knee biomechanics


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knee biomechanics

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knee biomechanics

  1. 1.  The knee is a mechanism of three joints and Four bones - the femur, tibia, patella and fibula  Interact in separate joints - the tibiofemoral & patellofemoral  The function of these joints is to allow certain movements, restrict others, and to provide load transfer through the lower limb.
  2. 2. Tibiofemoral joint rotations translations screw home mechanism  Axial & rotational alignment of knee  Patello femoral joint  Joint forces 
  3. 3.  Rotations › Flexion/extension-0 to 1350 › varus valgus - 6-8o in extension › Int/ext rotation - 25 – 300 in flexion  Translations › AP 5 - 10mm › comp/dist 2 - 5mm › medio-lateral 1-2mm
  4. 4.  flexion axis varies in a helical fashion in a normal knee, with an average of 2 mm of posterior translation of the medial femoral condyle on the tibia during flexion compared with 21 mm of translation of the lateral femoral condyle.  Relevance :posterior rollback › as the knee flexes, the instant center of rotation on the femur moves posteriorly
  5. 5.  flexion axis as varying in a helical fashion
  6. 6. allows for increased knee flexion by avoiding impingement
  7. 7.    the external rotation of the tibia on the femur during extension and internal rotation of the tibia during knee flexion. cause › medial tibial plateau articular surface is longer than lateral tibial plateau(Medially based pivoting of the knee.) relevance › "locks" knee decreasing the work performed by the quadriceps while standing
  8. 8.  mechanical axis of the lower limb is defined as the line drawn on a standing long leg antero posterior radiograph from the center of the femoral head to the center of the talar dome  anatomical axes of the femur and the tibia form a valgus angle of 6 2 degrees.  the tibial articular surface is in approximately 3 0of varus with respect to the mechanical axis, and the femoral articular surface is in 90 of valgus.
  9. 9. "sliding" articulation › patella moves 7cm caudally during full flexion  maximum contact between femur and patella is at 45 degrees of flexion   The primary function of the patella is to increase the lever arm of the extensor mechanism around the knee, improving the efficiency of quadriceps contraction.
  10. 10.  The quadriceps and patellar tendons insert anteriorly on the patella, with the thickness of the patella displacing their respective force vectors away from the center of rotation of the knee .  This displacement or lengthening of the extensor lever arm changes throughout the arc of knee motion.  the extensor lever arm is greatest at 20 degrees of flexion, and the quadriceps force required for knee extension increases significantly in the last 20 degrees of extension
  11. 11.  The length of the lever arm varies as a function of the geometry of the trochlea, the varying patellofemoral contact areas, and the varying center of rotation of the knee.
  12. 12. 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 › lateral retinaculum  10% of total restraining force  dynamic restraint › quadriceps muscles 
  13. 13.   The angle between the extended anatomical axis of the femur & the line between the center of the patella & the tibial tubercle normal Q angle › in flexion  males  13 degrees  females  18 degrees › in extension  8 degrees
  14. 14.  Limbs with larger Q angles have a greater tendency for lateral patellar subluxation.  Because the patella does not contact the trochlea in early flexion, lateral subluxation of the patella in this range is resisted primarily by the vastus medialis obliquus fibers.
  15. 15. Position  Standing on both feet  Swing phase  u/l stance phase –  Jogging – force acting on joint equal to body wt 1/2 x b.wt 2-4 x b.wt 6x b.wt
  16. 16.  Walking › 0.3 x body weight  Ascending Stairs › 2.5 x body weight  Descending Stairs › 3.5 x body weight  Squatting › 7 x body weight
  17. 17. Prevent anterior tibial displacement on femur  Secondarily, prevents hyperextension, varus & valgus stresses  Least stress on ACL between 30-60 degrees of flexion  Anteromedial bundle tight in flexion & extension Posterior lateral bundle tight only in extension
  18. 18. Primary stabilizer of the knee against posterior movement of the tibia on the femur  resists rotation, esp.internal rotation of tibia on femur  Two bundles  Anterolateral, taut in flexion  Posteromedial, taut in extension