The influence of wrist posture, grip type, and grip force on median nerve shape and cross‐sectional area

Abstract

During grasping, the median nerve undergoes mechanical stress in the carpal tunnel which may contribute to carpal tunnel syndrome. This study investigated the effects of wrist posture, grip type, and grip force on the shape and cross-sectional area of the median nerve. Ultrasound examination was used to obtain cross-sectional images of the dominant wrist of 16 healthy subjects (8 male) at the proximal carpal tunnel during grasping. The cross-sectional area, circularity, and axis lengths of the median nerve were assessed in 27 different conditions (3 postures × 3 grip types × 3 force levels). There were no significant changes in median nerve cross-sectional area (P > 0.05). There were significant interactions across posture, grip type, and grip force affecting nerve circularity and axis lengths. When the wrist was flexed, increasing grip force caused the median nerve to shorten in the mediolateral direction and lengthen in the anteroposterior direction (P < 0.04), becoming more circular. These effects were significant during four finger pinch grip and chuck grip (P < 0.05) but not key grip (P > 0.07). With the wrist extended, the nerve became more flattened (less circular) as grip force increased during four finger pinch grip and chuck grip (P < 0.04) but not key grip (P > 0.3). Circularity was lower during the four finger pinch compared to chuck or key grip (P < 0.03). The findings suggest that grip type and wrist posture significantly alter the shape of the median nerve. Clin. Anat. 30:470-478, 2017. © 2017 Wiley Periodicals, Inc.

Comparing Shape Categorization to Circularity Measurement in the Evaluation of Median Nerve Compression Using Sonography

“…Regardless of the method used, there is mixed evidence for the correlation between measures of median nerve circularity and finger positions. Cowley and colleagues 15 noted a slight, nonsignificant decrease in median nerve circularity from 0.57 to 0.54 when the hand was in a neutral position versus a pinching position. Using the reciprocal formula, Yoshii and colleagues 19 found a nonsignificant change in nerve circularity that increased from 1.69 to 1.75 when measured in full finger extension to full finger flexion.…”

“…While we explored the usefulness of these shape categorizations for concurrent measures of circularity, we found that subjective categorization provided additional information related to nerve compression. As sonography is being increasingly used to identify risk factors contributing to median nerve injury 15 and for diagnosis of carpal tunnel syndrome, 22,23 these findings may be useful for advancing both research and practice. Circularity is a quantitative method to identify the roundness of an object in a 2D plane.…”

“…This concept has been translated into health care as a means for detecting deformation of the median nerve, with multiple studies using this formula. 10,15,16,24 Others have used a reciprocal version of this formula whereby the perimeter value is placed in the numerator and the area value is placed in the denominator. 18,19 In both methods, a value of 1.0 suggests a perfect circle; however, in the original method, lower values indicate a less circular object as opposed to higher values over 1.0 when using the reciprocal method.…”

“…Rather, the calculation requires additional steps beyond the measures directly reported on the machine or other imaging software. In addition, current studies using circularity have conflicting results, 10,15,[18][19][20] such that median nerves with a similar circularity can exhibit different shapes, which may correlate with different amounts of compression. The median nerve could be compressed by surrounding tendons and structures becoming flattened or irregular in shape during hand movement or have a different circular or ovoid shape than when the hand is at rest due to varied compression by different surrounding tissues (e.g., slightly taller than longer vs. slightly longer than taller).…”

The effect of wrist angle on ulnar nerve appearance at Guyon's canal in asymptomatic individuals utilising high‐resolution sonography

“…Much of the recent literature that employed ultrasound to study the effect of wrist angle on peripheral nerves at the wrist have focussed on the median nerve in the carpal tunnel . This prior research has extended the knowledge base documenting that the neutral wrist position caused lower compression of the median nerve in the carpal tunnel than with flexion or extension .…”