A Tour of Knee Anatomy
Learn all about this incredible joint and its complex and powerful anatomy
Bob Sallis, M.D., FASM Former President, American College of Sports Medicine
- Understanding basic anatomy is key to preventing injury and staying healthy
- Many other body parts come together at the knee joint to help it bear weight, extend your leg, and propel your body forward
- The knee’s anatomy has extensive protective measures built into it as we move through our range of motion. The knee’s components are designed to support, stabilize, and cushion.
- The knee is one of the most complex joints in the human body
- The knee is the largest joint in the human body
As athletes, there isn’t much we do that doesn’t involve using our knees in some way. They let us twist, turn, run, walk, start, stop, pedal, and sprint. The bending actions might seem simple, but the knee is an incredibly complex joint with equally complex anatomy.
The scientific name for the knee is the tibiofemoral joint. As one of the largest in the human body, the knee is known as a hinge joint. Hinge joints differ from other joints, such as ball and socket joints and saddle joints, in that movement occurs along one plane through flexion and extension. A hinge joint works in the same way as the hinge on a door. The elbow and ankle are also hinge joints.
Three main bones come together at the knee joint: the femur (thigh bone), tibia (shin bone), and patella (kneecap). The movement and action occurs between these three bones. The femur, or thigh bone, sits against the flat upper surface of the tibia and slides back and forth as your knee bends and straightens. This sounds worse than it actually is, since the bones are covered by articular cartilage that protects the surface of the bones, decreases friction, and helps to absorb shock. The patella sits at the front of your knee and articulates with the femur to protect the joint. Together with the surrounding muscles, it helps your knee straighten. The patella works sort of like a lever as the muscles contract when your knee straightens. It sits within the tendon where the quadriceps connect to the knee and is known as a sesamoid bone, or little bone.
MUSCULAR SUPPORT SYSTEM
The muscles that help support the knee come from the upper and lower legs and are responsible for initiating all of the movement within our knees. Each muscle helps the knee perform a different function. When these muscles are strong and stable, activity is less likely to damage any of the other ligaments or cartilage in your knee. Hamstrings The three hamstring muscles, the biceps femoris, the semimembranosus, and the semitendinosus, work together to help the knee flex. Your knee doesn’t only bend and straighten, however. Sometimes it might not feel like it, but the knee also rotates medially (toward the center of the body) and laterally (away from the center of the body) with the help of the hamstring muscles. Two of the hamstring muscles, the semitendinosus and the semimembranosus, help medially rotate the knee. The biceps femoris muscle, the third of the hamstring muscles, helps the knee rotate laterally. Quadriceps The four quadriceps muscles enable you to strengthen your leg. The quads are actually one of the strongest muscles in the human body and function to extend the knee out in front of you.
TENDONS: HOLDING IT TOGETHER
Tendons, or tough fibrous bands of tissues that connect muscle to bone, also provide stability to the knee joint. There are two major tendons in the knee joint: the quadriceps tendon and the patellar tendon. Quadriceps Tendon The quadriceps tendon connects the quad muscles to the patella on the front of the knee. The patellar tendon is actually an extension of the quadriceps tendon that connects the patella to the shin bone. Iliotibial Band The other tendon that we hear a lot about is the iliotibial, or IT, band. The IT band is a band of tough fibers that runs from the outside of the hip, down the outside of the thigh, and connects to the tibia just below the knee. The band works together with the leg muscles to provide stability to the leg and to the knee. The IT band can often be irritated in runners as it moves back and forth across the outside of the knee.
LIGAMENTS: LENDING STABILITY
The main job of the ligaments is to provide stability. Ligaments connect bones to each other and make sure our bones don’t slip out of place or that our knee doesn’t rotate too far in one direction or the other. There are four knee ligaments in total: two major ligaments and two collateral ligaments. ACL and PCL The two major ligaments are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL and PCL ligaments form a cross in the middle of the knee joint as they connect the femur and the anterior and posterior sides of the tibia to the femur. The ACL is commonly injured when there are excessive twisting motions. Both ligaments help to stabilize your knee and prevent your shin bone from moving too far forward or backward. MCL and LCL The two collateral ligaments in the knee, the medial collateral ligament (MCL) and the lateral collateral ligament (LCL), stabilize the sides of your knees. The MCL connects the interior surface of the femur to the tibia and prevents movement inwards. It is therefore susceptible to injury when the outside of the knee takes excessive force. On the other hand, the LCL connects the outer surface of the femur to the fibula and protects the knee from moving too far outward. When you consider common sports injuries, the ACL, through twisting motions, and the MCL, by sudden blows to the outside of the knee, are the most likely to be injured.
CARTILAGE: CUSHIONING AND SUPPORT
Other than the articular cartilage mentioned earlier, the most important cartilage in your knee is the meniscus. Cartilage, whose main purpose is to protect against bones rubbing against each other and provide cushioning, protects bones as they move through their range of motion. The knee has two crescent-shaped pieces of cartilage that lie on top of the tibia. The menisci ease friction and help distribute weight evenly across the joint. There is an inner (medial) and an outer (lateral) meniscus. The medial meniscus is less mobile than the lateral meniscus and more susceptible to injury. The cartilage connects to ligaments so it’s important to realize that an injury to one often means an injury to the other.
BURSAE: CUSHIONING AND SUPPORT
We don’t often hear much about these fluid-filled protective sacs. Each of our knees has 11 bursae. Bursae are small pad-like sacs located within the joint cavity that reduce friction and provide cushioning for your bones, muscles, and ligaments. Bursae are not as tough as tendons but with all of the actions and motions our knees go through it is important to have the added component of cushioning wherever possible.
With all that our knees let us do, the least we can do is know a little bit about how and why they work. It’s the first step in preventing injury and keeping our bodies strong and up for all the activities we ask of them. Anatomy can be daunting and not all of us are naturally inclined to grasping the mechanics of biology. Think of understanding anatomy as another form of training designed to keep you in top condition.
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