Imagine that after years of painful knee symptoms, you have a total knee replacement (TKR). Ahhh, relief at last! But within a couple of months, the knee starts making a loud "clunk" every time you straighten it from a fully bent position.
The problem could be the patellar clunk syndrome. This syndrome occurs when a fibrous nodule develops on the back of the kneecap (patella). When the knee bends, this fibrous bump gets trapped within a notch in the surface of the thighbone (femur). (The bottom of the femur meets the top of the tibia in the lower leg to form the knee joint.) As the knee straightens, the bump moves out of the notch. Knee pain and a "crunching" sound occur as the patella moves against the femur. At the same time, a "clunk" is usually heard.
Doctors think that two factors cause the patellar clunk syndrome: the design of the joint implant (on the femoral side) and the patient's knee-flexion angle. Generally, only patients with more than average knee flexion get this problem.
The authors of this study report this may be a problem of the past. The newest implant design offers a longer and deeper groove on the back of the kneecap. With this new implant, it takes much more knee flexion for the patella to drop into the notch in the surface of the femur. The chances of getting a nodule and a clunk are much lower with the design of the new implant.
TKRs have gone through many design changes over the years. Starting in 1978, there have been a series of changes that affect the kneecap. The second generation of implants was released in 1989. Another design made its way onto the market in 1995. The newest implant and next generation appears to have eliminated patellar clunk syndrome.
William J. Maloney, MD, et al. Femoral Component Design and Patellar Clunk Syndrome. In Clinical Orthopaedics and Related Research. May 2003. Vol. 410. Pp. 199-202.'