Fabricating a Carbon Ankle Seven

Content provided by The O&P EDGE
Current Issue - Free Subscription - Free eNewsletter - Advertise

Fabricating an Ottobock Carbon Ankle Seven (17CF1) presents a challenge: getting a standardized, dynamic component to fit within a custom orthosis. However, incorporating the Carbon Ankle Seven into an orthosis doesn't require any new skills. You just have to apply the right techniques at the right times.

Clinicians recommend the Carbon Ankle Seven for patients who exhibit weak plantarflexion or weak or absent quadriceps function. Manufactured with seven degrees of outward rotation, which is the natural line of a person's feet while walking and standing, the preformed carbon-fiber strut is designed for AFOs and KAFOs. The strut is rated for a patient weight from 22 to 220 pounds and varies in length, predefined angle, width, and thickness to match the weight and activity level of patients (Figure 1).

Figure 1

Figure 1: Because energy stores in the Carbon Ankle Seven at heel strike and returns at toe-off, the jointless design helps patients enjoy a more natural gait cycle without the plantar and dorsal stops associated with conventional ankle joints.

The Process

Make corrections to the mold. First, identify the landmarks on the mold to ensure that any corrections requested by the clinician are made appropriately.

Ideally, the carbon strut has three-point contact with the mold—at the midpoint of the calf, the forefoot, and the heel. This can be achieved by correcting the ankle flexion, with the clinician's consent (Figure 2).

Cut the carbon strut. Measure the distance from the medial tibial plateau to the floor, and mark the midpoint on the carbon strut. The strut will flex below the midpoint, so fasteners must be placed above this mark. Position the strut against the cast and mark the long end 1 in. below the proximal calf trim line. Cut there to ensure that any adjustments near the trim line will not interfere with the carbon strut. Mark the location of the metatarsal on the short end. Cut no shorter than the white line to ensure the proper lever arm. Buff and skive both ends of the strut, creating a smooth transition.

Figure 2

Figure 2: Making a correction in ankle flexion brings the cast closer to the contours of the carbon strut.

Figure 3

Figure 3: To determine the amount of liner material, measure the length of the mold, the circumference of the calf at its widest point, and the circumference at the heel, then add 1 to 2 in. to each dimension. Slice off narrow wedges from the ankle to the toes for easier handling on the foot.

Make the mold. A thick mixture of plaster minimizes leaking through your cast. I use warm water to make the plaster set faster. If you modify your mold while it still holds moisture, it will save time and help any addition of plaster to hold. This will make it easier to smooth the surface.

Form the liner. Ottobock recommends using a medium-density liner material that maintains shape after being heat formed to make a continuous, single-piece liner. We typically use a 1/4-in. thickness, knowing that heat, stretching, and vacuum will thin the material, resulting in a finished thickness of about 3/16 in.

Measure three areas of the mold:

  1. The length from the posterior top edge around the heel
    to the toes.
  2. The circumference at the widest part of the calf.
  3. The circumference at the heel.

Transfer the measurements to a sheet of material, adding 1/2 in. to each dimension. Too much material is difficult to handle, so cut the corners from the portion for the foot (Figure 3).

Pull a closed-toe nylon over the mold to keep the liner clean. Tie off the top of the nylon around the pipe. With the mold in a vacuum stand, position the mold with the toes down, and check that your stapler is loaded.

Heat the liner material at the manufacturer's recommended temperature. If using an infrared oven, turn the liner material over halfway through the heating cycle to ensure even heating. Be careful not to overheat, which contributes to compression and shrinking of the liner.

With the vacuum ready and heat-resistant gloves on, place the wide end of the heated liner 1/2-in. beyond the proximal end of the mold, and staple it in position (Figure 4). Use both hands to form the liner around the ankle. Use your staple gun to tack the liner in position. Be careful not to stretch the liner, which could make it too thin at the heel. From the ankle, work toward the foot, gathering and stapling near the midline as you go. Follow the same steps to work from the ankle up the calf. Use scissors to cut away excess so the edges of the material butt together along the shin (Figure 5). If needed, a heat gun can be used to re-form portions of the liner.

Figure 4

Figure 4: For the liner, start by centering and stapling the material at the top of the mold with a 1/2 in. overlap.

Figure 5

Figure 5: Starting at the ankle, staple close to the midline on both sides. Work down to the toes, then up the anterior calf. Cut away excess material with a pair of scissors.

Figure 6

Figure 6: Use a straightedge to outline and cut multiple lengths of liner material to backfill between the liner and carbon strut.

Figure 7

Figure 7: Gaps between the liner and carbon strut cause problems when thermoforming the plastic shell.

Figure 8

Figure 8: When grinding the high points, make sure the grinding wheel’s rotation goes toward the end of the strip to prevent separation from the liner.

Figure 9

Figure 9: Once grinding brings the liner closer to the contours of the carbon strut, use tape to secure the strut in position.

Match the contour of the carbon strut. Start by outlining the proper location of the carbon strut on the liner. With the strut in position, check for gaps between the liner and the strut.

