Instrumented socket inserts for sensing interaction at the limb-socket interface

Abstract

The objective of this research was to investigate a strategy for designing and fabricating computer-manufactured socket inserts that were embedded with sensors for field monitoring of limb-socket interactions of prosthetic users. An instrumented insert was fabricated for a single trans-tibial prosthesis user that contained three sensor types (proximity sensor, force sensing resistor, and inductive sensor), and the system was evaluated through a sequence of laboratory clinical tests and two days of field use. During in-lab tests 3 proximity sensors accurately distinguish between don and doff states; 3 of 4 force sensing resistors measured gradual pressure increases as weight-bearing increased; and the inductive sensor indicated that as prosthetic socks were added the limb moved farther out of the socket and pistoning amplitude decreased. Multiple sensor types were necessary in analysis of field collected data to interpret how sock changes affected limb-socket interactions. Instrumented socket inserts, with sensors selected to match clinical questions of interest, have the potential to provide important insights to improve patient care.

Keywords: CAD/CAM; Interfacial sensors; Lower-limb amputee; Prosthetic socket; Residual limb; Socket fit.

Conflict of interest statement

Competing interests: None declared.

Copyright © 2017 IPEM. Published by Elsevier Ltd. All rights reserved.

Figures

Fig. 1. Instrumented insert installed into the enlarged socket

(A) Close-up of socket interior, arrows pointing to 1 sample sensor of each type (not pictured: posterior proximal FSR). (B) Completed test prosthesis, with wires and data acquisition device (DAQ) protected by medical wrap.

Fig. 2. Proximity sensor don/doff test

(A) Proximity sensor locations in socket. (B) Representative data set from 1 cycle, after processing mean values at each don/doff condition. Threshold of 63,000 shown to indicate which sensors were considered ON versus OFF. (C) Following threshold application, ON/OFF status of each sensor during each condition.

Fig. 3. FSR variable weight-bearing test

(A) FSR placement in socket and expected high vs low loading at each location. (B) Representative pressure waveform of 1 cycle. (C) Test results for three FSRs (mean +/− SD). Posterior Proximal not included since it was unloaded during this test. NWB = No Weight-Bearing; QWB = Quarter WB; EWB = Equal WB; FWB = Full WB.

Fig. 4. Inductive sensor pistoning test

(A) Inductive sensor placement at distal end, shifted slightly anteriorly. (B) Raw data of entire test after conversion to mm. Key measures illustrated (Peak, Trough, and Pistoning Amplitude). (C) Pistoning amplitude during each 90s walk with varying sock thicknesses shows distinct decreases as thickness increased, and increased as thickness decreased.

Fig. 5. Field data

(A) Proximity sensors for don and doff status during field test. Black and white triangles indicate self-reported sock changes and self-reported start-of-day, respectively. High sensor values match closely with self-reported doff data. (B–D) Sensor measurements 10 min before and after each of 3 sock changes during field test, indicated by circles. (B) 1 to 2 ply sock change induced an increase in proximity of sock to socket and increased liner distance from the socket, but did not noticeably change pressure. (C) 2 to 3 ply sock change did not induce significant changes in sensor measurements at the distal end, but the mid limb proximity sensor measured a distinct increase in proximity of the sock to the socket wall. (D) 3 to 4 ply appears similar to B, with a notable pressure decrease following the change.