Thermoformed Ankle-Foot Orthoses
Melvin Stills, C.O. *
This work was carried out with partial support of the Rehabilitation Services Administration, Department of Health, Education, and Welfare under Grant #23P-55518.
Clinical and research groups have for many years felt that the use of plastics in orthotics had great potential for the development of improved designs. Attempts to use polyester and epoxy laminates that were adopted by prosthetists in the fabrication of artificial limbs more than 20 years ago largely met with failure when applied to orthoses that were subjected to repeated bending loads, owing no doubt to the relatively short fatigue life of the polyester and epoxy resins.
However, with the availability of thermoplastics such as polyvinyl chloride, polyethylene, and polypropylene in sheet form, and with good resistance to fatigue, a number of new designs emerged in the 1960s  . Most of the new designs were ankle-foot orthoses for use with "drop foot."
In 1970 the Committee on Prosthetics Research and Development identified 11 ankle-foot-orthosis designs for treatment of "drop foot" that seemed to have advantages over the conventional "short-leg brace" which consisted of two metal uprights and two ankle joints, and were attached to a special shoe or to one that was modified extensively. The Committee requested the Rehabilitation Engineering Center at Moss Rehabilitation Hospital to conduct laboratory and clinical studies of each of the 11 designs in order to provide clinicians with guidance in prescribing and applying the various devices.
The 11 orthoses (Fig. 1 ) were studied and the origins of their design are listed on page 42.
Where the need for an orthosis was minimal the VAPC shoe-clasp orthosis was found to be the most acceptable, especially in ease of application and cost . For more severe cases the TIRR molded polypropylene orthosis seemed to have advantage over the other designs. It is light in weight, almost unnoticeable, requires no shoe modifications, is relatively easy to fabricate and fit, and the amount of control provided about the ankle can be varied .
While these studies were being carried out at Moss Rehabilitation Hospital, the Rehabilitation Engineering Center at the Ontario Crippled Children's Centre (Toronto, Canada) was developing methods and equipment for forming sheet thermoplastics for orthotic and prosthetic applications. Because of the encouraging results obtained in the orthosis evaluation project with the TIRR AFO and other molded orthoses, the Rehabilitation Engineering Center at Moss Rehabilitation Hospital decided to extend the clinical study to include vacuum-forming of orthoses.
- AMBRL Posterior-Bar Drop-Foot Orthosis
AMBRL Two-Rod Drop-Foot Orthosis
- U.S. Army Medical Biomechanical Research Laboratory Walter Reed Army Hospital Washington, D.C.
- IRM Spiral Below-Knee Orthosis Posterior Solid-Ankle Orthosis
- Institute of Rehabilitation Medicine, New York University, New York, N.Y.
- NYU Insert Orthosis
- New York University Post-Graduate Medical School, New York, N.Y.
- RANCHO Polypropylene Drop-Foot Orthosis
- Rancho Los Amigos Hospital Downey, California
- TEUFEL Ortholen Drop-Foot Orthosis
- Teufel, 7 Neckarstrasse, 189-191 Stuttgart, West Germany
- TIRR Below-Knee Orthosis
- Texas Institute for Rehabilitation and Research, Houston, Texas
- UC-BL Dual-Axis Ankle Orthosis
- University of California, San Francisco Medical Center, San Francisco, California
- VAPC Shoe-Clasp Drop-Foot Orthosis
VAPC Single-Bar Below-Knee Orthosis
- Veterans Administration Prosthetics Center, New York, N.Y.
Because of delays in making commercially available the equipment designed by OCCC, a machine manufactured by Plastic Vac, Inc., but not designed specifically for orthotics, was purchased by Moss in 1973. Since that time molded lower-limb orthoses have become standard at Moss Rehabilitation Hospital. Development work is continuing but sufficient experience has been gained so that others can use some of the new designs with confidence.
MOLDED ANKLE-FOOT ORTHOSES
The molded ankle-foot orthosis (Fig. 2 ) is formed from a single piece of sheet thermoplastic. Polypropylene is usually the material of choice because of its high resistance to fatigue, low cost, and very light weight. It can be worked easily and can be welded, but will not take adhesives.
