Ankle-Foot Orthosis with Flange: Fabrication and Fitting Instructions*

Child Prosthetic and Orthotic Studies, New York University Post-Graduate Medical School, New York, New York


Introduction

This article describes the fabrication and fitting of the Ankle-Foot Orthosis with Flange (AFO/F). The design of this orthosis (Figure 1-A) (Figure 1-B) derives from the work of David N. Condie and C. B. Meadows1, who adapted the hemispiral orthosis originated by Hans Richard Lehneis2. Frank Twyeffort3 subsequently made further modifications. The purpose of the AFO/F is to reduce flexible pes valgus more effectively than valgus-correction pads or inserts utilized in connection with conventional double-bar metal braces. Condie and Meadows realized that stiffening a plastic ankle-foot orthosis would be unlikely to improve its effect upon severe valgus, unless the fit was modified to apply corrective forces which were more specific than those achieved by standard casting and fabrication procedures. The hemispiral orthosis imposes a specific three-point system of corrective forces to control pes equinovarus, but this nylon-acrylic orthosis is relatively difficult to fabricate, is expensive, and is awkward for the patient to don. The polypropylene AFO/F utilizes a similar three-point force system in a modified solid-ankle design to control either varus or valgus deformity. This approach has been adopted with particular success for children with pes valgus due to spina bifida or cerebral palsy.

The extent of correction which can be achieved may be assessed by manually applying the necessary three-point forces. Fixed deformity may preclude realigning the limb to the neutral position, either because there is insufficient joint mobility or because the force required to maintain full correction would be intolerable.

The theoretical basis of the AFO/F involves the application of sufficient forces to the foot-ankle complex to hold the bones in the best achievable alignment. The orthosis applies total contact, so forces are distributed over a sufficient area to keep skin pressures tolerable. Furthermore, since the materials are not alterable in shape in the supported areas when the orthosis is worn, the application of the corrective pressures remains constant over the entire area supported.

Total contact about the foot and ankle permits application of forces to control deformity, with pressures more evenly distributed than those applied by orthoses of conventional design. A padded medial flange, proximal to the malleolus, controls valgus deformity. The flange applies controlling forces over a broad but specific surface and thus prevents ulceration. This feature has proved most helpful, especially with insensate limbs such as those found in the myelodysplastic child. The AFO/F is usually set so that the leg is perpendicular to the floor when the subject wears the orthosis and a shoe. The curved, broad posterior segment reaches to the midshank to provide very rigid support of the ankle. Since stiffening the ankle tends to resist the anteroposterior rotational forces which normally occur during walking, and therefore transmits high stresses to the knee and hip, a cushion shoe-heel may be necessary to reduce any undesirable effect on gait or on the joints of the leg.

The primary three-point force system consists of medially directed forces on the calf at the proximal-band level and at the lateral side of the foot, and a laterally directed force above the medial malleolus, via the flanged extension.

There is a secondary force system. Body weight is transmitted to the bottom of the orthosis, stabilizing it in the shoe. Thus, the medial flange can maintain laterally directed forces against the supramalleolar area. These forces are counteracted by medially directed forces applied by the orthosis against the lateral side of the foot. The lower portion applies these secondary forces independently of the proximal portion of the orthosis and only in weight-bearing.

Materials and Equipment

    1. Elastic or Regular Plaster Bandage

Plaster-of-Paris bandage, 8 cm (3 in.) wide, is used. Depending on the size of the foot, one or two rolls are needed. Elastic bandage, although more expensive, makes it somewhat easier to achieve close molding.

    1. Tubing

Latex tubing or other suitably flexible material, about 45 cm (18 in.) long and 1 cm (3/8 in.) in diameter, is used to form a tunnel under the plaster wrap for the introduction of bandage scissors to remove the wrap.

    1. Tubular Gauze

A length of Tubegauz tubular gauze or stockinette equal to twice the distance from the toe to the knee, as measured along the posterior aspect of the limb, will separate the plaster from the skin.

