Development and pilot testing of HEXORR: hand EXOskeleton rehabilitation robot

Abstract

Background: Following acute therapeutic interventions, the majority of stroke survivors are left with a poorly functioning hemiparetic hand. Rehabilitation robotics has shown promise in providing patients with intensive therapy leading to functional gains. Because of the hand's crucial role in performing activities of daily living, attention to hand therapy has recently increased.

Methods: This paper introduces a newly developed Hand Exoskeleton Rehabilitation Robot (HEXORR). This device has been designed to provide full range of motion (ROM) for all of the hand's digits. The thumb actuator allows for variable thumb plane of motion to incorporate different degrees of extension/flexion and abduction/adduction. Compensation algorithms have been developed to improve the exoskeleton's backdrivability by counteracting gravity, stiction and kinetic friction. We have also designed a force assistance mode that provides extension assistance based on each individual's needs. A pilot study was conducted on 9 unimpaired and 5 chronic stroke subjects to investigate the device's ability to allow physiologically accurate hand movements throughout the full ROM. The study also tested the efficacy of the force assistance mode with the goal of increasing stroke subjects' active ROM while still requiring active extension torque on the part of the subject.

Results: For 12 of the hand digits'15 joints in neurologically normal subjects, there were no significant ROM differences (P > 0.05) between active movements performed inside and outside of HEXORR. Interjoint coordination was examined in the 1st and 3rd digits, and no differences were found between inside and outside of the device (P > 0.05). Stroke subjects were capable of performing free hand movements inside of the exoskeleton and the force assistance mode was successful in increasing active ROM by 43 +/- 5% (P < 0.001) and 24 +/- 6% (P = 0.041) for the fingers and thumb, respectively.

Conclusions: Our pilot study shows that this device is capable of moving the hand's digits through nearly the entire ROM with physiologically accurate trajectories. Stroke subjects received the device intervention well and device impedance was minimized so that subjects could freely extend and flex their digits inside of HEXORR. Our active force-assisted condition was successful in increasing the subjects' ROM while promoting active participation.

Figure 1

Pictures of a hand in HEXORR at different postures. (A) The hand flexed. (B) Palmar view of the hand in extension, highlighting the Velcro strap arrangement. (C) The hand extended, with the thumb in pure extension and (D) the hand extended with the thumb in abduction.

Figure 2

Linkage motion simulation and force analysis. (A) Finger and (C) thumb motion simulation with the initial flexion position linkage configurations bolded and the thumb linkage's slider shaft is shown as a dotted line (green). Finger and thumb images are superimposed at the flexed and extended positions. (B) For the fingers, mechanical advantage is output torque at the drive shaft joint that is aligned with the MCP divided by the input force located at the contact point between the linkage and the DIP joints. (D) For the thumb, mechanical advantage is the torque at the CMC joint divided by the force at the thumbtip. The x-axis of these plots is the angle of the driver link relative to the fully flexed initial position.

Figure 3

Assistance condition illustrations. (A) An example of the motor current needed to passively stretch a stroke subject's fingers, compared to gravity compensation. X-axis is the MCP extension angle relative to the fully flexed position. (B) Block diagram of the compensation provided for the active-unassisted and active-force assisted conditions. Stiction is provided when -0.1°/sec ≤ angular velocity ≤ +0. 1°/sec. Otherwise, kinetic friction compensation is provided.

Figure 4

Unimpaired subjects' ROM. The mean values of the unimpaired subjects' (A) MCP, (B) PIP and (C) DIP joints for digits 2-5 under 3 conditions: passive stretch, active-unassisted movements inside HEXORR and active movement outside of the exoskeleton. For the first digit, the joints are the (A) CMC, (B) MCP and (C) IP. Twelve of the fifteen tested joints showed no significant ROM differences between active movements outside and inside HEXORR.

Figure 5

Joint-pair coordination plots for an unimpaired subject. Plots display the 1^st digit (A) CMC-MCP pair (B) MCP-IP pair and 3^rd digit (C) MCP-PIP pair and (D) PIP-DIP pair (mean ± standard error). Paired t-tests indicate no significant differences between trajectories performed inside and outside of the exoskeleton. All joint angles are measured relative to the initial fully flexed posture of the hand.

Figure 6

Stroke subject performance. (A) Finger MCP ROM and (B) mean torques and (C) thumb CMC ROM and (D) mean torques are shown for both the active-unassisted and active force-assisted conditions. The provided assistance increased finger active ROM by 43% and reduced finger extension torque by 22%. For the thumb, active ROM was increased by 24%, reducing thumb extension torque by 30%. For the thumb, the mean torque for Subject 4 and 5 were negative. This indicates that the assistance forces were too high and extended the thumb

Figure 7

Extension movement performed by Subject 2. Flexion motion was halted by the motors and the subject was able to relax the flexors and then further extend the hand's digits.