Pressure-morphology relationship of a released carpal tunnel

Abstract

We investigated morphological changes of a released carpal tunnel in response to variations of carpal tunnel pressure. Pressure within the carpal tunnel is known to be elevated in patients with carpal tunnel syndrome and dependent on wrist posture. Previously, increased carpal tunnel pressure was shown to affect the morphology of the carpal tunnel with an intact transverse carpal ligament (TCL). However, the pressure-morphology relationship of the carpal tunnel after release of the TCL has not been investigated. Carpal tunnel release (CTR) was performed endoscopically on cadaveric hands and the carpal tunnel pressure was dynamically increased from 10 to 120 mmHg. Simultaneously, carpal tunnel cross-sectional images were captured by an ultrasound system, and pressure measurements were recorded by a pressure transducer. Carpal tunnel pressure significantly affected carpal arch area (p < 0.001), with an increase of >62 mm(2) at 120 mmHg. Carpal arch height, length, and width also significantly changed with carpal tunnel pressure (p < 0.05). As carpal tunnel pressure increased, carpal arch height and length increased, but the carpal arch width decreased. Analyses of the pressure-morphology relationship for a released carpal tunnel revealed a nine times greater compliance than that previously reported for a carpal tunnel with an intact TCL. This change of structural properties as a result of transecting the TCL helps explain the reduction of carpal tunnel pressure and relief of symptoms for patients after CTR surgery.

Figure 1

A schematic of the experimental set-up and a cross-section of the evacuated carpal tunnel with pressure transducer and catheter tubing in the tunnel. A–A′ indicates to the location of cross-section at the hook of the hamate level.

Figure 2

Ultrasound images showing the changes of a released carpal tunnel with separation of the TCL (dotted line) at 10mmHg (a) and 120 mmHg (b) at the level of the hook of hamate (H) and the ridge of the trapezium (Tm). The schematic (c) shows the following carpal arch parameters: carpal arch width (CAW): solid line between the bones, carpal arch length (CAL): dashed line, carpal arch height (CAH): dashed line with arrows and carpal arch area (CAA): gray shading. The carpal arch radius of curvature (CAR), not labeled in this figure, is based on a circular fit of the volar boundary of the TCL.

Figure 3

The volar boundary of the TCL at various pressure levels for a representative specimen.

Figure 4

Change in the carpal arch height with increasing carpal tunnel pressure.

Figure 5

Change in the carpal arch area with increasing carpal tunnel pressure. The linear regression lines provided show that the rate of increasing area changes at 70 mmHg (dashed line).