Computer Aided Design and Computer Aided Manufacturing of Foot Orthoses

Timothy B. Staats, M.A., C.P.
Michael P. Kriechbaum, B.A.

According to a 1983 Gallup Organization Survey entitled "Public Attitudes Toward Footcare," 75% of all Americans over the age of 18 say their feet hurt, and 62% think this condition is "normal."¹ With increased interest in running and walking sports activities there has been an increased awareness by patients and athletes that foot orthoses can help correct foot problems and assist in maintaining comfort during a variety of activities. The typical laboratory specializing in foot orthoses can be inundated with plaster casts of patients' feet. Volumes of plaster casts range from approximately 300 per month for a smaller laboratory to 9,000 for the largest. The casts are stored for future use and become a storage problem due to the very high volume this service typically provides (Figure 1) .

Cast impressions of feet taken by podiatrists, orthotists or ped-orthotists are usually either taken in plaster (Figure 2) or with the use of a foam impression block (Figure 3) into which the patient presses the foot. This impression would normally be filled with plaster of Paris, creating a master model which would be smoothed and corrected according to instructions taken at the time of casting. This process of expansion and correction differentiates each laboratory and is an essential component in determining proper patient fit as well as patient acceptance. The recent development and application of computer-aided-design and computer-aided-manufacturing (CAD-CAM) technology to foot orthotics has successfully been demonstrated and is a commercially viable method for replacing most of the normal manufacturing stages.

The finished cast impression of the foot is digitized using a wand digitizer. Essentially, the digitizing stylus or wand, working through magnetic dissonance, signals to the computer its position in space as it touches the inner surface of the cast impression. The location in space of the surface of the cast impression is transmitted to a computer file assigned to each patient. The digitization of the cast takes approximately three minutes (Figure 4 and Figure 5 ). The digital data is then expanded and corrected by the computer using a special computer program developed by American Digital Engineering.² This program was designed using cast modification parameters developed specifically for foot orthoses in conjunction with noted Podiatrists and technical assistance from the UCLA Prosthetics Education Program. The software or computer program allows over 35 different modifications in contour of the actual foot, and over 40 different modifications in the actual orthotic design. The modification or cast correction can be done automatically using pre-specified parameters or can be altered on an individual basis on the computer screen using a "menu driven" command system. The finished and modified electronic master model of the orthosis is then displayed on the computer monitor for review (Figure 6) . The completed orthosis can be viewed from a cross-sectional perspective or in a three-dimensional perspective. The three-dimensional view of the finished orthosis can be viewed from eight stan- dard views, but can also be altered by increasing relative viewing pitch (Figure 7) .

When the computer corrected orthosis is confirmed to be acceptable in the order review process, the completed digital data file is sent to a numerically controlled (computer driven) milling machine. The milling ma- chine cuts out the completed orthosis from a solid block of material according to the information received from the computer. A wide variety of materials can be used to create orthoses including: polyethylene, polypropylene, cork, Korax, sponge rubber, rubber crepes, and foamed plastics. Since this system creates completed orthoses by operating in "batch mode" (more than one function), each block yields approximately five different individual pairs of orthoses. This batch processing allows economies-of-scale production and significantly reduces the cost of prescription/custom orthoses. When the top surface milling has been completed on a plate of orthoses the material block is flipped over and the bottom surface is milled, completing the orthoses (Figure 8) .

The milled foot orthosis shown in Figure 9 shows the precision which is possible after one pass of the milling machine. Note the fine mill lines that have resulted from this process. Many laboratories will now be able to offer texturization as an option to control shear and slippage commonly found with traditional foot orthoses. Texturization can be set as fine as .25 millimeters and as wide as 10 millimeters. The orthosis may then undergo a brief further finishing (if required) which can yield a number of different devices, including a solid plastic shoe insert (Figure 10) ; a semi-rigid foot orthotic device covered in leather or Naugahyde with Neoprene overlay (Figure 11) ; or even a milled piece of foamed flexibile plastic such as Plastozote, Pelite or Evazote.

This technology has been tested and imat Paragon Podiatry Laboratories in Whittier, California³ over the past 18 months. A second system has been recently implemented at Custom Support Technologies.⁴ The foot orthotics market potential demand is such that this application of CAD/CAM is an important step in increasing speed of production, while increasing the accuracy of the orthotic devices fabricated.

Initial studies show that orthosis rejection has been reduced from an average of 3% to less than 1% and that production time has been reduced by over 50%. This reduced production time has resulted in reduced cost of orthotic devices. Initial estimates have seen a 25% to 50% cost savings. This cost savings is being translated into higher profitability for the fabrication laboratories. The ease of use and the increased profitability is creating a demand for the computerized production systems.

Currently under development is a low-cost optic scanning device that will be used in-office by practitioners to obtain optical "castings" (impressions) of patients' feet. These optical casts will be transmitted via computer modem to facilities using the Ortho-Cam production system, completely eliminating the need for traditional cast impression techniques. While a tremendous amount of progress has been made in this area, speed of development will be contingent upon acquisition of additional research funding. Information related to both hardware, software, or central fabrication possibilities of this technology is available from the authors.

Timothy B. Staats, M.A., C.P., Adjunct Assistant Professor of Orthopedic Surgery, Director of UCLA Prosthetics Orthotics Education Program, 1000 Veteran Avenue, Room 22-46, Rehabilitation Center, Los Angeles, California 90024.

Michael P. Kriechbaum, B.A., Vice President, Orthopedic Division, American Digital Engineering, 1901 Avenue of the Stars #1774, Century City. California 90067; (213) 553-3555.

References:

Public Attitudes Toward Foot Care 1983 Gallup Organization Survey for American Podiatry Association, 1983.
American Digital Engineering, 1901 Avenue of the Stars #1774, Century City, California 90067; (213) 553-3555
Paradon Podiatry, 12612 Penn Street, Whittier, California 90602; (213) 698-1444.
Custom Support Technologies, 2529 Maroon Belle Road, Chino Hills, California 91709; (213) 696- 4122.