P. Convery

Calibration problems encountered while monitoring stump/socket interface pressures with force sensing resistors: Techniques adopted to minimise inaccuracies

Force sensing resistors (FSR) have been used to measure dynamic pressures at the interface between appliance and patient. Inaccuracies using FSRs have been reported. This paper summarises both the calibration problems encountered and the techniques adopted to minimise inaccuracies. It is considered that, by calibrating the transducers attached to the socket, and by adopting a strict test protocol, FSRs may provide a guide to the dynamic pressure distribution applied to the trans-tibial stump during gait.

Socket/stump interface dynamic pressure distributions recorded during the prosthetic stance phase of gait of a trans-tibial amputee wearing a hydrocast socket.

Force sensing resistors (FSR) have been used to measure dynamic stump/socket interface pressures during the gait of a trans-tibial amputee. A total of 350 pressure sensors were attached to the inner wall of a hydrocast socket. Data were sampled at 150Hz during approximately 0.8 seconds of prosthetic stance of gait. The dynamic pressure distributions within a hand cast socket reported by Convery and Buis (1998) are compared with those monitored within a hydrocast socket for the same amputee. The pressure gradients within the hydrocast socket are less than that of the hand cast Patellar-Tendon-Bearing (PTB) socket. The proximal “ring” of high pressure in the hand cast PTB socket is replaced with a more distal pressure in the hydrocast socket.

Ultrasound study of the motion of the residual femur within a trans–femoral socket during gait

This study analyses the motion of the residual femur within a transfemoral socket during gait using ultrasound data from two simultaneously transmitting transducers connected to two ultrasound scanners. Calibration tests accurately established the orientation of the two transducers mounted on the lateral wall of the socket. Relative positions of the ultrasound image of the femur were measured on video playback. Motion of the residual femur, relative to the lateral wall of the socket, at any instant during gait may be estimated, if the relative positions of the two transducers and the motion of the ultrasound image are known. A consistent pattern of femoral motion during 10 gait cycles is displayed graphically. The femoral motion in this paper is expressed as abduction/adduction or flexion/extension relative to the socket. However, without a full gait analysis study, the orientation of the socket relative to the ground or relative to the pelvis cannot be determined. Only one ultrasound scanner may be available for clinical use. Hence data collection may be restricted to only one transducer during gait. In order to simulate the single transducer mode, the ultrasound data recorded during the 10 previous gait cycles, was averaged at any instant of the gait cycle. The angular orientation of the femur was calculated based on the averaged data. Similar patterns of femoral motion were obtained irrespective of the technique adopted.

Conventional patellar-tendon-bearing (PTB) socket/stump interface dynamic pressure distributions recorded during the prosthetic stance phase of gait of a transtibial amputee

Force sensing resistors (FSR) have been used to measure dynamic stump/socket interface pressures during the gait of a trans-tibial amputee. A total of 350 pressure sensor cells were attached to the inner wall of a patellar-tendon-bearing (PTB) socket. Data was sampled at 150 Hz during the approximate 0.8 seconds of prosthetic stance of gait. A total of 42,000 pressures were recorded during a single prosthetic stance. This paper describes the distribution of the pressure patterns monitored during the prosthetic stance phase of gait.

Test apparatus for the measurement of the flexibility of ankle-foot orthoses in planes other than the loaded plane

Previous publications have reported on the flexibility of ankle-foot orthoses (AFO) only in the same plane as the applied load. This paper reports on a test apparatus developed to detect the flexibility of an AFO in 5 degrees of freedom when subjected to a plantar/dorsiflexion moment, a medial/lateral moment or a torque. A moment applied to an AFO in one plane induces angulation and translation in all planes.

Measurement of the consistency of patellar-tendon-bearing cast rectification

The quality of fit of a trans-tibial patellar-tendon-bearing (PTB) socket may be influenced by consistency in casting, rectification or alignment. This paper quantifies, for the first time, the variations in the rectified casts between two experienced prosthetists and the variation between the rectified casts of each individual prosthetist. Prosthetists A and B observed the hand casting of a typical trans-tibial amputee. Each prosthetist was supplied with 5 previously measured duplicated plaster models. The two prosthetists rectified the supplied plaster models based on their own interpretation of basic rectification guidelines. Both prosthetists operated in isolation. The re-measured rectified plaster model data was compared with the unrectified data. The extent of rectification at each of 1800 locations per plaster model was calculated. In zones of major rectification, the mean difference between prosthetists was quantified as 2mm and the standard deviation (SD) about that mean was ±1mm for each prosthetist. The co-ordinates of the apex of the fibular head for the 10 modified casts indicated that the maximum variation was in the axial direction with a SD of 4.3mm for prosthetist A and a SD of 2.8mm for prosthetist B. The lengths of the 5 plaster models rectified by prosthetist A indicated a SD of 0.2mm whereas the lengths of the 5 plaster models rectified by prosthetist B indicated a SD of 2.9mm.

