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

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.

Automatic Control of Prosthetic Socket Size for People WithTranstibial Amputation: Implementation and Evaluation.

OBJECTIVE: The purpose was to design, implement, and test a control system for a motor-actuated, cable-panel prosthetic socket that automatically maintains socket fit by continuous adjustment of the socket size. METHODS: Sockets with motor-driven adjustable panels were fabricated for participants with transtibial amputation. A proportional-integral control system was implemented to adjust socket size based on Socket Fit Metric (SFM) data collected by an inductive sensor embedded within the socket wall. The sensed distance was representative of limb-to-socket distance. Testing was conducted with participants walking on a treadmill to characterize the system's capability to maintain a set point and to respond to a change in the set point. RESULTS: Test results from 10 participants with transtibial amputation showed that the Integral of Absolute Error (IAE) to maintain a set point ranged from 0.001 to 0.046 mm with a median of 0.003 mm. When the set point was changed, IAE errors ranged from 0.001 to 0.005 mm, with a median of 0.003 mm. An IAE of 0.003 mm corresponded to approximately a 0.08% socket volume error, which was considered clinically acceptable. CONCLUSION: The capability of the control system to maintain and respond to a change in set point indicates that it is ready for evaluation outside of the laboratory. SIGNIFICANCE: Integration of the developed control system into everyday prostheses may improve quality of life of prosthesis users by relieving them of the burden of continually adjusting socket size to maintain fit.

Fluid Volume Management in Prosthesis Users: Augmenting Panel Release with Pin Release.

BACKGROUND: Management of fluid in the limbs is a challenge faced by people with disabilities. In prosthetics, a means for transtibial prosthesis users to stabilize their residual limb fluid volume during the day may improve socket fit. OBJECTIVE: To determine if releasing the panels and locking pin of a cabled-panel adjustable socket during socket release significantly improved limb fluid volume recovery and retention over releasing the panels alone. DESIGN: Repeated-measures experiment to assess the effects on limb fluid volume retention. SETTING: Participants were tested in a laboratory setting while walking on a treadmill. INTERVENTION: Release of a locking pin tether during sitting as a limb volume accommodation strategy. MAIN OUTCOME MEASURE: Percent limb fluid volume retention for panel and pin release compared with panel release alone at 2 minutes (short term) and 50 minutes (long term) after subsequent activity. Limb fluid volume was monitored using bioimpedance analysis. RESULTS: Median percent limb fluid volume retention for the panel and pin release was significantly greater than panel release alone for both anterior and posterior regions for the long term (P = .0499 and .0096, respectively) but not the short term (P = .0712 and .1580, respectively). CONCLUSION: Augmenting panel release with pin release may be an effective accommodation strategy for prosthesis users with transtibial amputation to better retain limb fluid volume.

Thin Magnetically Permeable Targets for Inductive Sensing: Application to Limb Prosthetics.

The purpose of this research was to create a thin ferrous polymer composite to be used as a target for inductive sensing in limb prosthetics. Inductive sensors are used to monitor limb-to-socket distance in prosthetic sockets, which reflects socket fit. A styrene-ethylene-ethylene/propylene-styrene (SEEPS) polymer was mixed with iron powder at three concentrations (75, 77, 85 wt%), and thin disk-shaped samples were fabricated (0.50, 0,75, 1.00 mm thickness). For 85 wt% samples of 0.50 mm thickness, which proved the best combination of high signal strength and low target volume, inductive sensor sensitivity ranged from 3.2E5 counts/mm at 0.00-1.00 mm distances to 7.2E4 counts/mm at 4.00-5.00 mm distances. The application of compressive stress (up to 425 kPa) introduced an absolute measurement error of less than 3.3 µm. Tensile elasticity was 282 kPa, which is comparable to that of commercial elastomeric liners. Durability testing in the shoe of an able-bodied participant demonstrated a change in calibration coefficient of less than 3.8% over two weeks of wear. The ferrous polymer composite may facilitate the development of automatically adjusting sockets that use limb-to-socket distance measurement for feedback control.

Evaluation of Force Sensing Resistors for the Measurement of Interface Pressures in Lower Limb Prosthetics.

Understanding the pressure distributions at the limb-socket interface is essential to the design and evaluation of prosthetic components for lower limb prosthesis users. Force sensing resistors (FSRs) are employed in prosthetics research to measure pressure at this interface due to their low cost, thin profile, and ease of use. While FSRs are known to be sensitive to many sources of error, few studies have systematically quantified these errors using test conditions relevant to lower limb prosthetics. The purpose of this study was to evaluate FSR accuracy for the measurement of lower limb prosthetics interface pressures. Two FSR models (Flexiforce A201 and Interlink 402) were subjected to a series of prosthetic-relevant tests. These tests included: (1) static compression, (2) cyclic compression, and (3) a combined static and cyclic compression protocol mimicking a variable activity (walk-sit-stand) procedure. Flexiforce sensors outperformed Interlink sensors and were then subjected to two additional tests: (4) static curvature and (5) static shear stress. Results demonstrated that FSRs experienced significant errors in all five tests. We concluded that: (1) if used carefully, FSRs can provide an estimate of prosthetic interface pressure, but these measurements should be interpreted within the expected range of possible measurement error given the setup; (2) FSRs should be calibrated in a setup that closely matches how they will be used for taking measurements; and (3) both Flexiforce and Interlink sensors can be used to estimate interface pressures; however, in most cases Flexiforce sensors are likely to provide more accurate measurements.

An Inductive Sensing System to Measure In-Socket Residual Limb Displacements for People Using Lower-Limb Prostheses.

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.

Development of a magnetic composite material for measurement of residual limb displacements in prosthetic sockets.

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.