What is the knee joint and what does it do?
Your knee is one of the most important joints in your body. It is a major “weight-bearing” joint—which means it helps support your body’s weight. It plays a role in almost all of your movements.
The way your knee bends back and forth is a lot like a hinge on a door. Part of it also rotates whenever you bend it (this is called “flexion”) and straighten it (this is called “extension”).
Your knee is made up of four parts:
Bones of the knee
The bones of the knee are:
- The femur (thighbone)
- The tibia (shinbone)
- The patella (kneecap)
The femur and tibia meet to form a hinge with the patella in front of them, which provides protection for the joint. The patella moves in a sliding action up and down in a groove in the femur called the femoral groove. The sliding occurs whenever you bend or straighten your knee.
Ligaments of the knee
The job of the ligaments is to make sure all parts of the knee are held together and kept stable.
- The medial (inner) collateral ligament (MCL) and lateral (outer) collateral ligament (LCL) limit sideways motion of the knee
- The posterior and anterior cruciate ligaments (PCL and ACL) limit forward motion of the knee
The stability provided by these ligaments is very important; without it, you would feel that your knee is simply giving out from under you.
Cartilage of the knee
Cartilage is inside the joint and provides shock absorption whenever you walk, run, lift, climb stairs, or perform any other “impact” activity.
The knee has two types of cartilage:
- Menisci cartilage. This type sits between the femur and the tibia. Both strong and flexible, menisci act as shock absorbers and also stabilize and evenly distribute your weight across the knee joint. Often, a torn meniscus is called “torn cartilage”.
- Articular cartilage. This type is a smooth and very slick material. It covers the end of the femur, the femoral groove, the top of the tibia, and the underside of the patella. All of these parts enable the knee and its bones to move smoothly.
Cartilage has no blood vessels and does not regenerate naturally. This is why damaged cartilage is very hard to restore or repair. (1)
Tendons of the knee
The job of the tendons is to connect muscles to the knee:
- The quadriceps muscle at the front of the thigh is connected to the top of the patella by the quadriceps tendon
- The quadriceps tendon covers the patella and becomes the patellar tendon
- The patellar tendon attaches to the front of the tibia
- The hamstring muscles at the back of the thigh attach to the tibia at the back of the knee
- The quadriceps muscles straighten the knee
- The hamstring muscles provide the knee’s bending motion