The upper leg and lower leg bones meet in the knee joint. However, the surfaces which contact each other do not quite fit together. Both surfaces only have contact with each other at a few points. Both menisci create an all-important balance.
There is a somewhat larger medial meniscus and a smaller lateral meniscus. The medial meniscus is grown together with the medial ligament and is more prone to injuries for this reason. The blood supply to the meniscus diminishes with age. As a result, once injured, it will no longer heal as quickly or as well. A damaged meniscus should be treated since it provides an important cushion that protects the articular cartilage and helps to prevent knee arthrosis.
The meniscus is made of more resilient fibrous cartilage and has more or less the shape of a "C". It is wedge-shaped or triangle-shaped in the cross-section.
Simply take two apples to make a picture of the function of the meniscus yourself. Place one of the apples on a table top so that it easily rolls back and forth, and divide the other one into eight bite-size pieces with a knife. Now place two of these pieces under the apple so that it can no longer roll around: your meniscus model is completed. The table top corresponds to the joint surface of the tibia, the bottom part of the apple to the upper leg, and both pieces of apples have assumed the task of the medial meniscus and the lateral meniscus. You will easily recognize that the outer edge of the meniscus is thicker than the inside part.
The menisci fulfil two vital functions
They transfer the load from the upper leg to the lower leg and stabilize the knee during flexion, extension and during circular movements.
The menisci move during the backward flexion and the forward extension of the knee in order to balance the change of the articular surfaces. The medial meniscus moves by up to six millimetres forward and backward during flexion and extension, and it deforms in the process. The lateral meniscus moves even more. It slides up to twelve millimetres forward and back each time when we flex or extend the knee. It also deforms during each flexion and extension movement.
The meniscus function can be summarized as follows: the medial and lateral meniscus distribute emergent pressure onto the joint surfaces and cushion blows, and contribute to the lubrication and nourishment of the joint.
Additional Reading: Knee Stress.
Additional Reading: Knee Pain and Potential Causes