Patellar clunk syndrome (PCS) is a relatively uncommon postoperative complication of total knee arthroplasty (TKA) with a resurfaced patella. Despite being relatively uncommon, it is a common reason for revision of TKA.

Causes / typical injury mechanism: PCS occurs because of the growth of a fibrous nodule or scar tissue on the posterior aspect of the quadriceps tendon. Generally, the location of the nodule is just proximal to the base (superior pole) of the patella, and the symptomatic "clunk" is experienced as the nodule becomes lodged in the intercondylar notch of the femoral prosthesis with active knee extension.

While the exact etiology behind the formation of the nodule is unclear, it is thought to arise either due to the placement of a patellar button when the patella is resurfaced or due to the design and geometry of the intercondylar notch of the femoral prosthesis.

Classic history and presentation: On average, this postoperative complication arises within 7.4 months after TKA. Common symptoms include a painless or painful clunk and/or catching of the patella prosthesis as the knee is actively extended from a flexed position. Symptomatic pain is usually localized to the anterior aspect of the knee.

Generally, patients will complain of an anterior knee pain that occurs as the patient extends the knee from a fully flexed position. This is followed by a sometimes painful and/or audible clunk as the knee reaches the fully extended position. This clunk is usually felt and/or heard at 30-60 degrees from full extension.

There is a milder form of this syndrome that can present as a painless or painful crepitus as the knee is extended from a flexed position.

Prevalence: The estimated incidence ranges from 0%-20%, depending on the design and technique employed in TKA.

• Age – Elderly individuals are at higher risk for developing postoperative complications in general.
• Sex / gender – Male patients are at a higher risk of developing PCS.

Risk factors: Most risk factors are due to the patient's anatomy and/or the surgical technique / materials employed by the joint replacement surgeon during TKA. These include the following.

• Small patellar component size
• Patella baja
• Patella alta
• Femoral implants with an increased intercondylar box ratio
• Posterior stabilized TKA
• Smaller and flexed femoral components
• Thicker tibial polyethylene inserts
• Anterior placement of tibial implant
• Reduced patellar composite thickness
• Increased degree of postoperative knee flexion
• Previous knee surgery

Although these risk factors have been identified, how they specifically contribute to the disease is still unknown and likely multifactorial.

Pathophysiology: PCS is likely caused by excessive scar tissue formation at the posterior aspect of the patella following TKA.