The objective of this research was to assess the performance of an embedded sensing system designed to measure the distance between a prosthetic socket wall and residual limb. Low-profile inductive sensors were laminated into prosthetic sockets and flexible ferromagnetic targets were created from elastomeric liners with embedded iron particles for four participants with transtibial amputation. Using insights from sensor performance testing, a novel calibration procedure was developed to quickly and accurately calibrate the multiple embedded sensors. The sensing system was evaluated through laboratory tests in which participants wore sock combinations with three distinct thicknesses and conducted a series of activities including standing, walking, and sitting. When a thicker sock was worn, the limb typically moved further away from the socket and peak-to-peak displacements decreased. However, sensors did not measure equivalent distances or displacements for a given sock combination, which provided information regarding the fit of the socket and how a sock change intervention influenced socket fit. Monitoring of limb–socket displacements may serve as a valuable tool for researchers and clinicians to quantitatively assess socket fit.
Introduction
Wearable limb–socket displacement sensors may help patients and prosthetists
identify a deteriorating socket fit and justify the need for repair or
replacement.
Methods
A novel sensor using an inductive sensing modality was developed to detect
limb-to-socket distances. Key detection elements were a coil antenna placed
in the socket wall and a magnetic composite sheath worn over the outside of
the prosthesis user's elastomeric liner. The sheath was a nylon or cotton
prosthetic stocking coated with a polyurethane composite. The polyurethane
composite contained embedded iron particles (75 wt%).
Results
Brushing γ-glycidoxypropyltriethoxysilane onto the sheath fabric, coating it
first with unfilled polyurethane and then iron-filled polyurethane, enhanced
bonding between the sheath and the composite and overcame mechanical
degradation problems. A γ-glycidoxypropyltriethoxysilane-rich fumed silica
layer applied to the outside of the sheath reduced friction and improved
durability. Field testing demonstrated less than a 3% signal degradation
from four weeks of field use.
Conclusions
The developed wearable displacement sensor meets durability and performance
needs, and is ready for large-scale clinical testing.
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