Design, modeling, and preliminary evaluation of a simple wrist-hand stretching orthosis for neurologically impaired patients

Abstract

This work studies upper-limb impairment resulting from stroke or traumatic brain injury and presents a simple technological solution for a subset of patients: a soft, active stretching aid for at-home use. To better understand the issues associated with existing associated rehabilitation devices, customer discovery conversations were conducted with 153 people in the healthcare ecosystem (60 patients, 30 caregivers, and 63 medical providers). These patients fell into two populations: spastic (stiff, clenched hands) and flaccid (limp hands). Focusing on the first category, a set of design constraints was developed based on the information collected from the customer discovery. With these constraints in mind, a powered wrist-hand stretching orthosis (exoskeleton) was designed and prototyped as a preclinical study (T0 basic science research) to aid in recovery. The orthosis was tested on two patients for proof-of-concept, one survivor of stroke and one of traumatic brain injury. The prototype was able to consistently open both patients' hands. A mathematical model was developed to characterize joint stiffness based on experimental testing. Donning and doffing times for the prototype averaged 76 and 12.5 s, respectively, for each subject unassisted. This compared favorably to times shown in the literature. This device benefits from simple construction and low-cost materials and is envisioned to become a therapy device accessible to patients in the home. This work lays the foundation for phase 1 clinical trials and further device development.

Keywords: Intelligent orthotics; Rehabilitation robotics.

Figure 1.

Stretching orthosis prototype. the photos, clockwise from top left, show the prototype with (a) straps unfastened, (b) straps fastened, (c) flat hand, donned, (d) clenched hand, dorsal view, (e) clenched hand, angled view, and (f) clenched hand, palmar view.

Figure 2.

Inflated orthosis dimensions. The inflated orthosis is on the bottom, and dimensions to certain points are on the top diagram, profile view. The points are: (A) shrink distance of the center due to inflation, (B) the widest point of the hourglass (see Figure 1 a–c), (C) narrowest point of the hourglass (see Figure 1 a–c), and (D) wrist strap. The left end is the distal end, and the right is proximal.

Figure 3.

Testing sequence. This graphic outlines the experimental protocol for subject testing.

Figure 4.

Experimental setup. This block diagram shows the experimental setup. The arrows indicate the flow of power (red, solid), signal (green, dotted), and air (blue, dashed) for device inflation.

Figure 5.

Hand stretching cycle. The first photo shows a spastic hand, and the remaining three show how the hand opens as the orthosis inflates.

Figure 6.

Stretching pressures. These figures show the average air pressure in the orthosis over 10 cycles for (a) the first one-minute interval, and (b) the last (fifth) one-minute interval. Subject 1’s curve is green, and Subject 2’s curve is red. The gray bands indicate ± 1 standard deviation from the means. The standard deviation bands are too small to be seen on the fifth session (b) for both subjects, indicating that consistency increased between the first and last intervals.

Figure 7.

WHO modeling diagram. this diagram shows the 4-link model of the hand, along with states used to represent this system. The inflated orthosis is depicted in red along with the resultant pressure forces acting at the center of each link. The forearm link ( ) is grounded and not shown in this diagram.

Figure 8.

Mechanical hand. This figure shows the phantom hand prototype, constructed of 3D-printed ABS links, connected by revolute joints with torsional springs of known stiffnesses.

Figure 9.

Simulation results of final configuration static matching of the 3D printed mechanical hand. Known spring constants are used to show the predicted and actual mechanical hand configuration. The average rotation error is 4.1 deg (magnitude of 1.2, 3.5, 4.3, and 7.6 deg for each joint, from the wrist to the DIP, respectively).

Figure 10.

Subject dimensions. This figure shows the segment data for a subject’s hand prior to stretching. The black line is a known distance of 100 mm used for calibration, and the red lines indicate link lengths (forearm , dorsal palm , proximal finger segment , middle segment , and final segment ).

Figure 11.

Simulation results of final configuration static matching for a patient’s hand. The controller can drive the simulation (red dashed line) from the starting configuration (black) of the closed hand to the final (blue) configuration by identifying sufficiently accurate joint stiffness values. Here the average rotation error is 0.7 deg (0.2, 1.5, 0.39, and 0.59 deg for each joint, from the wrist to the DIP, respectively).