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Rolling and Gliding Mechanism

- See: Role of Knee Joint in Gait:
- Discussion:
- for reason of contour of femoral & tibial condyles, flexion & extension of knee joint are not simple hinge movements that occur at the 
     elbow joint.
- flexion & extension do not occur about a fixed transverse axis of rotation but rather about a constantly changing center of rotation,
     that is, polycentric rotation.
     - when plotted, path of this changing center of rotation describes J-shaped curve about the femoral condyles;
- in full flexion, posterior portions of femoral condyles are in contact w/ posterior portions of the tibial condyles.
- knee is extended, femoral condyles roll on tibial condyles & meniscii, movement resembling that of a rocking chair.
- Extension of Knee Joint:
- moving from flexion to extension, shorter, more highly curved lateral condyle exhausts its articular surface & is checked by ACL, whereas 
   larger & less curved medial condyle continues its forward roll & skids backward, assisted by tightening of PCL;
- because medial femoral condyle is larger than lateral fem condyle, distance from extreme flexion contact point to extreme extension 
   contact point of medial femoral condyle is about 17 mm > that of lateral femoral condyle;
- as tibia travels from flexion to extension the medial tibial plataeu must cover a greater distance;
- result is external rotation of tibia that tightens collateral ligaments, and joint is "screwed home;"
- ratio of rolling to gluiding:
   - ratio 1:2 in early flexion & to increase to one to four by end of flexion;
- Flexion of Knee Joint:
- flexion of extended knee is preceded by internal rotation of the tibia, produced by Popliteus muscle;
   - this rotation relaxes tension of collateral ligaments suffic to permit flexion.
- ratio of rolling to gluiding:
   - ratio 1:2 in early flexion & to increase to one to four by end of flexion;
- Rotation:
   - during nl gait, tibia undergoes Internal Rotation during swing phase and external rotation during stance phase