Knee

The knee is the largest and one of the most complex synovial hinge joints in the human body. It connects the thigh to the lower leg and allows movement while providing stability. The knee joint is formed by the articulation of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). It consists of two main articulations: the tibiofemoral joint (between the femur and tibia) and the patellofemoral joint (between the femur and patella).

Location

The knee joint is located in the lower limb, between the thigh and the lower leg. It lies at the junction of the femur (upper bone of the thigh) and the tibia (shin bone) and is covered anteriorly by the patella. It is positioned centrally on the anterior aspect of the leg.

Anatomy

The knee joint is the largest and most complex synovial joint in the human body. It functions as a modified hinge joint, primarily allowing flexion and extension while also permitting slight rotational movements. The knee joint is composed of bones, cartilage, ligaments, menisci, muscles, tendons, and a joint capsule, all working together to provide movement and stability. Below is a detailed description of its anatomy:

Bones of the Knee Joint

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

Femur (Thigh Bone):

• The distal end of the femur forms the upper part of the knee joint.
• It has two large rounded condyles:
• Medial condyle and lateral condyle, which articulate with the tibia.
• The intercondylar fossa (a deep notch) separates these condyles posteriorly.
• The anterior surface articulates with the patella to form the patellofemoral joint.

Tibia (Shin Bone):

• The proximal end of the tibia forms the lower part of the knee joint.
• It has two flattened articular surfaces (tibial plateaus):
• Medial tibial plateau and lateral tibial plateau, which articulate with the femoral condyles.
• The intercondylar eminence lies between these plateaus and serves as an attachment site for the cruciate ligaments and menisci.^[6]

Patella (Kneecap):

• A sesamoid bone embedded within the quadriceps tendon.
• The posterior surface of the patella articulates with the femur, forming the patellofemoral joint.

Joint Compartments

The knee joint consists of two major articulations:

Tibiofemoral Joint:

• Between the femoral condyles and the tibial plateaus.
• This is the primary weight-bearing articulation of the knee.

Patellofemoral Joint:

• Between the patella and the trochlear groove (anterior surface of the femur).
• This articulation helps the patella glide during knee movement.

Menisci

The knee joint contains two crescent-shaped fibrocartilaginous structures called menisci, which sit between the femoral condyles and the tibial plateaus:

• Medial Meniscus: C-shaped and larger, it attaches firmly to the medial collateral ligament (MCL).
• Lateral Meniscus: O-shaped and more mobile.^[4]

Functionally, the menisci act as shock absorbers, enhance joint stability, and improve congruence between the femur and tibia.

Ligaments

The knee joint is stabilized by several strong ligaments, categorized into extracapsular and intracapsular ligaments:

Extracapsular Ligaments (outside the joint capsule):

• Medial Collateral Ligament (MCL):
  • Runs from the medial femoral condyle to the medial tibia.
  • Stabilizes the knee against valgus forces (forces pushing the knee inward).
• Lateral Collateral Ligament (LCL):
  • Runs from the lateral femoral condyle to the head of the fibula.
  • Stabilizes the knee against varus forces (forces pushing the knee outward).
• Patellar Ligament:
  • Connects the patella to the tibial tuberosity.
  • Acts as the distal continuation of the quadriceps tendon.

Intracapsular Ligaments (inside the joint capsule):

• Anterior Cruciate Ligament (ACL):
  • Runs from the anterior intercondylar area of the tibia to the posterior part of the lateral femoral condyle.
  • Prevents anterior displacement of the tibia relative to the femur.
• Posterior Cruciate Ligament (PCL):
  • Runs from the posterior intercondylar area of the tibia to the anterior part of the medial femoral condyle.
  • Prevents posterior displacement of the tibia relative to the femur.

Joint Capsule

The knee joint is enclosed by a fibrous joint capsule, which:

• Encases the joint and stabilizes its structure.
• Is lined internally by a synovial membrane that produces synovial fluid, reducing friction and nourishing the joint structures.

