Posterior Knee Dislocation: Understanding Complications, Long-Term Effects, and Prognosis

What are the complications of a posterior knee dislocation?

A posterior knee dislocation is a severe and often limb-threatening orthopedic emergency characterized by the complete displacement of the tibia backward relative to the femur, leading to a high risk of significant neurovascular damage, extensive ligamentous tears, and long-term functional impairment.

Understanding Posterior Knee Dislocation

A knee dislocation, distinct from a patellar dislocation, involves the separation of the tibia from the femur at the tibiofemoral joint. Posterior dislocations, where the tibia moves behind the femur, are particularly concerning due to the anatomy of the popliteal fossa, which houses critical neurovascular structures. These injuries typically result from high-energy trauma, such as motor vehicle accidents, falls from height, or severe sports injuries, though lower-energy mechanisms can occur in individuals with extreme hyperlaxity. The immediate and long-term complications can be devastating, making prompt diagnosis and management crucial.

Life-Threatening and Limb-Threatening Complications

The most critical complications of a posterior knee dislocation involve damage to the major blood vessels and nerves that traverse the popliteal region.

• Popliteal Artery Injury: This is the most dangerous complication, occurring in a significant percentage of knee dislocations (up to 40-50%, with posterior dislocations having a higher incidence). The popliteal artery is tethered proximally and distally, making it highly susceptible to stretch, compression, or transection during dislocation.
  • Consequences: Arterial injury can lead to acute limb ischemia (lack of blood flow), necessitating emergent vascular repair. Delayed or missed diagnosis can result in irreversible muscle and nerve damage, compartment syndrome, and ultimately, limb amputation if blood flow is not restored within 6-8 hours. Even with successful repair, complications like pseudoaneurysm, arteriovenous fistula, or chronic claudication can arise.
• Nerve Damage: The common peroneal nerve (also known as the fibular nerve) is the most frequently injured nerve due to its superficial course around the fibular head.
  • Consequences: Peroneal nerve injury can cause foot drop (inability to dorsiflex the ankle and extend the toes), sensory loss over the dorsum of the foot and lateral leg, and weakness in eversion. While some nerve palsies may be transient (neuropraxia), others can be permanent, severely impacting gait and function. The tibial nerve is less commonly injured but can also be affected, leading to sensory loss on the sole of the foot and weakness in ankle plantarflexion.

Musculoskeletal System Damage

Beyond neurovascular threats, posterior knee dislocations invariably involve extensive damage to the knee's stabilizing structures.

• Extensive Ligamentous Injuries: Unlike isolated ligament tears, knee dislocations almost always involve multiple ligaments.
  • Posterior Cruciate Ligament (PCL) and Anterior Cruciate Ligament (ACL): These are the primary stabilizers of the knee, and both are frequently torn, making the knee grossly unstable. Posterior dislocations inherently involve significant PCL disruption.
  • Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL): These collateral ligaments, which provide varus and valgus stability, are also commonly injured, often in combination with cruciate tears.
  • Posterolateral Corner (PLC): The PLC is a complex group of structures (including the popliteofibular ligament, arcuate ligament complex, and fibular collateral ligament) that resists external rotation and varus stress. Injury to the PLC is common in posterior dislocations and is critical for posterolateral stability; if unrecognized, it can lead to persistent instability and failure of other ligament reconstructions.
• Meniscal Tears: The menisci, C-shaped cartilages that act as shock absorbers and secondary stabilizers, can be torn, crushed, or displaced during the dislocation event.
  • Consequences: Meniscal tears can contribute to pain, mechanical symptoms (clicking, locking), and increase the risk of developing osteoarthritis.
• Articular Cartilage Lesions: The smooth articular cartilage covering the ends of the femur and tibia can be directly damaged through impaction or shearing forces during the dislocation.
  • Consequences: Chondral or osteochondral defects can lead to chronic pain, swelling, and are a significant predictor of early onset post-traumatic osteoarthritis.
• Associated Fractures: While less common than soft tissue injuries, fractures can occur.
  • Consequences: Fractures of the tibial plateau, femoral condyles, patella, or fibular head (especially with PLC injury) can complicate surgical management and rehabilitation, potentially affecting joint alignment and stability.

Long-Term Sequelae

Even after successful acute management, individuals who experience posterior knee dislocations are at high risk for chronic complications.

• Post-Traumatic Osteoarthritis (PTOA): This is a nearly universal long-term complication due to the initial cartilage damage, persistent instability (even after surgery), meniscal injuries, and altered joint mechanics.
• Chronic Instability and Pain: Despite surgical reconstruction, some degree of residual instability or chronic pain can persist, particularly if multi-ligamentous injuries are not adequately addressed or if nerve damage is permanent.
• Arthrofibrosis and Stiffness: The extensive trauma, prolonged immobilization, and surgical interventions can lead to excessive scar tissue formation within the joint, resulting in significant loss of knee range of motion (stiffness or arthrofibrosis).
• Muscle Weakness and Atrophy: Disuse, nerve injury, and prolonged recovery periods often lead to significant weakness and atrophy of the quadriceps and hamstring muscles, which can be challenging to fully rehabilitate.
• Functional Limitations: The combination of pain, instability, stiffness, and weakness can severely limit an individual's ability to return to pre-injury activity levels, impacting sports participation, occupational duties, and daily living activities.

Prognosis and Rehabilitation Considerations

The prognosis following a posterior knee dislocation is highly variable and depends on the extent of initial injury, particularly neurovascular damage, the timeliness and effectiveness of treatment, and adherence to a rigorous rehabilitation program. A multidisciplinary approach involving orthopedic surgeons, vascular surgeons, physical therapists, and rehabilitation specialists is essential to optimize outcomes and mitigate the severe complications associated with this devastating injury. Early recognition, prompt surgical intervention for vascular compromise and ligament reconstruction, and dedicated, long-term rehabilitation are paramount to maximizing functional recovery and minimizing the long-term sequelae.