Patient Education · Knee

MPFL Reconstruction

For the kneecap that slides out of its groove. Here is the ligament that was supposed to stop it, and how we rebuild it.

Dr. Darin W. Allred Darin W. Allred, MD Board-certified orthopedic surgery & sports medicine
~1 hrTypical time for the operation
Day 1Full weight, with crutches until the leg holds you
6 weeksBefore the graft is properly healed in
4–6 moBack to sport

What happens when a kneecap dislocates

Your kneecap glides in a shallow groove at the end of your thigh bone. On its inner edge is a ligament — the medial patellofemoral ligament, or MPFL — which acts as a checkrein, holding the kneecap in the groove and stopping it sliding off to the outside.

When a kneecap dislocates, it goes outward, essentially always. The MPFL tears as it goes. Often the kneecap pops back in on its own when you straighten the leg, so people arrive describing "my knee went out and came back."

Illustration of a bent knee showing the kneecap in its groove
The knee bent, showing the groove the kneecap has to track along.

The knee swells quickly, and there is tenderness along the inner edge of the kneecap where the ligament tore. Sometimes a piece of cartilage chips off as the kneecap goes over the edge, which is one reason we image these carefully.

Why it keeps happening

Like a torn shoulder labrum, the MPFL does not reliably heal back tight. Once the checkrein is stretched or gone, the kneecap slips more easily — and the recurrence rate after a first dislocation is high in young patients, particularly teenagers.

Several things stack the odds further, and they are worth knowing about because they change the operation:

  • A shallow groove (trochlear dysplasia). If the track the kneecap runs in is flat rather than a valley, no ligament has an easy job.
  • A high-riding kneecap, which sits above the deepest part of the groove and has to travel further before it engages.
  • Naturally loose ligaments, or an alignment that pulls the kneecap outward.

If those factors are severe, an MPFL reconstruction alone may not be enough, and a bone procedure to realign the pull may be added. I look for these on imaging before promising you a result.

What the operation does

We rebuild the checkrein. A tendon graft is anchored to the inner edge of the kneecap and to a precise point on the thigh bone, recreating the ligament that tore.

Illustration of the kneecap seen from the front
The kneecap. The MPFL anchors onto its inner edge and acts as a checkrein against it sliding outward.

The graft is usually one of your own hamstring tendons, or donor tissue. The critical technical detail is the position on the thigh bone — a few millimeters off and the kneecap is either still loose or overly tight, and an overly tight kneecap grinds. That single point is what this operation lives or dies by.

The graft is tensioned to guide, not to clamp. It should hold the kneecap in its track while still letting the knee bend freely.

Who needs it

  • More than one dislocation. The clearest indication.
  • A first dislocation with a loose fragment of cartilage in the joint, which needs addressing anyway.
  • A first dislocation in someone with a shallow groove or other risk factors, where the odds of it happening again are high enough to act sooner.
  • Ongoing instability — the kneecap does not fully dislocate but slips, and you have stopped trusting the leg.

For a straightforward first dislocation without those features, a good therapy program aimed at quadriceps and hip strength settles many knees, and that is where I start.

How the operation goes

  1. Anesthesia. General anesthetic with a nerve block. You go home the same day.
  2. Look inside first with the camera — the cartilage on the kneecap and groove, and any loose fragment.
  3. Harvest and prepare the graft.
  4. Attach it to the kneecap along the inner edge.
  5. Find the point on the thigh bone. This is checked with imaging during surgery, because getting it right is everything.
  6. Tension and fix, then take the knee through its full range to confirm the kneecap tracks properly and the knee still bends freely.

Recovery

  • First 2 weeksFull weight on the leg, using crutches until the leg can safely support you — stop them as soon as it can. Work on getting the knee fully straight, and wake the thigh muscle up. Therapy starts within a few days. Keep the incisions dry 48 hours. Aspirin 81 mg twice daily for two weeks.
  • 2–6 weeksBending progresses steadily under your therapist's guidance. Walking normalizes. The graft is healing into bone during this window, so no twisting or pivoting.
  • 6–12 weeksStrengthening in earnest — quadriceps, and the hip muscles that control which way your knee points when you land.
  • 3–6 monthsRunning, then agility, then sport, on strength and control testing.

The hip work matters more than people expect. A kneecap that dislocates is often a kneecap on a thigh that rotates inward under load, and strengthening the muscles that control that is a large part of not doing this again.

The risks, honestly

  • It dislocates again Much less likely after reconstruction, but possible — most often where the groove is very shallow or the alignment was never addressed.
  • An overtightened graft Too tight and the kneecap is pressed hard into its groove, which causes pain and can damage cartilage over time. This is the reason for the care taken over graft position and tension.
  • Stiffness Losing bend is uncommon and largely prevented by moving early.
  • Kneecap fracture Rare, related to the fixation in the kneecap.
  • Ongoing front-of-knee pain If the cartilage was already damaged by previous dislocations, stabilizing the kneecap stops further damage but may not remove all the ache.
  • Numbness beside the incisions Common and usually fades.
This page is general education. It describes how I approach this and what most patients experience. If anything here does not match what you were told in clinic, ask — your own instructions always come first.