Performance of an auto-adjusting prosthetic socket during walking with intermittent socket release

Abstract

Introduction: A challenge in the engineering of auto-adjusting prosthetic sockets is to maintain stable operation of the control system while users change their bodily position and activity. The purpose of this study was to test the stability of a socket that automatically adjusted socket size to maintain fit. Socket release during sitting was conducted between bouts of walking.

Methods: Adjustable sockets with sensors that monitored distance between the liner and socket were fabricated. Motor-driven panels and a microprocessor-based control system adjusted socket size during walking to maintain a target sensed distance. Limb fluid volume was recorded continuously. During eight sit/walk cycles, the socket panels were released upon sitting and then returned to position for walking, either the size at the end of the prior bout or a size 1.0% larger in volume.

Results: In six transtibial prosthesis users, the control system maintained stable operation and did not saturate (move to and remain at the end of the actuator's range) during 98% of the walking bouts. Limb fluid volume changes generally matched the panel position changes executed by the control system.

Conclusions: Stable operation of the control system suggests that the auto-adjusting socket is ready for testing in users' at-home settings.

Keywords: adjustable socket; amputee; control system; interface stress; pistoning; pressure; prosthesis; prosthetic; residual limb; socket fit; socket release; trans-tibial; volume accommodation.

Figure 1.

Instrumented investigational prosthesis. Left: Inside view of socket showing sensors at two posterior mid-limb and one anterior distal location (white arrows). The tether (yellow arrow) connects to a short pin (not shown) that provides suspension. The red and green buttons at the top right are for operation of the powered tether system. Right: Motors supported to frames mounted to the socket move the panels radially inward and outward based on socket fit. The mechanism to control tether length (gray cylinder) is mounted beneath the socket.

Figure 2.

Diagram illustrating the design of the control system. The socket fit metric (SFM) is the mean of the measurements from the two posterior mid-limb sensors. Slopes of the green, red, and blue lines are the plant gain. Deviations from the set point reflect changes in socket fit. An increase in limb fluid volume (“A” to “B”) causes the controller to increase socket size to return to the set point (“B” to “C”). A decrease in limb fluid volume (“C” to “D”) causes the controller to decrease socket size to return to the set point (“D” to “E”). A goal of socket release/relock is to reduce volume loss during sitting, that is, retard the change from “C” to “D.”

Figure 3.

Test protocol. Eight cycles of sit and walk were conducted. The protocol used for cycles 1–4 and 6–8 is shown in the upper figure. The protocol used for cycle 5 is shown in the lower figure. During cycle 5 after the sit, the panels were tightened to the pre-sit socket volume plus 1.0% socket volume. S = Sit, W = Walk.

Figure 4.

Example plots illustrating control system performance during the eight walking bouts. Upper panel: SFM data (black) and control system set point (orange), both in mm. A consistent scale is not used across bouts so that the shapes of the curves are clearly visible. The SFM is closer to the set point at the end of the bout than at the outset, demonstrating that the control system is performing well. Lower panel: IAE in mm plotted over the course of each bout. The shape of the IAE curves over time, a rise and maximum followed by a decrease to a stable value, is typical for a properly functioning engineered control system.

Figure 5.

Integral of absolute error at the end of each bout. Dark bars are from bouts before the 1.0% socket size increase. Light bars are from bouts after the 1.0% socket size increase. Participants are ordered from low to high plant gains. There was no consistent trend across participants of a higher IAE after v. before the 1.0% socket size increase suggesting that control system performance was not sensitive to this perturbation.

Figure 6.

Panel position at the end of each walking bout. Participants #1, #3, and #4 experienced a socket reduction from the beginning to the end of the test session. Participants #2, #5, and #6 experienced a socket enlargement.

Figure 7.

Percent limb fluid volume during stance phase minima for each bout. Results from the anterior region (ant) and posterior region (post) are shown. Participants #1, #3, and #4 lost limb fluid volume over the session while participants #2, #5, and #6 gained. Differences are likely a result of participant physiological characteristics. All participants (#1 to #6) demonstrated an increase in limb fluid volume from the intervention (between bout 4 and bout 5), suggesting a common mechanism. However, the changes in both the anterior and posterior regions for participant #4 and the posterior region for participant #5 were less than for other participants.

Figure 8.

Change in socket comfort relative to cycle 4 for all participants. The central horizontal line represents the reference, that is, the socket comfort during cycle 4 right before the 1.0% relock socket size increase. The distance between adjacent tick marks on the y-axis is a relative socket comfort rating “unit change.” A positive unit change in RSCR is “a little better,” and a negative unit change in RSCR is “a little worse.” Upon the 1.0% relock socket size increase, four participants indicated a worsened RSCR, one no change, and one a more favorable RSCR.

Figure 9.

Relative socket comfort rating (RSCR) v. change in percent fluid volume. Participants who experienced >1.0 change in percent fluid volume between cycles 4 and 5 reported a worsened socket comfort rating while those with a change <1.0% reported no change or a more favorable socket comfort rating.