Knee Dislocation: Mechanisms, Anatomy, and Complications

What is the Mechanism of Knee Dislocation?

Knee dislocation is a severe and relatively rare injury characterized by the complete separation of the tibia from the femur, typically resulting from high-energy trauma that ruptures multiple stabilizing ligaments of the knee joint.

Understanding the Knee Joint Anatomy

To comprehend the mechanism of knee dislocation, it's crucial to first understand the intricate anatomy of the tibiofemoral joint. The knee is a complex hinge joint, allowing for flexion and extension, with limited rotation. Its stability relies heavily on a robust network of ligaments, menisci, and the surrounding musculature.

• Bones: The primary bones involved are the femur (thigh bone), tibia (shin bone), and patella (kneecap). The patella articulates with the femur but is not directly involved in tibiofemoral dislocation.
• Ligaments: These strong, fibrous bands connect bone to bone, providing static stability.
  • Cruciate Ligaments: Located within the joint capsule, crossing each other.
    • Anterior Cruciate Ligament (ACL): Prevents anterior translation of the tibia relative to the femur and limits hyperextension.
    • Posterior Cruciate Ligament (PCL): Prevents posterior translation of the tibia relative to the femur and limits hyperflexion.
  • Collateral Ligaments: Located on the sides of the joint.
    • Medial Collateral Ligament (MCL): Resists valgus (inward) forces.
    • Lateral Collateral Ligament (LCL): Resists varus (outward) forces.
• Menisci: Two C-shaped cartilaginous discs (medial and lateral menisci) sit between the femur and tibia. They act as shock absorbers, distribute load, and enhance joint stability.
• Joint Capsule: A fibrous capsule enclosing the joint, lined with a synovial membrane that produces lubricating fluid.

Defining Knee Dislocation

A true knee dislocation, also known as tibiofemoral dislocation, involves the complete displacement of the tibial plateau relative to the femoral condyles. This is distinct from a patellar dislocation, which only involves the kneecap. Tibiofemoral dislocations are considered orthopedic emergencies due to their potential for severe associated injuries, particularly to neurovascular structures. Unlike more common ligamentous sprains, knee dislocations almost always involve the rupture of at least two of the four major stabilizing ligaments (ACL, PCL, MCL, LCL), with the PCL being involved in the majority of cases.

Primary Mechanisms of Knee Dislocation

Knee dislocations typically result from significant forces that overwhelm the structural integrity of the knee joint. These forces can be categorized by their energy level:

• High-Energy Trauma: The most common cause, often seen in motor vehicle accidents, falls from height, or high-impact sports injuries. The immense forces involved lead to multi-ligamentous damage.
  • Hyperextension: A common mechanism where a direct blow to the anterior aspect of the distal femur or a force applied to the anterior tibia drives the knee into extreme hyperextension, tearing the PCL first, then often the ACL, leading to anterior dislocation (tibia moves anteriorly relative to the femur).
  • Direct Blow to the Proximal Tibia:
    • Anterior Blow: A force striking the anterior aspect of the tibia (e.g., "dashboard injury" in car accidents) drives the tibia posteriorly, rupturing the ACL and often the PCL, resulting in posterior dislocation.
    • Posterior Blow: A force striking the posterior aspect of the tibia drives the tibia anteriorly, causing anterior dislocation.
  • Valgus or Varus Stress with Rotation: A combined force where a significant blow to the side of the knee (valgus for medial, varus for lateral) combined with a rotational component can tear the collateral ligaments and then one or both cruciates, leading to medial or lateral dislocation.
• Low-Energy Trauma: Less common, but can occur in certain populations, often without a significant external force. This is typically seen in individuals with:
  • Morbid Obesity: The sheer mass of the thigh and leg can generate enough leverage during a seemingly innocuous fall or twist to dislocate the knee, even with minimal external force.
  • Neurological Conditions: Conditions causing muscle weakness or spasticity can predispose to dislocation.
  • Connective Tissue Disorders: Conditions like Ehlers-Danlos syndrome, which cause generalized ligamentous laxity, can make the knee joint more susceptible to dislocation from minor trauma.

