Overview
The kneecap rides in a groove on the front of the femur called the trochlea, held centered by the shape of that groove and by a ligament on its inner side, the medial patellofemoral ligament (MPFL). When a twist or blow drives the kneecap over the outer edge of the groove, it dislocates, tearing the MPFL on the way out. Patellar instability describes a kneecap that has done this once and is at risk of doing it again, or that slips repeatedly with everyday activity.
The decision that matters is whether this is a one-time event or the start of a pattern. That depends on the anatomy the patient was born with (a shallow groove, a high-riding kneecap, a tibial attachment set far to the outside) and on what the first dislocation damaged. A first dislocation in a normally shaped knee is treated in a brace and with rehabilitation. A second dislocation, or a first one that knocked a piece of cartilage loose, is treated surgically, because each episode risks more damage to the cartilage on the back of the kneecap.
Common Symptoms
- The kneecap visibly out of place on the outer side of the knee, often sliding back on its own as the leg straightens
- Rapid swelling after the episode from bleeding into the joint
- Tenderness along the inner edge of the kneecap where the MPFL tore, and on the outer femoral condyle where the kneecap struck it
- Apprehension — an involuntary bracing when the kneecap is pushed outward or the knee is bent in a twisted position
- In recurrent cases, a sense of slipping or giving way with turning, dancing or stairs, sometimes with little pain
Giving way with a pop after a plant-and-cut suggests an ACL tear rather than the patella. Persistent catching after the swelling settles suggests a loose cartilage fragment, and aching around the kneecap without any episode of slipping is patellofemoral pain.
Causes & Risk Factors
Most first dislocations happen in a twisting movement with the foot planted and the knee slightly bent and drifting inward — a cut in soccer or basketball, a turn in dance or cheerleading, a ski fall. A direct blow to the inner side of the kneecap causes the rest. The risk is weighted toward adolescents and young adults, girls and young women in particular, and toward knees with predisposing anatomy: a shallow trochlear groove (trochlear dysplasia), a high-riding kneecap (patella alta), an outward-set tibial tubercle, joint hypermobility and knock-kneed alignment. That is why a 14-year-old who dislocates once has a high chance of dislocating again, while a 30-year-old with normal anatomy who dislocates in a collision often never does.
How It Is Diagnosed
Once swelling allows, Dr. O'Donnell examines the kneecap's tracking and stability. The apprehension test (pushing the kneecap outward with the knee slightly bent) reproduces the patient's fear of dislocation and is the most reliable sign. Lateral patellar glide measures how far the kneecap can be pushed outward, a J-sign watches it jump laterally as the knee straightens, and the Beighton score screens for hypermobility.
X-rays include a lateral view to measure kneecap height and trochlear depth and a sunrise view to show tilt and any fracture fragment. MRI is ordered after every first dislocation, because it answers the question that decides surgery: whether a piece of cartilage or bone has been knocked off the kneecap or femoral condyle. It also shows the MPFL tear and the bone bruises that confirm a dislocation happened. When surgery is planned, a CT measures the tibial tubercle–trochlear groove (TT–TG) distance, which decides whether the bony attachment needs to be moved. Out-of-town patients can upload MRI or CT through mymedicalimages.com.
Treatment Options
Non-operative care
A first-time dislocation without a loose fragment is treated in a brace for comfort for a week or two, with weight bearing as tolerated, early motion, and a rehabilitation program that builds the quadriceps (particularly the inner portion), the hip abductors and external rotators, and control of the knee in landing and cutting. A patellar stabilizing brace or taping is used during the return to sport, and most athletes are back in 6 to 12 weeks. The trade-off is that the torn MPFL heals loose, so the risk of a second dislocation is substantial, and highest in adolescents with dysplastic anatomy.
MPFL reconstruction
For recurrent instability, or a first dislocation with a loose osteochondral fragment or an unacceptable risk profile, Dr. O'Donnell reconstructs the medial patellofemoral ligament with a hamstring or quadriceps tendon graft fixed to the kneecap and to the femur at the ligament's anatomic origin. Any cartilage fragment is fixed back in place if large enough or removed if not. Reconstruction restores the restraint that stops the kneecap sliding outward and allows a return to cutting and pivoting sport.
