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Overview
  • Biomechanically complex articulation between the patella and distal femoral condyles
  • Associated clinical conditions
    • trauma
      • patella dislocation
      • fracture
      • patella tendon rupture
      • quadriceps tendon rupture
      • OCD lesions
    • chondromalacia patellae
    • patellofemoral joint arthritis
    • patella instability
Anatomy
  • Osseous
    • bony constraint of the patella within the trochlear groove
      • intracondylar groove 
        • diameter of lateral femoral condyle > medial femoral condyle
        • bony constraint of groove is the primary constraint to lateral patellar instability when knee flexion is > 30 degrees
  • Ligaments 
    • static stability of the patella within the trochlear groove
      • medial patellofemoral ligament (MPFL)   
        • originates from the adductor tubercle to insert onto the superomedial border of the patella  
        • primary constraint to lateral patellar instability with knee flexion 0 to 20 degrees
      • patellotibial ligament
      • retinaculum
  • Muscles
    • dynamic stability of the patella within the trochlear groove 
      • vastus medialis = medial restraint to lateral translation
      • vastus lateralis = lateral restraint to medial translation
  • Tendon
    • angular difference between the quadriceps tendon insertion and patella tendon insertion creates a valgus axis (Q angle
    • creates a laterally directed force across the patellofemoral joint
  • Blood supply
    • superior, medial and lateral, geniculate arteries
    • inferior, medial and lateral, geniculate arteries
    • anterior geniculate artery
    • descending geniculate artery
Biomechanics
  • Function
    • transmits tensile forces generated by the quadriceps to the patellar tendon 
    • increases lever arm of the extensor mechanism
      • patellectomy decreases extension force by 30%
  • Biomechanics
    • patellofemoral joint reaction force 
      • up to 7x body weight with squatting 
      • 2-3x body weight when descending stairs
  • Motion
    • "sliding" articulation
      • patella moves 7cm caudally during full flexion 
    • maximum contact between femur and patella is at 45 degrees of flexion
  • Stability
    • passive restraints to lateral subluxation
      • medial patellofemoral ligament  
        • primary passive restraint to lateral translation in 20 degrees of flexion
        • 60% of total restraining force
      • medial patellomeniscal ligament
        • 13% of total restraining force
      • medial retinaculum 
        • 10% of total restraining force
    • dynamic restraint
      • quadriceps muscles
    • Q angle
      • definition
        • line drawn from the anterior superior iliac spine --> middle of patella --> tibial tuberosity 
      • normal Q angle
        • males = 13 degrees
        • females = 18 degrees
Imaging
  • Radiographs 
    • recommended views of the knee
      • AP, lateral, axial views
    • findings
      • AP = joint alignment, fracture, knee arthritis
      • lateral = patella alta vs baja, femoral condyle dysplasia, arthritis, transverse patellar fracture
      • axial = patella malalignment, trochlear groove depth, arthritis, vertical patellar fracture
    • measurements
      • patellar height (e.g. Insall-Salvati ratio)
      • lateral patellofemoral angle (normal is an angle that opens laterally )  
      • congruence angle (normal is -6 degrees)
  • CT 
    • indications
      • better visualization of the patellofemoral joint alignment
      • fracture
    • findings
      • trochlear geometry
      • TT-TG distance
  • MRI  
    • indications
      • best modality to assess articular cartilage
    • views 
      • T2 best sequence to assess cartilage

 

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