Introduction: A sensor that detects changes in prosthetic socket fit before they manifest clinically may be helpful towards rehabilitation after limb amputation.
Materials and Methods: Participants with transtibial amputation walked on a treadmill wearing a motor-driven, cabled-panel adjustable socket. Socket volume was slowly adjusted away from the neutral socket volume (panels flush). The percent socket volume change at which the practitioner, using visual assessment, first noted fit issues was compared with the percent socket volume change at which the sensor first detected a measurable change in fit.
Results: The median percent socket volume change at which the sensor first detected a measurable socket enlargement (0.50%) was significantly less than both the median percent socket volume change at which the research practitioner first noted a change in fit (1.69%) (P = 0.008) and the median percent socket volume change at which the participant would normally add a sock (2.30%) (P = 0.005). During socket reduction, the practitioner noted a change in fit before the participant would normally remove a sock in only 2 of 10 participants. The median percent socket volume change at which the sensor first detected a measurable socket reduction (−0.44%) was significantly smaller in absolute value than the median percent socket volume change at which the participant would normally remove a sock (−2.76%) (P = 0.005).
Conclusions: The sensor detected controlled socket fit changes before the research practitioner in this study. A next step will be to determine if the sensor detects unforced socket fit changes, and changes in user free-living environments. (J Prosthet Orthot. 2021;33:3–10)
Quality of socket fit is commonly acknowledged by both patients and prosthetists as the most important aspect of a prosthesis. Changes in socket fit over the course of the day are considered to be caused primarily by changes in volume of the residual limb, Socket fit changes are problematic because they distract the user, cause discomfort, and may require the prosthesis to be doffed to accommodate (e.g., change sock ply). Many prosthesis users avoid the inconvenience of removing their prosthesis to change socks and instead tolerate the poor fit. This behavior may cause discomfort that reduces activity or may lead to tissue injury or gait instability and a fall.
Several researchers are pursuing development of sockets that automatically adjust to maintain fit when a limb volume change occurs, thus relieving prosthesis users of the burden of accommodation strategies. Since the pioneering work of Greenwald et al., who in 2003 described a passive system to adjust socket volume using a series of bladders inside the socket, researchers have pursued various powered systems. Actuators used include liquid-filled bladders or coiled tubes, air-inflated bladders, size-adjustable mechanical patches and wafers, sockets with movable panels or struts, and elevated vacuum sockets. Sensors are incorporated into the socket, quantifying the socket fit in real time and using that for feedback to an onboard computer to control the actuator and adjust the socket. Ideally, these socket-fit sensors should be more sensitive than the prosthesis user, detecting changes in fit before the onset of discomfort. They should also detect if the residual limb is loose or tight in the socket, thus indicating the type of adjustment needed. To date, sensors used to accomplish this objective have measured primarily limb pressure and limb-to-socket distance within the socket. Researchers have conducted controlled studies in laboratory settings on people with limb amputation demonstrating changes in pressure and distance upon activation of actuators. Using a visual analog scale, Ogawa attempted to identify a pressure threshold for pain, but there was considerable variability among participants, making it difficult to specify an appropriate threshold pressure for use in automated control. Intermittently adjusting the actuator in response to the sensed variable to maintain clinical socket fit has yet to be accomplished.
The purpose of this study is to extend evaluation of distance sensing for use in automated socket control by comparing signals from inductive distance sensors with practitioner and participant assessments of socket fit. We conducted controlled socket volume adjustments away from users' preferred socket volume, and assessed if the sensor detected fit changes before the practitioner and participant noted fit issues from enlargement or reduction.