Device to monitor sock use in people using prosthetic limbs: technical report

Abstract

A device using radio frequency identification (RFID) technology was developed to continuously monitor sock use in people who use prosthetic limbs. RFID tags were placed on prosthetic socks worn by subjects with transtibial limb loss, and a high-frequency RFID reader and antenna were placed in a portable unit mounted to the outside of the prosthetic socket. Bench testing showed the device to have a maximum read range between 5.6 cm and 12.7 cm, depending on the RFID tag used. Testing in a laboratory setting on three participants with transtibial amputation showed that the device correctly monitored sock presence during sitting, standing, and walking activity when one or two socks were worn but was less reliable when more socks were used. Accurate detection was sensitive to orientation of the tag relative to the reader, presence of carbon fiber in the prosthetic socket, pistoning of the limb in the socket, and overlap among the tags. Use of ultra-high-frequency RFID may overcome these limitations. With improvements, the technology may prove useful to practitioners prescribing volume accommodation strategies for patients by providing information about sock use between clinical visits, including timing and consistency of daily sock-ply changes.

FIGURE 1. Sock monitor on a socket

The prototype was mounted on the anterior lateral surface with Velcro straps. The reader antenna (dashed line) was contained with the box in this prototype.

FIGURE 2. System layout

Components were housed within a box mounted to the external socket surface.

FIGURE 3. Unobstructed read distance results

The maximum read distance for the five tags tested was approximately proportional to the surface area of the tag’s antenna.

FIGURE 4. a,b. Fitting test results

(a) Subject #2 – During each of the four testing intervals the subject wore the number of socks indicated (1, 2, 3, or 4). (b) Subject #1 wearing four socks. Seven attempts were made, making adjustments to the antenna or tag positions, to detect all four socks but none were successful. The system was turned off between tests while the subject doffed the socket thus the time axes represent cumulative time during which data were collected.

FIGURE 5. a,b. Activity test results – Subject #1

The subject wore (a) three socks and (b) four socks. The reader well detected all socks when three socks were worn (in a), but failed to detect all socks when four socks were worn (in b).

FIGURE 6. Activity test results – Subject #2

The subject wore three socks during the entire test while he walked on different surfaces, stood, climbed stairs, or sat. The reader well detected sock presence for all activities.