The Essential Guide to KAFO Fabrication, Part 3

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By Pedro Salazar

Part 3: How to Make Modifications to a Plaster Cast

In "The Art of Mold Modification" (The O&P EDGE, September 2011), Tony Wickman, CTPO, did a terrific job of explaining various plaster modification techniques. However, in this final installment of my series, I'd like to offer some additional information -specifically on modifying the plaster cast for a KAFO.

Figure 1

Figure 1: The clearance between the joint and the plaster mold should conform to the manufacturer's specifications (Otto Bock recommends a clearance between 3/16 in. to 1/4 in. at the knee joint). If necessary, shave the adapter sleeve with a Surform to achieve the desired clearance.

In parts one and two of this series ("How to Prepare a KAFO Negative Wrap," The O&P EDGE, October 2011; and "How to Set Up Proper Alignment," The O&P EDGE, November 2011), I explained how to prepare a negative wrap and how to set up proper alignment of the knee and ankle joints. Making plaster modifications is your last chance to make corrections before assembling a test orthosis. At Otto Bock, we achieve consistent quality with the following process.

Remove the negative wrap. With the pipe running through the center of the mold secured in a vise, remove the plastic or tape used to extend the height of the mold. Then cut any tape or plaster bandage used to seal the negative wrap and remove the staples used to close the cut line. Starting at the thigh, splay the wrap and pull it away from the plaster mold. Be gentle as you approach the foot, where the plaster is most likely to chip or crack.

Establish minimum clearance at joints. Insert the knee joint or dummy joint with identical dimensions into the medial and lateral sides of the knee adapter sleeve. Measure the clearance between the joint and the plaster mold. The clearance should conform to the manufacturer's or practitioner's specifications (Otto Bock recommends 3/16 in. to 1/4 in. at the joint). If necessary, shave the adapter sleeve with a Surform to achieve the desired clearance (Figure 1). Often this clearance cannot be achieved if significant varus or valgus deformity is present. In that case, shave the adapter only until the joint rests against either the cast's thigh or calf (Figure 2).

Figure 1

Figure 2: Sometimes the recommended clearance cannot be achieved if signifi cant varus or valgus deformity is present. In that case, shave the adapter only until the joint rests against either the cast's thigh or calf.

Figure 1

Figure 3: Transfer every X and circle denoting a bony landmark from the negative wrap, including the calcaneus with its textbook teardrop shape. For a foolproof spacer, slip the appropriate bushing over a nail and drive it into the X to ensure the required height.

Insert the ankle or dummy joint into the medial and lateral sides of the ankle adapter sleeve and measure the clearance. Again, follow the clearance guidelines discussed above. If necessary, shave the adapter with a Surform to achieve the desired clearance.

Figure 4

Figure 4: Trim lines at the posterior bottom edge of the thigh section and posterior top edge of the calf section are equidistant from the knee. If the manufacturer specifi es the posterior calf section should be trimmed one inch below the neck of the fi bula, start by drawing a line parallel to the knee joint through that point. Then measure the distance from the trim line to the center of the knee joint. The trim line on the posterior bottom edge of the thigh section should be the same distance above the knee joint.

Figure 5

Figure 5: With the mold in an orthotic alignment jig, slip a sheet of rigid plastic under the foot. To ensure adequate length, add freshly mixed plaster to extend the forefoot 1/2 in. beyond the end of the toes. Maintain the shape of the foot, keeping the same medial-lateral (M-L) width of the foot and forming it to be a left or right foot.

Figure 6

Figure 6: If the patient was not bearing weight for the negative wrap, the foot most likely supinated, so the bottom of the foot will have no flat spots. For stability under the foot, correct the supination by filling any space under the forefoot with freshly mixed plaster. Use the negative wrap as a guide in recreating the shape of the bottom of the foot.

Identify bony landmarks. Transfer every X and circle from the negative wrap. Virtually every negative wrap pinpoints six bony landmarks: the base of the fifth metatarsal; the base of the navicular; the first and fifth metatarsal heads; the fibula head; and the calcaneus. Transfer any other bony landmarks or areas of concern identified by the practitioner. For a KAFO with a solid ankle, also mark the medial and lateral maleoli.

Next, check the instruction sheet for the height of each bony landmark. We use bushings in three heights-1/8, 3/16, and 1/4 in.-as foolproof spacers. Slip the appropriate bushing over the nail and drive it into the X to ensure the required height (Figure 3).

Establish trim lines. Unless the practitioner has a specific request, we follow Otto Bock standards for the trim lines:

  • 4cm anterior to the edge of the knee joint on the thigh section.
  • 1cm anterior to the edge of the knee joint and 1cm anterior to the edge of the ankle joint on the calf section. (For a solid ankle, use the practitioner's trim line.)
  • 5mm posterior to the first and fifth apices of the metatarsal heads on the medial and lateral sides of the foot.
  • The height of the cast on the lateral side and 1.5 in. lower on the medial side.
  • The exact measurement for the posterior bottom edge of the thigh section and posterior top edge of the calf section varies. But they are equidistance from the knee. For example, if the manufacturer specifies the posterior calf section should be trimmed one inch below the neck of the fibula, draw a line parallel to the knee joint through that point. Then measure the distance from the trim line to the center of the knee joint and mark a point that same distance above the center of the knee. Draw a line parallel to the knee joint through that point for the trim line on the posterior bottom edge of the thigh section (Figure 4).

Because the joint edges are perpendicular to the joint, you can use the joint or dummy joint to extend a perpendicular line above or below the joint. Lay a straight edge against the edge of the joint and draw a line extending the length of the cast. Then measure and mark the appropriate distance from the edge of the joint. Measure and mark that distance at the other end of the perpendicular line. Using the straight edge, connect the two points, drawing a perpendicular line that is the proper distance anterior to the joint.