Synovial Bursae

The knee joint has bursae (fluid-filled sacs) to reduce friction between tissues. Major bursae include:

• Suprapatellar bursa: Above the patella, beneath the quadriceps tendon.
• Prepatellar bursa: Over the patella, under the skin.
• Infrapatellar bursa: Below the patella, superficial and deep.
• Pes anserine bursa: Located medially, below the tibial plateau.

Muscles and Tendons

The knee joint is surrounded by several muscles and tendons that allow movement and stabilize the joint:

Anterior Compartment (Extensors)

Quadriceps Femoris Muscle: Includes four muscles — rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis.

• Their tendons unite to form the quadriceps tendon, inserting on the patella.

Posterior Compartment (Flexors)

• Hamstrings: Includes semitendinosus, semimembranosus, and biceps femoris.
• Gastrocnemius: Assists with knee flexion.

Medial Side

• Gracilis, sartorius, and semitendinosus (form the pes anserinus tendon).

Lateral Side

• Iliotibial Band (ITB): A thick band of fascia that runs laterally, stabilizing the knee.

Blood Supply

The knee joint receives a rich blood supply from the genicular arteries, which arise from the popliteal artery. These include:

• Medial and lateral superior genicular arteries
• Medial and lateral inferior genicular arteries

Nerve Supply

The knee joint is innervated by branches of:

• Femoral nerve (anterior aspect)
• Tibial nerve (posterior aspect)
• Common fibular nerve (lateral aspect)
• Obturator nerve (medial aspect)

These nerves provide both motor and sensory innervation.

Articular Cartilage

The surfaces of the femur, tibia, and patella are covered by hyaline cartilage, which reduces friction and absorbs shock during movement.^[1]

Function

The knee joint is a complex, modified hinge joint that primarily enables movement of the lower limb while bearing and distributing the body’s weight. It combines mobility and stability, making it essential for locomotion, weight-bearing, and posture. Below is a detailed explanation of its functions:

Primary Movements of the Knee Joint

The knee joint allows two primary movements along with some degree of rotation:

Flexion

• Definition: Bending the knee so that the angle between the femur and tibia decreases.
• Range of Motion: Approximately 135 degrees in normal conditions.
• Muscles Involved:
  • Hamstrings (semitendinosus, semimembranosus, and biceps femoris) are the primary flexors.
  • Assistants: Gracilis, sartorius, and gastrocnemius.
• Examples: Walking, sitting, squatting, and running.

Extension

• Definition: Straightening the knee so that the femur and tibia align in a straight line, increasing the angle between them.
• Range of Motion: Up to 0–10 degrees of hyperextension in some individuals.
• Muscles Involved:
  • Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) are the primary extensors.
• Examples: Standing up, kicking, and climbing stairs.^[8]

Medial and Lateral Rotation

• Definition: When the knee is flexed, the tibia can slightly rotate:
  • Medial Rotation: About 10 degrees.
  • Lateral Rotation: About 30–40 degrees.
• Muscles Involved:
  • Popliteus initiates medial rotation.
  • Biceps femoris assists in lateral rotation.

This rotational movement is critical for walking, pivoting, and adjusting foot placement.

Weight-Bearing and Load Transmission

• The knee joint bears and transmits the weight of the body from the femur to the tibia during standing, walking, running, and jumping.
• Distribution of Load:
  • The femoral condyles distribute the load across the tibial plateaus.
  • The menisci (medial and lateral) distribute forces evenly across the articular surfaces, reducing stress on the cartilage.
• During activities like running or jumping, the knee absorbs forces up to 4–6 times the body weight, making its load-bearing role critical.

Shock Absorption

The knee joint plays a vital role in absorbing impact forces during weight-bearing activities:

• Menisci act as shock absorbers by distributing forces and reducing stress on the bones and cartilage.^[7]
• The articular cartilage covering the femur, tibia, and patella cushions the joint and minimizes friction.
• The ligaments and joint capsule provide passive resistance to excessive movement, protecting the knee from injury.

