• According to gait studies during stance phase of normal gait, the knee joint bears about 2-6 times body weight of compressive force due to normal muscular contraction and body mass.
  
• The geometric axis of flexion by Braune and Fischer (= static center of rotation) of knee joint, and the actual axis of rotation of the knee during gait do not coincide; the dynamic axis of rotation (= instant center) moves as the knee flexes drawing a J curve (Fick's curve) (Fig.1 & 2).
  

• Since the instant center of the femur does not coincide with the contact point of the knee joint, the weight-bearing surface of the femoral condyles slide over the menisci and tibial surface (sliding contact, not rolling), therefore normal friction loss and wear may result. Three paticular points and relatively NORMAL instant center, and the direction of the velocities of three points on the femur are shown (Fig.3). Each velocity is tangential to the tibia and menisci, namely, neither separating nor compressing to the contact point but is sliding with normal free motion.

  If the instant center has been displaced because of INTERNAL DERANGEMENT, the directions of the velocities are no more tangential to the joint surface but rather penetrating or separating (Fig.4).

• As we know, the femoral condyles and mensci bear 2-6 times body weight during normal gait. This may double when you run, jump, twist, and ski. This can be tripled when you age and by osteoarthritic changes the friction coefficient of menisci/condyles increase (which means less slippery) and/or abnormal kinematic motion due to wear, tear, and trauma of knee joint components is introduced . Nearly 18 times body weight of compression/shear force may often cause more severe joint damage. (gif.MOVIE).

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