispastic position with forearm pronation, wrist flexion, and clenched fist. The images were compared for changes. In addition, a video recording was made at each site to track the movement of the artery, tendons, nerve, and muscles passing through or near the carpal tunnel during movement from position 1 to position 2. Standard clinical nerve conductions of the median and ulnar nerve were performed to rule out carpal tunnel syndrome. Results The median nerve at the proximal flexor retinaculum measured 0.08 cm2 in cross-sectional area (CSA) in the anatomic position (normal). Cross-sectional area increased to 0.11 cm2 in the pronated clenched fist hemispastic position (borderline abnormal). It was difficult to maintain the probe in the same position with the wrist flexed. Nerve conduction studies were normal. The lumbricals were not visible with the hand supinated and open but appeared in the distal carpal tunnel region in the hemispastic fist clenched position. Discussion Precaution should be taken in diagnosing carpal tunnel syndrome in nonanatomic neutral positions using CSA values on MSKUS. Possible reasons for the increase in CSA measured are discussed in this article. The images show that the lumbricals may contribute to crowding of the distal carpal tunnel. Conclusions It is hoped that this series of images will increase the accuracy and safety of diagnosis and needle placement for carpal tunnel syndrome in patients with hemispasticity.