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from the American Academy of Orthopaedic Surgeons

Diseases & Conditions



Staying Healthy

Patellar (Kneecap) Instability

When things are "in the groove," they're going smoothly. That certainly is the case with your patella (kneecap). As long as your patella stays in its groove (trochlea) in the knee, you can walk, run, sit, stand, and move easily. When the kneecap slips out of the groove, recurrent instability (the kneecap dislocates often) and pain may result.



The kneecap connects the muscles in the front of the thigh to the shinbone (tibia). As you bend or straighten your leg, the kneecap is pulled up or down. The thighbone (femur) has a V-shaped notch (femoral groove) at one end to accommodate the moving kneecap.

There are multiple restraints, such as ligaments, muscles, as well as the shape of the bones, that keep the patella well positioned in the trochlear groove.

But sometimes, there are issues that make the kneecap more likely to move out of position:

  • Damage to the medial patellofemoral ligament (MPFL) — a ligament that prevents the patella from moving too far sideways
  • A trochlea (groove) that is too flat to keep the patella in its proper position
  • Abnormal limb alignment, or being “knock-kneed”
  • Abnormal rotation in either the femur and/or tibia can cause the patella to shift out of the groove
  • Attachment site of the patellar tendon on the shin bone — if the place where the patellar tendon connects to the shin bone is too far to the lateral side (outside) of the knee, the tendon can pull the kneecap out of the groove


  • Patellar dislocations often result from a non-contact injury, such as a pivot, twist, or awkward fall.
  • A sharp blow to the kneecap could also pop the patella out of place.


Symptoms of recurrent instability can include:

  • You feel like the kneecap will dislocate or move out of position. This is called apprehension.
  • Actual dislocations of the kneecap
  • Pain or swelling from injured cartilage behind the kneecap

Doctor Examination

Physical Examination

During the physical examination, your doctor may:

  • Ask you to walk around
  • Ask you to straighten and bend your knee
  • Carefully feel the area around your kneecap and take measurements to determine if the bones are out of alignment or if the thigh muscles are weak

Imaging Tests

  • The doctor will likely order a series of X-rays.
  • They may also order an MRI scan to evaluate for loose fragments of bone or cartilage, or to evaluate the ligaments and anatomy of the knee.
  • Sometimes, the doctor may order a CT scan if they are concerned about the alignment of the knee.


If the kneecap has been completely dislocated out of its groove, the first step is to return the kneecap to its proper place. This process is called reduction. Sometimes, reduction happens spontaneously — meaning, on its own. Other times, your doctor will have to apply gentle force to push the kneecap back in place.

A dislocation often damages the underside of the kneecap and the end of the thighbone, which can lead to additional pain and arthritis.

Nonsurgical Treatment

For a first-time dislocation, the doctor will most likely recommend nonsurgical treatments, such as exercises and bracing, with a gradual return to full sport. Exercises will help strengthen the muscles in your thigh so that the kneecap stays aligned.

Cycling is often recommended as part of physical therapy. A stabilizing brace may also be prescribed. The goal is for you to return to your normal activities within 1 to 3 months.

Surgical Treatment

A recurring condition, in which the patella continues to be unstable, is often corrected with surgery. This can mean a procedure to:

  • Reinforce and balance the soft tissue restraints (the ligaments that keep the kneecap in place within the trochlear groove)
  • Correct poor bony alignment
  • Reshape the trochlear groove

Recurrent patellar instability may also increase risk of cartilage injury to the kneecap, which is another reason your doctor may recommend surgery.


Generally speaking, nonsurgical management of first-time dislocations often results in good outcomes with a relatively low recurrence rate (chance that the kneecap will dislocate again). However, some patients may be at higher risk for recurrence, and this should be discussed with your doctor.

Surgery is typically reserved for:

  • Very high-risk first-time dislocations.
  • Patients who have recurrent instability.
  • Patients whose patella is severely out of position during range of motion.  This means that the movement of the kneecap is not aligned, also known as maltracking.

Outcomes for these procedures are very good, with a re-dislocation rate of less than 10%.

Last Reviewed

November 2023

Contributed and/or Updated by

Michael J. Alaia, MD, FAAOS

Peer-Reviewed by

Thomas Ward Throckmorton, MD, FAAOS

AAOS does not endorse any treatments, procedures, products, or physicians referenced herein. This information is provided as an educational service and is not intended to serve as medical advice. Anyone seeking specific orthopaedic advice or assistance should consult his or her orthopaedic surgeon, or locate one in your area through the AAOS Find an Orthopaedist program on this website.