Molding is carried out over a positive plaster-of-Paris model of the foot, ankle, and shank, usually with the use of a vacuum system.
Although the molded AFO is a single unit, it has two basic components—the posterior calf enclosure and the foot enclosure (Fig. 3 ).
The perimeter is referred to as the "trimline," and consists of five parts:
- Proximal trimline
- Anterior trimline
- Ankle trimline
- Foot trimline
- Metatarsal trimline
FUNCTION
The function provided by a molded AFO is largely dependent upon the degree of resistance that is provided to rotation about the ankle joint. The resistance to rotation about the ankle joint, or rigidity of the ankle area, for any given material varies with the amount and distribution of material. Obviously, the thicker the walls the more rigid the orthosis will be for a given cross-section geometry. Not so obvious is the effect of the shape of the transverse cross-section.
Because the entire inner surface of the molded AFO is in contact with the limb, the shape of the foot, ankle, and calf of the patient dictates the basic geometry of the transverse cross-section. Everything else being equal, rigidity in the sagittal plane varies inversely with the ankle between the two sides of the orthosis in the ankle area (Fig. 4 ).
The rigidity provided by the shape of the anatomy can be modified by: location of the trimline; thickness of the material; and introduction of corrugations.
Thin walls are generally desirable and the orthotist will try to make the walls as thin as possible. They should be no more than 4 mm. thick at the thickest part, tapering down to 2 mm. However, corrugations are seldom needed because the orthotist can usually obtain the desired amount of flexibility by location of the trimlines. The effect of the orthosis on the patient will also depend upon the location of the weight line (Fig. 5, center), which in turn will depend upon the position of the foot part of the orthosis with respect to the shank part of the orthosis for a given heel height.
The location of the weight line must be considered at the time of casting and not left until the time of fitting with the idea that it can be controlled by heel height changes (Fig. 5 ).
Fig. 5, Fig. , Fig. , Fig. , Fig. , Fig. , Fig. , Fig. , Fig. , Fig. , Fig. , Fig.
THE TRIMLINES
Location of the trimlines will depend upon the results of the assessment of the patient, thus on the patient's individual needs. The following rules apply in general:
- The proximal trimline (Fig. 2 and Fig. 6) is located approximately 3.8 cm. (1 1/2 in.) distal to the head of the fibula so that it will be clear of the peroneal nerve. The proximal trimline should encircle about 3/4 of the calf.
- The anterior trimline extends from the bottom of the junction of the Velcro strap and the orthosis to the midpoint of the shank. The anterior trimline should curve posteriorly as it proceeds distally to the midline of the shank (Fig. 2 and Fig. 6).
- The location of the ankle trimline affects the rigidity about the ankle joint more than any other single factor. Obviously, the further anterior that the trimline is located, the more resistance there is to rotation about the ankle.
- The foot trimline on the medial side extends through or slightly above the apex of the navicular. On the lateral side, the foot trimline extends slightly above the shaft of the fifth metatarsal.
- The metatarsal, or terminal, trimline (Fig. 2 and Fig. 6) lies along the apices of the metatarsal heads.
- All trimlines should be contoured and blended together smoothly. When a trimline is located slightly below a bony prominence, the area often becomes irritated, but when the trimline is located at the level of or slightly above the apex of a bony prominence, pressure areas do not occur as a rule.
Fig. 6
Until more definitive guidelines can be established for arriving at the rigidity desired, the location of the trimlines must be a result of the judgment of the orthotist. Some variations in trimlines to achieve desired effects can be seen in Fig. 7. An attempt is being made to develop more definitive guidelines, especially for teaching purposes. Meanwhile, it is felt that competent orthotists will find that, with a little experience, quite satisfactory results can be achieved.
*Minnesota Mining and Manufacturing Co.
References:
- Committee on Prosthetics Research and Development. Report of seventh workshop panel on lower-extremity orthotics, National Academy of Sciences. May 1970.
- Finley, F. Ray, and Ray G. Wirta. Rehabilitation biomedical engineering: orthotics design, Krusen Center for Research and Engineering, Moss Rehabilitation Hospital, Philadelphia. Pennsylvania, May 1972.
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