Causes / typical injury mechanism and pathophysiology: Knee joint dislocation is considered an orthopedic emergency. It is an uncommon condition; however, the accompanying arterial injury that occurs after knee joint dislocations cannot be missed. The popliteal artery is most at risk. The incidence of popliteal artery injury necessitating surgery is reported to be 13%.

Risk of injury is due to anatomic constraints involving fibrous tethering proximally at the adductor hiatus and distally at the soleus arch. Vascular injury also predisposes patients to compartment syndrome, another orthopedic emergency.

Similarly, the common peroneal nerve is at risk due to anatomic constraints, proximally around the fibular neck and distally at the intermuscular septum. Incidence of peroneal nerve injury is reported to be about 25%. The tibial nerve is less commonly injured.

About half of patients self-reduce, often on the way to the ED.

Knee structures susceptible to injury include the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the posteromedial corner (PMC), and the posterolateral corner (PLC).

Classic history and presentation: Knee joint dislocations are often the result of high-energy trauma, with the most common mechanism of injury being motor vehicle collisions. Often, these patients also sustain fractures and ipsilateral joint dislocations. Less commonly, knee joint dislocations are seen after sports injuries and falls. A smaller group presents after an ultra-low energy mechanism. These patients are more likely to present with nerve injury than their higher-energy trauma counterparts, with an incidence of nerve injury of approximately 44%. This patient population is commonly composed of obese patients who present after a ground-level fall. This particular mechanism is more common in female patients.

Open knee joint dislocation occurs in between 15% and 35% of cases and is associated with poorer long-term outcomes.

Prevalence: Knee joint dislocation comprises 0.002% of all musculoskeletal injuries.

• Age – This typically affects patients between the ages of 20 and 50 years.
• Sex / gender – Knee joint dislocation is more common in male patients.

Risk factors:

• High-energy trauma
• Obesity

Grade / classification system: Several classification systems for knee joint dislocations exist.

The Schenck classification is based on ligamentous injuries and is currently the most widely used and accepted classification system –

• Grade I: ACL or PCL
• Grade II: ACL and PCL
• Grade III: ACL, PCL, and either MCL or LCL
  • Grade IIIM: ACL, PCL, and MCL
  • Grade IIIL: ACL, PCL, and LCL
• Grade IV: ACL, PCL, MCL, and LCL
  • Highest rate of vascular injury
• Grade V: Fracture dislocation

The Kennedy classification is based on how the tibia is translated relative to the femur –

• Anterior
  • Most common
  • Hyperextension injury
  • Highest rate of peroneal nerve injury
  • Common: PCL injury
• Posterior
  • Axial load on flexed knee
  • Highest rate of popliteal artery injury
• Medial
  • Varus / valgus mechanism
  • Common: PLC and PCL injury
• Lateral
  • Varus / valgus mechanism
  • Common: ACL and PCL injury
• Rotatory
  • Usually posterolateral
  • Often irreducible, dimple sign

McCoy et al and Shelbourne developed a classification based on energy of injury mechanism (high, low, ultralow), noting that high-energy mechanisms pose greater risk to vasculature and soft tissues. Palmer and Boisgard et al divided injuries based on chronicity, with 3 weeks from time of injury to management dividing acute from chronic injuries.