The knee has the difficult task of transferring the load of the body while at the same time having the freedom to allow rapid change of direction and speed. This necessary twisting and rotating adds to the stress the joint endures. With certain activities, the knee is subjected to nearly 10 times body weight (another reason to keep body weight optimal).

The Knee Joint
The knee joint is made up of three bones, four major ligament groups, and two different types of cartilage. The bones in the knee are:

Femur (thigh bone): At the knee joint there are two separated prominences called condyles. Touching your knees together brings the inside condyles closer together—the medial femoral condyles. The outer condyle is the lateral femoral condyle.

Tibia (shin bone): The tibia meets the femur at the knee with two areas on which the two femoral condyles “ride”. These are the medial and lateral tibial plateaus.

Patella (knee cap) - Rides in a shallow groove (or sulcus) over the front part of the femur called the trochlea.

The ends of these bones at the joint are covered by articular cartilage. This glistening white substance has somewhat the consistency of firm rubber but is actually a mixture of collagen and special large sponge-like molecules all maintained by living cells (chondrocytes). With normal joint fluid, the surface is more slippery than water on ice. This allows for normal smooth and easy joint motion.

The other type of knee cartilage is the meniscal cartilage. These C-shaped pads are found between the thigh bone and shin bone—one at each side—thus called the medial meniscus (inner thigh aspect) and lateral meniscus (outer). These are attached predominantly to the tibial plateaus. They serve to cushion and transfer joint force. They accomplish this by distributing joint forces over a larger area of the joint—transferring force from the curved femoral condylar margins to the flatter tibial plateaus.

An example: You obviously weigh the same, yet stepping on someone’s foot with an ice skate runner is more harmful than a tennis shoe. This illustrates force (your body weight) distributed through two different areas. Stress = Force ÷ Area. As the area decreases, the stress increases. This increase in stress also occurs when the area of load transfer decreases on the basis of local joint loss of cartilage, whether it is meniscal or articular.

You may now appreciate that injury to either type of cartilage can upset the normal loading of the joint. This “injury” is not limited to trauma. It can occur with normal daily activities. Once the delicate balance of the knee is upset, the resulting abnormal loads then lead to “overload damage”. Over time, initially small defects in the articular cartilage or tears in the meniscal cartilage can progress. This gradual deterioration can lead to degenerative joint disease - a form of osteoarthritis.