Home Use of a Neural-connected Sensory Prosthesis Provides the Functional and Psychosocial Experience of Having a Hand Again

Abstract

The loss of a hand has many psychosocial repercussions. While advanced multi-articulated prostheses can improve function, without sensation, they cannot restore the full experience and connection of a hand. Direct nerve interfaces can restore naturalistic sensation to amputees. Our sensory restoration system produced tactile and proprioceptive sensations on the hand via neural stimulation through chronically implanted electrodes. In this study, upper limb amputees used a sensory-enabled prosthesis in their homes and communities, autonomously and unconstrained to specific tasks. These real-life conditions enabled us to study the impact of sensation on prosthetic usage, functional performance, and psychosocial experience. We found that sensory feedback fundamentally altered the way participants used their prosthesis, transforming it from a sporadically-used tool into a readily and frequently-used hand. Functional performance with sensation improved following extended daily use. Restored sensation improved a wide range of psychosocial factors, including self-efficacy, prosthetic embodiment, self-image, social interaction, and quality of life. This study demonstrates that daily use of a sensory-enabled prosthesis restores the holistic experience of having a hand and more fully reconnects amputees with the world.

Figure 1

Take home sensory restoration system. (A) Participants wore their own prosthetic socket with standard agonist/antagonist myoelectric control. They were provided a VariPlus Speed™ prosthetic hand (Ottobock, Vienna, Austria), which had been augmented with Flexiforce™ pressure sensors (Tekscan, Inc., Boston, MA) embedded in silicone in the pads of the thumb, index, and middle fingers. A custom aperture sensor underneath the cosmetic glove encoded the position of the prosthetic hand’s single degree of freedom. The sensor information was sent through a cable to the external nerve stimulator, which converted the sensor information into electrical stimulation pulses. The stimulation traveled to the nerve via percutaneous leads to Flat Interface Nerve Electrodes (FINEs) implanted on the participants’ median nerves. Illustration courtesy of Cleveland FES Center. (B) Image of a subject wearing the sensory restoration system. (C) Study timeline. The study was a three-stage crossover design, in which the subjects used the system at home either without sensation (stages 1 and 3) or with sensation (stage 2). Functional metrics were administered in laboratory testing sessions at the start of the study and after each stage. (D) Study stage durations for subject 1 (left) and subject 2 (right). Subject 2 wore the system for nearly twice as many days per stage as subject 1. Subject 1 experienced 3 days of interruption per stage due to component breakage (see Methods). (E) Locations of sensory percepts associated with each prosthetic hand sensor. Sensation locations reported daily throughout the study were overlaid such that regions of higher opacity were more-frequently reported. (F) Stimulation charge delivered to each channel for subject 2 over the course of the sensory-enabled stage of the study. Participants could calibrate stimulation settings (pulse amplitude and pulse width) whenever they chose. Filled dots are stimulation settings at each recalibration; color corresponds to the percept locations in subpanel E. The slopes of the regressions for the thumb, index, and middle channels (navy blue, teal, and magenta) were not significantly different from zero (regression slope test, p > 0.1 for all). The slope for the aperture channel (brown) was −0.11 nC/hr and was significantly less than zero (p = 0.002).

Figure 2

Impact of sensation at home on prosthesis usage. (A) Daily diary reports of system wear time (S1: n = 6, 7, 7 for stages 1, 2, 3, respectively; S2: n = 11, 13, 14 for stages 1, 2, 3, respectively). (B) Stimulator log of system wear time (S1: n = 5, 4 for stages 1, 2, respectively; S2: n = 12, 13, 15 for stages 1, 2, 3, respectively). (C) Tasks performed with the prosthesis each day out of standard task list based on the UEFS (S1: n = 6, 7, 7 for stages 1, 2, 3, respectively; S2: n = 11, 13, 14 for stages 1, 2, 3, respectively). (D) Active usage of the prosthesis for grasp-related activities that involved triggering the pressure sensors. Raw sensor data was analyzed for peaks in force, which indicated instances that the prosthesis was used to manipulate or grasp objects (S1: n = 5, 4 for stages 1, 2, respectively; S2: n = 9, 13, 15 for stage 1, 2, 3, respectively). All subpanels: Error bars indicate standard error of the mean. Single asterisk indicates significance of p < 0.05. Double asterisks indicate significance of p ≤ 0.001.

