Dislocation of the Patella (Dislocated Kneecap)

What happens when the kneecap dislocates

The patella sits in a groove at the end of the thighbone (the trochlea). Muscles and ligaments hold it in place as it glides up and down when you bend your knee. The most important restraint on the inner side is the medial patellofemoral ligament (MPFL).

When the knee twists inward while the foot is planted and the thigh muscle contracts hard, the kneecap is pulled sideways out of the groove. The MPFL tears in most first-time dislocations. As the kneecap snaps back, it can knock off small pieces of cartilage or bone from the kneecap or femur.

This injury is different from a dislocated knee joint (where the thighbone and shinbone separate), which is a severe injury that can damage arteries. See dislocation of the knee.

Symptoms

• A pop and a sudden buckling of the knee
• Severe knee pain, often at the inner edge of the kneecap where the ligament tore
• The kneecap visibly sitting on the outside of the knee, with the knee held bent
• Rapid knee swelling, often within an hour or two, from bleeding into the joint
• Trouble straightening the knee or walking
• Later: a feeling that the kneecap might slip again (apprehension)

What to do right away

1. Stop activity and do not put weight on the leg.
2. If the kneecap is still out, support the leg in the most comfortable position and get medical help. Do not force it back. It often relocates on its own when the leg is gently straightened, but a clinician should do this, usually after pain relief.
3. Apply ice wrapped in a cloth for 15 to 20 minutes.
4. Get the knee checked the same day, even if the kneecap has gone back.

Risk factors

• Female sex and the teenage years
• Previous dislocation, or a family history of unstable kneecaps
• A shallow trochlear groove (trochlear dysplasia)
• A high-riding kneecap (patella alta)
• Loose joints (hypermobility), knock knees or an outward-pulling quadriceps alignment
• Sports with pivoting and cutting: soccer, basketball, gymnastics and dance

Diagnosis

A doctor examines the knee for tenderness along the inner edge of the kneecap, swelling and how far the kneecap can be pushed sideways. Many patients tense up when the kneecap is pushed outward, a sign called apprehension.

• X-rays check for fractures, loose fragments and the shape of the groove and kneecap height.
• MRI is often ordered after a first dislocation, especially with a large swelling. It shows MPFL tears, cartilage damage and bone bruises, and helps predict the risk of it happening again.

Treatment

Non-surgical treatment (most first dislocations)

• Relocation if the kneecap is still out.
• Draining the joint with a needle sometimes, if a tense swelling causes severe pain.
• A brace or knee immobilizer for comfort for a short period, then a hinged or patellar-stabilizing brace. Long rigid immobilization is avoided because it causes stiffness.
• Crutches until you can walk without a limp.
• Physical therapy: restoring motion, then strengthening the quadriceps (especially the inner thigh muscle), hips and core, and balance training.
• Pain relief: ice, and over-the-counter pain relievers following the label dose.

Surgery

Surgery is usually advised when:

• A piece of cartilage or bone has broken off and is loose in the joint
• The kneecap dislocates again (recurrent instability)
• Anatomical risk factors make another dislocation very likely

Common procedures include MPFL reconstruction (rebuilding the ligament with a tendon graft), tibial tubercle osteotomy (moving the bony attachment of the kneecap tendon to realign it) and, in selected cases, deepening the groove (trochleoplasty). Return to sport after reconstruction usually takes about 6 to 9 months.

Recovery time

Complications and outlook

The biggest problem is recurrence. Roughly a third of people dislocate again after a first episode treated without surgery, and the risk is higher in teenagers with shallow grooves or high-riding kneecaps. Repeated episodes can damage cartilage and lead to long-term patellofemoral pain or early arthritis. For ongoing slipping, see patellar instability.

Frequently asked questions

How long does a dislocated kneecap take to heal?

Most people walk normally within 2 weeks and return to sport in about 6 to 12 weeks after a first dislocation treated with a brace and physical therapy. Recovery from surgery to rebuild the ligament takes longer, usually 6 to 9 months before full return to pivoting sports.

Can a dislocated kneecap go back in by itself?

Yes. It often pops back into place when the leg is straightened. That does not mean the knee is fine. The supporting ligament usually tears and fragments of cartilage can break off, so the knee should still be examined and often X-rayed the same day.

Is a dislocated kneecap the same as a dislocated knee?

No. In a kneecap dislocation, only the patella slips out of its groove. In a knee dislocation, the thighbone and shinbone separate, which can tear several ligaments and damage the main artery behind the knee. A knee dislocation is a limb-threatening emergency.

Do you need surgery for a dislocated kneecap?

Usually not after a first dislocation. Surgery is recommended when a loose fragment of bone or cartilage is present, when the kneecap keeps dislocating, or when anatomy makes recurrence very likely. The most common operation is MPFL reconstruction.

How can I prevent my kneecap from dislocating again?

Complete the physical therapy program, with strengthening of the quadriceps, hip and core muscles and balance training. A patellar-stabilizing brace can help during sport. Return to cutting and pivoting only when strength and control are near equal to the other leg. Repeated dislocations are a reason to see an orthopedic specialist.