Knee Joint: Structure, Function, and Biomechanics

What is the structure and function of the knee?

The knee is a complex hinge joint, intricately designed for both mobility and stability, facilitating movement like walking, running, and jumping while bearing the body's weight and absorbing significant forces.

The Bony Architecture

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

• Femur (Thigh Bone): The distal end of the femur features two rounded condyles (medial and lateral) that articulate with the tibia.
• Tibia (Shin Bone): The proximal end of the tibia presents a relatively flat surface, known as the tibial plateau, which receives the femoral condyles.
• Patella (Kneecap): A sesamoid bone embedded within the quadriceps tendon, the patella glides in a groove on the front of the femur (trochlear groove) and serves as a mechanical pulley, enhancing the leverage of the quadriceps muscle.
• Fibula (Calf Bone): While the fibula runs parallel to the tibia, it does not directly participate in the knee joint's primary articulation but serves as an important attachment point for muscles and ligaments.

Articular Cartilage

Covering the ends of the femur, tibia, and the posterior surface of the patella is articular cartilage, a smooth, slippery, and resilient connective tissue. This hyaline cartilage reduces friction between bones during movement and acts as a shock absorber, distributing forces evenly across the joint surfaces.

Ligamentous Support

Ligaments are strong, fibrous bands of connective tissue that connect bones to other bones, providing crucial stability to the knee joint. They prevent excessive movement and guide the knee through its normal range of motion.

• Cruciate Ligaments: Located deep within the joint, these ligaments cross each other, forming an "X" shape.
  • Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding too far forward relative to the femur and limits rotational movements.
  • 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): Runs along the inner side of the knee, preventing excessive valgus (knock-kneed) forces.
  • Lateral Collateral Ligament (LCL): Runs along the outer side of the knee, preventing excessive varus (bow-legged) forces.

The Menisci

The menisci (medial and lateral meniscus) are two C-shaped wedges of fibrocartilage located between the femoral condyles and the tibial plateau.

• Function: They deepen the tibial plateau, improving congruency between the bones, thus enhancing joint stability. They also play a vital role in shock absorption, distributing compressive loads across the joint, and aiding in joint lubrication and nutrition.
• Medial Meniscus: Larger and more C-shaped, it is firmly attached to the MCL, making it more susceptible to injury.
• Lateral Meniscus: Smaller and more O-shaped, it is less firmly attached and thus more mobile.

Muscular and Tendinous Contributions

Muscles surrounding the knee provide dynamic stability and enable movement. Their tendons cross the joint, attaching to the bones.

• Quadriceps Femoris: A group of four muscles on the front of the thigh (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius). They converge into the quadriceps tendon, which encases the patella and continues as the patellar tendon to insert on the tibia. Their primary function is knee extension.
• Hamstrings: A group of three muscles on the back of the thigh (biceps femoris, semitendinosus, semimembranosus). Their primary function is knee flexion and hip extension.
• Gastrocnemius: A calf muscle that crosses the knee joint, contributing to minor knee flexion.
• Popliteus: A small muscle located behind the knee that "unlocks" the knee from full extension by internally rotating the tibia relative to the femur (or externally rotating the femur on the tibia).

Bursae and Joint Capsule

• Bursae: Small, fluid-filled sacs located around the knee joint, acting as cushions between bones, tendons, and muscles. They reduce friction during movement, preventing irritation and inflammation.
• Joint Capsule: A strong, fibrous capsule encloses the entire knee joint, providing additional stability. The inner lining of the capsule, the synovial membrane, produces synovial fluid, which lubricates the joint, nourishes the articular cartilage, and helps absorb shock.

Primary Functions of the Knee

The intricate structure of the knee enables it to perform several critical functions:

• Weight-Bearing: The knee is a primary weight-bearing joint, transmitting forces from the trunk and upper body to the lower leg and foot.
• Mobility: It allows for a wide range of motion, primarily flexion (bending) and extension (straightening), essential for activities like walking, climbing, and squatting.
• Shock Absorption: Through its cartilage and menisci, the knee efficiently dissipates forces generated during impact activities, protecting the bones and other structures.
• Stability: A combination of ligaments, muscles, and the joint's bony configuration provides robust stability, preventing unwanted movements and dislocations.
• Rotational Movement: While primarily a hinge joint, the knee also allows for slight internal and external rotation when flexed, which is crucial for activities like pivoting and turning.

Biomechanics of Knee Movement

• Flexion and Extension: These movements occur as the femoral condyles roll and glide over the tibial plateau.
• Screw-Home Mechanism: As the knee extends fully, the tibia externally rotates slightly (or the femur internally rotates on a fixed tibia). This "locking" mechanism increases stability in full extension, reducing the need for continuous muscle activity to maintain an upright stance. To initiate flexion from full extension, the popliteus muscle "unlocks" the knee by internally rotating the tibia.
• Patellar Tracking: The patella glides smoothly within the trochlear groove of the femur during knee movement. Proper patellar tracking is crucial for efficient quadriceps function and joint health.

Understanding the complex interplay of bones, cartilage, ligaments, and muscles within the knee joint is fundamental to appreciating its remarkable capacity for movement, stability, and load bearing in daily life and athletic endeavors.