Effect of Wrist Deviation on Median Nerve Cross-Sectional Area at Proximal Carpal Tunnel Level

IntroductionComputers are widely used in daily work. They have become essential tools in the workplace. Re- petitive and awkward positions are associated with musculoskeletal disorders of the upper extremities among computer users (1). Long hours of com- puter use in daily life have increased the discom- fort levels of the neck, shoulder, elbow, and wrist and hand regions. Studies have shown that ap- proximately 20% of computer users experience musculoskeletal disorders of the upper extremities (2, 3). Intensive use of a computer keyboard and a mouse may increase the risk of carpal tunnel syndrome (CTS), which is one of the most com- monly reported musculoskeletal disorders of the upper extremities, in which the median nerve is affected at the carpal tunnel as a result of nerve compression (1). CTS has a multifactorial etiology and mechanical compression stress on the median nerve is one of the relevant factors.The carpal tunnel is located at the volar wrist and formed by carpal bones as the floor, transverse carpal ligament as the roof and the proximal car- pal tunnel defined by the edge of the transverse carpal ligament at the pisiform level (4). The me- dian nerve and a total of nine tendons enter through the carpal tunnel, which are the flexor policies longus (FPL), four flexor digitorum su- perficialis (FDS) and four flexor digitorum pro- fundus (FDP). The median nerve is located near the flexor retinaculum and is vulnerable to com- pression stress. Wrist movements comprise flex- ion, extension, radial deviation and ulnar devia- tion. Finger dynamic movements were previously shown to impose compression stress on the medi- an nerve from FDS, FDP and FPL gliding motion (5). Dynamic kinematic movements of carpal bones may also influence the carpal tunnel volume during wrist flexion-extension motion (6). There- fore, different wrist angles may cause different levels of compression stress on the median nerve. A comfortable workspace that allows computer users to perform daily work in a good posture is necessary to reduce repetitive strain injuries. How- ever, body posture and joint angles change over time. For example, wrist motions during computer work are dynamic as the wrist changes between flexion and extension simultaneously with radial- ulnar deviation. In a previous study, most com- puter users showed a higher percentage of wrist extension at 20° with 20° ulnar deviation during computer use (7). Therefore, different wrist angles while working may cause different levels of com- pression stress on the median nerve.Ultrasound (US) examination has been widely used in different fields, including outside of medi- cine. In particular, the mechanical compression of carpal tunnel region structures during dynamic wrist motion can be observed by US. Compared with other imaging techniques, US is relatively inexpensive and requires a shorter time to exam- ine the wrist region (1). The objective of this study was thus to investigate the effect of wrist flexion-extension with radial-ulnar deviation on median nerve cross-sectional area (MNCSA) by using high-resolution US images.Materials and MethodsEight right-handed healthy male adults were re- cruited for this study (Table 1). All the partici- pants were free from signs and symptoms of CTS, as indicated by a screening test. Phalen's Test, CTS Tinel test and Boston Carpal Tunnel Ques- tionnaire (BCTQ) were conducted for all partici- pants to rule out CTS (8-10). In total, sixteen wrists were included for median nerve ultrasound (US) examination.US examination was performed with the GE Healthcare Ultrasound System (Model LOGIQ e) with a 5-13 mHz transducer (Model 12L-RS), and a gel pad (thickness 7.0 mm; Sonar Pad, Nippon BXI Inc., Japan) was used as the coupling medium for the US examination.Participants sat upright with the forearm resting on a forearm support on a table during the US examination. A total of nine wrist positions were examined, including WN, WF30°, and WE30°, together with three wrist deviation conditions, namely, without radial deviation (RD) and ulnar deviation (UD), with maximal RD and with maxi- mal UD (Fig. …