Knee motion is more than just bending and straightening. The two main bones that form the knee (femur and tibia) also glide, slide, tilt, and rotate. The combination of all these motions is called kinematics.

Studies show major changes in knee kinematics after a total knee replacement. There may be several reasons for this. One is the change in the shape of the bony surface of the knee that occurs when the implant is put in place. Another is the loss of the cruciate knee ligaments that criss-cross inside the joint to hold it together. Sometimes one or both of these ligaments are cut to insert the implant.

Finally, the way nerves and muscles work together can be altered when other diseases or conditions are present. This can include Parkinson's, multiple sclerosis, the results of a stroke, and many others.