Kneecap Dislocation and MPFL Reconstruction: 37 Questions Patients Ask

The kneecap (patella) normally glides up and down in a groove at the end of the thigh bone called the trochlea. In a dislocation, the kneecap slips completely out of the groove, almost always toward the outside of the knee. It often pops back in when the knee is straightened. See kneecap dislocation and patellar instability.

Patellar instability means the kneecap does not stay securely in its groove. It includes full dislocations and subluxations, where the kneecap slips partway out and back in. Some people feel the knee “give way” or feel the kneecap shift during twisting or cutting.

The medial patellofemoral ligament (MPFL) is a band of tissue on the inner side of the knee that connects the kneecap to the thigh bone. It is the main restraint that keeps the kneecap from sliding to the outside. It is torn or stretched in nearly every kneecap dislocation.

Mostly teenagers and young adults, often during sports that involve twisting, cutting or jumping. In a large study of acute patellar dislocations, the risk was highest in females aged 10 to 17.¹

Sudden pain, a visible shift of the kneecap to the outside, and often a pop. The knee usually swells quickly. Afterward the knee may feel weak or unstable, and some people are nervous to bend or twist it.

Yes. As the kneecap slides out and snaps back, it can chip cartilage or bone off the back of the kneecap or the end of the thigh bone. These pieces can float in the knee and cause locking. A loose piece may need to be removed or fixed. See cartilage defects.

Many people who dislocate once never do again, but the risk depends heavily on your age and anatomy. People with several risk factors have a much higher chance. In one study described in a review, recurrence was about 43% in patients whose growth plates were still open compared with about 22% in skeletally mature patients.²

Trochlear dysplasia means the groove for the kneecap is shallow or flat, so there is less of a track to hold the kneecap in place. It is one of the strongest risk factors for repeat dislocation, with studies reporting odds 2.6 to 23.7 times higher.²

Patella alta means the kneecap sits higher than usual. A high kneecap does not engage the groove until the knee is bent further, so it is less protected in the early part of bending. When trochlear dysplasia, patella alta and a younger age occur together, one model estimated the 5-year risk of recurrent instability at about 70%.²

TT-TG stands for tibial tubercle to trochlear groove distance. It is a measurement on MRI or CT of how far to the outside the patellar tendon attaches on the shin bone compared with the kneecap groove. A larger distance means the tendon pulls the kneecap outward more strongly. In one classic study described in a review, 56% of patients with instability had a distance over 20 mm compared with 3.5% of people without instability.²

Yes: open growth plates (still growing), generalized loose joints, knock-knees, twisting of the thigh or shin bones, and having already dislocated before. Skeletally immature patients had more than twice the risk of recurrent instability in one study.² Dr. Dold looks at all of these together, because the plan depends on the combination.

Dr. Dold will ask how the injury happened and whether it has happened before, then examine the kneecap’s position and movement, the knee’s alignment, and your hip, core and thigh strength. X-rays taken in our office show the bones, the height of the kneecap and any loose fragments.

An MRI is often recommended after a first dislocation, especially if the knee is very swollen or locking. It shows the MPFL, cartilage damage and loose pieces, and allows measurement of trochlear dysplasia, kneecap height and the TT-TG distance. A CT scan is sometimes used to check rotation or alignment.

Most first-time dislocations without a loose fragment are treated without surgery: a short period of protection in a brace with crutches as needed, rest, ice, compression and elevation, then physical therapy to restore motion and strengthen the quadriceps, hip and core muscles. Many people return to normal activities within about 1 to 3 months. See kneecap instability.

Not necessarily for everyone. A 2023 Cochrane review of 10 trials with 519 participants found that surgery may lower the number of people with a repeat dislocation (about 157 fewer per 1,000 people over 2 to 9 years), but the certainty of the evidence was very low.³ That is why the decision is individualized.

Surgery may be recommended sooner when a large piece of cartilage or bone has broken off and needs to be fixed or removed, or when anatomy and age suggest a very high risk of repeat dislocation. Dr. Dold will discuss the options based on your imaging and goals.

A patellar stabilizing brace or tape may help some people feel more secure during sport, especially early on. Braces do not replace strengthening, and the evidence that they prevent dislocation is limited. Your physical therapist can help you decide whether one is worth using.

Regaining full motion, then strengthening the quadriceps (thigh), hip and core muscles, improving balance and control during landing and cutting, and a gradual return to running and sport. Hip strength matters because weak hips let the knee collapse inward, which pulls the kneecap outward.

