Anatomy of the Knee

Learn About Knee Joint Anatomy

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The knee joint is part of the lower extremity. It is the junction of the thigh and the leg, and is a hinge joint. A hinge joint bends back and forth in one plane (unlike the ball-and-socket joint of the hip). The knee joint is commonly injured. Understanding the normal anatomy of the knee joint can help you understand the conditions that cause problems of the knee.

Bones Around the Knee

There are three bones that come together at the knee joint.

The shin bone (tibia), the thigh bone (femur), and the kneecap (patella) are each important parts of the knee joint. A fourth bone, the fibula, is located just next to the shin bone (tibia) and knee joint, and can play an important role in some knee conditions.

The tibia, femur, and patella, all are covered with a smooth layer of cartilage (see below) where they contact each other at the knee joint. There is also a small bone called a fabella, that is often located behind the knee joint. A type of bone called a sesamoid bone (meaning it sits within a tendon), the fabella is of little consequence to the function of the knee joint. It is found in about 25% of the population.

Cartilage of the Knee

There are two types of cartilage of the knee joint. Articular cartilage is the smooth lining that covers the end of the bone. When the smooth articular cartilage is worn away, knee arthritis is the result.

Cartilage is a resilient structure that resists damage, but when injured it has a difficult time healing.

The other type of cartilage in the knee joint is called the meniscus. When people talk about 'cartilage tears,' they are usually referring to a meniscus tear. The meniscus is a shock absorber that sits between the end of the thigh bone and the top of the shin bone.

Ligaments of the Knee

Ligaments are structures that connect two bones together. There are four major ligaments that surround the knee joint. Two of these ligaments are in the center of the joint, and they cross each other. These are called the cruciate ligaments, and consist of the anterior cruciate ligament, and the posterior cruciate ligament.

One ligament is on each side of the knee joint; the medial collateral ligament on the inner side, and the lateral collateral ligament on the outer side. Ligament injuries typically result in complaints of instability of the knee joint.

Muscles and Tendons Surrounding the Knee

Muscles propel the knee joint back and forth. A tendon connects the muscle to the bone. When the muscle contracts, the tendons are pulled, and the bone is moved. The knee joint is most significantly affected by two major muscle groups. The quadriceps muscles provide strength and power with knee extension (straightening) and the hamstrings muscles allow for strength and power in flexion (bending). The patellar tendon on the front of the knee is part of the quadriceps mechanism. Other smaller muscles and tendons surround the knee joint as well.

Synovium and Synovial Fluid

The synovium is the lining of the joint space.

The synovium is a layer of tissue that defines the joint space. The synovial cells produce a slippery, viscous fluid called synovial fluid within the joint. In conditions that cause inflammation of the joint, there can be an abundance of synovial fluid produced, that leads to swelling of the knee joint.