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Patello-Femoral Instability

Patello-femoral Joint Mechanics

  • Patella increases moment ( lever ) arm of extensor muscles
  • Slides 7 cm in trochlear groove
  • Patello-femoral contact minimal until 20 deg flexion
  • Contact area moves from proximal -> distal in trochlea  & distal to proximal on patella
  • > 90deg flexion quads tendon is in contact with the trochlea
  • 10deg flexion -> lever arm increased 10 %
  • 45deg -> 30 % then decreases
  • Patello-femoral joint reaction force determined by quads force and amount of knee flexion
  • Normal walking -> joint compressive forces = half bodyweight
  • Up stairs -> 2.5 - 3.3 times body wt
  • Deep knee bends -> 7-8 x body wt

 

Presentation

PFJ instability may present as:

  1. Anterior Knee Pain
  2. Patello-femoral subluxation
  3. Patello-femoral Dislocation

 

Anterior Knee Pain

Other causes of anterior knee pain include:

  1.   Patello-femoral overload (Chondromalacia)
  2.   Plica Syndrome
  3.  'Jump' knee (enthesitis of patella tendon origin)
  4.  Sindig-Johansson-Larsen's disease (traction tendinitis at lower pole of patella, w/ calcification)
  5.  Torn Meniscus
  6.  Discoid Meniscus
  7.  Osteochondritis Dissecans
  8.  Patella Bursitis
  9.  Bipartite Patella
  10.  Patella cysts or tumours

 

Risk Factors / Causes (Hedden (Toronto); Curr Orth. 9:249-52. 1995):

A. Bony (Static Stability)
  • Shallow femoral trochlea (Dejour et al.)
  • Hypoplastic lateral femoral condyle
  • Patella Shape (Shutzer et al.)
  • Patella Alta 

B. Malalignment

  • External tibial torsion
  • Incr. femoral anteversion
  • Incr. genu valgum
  • Incr. Q angle (unreliable)

 

C. Soft Tissue (Dynamic stability)

  • Ligamentous laxity
  • contribution of distal oblique portion of vastus medialis muscle is critical
  • (tight hamstring & gastrocnemius w/ pronated feet)

 

Clinical

Mechanism of injury, acuity, previous treatment, and status of the opposite knee. 

Q angle

The integrity of the ACL (because similar derotation, deceleration mechanisms that cause patellar instability can cause ACL injury) 



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