INTRODUCTION
Sustained elevated carpal tunnel pressure may lead to the development of carpal tunnel syndrome. The following laboratory experiment was performed in an effort to correlate carpal tunnel pressure with thumb posture.
REVIEW AND THEORY
Carpal tunnel syndrome (CTS) is the most common work-related entrapment neuropathy. There is indirect evidence that the pathophysiologic mechanism of activity-related CTS involves repeated and prolonged elevations of carpal tunnel pressure (CTP)(Szabo & Gelberman, 1987; Fuchs et al., 1991). Elevated pressure leads to ischemia, flexor tendon synovial tissue edema, further elevations in CTP, and slowed nerve conduction.
It has been shown that CTP varies with wrist flexion/extension, ulnar/radial deviation (Weiss et al., 1995), forearm pronation/supination, and metacarpophalangeal angle (Rempel et al., 1995). The current study examines the relationship of CTP to thumb posture.
METHODS
Sixteen volunteers who demonstrated no evidence of carpal tunnel syndrome based on history, examination, and nerve conduction testing participated in this study. Carpal tunnel pressure measurement was accomplished via a saline-filled catheter inserted percutaneously into the carpal tunnel, and attached at the proximal end to an in-line pressure transducer (Rempel et al., 1994). CTP was measured continuously on a digital computer. The transducer was secured to the mid forearm at the same elevation as the carpal tunnel.
After catheter insertion, the subject was methodically directed through a range of wrist and forearm motions to determine the position of lowest CTP (defined as the rest position). The rest position provided the starting and ending point for all subsequent maneuvers.
Beginning in the neutral wrist and forearm position the subject positioned their fingers until an MP angle of 45 degrees was reached. The subject then adducted the thumb and held the posture for 5 seconds. The subject then extended, abducted, opposed, and flexed the thumb sequentially pausing for 5 seconds at each posture. The CTP value stabilized within 1 to 2 seconds after each new position was achieved. The experimenter continuously observed the subject's thumb position and time stamped the data at the beginning and end of each 5 second pause.
A repeated measures ANOVA was performed on the data with a single factor, thumb position, with five levels. Tukey's method of multiple comparisons was performed using the repeated measures results to determine significant differences between the various levels of the factors.
RESULTS
The CTP means and standard errors can be seen in Figure 1. The repeated measures ANOVA revealed that the thumb posture had a significant effect on CTP (p=0.0001). Tukey's method detected significant differences in the CTP for several of the positions (Figure 1).
Figure 1 : CTP means and standard errors. Positions with a common letter (a, b, c) were not significantly different.
DISCUSSION
Carpal tunnel pressure is known to vary with wrist, forearm, and metacarpophalangeal postures (Weiss et al., 1995; Rempel et al., 1995). The results of the current study reveal that thumb posture influences CTP as well. If CTP plays a role in the development of activity-related CTS, then redesigning tools and tasks to minimize CTP should decrease the risk of developing CTS. Based upon the results of this study, tasks requiring prolonged thumb opposition or flexion should be minimized.
REFERENCES
Szabo R.M. and Gelberman R.H., J. Hand Surg., 12a, 880-884, 1987.
Fuchs P.C. et al., J. Hand Surg., 16a, 753-758, 1991.
Rempel D.M. et al., J. Hand Surg., 19a, 106-110, 1994.
Rempel D.M. et al., Proceedings of the 50th Annual Meeting of the Amer. Soc. for Surg. of the Hand, 1995.
Weiss, N. et al., J. Bone Joint. Surg., 77-A(11), 1695-1699, 1995.
ACKNOWLEDGEMENTS
Thanks to Ron Tal for his contributions to this research project. This study was supported in part by grant K01OH--121-01 the National Institution of Occupational Safety and Health of the Centers for Disease Control.
