From a Jig to a Joint: Replacing a KAFO Knee Joint Made Easy
April 2012 Issue
Here's an interesting job that came into the lab recently: a well-worn KAFO with a broken knee joint.
Some might assume that it was time for a new KAFO, but for a number of different reasons that wasn't going to happen, so we had to figure out a way to replace the broken part. This happens frequently, so to make our lives easier we have devised a simple jig to make sure everything goes back together just like new.
What we need to do first is establish the extent of the damage. There are basically two ways a knee joint fails. The least common is the single-event catastrophic failure-the result of a one-time occurrence such as when one side locks and the other does not. This usually causes the locked joint to twist, torque, and snap. This is a difficult repair job because, more often than not, both joints become grossly misaligned in the process, and sometimes even the joint that did not fail is damaged to the point where it may be compromised.
The second and more common failure is the repeated stress failure. A repeated stress failure results from a repeated-bending load on the joint beyond its design parameters. This will eventually fracture a bar and it will continue to crack a little bit at a time until ultimately it will fail completely. Sometimes you can even see the individual micro fractures in the broken part. They look like a series of little waves that typically end in one big wave. That last big wave usually occurs just before you get a phone call from the patient!
In either event, the first two things you will need to do are to confirm that the remaining components are as square as you can make them, and verify the components that have not failed are in good condition.
Before you begin your repair, I recommend building a jig to assist you with the process. This jig is simple to build and costs just a few dollars using some parts from a hardware store. You'll need the following materials:
- One length of 3/8 in.-16 threaded rod.
- Four fender washers (3/8 in. id x 1 in. od).
- Eight 3/8 in.-16 nuts.
First, cut the 3/8 in.-16 threaded rod to about eight inches in length. This is usually long enough, and since the threaded rod is generally available in three-foot lengths, you will have plenty left over to make additional jigs of different lengths at a later time. Next, spin two of the nuts onto each end of the rod and then place a fender washer onto each end. These washers will each lay flat against the inside of each joint face.
Once the joints are as square as possible (as mentioned above), remove the joint center-axis nuts from the joints and install the jig by sliding the threaded rod into the holes. It is okay if there is a little slop in the holes; just make sure you do not have to force the rod in.
Finally, once the rod is in place, move the outermost nuts out until the fender washers touch the inside of the joint face. Then you can spin the innermost nuts out to lock the outer nuts in place. This is important because you don't want the spacing to change as you repair the joint.
On the side with the intact joint, you will need to apply another fender washer and two more nuts to the outside of the joint. This will lock the jig in place and keep the jig square relative to the knee joint. If everything is locked down tight, you can remove the broken joint and begin the repair.
By removing the joint center-axis nut from the new joint, you can slide the new joint on and off the jig easily and begin bending, removing, and replacing the joint on the jig as needed.
You can use this same method to replace both joints. Replace one side first, lock that side down with the outside nuts and washers, and then remove the outside nuts from the other side and repeat the process.
Broken joints are the kind of thing we work hard to avoid, but sometimes they happen. The best thing we can do is to make sure that we can quickly replace the part and get the patient back on his or her way. For about eight dollars you can build this jig and make that job a whole lot easier!
Tony Wickman, CTPO, is the CEO of Freedom Fabrication, Havana, Florida. He can be reached at