Dislocated Knee

A dislocated knee causes severe pain.

• Knee dislocations are a result of significant force or trauma.
• With a dislocated knee, the relationship between the femur and tibia is lost and at least three of four ligaments that hold the knee stable must be torn.
• Fractures often accompany the dislocation.
• Surgeons most often repair the torn knee ligaments to allow the knee to be stable and function again.
• Knee dislocations are a potential vascular emergency and can be associated with damage to the popliteal artery in the back of the knee. If the artery injury is not repaired within a few hours, there is high risk of leg amputation.
• Compartment syndrome is also a common complication.
• Physical therapy and postoperative rehabilitation is a long process and can last more than a year.

The knee is the leg joint where the femur (thighbone) meets the tibia (shinbone). Four ligaments cross the joint attaching to both the femur and tibia, stabilizing 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 of the thigh add to the knee’s stability.

A knee dislocation is a relatively rare injury. It takes significant force and energy to dislocate the knee, and at least three of the four stabilizing ligaments need to be torn to lose the relationship between the femur and tibia. Common causes include motor vehicle crashes, car-pedestrian accidents, industrial accidents, athletic injuries, and falls. Fractures often accompany a knee dislocation.

The knee most commonly dislocates anteriorly or posteriorly, where the tibia is shoved forward or back compared to the femur. This can tear the popliteal artery that runs behind the knee and supplies the leg and foot with blood. The popliteal artery can suffer a partial or complete tear, and if medical professionals do not recognize and repair the damage quickly, the leg may require amputation.

An associated complication is compartment syndrome. Muscles in the calf and shin are in compartments tightly bound by fascia (thick tissue). If there is swelling or bleeding in a compartment, the pressure can rise high enough to damage muscle tissue and prevent blood from flowing through the compartment to the foot.

Another complication of knee dislocation is damage to the peroneal nerve, which supplies the muscles that dorsiflex, or lift the toes and foot off the ground when we walk. Damage to this nerve can cause foot drop and numbness of part of the foot.

A knee dislocation is an orthopedic emergency and is different from a kneecap (patellar) dislocation, where the kneecap, located in front of the knee, slides laterally out of place. The knee joint itself is not involved in a kneecap dislocation. The treatment is to pop the kneecap back into place and start physical therapy to strengthen the muscles around the kneecap to prevent recurrence.

A knee dislocation is very painful, and marked swelling and deformity often accompany the injury. About half of knee dislocations will reduce or realign themselves spontaneously. The bones may look aligned, but the joint remains very unstable. The patient will have too much pain to lift the leg off the stretcher or to try and walk at all.

If there is damage to the peroneal nerve, the patient may complain of numbness in the foot and be unable to dorsiflex the foot or flex the toes in the direction of the nose.

If there is damage to the popliteal artery and no blood is pumping to the leg, the foot may be cold and develop increasing pain.

The initial diagnosis of knee dislocation occurs by history and physical exam. The care provider will learn about mechanism of injury and the appearance of the leg at time of injury. Knee examination will look for swelling, areas of tenderness, and stability of the knee ligaments. Knee dislocations are associated with significant swelling and bleeding, and it may be difficult to appreciate on physical exam how unstable the knee might be. The provider needs to have a high index of suspicion to make the diagnosis.

The provider will also look for nerve and artery damage.

Medical professionals test for peroneal nerve damage by looking for decreased sensation on the top of the foot between the big and second toe and by assessing the ability for the patient to dorsiflex the foot (bend the toes back toward the nose).

Popliteal artery injury is an important concern. Doctors will check pulses in the foot (dorsalis pedis and posterior tibial arteries) and in the back of the knee (popliteal artery). Hard signs of a popliteal injury include

• loss of pulses,
• enlarging hematoma (blood clot in the back of the knee),
• bruits (an audible sound heard with a stethoscope due to turbulent blood),
• thrills (palpable turbulent blood flow under the skin), and
• if there is a wound, pulsatile bleeding.

Hard signs of popliteal artery injury will cause the patient to be taken immediately to the operating room for artery repair.

Because the risk of popliteal injury is so high, if hard signs aren’t present, medical professionals may perform other testing to look for artery damage:

• The ankle-brachial (ABI) index compares blood pressures in the arm and leg. Lower leg blood pressure could signal artery damage.
• Doppler ultrasound of the leg blood vessels
• CT angiogram
• Doctors no longer routinely perform arteriography, where dye is injected directly into the artery.

If there is concern for compartment syndrome, medical professional can insert needles into each of the four muscle compartments in the leg to directly measure the pressure within each compartment.

Medical professionals take plain X-rays to look for broken bones (fractures) and to determine the misalignment of the femur and tibia.

If there is no arterial damage and a doctor admits a patient to the hospital for continued observation for the potential of delayed injury, a medical professional may perform a knee MRI to better understand the extent of the injuries.

Initial treatment and stabilization includes looking after the whole patient, including assessing and treating any other injuries that might be present.

If the knee is dislocated when the patient comes to the emergency department, a medical professional might attempt to relocate the knee dislocation, if possible, to minimize the risk of continued popliteal artery damage.

Knee dislocations are unstable. Most patients will require repair of their fractures and reconstruction of their torn ligaments. This requires an orthopedic surgeon to perform an operation.

Vascular surgeons are also actively involved in the care of knee dislocations because popliteal artery injuries can be catastrophic.

• If there are hard signs of popliteal artery injury, a patient will go immediately to the operating room for artery repair.
• If hard signs are not present, medical professionals may evaluate of the artery with ABI measurements, ultrasound, or CT angiography. If there is evidence of injury, then the patient will go to the operating room for repair.
• Doctors often perform fasciotomy along with the artery repair to prevent the development of compartment syndrome. Physicians split open the thick tissues that divide the muscles in the lower leg to allow swelling to occur.
• If there is no arterial injury, doctors will observe a patient for 2-3 days in the hospital, with repeated testing to make certain that a delayed injury to the artery does not develop.

Observation for compartment syndrome is also an important part of the care. If this develops, the patient needs to go to the operating room to have a fasciotomy to relieve the pressure and save the muscle from damage.

Repairing damage to the popliteal vein or the peroneal nerve is controversial as to whether that repair is beneficial in the long term. The surgeon usually decides along with the patient as to the best course of action.

• Orthopedic surgeons operate to repair any bone injuries and fractures, as well as torn knee ligaments.
• Vascular surgeons are responsible for repairing artery injuries.
• Physical therapists help the patient recover from the injury, including recovering strength and range of motion in the leg.

After surgery, physical therapy, rehab treatment, and recovery time take months, lasting up to a year or more.

The major complications of knee dislocation include the following:

• Ligament damage to the knee: Three of the knee four ligaments (ACL, PCL, MCL, LCL) must tear for the knee to dislocate.
• Popliteal artery injury
• Popliteal vein injury
• Peroneal nerve damage
• Compartment syndrome
• Infection
• Venous thromboembolism (deep vein thrombosis, pulmonary embolus)

• About 60%-70% of patients with reconstructed knee dislocations have a good result with a functional painless knee.
• Approximately 10%-15% will have adequate function.
• Another 10%-15% will have a chronically unstable and painful knee. Patients may complain of instability with the knee giving way, recurrent swelling, decreased range of motion, and pain.
• In those patients who have a popliteal artery injury repaired more than 8 hours after injury, 80% will need a leg amputation. That complication falls to 20% if the artery repair occurs sooner.