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Anatomy and Kinematics of the Knee Joint

- See: role of knee joint in locomotion and mechanical axis

- Discussion:
    - bones forming the knee joint are femur, tibia, & patella;
    - both left and right femurs converge toward knee and each tibia is nearly vertical, femur and tibia meet angle of some 5-12 deg;
            - greater angle results in genu valgum;
            - lesser angle results in genu varum;
    - patello-femoral function:
           - knee from 0-20 deg: involves internal rotation of tibia, which decr Q angle & lateral directed quadriceps muscle vector;
           - patella is drawn into trochlear notch of femur, & patellofemoral contact is made from 0 to 20 deg of knee flexion;
           - the initial contact is made at the lateral facet of the patella;
           - further flexion of knee moves patella anterior relative to center of rotation of the knee, which improves the
                   mechanical advantage of quadriceps mechanism;
           - patella continues to move laterally at 90 deg of knee flexion, and lateral border of the patella provides the primary loading site;
    - condyles:
           - femur ends in two rounded condyles joined anteriorly to from patellar articular surface and separated posteriorly by a deep
                     intercondylar fossa;
           - condyles are almost in line w/ front of shaft, but they project backward well beyond shaft, as in letter J, circumstance of
                     significance in movement of the joint;
           - medial condyle is larger, more curved, & projects further than lateral condyle, accounting for angle between femur & tibia;
           - sides of condyles are roughened and project somewhat as medial and lateral epicondyles;
           - the bigger the posterior bulge of the femoral condyle, the greater can be ROM into flexion as tibia glides around convexity;
                    - this allows full flexion w/o contact between the posterior joint margins of the tibia and femur;
           - thick cartilaginous surfaces of knee helps to spread out joint reactive load over a wide area and helps contribute to cam shape
                   of condyles which maximizes the extensor lever arm;
           - in degenerative arthritis the quality of the articular cartilage is lost;
                  - as wear occurs, the patello femoral joint is reduced to a cylindrical outline;
                  - the mechanical outline is lost, but wear in the bone to bone contact area is reduced;
    - tibial plateua:
         - on its expanded upper end, tibia has 2 slightly concave condyles separated by an intercondylar eminence and the sloping areas
                  in front and behind it;
         - low elevation, tibial tuberosity, for insertion of quadriceps, is situated at the junction of anterior border of shaft with expanded
                  upper end of the tibia;

- Roll Back and Sliding of the Knee Joint: (role of knee joint in locomotion)
    - contour of femoral & tibial condyles, flexion & extension of knee joint are not simple hinge movements that occur at 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 a J shaped curve about the femoral condyles;
    - for example, 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.
    - there is also sliding of the femur backward.
           - as extension progresses, shorter, more highly curved lat 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;
           - result is medial rotation of femur that tightens collateral ligaments, and the joint is "screwed home;"
    - flexion of extended knee is preceded by lateral rotation of femur (or medial rotation of tibia), usually produced by popliteus muscle;
           - this rotation relaxes tension of collateral ligaments suffic to permit flexion;
    - exact ratio of rolling to gluiding differs between individuals and does not remain constant thruout all deg of flexion;
           - it is estimated to be one to two in early flexion & to increase to one to four by end of flexion;
    - during normal gait, tibia undergoes internal rotation during swing phase and external rotation during stance phase;
    - because the medial femoral condyle is larger than lateral fem condyle, the 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;
    - ref: Does the femur roll-back with flexion?

- Knee Rotation:
     - as extension progresses, shorter, more highly curved lateral condyle exhausts its articular surface and is checked by ACL, whereas
             larger and less curved medial condyle continues its forward roll & skids backward, assisted by tightening of PCL;
     - result is a medial rotation of femur (external tibial rotation) that tightens collateral ligaments, & joint is "screwed home," to use
             mechanical phraseology;
     - flexion of extended knee is preceded by lateral rotation of femur (or medial rotation of tibia), usually produced by popliteus;
     - this rotation relaxes the tension of the collateral ligaments sufficiently to permit flexion

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