Dislocated Knee

The knee is where the femur (thighbone) meets the tibia (shinbone). Ligaments stabilize the knee:

• Medial and lateral collateral ligaments (MCL, LCL)
• Anterior and posterior cruciate ligaments (ACL, PCL)

The quadriceps muscles on the front of the thigh and the hamstrings on the back add to the knee’s stability.

A knee dislocation is a rare injury. It requires significant force and energy, and at least three of the four stabilizing ligaments need to be torn to lose the relationship between the femur and tibia. Causes include car accidents, falls, and athletic injuries. Fractures often accompany a knee dislocation.

The knee commonly dislocates anteriorly or posteriorly, tearing the popliteal artery that supplies blood to the leg and foot. The artery can suffer a partial or complete tear, requiring immediate repair to avoid amputation.

Compartment syndrome is another complication. If there is swelling or bleeding in a compartment, the pressure can damage muscle tissue and prevent blood flow to the foot.

Damage to the peroneal nerve, which controls foot movement, is also a complication of knee dislocation.

A knee dislocation is an orthopedic emergency and is different from a kneecap dislocation. The treatment requires relocating the kneecap and starting physical therapy.

Symptoms of a dislocated knee

Knee dislocation is painful, with swelling and deformity. About half of knee dislocations reduce themselves, but the joint remains unstable. The patient will be unable to lift the leg or walk.

If there is damage to the peroneal nerve, the patient may experience numbness in the foot and an inability to move the toes.

If there is damage to the popliteal artery, the foot may become cold and painful.

Diagnosis of a dislocated knee

The diagnosis is based on history and physical exam. Swelling and tenderness are common. Nerve and artery damage are assessed in the examination.

Medical professionals test for peroneal nerve damage by looking for decreased sensation and the ability to move the foot.

Injuries to the popliteal artery are checked using various tests.

If there is concern for compartment syndrome, doctors measure the pressure within each compartment.

Plain X-rays and MRI scans are taken to assess the extent of the injury.

Treatments for a dislocated knee

Treatment includes assessing and treating any other injuries present.

If the knee is dislocated, medical professionals will attempt to relocate it to minimize artery damage.

Knee dislocations require surgery to repair fractures and torn ligaments. Vascular surgeons may also be involved in cases of artery injury.

If there are signs of artery injury, immediate repair is necessary. Other tests may be performed to confirm artery damage.

Observation and testing are performed if there is no arterial damage. Complications such as compartment syndrome are closely monitored.

Repairing damage to the popliteal vein or peroneal nerve is decided on a case-by-case basis.

Orthopedic surgeons repair bone injuries and torn ligaments, while physical therapists assist with recovery.

Recovery time for a dislocated knee

Physical therapy, rehab, and recovery can take months or longer.

Complications of a dislocated knee

Complications include ligament damage, artery and vein injury, nerve damage, compartment syndrome, infection, and venous thromboembolism.

Prognosis for a dislocated knee

A good outcome with a functional painless knee is seen in 60%-70% of cases. Approximately 10%-15% have adequate function, while 10%-15% have chronic instability and pain. Delayed artery repair leads to a higher amputation rate.

Knutson, T., J. Bothwell, and R. Durbin. "Evaluation and Management of Traumatic Knee Injuries in the Emergency Department." Emerg Med Clin North Am 33.2 (2015): 345-362.

Sillanpaa, P.J., P. Kannus, and S.T. Niemi, et al. "Incidence of Knee Dislocation and Concomitant Vascular Injury Requiring Surgery: A Nationwide Study." J Trauma Acute Care Surg 76.3 (2014): 715.