Fabricating a Dynamic Test Orthosis
December 2012 Issue
In most cases, you would not fabricate a definitive lower-limb prosthesis without first making a check socket. The same principle applies to orthotic devices. With stance-control KAFOs, it is just as important to verify the fit and function before proceeding with the definitive fabrication.
At Ottobock, we recommend making a dynamic test orthosis (DTO) for stance-control KAFOs and any other device that may require a rigid laminated structure. Unlike traditional thermoplastic KAFOs, a laminated device cannot be modified easily to adjust the fit. Using the DTO, the orthotist is able to verify the fit and the functionality of the device. That information allows the practitioner to either make adjustments in-house or identify changes to be made before continuing with the definitive fabrication. Ultimately, the DTO helps to achieve the best possible patient outcome.
To fabricate a DTO, begin with a plaster mold that has been modified and set up with an aligned adapter sleeve with proper clearance at the knee and ankle. Then follow these steps:
Cut the metal. With the mold secured horizontally in a vacuum stand, insert the dummy joints that came with the joint kit or the real joints. Choose the appropriate bar stock for the patient's weight and joint selection. You will need six pieces, but complete them one at a time to avoid confusion. For the thigh section, measure from the top of the knee joint to a point 1 to 1½ inches below the proximal trim line. Remember to account for the amount of bar that attaches into the joint. Note that the medial and lateral measurements probably will differ. Cut the metal with a hacksaw or drop saw. As a standard measurement, we use 6-inch bar stock for the medial knee joint and proximal ankle. For the definitive orthosis, metal usually is reduced and combined with a polyvinyl chloride (PVC) composite for weight reduction.
Round and skive the metal. Use a belt sander to round and skive one side of the bar (Figure 1). Insert the other end of the metal into the dummy joint. Draw a line on the bar along the top edge of the joint to show the end point for any bending. Then score the bar for the attachment hole(s), using the hole in the dummy joint as a guide, and drill lightly. Use the manufacturer-specified drill bit, which for Ottobock joints is a 3.2mm bit with an m4 tap, in a drill press to make perpendicular holes.
Bend the metal. The metal should follow the contour of the mold. We aim for a 3mm space between the metal and mold (Figure 2), so the final product is close to the body but not so close that the patient feels the metal. With a bending fork and bending irons, work from the joint to the end of the bar; the tapered side should be away from the mold. Return the bar to the joint after every bend to check your work and identify where the next bend should be. For a smooth curve, make a series of small bends. Once you're satisfied, secure the metal in the joint with the attachment screw. Repeat these three steps for each piece of metal.
Backfill the metal. Overfill the space under the metal with a soft, heat-resistant putty-one that won't melt or bond to the plaster. With a tongue depressor, make a perpendicular slice along each edge of the metal and scrape away the excess putty (Figure 3). The putty will prevent the plastic from forming under the metal.
Apply vacuum socks. If necessary, fill significant imperfections in the mold and smooth the surface. Pull two layers of stockinette over the mold and metal, securing them beyond the proximal end of the mold with tape around the vacuum pipe. We use closed-toe socks for simplicity. The stockinette covers minor imperfections in the cast.
Cover joints. Cut a piece of 5mm Volara® foam, a heat-formable padding material, slightly less than the length of the joint and about three times the width of the joint. Staple the material over each side of each joint (Figure 4).
Thermoform. For a DTO, we use polyethylene terephthalate glycol-modified (PETG), a type of clear thermoplastic. Compared to polypropylene and other copolymers, PETG has a lower process temperature, cools more quickly, and does not shrink-factors that save time and ensure an accurate fit. Its rigid structure provides a feel similar to a definitive device, and its transparency makes it easy for the orthotist to identify pressure points. It is also easy to heat and form so the practitioner can change the shape or add flares.
Place a sheet of 6mm PETG in the oven at 325 degrees Fahrenheit for 15 to 20 minutes. After 15 minutes, or when the plastic becomes pliable, use scissors to cut a wedge from each side of the lower half to reduce the amount of excess below the knee. For the best results, we recommend having two people work on the KAFO. Work quickly because you only have a matter of minutes before PETG cools and no longer forms or adheres to itself. With the mold horizontal and the foot down, turn on the vacuum. When the plastic is ready, drape the PETG over the mold, making sure the plastic extends beyond the proximal and distal ends of the mold. Create a seal by pressing the plastic together around the foot, around the pipe, and along the underside of the mold (Figure 5). Vacuum makes the plastic hug every contour. Use a shop knife to cut away excess plastic before it cools (Figure 6).
Transfer trim lines. You can see the original trim lines through the clear PETG. Trace or sketch the trim lines with a marker (Figure 7).
Drill the attachment holes. Once the plastic is cool to the touch, drill through the plastic with the specified bit to mark the metal for precise placement of screw holes (Figure 8). Drill two holes in flat areas in each piece of metal above the ankle and three holes in each ankle stirrup.
Cut the plastic. When the PETG has cooled, cut out the plastic with a cast saw, staying at least ½ inch outside the trim lines (Figure 9). Remove the foot, calf cuff, and thigh cuff.
Finish the plastic. Grind each piece using a buffing cone to remove excess plastic until reaching the desired trim and then finish with a polishing cone. Use isopropanol with a small cloth to remove marker, putty, and dust from grinding. Carefully use a propane torch to heat treat the trim lines for a nice finish (Figure 10).
Drill. Use the specified bit in a drill press to complete the attachment holes in all the metal. Clean the metal, and clean all holes in the three plastic pieces.
Assemble. Use the hardware from the joint kit to attach metal to the joints and to attach the plastic shells to the metal. Tighten all fasteners, and add the straps. When it is completed (Figure 11), test the functionality of the joints to make sure the axis flexes freely. For stance-control orthoses, we also do preliminary programming to make sure the sensors are responding.
The DTO is now ready for the orthotist to check correct alignment and proper fit. Whether the practitioner makes changes, requests adjustments, or approves it as is, the DTO serves as the model for the definitive KAFO. In taking this extra step, we build confidence that the final device is exactly what the patient needs.
Justin Eitel is the technical orthopedics lead for Ottobock US. He oversees all orthotic fabrication at the Ottobock technical center, Minneapolis, Minnesota.