Shock absorption helps protect the knee joint and surrounding structures during activities such as running, jumping, or descending stairs.

Stability of the Lower Limb

The knee joint provides static and dynamic stability, enabling individuals to maintain balance and posture:

Static Stability

• Provided by the ligaments:
  • Anterior Cruciate Ligament (ACL): Prevents forward displacement of the tibia relative to the femur.
  • Posterior Cruciate Ligament (PCL): Prevents backward displacement of the tibia.
  • Medial Collateral Ligament (MCL): Resists valgus forces (inward stress).
  • Lateral Collateral Ligament (LCL): Resists varus forces (outward stress).

Dynamic Stability

• Maintained by surrounding muscles and tendons, especially the quadriceps (anteriorly) and hamstrings (posteriorly).^[5]
• The patellar ligament and iliotibial band (lateral side) reinforce dynamic stability.

This stability is essential for standing, walking, and performing movements that involve changing directions.

Facilitating Locomotion

The knee joint is essential for locomotion by enabling a smooth gait cycle. Its role includes:

• Heel Strike: The knee absorbs impact by slightly flexing during initial contact with the ground.
• Midstance: The knee stabilizes the body while transferring weight over the leg.
• Push-Off: Extension of the knee generates propulsion, driving the body forward.
• Swing Phase: The knee flexes to allow the foot to clear the ground, followed by extension to prepare for the next step.

This coordinated movement allows for energy-efficient walking, running, and other activities.

Facilitating Activities of Daily Living

The knee joint enables many essential daily activities, such as:

• Sitting: Requires knee flexion to lower the body.
• Standing: Knee extension stabilizes the lower limb.
• Climbing Stairs: Combines flexion and extension for upward and downward movement.
• Squatting: Involves deep flexion of the knee for activities like lifting objects or sitting on the ground.

Proprioception and Balance

The knee joint is equipped with sensory receptors in the ligaments, tendons, and joint capsule that provide proprioceptive feedback.

• Proprioception helps the brain sense the position, movement, and forces acting on the knee joint.
• This information is crucial for:
  • Maintaining balance and posture.
  • Coordinating movements during activities like walking, running, and jumping.

Integration with the Hip and Ankle Joints

The knee joint works in close coordination with the hip joint and ankle joint to ensure smooth lower limb movement:

• The hip provides strength and power during locomotion.
• The knee absorbs shock and transmits forces.
• The ankle adjusts foot placement for balance and propulsion.

This integration ensures proper alignment and force distribution during weight-bearing and dynamic activities.^[3]

Clinical Significance

The knee joint is the largest and most complex joint in the human body, making it vulnerable to a wide range of injuries and disorders due to its role in weight-bearing and movement. Key clinical aspects include:

Ligament Injuries

• Common in athletes, involving tears to the Anterior Cruciate Ligament (ACL), Posterior Cruciate Ligament (PCL), Medial Collateral Ligament (MCL), or Lateral Collateral Ligament (LCL).
• ACL injuries are particularly frequent due to twisting motions or sudden stops.

Meniscal Tears

Tears of the medial or lateral meniscus occur due to twisting injuries and cause pain, swelling, and locking of the knee.

Osteoarthritis

Degenerative joint disease is common in older adults, leading to pain, stiffness, and reduced mobility due to cartilage wear.

Patellofemoral Disorders

Conditions like patellar dislocation or chondromalacia patellae cause pain and instability of the knee cap.

Fractures

Injuries to the distal femur, proximal tibia, or patella are often caused by trauma or falls.^[2]

Tendon Injuries

Inflammation or rupture of the patellar tendon or quadriceps tendon can severely impair knee extension.

Bursitis

Inflammation of bursae, such as prepatellar bursitis, causes localized pain and swelling.