Knee

The knee is the largest joint in the human body.

It is a vital structure for movement and stability. The knee allows humans and other primates to carry their body weight. This movement happens in horizontal directions, like walking or running. It also works in vertical directions, such as jumping. Because it handles so much force, the knee is vulnerable to injury. It is also prone to developing osteoarthritis, a condition where the joint wears down.

Technically, the knee is a modified hinge joint. This means it functions much like a door hinge. It primarily permits flexion, which is bending, and extension, which is straightening. It also allows for slight internal and external rotation. The knee is often called a compound joint. This is because it consists of two main parts: the tibiofemoral joint and the patellofemoral joint.

The tibiofemoral joint connects the femur, or thigh bone, to the tibia, the main lower leg bone. The patellofemoral joint involves the femur and the patella, commonly known as the kneecap.

To understand the mechanism, we must look at the articular bodies. The femur has two large rounded ends called the lateral and medial condyles. These condyles have a specific curvature in the sagittal plane. As the curvature moves toward the back, the radius of the curve becomes smaller. This creates a series of midpoints on a spiral. This shape allows for a sliding and rolling motion when the knee flexes. This design also ensures that the collateral ligaments remain loose enough to allow rotation. The tibia has its own condyles, which are separated by a feature called the intercondylar eminence.

The joint is protected by a structure called the articular capsule. This capsule contains a synovial membrane and a fibrous membrane. Inside this capsule, the joint is bathed in synovial fluid. This fluid acts as a lubricant to help the bones glide. The synovial membrane also forms various extensions called bursae. These are small, fluid-filled sacs that surround the joint. The largest is the suprapatellar bursa, located above the patella. Other bursae, like the semimembranosus and popliteal bursae, are found at the back of the knee.

Cartilage plays a critical role in protecting the bone surfaces. There are two main types of cartilage in the knee. Hyaline cartilage covers the surfaces where the bones move against each other. It has no blood vessels and receives nutrition through diffusion from the synovial fluid. Fibrous cartilage is found in the menisci, which are two articular disks. The medial meniscus is larger, thinner, and less curved than the lateral meniscus. The lateral meniscus is smaller, more circular, and more curved. These menisci help manage pressure within the joint space.

Stability is provided by a complex system of ligaments. Some ligaments are intracapsular, meaning they are inside the joint capsule. The anterior cruciate ligament (ACL) prevents the tibia from sliding too far forward. The posterior cruciate ligament (PCL) prevents the tibia from sliding too far backward. Other ligaments are extracapsular, located outside the capsule. The medial collateral ligament (MCL) protects the inside of the knee from stress. The lateral collateral ligament (LCL) protects the outside of the knee. The patellar ligament connects the kneecap to the tibia to provide mechanical leverage.

Muscles drive the movement of the knee through different compartments. The extensors, which straighten the leg, generally belong to the anterior compartment of the thigh. The flexors, which bend the leg, generally belong to the posterior compartment. There are a few exceptions to these rules. For example, the sartorius muscle is a flexor but is located in the anterior compartment. Additionally, some muscles in the lower leg, like the gastrocnemius, can provide weak knee flexion. This coordinated effort between bones, ligaments, and muscles allows for complex human locomotion.