Patellar Instability

What is it?

Patellar instability, or as it’s more commonly known, knee dislocation, is instability within the kneecap or a loose kneecap. If you have a loose kneecap or it partially slips out of the joint (sublimes) or completely slips out of the joint (dislocates), this can cause a massive problem. Generally, when patients have kneecap subluxation or dislocation, it is because they have some kind of underlying anatomical problem, such as too shallow of a groove where the patella hits or having the patella in the wrong position.

The kneecap is a separate bone controlled by the quadriceps muscle and it attaches to the shinbone via the patellar tendon. When the knee is bent, the kneecap rests in a groove at the end of the thighbone, the trochlear groove. However, when the kneecap is out straight, it doesn’t sit within these confines and it is most vulnerable to slipping to the outside.

How does it get hurt/damaged?

May anatomical factors can predispose you to having a knee dislocate. If the kneecap sits up higher than normal, it is called patella alta. If the bony confine itself is relatively flat, or flatter than usual, the kneecap is also more likely to slip out which is called trochlea dysplasia. Another issue could be the angle between the quadriceps muscle, the kneecap, and the patellar tendon’s attachment on the shinbone. This is called tibial tubercle to trochlea grove (TTTG) angle or distance. If the TTTG is high, or above twenty millimeters, the knee has a much higher likelihood of slipping to the outside or dislocation when the knee is straight. Another issue can occur with the ligament that attaches the inside part of the thighbone to the inside part o the kneecap. This ligament, called the medial patellofemoral ligament (MPFL), stabilizes the kneecap. This ligament needs to be intact when the knee is out straight or there is a higher risk of stress causing the kneecap to slip to outside.

How common is a PCL tear?

It’s difficult to know how common patellar instability is, but it is known that cases are increasing in frequency. Some patients may not even realize that they have this diagnosis due to mellow or undetectable symptoms.

When should you be worried about a PCL tear and what should you do initially?

If the kneecap self-reduces, or pops back in without any manipulation, then this injury isn’t an emergency and can be seen by a primary care provider or an orthopedic surgeon. Many patients with instability will have recurring bouts of dislocation that relocate spontaneously.

What is the severity of the injury and the treatment options?

The treatment of kneecap subluxation or dislocation depends upon many factors. In general, as long as no piece of bone and / or cartilage has been knocked off the kneecap or trochlear surface with a dislocation, rehabilitation is encouraged rather than surgery. First, the patient should get the swelling down so the quadriceps muscles recover. Then, they’ll work on strengthening the quadriceps muscles, so they can better hold the kneecap in place. If the patient has a relatively normal kneecap position - that is, without patella alta - and the end of the thighbone is relatively deep, without any significant trochlea dysplasia, this nonoperative treatment program has a 90% success rate.

If you do have significant patella alta or trochlea dysplasia, all bets are off. While your physician still may prescribe a program of rehabilitation to attempt to strengthen the quadriceps and better hold the kneecap in position, because the MPFL is torn and your kneecap sits up high or in a flat trochlea, there is a much higher risk that the kneecap will continue to slip or dislocate. While it’s worth a shot to try rehabilitation, in general, the patient has a much higher risk of dislocations in the future. Once the kneecap has dislocated twice or three times, the patient almost always needs surgery to address the problem. 

What is my recovery timeline and the anticipated outcome?

After suffering a patella subluxation or dislocation for the first time, it usually takes at least 4-6 weeks of rehabilitation to ensure that motion returns, the quadriceps muscles are strong enough, and the kneecap is balanced within the end of the trochlea prior to returning back to activities. 

If you do have surgery, the treatment depends on the overall bony architecture as well as the ligament on the outside of the knee. Studies show that repairing the MPFL does not result in nearly as good an outcome as completely reconstructing and replacing it, so most surgeons perform an MPFL reconstruction as part of their kneecap stabilization procedures.

Rehabilitation programs after these procedures are almost all the same. In general, the patients don’t bear weight for six weeks on crutches. They are allowed early motion to ensure that their knee does not become stiff, with a goal of flexing past 90 degrees at about two weeks after surgery. For patients who have solely an MPFL reconstruction, they may work on the quadriceps by performing straight leg raises in a brace. However, if the tibial tubercle was moved, patient shouldn’t perform straight-leg raises for the first six weeks because a sudden quadriceps pull could dislodge the screws that are hold the tubal tubercle in place and lead to the need for further surgery.

If a trochleoplasty was performed with these other procedures, patients need to be careful about putting any significant weight on the trochleoplasty until the cartilage surfaces have had time to heal. In general, when a patient has a tibial tubercle osteotomy or trochleoplasty, or both, x-rays are taken at the six week point to ensure that there is sufficient healing to allow for weight bearing and simple exercises, such as riding a stationary bike.