Orthoteers homepage Advertise on Orthoteers
Orthoteers Junior Orthoteers Orthopaedic Biomechanics Orthopaedic World Literature Society Educational Resources Image Gallery About Orthoteers Orthoteers Members search

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



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)



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) 

This is a preview of the site content. To view the full text for this site, you need to log in.
If you are having problems logging in, please refer to the login help page.

© 2005-2007 Orthoteers.co.uk - last updated by Len Funk on 01 October 2007Medical Merketing and SEO by Blue Medical 
Biomet supporting orthoteersThe British Orthopedic Association supporting OrthoteersOrthoteers is a non-profit educational resource. Click here for more details
Total Knee Replacement
Anterior Cruciate Ligament Injuries
Knee - Osteotomies
Knee Anatomy
Knee Arthrodesis
Knee Ligament - Sectioning Studies
Knee Ligament Injuries
Knee Mechanics
Lateral Meniscus Syndrome:
Medial Meniscus Syndrome
Meniscal Injuries
Meniscal Special Tests
Osteochondritis dissecans
Patellofemoral Disorders
Patello-Femoral Instability
Posterior cruciate ligament injury
Spontaneous Osteonecrosis of the Kn...
Hide Menu