Changes in the carpal tunnel due to action of the flexor tendons: visualization with magnetic resonance imaging.

Successive cross-sectional areas (CSA) of the carpal tunnel were measured with the fingers in both extension and full flexion in 12 healthy volunteers using magnetic resonance imaging. During flexion, lumbrical muscles could be observed to move into the carpal tunnel up to different levels in all volunteers. For each of the volunteers, the level of the hook of hamate was used as the reference level. The mean CSA measured at this level was considerably larger in flexion than in extension: 191 mm2 (SD, +/- 26) and 169 mm2 (SD, +/- 15), respectively (p = .004). In three volunteers, no difference in CSA between extension and flexion was measured at the hamate level, despite the presence of lumbrical muscles, whereas in these same volunteers at levels more distal, the CSA clearly increased during flexion. The mean CSA for extension and flexion distal and just proximal to the smallest level differed significantly, but the absence of expansion was noticed only at the smallest level. Other changes that were frequently observed during flexion were fat compression, flattening and displacement of the median nerve, and pressure on the superficial and deep flexor tendons.