Anatomy of the Knee Joint

Updated on October 11, 2016
x-ray of left knee.
x-ray of left knee. | Source

The knee joint is the largest joint in the human body. It is designed to support the full weight of the body, allowing us to stand, walk, run or dance with ease, grace and fluidity. The knee is also a very common area for injury.

Damage to any structure of the knee anatomy will impact normal movement of the leg. It is important to understand the anatomy of knee joint to know how different structures work together to ensure the normal functioning of the knee.

Positions of Anatomical Features of Knee

It is easy to understand the anatomy of the knee if you know how the names of its features were derived.

Terms used to describe the position of anatomical features of the knee are in relation to an imaginary line (mid line) drawn through the whole length of the body. This line separates the body in two equal halves such that parts of the body that occur in pairs will be on opposite sites of this mid line.

In knee anatomy, a term with medial as part of its name refers to a position closer to the mid line. In this case, it will be a position closest to the other knee. A term with Lateral as part of its name refers to a position of the knee away from the mid line. In this case, it will be a position farthest away from the other knee. A term with anterior or posterior in its name refers to the front or the back of the body respectively.

Diagram to Illustrate the positions of medial and lateral features of the knee
Diagram to Illustrate the positions of medial and lateral features of the knee | Source

Anatomical Features of the Knee

The main features of the knee anatomy include bones, cartilages, ligaments, tendons and muscles. In the knee joint, the femur articulates with the tibia and the patella. The knee joint is a synovial joint this means it contains a fluid that lubricates it. This fluid is known as the synovial fluid. Consult the diagrams provided to better digest the descriptions of the features below.

Diagram of the anatomy of the knee
Diagram of the anatomy of the knee | Source

Bones of the knee

3 bones meet at the knee joint, namely Femur, Tibia and Fibula. There is one other bone called the Patella, it covers the anterior surface of the joint.

The femur is the biggest bone of the body. This bone extends from the knee joint superiorly. At the knee joint the femur ends at the lateral condyle, the medial condyle and the intercondylar notch.

The tibia extends from the knee joint inferiorly towards the ankle. At the knee joint, the tibia ends at the medial condyle, lateral condyle and the articular facet of the fibula.

The fibula is a thin bone lateral to the tibia and does not really enter the knee joint. It has a small joint that links it to the side of the tibia. The joint between the tibia and fibula moves only slightly.

The patella also known as the kneecap is located directly above the knee joint space in the Patellofemoral Groove. It is held in place by a band of connective tissue (the quadriceps tendon and the patellar tendon).

Cartilages of the Knee

Cartilages are white stiff flexible connective tissues that protect bony surfaces when they rub against each other. They also act as shock absorbers and ensure smooth movement during motion of the joint. In the knee anatomy, the surfaces of the bones inside the knee joint are covered by two different types of cartilages.

The smooth Articular cartilage covers the head of the femur and the tibial plateau inside the knee joint.

The Menisci (the lateral and medial menisci cartilages) sit on top of the articular cartilage of the tibial plateau. The structure of the menisci equalizes the pressure across the uneven surface of the femur and this helps to even the weight distribution across the surface between the femur and the tibia.

Tendons of the Knee

Tendons are tough fibrous connective tissues that attach muscles to bones. There are two tendons in the anatomy of the knee.

Quadriceps tendon extends from the patella superiorly (up) to the quadriceps muscles. It connects the quadriceps muscle to the patella. It continuous downwards and blend to the patella tendon.

The patellar tendon (also known as the patellar ligament) is a downwards continuation of the quadriceps tendon. It extends from the patella down inferiorly (down) to the tibia.

Ligaments of the Knee

Without ligaments, the bones of the knee will be very loose. Ligaments tie the femur to the tibia and provide stability. Ligaments permit flexion (bending the knee) and extension (straightening the knee) motions of the knee. In the anatomy of the knee, there are 4 ligaments that hold the knee together.

The Anterior Cruciate Ligament (ACL) is located inside the knee joint, in front of the Posterior Cruciate Ligament (PCL). It prevents the tibia from moving excessively forward with respect to the femur. The ACL extends from the posterior lateral femur through the intercondylar notch down to the anterior medial tibia. It crosses the PCL and they form an ‘X’.

The Posterior Cruciate ligament (PCL) is located inside the knee joint, posterior to the ACL. It prevents the tibia from moving excessively backward with respect to the femur. The PCL extends from the anterior medial femur down to the posterior lateral tibia.

The Medial Collateral Ligament (MCL) extends from the medial side of the femur down the tibia. Together with the Lateral Cruciate Ligament it prevents excessive motions of the knee joint by limiting joint mobility in the side-to-side direction.

The Lateral Collateral Ligament (LCL) extends from lateral side the femur down to the lateral fibula. Together with the MCL it prevents excessive motions of the knee joint by limiting joint mobility in the side-to-side direction.

Rough illustration of knee flexion and extension in knee anatomy
Rough illustration of knee flexion and extension in knee anatomy | Source

Muscles of the Knee

Muscles in the anatomy of the knee include quadriceps and hamstrings muscles.

The quadriceps sits directly on top on the anterior side of the femur. They help in the extension motion of the knee. It is made up of 4 groups of muscles: Vastus Medialis, Vastus Intermedius, Vastus Lateralis and Rectus Femoris.

In the knee anatomy, the hamstring muscles run down the posterior side of the femur. They are involved in flexion of the knee (knee bends when the hamstring muscles contract). It is composed mainly of 5 muscle groups: Satorius, Gratilis, Semimembranosus, Semitendinosus and Biceps Femoris.

Animated Tutorial on the Anatomy of the knee

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