    1. Contoured Standing Surface

Since the AFO/F must fit the shoe as well as the foot, it is necessary to use a standing surface which has a suitable configuration. A prefabricated, plastic standing surface, with the heel and forefoot portions planar, is satisfactory. The contralateral foot must be supported on a surface that allows symmetricalstance. A typical standing surface is shown in (Figure 2) , and the actual contour is shown in (Figure 3) .

    1. Casting Stand (optional)

A casting stand (Figure 4) is employed to minimize the need for moving the foot once the patient is brought to the standing position. It consists of a revolving platform to which a pipe support has been added, and the assembly is mounted on a suitable base. The stand enables the patient to remain upright and to be rotated 180 degrees so that the orthotist can observe the calcaneus and the Achilles tendon while the foot is maintained in a corrected weight-bearing position as the plaster hardens.

    1. Rigid Plastic

Thermoplastic polymer is used for the major portion of the AFO/F. The most suitable plastic is orthopedic-grade polypropylene (polypropylene alloyed with small amounts of butyrate and polyethylene), 0.3-0.5 cm (1/8-3/16 in.) thick, depending on the strength needed. A sheet approximately 50 x 20 cm (20 x 8 in.) will be required.

    1. Closed-Cell Foam

PE LITE or similar closed-cell foam sheeting, 0.5 cm (3/16 in.) thick, is used for the supramalleolar pad. A rectangle approximately 6 x 5 cm (2 and 1/2 x 2 in.) will be required.

Fabrication and Fitting Procedures

    1. Examination of Limb

Note the amount of positional correction which can be obtained manually without loss of mediolateral alignment. Maximum control may be sacrificed to avoid excessive skin pressure. After a period of wearing the orthosis, a second AFO/F with additional control can be made. Note also the changes which occur in the foot when the subject goes from nonweight-bearing to weight-bearing. Measure the calf circumference and the widths of the forefoot and heel. Record the results of the examination on the Orthotist's Information Form (Figure 5) .

With indelible pencil mark the skin over the following areas: distal border of the head of the fibula, medial malleolus, lateral malleolus, navicular tuberosity, first metatarsal head, fifth metatarsal head, and any calluses and other pressure-sensitive areas.

    1. Establishment of Trim Lines (Figure 6-A) (Figure 6-B)

With an indelible pencil, draw trim lines on the skin. The plantar trim line is at the metatarsophalangeal crease. The distomedial and distolateral trim lines begin at the medial and lateral ends of the metatarsophalangeal crease and continue dorsally two-thirds of the way toward the dorsa of the first and fifth metatarsals.

The anteromedial and anterolateral trim lines run proximally from the upper ends of the distomedial and distolateral trim lines, respectively, and parallel to the respective metatarsals.

The lateral trim line curves to bisect the apex of the lateral malleolus and proceeds upward along the lateral midline of the leg. The upper 5 cm (2 in.) of the lateral trim line extends forward beyond the lateral midline of the leg 2—3 cm (1 to 1 and 1/4 in.), depending on the size of the subject. If this portion is too small, it will apply excessive pressure; if it is too extensive, the orthosis will be difficult to don. The anterior opening should be sufficient only to allow insertion of the narrowest part of the leg during donning.

The medial trim line curves to bisect the apex of the medial malleolus and proceeds upward along the medial midline of the leg. At the distal edge of the middle third of the leg, the trim line extends anteriorly 2—3 cm (1 to 1 and 1/4 in.), close against the tibia. The anterior projection contacts the middle third of the leg. At the proximal edge of the middle third, the trim line returns to the medial midline of the leg. The upper 5 cm (2 in.) of the medial trim line extends forward beyond the medial midline of the leg 2—3 cm (1 to 1 and 1/4 in.).

The proximal border of the orthosis is at a level 2 cm (1 in.) distal to the distal border of the fibular head.

    1. Application of Gauze

The subject should sit comfortably on an examining table or its equivalent, with his bare foot and leg extended toward the orthotist.

Apply one or two layers of tubular gauze or stockinette between the knee and a point 3 cm (1 and 1/4 in.) distal to the toes, eliminating wrinkles. If stockinette is used, draw the trim lines on the outside of the fabric.

