A dislocated kneecap (patellar dislocation) is when the kneecap slips out of its groove, almost always toward the outside of the knee. It often pops back in by itself when you straighten the leg. After a FIRST dislocation, most people are treated without surgery and are back to normal activities in about 1 to 3 months.
- First time: physio and a gradual return to sport, unless a piece of bone or cartilage has broken off inside the knee
- It happens again: about 1 in 3 people dislocate again after a first dislocation treated without surgery. After repeated dislocations, surgery to stabilize the kneecap, usually MPFL reconstruction, becomes the main option
It is most common in teenagers and young adults, often during a twist or pivot in sport or dance.
What happens when the kneecap dislocates
- The kneecap is pulled out of its groove at the end of the thighbone, usually when the foot is planted and the body twists
- The MPFL tears: the medial patellofemoral ligament is a band on the inner side of the kneecap that holds it in place. It is usually damaged in a dislocation
- Cartilage can be chipped as the kneecap slides out and back, sometimes leaving a loose fragment inside the joint
- The knee swells quickly, often within hours, because of bleeding inside the joint
A knee that gives way and swells fast after a twist can also be an ACL tear, so it needs a proper examination. See ACL reconstruction recovery.
First-time dislocation: what to expect
- X-ray, and often an MRI: mainly to look for a broken-off piece of bone or cartilage and to check the shape of the groove
- A brace or splint for a short time: for comfort and protection while the swelling settles. Crutches if you need them
- Physio early: getting the knee straight, waking up the quadriceps (thigh muscle), then building hip and thigh strength and balance
- Return to sport: usually once swelling has gone, strength is close to the other leg, and you can hop, land and change direction confidently. For most people that is about 1 to 3 months
Will it dislocate again?
For many people, a single dislocation is the only one. The chance of it happening again is higher with:
- Younger age, especially before the growth plates have closed
- A shallow groove on the thighbone (trochlear dysplasia), so the kneecap has less to sit in
- A high-riding kneecap (patella alta), which engages the groove later as the knee bends
- A previous dislocation in either knee. Each dislocation makes the next one more likely
One dislocation is usually a rehab problem. Repeated dislocations are usually a surgical one.
When is MPFL reconstruction considered?
- Recurrent dislocation: the kneecap has come out more than once, or keeps feeling like it is about to
- A first dislocation with high risk: for example a very shallow groove in a young athlete, where the chance of it happening again is high
- A loose fragment: surgery is often done soon after a first dislocation to fix or remove it, and the kneecap may be stabilized at the same time
MPFL reconstruction replaces the torn ligament with a tendon graft, usually taken from the hamstrings. If the bones are a big part of the problem, the surgeon may also move the bony bump below the knee where the kneecap tendon attaches, or deepen the groove.
- It works well: fewer than 1 in 10 people dislocate again after surgery, compared with about 1 in 3 after a first dislocation treated with rehab alone
- It takes time: on average about 7 months to return to sport
- Most get back to sport, but not everyone returns to the same level. About 2 in 3 do
For more on treatment options, see OrthoInfo’s guide to the unstable kneecap.
Recovery after MPFL reconstruction
- Weeks 0 to 6: a brace and crutches at first. Focus on getting the knee straight, controlling swelling and gradually regaining bend
- Weeks 6 to 12: normal walking, then strength work, cycling and balance
- Months 3 to 6: jogging, then jumping, landing and change-of-direction drills
- Around 6 to 9 months: return to pivoting sport once strength and hop tests are close to the other leg. Your surgeon may use a different timeline. Follow theirs
Go to the emergency department if
- The kneecap is still out of place. Don’t try to force it back
- You can’t straighten the knee or put any weight on it
- The leg or foot goes numb, cold or pale
Get it checked soon if
- The knee swelled quickly after the kneecap popped back in. It needs an X-ray to rule out a broken-off fragment
- The knee locks, catches or clunks after the injury
- The kneecap keeps feeling like it will slip out despite a few months of rehab
- After surgery: increasing redness, heat, wound discharge, fever or calf pain and swelling
Front-of-knee aching without the kneecap slipping out is more often runner’s knee.
The first dislocation calls for good rehab. The second calls for a surgeon’s opinion.