CARPAL TUNNEL PRESSURE: EFFECTS OF WRIST FLEXION/EXTENSION

P.J. Keir (1), J.M. Bach (1), J.W. Engstrom (2) and D.M. Rempel (1)
(1) Ergonomics Program, Department of Medicine,
University of California, San Francisco, Richmond, CA 94804
(2) Department of Neurology, University of California,
San Francisco, San Francisco, CA, 94143

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

The relationship between wrist angle and elevated carpal tunnel pressure (CTP) has been reported for the end ranges of flexion and extension as far back as 1947 (Brain et al. 1947). However, the dose-response relationship of wrist angle to CTP has not yet been determined. This study examined, in detail, the relationship between wrist flexion-extension angle and CTP by simultaneous measurement in 20 healthy volunteers.

REVIEW AND THEORY

Carpal tunnel syndrome (CTS), the most common upper extremity neuropathy in the workplace, has been strongly associated with a sustained elevation of carpal tunnel pressure (Szabo & Gelberman, 1987). Awkward work postures are commonly reported in the workplace and may lead to sustained elevated CTP.

The relationship between wrist posture and CTP has typically been examined using a minimum of data points, i.e. neutral wrist posture, end range flexion, and end range extension. These postures are not often reached in daily activities and do not allow determination of a dose-response relationship.

There are several factors which have the potential to increase CTP. Cadaveric research has demonstrated that the movement of the lumbricals can increase pressure by occupying space within the carpal tunnel (Cobb et al., 1994) and that muscles of the forearm have a large effect on carpal tunnel pressure by acting on and within the carpal tunnel (Keir et al., 1995). Static fingertip loading has also been shown to increase CTP (Rempel et al., 1994).

From our external view point , one can not ascertain the cause of a given increase in CTP, the pressure may be elevated from its minimum or baseline level due to muscle activity above and beyond the increase due solely to postural changes. In this paper, we address the response of CTP to a simple active wrist flexion-extension task and discuss the nature of the response.

PROCEDURES

Twenty subjects who had no signs, symptoms or electrodiagnostic evidence of CTS took part in this study. An 18 gauge needle was inserted at a 45?angle about 5 mm proximal to the distal wrist crease. A saline-solution-filled 20-gauge catheter with 3 side holes was threaded through the needle into the carpal tunnel, the needle was removed, and the catheter was secured with a suture. A biaxial wrist goniometer (Penny & Giles, Santa Monica, CA) was used to collect simultaneous wrist angle data. For all motions, the metacarpophalangeal joints were flexed to 45?as previous research in our lab determined that the lowest CTPs were found in that posture. After determining the wrist posture of lowest CTP (cf. Weiss et al., 1995), the subject moved their wrist through a comfortable range of flexion-extension motion, keeping the motion in the other planes constant. Each test started from the position of lowest CTP, then the subject slowly flexed the wrist, slowly extended the wrist, and then the cycle was repeated, finishing at the posture of lowest CTP.

The data were analyzed at increments in wrist angle of ten degrees. Based on previous studies evaluating the goniometer, the data were averaged for each wrist angle plus or minus two degrees. Only cycles that were continuous from full flexion to full extension, and vice versa, were included in the analysis (three values at each angle for each subject). For each subject, the minimum, mean, and maximum pressures at each wrist angle were calculated from the three passes of each angle. Group means and standard errors of the mean of each of these values (minimum, mean, and maximum) were subsequently calculated.

RESULTS

The curves from individual subjects indicated that some subjects had substantial differences in CTP within the same trial (see Figure 1). These increases typically occurred at the end of the range of motion of flexion and/or extension and were, in some instances, up to 50 mmHg.

Figure 2 shows the study grand means of the minimum and maximum pressures seen for each subject at each wrist angle, as well as the mean of all three trials per subject. Thus the "minimum" curve on Figure 2 represents the mean of the each subject's lowest CTP at each wrist angle. These values are on the order of 10-15 mmHg lower than the "maximum" data points which reflect the CTPs of highest contamination of other factors.

Figure 1. Data for an individual subject showing the differences in the three passes in a single trial. (Each trial took approximately 30 seconds to complete)

Figure 2. Grand means of the minimum, mean and maximum CTPs found at each posture. (n= 20). Standard error bars have been included for the mean CTP data.

DISCUSSION

This study sheds new light on the relationship between posture and pressure. During the analysis of the data it became apparent that the CTPs were not identical when measured at the same wrist angle at different times. Our technique of simultaneous posture and CTP measurement allowed us to examine the differences found in successive wrist movements which have not been reported previously.

The curve in Figure 2, identified as "minimum" represents what we consider to be the best estimate of the posture-CTP relationship. Because we know of only pressure elevating mechanisms intrinsic to the hand/wrist system, we must attribute these minimal pressures to posture alone. The increase above the minimum can be attributed to other factors within the tunnel (e.g. muscle/tendon tension). Thus the mean and maximum represent the combined effects of posture and muscle tension.

The time course of the pressure changes (as seen in Figure 1) was a matter of seconds, thus factors which may alter pressure over time cannot be deemed as plausible for explaining the changes. As the changes occurred typically near the end of the subject's unresisted range of motion or as the direction of wrist motion changed, it is plausible that changes in muscle activity and thus force may be responsible for the large changes in CTP. Okutsu et al. (1989) demonstrated that CTP is increased by grip force, extension force, and flexion force, the latter two being corroborative to our data. The increases in CTP variance were found to be greatest at the end ranges of motion (as seen in Fig. 1 for an individual and in Figure 2 for between subjects). This could be caused by a cocontraction associated with the change in direction of wrist motion.

The carpal tunnel pressures in Figure 2 appear lower than previous studies. The range of motion seen in this study was much less than in other studies thus the data are not directly comparable. The nature of the protocol used here was to elicit the lowest pressures possible, that is, to limit the activity of the musculature beyond that required to move the wrist in order to characterize the pressure changes in functional wrist postures.

This study has used a large number of healthy wrists to examine the effects of posture on CTP. We have determined a dose-response profile that we feel just in attributing to posture alone. We have also demonstrated what can be considered typical contamination due to intervening factors by also quantifying the highest CTPs found in each trial.

REFERENCES

Brain et al., The Lancet, 1, 277-282, 1947

Cobb, T.K. et al.., J. Hand Surg. 19B, 434-438, 1994.

Okutsu, I. et al., J. Bone Joint Surg. 71A, 679-683, 1989.

Keir, P.J. & Wells, R.P. ASB 1995, pp. 129-130, 1995.

Rempel, D. et al. In: Proceedings of the 40th Annual Meeting of the Orthopaedic Research Society, 1994.

Szabo, R.M. and Gelberman,R.H., J. Hand Surgery, 12A(5), 880-884, 1987

Weiss, N.D. et al, J. Bone Joint Surg , 77A(11), 1695- 1699, 1995.