MPFL reconstruction rebuilds the torn ligament with a tendon graft, either your own tendon (often a hamstring tendon) or a donor tendon. The graft is attached to the kneecap and the thigh bone to restore the main restraint against outward sliding. It is usually done through small incisions as outpatient surgery. See kneecap instability.

In a meta-analysis of 14 studies, isolated MPFL reconstruction for recurrent dislocation had a redislocation rate of about 1.2%, a reoperation rate of about 3.1%, and about 84% of patients returned to sport.⁴ A later meta-analysis of 930 patients reported recurrent instability in about 1.9%.⁵ Individual results vary.

In a tibial tubercle osteotomy (TTO), the bony bump on the shin bone where the patellar tendon attaches is cut, moved and fixed with screws to realign the kneecap. It can move the attachment inward to reduce the outward pull, downward for a high kneecap, or forward to take pressure off damaged cartilage. See knee osteotomy.

A TTO may be added when there is a large TT-TG distance, a high kneecap (patella alta), or cartilage damage on the kneecap that would benefit from unloading. It is generally not done while the growth plates are still open. In a meta-analysis, the return-to-sport rate was not significantly different with or without an added osteotomy.⁵

For severe trochlear dysplasia, a surgery called trochleoplasty deepens the groove. It is a larger operation used in selected cases, usually after the growth plates have closed. Dr. Dold will explain whether your anatomy calls for any additional procedure.

Loose pieces may be removed or fixed back in place during surgery. Larger areas of damage may need a cartilage procedure. See cartilage defects and cartilage transplantation.

All surgery carries risks such as infection, stiffness, blood clots, numbness around the incision and problems with anesthesia. After MPFL reconstruction, problems include stiffness, kneecap pain, a graft that is too tight, and, rarely, fracture of the kneecap. A TTO adds risks related to bone healing and the screws, which sometimes need removal later.

You will use crutches and a brace at first, with weight bearing as instructed. Physical therapy starts early to regain motion, then strength, balance and running, followed by sport-specific training. Progress is based on milestones, not just the calendar.

Return to sport after MPFL reconstruction is often around 4 to 6 months, once strength, motion and confidence have returned. In a meta-analysis of 930 patients, about 93% returned to sport and about 71% returned to their previous level or higher, at an average of about 6.7 months.⁵ Recovery is often longer when an osteotomy is added, because the bone needs time to heal. See knee osteotomy.

If surgery was on your left knee and you drive an automatic car, you may be able to drive within a couple of weeks once you are off narcotic pain medicine. Surgery on the right knee usually takes longer, until you have the strength and control to brake quickly. Dr. Dold will advise you.

Students and desk workers often return within one to two weeks, using crutches. Jobs that require standing, kneeling, climbing or heavy lifting may need several months, or modified duty in the meantime.

It can, but the risk is low after MPFL reconstruction.^4,5 The risk is higher when risk factors such as severe trochlear dysplasia or patella alta are not addressed. Completing your therapy and returning to sport only when you meet the goals helps protect the repair.

Young athletes have the highest risk of repeat dislocation, and open growth plates raise that risk further.² Treatment usually starts with therapy after a first dislocation, but recurrent instability in a teen is often treated with surgery to protect the cartilage and allow a return to sport.

Yes. When the growth plates are still open, the surgery is modified to avoid the growth plate near the thigh-bone attachment. The surgical technique is chosen based on the patient’s age and whether the growth plates are still open. See kneecap instability.

Generally no. Cutting and moving the tibial tubercle while the growth plate is open can disturb growth of the shin bone. In growing children, alignment problems may be addressed in other ways or later, after growth is complete.

Take a first dislocation seriously, have the knee checked for loose pieces, and complete the full course of therapy. Watch for repeated “giving way,” locking or swelling, and let us know if they happen. Other common knee injuries in young athletes include ACL tears.

Call our office for fever above 101°F, increasing redness, warmth or drainage from the incisions, calf pain or swelling, or pain that is not controlled by your medicine. For chest pain or trouble breathing, call 911.

Our staff will call and text you within an hour (Monday–Friday, 8 am–5 pm) after you request an appointment. You can also call 469-850-0680. Bring any X-rays or MRI reports you already have.

Dr. Dold accepts most major insurance plans, including Medicare and Tricare. Please call 469-850-0680 to confirm your coverage. See our insurance page and surgery scheduling and insurance questions.