Effects of carpal tunnel syndrome on dexterous manipulation are grip type-dependent

Abstract

Carpal tunnel syndrome (CTS) impairs sensation of a subset of digits. Although the effects of CTS on manipulation performed with CTS-affected digits have been studied using precision grip tasks, the extent to which CTS affects multi-digit force coordination has only recently been studied. Whole-hand manipulation studies have shown that CTS patients retain the ability to modulate multi-digit forces to object mass, mass distribution, and texture. However, CTS results in sensorimotor deficits relative to healthy controls, including significantly larger grip force and lower ability to balance the torques generated by the digits. Here we investigated the effects of CTS on multi-digit force modulation to object weight when manipulating an object with a variable number of fingers. We hypothesized that CTS patients would be able to modulate digit forces to object weight. However, as different grip types involve the exclusive use of CTS-affected digits ('uniform' grips) or a combination of CTS-affected and non-affected digits ('mixed' grips), we addressed the question of whether 'mixed' grips would reduce or worsen CTS-induced force coordination deficits. The former scenario would be due to adding digits with intact tactile feedback, whereas the latter scenario might occur due to a potentially greater challenge for the central nervous system of integrating 'noisy' and intact tactile feedback. CTS patients learned multi-digit force modulation to object weight regardless of grip type. Although controls exerted the same total grip force across all grip types, patients exerted significantly larger grip force than controls but only for manipulations with four and five digits. Importantly, this effect was due to CTS patients' inability to change the finger force distribution when adding the ring and little fingers. These findings suggest that CTS primarily challenges sensorimotor integration processes for dexterous manipulation underlying the coordination of CTS-affected and non-affected digits.

Conflict of interest statement

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

Figures

Figure 1. Grip device and experimental variables.

Panel A shows the front view of the grip device used for each grip type condition. All dimensions shown are in cm. Force/torque (F/T) sensors are mounted on both sides of the device to measure forces and moment of forces exerted by each digit involved in a given grip type condition (thumb, index, middle, ring, and little fingers: T, I, M, R, and L, respectively). A mass (“G”; 100 g or 400 g) was inserted in the midpoint at the bottom of the grip device (“Light mass” and “Heavy mass”, respectively). For the 2- and 4-digit grip types, the center of the thumb sensor was collinear with the center of the index or middle finger sensor, respectively. For the 3- and 5-digit grip types, the center of the thumb sensor was collinear with the mid-point between the centers of the index and middle finger or middle and ring finger sensors, respectively. Note that for each grip type, all five sensors were mounted on the grip device to maintain a constant mass for a given mass condition, thus changes in grip types were implemented by changing the relative position of F/T sensors. A magnetic tracker (not shown) was used to track the object position and orientation of the object during the manipulation. ‘O’ denotes the point about which moments were computed (see Methods for more details). Panel B shows, from top to bottom, the time course of the sum of all digit grip forces (FG), the sum of all digit tangential forces (FT), and the derivatives of FG and FT. Data are aligned with object lift onset (vertical dashed line, a). Note that analysis of digit forces during object hold was performed on data averaged over the last 2 seconds of the hold (horizontal bar, b). Data are from one representative CTS patient (S3) and her matched control (left and right column, respectively) performing the task on the seventh trial for the “light mass” condition and two grip types (two- and five-digit, 2D and 5D, respectively).

Figure 2. Time course of grip force as a function of grip type in CTS and controls.

The time course of grip force (FG) from contact to object release is shown for a representative CTS patient (S3) and her control subject (left and right column, respectively) for the light and heavy mass conditions (top and bottom row, respectively). Data are from the last trial of each block performed with each grip type (two-, three-, four- and five-digit grasps are denoted by 2D, 3D, 4D, and 5D, respectively) and are aligned relative to object lift onset (vertical dashed line).

Figure 3. Grip force at object lift onset and during object hold.

Grip force (FG) measured at object lift onset and during object hold (top and bottom row, respectively) is shown for each subject group and grip type. Data are mean values averaged across trials 3 through 7 for each subject group and mass condition (left and right column). Vertical bars denote standard errors.

Figure 4. Individual digit normal forces at object lift onset.

The normal force exerted by each digit at object lift onset is shown for each grip type, mass, and subject group (CTS and controls on the left and right column, respectively). Two-, three-, four- and five-digit grasps are denoted by 2D, 3D, 4D, and 5D, respectively. Data are mean values averaged across trials 3 through 7 for each subject group.

Figure 5. Compensatory moment at object lift onset and its components.

The absolute value of the moment exerted on the object (Mcom) at object lift onset is shown for the light and heavy mass (left and right column, respectively) for each grip type and subject group. Data are mean values averaged across trials 3 through 7 for each subject group. Vertical bars denote standard errors.