Updated: 8/12/2021

Patellofemoral Joint

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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
        • leads to increased contact pressures in lateral patellar facet between 40-90 degrees 
    • 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 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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