Thumb Stabilization and Assistance in a Robotic Hand Orthosis for Post-Stroke Hemiparesis

Abstract

We propose a dual-cable method of stabilizing the thumb in the context of a hand orthosis designed for individuals with upper extremity hemiparesis after stroke. This cable network adds opposition/reposition capabilities to the thumb, and increases the likelihood of forming a hand pose that can successfully manipulate objects. In addition to a passive-thumb version (where both cables are of fixed length), our approach also allows for a single-actuator active-thumb version (where the extension cable is actuated while the abductor remains passive), which allows a range of motion intended to facilitate creating and maintaining grasps. We performed experiments with five chronic stroke survivors consisting of unimanual resistive-pull tasks and bimanual twisting tasks with simulated real-world objects; these explored the effects of thumb assistance on grasp stability and functional range of motion. Our results show that both active- and passive-thumb versions achieved similar performance in terms of improving grasp force generation over a no-device baseline, but active thumb stabilization enabled users to maintain grasps for longer durations.

Keywords: Physically Assistive Devices; Prosthetics and Exoskeletons; Rehabilitation Robotics; Wearable Robotics.

Fig. 1.

The hand orthosis assists hand-opening for stroke survivors who are unable to actively extend their digits. The active-thumb version has a linear actuator mounted in-line with the thumb’s extension cable (blue). A fixed-length cable replaces the linear actuator in the passive version. Thumb ab/adduction is controlled by an adjustable passive cable (pink) that wraps around the hand to terminate on the splint at the wrist. A padded tube prevents the cable from digging into the skin as it bends around the base of the hand. A motorized winch pulls a separate tendon network (green) to extend the fingers.

Fig. 2.

Dual-cable thumb opposition mechanism. Top: stroke subject demonstrating hand configurations. Bottom: working principle with passive abduction tendon (pink) and active extension tendon (blue). The default position for the paretic hand when both tendons are slack, i.e. disengaged, is a closed fist (Scenario A) as the stroke subject has voluntary flexion but not extension. Without the abduction constraint, active extension pulls the thumb away from the fingers (Scenario B). With a passive abduction tendon, the thumb is stabilized in opposition. The abduction tendon blocks the actuator from overextending the thumb; instead, the thumb rotates about the CMC joint (Scenario C). From this hand position, gradually releasing the actuator under body-powered flexion guides the thumb into a pad-to-pad grasp against the fingertips (Scenario D). When both tendons are completely slack, the thumb buckles against the fingers and returns to the original closed fist. The lengths of the extension and abduction cables are calibrated per subject at the start of the experiment.

Fig. 3.

Healthy-subject demonstration to show index-thumb range of motion supported by the device. Top: photos left-to-right showing fully-closed, half-open, and fully-open hand poses. Bottom: measurements of pad-to-pad distances with three healthy subjects.

Fig. 4.

Photos of subjects completing unimanual pull tasks (A) and bimanual twist tasks (B), along with a photo of the objects used in this study (C).

Fig. 5.

Unimanual-pull peak force (Top) and duration (Bottom) results aggregated across all subjects. Displayed bars indicate median; p-values and bars indicate pairwise differences where significant or equaled α = 0.05. Sample sizes were n = 15 per object, and n = 45 across all objects.

Fig. 6.

Bimanual-twist peak torque (Top; n = 15 per object, n = 45 for all objects) and elapsed time to stabilize (Bottom; total n = [39, 41, 42] for [unassisted, passive, active]) results aggregated across all subjects. Displayed bars indicate median; p-values and bars indicate pairwise differences where significant. Time measurements could not be determined for [3, 1, 1] pill-bottle and [3, 3, 2] marker trials, which were excluded from analysis. Unlike the other performance metrics, elapsed-time evaluates whether assistance facilitated task speed (less time), thus is plotted with a different color-shading scheme.