Do you recall the song, “Them bones, them bones, them dry bones”? As we consider how a healthy knee works, let’s sing along.
“The thigh bone’s connected to the knee bone, the knee bone’s connected to the shin bone.” (OK, enough singing, already.) The thigh bone is called the femur. The knee bone is the pettella, and the shin bone is the tibia. The fibula is the other bone in the lower leg. It is under the tibia. Those two lower leg bones connect just below the knee, but the smaller fibula does not connect to the patella, the knee bone.
We know from the song that they are all connected, but how? Who just said, “ligaments and tendons?” That is correct. When you think of ligaments, think of large rubber bands, because they’re big and they stretch. Tendons aren’t as large, or as elastic as ligaments.
There are 2 main ligaments in each knee. They are known as cruciate ligaments; anterior and posterior (which you probably know, means front and back.) They are referred to as PCL and ACL; the latter is the one that so often gets injured by — and often ends the careers of — football players. Both of these ligaments connect the femur and the tibia together. They play a huge role in the proper movement of the knee joint, and they prevent it from moving too far in the wrong direction.
Tendons are heavy duty cords of tissue which attach to the muscles that are attached to the knee bone. Together, they make movement of this joint possible. The main muscle groups involved are the quadrants, at the front of the thigh, and the hamstrings, at the back of the thigh. Ligaments from the tibia, just below the knee joint, connect to the hamstring muscles.
The actual joint is found in the cavity which is in what is known as the knee capsule. It’s made up of a fibrous membrane, which, produces synovial fluid to lubricate the knee, much like you would put oil on a door hinge. Underneath the patella and on the ends of the femur and tibia there is cartlidge tissue. This prevents all those bones from grinding together. The cartlidge also absorbs some of the shock stress from walking and running that would otherwise go to the joint.
The knee bends and straightens by way of a hinge joint. Additionally, there is the ability for slight rotation.
Here is the breakdown of what happens when you straighten your knee or leg. Starting with your front thigh muscles, the quadraceps, force is exerted which pulls on the quadraceps tendon. That, in turn, pulls on the patella, and the end result is the extension of your knee.
Here is what happens when you bend your leg. This motion starts with the rear thigh muscles, the hamstrings. When they contract, the tibia is pulled back, in the direction of the back of the thigh, which brings about the flexing of the knee. As this happens, muscles, ligaments and tendon and all work together to keep the joint stable and hold the joint together.