The knee is the largest joint in the human body, comprising of four bones, four main ligaments, two main tendons, and cartilage. It supports the weight of the body and functions as a hinge, allowing lower leg movement. Crucial movements such as standing, walking, running, and squatting rely on healthy knees.
Read on to learn about the structure and function of the knee.
The Four Bones of the Knee
Most people think of the knee solely as the kneecap, a.k.a. patella. However, the patella is one of four bones that comprise the knee joint. The other three bones are the femur (thigh bone), tibia (shin bone), and fibula (calf bone). Tendons and ligaments join the patella to the femur and tibia. The patella acts as a shield, protecting the front of the knee and facilitating movement.
Since most knee functions involve the tibia and femur, the knee joint is often called the tibiofemoral joint. As a synovial joint, the knee contains a capsule that encases the bones of the knee. Strong connective tissue on the outside of the joint holds it in place. The synovial membrane produces synovial fluid that fills the space between the bones, providing cushion and lubrication.
The knee joint is protected by a layer of cartilage – flexible connective tissue that helps absorb shock. Articular cartilage surrounds the ends of the knee bones and disperses force as the knee moves. The smoothness of cartilage also minimizes friction during knee flexion and extension (i.e., bending and straightening).
Located between the femur and tibia is the meniscus, rubbery cartilage that absorbs shock and helps stabilize the knee. The meniscus allows for high-impact movements such as running and jumping.
Cartilage in the knee is generally about a quarter of an inch thick. People with worn out knee cartilage may experience constant knee discomfort due to reduced impact absorption and grinding of the joint.
What Causes Cartilage Damage?
Many factors can cause damage to knee cartilage, with the most common being injury, arthritis, and joint deformities. While we all experience some degree of cartilage deterioration as we age, osteoarthritis can speed up the process. Trauma can cause joint inflammation, leading to fluid buildup and swelling. If you experience any of these issues, consult a doctor immediately.
The tibia and femur bones of the knee are joined by ligaments. The knee joint has two cruciate ligaments and two collateral ligaments. These structures stabilize the knee and prevent it from moving the wrong way.
The two cruciate ligaments of the knee cross on another and keep the upper leg bone (femur) and lower leg bone (tibia) from moving too far forward or backward. The anterior cruciate ligament (ACL) joins the front of the tibia to the back of the femur, preventing forward movement. The posterior cruciate ligament (PCL) joins the back of the tibia to the front of the femur, preventing backward movement. The PCL also stops the knee from bending awkwardly.
The collateral ligaments govern the sideways (lateral) movements of the knee and are located on the sides of the knee. The medial collateral ligament (MCL) connects the femur to the tibia on the inside of the knee, protecting the knee from collapsing inward. The lateral collateral ligament (LCL) connects the femur to the tibia on the outside of the knee, preventing the knee from being pushed outward.
Tendons in the Knee
Just as the ligaments of the knee join the femur and tibia, the tendons of the knee join the patella to leg muscles. The quadriceps tendon joins the quadriceps muscles to the patella, which then merges with the shin muscles via the patellar tendon.
The patellar tendon is a common place of injury for many athletes (we’ll write a post about them soon)!
Other Components of the Knee Joint
Surrounding the knee are bursae, pockets of synovial fluid that minimize friction as tendons move across the joint. Also, articular fat pads serve as cushioning to protect the knee from an external force.
Potential Knee Issues
While it might be hard to imagine, the knee is the largest joint in the human body. It absorbs a great deal of force, making it susceptible to injury. Common causes of knee issues include trauma, fractures, dislocations, ligament tears, tendon tears, wearing out of the cartilage, obesity, arthritis, and calcium and vitamin D deficiency.
When it comes to knee issues, prevention is the best cure. If you have a history of knee issues, consult a professional to learn ways to keep your knees strong and healthy.