Extend the forefoot. If the insert in the patient's shoe is too short, the foot will slide around in the shoe, which can cause blisters and a feeling of instability.

Place the mold in an orthotic alignment jig and slip a sheet of rigid plastic under the foot. To ensure adequate length, check the instruction sheet and add freshly mixed plaster to extend the forefoot ½ in. beyond the end of the toes. It's better to err on the side of excess length. The insert can always be trimmed later, but you can't extend an insert that is not long enough. Maintain the shape of the foot, keeping the same medial-lateral (M-L) width of the foot and forming it to be a left or right foot (Figure 5). If the patient was not bearing weight for the negative wrap, the foot most likely supinated, so the bottom of the foot will have no flat spots. For stability under the foot, the KAFO needs three points on the same plane. The heel provides one point. The other two points must be in the forefoot. Correct the supination by filling any space under the forefoot with freshly mixed plaster. Use the negative wrap as a guide in recreating the shape of the bottom of the foot (Figure 6).

Figure 7

Figure 7: Find low areas by moving a straight edge across the mold's surface. Any low area more than 1/2-in. deep probably created a corresponding high area. If you didn't already remove the high area, file it down about halfway. Fill all low areas with freshly mixed plaster. Once it cures, file and sand it to create a natural, symmetrical shape. Match the practitioner's measurements of the leg's circumference and M-L width.

Figure 8

Figure 8: Create flat edges along medial and lateral trim lines to make the orthosis more comfortable to wear and easier to don and doff. For a thin limb, add freshly mixed plaster to build up along the trim lines. File lightly along the trim line to flatten the surface. Check your progress with a straight edge.

Figure 9

Figure 9: After covering the nail at a bony prominence with plaster, shape the build-up to the desired contour. Start by filing down to the nail marking the highest point. Next, file along the perimeter of the shape to expose the ink. Then file down the excess until the prominence has an anatomical shape.

Remove high spots. After the new plaster has cured, return the mold to the vise, clamping it on the pipe. Using a Surform, file off any obvious high spots from the thigh and the calf.

Fill low areas. Find low areas-ridges, rolls, and deformities in the cast and impressions left by the practitioner's hands correcting alignment while making the negative wrap-by moving a straight edge across the mold's surface. For patients with little muscle tone, these imperfections result from pressure applied to residual tissue.

Any low area more than 1/2-in. deep probably created a corresponding high area. If you didn't already remove the high area, file it down about halfway. Fill all low areas with freshly mixed plaster.

Watch for severe roping. If tight wrapping of the limb created a deep scallop pattern on the mold, it may have caused residual tissue to shift to one side. If the limb is not symmetrical, add plaster where tissue should be. Once it cures, file and sand to create a natural, symmetrical shape (Figure 7).

If you're ever unsure whether to remove or add plaster, match the practitioner's measurements of the leg's circumference and M-L width. Keep an M-L gauge nearby for frequent checks.

Create flat edges along medial and lateral trim lines. Flat edges make the orthosis more comfortable to wear and easier to don and doff. With a thin limb, add freshly mixed plaster to build up along the trim lines. A limb with residual tissue needs no more plaster. File lightly along the trim line to flatten the surface. Check your progress with a straight edge, and make sure you maintain the correct M-L width (Figure 8).

Start with the thigh. Move to the calf. Finish with the foot. Trim lines on the foot are prone to cupping and then the patient feels the plastic of the orthosis digging into the foot. To prevent cupping, add plaster along the trim lines. Then shave the cured plaster to flatten the trim line.

Build up bony prominences. Each bony prominence has its own textbook shape, such as a half-circle below the base of the navicular and a teardrop around the calcaneus. Mark the appropriate shape on the cast in indelible ink. Add enough freshly mixed plaster to cover the nail and extend beyond the outline of the shape to help create a smooth transition. Unless the KAFO will have a solid ankle, there's no need to build up for the malleoli because the ankle joint provides clearance.

Let the plaster cure, then shape the build-ups to the desired contour. Start by filing down to the nail marking the highest point. Next, file along the perimeter of the shape to expose the ink. Then file down the excess until the prominence has an anatomical shape (Figure 9).

Sand the entire plaster mold. Use dry/wet sanding screen-I prefer 180 grit-to remove Surform marks, do final contouring, and smooth the surface.

Re-draw trim lines. Creating flat edges along the trim lines may have covered or erased parts of the original trim lines. For a missing segment, use a straight edge to draw a line spanning the gap. If a trim line is missing, measure and mark two points the appropriate distance from the line extending the length of the plaster cast, then use a straight edge to draw the line.

You're ready now to pull plastic, attach metal, and install joints. Any corrections (plantar flexion, dorsiflexion, knee extension/flexion, varus/valgus) performed to the negative wrap may affect the fit of the definitive device. For that reason, Otto Bock recommends the use of a test orthosis prior to fabricating a laminated device. Using a dynamic test orthosis (a fully functional test orthosis with joints attached) on the patient, the practitioner can make sure the joints are in alignment and that the orthosis fits properly.

Nearly every test orthosis we fabricate receives approval from the practitioner with no corrections or only minor adjustments, such as a flair for comfort at the thigh or posterior calf. Then we produce the definitive orthosis. Following the process outlined in this series, you too can achieve a high success rate when fabricating KAFOs.

Pedro Salazar

Pedro Salazar is an orthotics fabrication specialist at Otto Bock U.S. HealthCare, Minneapolis, Minnesota. He oversees all orthotic fabrication and provides training to new fabrication technicians at Otto Bock.