Anatomical Description of the Knee

The knee is one of the largest and most complex joints in the body. It plays a crucial role in movement related to walking, running, and jumping. The knee joint is made up of bones, ligaments, tendons, cartilage, muscles, and other structures.

The knee’s structure allows it to withstand great pressure from activities like running and jumping. It is, however, prone to injury and wear-and-tear, which can lead to conditions like osteoarthritis.

Bones: The knee joint involves three bones. The lower end of the femur (thighbone) meets the top of the tibia (shinbone) to form the main joint. This joint has two compartments: the medial compartment (inside of the knee) and the lateral compartment (outside of the knee). The patella (kneecap) sits in front of the joint to provide some protection.

Muscles: There are two primary muscle groups responsible for knee motion: the quadriceps and the hamstrings.

  1. Quadriceps: Located on the front of the thigh, this group includes the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. These muscles extend the knee and are responsible for actions such as kicking or jumping.
  2. Hamstrings: Located on the back of the thigh, this group includes the biceps femoris, semitendinosus, and semimembranosus. These muscles flex the knee and extend the hip, and are key in actions such as running and jumping.

The popliteus muscle, located at the back of the knee, helps in unlocking the knee from the extended position.

Cartilage: Two types of cartilage are found in the knee. The menisci are tough, rubbery cartilage that sit between the femur and tibia to cushion and stabilize the joint. Articular cartilage is a smooth, slippery substance that covers the ends of the bones and helps reduce friction in the knee.

Ligaments: The knee joint is stabilized by several ligaments:

  1. Anterior cruciate ligament (ACL): This ligament prevents the tibia from moving too far forward in relation to the femur and controls the knee during rotation.
  2. Posterior cruciate ligament (PCL): This ligament prevents the tibia from moving backward excessively relative to the femur.
  3. Medial collateral ligament (MCL): This ligament provides stability against valgus force and prevents the knee from bending inward.
  4. Lateral collateral ligament (LCL): This ligament provides stability against varus force and prevents the knee from bending outward.

Tendons: Tendons are fibrous tissues that connect muscle to bone. The quadriceps tendon connects the muscles at the front of the thigh to the patella, and the patellar tendon connects the patella to the tibia.

Synovial Membrane: This is a thin lining that covers the knee joint. It produces synovial fluid, which lubricates the knee, reducing friction and allowing smooth movement.

Blood Vessels: The knee receives blood from the genicular arteries, which are branches of the popliteal artery.

Nerves: The knee is innervated by the femoral, tibial, and common fibular nerves.

Fascial Compartments: The thigh is divided into three fascial compartments: anterior, posterior, and medial.

  1. Anterior compartment: It includes the quadriceps and sartorius muscles.
  2. Posterior compartment: It contains the hamstring muscles and the popliteus.
  3. Medial compartment: It includes the gracilis and adductor muscles.

Joint Anatomy: The knee is a hinge joint that primarily allows for flexion and extension, but it also allows a small degree of rotation and lateral and medial displacement. The articulating surfaces of the femur, tibia, and patella are covered with hyaline cartilage, reducing friction and wear. Two fibrocartilaginous discs, the medial and lateral menisci, sit on the tibial plateau and provide shock absorption.

Bursa Sacs: There are approximately 14 bursae around the knee joint that help reduce friction. Some of the main ones include:

  1. Prepatellar bursa: Located between the skin and the patella, it can become inflamed in certain knee injuries.
  2. Infrapatellar bursa: Located below the patella, it can be superficial (between the skin and patellar ligament) or deep (between the patellar ligament and the tibia).

Kinesiology: The primary movements of the knee joint are flexion and extension:

  1. Flexion (bending): This movement is mainly produced by the contraction of the hamstring muscles, as well as the gracilis, sartorius, and popliteus.
  2. Extension (straightening): This movement is primarily produced by the contraction of the quadriceps muscles.

A small amount of medial and lateral rotation can also occur when the knee is flexed. The knee also plays a key role in shock absorption during activities such as walking and running.