Knee Anatomy: The Posterolateral Corner, Its Ligaments, and Injuries

What is the lateral ligament behind the knee?

While the Lateral Collateral Ligament (LCL) is the primary lateral knee ligament, the phrase "lateral ligament behind the knee" most accurately refers to the complex of structures in the Posterolateral Corner (PLC) of the knee, particularly the Popliteofibular Ligament, which are crucial for rotational and posterior stability.

Understanding the Knee's Lateral Aspect

The knee joint, a complex hinge and pivot joint, relies on a network of ligaments for its stability. On the lateral (outer) side of the knee, the most prominent ligament is the Lateral Collateral Ligament (LCL), also known as the Fibular Collateral Ligament (FCL). This strong, cord-like structure originates from the lateral femoral epicondyle (on the outside of the thigh bone) and inserts onto the head of the fibula (the smaller bone of the lower leg).

The LCL's primary role is to resist varus stress – a force that pushes the knee inward, creating a "bow-legged" appearance. While it is a lateral ligament, its position is more on the direct side of the knee rather than strictly "behind" it. When the knee is fully extended, the LCL becomes taut, providing significant stability.

The Posterolateral Corner (PLC): A Complex Region

The "behind the knee" aspect of your query specifically points to the Posterolateral Corner (PLC). This is a crucial, yet often overlooked, anatomical region on the outside-back of the knee. The PLC is a highly complex area, consisting of a synergistic group of ligaments, tendons, and capsular structures that work together to provide stability, especially against rotational forces and hyperextension. Injuries to this area are often debilitating and frequently occur in conjunction with other major ligamentous tears, such as the Anterior Cruciate Ligament (ACL) or Posterior Cruciate Ligament (PCL).

Key Ligaments of the Posterolateral Corner

While the LCL is a part of the overall posterolateral stability, several other structures are more directly "behind" and lateral:

• Popliteofibular Ligament (PFL): This is arguably the most direct answer to "lateral ligament behind the knee" when considering the PLC. It's a strong ligament that runs from the popliteus muscle tendon (a muscle located behind the knee) to the head of the fibula. The PFL is a primary static stabilizer of the PLC, playing a critical role in preventing excessive external rotation and posterior translation of the tibia relative to the femur.
• Popliteus Tendon: Although a tendon (connecting muscle to bone), the popliteus muscle and its tendon are integral dynamic stabilizers of the PLC. It originates from the lateral femoral condyle and inserts onto the posterior aspect of the tibia. Its primary action is to internally rotate the tibia on the femur, unlocking the knee from its fully extended position, and to assist in posterior stability.
• Arcuate Ligament Complex: This is a Y-shaped thickening of the posterolateral joint capsule. It has two main arms:
  • Medial Arm: Blends with the popliteal muscle fascia.
  • Lateral Arm: Extends over the popliteus tendon to the fibular head. The arcuate ligament contributes to overall posterolateral stability and reinforces the joint capsule.
• Posterolateral Joint Capsule: The fibrous outer layer of the joint capsule in this region also provides significant passive stability.
• Lateral Gastrocnemius Tendon: The lateral head of the gastrocnemius muscle (calf muscle) originates from the lateral femoral condyle and contributes to the posterior stability of the knee, especially during hyperextension.

Functions of the Posterolateral Ligaments

The collective function of the ligaments and structures within the PLC is vital for comprehensive knee stability:

• Resisting Varus Stress: Primarily the LCL, preventing the knee from bowing inward.
• Preventing External Tibial Rotation: The PFL and popliteus are crucial in limiting the tibia from rotating excessively outward, especially when the knee is partially flexed.
• Preventing Posterior Tibial Translation: These structures, particularly the PFL, work in conjunction with the PCL to prevent the tibia from sliding too far backward under the femur.
• Overall Rotational Stability: They prevent a combination of hyperextension and external rotation, a common mechanism for severe knee injuries.

Common Injuries and Mechanisms

Injuries to the PLC are often complex and typically result from high-energy trauma. They rarely occur in isolation and are frequently seen alongside tears of the ACL or PCL.

• Mechanism of Injury:
  • Direct blow to the anteromedial knee: A force applied to the front-inside of the knee, pushing it outward and backward.
  • Hyperextension with varus or external rotation force: The knee is forced beyond its normal range of motion, often with a twisting component.
  • Dashboard injuries: In car accidents, the shin hitting the dashboard can cause combined posterior and rotational forces.
  • Sports injuries: Especially in contact sports or activities involving rapid changes in direction.

Symptoms of PLC Injury

Recognizing a PLC injury can be challenging due to its complexity and frequent association with other ligamentous damage. Common symptoms include:

• Pain: Localized on the outside and back of the knee.
• Swelling: Often significant due to the trauma.
• Instability: A feeling of the knee "giving way," particularly during activities involving pivoting, cutting, or walking downhill.
• Hyperextension: The knee may feel like it "bows out" or hyperextends.
• Foot Drop: In severe cases, the common peroneal nerve, which runs close to the fibular head, can be stretched or damaged, leading to weakness in ankle dorsiflexion (lifting the foot) and toe extension.
• Difficulty with knee extension or flexion: Due to pain, swelling, and mechanical instability.

Diagnosis and Treatment

Diagnosing a PLC injury requires a thorough physical examination by an orthopedic specialist, involving specific tests to assess varus stability, external rotation, and posterior translation (e.g., varus stress test, external rotation recurvatum test, dial test). Magnetic Resonance Imaging (MRI) is the gold standard for definitively visualizing the extent of ligamentous and soft tissue damage.

Treatment for high-grade PLC injuries (Grade II or III) is often surgical due to the poor healing potential of these structures and their critical role in overall knee stability. Surgical reconstruction aims to restore the anatomical integrity of the damaged ligaments. Following surgery, a comprehensive rehabilitation program is essential to regain strength, range of motion, and stability.

Importance for Movement and Stability

The posterolateral corner structures, including the Popliteofibular Ligament, are paramount for maintaining normal knee kinematics. Their integrity ensures that the knee moves smoothly and predictably, preventing abnormal translation and rotation that could lead to further injury, chronic instability, or premature degenerative changes like osteoarthritis. Understanding these complex structures is key for anyone involved in fitness, sports, or physical therapy, highlighting the intricate design required for optimal human movement.