A patella dislocation occurs when the knee cap pops sideways out of its vertical groove at the knee joint. It’s usually caused by force, from a collision, a fall or a bad step. A dislocated patella is painful and will prevent you from walking, but it’s easy to correct and sometimes corrects itself.
A patella dislocation is a dislocation of the kneecap — the patella — from its groove at the knee joint. The knee joint is a meeting of three bones: the thighbone, the shinbone and the kneecap in the middle. Normally, when you bend and straighten your leg, the kneecap slides up and down inside a vertical groove between the bottom end of the thighbone and the upper end of the shinbone (the trochlear groove). A network of tendons and ligaments secure the kneecap within the groove, flexing as it moves.
When the patella dislocates, it’s forced outside of the trochlear groove and can no longer move up and down. This locks the knee and pulls the ligaments out of place, often tearing them. Most frequently (93% of the time), the kneecap pops out laterally, to the side of the groove. Patellar dislocation is usually an acute injury caused by impact or by a sudden turn and twist. Like any dislocation, it is painful and debilitating until it is corrected. But the dislocated knee cap will sometimes correct itself.
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Most of the time, a patella dislocation is an acute injury caused by force. This is called acute patella dislocation. It’s a relatively common injury, since the kneecap takes less force to dislocate than some other joints do. It’s also relatively easy to pop back in. Rarely, patella dislocation can also occur developmentally, from a condition called congenital patella dislocation (or trochlear dysplasia). When the patella develops outside of the trochlear groove, it can’t be moved back into place. These cases require surgery to correct.
Some people might think they have a patella dislocation when they actually have a patella subluxation. A subluxation is a partial dislocation. It means that the bone is unstable in the joint and may have strayed a little out of its proper place, but it hasn’t popped all the way out. When you have a patella subluxation, the kneecap still tracks in the groove and you can still walk, but it may feel uncomfortable or unsteady, and you may hear a popping noise as it moves. A patellar subluxation can result from injury or from general looseness in the joint (patellar instability).
A patella dislocation is a dislocation of the knee cap. A “dislocated knee” involves the other two bones that make up the knee joint: the thighbone (femur) and the shinbone (tibia). When your knee is dislocated, the femur and tibia no longer connect at the knee joint. One of the bones has been forced backward or forward relative to the other bone. A dislocated knee (tibiofemoral dislocation) is rarer and more serious than a dislocated kneecap, because of the force required to misalign the leg bones and the damage it does to the ligaments.
Acute patellar dislocation is caused by force, either from a direct impact or a bad step that uses your own body weight against you. A heavy fall or collision can knock the kneecap out of place. However, it doesn’t always take that much. It may be caused by something as simple as a sudden turn that twists the knee while the lower leg is still firmly planted. Athletes and dancers, who are prone to quick pivots, are common victims of this.
Some people have patellar instability, which means that the tendons and ligaments that hold the kneecap in place are already loose and unstable. This might be caused by a previous injury or by another preexisting anatomical condition. An unstable kneecap will dislocate more easily.
People with congenital patellar dislocation (trochlear dysplasia) are born with the condition. It is often, but not always, related to other developmental abnormalities.
Anyone can dislocate their patella through injury. However, certain people are more at risk, including:
Doctors don’t know what causes congenital patella dislocation, but a higher incidence among family members suggests a genetic link. Certain other congenital conditions are also associated with it, including:
Dislocations are often very painful, but there is a range. It depends how far out of joint the bone has been forced, and how much the surrounding muscles and ligaments, blood vessels and nerves have been injured. It will always be painful to move the dislocated joint or bear weight on it. You won’t be able to use the limb normally until the joint has been corrected.
If your dislocated patella corrects itself, your pain and mobility may improve. But you should still see your healthcare provider. They’ll check for secondary injuries to the ligaments and guide you through the longer rehabilitation process.
No. The knee will either be locked and unable to straighten or bend, or it will catch and pop when you try to bend it. The joint will be unstable and buckle when you try to bear weight on it. It will also be painful to move it. If you can walk, you may only have a patella subluxation.
