Planar Motion

The planar motion of the two adjacent body segments can be described by the concept of the instant center of motion. As one body segment rotates about the other, at any instant, there is a point that does not move. This point has zero velocity and acts as a center of rotation. This technique yields a description of motion at one point only and is not applicable if motion of 15 degree or greater exists in other planes. When the instantaneous center of rotation is at the contact point between femur and tibia, the instantaneous velocity is zero and the tibia is rolling around the femoral surface. An understanding of the motion between the articulating surfaces of the knee joint is important for understanding causes of wear, instability and loosening

KNEE JOINT STABILITY
The muscles, ligaments, menisci, osseous geometry and joint capsule all combine in a complex manner to produce joint stability. If any of these structures malfunction or disrupted, knee joint instability occurs. These factors are all interdependent and serve the function of both determining normal motion and limiting motion beyond a certain point.
JOINT SURFACE
The constraints provided by the femoral and tibial joint surfaces are not adequate for functional stability. The distal femur is convex, where as the proximal tibia is partially flat, medially slightly concave and convex laterally. However, the tibial intercondylar eminence and the articular geometry added some potential for stability. Hsieh and Walker concluded that geometric conformity of the condyles was the most important criteria to decrease laxity under load bearing. They also came to the conclusion that, to perform anterior or posterior, rotatory and medial or lateral movements, the femur must glide upward on the tibial curvature. Similarly, to rotate the femur "screws out", giving an upward movement. Medial/lateral motion produces this effect to an even greater degree because of the tibial spines. This is called the "uphill principle". These authors concluded that under low loading conditions, the soft structures (ligaments, capsule and meniscus) provided joint stability and whenever there is increase in the load; the conformity of femoral condylar surface becomes the most important factor for joint