The calibration of ultrasound transducers used to monitor motion of the residual femur within a trans–femoral socket during gait

During gait the motion of the residual femur within a transfemoral socket may be estimated using video recorded data from two ultrasound transducers attached to the socket wall. This paper reviews possible measurement errors and identifies the magnitude of the inaccuracies. Inaccuracies due to equipment limitations and those due to human error are identified and quantified. Ranges of flexion/extension and abduction/adduction of the residual femur within the socket during gait have been estimated with a cumulative level of inaccuracy of <1°.

A three centre study of the variability of ankle foot orthoses due to fabrication and grade of polypropylene

This study investigates if fabrication techniques employed at different orthotic centres affect the characteristics of the manufactured plastic orthoses. Plaster models were formed from the same master mould. The thickness and bending stiffness of the supplied polypropylene sheets were measured prior to fabrication. An orthotic technician at each of the 3 orthotic centres manufactured 4 homopolymer and 4 copolymer polypropylene ankle-foot orthoses (AFOs), following each centres fabrication practice. Another technician at one of the orthotic centres manufactured an additional 4 homopolymer and 4 copolymer AFOs. The thickness, the dorsiflexion stiffness and plantar-flexion stiffness of the 32 fabricated AFOs were monitored and compared. Analysis of the results suggests: Copolymer polypropylene sheets are supplied marginally thicker than homopolymer polypropylene sheets The difference between the thickness of the 16 copolymer and 16 homopolymer AFOs was not significant. The thickness of the AFOs manufactured in copolymer was less consistent than homopolymer. Dorsiflexion stiffness of the copolymer AFOs was less consistent than homopolymer AFOs. Although the bending stiffness of the copolymer and homopolymer sheets differed significantly, there was no significant difference between the dorsiflexion stiffness of the copolymer and homopolymer AFOs. Plantarflexion stiffness was consistent for both the copolymer and the homopolymer AFOs and there was no significant difference between the plantarflexion stiffness of the copolymer and homopolymer AFOs. The thickness and flexural stiffness of the AFOs manufactured by 2 technicians at the same centre did not differ. These results are useful benchmarks for the flexural stiffness of AFOs.

Pilot study: Data‐capturing consistency of two trans‐tibial casting concepts, using a manikin stump model: A comparison between the hands‐on PTB and hands‐off ICECAST compact® concepts

The quality of fit of a transtibial patellar tendon bearing (PTB) socket may be influenced by consistency in casting, rectification or alignment. For this study two distinctive different datacapturing concepts were tested in relation to prosthetist performance. The handson PTB and handsoff ICECAST compact® concept were studied and compared for inter and intraprosthetist consistency using a specially designed manikin stump model. A purpose designed digitiser was used to scan a selected surface area of the produced models, 5 for each concept, 10 in total. The extent of casting consistency at each of 936 locations per plaster model was calculated and the level of consistency was quantified. This study has shown that by using the manikin model there is a clear indication that the investigated handsoff concept produces more consistent results than the handson concept.

Ultrasound study of the motion of the residual femur within a trans-femoral socket during daily living activities other than gait

This study analyses the residual femur motion of a single amputee within a trans-femoral socket during a series of daily living activities. Two simultaneously transmitting, socket mounted transducers were connected to two ultrasound scanners. Displacement measurements of the ultrasound image of the femur were video recorded and measured on “paused” playback. Abduction/adduction and flexion/extension of the residual femur within the socket at any instant during these activities were estimated, knowing the relative positions of the two transducers and the position of the residual femur on the ultrasound image. Consistent motion patterns of the residual femur within the trans-femoral socket were noted throughout each monitored daily living activity of the single amputee studied. Convery and Murray (2000) reported that during level walking, relative to the socket, the residual femur extends 6° and abducts 9° by mid-stance while flexing 6° and adducting 2° by toe-off. Uphill/downhill, turning to the right and stepping up/down altered this reported pattern of femoral motion by approximately 1°. During the standing activity from a seated position the femur initially flexed 4° before moving to 7° extension, while simultaneously adducting 6°. During the sitting activity from a standing position the femur moved from 7° extension and 6° adduction to 3° flexion and 1° abduction. The activity of single prosthetic support to double support introduced only minor femoral motion whereas during the activity of prosthetic suspension the femur flexed 8° while simultaneously adducting 9°. Additional studies of more amputees are required to validate the motion patterns presented in this investigation.