Ligamentous Involvement and Direction of Dislocation

The direction of the tibial displacement relative to the femur is determined by the specific ligaments that rupture and the vector of the applied force.

• Anterior Dislocation:
  • Mechanism: Typically from hyperextension or a direct blow to the posterior aspect of the tibia.
  • Ligamentous Involvement: Usually involves rupture of the PCL (primary restraint to posterior translation), often followed by the ACL. The posterior capsule may also be torn. This is the most common type of knee dislocation.
• Posterior Dislocation:
  • Mechanism: Primarily from a direct blow to the anterior aspect of the tibia when the knee is flexed (e.g., dashboard injury).
  • Ligamentous Involvement: Involves rupture of the ACL (primary restraint to anterior translation), often followed by the PCL.
• Medial/Lateral Dislocation:
  • Mechanism: Result from significant valgus (force pushing knee inward) or varus (force pushing knee outward) stress, often with a rotational component.
  • Ligamentous Involvement: Involves the MCL (for medial) or LCL (for lateral), along with at least one, and often both, cruciate ligaments.
• Rotatory Dislocation:
  • Mechanism: Involves a combination of forces leading to complex rotational displacement.
  • Ligamentous Involvement: Typically involves extensive damage to multiple ligaments and the joint capsule.

Associated Injuries and Complications

The severity of forces required to dislocate the knee means that associated injuries are very common and can be limb-threatening.

• Neurovascular Compromise: This is the most critical complication.
  • Popliteal Artery Injury: The popliteal artery, which supplies blood to the lower leg, runs directly behind the knee joint. Dislocation can stretch, tear, or thrombose this artery, leading to acute limb ischemia. This is an emergency requiring immediate vascular surgery.
  • Peroneal Nerve Injury: The common peroneal nerve (a branch of the sciatic nerve) wraps around the fibular head near the knee. It can be stretched or compressed, leading to foot drop (inability to lift the foot at the ankle) and sensory deficits.
• Cartilage Damage: The articular cartilage surfaces of the femur and tibia can be damaged.
• Meniscal Tears: The menisci are frequently torn during dislocation.
• Fractures: Avulsion fractures (where a ligament pulls off a piece of bone) or other bone fractures can occur.

Immediate Management and Long-Term Rehabilitation Considerations

Given the potential for severe complications, knee dislocation is an orthopedic emergency.

• Immediate Medical Attention: Prompt reduction (re-alignment) of the joint is crucial, ideally in a medical setting, to minimize soft tissue damage and reduce the risk of neurovascular compromise. A thorough neurovascular assessment (checking pulses, sensation, and motor function) must be performed immediately upon injury and after reduction.
• Treatment Approach: Once stable, treatment often involves surgical reconstruction of the damaged ligaments. The decision for surgery versus non-surgical management depends on the number and severity of ligamentous injuries, patient activity level, and presence of other complications.
• Rehabilitation: Long-term rehabilitation is extensive and critical for restoring knee stability, strength, range of motion, and proprioception. It typically involves:
  • Initial Protection: Bracing and limited weight-bearing.
  • Gradual Range of Motion: Carefully restoring movement without stressing healing structures.
  • Progressive Strengthening: Targeting quadriceps, hamstrings, and calf muscles.
  • Neuromuscular Control and Proprioception Training: Exercises to improve balance and joint awareness.
  • Return to Activity: A carefully phased return to sports or high-impact activities, often taking 9-12 months or more.

Understanding the mechanisms of knee dislocation emphasizes the extraordinary forces involved and the profound impact on joint stability and overall limb function. It underscores the importance of immediate, expert medical intervention and a comprehensive, dedicated rehabilitation program for optimal recovery.