Human Knee: Bones, Ligaments, Tendons, Cartilage, and Bursae

What are the parts of the human knee?

The human knee is a marvel of biomechanical engineering, functioning as a complex hinge joint that facilitates movement and bears significant load. It is an intricate interplay of bones, ligaments, tendons, cartilage, and bursae, all working synergistically to provide stability, mobility, and shock absorption.

The Bones of the Knee

The knee joint is primarily formed by the articulation of three bones:

• Femur: The thigh bone, specifically its distal (lower) end, which features two rounded condyles that articulate with the tibia.
• Tibia: The larger of the two lower leg bones, its proximal (upper) end, known as the tibial plateau, provides the surface for articulation with the femoral condyles.
• Patella: Commonly known as the kneecap, this is a sesamoid bone embedded within the quadriceps tendon. Its primary roles are to protect the joint and to improve the leverage of the quadriceps muscle, enhancing its ability to extend the knee.
• Fibula: While the fibula, the smaller lower leg bone, is located alongside the tibia, it does not directly participate in the knee joint's primary articulation. However, it serves as an important attachment point for some knee ligaments and muscles.

Key Ligaments for Stability

Ligaments are strong, fibrous bands of connective tissue that connect bone to bone, providing crucial passive stability to the knee joint. The knee has four primary ligaments:

• Cruciate Ligaments: These ligaments are named for their "cross" shape within the knee joint.
  • Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding too far forward on the femur and helps control rotational stability. It is a common site for athletic injuries.
  • Posterior Cruciate Ligament (PCL): Prevents the tibia from sliding too far backward on the femur. It is generally stronger and less frequently injured than the ACL.
• Collateral Ligaments: These ligaments are located on the sides of the knee.
  • Medial Collateral Ligament (MCL): Located on the inner side of the knee, it resists excessive valgus (inward) stress, preventing the knee from bowing inward.
  • Lateral Collateral Ligament (LCL): Located on the outer side of the knee, it resists excessive varus (outward) stress, preventing the knee from bowing outward.

Important Tendons

Tendons are tough, fibrous cords that connect muscle to bone, enabling the transmission of force for movement. Key tendons around the knee include:

• Quadriceps Tendon: This large tendon connects the quadriceps muscles (on the front of the thigh) to the top of the patella. It is essential for knee extension.
• Patellar Tendon (or Patellar Ligament): Although often referred to as a tendon, it technically connects the patella to the tibial tuberosity (a bony bump on the front of the tibia). It is a continuation of the quadriceps mechanism and plays a vital role in extending the lower leg.
• Hamstring Tendons: The tendons of the hamstring muscles (on the back of the thigh) attach around the knee joint (e.g., to the tibia and fibula), facilitating knee flexion (bending) and hip extension.

Cartilage Structures

Two main types of cartilage are found within the knee, each with distinct functions:

• Menisci: These are two C-shaped (medial meniscus) and O-shaped (lateral meniscus) pieces of tough, rubbery fibrocartilage located between the femoral condyles and the tibial plateau.
  • Function: They act as shock absorbers, distribute weight evenly across the joint surfaces, enhance joint stability, and aid in joint lubrication.
• Articular Cartilage (Hyaline Cartilage): This smooth, slippery tissue covers the ends of the femur, the top of the tibia, and the back surface of the patella.
  • Function: It reduces friction between the bones during movement, allowing for smooth, gliding motion, and also helps to absorb shock. Damage to this cartilage can lead to osteoarthritis.

Bursae for Friction Reduction

Bursae are small, fluid-filled sacs strategically located around the knee joint.

• Function: They act as cushions, reducing friction between bones, tendons, and muscles as they move against each other. Examples include the prepatellar bursa (in front of the kneecap), infrapatellar bursa (below the kneecap), and suprapatellar bursa (above the kneecap). Inflammation of a bursa is known as bursitis.

Surrounding Musculature

While not technically "parts" of the joint itself, the muscles surrounding the knee are integral to its function, providing dynamic stability and facilitating movement.

• Quadriceps Femoris: Group of four muscles on the front of the thigh, primarily responsible for knee extension.
• Hamstrings: Group of three muscles on the back of the thigh, primarily responsible for knee flexion and hip extension.
• Gastrocnemius: The main calf muscle, which crosses the knee joint and assists in knee flexion.

Nerves and Blood Vessels

The knee joint is richly supplied by nerves that transmit sensory information (pain, position) and motor commands to the muscles, and by blood vessels that provide essential nutrients and oxygen.

• Major nerves include branches from the femoral nerve and sciatic nerve (tibial and common peroneal nerves).
• The primary blood supply comes from the popliteal artery, a continuation of the femoral artery.

Understanding the intricate anatomy of the knee is fundamental for appreciating its complex function, identifying potential vulnerabilities, and developing effective strategies for injury prevention, rehabilitation, and performance optimization.