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Patellar Instability / Kneecap Dislocation (Pediatric)

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What is patellar instability?

The kneecap or patella bone is the largest sesamoid bone in the human body. A sesamoid bone is embedded in a tendon and functions to modify friction, diminish pressure, and control the direction of muscle pull. The patella sits in a groove in the thigh bone (femur). When the geometry of the groove or how the patella sits is abnormal or a traumatic accident occurs, the patella dislocates and no longer tracks appropriately in the groove. This is referred to as patellar instability.

Who is affected by patellar instability?

Patellar instability is one of the most common knee injuries in the pediatric and adolescent patient populations. Data includes:

  • Incidence of primary patellar dislocation is 5.6 per 100,000 and can increase to 29 in patients ages 10-17 years old.
  • Established risk factors for patellar instability are being a young female and having a history of previous patellar dislocation
  • Recurrence rate for dislocation is 15-44 percent (17 percent is commonly quoted).
  • If there is another subsequent dislocation, rate of recurrence jumps to 50 percent.

What causes patellar instability?

Traumatic injury: This type of patellar dislocation occurs with an outward twisting force at the knee with the foot planted. It occurs with jumping or decelerating and is common in females playing soccer. This type of dislocation usually goes back in place on its own. Young athletes may experience a lot of pain and swelling in the front of their knee and have difficulty sitting. They may also experience pain and have feelings of instability or catching while walking.

Patella dislocates on its own: The kneecap slides in and out with knee motion. This may or may not be painful. If painful symptoms start, have your child see one of our doctors as soon as possible.

Patellar instability can damage the cartilage of the kneecap and thigh bone, and stretch and damage the soft tissue on the medial side of the knee. This can lead to early arthritis of the knee joint in the long term.

What are the symptoms of patellar instability?

Symptoms are:

  • Patella tracks off to the side.
  • Pain and swelling occur in the front of the knee.
  • Knee motion results in popping or creaking sounds.
  • There is tenderness to palpation along the medial border of the patella.
  • Stiffness and pain occur when straightening the knee.

How is patellar instability diagnosed?

Our doctors do a thorough history and physical examination, which looks at the rotational profile of the lower extremities underlying looseness or laxity of the ligaments and muscles, as well as a focused examination of the knee. Emphasis is placed on how the patella tracks with range of motion of the knee. Radiographs are also essential to our initial evaluation.

In addition, an MRI may be done after a traumatic dislocation if the knee remains unstable on range of motion or there is concern of an injury to the cartilage of the kneecap or femur loose in the joint.

What is the treatment for patellar instability?

Step-by-step nonsurgical treatment entails:

  • If your child's kneecap dislocates traumatically, go to an emergency room if the knee remains out of place. Most of the time it will slide back into the groove with little to no assistance.
  • If the knee is still out of place, our doctors might be able to relocate the kneecap in its groove. If not, we may recommend sedation to help put the kneecap back in place.
  • After the kneecap is relocated, your child will be placed in a knee immobilizer to keep the knee straight for two weeks. Your child can walk around using the knee immobilizer and crutches. Rest, ice, and elevation of the leg and knee help to reduce pain and swelling.
  • At the two-week follow-up, your child will be examined again and transitioned to a different type of knee brace that allows for more motion in the knee.
  • Rehabilitation exercises and a visit to physiotherapy address strengthening the core, hip, and thigh muscles and work on regaining complete motion in the knee.
  • The final steps of rehabilitation include more specific training to help with a return to sports. Recovery times vary but usually take around three months.
  • If the patella remains unstable with motion and continues to slide to the side, or if pain and swelling have not improved, an MRI may be done to look for damage to the cartilage or soft tissue structures of the knee that occurred from the dislocation.

Surgical intervention involves:

  • If an MRI shows loose cartilage or tears in the ligament, your child may need surgery to tighten the ligaments and strengthen the area around the knee. Surgery can also be done to stabilize the damaged cartilage, or if irreparable, remove it.
  • In addition, measurements taken from X-rays and an MRI are used to determine if any other surgery is necessary to improve the patella's geometry as it sits in the groove.
  • After surgery, your child will likely wear a brace and walk with crutches for six weeks and go to physical therapy.

Our goal is to have your child return to sports in 4-6 months. Recovery depends on the type of surgery needed to reconstruct and repair the patellar instability. A child usually returns to sports after completion of a stepwise rehabilitation program. We also recommend use of a patellar knee brace for high impact sporting activities. Maintaining good core strength is essential to minimize recurring dislocations in the future.