Making an Orthotic Carbon-Fiber Footplate with a Flexible Forefoot
Repeatedly flexing a thin strip of rigid material will eventually cause it to break. This principle has been demonstrated countless times with orthotic footplates. When the footplate breaks, it always does so at the point of maximum flexion- the metatarsal.
For footplates that fit inside a shoe, a flexible forefoot lasts longer and performs better. About 85 percent of foot orthoses fabricated at Ottobock, Minneapolis, Minnesota, have a flexible forefoot, and we strongly recommend it.
Based on our own studies of carbon fiber, we have developed a stronger footplate structure that has just the right amount of flexion, torsion, and rigidity in all the right places (Figure 1). This carbon-fiber matrix provides a dynamic structure that allows the patient's forefoot to roll over smoothly, resulting in a more natural gait so the patient is able to walk with greater ease for a longer period of time. The flexion force also helps stance-control devices to perform as intended.
This is how we fabricate a flexible forefoot in a footplate:
Incorporate heel height and toe pitch. We encourage clinicians to use a foot-casting aid (such as Ottobock's 743A9 aid), which uses interchangeable wedges to match the heel and toe pitch of the patient's shoe when making a plaster cast (Figure 2). Taking time to produce a good cast saves time later on adjustments.
Carbon-fiber structures have limited flexibility ranges. If the foot is not properly casted or the cast is not properly modified, additional forces may be exerted on the carbon-fiber matrix structure, which may lead to premature failure.
Identify the toe break. Mark the first and fifth metatarsals and then draw a dark line through those points on the plantar side of the cast (Figure 3). To allow the carbon fiber to respond and to prevent premature failure, the plantar surface of the forefoot must be flat.
Set up for lamination. Secure the dried cast upright on a vacuum stand. Sharp edges or holes in the plaster could tear a PVA bag, so fill in low spots with clay or plaster and chip off or sand any sharp points.
Pull a PVA bag over the cast. After the bag soaks in a wet towel for ten minutes, pat it with a dry cloth to remove any excess moisture. Align the bag to keep the seam outside of the desired trim. Pull the bag onto the cast and tie off the distal end of the bag. With the vacuum on, remove all of the wrinkles. Seal the bag around the pipe and cut away any excess.
Apply stockinette. Apply two layers of stockinette. We like to use T-3 Perlon stockinette because it has a tighter weave and absorbs more resin per cubic inch. Cut the stockinette slightly more than twice the length of the cast. Pull the first layer beyond the proximal end of the cast and then twist or tie the stockinette at the distal end and reverse direction, spreading the stockinette over the foot and pulling it beyond the proximal end of the cast.
Apply carbon-fiber weave. Apply three layers of carbon-fiber weave, staggering the layers to prevent weak points and to give each layer different flexion and torsion. Because unidirectional carbon fiber comes in 2-inch or 3-inch rolls, some layers will require multiple pieces. Use double-faced tape to position the pieces with minimal adhesive.
- 90-degree orientation. Unidirectional carbon-fiber weave flexes on a horizontal plane and has little torsion. Cut a length of carbon-fiber tape so that it extends from 1/8 inch behind the metatarsus, which should be visible through the stockinette, to 1 inch up the heel. Use double-faced tape along the edges to secure the carbon-fiber tape. Make a vertical incision at the heel and fold one side over the other for a tight, flat fit. Use one long strip along the medial side of the cast, around the heel, and along the lateral side of the cast. Align this strip below the trim line, overlapping the first strip. Cut away carbon-fiber weave as necessary to keep it below the trim line (Figure 4).
- 45-degree orientation. Bidirectional carbon-fiber weave is more flexible when properly oriented and has almost no torsion, which is ideal for the forefoot. From a 3-meter roll, cut a section so that it overlaps the first layer at least 2 inches and extends slightly beyond the toes. For proper orientation, the arrow pattern in the carbon-fiber weave must follow the line of progression of the desired flexion angle-pointing toward the heel (Figure 5). Cut a wedge from the left and the right at the heel end of the carbon-fiber weave, creating a peak at the center, similar to a roofline. Apply double-faced tape along every edge to stop fraying. Secure the carbon-fiber tape by wrapping the edges around the cast.
- 0-degree orientation. Unidirectional carbon-fiber weave flexes on a vertical plane when it's positioned in the 0-degree orientation and has little torsion. Duplicate the first layer but this time extend the layer a bit closer to the trim line. Cut a length of carbon-fiber weave to extend from the medial trim line to the lateral trim line, and use double-faced tape at the ends to position it along the toe break. Cut and tape additional strips, making sure they butt up to each other (Figure 6). Do not overlap the strips because that would create a ridge. The flexible bidirectional carbon-fiber weave in the forefoot should now be sandwiched between two unidirectional layers that strengthen the structure but stop at the metatarsal.
Apply stockinette. Verify proper placement of the carbon fiber and then carefully pull another two layers of Perlon stockinette beyond the proximal end of the cast, making sure to hold the carbon fiber in place.
Pull a second PVA bag over the cast. For a dull finish, use the bag as is. For a shiny finish, turn the bag inside out. Again, use a dry cloth to remove any excess moisture and align the seam to keep it outside of the desired trim. Pull the bag onto the cast and seal it around the pipe. Twist the distal end of the bag and pinch it with a Yates clamp, leaving enough bag to hold the required volume of resin.
Mix and pour the resin. Mix the recommended ratio of resin, hardener, and pigment. You'll need 200 to 300 grams of resin for a foot, 500 to 600 grams for an AFO, and 900 to 1,400 grams for a KAFO. We use a thinner resin to saturate the carbon fiber better. Its longer cure time also helps when fabricating a KAFO.
Using a funnel, pour the resin into the bag. Tie off the end to create a seal. Clean any spills on the outside of the bag with 99-percent isopropyl alcohol. Beware of 76-percent isopropyl alcohol solution, which contains water and could make a hole in the bag.
Work the resin. We laminate horizontally, clamping the plaster mold at a slight downward angle. Remove the Yates clamp and untwist the bag. Turn on the vacuum, and any air in the bag is the first thing to go. Slowly work the resin up the cast. Move resin away from the heel, where the bag tends to thin out (Figure 7). A KAFO involves moving at least 1,000 grams of resin through the foot. Otherwise, don't overwork the foot. Use minimal pressure when rolling with parachute cord or rubber hose to string the lamination. Too much pressure will starve an area by preventing resin from adequately saturating the carbon fibers.
Let the resin cure. To ensure the resin and promoter undergo the appropriate chemical reactions, keep the job under vacuum for 45 to 60 minutes.
Finish. Using a cast saw, cut the footplate at the trim line. We recommend a 90-degree angle at the metatarsal heads to allow flexibility and ensure durability and longevity (Figure 8). Grind to the desired finish.
The completed footplate will be long-lasting and, because it has a flexible forefoot, make walking easier for the patient.
Justin Eitel is the technical orthopedics lead for Ottobock US Healthcare. He oversees all orthotic fabrication at the Ottobock technical center, Minneapolis, Minnesota.