To bring the strut to the mold, you may need to backfill with more than one layer of liner material. Use a straightedge to outline the full length of the carbon strut plus 2 in. Cut out a strip of liner material (Figure 6). Round and skive the ends for a smooth transition.

Place the strip along the liner. Lay the carbon strut over the strip and reposition the strip so it extends 1 in. beyond each end of the strut. Outline the strip, then brush Master's glue, which is thin for easy spreading, on the non-skived side of the strip and on the outlined area of the liner to ensure a strong bond. Align and press the strip to make it adhere to the liner.

Use the strut to check for high points where padding touches the strut (Figure 7). Then, using a coarse grinding wheel, remove those areas of padding. Check repeatedly for high points and continue to correct (Figure 8). Avoid removing material from the original liner.

If needed, repeat with another layer. Use a buffing cone to blend layers and break edges for a smooth transition. Minimal gapping is okay because the padding will compress under vacuum. Too much gapping, though, could allow vacuum forming to pull plastic between the carbon strut and the liner.

Use thinner to remove excess glue from the liner. Secure the strut in position against the liner with one strip of tape near each end of the strut, where tape marks won't show (Figure 9).

Prepare the mold. With the mold in a vacuum stand, pull a closed-toe nylon or stockinette over the liner. Next, pull over a wet, thicker Perlon material to help separate the plastic from the mold after thermoforming.

To ensure a good vacuum seal, first wrap a heated piece of Volara® foam around the fabric and pipe, then tape off the Volara foam and pipe.

Thermoform. At Ottobock, we recommend using a copoly plastic that has a lower shrink rating and is less likely to crack under stress. Copoly also requires a shorter cure time than polypropylene, which helps reduce fabrication time.

Heat the copoly in an oven at 365 degrees for ten to 15 minutes. Cut away excess material for the foot, similar to the cuts made to the liner material. Now is the time to apply a transfer that reflects the patient's personality, if one has been requested.

Turn on the vacuum. While wearing thermal gloves, form the plastic over the mold. Seam together the plastic around the ankle. Continue to seam the plastic along the anterior calf and the foot (Figure 10). Use scissors to remove excess plastic, cutting close to the midline. Let the copoly cure for one hour to minimize volume change.

Drill attachment holes. While the shell and carbon strut are formed in place, drill holes for the attachment hardware. Mark two holes above the midpoint on the calf and centered over the carbon strut. Mark another two holes in a flat section under the foot away from the heel. Using a drill bit the same size as the screws provided (sizing varies based on strut selection), drill through the plastic and score the carbon strut.

Cut off the shell. Once the plastic has cured, sketch trim lines on the mold. Use a cast saw to cut along both sides of the mold, staying close to the midline to avoid cutting into the liner (Figure 11). (Author's note: Be very cautious with this step, as damaging the liner at this point will require starting over.)

Finish the shell sections. Pull off the thermoplastic shell, separating it from the carbon strut and liner. Use a cast saw or cutting wheel to cut close to the trim lines, separating the foot and calf sections. Grind each section to the final shape on a drum sander, switching to a Trautman™ cone to create a tighter radius. Next, round the inside and outside edges with a polishing cone, then soften the edges with a Tycro cone. With all grinding and buffing, use light pressure, letting the abrasive do the work.

Figure 10

Figure 10: Seam the draping plastic at the ankle, then continue along the foot and the anterior calf to complete the vacuum seal.

Figure 11

Figure 11: Stay close to the midline when cutting off the shell to avoid damaging the liner.

Figure 12

Figure 12: The finished product has a narrow rise under the heel. Posting the heel creates a level plane to help stabilize the plantar surface.

Attach the shell to the carbon strut. On a drill press, bore four holes at the score marks on the carbon strut. Deburr the holes to eliminate slivers. Insert T-nuts through the proximal side of the carbon strut, then temporarily attach the shell sections to test the length of screws. If any screws are too long, use a thread cutter to shorten them. Detach both shell sections, and apply medium-strength Loctite adhesive to the threads of each screw for a permanent bond. Tighten the screws while holding the T-nuts aligned with the length of the strut.

Trim the liner. Place the assembled Carbon Ankle Seven over the liner. Use a fresh blade to cut out the liner, leaving 1/4 in. of padding material extending beyond the plastic. Create a continuous curve to span the gap between the two pieces of shell. At the toes, cut the liner flush with the shell.

Remove the liner, and lightly grind with a Tycro cone to round the edges and smooth transitions.

Wash the liner with soap and water. Rinse inside and outside, and dry with a towel.

Add the finishing touches. Attach straps, keeping the chafes behind the trim line to avoid pinching.

The carbon strut creates a narrow rise under the heel (Figure 12). Post the heel, building in the ability to wear the orthosis with any shoe. At Ottobock, we typically use rigid foam or high-density crepe to create a level plane that follows the patient's anatomical shape.

Visit www.oandp.com/link/241 to view an instruction video for further guidance on fabrication and casting techniques for the Carbon Ankle Seven.

Justin Eitel is the technical orthopedics lead for Ottobock US. He oversees all orthotic and prosthetic fabrication at the Ottobock technical center, Minneapolis, Minnesota.

Bookmark and Share