Effect of finger position on the median nerve area within the carpal tunnel

An ultrasound imaging study

Carpal Tunnel Syndrome (CTS) is caused by prolonged, abnormal pressure on the median nerve at the wrist as the nerve runs through the carpal tunnel. It is the most common nerve entrapment disorder in the upper extremity, and presents with numbness, tingling, pain and weakness in the hand. The digits most commonly affected are the thumb, index and middle finger, and sometimes on the radial aspect of the ring finger.

Authors of this study performed ultrasound imaging on the median nerve cross sectional area (MNA) to measure what effect finger movement had on the nerve and the cross sectional area.
Previous studies have investigated the association between median nerve compression and change in carpal tunnel pressure during different wrist positions.
However this study solely focuses on measuring the cross sectional area of the median nerve and not the pressure within the carpal tunnel itself.

31 healthy, non symptomatic patients were involved in this study. All patients were right hand dominant, and MNA was measured at the level of hook of hamate. This site is the most restrictive area of the carpal tunnel; therefore compression is expected to be maximal in this area.

Ultrasound images were taken with the patients elbow extended, forearm supinated and the wrist in neutral.
The MNA was measured with the hand in four different hand positions; full finger extension, midpoint finger flexion, full finger flexion, and forceful finger flexion.

Results showed that this technique of measuring the MNA was both reliable and reproducible and demonstrated a significant difference in finger positions and the MNA. The greatest difference in MNA was that found between full finger extension and forceful finger flexion.

Results indicate finger movement directly affects the MNA, decreasing it as the fingers move towards a flexed position. Therefore, suggesting finger flexion may be a contributing factor to median nerve compression.
This also suggests there could be possibility of resisted functional activities, such as gripping, affecting median nerve compression.

Nadar. M.S., Asbeutah. A., and Al-Obaidi. S.
The Journal of Hand Therapy, 2010; 15: 26-30