Quick summary: Patellar instability happens when the kneecap (patella) slips out of the groove at the front of the knee. It ranges from a partial slip (subluxation) to a full dislocation. It is common in active teens and young adults and accounts for about 2 to 3 percent of all knee injuries. After a first dislocation, the kneecap can slip again in up to 60 percent of people, with the highest risk in those under 20. Many first-time cases settle with bracing and physical therapy. Repeated dislocations often need surgery to rebuild support and correct the underlying anatomy of the knee.
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Patellar instability is one of the most common knee problems I see in active young people, and it is also one of the most misunderstood. In my practice, patients often arrive certain that a single bad landing caused everything. The truth is usually quieter than that. Most of the time, the kneecap was set up to slip long before the day it finally did.
After more than twenty years treating knee problems as a board-certified, fellowship-trained surgeon, here is what I want you to walk away knowing: a kneecap that slips once deserves a careful look, and a kneecap that slips again almost always has a reason you can find and treat.
What is patellar instability?
Patellar instability means the kneecap does not stay centered in its groove when you bend and straighten your knee. Normally the patella glides up and down inside a V-shaped channel at the end of the thighbone called the trochlear groove. When that tracking goes wrong, the kneecap can slide toward the outside of the knee.
There are two ways this shows up. A subluxation is a partial slip, where the kneecap shifts but does not fully come out. A dislocation is a complete slip, where the kneecap pops all the way out of the groove, usually toward the outer side of the knee. Both sit on the same spectrum, and both point to the same underlying issue: the knee is not holding the patella the way it should.
Patellar instability ranges from a healthy, centered kneecap to partial slipping (subluxation) and full dislocation.What causes a kneecap to keep slipping out?
Most patellar instability comes down to anatomy, not bad luck. A twist of the knee with the foot planted can trigger the first dislocation, but the shape and balance of the knee usually decide whether it happens at all, and whether it happens again.
The anatomy that sets the stage
Several features tend to show up together in unstable kneecaps. I look for all of them in every patient:
- A shallow trochlear groove (trochlear dysplasia). If the channel the kneecap rides in is flat instead of deep, there is little to hold the patella in place. This is the single most important factor in chronic cases.
- A high-riding kneecap (patella alta). A patella that sits too high does not drop into the groove until later in the bend, so it is unprotected early on.
- A torn or stretched MPFL. The medial patellofemoral ligament (MPFL) is the main soft-tissue strap that holds the kneecap from the inside. It tears in nearly every full dislocation, most often where it attaches to the kneecap.
- Rotational malalignment. When the thighbone turns inward, the knees angle together, and the shinbone turns outward, the whole system pulls the kneecap toward the outside. Doctors sometimes call this combination “miserable malalignment,” and the name fits.
Who is most at risk?
Patellar instability shows up most often in the teens and twenties, and it is more common in girls and women, who tend to have looser ligaments. Athletes in cutting and pivoting sports such as basketball, soccer, and cheerleading carry higher risk. People with naturally loose joints, including those with connective tissue conditions, are also more prone to it. If a close family member has an unstable kneecap, your odds go up too.
What are the symptoms of an unstable kneecap?
The clearest sign is the feeling that your kneecap has shifted or popped out of place. Many patients describe a visible bump on the side of the knee during a dislocation, followed by the kneecap sliding back as the leg straightens.
Other common symptoms include:
- A buckling or giving-way feeling, as if the knee cannot be trusted
- Swelling soon after an injury, which often means bleeding inside the joint
- Pain across the front or inner side of the kneecap
- Catching, clicking, or a sense that the kneecap is sliding side to side
- A “J sign,” where the kneecap veers outward as you straighten the knee
Here is what I tell my patients: even if the kneecap pops back in on its own, the episode still matters. A first dislocation can knock loose a piece of cartilage or bone, and that changes the plan.
How is patellar instability diagnosed?
