The Essential Guide to KAFO Fabrication, Part 2
November 2011 Issue
How to Set Up Proper Alignment
They're coming. Scanning tools, software, and milling machines will one day be able to duplicate every contour of a patient's leg, making it possible to fabricate a KAFO with unmatched precision.
This technology will need to be refined and adapted to our industry before scanners and carving machines can handle the sheer size of a whole limb, the complexity of the dorsum, and plantar surfaces of the foot. This cutting-edge technology, however, may not be affordable to most O&P facilities for many years.
Until then, we have to fabricate KAFOs the old-fashioned way, which requires a plaster cast. Last month, I described how to prepare a negative wrap ("The Essential Guide to KAFO Fabrication. Part 1: How to Prepare a KAFO Negative Wrap," The O&P EDGE, October 2011). How well you mark and correct the cast can either simplify or complicate the setup for proper alignment of the knee and ankle joints—the subject of this article.
Earlier in my career, I had to measure repeatedly and rely on my eyes to achieve an alignment that was roughly level, parallel, or perpendicular. However, after joining Otto Bock, I began using an orthotic alignment jig, which is efficient and provides professional results.
Designed for instant horizontal positioning at precise heights and built-in vertical guides, the alignment jig makes work quicker and easier while enabling you to consistently fabricate a device exactly to the practitioner's specifications. Most alignment jigs use plumb lines. We use an alignment jig mounted on an Otto Bock laser alignment table (no longer in production) to project vertical lines on the negative wrap. Either setup will help you master three-dimensional alignment of a test orthosis.
Place the negative wrap in an orthotic alignment jig. Adjust the knee and ankle yokes to the heights listed on the technical sheet provided by the practitioner. Assuming the negative wrap was prepared with the patient bearing weight, the foot has flat areas that can stabilize it on the foot platform, unless the practitioner specifies a different alignment.
Mark the anterior alignment line. Position the negative wrap so the jig's anterior vertical reference line runs through the center of the patella. Given the natural rotation of the foot, the vertical line also should run between the first and second ray (Figure 1). Screw in the alignment pins of the knee yoke until they just touch the negative wrap. Double-check that the heel, the gap between the rays, and the knee are still in alignment. If not, loosen the alignment pins, reposition the negative wrap, and reset the pins (Figure 2). Then screw in the alignment pins on the ankle yoke until they just touch the negative wrap (Figure 3).
The alignment pins touch the point where intersecting lines mark the joint axis on both sides of the knee and ankle. The height must be exact. The horizontal position must be within a few millimeters anterior or posterior. If not, check the technical sheet and the yoke heights and make additional corrections at the joints. Draw an anterior vertical line that extends the length of the negative wrap.
The standard alignment used at Otto Bock, and preferred by most practitioners, is to have joints parallel to the frontal plane. This assures that all joints work in the same plane and not against each other. However, some practitioners favor an alignment at the KAFO ankle joint that is more in line with the anatomical ankle joint, which gives the foot a normal external rotation of 7 to 12 degrees. If this is requested by the practitioner, loosen the rear knob on the ankle yoke, rotate the yoke to the desired external rotation, and tighten the knob.
Mark lateral and medial alignment lines. The jig's lateral and medial vertical reference lines should be anterior to the alignment pins on the knee yoke. If necessary, adjust the yoke to move the alignment lines anterior to the pins. Draw lateral and medial alignment lines that extend the length of the negative wrap.
You now have recorded the negative wrap's alignment in the frontal and sagittal planes, plus the horizontal plane of the joint axis for both the knee and ankle. Here's the added benefit of an orthotic alignment jig: Each job has its own components, and the component settings save the alignment throughout the process.
Insert the knee adapter in the negative wrap. Remove the negative wrap from the alignment jig. Place a pedillen adapter sleeve on one joint axis for the knee, centering the bumps on opposite sides of the sleeve along one of the intersecting lines. Later, those bumps will keep the sleeve from sliding when you pour plaster. Trace around the sleeve, keeping the barrel of the pen parallel to the sleeve, so the square is close to one inch on each side (Figure 4). Repeat on the other joint axis. Cut out the squares with a cast saw (Figure 5). Slip the sleeve through the holes to make sure it does not bind. Rub an aluminum jig adapter with a lubricant, such as petrolatum, to minimize resistance, and slide the adapter into the pedillen sleeve. Then insert the adapter and sleeve into the holes on the negative wrap.
