Knee Joint: Anatomy, Function, and Key Components

What is moving in my knee?

The knee is a marvel of biomechanical engineering, a complex hinge joint where multiple bones, ligaments, tendons, muscles, and specialized structures work in concert to facilitate movement, bear weight, and provide stability.

Introduction to the Knee Joint

The knee joint, or tibiofemoral joint, is the largest and one of the most complex joints in the human body. Primarily acting as a hinge joint, it allows for significant flexion and extension, with a small degree of rotation when the knee is flexed. Its intricate design enables essential movements like walking, running, jumping, and squatting, making it fundamental to human locomotion. Understanding its constituent parts is key to appreciating its function and protecting its health.

Bones of the Knee

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

• Femur (Thigh Bone): The longest and strongest bone in the body. Its distal end features two rounded condyles (medial and lateral) that articulate with the tibia.
• Tibia (Shin Bone): The larger of the two lower leg bones, bearing the majority of the body's weight. Its superior surface, known as the tibial plateau, has two shallow depressions that receive the femoral condyles.
• Patella (Kneecap): A sesamoid bone embedded within the quadriceps tendon. It glides over the front of the femur (in the trochlear groove) and serves to increase the mechanical advantage of the quadriceps muscle.
• Fibula (Calf Bone): While the fibula runs parallel to the tibia in the lower leg, it does not directly articulate with the femur to form the main knee joint. However, its head provides an attachment point for the lateral collateral ligament and some muscles, indirectly contributing to knee stability.

Ligaments: The Stabilizers

Ligaments are strong, fibrous bands of connective tissue that connect bones to other bones, providing stability and limiting excessive movement. The knee has several crucial ligaments:

• Cruciate Ligaments: Located deep within the knee joint, crossing each other like an "X."
  • Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding too far forward relative to the femur and limits hyperextension.
  • Posterior Cruciate Ligament (PCL): Prevents the tibia from sliding too far backward relative to the femur.
• Collateral Ligaments: Located on the sides of the knee.
  • Medial Collateral Ligament (MCL): Connects the femur to the tibia on the inner side of the knee, resisting valgus (inward) forces.
  • Lateral Collateral Ligament (LCL): Connects the femur to the fibula on the outer side of the knee, resisting varus (outward) forces.

These ligaments work together to control the knee's range of motion and prevent dislocation.

Menisci: Shock Absorbers and Load Distributors

Two C-shaped pieces of fibrocartilage, known as the menisci, sit between the femoral condyles and the tibial plateau:

• Medial Meniscus: Larger and more C-shaped, located on the inner side of the knee.
• Lateral Meniscus: Smaller and more O-shaped, located on the outer side.

The menisci serve several vital functions:

• Shock Absorption: They cushion the impact between the femur and tibia during movement.
• Load Distribution: They help distribute the weight evenly across the joint surface, reducing stress on the articular cartilage.
• Joint Congruency: They improve the fit between the rounded femoral condyles and the relatively flat tibial plateau, enhancing joint stability.

Muscles: The Movers

Muscles are the primary drivers of knee movement, contracting to pull on tendons and move bones.

• Quadriceps Femoris: A group of four muscles on the front of the thigh (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius). Their primary action is knee extension (straightening the leg). The rectus femoris also crosses the hip joint, contributing to hip flexion.
• Hamstrings: A group of three muscles on the back of the thigh (biceps femoris, semitendinosus, semimembranosus). Their primary action is knee flexion (bending the leg). They also contribute to hip extension.
• Gastrocnemius: The large calf muscle, which crosses the knee joint. It assists with knee flexion but is primarily known for ankle plantarflexion.
• Popliteus: A small muscle located at the back of the knee that helps "unlock" the knee from full extension by initiating internal rotation of the tibia.

Tendons: Connecting Muscle to Bone

Tendons are strong, cord-like tissues that connect muscles to bones, transmitting the force generated by muscle contraction to produce movement.

• Quadriceps Tendon: Connects the quadriceps muscles to the top of the patella.
• Patellar Tendon (often referred to as a ligament due to connecting bone to bone - patella to tibia): Connects the bottom of the patella to the tibial tuberosity (a bony prominence on the front of the tibia). This structure is crucial for transmitting the quadriceps' force to extend the knee.
• Hamstring Tendons: Connect the hamstring muscles to the tibia and fibula.

Bursae: Reducing Friction

Bursae are small, fluid-filled sacs located around joints. In the knee, they act as cushions between bones, tendons, and muscles, reducing friction during movement and preventing irritation. There are numerous bursae around the knee, including the prepatellar bursa (in front of the kneecap) and the infrapatellar bursa (below the kneecap).

Types of Movement at the Knee

The knee joint primarily performs two types of movement:

• Flexion: Bending the knee, decreasing the angle between the thigh and lower leg (e.g., bringing your heel towards your glutes). This movement is primarily driven by the hamstrings.
• Extension: Straightening the knee, increasing the angle between the thigh and lower leg (e.g., kicking a ball). This movement is primarily driven by the quadriceps.
• Internal and External Rotation: When the knee is flexed (bent), a limited degree of rotation is possible. This rotational movement is crucial for "locking" and "unlocking" the knee during walking.

The Patellofemoral Joint: A Crucial Player

While often considered part of the overall knee, the articulation between the patella and the femur forms a distinct joint known as the patellofemoral joint. The patella acts as a pulley, increasing the leverage of the quadriceps muscle by positioning its tendon further away from the knee's axis of rotation. This mechanical advantage allows for more efficient knee extension. Proper tracking of the patella within the femoral groove is vital for pain-free movement.

Proprioception and Knee Control

Beyond the physical structures, the nervous system plays a critical role in knee movement and stability through proprioception. Specialized sensory receptors (mechanoreceptors) within the joint capsules, ligaments, and muscles send continuous feedback to the brain about the knee's position, movement, and forces acting upon it. This information allows for precise, coordinated muscle contractions, enabling balance, agility, and injury prevention.

Conclusion: A Symphony of Structures

When you move your knee, it's not just one structure in isolation, but a dynamic interplay of bones providing leverage, ligaments ensuring stability, menisci cushioning and distributing load, muscles generating force, and tendons transmitting that force. This complex biological machine, orchestrated by the nervous system, allows for the incredible range and power of human lower limb movement. Understanding these components is the first step toward maintaining knee health and optimizing performance.