For most people, the kneecap, or patella, glides nicely within a V-shaped groove at the end of the thigh bone called the trochlea. As you bend and straighten the knee, the patella moves up and down and is stable within the trochlea, allowing you to stand, sit, walk and run without a problem.
Unfortunately, for others, things can go awry. “Sometimes we think of the patella as the train, and the notch it sits in, called the trochlear groove, as the track,” says Theresa Chiaia, PT, DPT, a physical therapist at HSS. “The issue for patients who dislocate their patella is that the train comes off the track, leading to the knee giving way, as well as pain and swelling.”
In a complete dislocation, the patella comes out of the track completely. Some people may experience more subtle issues with instability, which is known as subluxation, or partial dislocation. A partial dislocation also results in pain, swelling, popping or cracking sensations or stiffness, but these are much less severe, and the recovery is usually days instead of weeks or months.
Whether a person has a partial or a complete dislocation, the soft tissues of the knee, including the ligaments and cartilage, are often injured. If the ligament that keeps the patella in place, called the medial patellofemoral ligament (MFPL), is stretched or torn, it increases the chance that dislocation will continue to happen.
If you have a partial dislocation, the signs may be subtle. You may have pain around the kneecap, or it may feel like the knee is popping out and popping back in.
A dislocation is usually more obvious. The affected knee will buckle and be unable to support your weight, and you may have significant swelling, pain and stiffness. The kneecap may pop itself back in, or you may need help from someone else to get it back in place. Even if the kneecap shifts back into place on its own, pain and swelling can remain.
While these issues can happen in men and women, young and old, they happen most frequently to young women in their teens or early 20s. People who have patella dislocations usually have some physical risk factors, such as a notch (trochlea) that is not shaped correctly; significant laxity, or looseness, in their soft tissues; or a kneecap that sits too high on the thighbone, all of which can be hard for someone to know without being told by a doctor.
After listening to the story of how the injury happened, your doctor will perform a physical exam. The exam will begin with you seated and moving your knee. Then you’ll probably lie down and your doctor will bend your knee slightly and guide your kneecap outward. This feels uncomfortable and potentially painful for people with patellar instability. Your doctor may take measurements to see how the bones are aligned, and he or she may watch you walk as well.
Patients typically will have an X-ray to see how the kneecap fits in the groove, though if the kneecap has already returned to the correct position, the X-ray may look normal. This is why the physical exam is so important. You may also need an MRI to look at the ligaments and cartilage to check for damage, which to some degree is always seen after a dislocation. However, if that damage is minimal and it is the first time it has happened, then you may be treated without surgery.
People over the age of 25 who dislocate their patella for the first time have a fairly low chance of it happening again if they are treated appropriately with a brace, crutches and physical therapy. These people often don’t need surgery. People who are under 25 and who have a shallow groove have a much higher chance of the problem continuing to happen. These patients may need surgery. “That’s why it’s so important to manage these issues right away,” says Chiaia. “If you stop the kneecap from dislocating, you can stop further damage from happening.”
If the kneecap is dislocated and hasn’t returned to its proper place on its own, it needs to be put back where it belongs. Your doctor will apply gentle pressure to push it back, a process called reduction. Afterwards, rest, a knee brace and crutches will help the swelling to go down and allow the knee to heal. If the knee is very swollen, draining the knee of fluid can help reduce the discomfort. After a week or two of rest, physical therapy can help strengthen the muscles around the kneecap to help keep it aligned in the trochlear groove, and help the knee get back to a normal range of motion.
If an MRI shows that bone or cartilage has been damaged on the thigh bone or on the underside of the kneecap, arthroscopic surgery may be necessary. Both of these issues can cause locking, buckling or additional pain in the knee. In people who have had more than one dislocation, surgery can recreate the ligament that was torn. Sometimes in addition to ligament surgery, the doctor will recommend realigning the bone in a procedure called a tibial tubercle osteotomy to help keep the kneecap on track. The success rate of surgery for patellar instability is very high. More than 95% of patients have no more dislocations, and more than 85% return to sports they played before their injury at the same or a higher level.