Diagnosis starts in the exam room and is confirmed with imaging. In the office, I check how far the kneecap can be pushed to the side, watch how it tracks as you bend and straighten, and look for an apprehension response, where gently nudging the kneecap outward makes you guard. These simple tests tell me a lot about how the patella is behaving.
Imaging fills in the why. X-rays show the shape of the groove and whether the kneecap sits too high. A CT scan measures the distance from the tibial tubercle to the trochlear groove (the TT-TG distance), which is normally about 9 to 13 mm; a value over 20 mm points to real malalignment. An MRI checks the MPFL, the cartilage, and any loose fragments. Mapping all of this before any treatment is the part that protects your result, because the fix has to match the cause.
What are the treatment options for patellar instability?
Treatment depends on whether this is a first slip or a pattern, and on what the imaging shows. The goal is always the same: keep the kneecap centered and protect the cartilage for the long run.
Nonsurgical treatment
Most first-time dislocations without a loose fragment do well without surgery. After the kneecap is back in place, the plan usually includes a short period in a brace, anti-inflammatory medicine for pain and swelling, and crutches for comfort. The real work is physical therapy. Strengthening the quadriceps, the hip muscles, and the core improves how the leg lines up and how the kneecap tracks. Many people return to daily activity in 6 to 8 weeks and to sport in 3 to 4 months. You can read more about physical therapy after a knee injury in my related guide.
When does patellar instability need surgery?
Patellar instability needs surgery when the kneecap keeps dislocating despite good rehabilitation, or when a dislocation has left a loose piece of cartilage or bone in the joint. A first-time dislocation with a displaced fragment is also a reason to operate early. The aim is to rebuild stability and, just as important, to correct the anatomy that caused the problem.
The most common procedures include:
- MPFL reconstruction. A new ligament is built using a tendon graft, restoring the inner strap that holds the kneecap. This is the cornerstone of modern surgical care. The graft is meant to act as a check-rein that restores normal support, not a leash that drags the kneecap inward, since overtightening can wear out cartilage over time.
- Tibial tubercle osteotomy. The bony bump where the kneecap tendon attaches to the shinbone is shifted to improve alignment and unload damaged cartilage. This is added when the TT-TG distance is high or the kneecap sits too high.
- Trochleoplasty. In severe cases of a flat groove, the groove itself is deepened. This is a specialized procedure reserved for select patients.
The modern approach is individualized. No single operation fits every knee, and the best results come from matching the procedure to the specific anatomy. When a flat groove is the main problem, rebuilding soft tissue alone often is not enough, which is why a careful workup matters so much. Some patients also have related issues such as ligament injuries that need attention at the same time.
How long does recovery take?
Recovery depends on the treatment. Nonsurgical care usually means a return to normal life in about 6 to 8 weeks and a return to sport in 3 to 4 months. After surgery, most people are looking at 6 to 12 months before they are fully back to athletics, with the longest timelines after bony procedures. Physical therapy is not optional in either path; it is the difference between a knee that holds and one that slips again.
When should you see a knee surgeon?
See a knee surgeon if your kneecap has dislocated, keeps feeling like it might give way, or slips more than once. Early evaluation finds loose fragments and anatomy problems while they are easiest to treat, and it lowers your risk of cartilage damage and arthritis down the road.
Choosing a fellowship-trained surgeon at a high-volume center is one of the most evidence-backed decisions you can make for a complex knee. The judgment to know which tools your knee actually needs is what protects your result. To have your knee evaluated by Dr. Cory Calendine, you can request an appointment online or call the office.
Bone and Joint Institute of Tennessee
3000 Edward Curd Lane, Franklin, TN 37067
(615) 791-2630
Sources and references
References are pending Dr. Calendine’s review and sign-off before publication. Authoritative sources used for this article include the American Academy of Orthopaedic Surgeons (OrthoInfo), Cleveland Clinic, Johns Hopkins Medicine, and the National Library of Medicine (StatPearls).
Medical disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified orthopaedic provider about your individual condition. Individual results vary based on personal health circumstances.
