Patellar Clunk Syndrome

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Introduction
  • Definition
    • 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
  • Epidemiology
    • 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
  • 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
Differential
  • 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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