Clunking Knee after Knee Replacement

Some patients may experience a painful clunking of the knee on straightening the knee after a knee replacement. The symptom may be a sign of patellar clunk syndrome. Patellar clunk syndrome occurs as a result of the formation of a fibrous nodule on the undersurface of the lower end of thigh muscles. The symptom is associated with the use of older implants and is rare with the designs of modern implants.

During knee replacement surgery, the surgeon replaces the diseased/arthritic parts of the knee joint with prosthetic parts. The lower end of the thigh bone (femur) is replaced with a metallic component fixed with bone cement. Similarly, the upper part of the shin bone (tibia) is replaced with a metallic component fixed similarly with bone cement.

X-ray showing total knee replacement and the site of fibrous tissue in patellar clunk syndrome.

X-ray showing total knee replacement and the site of fibrous tissue in patellar clunk syndrome.

The undersurface of the kneecap (patella) is replaced with a high-grade plastic component. The femoral component has a groove on the upper surface for the patella to glide smoothing as the knee bends and straightens.

The thigh muscles (quadriceps) in front of the thigh are attached to the shin bone (tibia) with the patellar tendon. The quadriceps muscle is attached to the patella and the patellar tendon extends from the patella to the tibial tuberosity.

The exact cause of the formation of fibrous tissue underneath the quadriceps muscle near the patella is unknown. The scar formation has been hypothesized due to direct injury to the quadriceps muscle during patellar cutting and as a result of the small size of the patellar component.

The patellar clunk syndrome occurs exclusively in posterior stabilized implants as the fibrosis tissue is prone to get engaged in box shaped wedge of the femoral component.

Risk factors

Preoperative patient factors such as low lying patella, a history of prior knee surgery, scar tissue/fibrous prior to knee replacement, knock knee deformity before surgery, etc. are risk factors for the development of patellar clunk syndrome.

A smaller patellar component or low placement of the patellar component also increases the risk of patellar clunk syndrome.
Mal-position of the femoral component and large size of polyethylene insert may also increase the risk of patellar clunk syndrome.


The symptoms of patellar clunk syndrome are reported in the form of a painful clunk experience on straightening a bent knee. The patients may describe the clunk as a popping, snapping, or catching sensation. The patients typically present after a year of knee replacement surgery.


The diagnosis of patellar clunk syndrome involves a physical examination and radiological studies. A palpable and painful clunk may be felt on extending the knee at about 40-50 degrees of motion. The physician may request imaging studies in the form of ultrasound, X-ray, CT scan, or MRI.


The management of patellar clunk syndrome involves surgical removal of the fibrous tissue. The removal of the fibrous tissue may be done through an open incision or using an arthroscope.

Arthroscopic removal involves the use of miniature instruments. The surgeon gives keyhole incisions and inserts a miniature camera with a light source. The camera feed is displayed on a large screen outside for the surgeon. Miniature instruments are then guided through another keyhole incision to remove the fibrous tissue.

The open removal involves the removal of the fibrous tissue under direct vision. The surgeon gives a small incision and cuts/separates tissues to reach the undersurface of the quadriceps muscle. The fibrous tissue is then resected. The results of surgical removal are excellent and patients report relief from the symptoms.

My name is Dr. Suhirad Khokhar, and am an orthopaedic surgeon. I completed my MBBS (Bachelor of Medicine & Bachelor of Surgery) at Govt. Medical College, Patiala, India.

I specialize in musculoskeletal disorders and their management, and have personally approved of and written this content.

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