summary Patellar Clunk Syndrome is a painful, palpable "clunk" that can occur at the patellofemoral articulation of a posterior stabilized TKA caused by a fibrous nodule of scar tissue. Diagnosis can be made clinically with the presence of a painful, palpable "pop" or "catch" as knee extends (~40° of flexion). Treatment is observation for patients with mild symptoms. Surgical resection of the fibrous nodule is indicated for patients with severe symptoms. Epidemiology Incidence prevalence reported at 3.5% even less frequent with newer component designs Risk factors preoperative factors preop patella baja valgus pre-operative alignment preoperative fibrosynovial proliferation at quadriceps insertion into superior pole of patella resect this at the time of surgery previous knee surgery component factors patellar component login to view 8 more bullets femoral component login to view 6 more bullets thick polyethylene insert login to view 1 more bullet Etiology Mechanism A painful, palpable "clunk" at the patellofemoral articulation of posterior stabilized TKA caused by a fibrous nodule of scar tissue at the posterior surface of the distal quadriceps tendon/superior patellar pole catching on the box of the femoral component during knee extension Pathophysiology cause of scar tissue is unknown, but may be related to: direct trauma to quadriceps tendon during patella resection during TKA impingement of the quadriceps tendon on the femoral component due to an undersized patellar component scar is entrapped within the intercondylar notch during flexion the scar is forced out of the notch during active knee extension the painful snap or clunk is usually felt between 30-45 degrees Presentation Presents an average of 12 months after TKA Symptoms patellofemoral knee pain complaints of knee "popping" and "catching" Physical exam painful, palpable "pop" or "catch" as knee extends (~40° of flexion) Imaging US shows suprapatellar fibrous tissue Patellar crunch syndrome occurs when scar accumulates around the patellar component, causing a crunching sensation when bringing the knee from extension to flexion Treatment Operative arthroscopic vs open resection of fibrous nodule indications login to view 1 more bullet outcomes of arthroscopic resection are good, with very low rates of recurrence and improvement in knee society scores