Anatomy of the Knee

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, so understanding its anatomy can help you understand the conditions that cause problems, so you stay safe and prepared.

Bones Around the Knee

There are three important bones that come together at the knee joint:

1. The tibia (shin bone)
2. The femur (thigh bone)
3. The patella (kneecap)

A fourth bone, the fibula, is located just next to the 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 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 only found in about 25% of the population.

Cartilage of the Knee

There are two types of the cartilage of the knee joint:

1. 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 generally a resilient structure that resists damage, but when injured it has a difficult time healing. It can also wear down over time with age.
2. The other type of cartilage in the knee joint is called the meniscus. 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 the instability of the knee joint.

Muscles and Tendons

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:

1. The quadriceps muscles provide strength and power with knee extension (straightening).
2. 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.

Joint Capsule and Lining

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, which leads to swelling of the knee joint.

Joint Bursa

A bursa is a structure in your body that is placed between two moving parts. In your knee, there is a prominent bursa just in front of your knee and underneath the skin.

The bursa in front of the kneecap is prone to swelling, especially when people injure their knee or perform activities that involve kneeling on hard surfaces. Inflammation of the bursa, called prepatellar bursitis, is common in people who do flooring work or cleaning work and have to spend a lot of time kneeling.

Knee Joint Function

Knee function is determined in large part by the anatomy of the joint. The primary function of the knee is to hinge at the lower extremity.

However, the knee does not only bend back and forth. There are also rotational movements at the knee joint.

In order for the knee joint to function properly, there needs to be a good stability of the joint throughout its range of motion. If there are restrictions in mobility or instability of the knee joint, the function will not be normal.

A normally functioning knee joint will allow the following:

• Lower extremity support when standing
• Strength and power with movements such as standing up, squatting, or climbing
• Efficient movement when walking or running
• Power to propel your body more when you move
• Shock absorption when walking or landing from a jumping position

These are just some of the important functions that the knee joint allows. In order for any one of these functions to behave normally, all of the aforementioned structures need to be working together—and functioning normally.

Common Knee Conditions

Knee pain, decreased range of motion, and functional problems can be due to a number of conditions, including:

• Arthritis: Arthritis occurs when there are inflammation and damage to the cartilage of the knee joint. Arthritis can lead to swelling, pain, and difficulties with activities.
• Ligament injuries: Some of the most common sports-related injuries to the knee joint are ligament injuries. The most commonly injured ligaments are the anterior cruciate and the medial collateral ligaments.
• Meniscus tears: Tears of the meniscus, the cushion between the bones, can occur as the result of an injury, or as a result of wear and tear. Not all tears cause pain or functional problems.
• Tendonitis: Inflammation to the tendons that surround the joint can lead to a common condition known as tendinitis. Some of the tendons around for more prone to developing inflammation.

A Word From Verywell

The knee joint is a complex structure that involves bones, tendons, ligaments, muscles, and other structures for normal function. When there is damage to one of the structures that surround the knee joint, this can lead to discomfort and disability. Understanding the normal function of the knee joint can help you address some of these common conditions.