If your dislocated patella pops back into place, you may be able to walk afterward. But the knee will still be swollen and painful from the trauma. You shouldn't try to walk if it’s too painful. Always see your healthcare provider anyway to check for any secondary injuries. They’ll likely recommend crutches and a brace when you begin to walk again.
Knowledgeable healthcare providers can usually diagnose a dislocated kneecap by physically examining the knee and asking you questions about the injury. However, they’ll order radiographic imaging tests to check for any related injuries, such as torn ligaments, cartilage injury or fractures. With patellar dislocation, it is safe to correct the joint first and take pictures after.
If your dislocated patella corrected itself, you might not realize that it was dislocated. A dislocation that corrects itself is called “transient.” Afterward, your knee will still be sore and swollen, but it may look like many other more common knee injuries. In this case, imaging tests can show evidence after the fact that there was a dislocation, along with secondary injuries.
Possibly. The kneecap has been known to pop back into place on its own when the leg is carefully extended. If it’s too painful to extend the leg, your healthcare provider can give you medication to make it easier. If you do fix it yourself, see your healthcare provider after. They’ll want to check for fractures and for damage done to the cartilage and ligaments.
Reduction: As long as the diagnosis is clear, a knowledgeable healthcare provider will manually relocate the kneecap as soon as possible. This is called a reduction. A patellar dislocation injury that occurs on the playing field can be reduced immediately by a trained healthcare provider if there is one on-site. If you go to the emergency room, they may give you sedatives and pain medication first. They’ll usually correct the joint first and then look at it on an X-ray.
Imaging: Healthcare providers will take imaging tests to see that the kneecap has been properly replaced and plan any additional treatment. X-rays and CT scans can help reveal any preexisting anatomical conditions that may have contributed to the dislocation, as well as any additional injuries. An MRI can give more detailed information on the cartilage and ligaments if it’s needed. Sometimes an MRI will reveal a previous transient dislocation that wasn’t suspected before.
Surgery: If there is significant damage to the bone or to the cartilage and tendons of the knee, your healthcare provider may recommend surgery to repair it. Surgery may also be recommended if you have recurrent patellar dislocations or chronic patellar instability. Repairing and strengthening the cartilage and ligaments is a preventative measure to restabilize the knee. When patellar dislocation is congenital, the joint can only be repaired through surgery.
Rehabilitation: You’ll be sent home with painkillers and a splint for the first few days. Periodically elevating and icing the joint can help keep the swelling down. You’ll begin walking again gradually with crutches and a brace to hold the joint in place. Physical therapy is very important to restrengthen the muscles while limiting the range of motion until the joint is restabilized. It takes about six weeks to three months to fully recover from a dislocated patella.
All dislocations stretch the ligaments and deteriorate the cartilage of the joint to some extent. Once your patella has dislocated, it’s more likely to happen again if it’s similarly injured. Accidents are hard to prevent, but sometimes there are contributing factors that we can try to reduce. Depending on what made your patella dislocate in the first place, you may want to take one or several of these preventative measures:
If you’ve been treated for a first-time patella dislocation injury, your prognosis is good. Most people will recover fully within six weeks. Long-term complications may include a less stable knee joint and deteriorated cartilage, which can contribute to osteoarthritis later in life.
If you have chronic patellar instability or recurrent patella dislocations, you may be a good candidate for surgery to help stabilize the joint. Surgery has a good success rate for stabilization, but it is associated with osteoarthritis down the road.
A note from Cleveland Clinic
A dislocated patella can be scary and painful, but it’s not as serious as other dislocation injuries. It takes less force to dislocate the patella than other bones, which means there is less likely to be collateral damage to the blood vessels or nerves. It also relocates more easily, sometimes by itself.
If you dislocate your kneecap, the first thing to worry about is putting it back in place. You or a trained professional may be able to fix it on site. If not, your healthcare provider can do it for you with medication to make it less painful. After that, rest and rehabilitation should have you back on your knee in about six weeks.
Last reviewed by a Cleveland Clinic medical professional on 08/03/2021.
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