Patellar Instability Guide
A kneecap that dislocates is frightening — but most first dislocations do not need surgery, and the real question is whether it will happen again.
What Is It?
Your kneecap — the patella — is a bone embedded in the quadriceps tendon that glides in a groove on the front of your thigh bone, called the trochlea. As the knee bends, the kneecap settles into that groove and is held there by its shape.
Patellar instability means the kneecap comes out of that groove — either fully, a dislocation, or partway before sliding back.
Stability comes down to three things working together
1. The depth of the bony groove itself, since a deep groove is inherently stable and a shallow one isn't
2. The medial patellofemoral ligament (MPFL), which runs from the inner side of the kneecap to the thigh bone and acts like a tether holding the knee cap in place before it settles into the grovoe
3. The alignment of the extensor mechanism — how well the pull of your thigh muscle (quadriceps) lines up with the groove, since a pull that is angled too far outward will drag the knee cap off course.
Why Does It Happen?
Most dislocations are non-contact. The knee is slightly bent, the foot is planted, and the body rotates inward over the leg — driving the kneecap sideways, out of its groove. It typically relocates when the knee straightens, sometimes on its own, sometimes with help.
As the kneecap goes out and comes back, its inner surface strikes the outer part of the thigh bone. This commonly injures cartilage, and can even shear off a small fragment of bone or cartilage — which is why we image after a first event rather than assuming it was a simple sprain.
Some dislocations happen with barely any force at all, in knees whose anatomy predisposes them to it. When that's the case, the dislocation itself is telling us something about how that knee is built. More than most conditions we treat, patellar instability is a problem of anatomy — a shallow groove, a kneecap that rides high, or a pull from the thigh muscle that's angled too far outward.
Common Symptoms
The hallmark is the kneecap visibly shifting out of place, usually to the outside of the knee, often with a sense that the knee gave out from under you. Swelling typically follows, along with pain and tenderness along the inner edge of the kneecap. Sometimes the kneecap needs to be put back manually; other times it slides back on its own.
Many people also develop apprehension — a persistent fear that it will happen again, especially in certain positions — and for some, later episodes require less and less force to trigger.
How Is It Diagnosed?
We start with your story. Did the kneecap actually shift out of place, and did it need to be put back? A true dislocation and a sense of the knee giving way are different things, and telling them apart matters. We also ask about prior episodes, family history, and whether you're generally loose-jointed, since all of these affect your risk.
On exam, we check how the kneecap tracks as the knee moves, look for tenderness and swelling along the inner edge, and assess your overall leg alignment.
Imaging starts with X-rays to rule out a fracture and get a first look at the groove and kneecap height. We get an MRI after nearly every first-time dislocation — it often reveals cartilage or bone fragments, and confirms whether the MPFL ligament was injured.
It also lets us measure the groove itself, how high the kneecap sits, and how far off-course the pull of the thigh muscle runs — measurements that determine what kind of surgery, if any, makes sense.
How Do We Think About Treatment?
After a First Dislocation
Non-operative management is the default after a first dislocation. Rehab focuses on quadriceps strength, hip and trunk control, and movement mechanics during landing and cutting, with a graded return to activity. Bracing may help early on for comfort and confidence. The goal is to give the surrounding muscles the best chance of controlling a kneecap whose passive restraint — the MPFL — is now compromised.
Two things change that default: a loose cartilage or bone fragment in the joint, which usually needs surgical attention, or anatomy severe enough to prompt an earlier conversation about stabilization.
A meaningful number of first-time dislocations happen again even with good rehab. Your individual risk depends heavily on your anatomy and your age, which is part of what the imaging tells us.
When Instability Becomes Recurrent
Once the kneecap has dislocated more than once, the calculation shifts. Surgery meaningfully lowers the chance it happens again compared with continued rehab alone. But the bigger issue isn't just recurrence — it's that each episode is another opportunity for cartilage damage, and cartilage doesn't grow back. That accumulating cost, more than the dislocations themselves, is what tips the decision toward surgery.
When the underlying anatomy is broadly favorable — a reasonable groove, a kneecap that doesn't sit too high — reconstructing the MPFL alone is usually enough. The ligament is rebuilt using a tendon graft anchored between the kneecap and thigh bone.
When the anatomy is working against us — particularly a high-riding kneecap or a shallow groove — we may also need to realign the extensor mechanism itself, repositioning where the tendon attaches on the shin bone. That adds a bone-healing phase to recovery, but it addresses the architecture the ligament alone can't overcome.
When Is Surgery the Right Choice?
Not because you dislocated once, and not simply because the MRI shows a torn MPFL — it's torn in the large majority of dislocations, and most first-timers do fine without surgery.
Surgery becomes the right call when the kneecap has gone out more than once, when there's a loose fragment in the joint that needs attention, or when the underlying anatomy makes another episode unusually likely. Muscle strength genuinely helps, but it can't fully compensate for a shallow groove or a kneecap being pulled off course — that's not a reason to skip rehab, it's a reason to be honest about what rehab alone can and can't fix.
Recovery
Two things heal on their own timelines after stabilization, and they don't move at the same pace.
The first is the graft itself settling in. It goes through a normal process — getting its blood supply re-established early on, then gradually strengthening over the following months, continuing to mature through the first year. Early on, the repair is being held together mostly by the stitches and anchors, not by the tissue itself, so protection really matters in those first several weeks.
The second is strength. Your quad takes real time to catch back up to how the knee feels — often lagging behind well into the second half of the first year.
There's a third thing worth calling out on its own: apprehension, the lingering fear that the kneecap will go out again. This is a real, lasting part of recovery — not just nerves — and it often sticks around longer than pain or weakness does. It gets better through gradually rebuilding confidence with activity, not just through reassurance.
Return to Sport
Most people who have reconstruction do get back to sport, typically somewhere around seven to ten months out. Fewer return at the exact same level they were playing at before — which is worth knowing going in, so the goal isn't just "back on the field," but back and playing the way you want to.
That timeline lines up with the two clocks from recovery — by around six months the graft has done a lot of its healing and the quad is closing in on full strength, but confidence and sport-specific control usually take a bit longer to catch up.
Before clearing pivoting or contact sport, we want to see full motion, strength that matches the other leg, solid performance on hop testing, and real confidence in the knee — not just the absence of pain. These are benchmarks to hit, not dates that arrive on their own.
Common Questions
Will it happen again?
Recurrence is meaningfully lower after surgery than with rehab alone, but no procedure brings the risk to zero. Your individual risk depends on your anatomy and age, which is part of why we look closely at your imaging before deciding on a plan.
My MRI shows a torn MPFL. Doesn't that mean I need surgery?
Not by itself. That ligament tears in the large majority of dislocations, including plenty that do fine without surgery. Recurrence — not the first event or the MRI finding — is what drives the decision.
Can't I just strengthen my quad instead?
Quad strength genuinely helps and is a core part of rehab. But muscle can't fully make up for a shallow groove or a kneecap being pulled off course by the thigh muscle. Anatomy sets a ceiling that strength alone can't get past.
How long until I'm back to sport?
Most people return somewhere around seven to ten months, though it depends on meeting real benchmarks — motion, strength, hop testing, and confidence in the knee — rather than a fixed date. Fewer people return at the exact same level as before, which is worth knowing from the start.
What Happens Next?
After a first dislocation, we get imaging, address any loose fragment, and start structured rehab — with a clear sense of your individual recurrence risk. If instability comes back, we revisit that imaging and talk through reconstruction, with or without a bony correction depending on what it shows.