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Title: Dynamic interface pressure measurement : comparing two trans-tibial socket concepts
Author: Buis, Arjan W. P.
ISNI:       0000 0001 3508 349X
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 1997
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The aim of this study was to compare two socket designs for the trans-tibial prosthetic case. Pressure distribution studies, comfort and functionality are potential methods of assessing and distinguishing the conceptual differences. This study investigated pressure distribution at the stump-socket interface, using a technique able to give precision over most of the area. Two socket designs were studied, the worldwide accepted "conventional" hand casted socket incorporating Patella Tendon Bearing (PTB) and a "hands off' pressure casting method represented by the University of Strathclyde Hydrocast socket. The sockets used in this study differed in how pressure was applied during casting (uniform pressure versus localized pressure). Investigations of the stump-socket interface conducted in the past were limited due to an inability to accurately monitor interface pressure during gait. There were no transducers that could measure pressure distribution over large areas or identify local pressures in socket regions with localised changes of curvature. The Tekscan pressure measurement system, based on force sensing resistor (FSR) technology was selected for this study due to the following system characteristics: • A complete commercially available pressure measurement system, which incorporates transducers suitable for stump-socket pressure investigations. • The high degree of flexibility of the 0.017 mm thick transducer. • The resolution and sensing surface of a single transducer array ( 96 individual sensors, covering a total sensing area of 15.500 mm2 ).• Relative cheapness of the system and replacement transducers. The accuracy and reliability of similar Tekscan systems has been questioned by researchers using them in insole applications. Although the Tekscan system represents an innovative method of multi-point pressure measurement, a number of potential inaccuracies, typical for FSR technology, were investigated in this thesis and evaluated to confirm the reliability of this technology. The main areas of concern were: • non linearity. • drift. • temperature sensitivity. • dynamic response. • response to shear. • hysteresis. • crosstalk. • sensor wear. • repeatability. • calibration. A series of dynamic and static tests were performed to investigate the main areas of concern. Static tests were discontinued due to unacceptably high drift. Dynamic tests, however, indicated a high degree of repeatability after a preconditioning period of approximately 10 loading and unloading cycles. Equipment and test methods were developed and used to identify the limitations of the selected pressure measurement system. Tests varied from single cell pressure testing by means of indentors to full transducer testing by means of a dynamic pressure rig. Compressive loads were applied to individual sensors of a transducer, using an Instron testing instrument. System behaviour was tested for repeatability, response to curvature, drift, dynamic response and hysteresis. A shear rig was used to investigate the response of the transducer to a variety of shear loads. The dynamic pressure rig developed in this work, was used to calibrate and test the transducers for repeatability and validity. A typical hysteresis error of 18% was noted as well as inaccuracies due to shear (15%) and 3 dimensional curvated areas (50%). Sensor wear and 3D curvature effects were minimised by bonding the transducers to the rigid inner socket wall and calibrating the transducers "in-situ". A gel filled "condom" was fitted into the socket, the brim of which was sealed, and the gel pressurised according to a predetermined dynamic loading sequence. The transducers, when calibrated, demonstrated consistent pressure output irrespective of socket curvature. This developed technique provided acceptable output validity, when subjected to a pressure range between 25 and 200 kPa. An average variation of ±2% and a maximum variation of ±1 0% for any individual sensor in the transducer array was observed. It was confirmed that the inaccuracies of FSR technology must be recognised if fidelity is required. By selective application and by adopting strict test protocols it was possible to minimise inaccuracies to such a level that a satisfactory pressure distribution pattern was monitored. Transducers were attached to the anterior, posterior, medial and lateral walls of both socket designs, with some sensors located at the distal end of each socket. These 4 transducers provided approximately 350 individual sensing cells covering 90% of the load bearing socket area. The 350 sensors were sampled at 150 Hz. for approximately 0.8 seconds of prosthetic stance providing 42,000 pressure results for a single prosthetic step. Gait studies investigated the consistency of the subject's walking performance. This was essential because only two transducers per socket could be recorded for a particular walk. The trans-tibial amputee's preferred walking speed and consistency with his existing prosthesis was verified with respect to walking speed and the ground reaction force (GRF). The GRF was monitored with a force plate located in a 9 metre walkway. The statistical analysis of 15 constitutive walks, with and without a metronome, indicated a highly consistent gait. Similar studies were repeated with both socket designs. Similar statistical results were noted. The test subject's preference for assistance from a metronome, together with an anticipated velocity difference between the two socket designs, supported the recommendation that future test protocol for this subject adopt metronome assisted walking speeds. This pre-determined walking speed, should match the subjects preferred walking speed with his existing prosthesis. A strict pressure study protocol was adopted to optimise accuracy and reliability of the recorded pressure data. Simultaneous data of walking velocity, GRF and pressure data of a maximum of 2 transducers were recorded. This procedure was repeated 15 times monitoring the two transducers attached to the anterior/posterior aspects of the sockets and 15 times monitoring the two transducers attached to the media/lateral aspects of the sockets. Statistical analysis of both force plate and pressure data revealed that they were highly correlated. As a consequence, pressure data obtained from a walk with the transducers placed in the anterior\posterior socket region was combined with pressure data from another walk with the transducers placed in the medial/lateral socket region. This created a complete picture of socket pressure distribution during gait. A 3D computer model was developed which enabled the output from all four transducers, and hence the pressure distribution within the socket, to be displayed at any instant of gait. The computer model also illustrated the line of action of the GRF relative to the socket. Significant differences in pressure distribution and peak pressures were observed for both socket designs. The "Hydrocast" pressure casting concept indicated a 30% reduction in measured pressure compared with the PTB concept. Lower peak pressures were monitored in the pressure cast socket during gait (143 kPa versus 417 kPa. for the PTB socket); fewer localised pressure zones were noted within the Hydrocast socket. A greater number of subjects must be investigated to confirm the apparent effectiveness of the socket designs. The 3D computer model enabled the mass of pressure data to be easily observed. A wide range of future pressure studies may be undertaken using the developed system, equipment, methods and protocols described in this study. The effects of alignment modifications on socket pressure may be reinvestigated now that the total socket may be studied rather than "selected" locations. The variation of socket pressure with respect to time may be studied in conjunction with patient's stump volumes.
Supervisor: Convery, Peter Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Artificial legs; Prosthetics; Prostheses