Patellar Clunk Syndrome - Recon

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

Action	Numeric Key	Letter Key	Function Key
Show Bullets		S	Enter (frontside only)
20%	1	N
40%	2	H
60%	3	F	Enter (backside only)
80%	4	E
100%	5	M
Toss	0	T