Size the sleeve. Remove the sleeve and pull out the adapter. Check the manufacturer's specifications for clearance on each side of the joint. Add at least another ¼ inch on each side to ensure you have enough material to shave down during the modification process. If you're not experienced with this process, add length as a precaution. Just be aware that the more excess you have now, the more you will have to cut or shave later. Add those amounts plus the cast's width at the joint, then measure and mark that length on the sleeve (Figure 6). Cut into the four corners with a knife, grab the sleeve on each side of the notches, and snap off the excess. Reinsert the sleeve and adapter into the negative wrap.
Align and secure the knee adapter. Remove the alignment pins for the knee from the alignment jig. Return the negative cast to the alignment jig, placing the knee adapter in the same slots that held the alignment pins on the knee yoke (Figure 7). Position the joint axis marks on the ankle between the alignment pins on the ankle yoke. Then match the vertical alignment lines on the negative cast to the jig's anterior, lateral, and medial vertical reference lines. Check to make sure the adapter is resting flat on the jig, the sleeves are flat and have equal clearance on each side, and there is no binding at the holes in the wrap. Use light putty, mixed according to the manufacturer's guidelines, on each side of the knee to secure the sleeve in that alignment (Figure 8). The bigger the hole, the more putty you need, so you might want to cut smaller squares as your technique improves.
Insert, align, and the secure ankle adapter in the negative wrap. Remove the negative wrap from the alignment jig, and repeat the previous three steps for the ankle after checking the instruction sheet for any requests from the practitioner.
Seal the negative cast. Seal all visible openings in the negative cast with plaster bandage to avoid having leaks when you pour the plaster. Let it cure until firm.
Extend the height. Check the instruction sheet for the overall height of the brace. Extend the thigh section to the requested height plus two inches. The lateral side may be close to the overall height, but the medial side probably will need more height to contain the plaster.
If the height requires less than two more inches, you can use duct tape. Tear off a strip a couple inches longer than the top circumference of the wrap and lay it adhesive-side up on a countertop. Tear off another strip the same length, and lay it adhesive-side up overlapping half of the first strip. Add another overlapping strip or two, if necessary. Then wrap the sheet of duct tape around the cast, making sure that both the medial and lateral sides reach the desired height. Duct tape is not a good choice for larger extensions because it tends to narrow at the top. For an extension of more than two inches, use a 1mm sheet of plastic, such as polyethylene, because it extends the shape of the thigh (Figure 9). Wrap the plastic around the top of the cast, and secure it with staples or duct tape.
Prepare a pipe. Choose a pipe longer than the negative wrap. Hammer the last few inches on one end flat, which will keep the pipe from rotating in the plaster. Bend a six-inch flat strip of aluminum in a vise to mimic the contour of the cast at the foot. Use duct tape to secure the aluminum strip to the flattened pipe (Figure 10). Make sure the strip extends into at least one-fourth the length of the foot because the plaster will need strength for modifying later. Cut unwanted excess pipe with a band saw.
Pour plaster. We use a sandbox—a small bin filled with sand—to hold the cast upright and to contain the plaster (Figure 11). Clear out one corner of the bin. Rest the cast in the corner and hold it in place while filling in around the cast with sand, which supports but does not crush the cast.
Insert the pipe into the cast behind the ankle adapter. It's important to maintain a 1/2-inch clearance from the bottom because plaster tends to break off where the pipe is near or touches the cast. We solved that challenge with a giant clothespin, which is just two two-foot pieces of plastic fastened together at both ends by duct tape. When we slide the separated pieces of plastic over the pipe and slide them down to rest atop the cast, there's enough pressure to hold the pipe in place, and we have a clear view of the action as we adjust the clearance from the bottom. After ensuring there is enough clearance, center the pipe at the bottom and the top.
Mix the plaster according to package instructions and pour it into the negative cast. If the plaster moves the pipe, try to center it again. Allow the plaster to harden until firm.
You now have a plaster cast with perfect alignment. In the final article in this series, we'll complete the process of fabricating a KAFO by making modifications.
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.