Cross-sectional area of the carpal tunnel in different wrist positions

The potential for median nerve compression in the carpal tunnel depends on the tunnel's cross-sectional area. Technologies like MRI and ultrasound permit careful measurement of the tunnel's area as the wrist's position changes. In either wrist flexion or extension, the cross-sectional area decreases in some part of the tunnel. The probability of median nerve compression increases as the area decreases. This is especially true when the nerve's cross-sectional area is increased, a finding that is typical when the nerve is inflamed.

The cross-sectional area decreases in some region of the carpal tunnel with either wrist flexion or extension of as little as 45 degrees (Yoshioka, Okuda, Tamai, Hirasawa, & Koda, 1993; Allmann, Horch, Uhl, Gufler, Altehoefer, et al., 1997). Wrist flexion decreases the canal's cross-sectional area at the levels of the hamate and pisiform. When the wrist flexes, the tunnel's width narrows, and its cross-sectional area decreases at both the hook of the hamate and at the pisiform (Horch, Allmann, Laubenberger, Langer, & Stark, 1997).

Wrist extension decreases the canal's cross-sectional area only at the level of the pisiform. The decrease in area is associated with a decrease in its (A-P) depth, even as the canal's width increases. Simultaneously, the canal's cross-sectional area increases at the level of the hook of the hamate (Horch, et al., 1997). Measurements in normal carpal tunnels at the level of the hook of the hamate reveal that mean cross-sectional areas vary between 1.52 cm2 when the wrist is positioned in neutral, 1.75 cm2 when the wrist is extended, and 1.36 cm2 when the wrist is flexed (Skie, Zeiss, Ebraheim, & Jackson, 1990).

Not only does wrist flexion decrease the carpal tunnel's cross-sectional area at both the pisiform and hamate, it "rearranges" the flexor tendons in such a way that they may be more likely to compress the median nerve (Skie et al., 1990). This finding supports use of the modified Phalen's test (Hertling & Kessler, 1996, p. 567) as a provocative maneuver in people with suspected median nerve involvement.

References:

Allmann, K.H., Horch, R., Uhl, M., Gufler, H., Altehoefer, C., Stark, G.B., & Langer, M. (1997). MR imaging of the carpal tunnel. European Journal of Radiology, 25, 141-5.

Hertling, D., & Kessler, R. M. (1996). Management of common musculoskeletal disorders: Physical therapy principles and methods. (3rd ed.). Philadelphia: J.B. Lippincott.

Horch, R.E., Allmann, K.H., Laubenberger, J., Langer, M., & Stark, G.B. (1997). Median nerve compression can be detected by magnetic resonance imaging of the carpal tunnel. Neurosurgery, 41, 76-82.

Skie, M., Zeiss, J., Ebraheim, N.A., & Jackson, W.T. (1990) . Carpal tunnel changes and median nerve compression during wrist flexion and extension seen by magnetic resonance imaging. Journal of Hand Surgery [Am]. 15, 934-9.

Yoshioka, S., Okuda, Y., Tamai, K., Hirasawa, Y., & Koda, Y. (1993). Changes in carpal tunnel shape during wrist joint motion. MRI evaluation of normal volunteers. Journal of Hand Surgery [Br], 18, 620-3.

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