Knee dislocations are classified by tibial displacement relative to the femur (anterior, posterior, medial, lateral, or rotatory). They invariably cause multiple ligamentous injuries and are usually the result of motor vehicle collisions, falls, sports, and industrial injuries. Anterior dislocations are more common and usually occur after high-energy hyperextension injuries. Knee dislocations are associated with popliteal artery and common peroneal and tibial nerve injuries. Popliteal artery injury can result from both anterior and posterior dislocations and is more common than nerve injury. Injury can be present despite normal pulses, and if not identified and repaired within 8 hours, amputation may be necessary. Common peroneal nerve injury can cause decreased sensation on the lateral foot, impaired dorsiflexion and eversion, and impaired sensation over the 1st dorsal web space. Knee dislocations can spontaneously relocate, so the physician must maintain a high index of suspicion. Injuries are painful and visually striking. An effusion will often be absent since the capsule has been violated. On exam, the knee will be grossly unstable since dislocations tend to injure most of the surrounding ligaments.
Anterior Knee Dislocation. Anterior displacement of the tibia in relation to the femur. (Photo contributor: Selim Suner, MD, MS.)
Management and Disposition
Management includes early reduction, immobilization, assessment of distal neurovascular function, and emergent orthopedic referral. Reduction of anterior dislocation is accomplished by flexing the hip 20 degrees and having an assistant apply longitudinal traction on the leg while keeping one hand on the tibia and simultaneously lifting the femur back into position. A posterior splint with the knee in 20 degrees of flexion is used for immobilization and to avoid tension on the popliteal artery. If there is concern for possible arterial injury, consider calculating an ankle-brachial index; a reading of less than 0.9 is abnormal and should raise concern. The patient should be admitted for observation and, likely, angiography.
Posterior Knee Dislocation. Posterior displacement of the tibia in relation to the femur. (Photo contributor: Paul R. Sierzenski, MD.)
Posterior Knee Dislocation. Posterior displacement of the tibia in relation to the femur. (Photo contributor: David Effron, MD.)
Knee dislocations are often associated with a fracture of the tibial plateau.
The presence of distal pulses in the foot does not rule out an arterial injury.
Vascular repair after 8 hours of injury carries an amputation rate of greater than 80%.