Bony realignment
When the tibial tubercle sits too far to the outside (a TT–TG distance of about 20 mm or more) or the kneecap rides too high, a soft-tissue reconstruction alone is overloaded and fails. In those knees a tibial tubercle osteotomy moves the bony attachment inward and, when needed, downward, fixed with screws in the same operation as the MPFL reconstruction. Trochleoplasty, which deepens a severely flat groove, is reserved for the few patients whose dysplasia is the dominant problem.
Recovery & What to Expect
After MPFL reconstruction the graft is protected from stretching while it heals into bone, but the knee is moved early to avoid stiffness. Adding a tubercle osteotomy slows the early weight-bearing phase while the bone heals. There is no fixed timetable for this operation; the sequence below is a conservative guide, and Dr. O'Donnell advances each phase on strength, motion and swelling rather than on a date.
| Phase | Timing | What happens |
|---|---|---|
| Protection & motion | Weeks 0–2 | Brace locked in extension for walking, weight bearing as tolerated with crutches (protected for about 6 weeks if an osteotomy was added). Quad sets, straight-leg raises, patellar mobilizations, heel slides to 90°. |
| Motion & early strength | Weeks 2–6 | Brace unlocked and weaned as quadriceps control returns; crutches discontinued with normal gait. Flexion to full, stationary bike, closed-chain and hip strengthening. |
| Strengthening | Weeks 6–12 | Progressive resistance, single-leg control, landing mechanics, low-impact conditioning. Advance at about 80% strength of the other leg with no swelling. |
| Return to sport | Months 3–6 | Running progression, plyometrics, cutting and sport-specific drills. Full return typically 4–6 months, on symmetric strength and hop testing. |
Desk work and school are possible within a week; driving a right-knee patient resumes once off crutches with the brace unlocked, usually 3 to 4 weeks; running from about 3 months and sport at 4 to 6 months. First-time dislocations treated without surgery follow the same strengthening sequence over 6 to 12 weeks.
Frequently Asked Questions
Will my kneecap dislocate again?
It depends on anatomy and age. A young athlete with a shallow trochlear groove, a high-riding kneecap or hypermobility has a high chance of a second dislocation after non-operative treatment; an adult with normal anatomy who dislocated in a collision often never does. MRI and X-rays after the first episode show which group you fall into, and that drives the recommendation.
Do I need surgery after a first kneecap dislocation?
Not usually. A first dislocation without a loose fragment is treated with a brace and 6 to 12 weeks of rehabilitation. Surgery is recommended after a first episode when MRI shows a piece of cartilage or bone has been knocked off, or when the anatomy makes recurrence very likely, and after a second dislocation in almost every case.
What is MPFL reconstruction?
The medial patellofemoral ligament is the main restraint that stops the kneecap sliding outward, and it tears in every dislocation. Reconstruction replaces it with a tendon graft fixed to the kneecap and to its anatomic origin on the femur, so the kneecap is held in its groove again. It is an outpatient operation, sometimes combined with a tibial tubercle osteotomy when bony alignment is also off.
How long is recovery after patellar stabilization surgery?
Crutches and a brace for the first 2 to 4 weeks, full motion by about 6 weeks, strengthening through 3 months, and return to sport at 4 to 6 months once strength and hop testing are symmetric. If a tubercle osteotomy is added, weight bearing is protected for about 6 weeks while the bone heals and the timeline shifts back a few weeks.
What is the difference between a patellar dislocation and a subluxation?
A dislocation is the kneecap fully leaving its groove, usually with a visible deformity until it reduces. A subluxation is a partial slip that catches itself and pops back. Both tear or stretch the MPFL, both cause the same apprehension on exam, and repeated subluxations are treated the same way as dislocations.
This page is for general education and is not a substitute for a medical evaluation. Treatment recommendations depend on your individual diagnosis and goals. Kevin O'Donnell, MD sees patients from Coral Gables, Miami, Brickell, Pinecrest and Coconut Grove at 475 Biltmore Way, and out-of-town patients by telemedicine.