Figure 3

Impact of sensation on functional performance within sessions (A–E) and over time (F,G). (A) Impact of sensation on the nine hole peg test. Faster completion times indicate better performance (n = 4 for each condition (sensation on/off)). (B) Impact of sensation on the clothespin relocation task. More clothespins moved indicates better performance (n = 12 for each condition). (C) Impact of sensation on the magnetic table test. Greater numbers of blocks removed in the allotted time (2 min) indicates better performance. Fewer grasping errors and more corrections also indicate better performance (all metrics: n = 12 for each condition). (D) Impact of sensation on the foam block identification test. Greater percentages of blocks identified correctly indicate better performance (n = 8 for each condition). (E) Impact of sensation on the AM-ULA. Higher scores indicate better performance (The AM-ULA was only scored for 3 of 4 testing sessions for each subject (see Methods); n = 3 for each condition). Panels A–E: Two sample t tests were used to compare performance with and without sensation. Single asterisk indicates significance of p ≤ 0.05. Double asterisks indicate significance of p ≤ 0.001. (F) Changes in sensation-enabled functional performance over time for Subject 1 and (G) Subject 2. Data is pooled within sessions (n = 3 for each testing session for the clothespin relocation test and magnetic table test; n = 2 for the foam block test; n = 1 for the nine hole peg test and AM-ULA). For F,G, percentage change due to sensation was calculated as the difference between performance with sensation on and off, normalized by the maximum possible score on the metric (magnetic table test: maximum of 10 blocks; foam block test: maximum of 100% accuracy; AM-ULA: maximum score of 40). The nine hole peg and clothespin relocation test scores were normalized to the maximum score reported across all testing sessions and subjects (see Methods). Points above the black line at zero indicate that performance was better with sensation than without. Points below the black line indicate that performance was worse with sensation than without. All panels: Error bars indicate the standard error of the mean.

Figure 4

Impact of home use with sensation on end of stage psychosocial measures. (A) Patient-specific functional scale (PSFS) scores over time. Higher scores indicate greater perceived ability. (B) QuickDASH disability score over time. Higher scores indicate higher perceived disability. (C) OPUS Quality of life score over time. Higher scores indicate better perceived life. (D) Patient experience scales (long form) scores over time. Scale scores were computed by averaging ratings on items within each scale. Higher ratings indicate better outcomes. Statistical comparisons were made between individual items scores within each scale using paired t-tests. (E) RHI embodiment survey. Agreement ratings on the embodiment-related statements are shown as filled bars. Ratings on control statements are shown as striped bars. Embodiment-related statements were compared to control statements with two-sample t-tests. Error bars indicate standard error of the mean. All panels: Each subject completed each survey once per testing session. Single asterisk indicates significance of p < 0.05. Double asterisks indicate significance of p ≤ 0.001. Double cross indicates change greater than the minimal detectable change (MDC-90) reported for that test.

Figure 5

Impact of sensation on daily psychosocial measures. (A) Patient experience scale (short form) scores by study stage. Higher scale scores indicate better perceived outcomes in each domain (S1: n = 6, 7, 7 for stages 1, 2, 3, respectively; S2: n = 11, 13, 14 for stages 1, 2, 3, respectively). (B) Reported difficulty in task performance by study stage. Each day, participants rated their perceived difficulty in doing each of 28 tasks on a list based on the OPUS UEFS. Higher scores indicate greater perceived difficulty (S1: n = 6, 7, 7 for stages 1, 2, 3, respectively; S2: n = 11, 13, 14 for stages 1, 2, 3, respectively). (C) Perceived phantom limb length by study stage. The difference between the perceived phantom limb length and the contralateral limb length indicates the degree of limb telescoping experienced. Lower values of limb length difference correspond to a more natural phantom limb length and thus less disturbance in body image (S1: n = 6, 7, 8 for stages 1, 2, 3, respectively (n = 8 for stage 3 because we included one report from a day of interruption); S2: n = 11, 13, 15 for stages 1, 2, 3, respectively). Note that subject 1’s residual limb is ~20 cm shorter than his contralateral side, while subject 2’s residual limb is ~35 cm shorter than his contralateral side. All panels: Error bars indicate standard error of the mean. One-way ANOVAs with Tukey pairwise comparisons were used to compare outcomes across stages. Single asterisk indicate significance of p < 0.05. Double asterisks indicate significance of p ≤ 0.001.