What is a dislocated knee?
Your knee contains four bones—the thigh bone (femur), kneecap (patella), shinbone (tibia), and a smaller lower leg bone (fibula). A knee dislocation occurs when one of these bones moves out of its normal position. In most cases, it is a dislocated kneecap. However, it is possible for the tibia and fibula to dislocate from the joint. This injury is much more rare and severe than a dislocated kneecap. Another name for this injury is a femorotibial dislocation. It accounts for less than 1% of orthopedic injuries.
The knee is a very stable joint. There are four main ligaments that stabilize the knee joint and hold the bones in position. The two collateral ligaments are on either side of the knee and control sideways movements. The two cruciate ligaments cross each other inside the knee joint. They control back and forth movements. An injury that tears these ligaments can result in the tibia and fibula dislocating. The type of dislocation depends on which ligaments tear. Doctors classify knee dislocations based on the position of the tibia in relation to the femur:
Anterior is the most common type. It usually involves hyperextension of the knee joint, which tears the PCL (posterior cruciate ligament). The tibia dislocates in front of the femur and knee joint.
Posterior is the second most common type. It typically occurs when there is blunt force to a flexed knee, such as hitting a car’s dashboard. This forces the tibia behind the femur and knee joint. There is a high risk of tearing the popliteal artery with this dislocation. The popliteal artery is a major leg artery that runs behind the knee.
Lateral dislocation occurs when the tibia moves outward in relation to the femur and knee joint.
Medial dislocation happens when the tibia moves inward in relation to the femur and knee joint.
Rotary dislocation results from twisting forces on the tibia.
Knee dislocations are severe injuries with obvious deformities. About half the time, the bones move back into position by themselves and the joint looks normal. However, knee dislocations often occur along with fractures of the femur or tibia. There is also a high rate of artery, nerve, tendon and muscle damage. This makes them medical emergencies that can lead to loss of the limb or even life. Seek immediate medical care (call 911) for any injury involving the knee.
What are the symptoms of a dislocated knee?
Knee dislocations usually result in severe deformities of the knee and leg. However, the joint may have a normal appearance if the bones spontaneously move back into position. This happens in about half the cases.
Common symptoms of a dislocated knee
The most common symptoms of a knee dislocation are:
Bruising, which can be extensive
Difficulty moving the knee and pain with any amount of movement
Inability to bear weight on the leg
Severe pain even at rest
Swelling, which can be quite significant
Serious symptoms that might indicate a life-threatening condition
In some cases, a knee dislocation can lead to the need for amputation and even be life threatening. Seek immediate medical care (call 911) if you, or someone you are with, have any of these limb-threatening symptoms including:
Complete inability to move the knee
Lack of a pulse below the knee, which can cause changes in skin color and coolness to the touch
Loss of feeling or movement below the knee
The longer it takes to get medical treatment, the higher the risk of amputation. Do not delay contacting emergency services even if the knee looks normal. Damage to blood vessels and nerves may still be present, and serious complications can come on quickly.
What causes a dislocated knee?
Trauma is the cause of a dislocated knee. Doctors usually classify the trauma resulting in a dislocation as high-energy or high-velocity and low-energy or low-velocity. The most common high-energy cause of knee dislocations is a motor vehicle accident. Falls and industrial accidents can also cause high-energy dislocations. Low-energy dislocations generally involve sports injuries. There tends to be less damage in low-energy dislocations. The prognosis is usually better for low-energy vs. high-energy knee dislocations.
What are the risk factors for a dislocated knee?
There are a few factors that increase the risk of dislocating a knee including:
Being obese, which puts excess stress on the stabilizing muscles and structures of the knee with movement. The knee bears forces of up to four times a person’s body weight with motion. As a result, even simple movements can cause low-energy knee dislocations in an obese person.
Having a previous knee injury that weakens or destabilizes the joint
Playing contact sports
Working at heights or otherwise being at risk of falls from heights
Reducing your risk of a dislocated knee
It is difficult to prevent a dislocated knee because they are usually traumatic accidents with no warning. You can keep your knees healthy and lower your risk of having a dislocated knee by maintaining a healthy body weight. You should also use proper safety equipment when driving, riding in a motor vehicle, or playing sports.
How is a dislocated knee treated?
The first treatment goal for a dislocated knee is to put the bones back in the correct position, called reduction. Doctors can often accomplish this without surgery. However, posterior dislocations—when the shinbone moves behind the thigh bone and knee joint—almost always require surgery to reposition the bones.
Your doctor also needs to evaluate the status of your blood vessels, nerves, and ligaments. Emergency surgery may be necessary if the blood supply to your lower leg is not adequate. Surgery for nerve injury or to repair or reconstruct ligaments is also usually necessary. In general, this takes place within a couple of weeks of the injury to allow swelling to resolve as much as possible. In some cases, doctors may recommend conservative treatment by just immobilizing the joint. This may be the case for elderly or sedentary people with a relatively stable joint.
Dislocated knee recovery time can be quite lengthy and the recovery itself is difficult. It typically involves a long period of orthopedic rehabilitation and physical therapy. A comprehensive program will use bracing and progressive range of motion, strength, and agility exercises. It can take up to a year to return to normal activities.
What are the potential complications of a dislocated knee?
Due to the violence and severity of the injury, complications are relatively common with a dislocated knee. It is possible to have chronic pain and stiffness after recovering from a dislocated knee. Other complications include post-traumatic arthritis and persistent joint instability.
The velocity or energy of the dislocation will play a role in your long-term outlook. Low-energy dislocations generally have a better prognosis. Most people with this injury will have a stable, pain-free knee after recovery and will have near normal function. However, only 1 in 5 athletes will return to their previous performance level after a low-energy dislocation. High-energy dislocations have more extensive knee damage. It is unlikely that people suffering this injury can return to full athletic activities. This is especially true if nerve damage is present.
Serious complications can also occur. This includes amputation above the knee, DVT (deep venous thrombosis), and compartment syndrome. Recognizing the injury and seeking immediate medical care can help prevent these potentially devastating complications.