    1. Plaster-Wrap Procedure

Prior to making the plaster wrap, place a sheet of paper on the standing surface on which the wrapped foot will rest, to aid in removal of the cast. If the casting stand is not used, an assistant or a walkerette should be available to assist the subject in standing and in maintaining his balance.

Also, before the wrap is made, insert a strip of flexible tubing, coated with petrolatum or silicone spray, between the skin and the fabric on the dorsum of the foot and anterior leg. If an oscillating-blade cast-cutter is used, it is unnecessary to lubricate the strip.

The subject is seated with the leg and foot hanging freely. Wrap a circumferential spiral from the knee to the toes as far as the distal phalanges. It is not necessary to encase the toe tips. Exert only enough tension on the bandage to reproduce the limb's contours. Too much tension will produce grooves in the plaster, especially over fleshy areas, and will bridge the bandage over bony areas, producing hollows in the cast and in the orthosis.

    1. Alignment and Corrections During Weight-Bearing

While the plaster is still pliable, assist the subject to the standing position with his back to the orthotist. The subject maintains his balance with the aid of the casting stand, a walkerette, or an assistant. The orthotist carefully positions the wrapped foot on the shaped surface, locating the entire heel on the upper horizontal surface of the standing block.

If weight-bearing is not possible because the subject cannot stand without an orthosis or because an assistant is unavailable, the subject can remain seated. After wrapping the limb, press a casting board against the sole to give proper contour to the bottom of the cast; and manually hold as much correction as possible. This technique is not as accurate, and the cast will require more modification.

The ankle should be dorsiflexed by rotating the leg forward to the position desired in the finished orthosis, generally such that the leg is perpendicular to the floor. The foot may have to be plantarflexed slightly to avoid pes valgus or skin breakdown. Plantarflexion requires the addition of a heel lift inside and, at times, also outside the shoe.

To raise the arch, the orthotist externally rotates the leg with one hand and maintains the metatarsal heads flat on the standing surface with the other (Figure 7) . If necessary, use the thumb or fingers of the near hand to press the medial malleolar area to attain full correction. Observe the effect of these corrections on the Achilles tendon, a useful indicator of the relation of the heel to the leg. The Achilles tendon is straight when the foot is well aligned.

The subject should bear less weight on the foot until the correction is made. He can then gradually apply equal weight to both feet. It is relatively easy to hold the foot in an already corrected position in full weight-bearing.

After the plaster sets, slide the wrapped foot sideways to break any seal between the plaster and the standing surface. The subject is instructed to ease his weight carefully off the wrapped foot before sitting. CAUTION: Do not lift, or allow the subject to lift, the foot from the standing surface until the seal has been broken by the aforementioned sideways movement, or the plantar surface of the cast may be deformed.

Before cutting the wrap, draw several horizontal reference lines over the tubing to aid in reassembling the cast after removal. Then remove and close the cast in the usual manner.

    1. Wrap Modification

Reduce residual valgus and incorporate any plantar or dorsiflexion that could not be achieved when the wrap was taken, modifying the cast to the extent that judgment dictates will result in an effective orthosis which the skin can tolerate. This is done by removing wedges of the plaster wrap and bending the cast to the desired position. One should be extremely careful to remove wedges at the level of, or slightly distal to, a line between the sagittal midlines of the malleoli at the level of the distal border of the medial malleolus, parallel to the ankle axis. Wedges should be removed both anteriorly and posteriorly, so that the pivot point is midline. Removing wedges elsewhere will distort the relationship of bony prominences. Wedges should equal the amount of correction desired and should be measured in degrees. Once these modifications have been made, fix these positions and seal the openings with plaster splints.

    1. Modification of Plaster Model

The model is poured in the standard manner. When the wrap is removed, small surface imperfections caused by air entrapment can be removed by light sanding. Rasp smooth any discrepancies where the bandage may have constricted soft tissue. Remove 50 per cent of the convexities from the area surrounding the constrictions, then fill the remaining portion of the constricted concavities.

Modify the model by applying 0.3 cm (1/8 in.) plaster buildups to the pressure-sensitive areas. Drive a tack into the apex of each malleolus, leaving a 0.3 cm (1/8 in.) protrusion. Cover the tack for the buildup. Smooth the buildups to feather their edges.

