Automated Fabrication of Mobility Aids: Clinical Demonstration of the UCL Computer Aided Socket Design System
David A. Boone, B.S.P.O., C.P.
Ernest M. Burgess, M.D.
Introduction
In 1987, Prosthetics Research Study organized three research projects under the common title Automated Fabrication of Mobility Aids,
AFMA. The term AFMA encompasses the Computer Aided Design and Manufacture of the term CAD/CAM, while recognizing many
kinds of prostheses and orthoses as mobility aids which may be fabricated using computerized systems.
In this paper we present a study of the use of an AFMA system for prostheses being developed at the Bioengineering Centre of
University College London, UCL.1,2 Through clinical experience this project was intended both to guide our future research efforts and provide a forum for introduction of AFMA to American prosthetists.
Four specific goals were formulated for this project:
To use an AFMA system in fitting definitive below-knee prostheses for normal use.
Measure the efficiency of the UCLCASD fitting process in terms of the number of socket iterations necessary to provide a fit satisfactory upon clinical examination and for extended use by the amputee.
Demonstrate the electronic modification of prosthetic sockets held in a computer database, accounting for individual subject variations.
Demonstrate the feasibility of remote manufacturing on the basis of computer data transmitted electronically.
Overall our goal was to demonstrate, and in doing so, we have established a knowledge base of the advantages and weaknesses of an
AFMA system in general. Since it would be counterproductive at this time to over emphasize specific failings of this or any system under
development, only the general results of the socket fittings will be discussed.
Methods
Selection of a subject required that the subject have a below-knee residual limb of at least 24 months post-amputation. All subjects had
to currently wear a prosthesis and be suitable for prescription of a standard PTB socket design.
After inclusion into the study, the fitting of a standard PTB socket proceeded through remote use of the UCL system, (Figure 1)
. The
steps were:
Computer digitization of the residual limb shape of the subject. The digitization could take place using equipment in Seattle or in
London. In either case the limb was digitized from the inner surface of a "passive" plaster wrap cast of the limb.
Remote (London) design of a PTB below knee socket based on the residual limb digitization. The initial socket tested was designed with a standard rectification built into the UCL design software. Modifications to accommodate individual subject variations were not made to the first socket.
Computer controlled manufacturing of a plaster socket model.
Return of the socket model to Seattle via next day air freight.
Fabrication of a transparent plastic check socket over the plaster model.
Clinical prosthetic evaluation of the fit of the check socket under static and dynamic conditions.
Subsequent modifications or definitive socket models which duplicated successful check sockets were requested from London via computer electronic mail.
Results
Thirteen subjects were chosen to participate in this study (Figure 2)
. Through remote use of the computer based design and manufacturing system in London, PRS has been able to
"successfully" fit below-knee prostheses to 11 subjects in Seattle. Successful fittings were those in which the PRS prosthetist evaluated the fit of the prosthesis in the same way as a conventional fitting and deemed it suitable for delivery for permanent use by the subject. The subject then wore the limb for at least
one month of daily use.
In the fitting process, the UCL computer system designed the first socket with limited information about the subject. Specifically, the digitized residual limb shape, location of the mid-patellar
tendon and identification were entered into the computer in London.
The plaster models were not modified in any way. Hand modification of the transparent check sockets was used only as a diagnostic procedure to clarify what changes were to be made
using the computer software. Subsequent modifications to the initial UCL designed socket were requested electronically by PRS prosthetists in Seattle.
All of the specific goals outlined in the introduction have been achieved.
The AFMA techniques were adequate to provide a clinically satisfactory fit in 85% (11 of 13) of our subject population.
The number of iterations of socket designs needed, as modifications were required to provide an adequately fitting prosthesis, averaged 2.89. This does not include the initial UCL CASD standard rectification.
Remote electronic modification of the prosthetic socket design was achieved and proved to be an intuitive process for PRS prosthetists.
Remote manufacturing on the basis of electronically collected, transmitted, and stored data was achieved. In addition to the sockets made using the UCL CASD software, several conventionally hand modified plaster socket models were digitized at PRS and reproduced by the UCL system.
Discussion
In designing this trial, it was difficult to determine the criteria by which to measure our progress. Taking into account the rather general
purpose for this trial, and our desire to keep results clinically relevant to practicing prosthetists, we decided that each fitting would simply
be considered either successful or not successful. It was also decided that to reflect clinical reality, the success of a fit is best based at least
partially on a subjective decision made by the prosthetist responsible for the fitting.
The prosthetists were instructed that each socket should be evaluated on the basis of function and not individual stylistic preferences
of the prosthetist. Comfort being a central component of function, patient feedback was at least indirectly part of deciding if a socket was
successful.
Of those sockets deemed successful, the prosthetists did not note any general preference for either the AFMA prosthesis or the subjects'
previously fit conventional prosthesis. Of the two fittings which were not successful, one subject removed himself from the study due to
personal time conflicts. In the other, the prosthetist was not able to use the design software to accommodate a prominent bone spur
protruding perpendicularly from the distal medial aspect of the subject's tibia.*
* Since the completion of this project, advances in the capabilities of the software have provided the necessary tools to design the required modification to accommodate the bone spur.
** A Computer Aided Socket Design workshop for Prosthetics Research Study was organized at the Medical Engineering
Resource Unit of the University of British Columbia in Vancouver, Canada. During the two-day workshop, a PRS subject was fit with a prosthesis designed using the MERU software.
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Remote use of the UCL software was accomplished. Paper modification forms were provided by UCL which duplicated the com
puter screen. Using these forms, the prosthetist could visualize the changes that would be made on the computer screen in London before
sending them. Remote use of the software was also facilitated by the use of predefined regions of modification in the UCL software. These
regions are defined by their location and contour and could be modified in their degree. For example, the prosthetist could request two
millimeters greater relief over the fibular head by simply requesting "+ 2mm FH". Twelve different parameters of the PTB design could
be altered in this way. While the UCL software lended itself to remote modifications, other systems being developed could have been used
for a clinical demonstration.**
In summary, a computer system for prosthetic socket design and manufacture has proven itself as a workable method for clinical
prosthetics. Focusing on clinical demonstration of the system has quickly illuminated the directions for future research and development
that would be of clinical use. As a result of this experience, Prosthetics Research Study has already engaged in those projects that it is felt
will best help AFMA to enter clinical service.
The use of computer technology in prosthetics and orthotics is accelerating. Office tasks such as accounting or writing letters are
commonly accomplished with a personal computer. Now the same personal computer is proving to be sophisticated enough to handle jobs
in the fabrication lab. This does not mean that the personal computer replaces a trained professional in any way. Clinical experience,
evaluation, and judgement are not reproducible by today's computer. In our experience, the prosthetist has a new and possibly revolutionary
tool for efficiently providing a functional mobility aid.
References:
- Dewar, M, P. Jarman, D. Reynolds, H. Crawford, J. MacCoughlan, J. Wilkinson, and A. Crew, "Computer Aided Socket Design (CASD): UCL System Based on Full Shape Sensing," Bioengineering Centre Report 1985, University College London, 1985, pp. 19-30.
- Dewar, M, J. Wilkinson, P. Jarman, and K. Jones, "Clinical Trial of the UCL Computer Aided Socket Design System," Bioengineering Centre Report 1986, University College London, 1986, pp. 13-16.
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