(See also Overview of Dislocations.)
Knee dislocations occur when the end of the thighbone (femur) loses contact with the shinbone (tibia). Knee dislocations can disrupt the blood supply to the lower leg, sometimes leading to amputation.
Usually, the knee is dislocated when a great force pushes or bends the knee beyond its normal limits.
The knee is often obviously out of place and is always painful and swollen, and people cannot walk.
Usually, doctors can diagnose a dislocated knee by examining the joint, but x-rays are taken from several angles to confirm the diagnosis.
Doctors must check for injuries to arteries, which can accompany a knee dislocation, because if blood flow is disrupted, surgery should be done immediately.
After giving the person drugs to make the procedure more tolerable, doctors put the joint back in place and splint the knee, but later the knee must usually be repaired surgically.
Knee dislocations differ from kneecap dislocations and are much more serious.
The knee may be dislocated when the lower leg is pushed forward beyond the normal limits of the knee joint (hyperextending the knee). In such cases, the shinbone is pushed in front of the thighbone. The shinbone may also be pushed behind the thighbone or to either side. Most knee dislocations result from great force, as occurs in high-speed car crashes. But sometimes slight jarring, as when stepping in a hole, can dislocate the knee if the joint is twisted at the same time. Also, if people are obese, knee dislocations may occur simply falling to the ground.
A knee dislocation always damages structures that support the knee joint (such as ligaments and tendons), making the joint unstable, sometimes permanently.
Arteries and nerves are also often injured. If an artery is injured, the blood supply to the lower leg may be disrupted, and tissues may die. If this problem is not identified and treated, the leg may have to be amputated.
Usually, the knee is obviously out of place. The knee is painful and swollen, and people cannot walk.
The knee sometimes slips back into place on its own before people can see a doctor, but the knee remains swollen and unstable.
The lower leg may feel numb and look pale. These symptoms may indicate that an artery is damaged and the blood supply is disrupted or that a nerve is damaged.
If people suspect that their knee is dislocated, they should go to an emergency department right away.
Doctors can usually identify a dislocated knee when they examine the joint. However, x-rays are taken from several angles. Unless the knee has slipped back into place on its own, x-rays can confirm the diagnosis. X-rays can also identify fractures.
To determine whether arteries are damaged, doctors check for a pulse in the lower leg, usually several times over a period of time. They may also compare blood pressure in the affected leg with that in the arm. However, CT angiography is the best way to check for damage to arteries after the knee is dislocated.
If any symptoms and findings suggest that tissues are not receiving enough blood, doctors consult with a surgeon about repairing the damaged arteries. Identifying damaged arteries promptly is essential because if the tissues in the lower leg are deprived of oxygen for more than 8 hours, amputation is much more likely to be needed.
If arteries are not damaged, doctors gently move the joint to determine how badly the ligaments are damaged.
Doctors also check for nerve damage—for example, by asking the person to move the foot up and down and to turn the foot in and out and by testing for numbness.
Doctors immediately put the joint back in place (called reduction). Before this maneuver, people are given a sedative and pain reliever, but they remain conscious. After reduction, the knee is immobilized with a splint.
Damaged arteries are surgically repaired immediately. If the arteries are not damaged, surgery to repair damaged ligaments in the knee is usually done after the swelling has gone down.
If the knee is very unstable, an external fixator may be applied. This device is a frame of rods that is attached to the outside of the leg by stainless steel pins that are inserted through the skin into the bone.