Effects of carpal tunnel syndrome on reach-to-pinch performance

Abstract

Background: Carpal tunnel syndrome (CTS) compromises fine sensorimotor function during activities of daily living. Reach-to-pinch for a small object requires not only dexterity of the grasping digits, but also coordinated transport of the hand to the target. This study examined the effects of CTS on the kinematic performance of reach-to-pinch maneuver.

Methods: Eleven CTS subjects and 11 able-bodied (ABL) controls donned markers for motion capture of the hand, thumb and index finger during reach-to-pinch. Subjects were presented with a virtual target they could see without seeing their reaching upper-extremity. Subjects were instructed to reach to and grasp a virtual object as accurately and precisely as possible. Performance was assessed by variability of the movement trajectories of the digits and hand, the accuracy relative to the target, and precision of pinch contact over repetitive trials.

Findings: The CTS group demonstrated significantly increased movement variability in inter-pad distance, joint angles, and transport of the hand compared to ABL controls (p<0.01). CTS subjects also exhibited reductions in accuracy (41%) and precision (33%) of their pinch contact location (p<0.05).

Interpretation: CTS adversely affects the ability to execute the reach-to-pinch maneuver. Reduced performance was shown in terms of increased variability for both grasp and transport and the ability to locate the grasping digits relative to a target-object. These performance indices could be used for diagnostic and evaluative purposes of CTS.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Experimental set-up.

LEFT: Markers utilized for motion tracking and computing digit kinematics RIGHT: Calibration using a digit alignment device .

Figure 2. Subject performs reach-to-pinch maneuver towards “virtual” target with no visual feedback of moving hand and grasping digits.

Figure 3. Inter-pad distance.

LEFT: Inter-pad trajectory over pinch cycle for ABL and CTS subject groups. Solid line denotes mean trajectory, and dotted line denotes variability (±1 s.d.) about mean trajectory. RIGHT: Comparing variability between ABL and CTS groups. Note: ***p<0.001, Distances self-normalized by subject “palm width”.

Figure 4. Digit path-length accumulated over pinch cycle.

TOP: Mean trajectory and variability of thumb path-length for ABL and CTS groups. BOTTOM: Mean trajectory and variability of index path-length for ABL and CTS groups. Note: ***p<0.001, Distances self-normalized by subject “palm width”.

Figure 5. Differences (Δ) in mean-value (M) and variability (V, ±1s.d. about mean trajectory) of mean angular trajectories of digits between ABL and CTS groups.

Figure 6. Hand transport accumulated over pinch cycle.

LEFT: Hand transport for ABL and CTS subject groups. Solid line denotes mean trajectory, and dotted line denotes variability (±1 s.d.) about mean trajectory. RIGHT: Comparing variability between ABL and CTS groups. Note: ***p<0.001, Distances self-normalized by subject “palm width”.

Figure 7. Mean pinch contact location for individual ABL and CTS subjects relative to virtual target, pictured with stand.

Sphere center denotes mean pinch contact location and radius denotes pinch precision (1 s.d.) about that location. Pinch contact spheres shown relative to pinch target silhouette with respect to global coordinate system (X  =  +forward, Y  =  +left, Z  =  +up). Note: Color bar denotes height and transparency overlay indicates perceptive depth for 3-D visualization.