Lumbrical muscle incursion within the carpal tunnel has been implicated as a possible cause of carpal tunnel syndrome. During finger flexion, surgeons have observed the presence of lumbrical muscles in the carpal tunnel. However, the significance of this incursion has not been evaluated. To evaluate the effect of lumbrical muscle incursion within the carpal tunnel as a cause of carpal tunnel syndrome, carpal canal pressures were measured in cadaver hands at the level of the hamate hook for four finger positions: (1) 100% finger flexion; (2) 75% finger flexion; (3) 50% finger flexion; and (4) full extension. After measuring carpal tunnel pressures for each position, the lumbrical muscles were excised and the pressures were again recorded. A progressive increase in carpal tunnel pressure was noted for each degree of finger flexion in the group with intact lumbricals. This is in sharp contrast to a relatively stable carpal tunnel pressure during finger flexion for the group without lumbrical muscles. Two-way repeated measures analysis of variance revealed a significant difference in carpal tunnel pressure for both variables, lumbrical muscles and finger position. One-way repeated measures analysis of variance for carpal tunnel pressures demonstrated that the effect of finger position was significant for the group with intact lumbricals but not for the group with lumbricals removed. We conclude that lumbrical muscle incursion into the carpal tunnel can result in elevation of carpal tunnel pressure in cadaver hands and could be a variable in the cause of work-related carpal tunnel syndrome.
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