Knee Anatomy: Bones, Ligaments, Menisci, and Musculature

What is the Orthopedic Anatomy of the Knee?

The knee is a complex synovial hinge joint, primarily responsible for flexion and extension, with a crucial role in weight-bearing and locomotion, formed by the articulation of the femur, tibia, and patella, stabilized by a intricate network of ligaments, menisci, and musculature.

The Knee: A Masterpiece of Biomechanical Engineering

The knee joint, one of the largest and most intricate joints in the human body, is a marvel of biomechanical design, balancing mobility with stability to facilitate a wide range of movements essential for daily life and athletic performance. Understanding its orthopedic anatomy is fundamental for anyone involved in fitness, rehabilitation, or sports, as it provides the basis for comprehending movement, injury mechanisms, and effective training strategies.

Bony Architecture

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

• Femur (Thigh Bone): The distal end of the femur features two large, rounded condyles (medial and lateral) that articulate with the tibia. Above these are the epicondyles, serving as attachment points for ligaments and muscles. The intercondylar fossa, a notch between the condyles, houses the cruciate ligaments.
• Tibia (Shin Bone): The proximal end of the tibia forms the tibial plateau, which has two slightly concave surfaces (medial and lateral tibial condyles) that articulate with the femoral condyles. The tibial tuberosity, located anteriorly, is the attachment point for the patellar ligament.
• Patella (Kneecap): This is a sesamoid bone embedded within the quadriceps tendon, situated anterior to the knee joint. Its primary function is to increase the mechanical advantage of the quadriceps muscle, protecting the joint and distributing forces across the femur.
• Fibula (Calf Bone): While the fibula runs parallel to the tibia and its head is near the knee joint, it does not directly articulate with the femur or patella and is not considered a primary bone of the knee joint itself. However, it serves as an important attachment point for the lateral collateral ligament and various muscles.

Articular Cartilage

The ends of the femur and tibia, as well as the posterior surface of the patella, are covered with articular (hyaline) cartilage. This smooth, slippery tissue reduces friction between the bones during movement and acts as a shock absorber, distributing forces across the joint surfaces. Damage to this cartilage (e.g., osteoarthritis) can severely impair knee function.

Ligamentous Stabilizers

Ligaments are strong, fibrous connective tissues that connect bones to other bones, providing passive stability to the knee joint. They prevent excessive motion and guide movement.

• Cruciate Ligaments (Intracapsular): Located deep within the knee joint, these ligaments cross each other like an "X."
  • Anterior Cruciate Ligament (ACL): Originates from the posterior-medial aspect of the lateral femoral condyle and inserts into the anterior intercondylar area of the tibia. It primarily prevents anterior translation of the tibia on the femur and limits rotational movements.
  • Posterior Cruciate Ligament (PCL): Originates from the anterior-lateral aspect of the medial femoral condyle and inserts into the posterior intercondylar area of the tibia. It primarily prevents posterior translation of the tibia on the femur.
• Collateral Ligaments (Extracapsular): Located on the sides of the knee joint.
  • Medial Collateral Ligament (MCL): Runs from the medial epicondyle of the femur to the medial aspect of the tibia. It resists valgus (knock-kneed) stress, preventing the knee from collapsing inward. It is intimately connected to the medial meniscus.
  • Lateral Collateral Ligament (LCL): Runs from the lateral epicondyle of the femur to the head of the fibula. It resists varus (bow-legged) stress, preventing the knee from collapsing outward. It is distinct from the joint capsule and lateral meniscus.

The Menisci

The menisci are two C-shaped wedges of fibrocartilage located between the femoral and tibial condyles. They enhance joint congruity, absorb shock, distribute forces, and contribute to joint stability.

• Medial Meniscus: C-shaped, larger, and less mobile than the lateral meniscus. It is firmly attached to the MCL and the joint capsule, making it more susceptible to injury.
• Lateral Meniscus: More O-shaped and more mobile than the medial meniscus, with looser attachments, particularly from the LCL.

Musculotendinous Units

Muscles surrounding the knee provide dynamic stability and are responsible for its movement. Their tendons connect them to the bones.

• Quadriceps Femoris: A group of four muscles (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) on the anterior thigh. They converge to form the quadriceps tendon, which encompasses the patella and continues as the patellar ligament (or tendon) to insert on the tibial tuberosity. The quadriceps are the primary extensors of the knee.
• Hamstrings: A group of three muscles (biceps femoris, semitendinosus, semimembranosus) on the posterior thigh. They are the primary flexors of the knee and also contribute to hip extension and tibial rotation.
• Gastrocnemius: The superficial calf muscle that crosses the knee joint. It assists with knee flexion, particularly when the foot is dorsiflexed.
• Popliteus: A small muscle located at the back of the knee, crucial for "unlocking" the knee from full extension by internally rotating the tibia on the femur (or externally rotating the femur on the tibia).
• Iliotibial Band (ITB): A thick band of fascia running down the lateral thigh, originating from the tensor fascia latae and gluteus maximus, inserting on the lateral tibia (Gerdy's tubercle). It provides lateral stability to the knee.

Bursae of the Knee

Bursae are small, fluid-filled sacs that reduce friction between bones, tendons, and muscles. Several bursae are located around the knee:

• Prepatellar Bursa: Between the skin and the anterior patella.
• Infrapatellar Bursae: Superficial (between skin and patellar ligament) and Deep (between patellar ligament and tibia).
• Anserine Bursa: Located on the medial side of the knee, beneath the pes anserinus tendons (sartorius, gracilis, semitendinosus).
• Suprapatellar Bursa: Superior to the patella, between the quadriceps tendon and the femur.

Inflammation of these bursae (bursitis) can cause pain and limit movement.

Neurovascular Supply

The knee joint receives its primary blood supply from branches of the femoral artery and popliteal artery, forming an extensive genicular anastomotic network around the joint. Innervation is provided by branches of the femoral nerve, tibial nerve, and common fibular (peroneal) nerve, supplying sensory feedback and motor control to the surrounding musculature.

Functional Significance and Biomechanics

The intricate orthopedic anatomy of the knee allows for its primary movements of flexion and extension, along with limited internal and external rotation when the knee is flexed. The interplay between its bony structures, strong ligaments, meniscal shock absorbers, and powerful musculature ensures both dynamic stability and efficient movement. An understanding of these components is crucial for:

• Injury Prevention: Identifying vulnerable structures and implementing appropriate strengthening and conditioning programs.
• Rehabilitation: Designing targeted exercises to restore function after injury or surgery.
• Performance Enhancement: Optimizing movement patterns and training loads to maximize athletic potential while minimizing risk.

By appreciating the sophisticated design of the knee, individuals can better care for this vital joint and maintain optimal mobility throughout their lives.