The plantar surface of the orthosis should fit the patient's foot and his shoe, accommodating the heel height of the shoe that is to be worn without distorting the relative position of plantar or dorsiflexion which exists in the heel. The forefoot and heel should appear slightly flattened on the plantar surface of the model and should be in parallel planes, with the difference between forefoot and heel height determined by the shoe.

Adjust the width of the heel and forefoot to ~correspond to the recorded measurements.

For valgus control, remove plaster on the medial side of the model in a rectangular area. The rectangle is shaped as follows: a) the bottom edge lies just above the superior edge of the medial malleolus with the top edge being 6 cm (2 and 1/2 in.) above the bottom edge. In order that the rectangle project anteriorly from the usual medial border of a solid ankle AFO, b) the anterior edge approaches the anterior aspect of the model, with the posterior edge lying approximately 5 cm (2 in.) posterior to the anterior edge. The rectangle has a maximum depth of 0.5 cm (3/16 in.) and parallels the approximate curvature of the medial side of the model from where it is removed.

    1. Shaping the Supramalleolar Pad

Cut a piece of foam 0.5 cm (3/16 in.) thick into a rectangle 6 cm (2 and 1/2 in.) x 5 cm (2 in.) or slightly smaller. The pad should be large enough for tolerable pressure and small enough to focus the three-point force system. Skive the posterior, superior, and inferior edges.

    1. Thermomolding

Place the mandrel of the model into the suction pipe which is attached to the vacuum system. Pull nylon hose over the model (nylon prevents suction from being omitted from any portion of the model, because the fabric pores allow air withdrawal from the space between the model and the plastic). Fasten the nylon with pressure tape around the suction pipe; cut off excess fabric. Glue the supramalleolar pad with rubber cement, with the skived edges facing away from the model so that the skiving marks will be on the outside of the model. Position the pad over the flat supramalleolar area of the model, with the longer dimension parallel to the long direction of the leg.

Cut a rectangle of rigid plastic. The length of the plastic equals the distance along the posterior aspect of the model from the toes to the heel to the top of the model. The width equals the greatest circumference of the model plus 2 cm (1 in.). If orthopedic-grade polypropylene is used, heat it in an oven preheated to 205 degrees C (400 degrees F) approximately six minutes, until transparent. Follow manufacturer's instructions for other, similar, thermoplastics. Remove the plastic from the oven and drape it over the model. With the fingers (protected by suitable gloves), seal the edges as well as the junction around the pipe beyond the tape. Apply suction until the plastic hardens and becomes translucent, about 1—2 minutes on a wet model and 2—3 minutes on a dry model. Premature removal of suction from a wet model may cause steam to pucker the plastic surface, creating a rough interior on the orthosis. Early cessation of suction from a dry model will prevent the plastic from conforming intimately to the model. Let the plastic cool completely to room temperature, approximately 15—30 minutes. Cut the plastic with an oscillating-blade cast-cutter. If the plastic is cut while warm, it may warp. Follow the trim lines which transferred from the skin to the wrap cast to the positive model. The heat of the plastic causes adhesion between it and the foam pad. Smooth the edges of the orthosis with a router, first with a sanding cone and then with a felt cone.

    1. Preliminary Fitting

Check the orthosis on the subject for fit and comfort, and trim it further or modify it as necessary, paying particular attention to the trim lines, areas of high pressure, and ease of donning.

    1. Application of Strap

Rivet a Velcro strap to the proximal portion of the orthosis.

    1. Fitting

With the strap tightened, again check the orthosis on the patient. After a few minutes of wear, check again for comfort and areas of high pressure. Modify the orthosis if necessary.

References

 

  1. Condie, David N. and C. B. Meadows, Some biomechanical considerations in the design of ankle-foot orthoses. Orthot and Prosth, 31:45-53, September 1977.
  2. Lehneis, Hans Richard, Plastic spiral ankle-foot orthoses. Orthot and Prosth, 28:3-13, June 1974.
  3. Twyeffort, Frank H., III, AFO designs and modifications. Paper presented at Association of Childrens Prosthetic-Orthotic Clinics Annual Meeting, Memphis, Tennessee, April 28, 1979.