Knee Dislocation: Mechanisms, Causes, and Complications

How Does a Knee Dislocate?

A knee dislocation, distinct from a patellar dislocation, is a severe and rare orthopedic injury where the femur and tibia completely lose their normal alignment, typically resulting from high-energy trauma that extensively damages the knee's stabilizing ligaments.

Understanding Knee Anatomy and Stability

To comprehend how a knee dislocates, it's essential to first grasp the intricate anatomy that provides its stability. The knee joint, a modified hinge joint, is formed by the articulation of three bones:

• Femur: The thigh bone.
• Tibia: The larger of the two lower leg bones.
• Patella: The kneecap, which glides in a groove on the femur.

While the bony architecture offers some stability, the primary restraints against dislocation come from a complex network of soft tissues:

• Ligaments: These strong, fibrous bands connect bones and are crucial for joint stability. The knee has four major ligaments:
  • Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding forward on the femur and limits hyperextension.
  • Posterior Cruciate Ligament (PCL): Prevents the tibia from sliding backward on the femur and limits hyperflexion.
  • Medial Collateral Ligament (MCL): Resists valgus (inward) forces, stabilizing the inner aspect of the knee.
  • Lateral Collateral Ligament (LCL): Resists varus (outward) forces, stabilizing the outer aspect of the knee.
• Menisci: Two C-shaped cartilage pads (medial and lateral) that sit between the femur and tibia, acting as shock absorbers and improving joint congruity.
• Joint Capsule: A fibrous sac enclosing the joint, reinforced by ligaments and muscles.
• Muscles and Tendons: The powerful quadriceps (anterior thigh) and hamstrings (posterior thigh) muscles, with their respective tendons, actively contribute to dynamic knee stability.

What Constitutes a Knee Dislocation?

A knee dislocation, medically termed a tibiofemoral dislocation, is a catastrophic injury involving the complete loss of articulation between the distal femur and the proximal tibia. This is a critical distinction from a patellar (kneecap) dislocation, which is far more common and less severe. In a true knee dislocation, the integrity of multiple major knee ligaments (typically three or more, including both cruciates) is compromised, leading to gross instability.

Primary Mechanisms of Knee Dislocation

Knee dislocations are almost exclusively the result of significant force applied to the knee, overwhelming the strength of its ligamentous stabilizers. The mechanisms can be broadly categorized:

• High-Energy Trauma: This is the most common cause and often involves:

  • Motor Vehicle Accidents (MVAs): Direct impact to the dashboard during a collision, leading to a "dashboard injury" where the tibia is driven backward (posterior dislocation).
  • Falls from Significant Heights: Landing directly on the knee or with extreme force.
  • High-Impact Sports Injuries:
    • Direct Blows: A forceful impact to the front, back, or side of the knee.
    • Hyperextension: The knee is forced beyond its normal range of extension, often tearing the ACL, PCL, and posterior capsule.
    • Hyperflexion: The knee is forced beyond its normal range of flexion, primarily tearing the PCL and potentially the ACL.
    • Extreme Varus or Valgus Stress: Excessive inward (valgus) or outward (varus) force applied to the knee, tearing the MCL/LCL, and often the cruciates.
    • Rotational Forces: A combination of twisting and other forces, leading to complex ligamentous disruption.

• Low-Energy Trauma (Less Common): While rare, knee dislocations can occur with less force in specific circumstances:

  • Extreme Obesity: The increased soft tissue around the knee can act as a fulcrum, and the sheer weight can contribute to ligamentous failure with relatively minor trauma.
  • Underlying Ligamentous Laxity: Individuals with genetic conditions causing hypermobility (e.g., Ehlers-Danlos syndrome) may be more susceptible.
  • Specific Positions: A combination of an awkward joint position and a minor external force can, in rare cases, lead to dislocation.

Biomechanical Forces and Ligamentous Disruption

The specific direction of the dislocating force dictates which ligaments are most severely affected:

• Anterior Dislocation: The tibia moves forward relative to the femur. This usually results from a hyperextension injury, often tearing the PCL, ACL, and posterior capsule.
• Posterior Dislocation: The tibia moves backward relative to the femur. This is often seen in "dashboard injuries" and typically tears the ACL, PCL, and posterior capsule.
• Medial Dislocation: The tibia shifts medially. This indicates significant valgus stress, leading to tears of the LCL, PCL, and often the ACL.
• Lateral Dislocation: The tibia shifts laterally. This indicates significant varus stress, leading to tears of the MCL, PCL, and often the ACL.
• Rotary Dislocation: A combination of rotational and other forces, leading to highly complex and extensive ligament damage.

In virtually all true knee dislocations, there is extensive damage to at least two, and often all four, of the major knee ligaments, along with potential damage to the joint capsule, menisci, and surrounding soft tissues.

Associated Injuries and Complications

The severity of a knee dislocation extends far beyond ligamentous damage. Due to the high forces involved and the anatomical proximity of vital structures, significant complications are common and can be limb-threatening:

• Neurovascular Damage: This is the most critical concern. The popliteal artery (the main artery supplying the lower leg) and the peroneal nerve (which controls foot movement and sensation) are highly vulnerable to stretching, tearing, or compression during dislocation.
  • Popliteal Artery Injury: Can lead to acute limb ischemia, requiring immediate surgical repair to prevent amputation.
  • Peroneal Nerve Injury: Can cause foot drop and sensory loss in the lower leg and foot.
• Meniscal Tears: The cartilage pads are frequently torn.
• Fractures: Bone fractures of the femur, tibia, or patella can occur concurrently.
• Articular Cartilage Damage: The smooth cartilage surfaces of the joint can be severely bruised or damaged.
• Compartment Syndrome: Swelling within the fascial compartments of the lower leg can compromise blood flow and nerve function.

Risk Factors

While most knee dislocations are due to unpredictable high-energy trauma, certain factors can increase susceptibility:

• Participation in high-impact sports: Football, rugby, skiing, etc.
• Pre-existing ligamentous laxity: Conditions causing hypermobility.
• Previous knee injuries: Weakened structures from prior trauma.
• Obesity: Can contribute to the mechanism of injury in some cases.

Prevention Strategies

Given that most knee dislocations result from severe trauma, complete prevention can be challenging. However, strategies to enhance knee resilience and reduce risk include:

• Strengthening Surrounding Musculature: Robust quadriceps, hamstrings, and gluteal muscles provide dynamic stability to the knee joint.
• Proprioceptive Training: Exercises that improve balance and joint position sense can enhance the body's ability to react to sudden forces.
• Proper Technique in Sports: Learning and executing correct movement patterns can reduce undue stress on the knee.
• Appropriate Protective Gear: Wearing recommended safety equipment in high-risk activities.

Conclusion

A knee dislocation is a grave orthopedic emergency requiring immediate medical attention. It represents a complete failure of the knee's primary stabilizing structures, usually multiple ligaments, due to extreme forces. Understanding the underlying anatomy and biomechanics of these injuries underscores the importance of prompt diagnosis and treatment to mitigate devastating complications, particularly neurovascular compromise, and to optimize long-term outcomes.