The knee joint is located where the thigh bone (femur) and shin bone (tibia) meet. At the end of the femur are two rounded bumps, called condyles, and the gliding surface of the tibia is called the plateau.

  • Inner knee. Where the tibia’s medial plateau and femur’s medial condyles meet forms the inside “compartment” of a knee. (Standing with legs together, the left and right knees’ medial condyles are next to one another.)
  • Outer knee. The outer knee, or outside compartment, is comprised of the tibia’s lateral plateau and femur’s lateral condyles.
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When degeneration happens in just one compartment of the knee joint, it is called one-sided knee arthritis, unicompartmental knee arthritis, or unilateral knee arthritis.

To understand how one-sided arthritis often happens, imagine two people carrying a couch down a set of stairs. The person at the bottom of the stairs bears the greatest weight burden. A person with one-sided knee arthritis is similar; one side of the knee joint carries a bigger load and is, therefore, more prone to wear-and-tear.

Knock-kneed or Bow-legged Alignment

In a normal leg, an imaginary straight line drawn from the center of the hip’s femoral head (the “ball” of the hip’s ball and socket) to the center of the ankle will pass through the center of the knee. This straight line, called the mechanical axis, shows how the body bears weight when standing.

A difference of just millimeters in where the mechanical axis passes through the knee can make a big difference in the wear-and-tear on a knee joint.

    Bow-legged alignment. If the mechanical axis passes 15 mm or more inside the center of the knee, a person is considered bow-legged (varus deformation). People who are bow-legged tend to have more wear on the inside (medial side) of their knees. High tibial osteotomy is more often recommended in these cases.

    Knock-kneed alignment. If the mechanical axis passes 10 mm or more to the outside of the knee’s center, the person is considered to have knock knees (valgus deformation). People with knock knees tend to have more cartilage wear in the outside (lateral side) of their knees. Femoral osteotomy is more often recommended in these cases.

A knee osteotomy can change bone alignment so that the mechanical axis passes though the center of the knee and weight is more evenly distributed. (In the couch moving analogy, both movers would stand on more level ground.) This surgery will not repair the joint degeneration that has already occurred on the “bad” side, but rather it is done to transfer stress to the good side of the joint to relieve pain and mechanical stress.