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Updated: Jun 11 2021

Patellar Clunk Syndrome

  • 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
          • small patellar component
            • because of decreased quads tendon contact forces against superior aspect of intercondylar box, allowing fibrosynovial proliferation and entrapment in intercondylar region of the box
          • patellar component placed low on the patella
            • exposes unresurfaced superior pole and quadriceps tendon to contact with femoral component
          • patellar overresection and thin patellar button
            • reduces offset of quadriceps tendon from top of trochler groove
          • exposure of cut patellar bone that is not covered by patellar component
            • resect uncovered lateral patellar facet
        • femoral component
          • PS design
          • increased posterior femoral condylar offset
          • smaller femoral component
          • femoral component in flexed position
            • causes by more posterior entry point for intramedullary distal femoral cutting jig, because of anterior bow of femur
          • femoral component with higher intercondylar box ratio (trochlear groove extended more proximal and anterior)
        • thick polyethylene insert
          • raises joint line, creates relative patella baja
  • 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
          • severe symptoms
        • outcomes of arthroscopic resection are good, with very low rates of recurrence and improvement in knee society scores
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