Patellar Clunk Syndrome: Complication Following Knee Replacement Surgery
In general, Total Knee Replacement (TKR) is a highly successful procedure. In fact, according to the American Academy of Orthopaedic Surgeons, more than 90% of patients have well functioning implants 15 years after surgery.1 Unfortunately, complications, although relatively rare, can sometimes occur. One such complication is known as Patellar Clunk Syndrome.
Patellar clunk syndrome can result in the following symptoms:
- “Patello-femoral” knee pain (pain in front of knee in region of knee-cap)
- Knee “popping” or catching during bending and straightening as in rising from a chair
To understand this condition, it helps to first understand a little bit about normal knee anatomy and function (Figure 1).
During knee flexion (bending) and extension (straightening), the patella (knee-cap) rides within a “track” or groove in the femur (thigh bone), known simply as the patellar groove. This motion is smooth and no pain or disruption in this motion is felt during normal activity. In a patient with Patellar Clunky Syndrome, however, this motion is not smooth and can become painful. So how does this occur?
During Total Knee Replacement, the worn ends of the bones are “resurfaced” with metal and plastic implants. The patellar groove on the femur (thigh bone) is part of the area that is replaced and is part of the metal femoral component that is implanted onto the prepared thigh bone during the total knee replacement procedure. At the end of this groove there is a transition between the metallic implant and the native bony surface.
Refer to the X-ray image below of a knee where a total knee replacement prosthesis has been implanted (Figure 2-patella is not visible in this view).
Following knee replacement, scar tissue can sometimes form at the top or “superior pole” of the patella. During particularly deep flexion, this scar tissue may move below the end of the groove in the femoral component and then “catch” on the end of the groove as the patella moves back with knee extension. It is this catching and then forceful release with extension that results in the “clunk” and pain characteristic of this condition.
This condition is more prevalent in a type of knee replacement known as Posterior Stabilized, or “PS” knee replacement, although it has also been reported in Cruciate Retaining (CR) knee designs as well.2 In PS knee designs, where the posterior cruciate ligament is removed, the patellar groove tends to be shorter to avoid contact (in extension) between the end of the groove and a plastic post on the tibial component found in these types of designs. Consequently, the patella can come off the end of the groove during lesser degrees of flexion.
The article at this link compares the incidence of patellar clunk in two total knee replacement implant designs. It contains some illustrative images of knee replacement implants and discusses the parameters in the design that can help to reduce the likelihood of patellar clunk.
The publication cited at the end of this article provides more detailed information and a review of some of the literature on patellar clunk.
1″Total Knee Replacement-OrthoInfo – AAOS.” Total Knee Replacement-OrthoInfo – AAOS. N.p., Dec. 2011. Web. 07 Nov. 2013.
2 Niikura T, Tsumura N, Tsujimoto K, Yoshiya S, Jurosaka M, Shiba R. Patellar Clunk Syndrome After TKA with Cruciate Retaining Design: A Report of Two Cases. Orthopedics. 2008: 31:90