Effects of Carpal Tunnel Syndrome on Adaptation of Multi-Digit Forces to Object Weight for Whole-Hand Manipulation

Abstract

The delicate tuning of digit forces to object properties can be disrupted by a number of neurological and musculoskeletal diseases. One such condition is Carpal Tunnel Syndrome (CTS), a compression neuropathy of the median nerve that causes sensory and motor deficits in a subset of digits in the hand. Whereas the effects of CTS on median nerve physiology are well understood, the extent to which it affects whole-hand manipulation remains to be addressed. CTS affects only the lateral three and a half digits, which raises the question of how the central nervous system integrates sensory feedback from affected and unaffected digits to plan and execute whole-hand object manipulation. We addressed this question by asking CTS patients and healthy controls to grasp, lift, and hold a grip device (445, 545, or 745 g) for several consecutive trials. We found that CTS patients were able to successfully adapt grip force to object weight. However, multi-digit force coordination in patients was characterized by lower discrimination of force modulation to lighter object weights, higher across-trial digit force variability, the consistent use of excessively large digit forces across consecutive trials, and a lower ability to minimize net moments on the object. Importantly, the mechanical requirement of attaining equilibrium of forces and torques caused CTS patients to exert excessive forces at both CTS-affected digits and digits with intact sensorimotor capabilities. These findings suggest that CTS-induced deficits in tactile sensitivity interfere with the formation of accurate sensorimotor memories of previous manipulations. Consequently, CTS patients use compensatory strategies to maximize grasp stability at the expense of exerting consistently larger multi-digit forces than controls. These behavioral deficits might be particularly detrimental for tasks that require fine regulation of fingertip forces for manipulating light or fragile objects.

Is the Control of Applied Digital Forces During Natural Five-digit Grasping Affected by Carpal Tunnel Syndrome?

“…However, the ability to control digit forces in the functional grasping configuration has not been reported in very many studies [21,23,29,[42][43][44][45]. We therefore examined the deficiencies in digit force control during five-digit grasping in patients with CTS.…”

“…During the last decade, numerous devices have been used to record the forces applied by the digits during various grasp configurations [1,21,29,39,43,45]. Studies show that patients with CTS exhibit diminished efficiency when pinching objects, implying that the affected digits are unable to accurately adjust the forces applied in response to the given external load [21,29,39].…”

“…Studies show that patients with CTS exhibit diminished efficiency when pinching objects, implying that the affected digits are unable to accurately adjust the forces applied in response to the given external load [21,29,39]. As for five-digit grasping, several studies also examined the altered controls of grasping force with respect to grasping objects with different weights [45], changed center of masses [43], and various textures [1]. These studies indicated that patients with CTS grasped with excessive forces and showed an inability to discriminate weight accurately, performed less modulation of force despite changes in the center of mass, and poorly adjusted the forces applied across various textures.…”

“…Greater variability of digit force was observed in men compared with women [14], in the nondominant hand compared with the dominant [13], and in the elderly (60-69 years) compared with the young (20-29 years) [38]. However, few studies have examined how abnormal sensory functions affect force variability [9,27], especially during five-digit grasping [45]. The synergies among the various roles of the fingers, which are described by the force relationships among thumb-finger pairs [8,24,36], may provide insights into the coordination that occurs among the involved and intact digits of patients with CTS.…”

“…The synergies among the various roles of the fingers, which are described by the force relationships among thumb-finger pairs [8,24,36], may provide insights into the coordination that occurs among the involved and intact digits of patients with CTS. In addition to the altered patterns of grasping force in patients with CTS, the patterns of force applied also depend on the contact positions of the digits during grasping, especially the thumb [8,21,25,43,45]. To assess how the impairments resulting from CTS affect daily grasping, the natural grasping configuration of patients should be considered in an experimental setting [11,30].…”