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PATELLA DISLOCATION AND INSTABILITY IN CHILDREN

Your child’s kneecap (patella) is usually right where it should be—resting in a groove at the end of the thighbone (femur). When the knee bends and straightens, the patella moves straight up and down within the groove. Sometimes, the patella slides too far to one side or the other. When this occurs — such as after a hard blow or fall — the patella can completely or partially dislocate.

When the patella slips out of place — whether a partial or complete dislocation — it typically causes pain and loss of function. Even if the patella slips back into place by itself, it will still require treatment to relieve painful symptoms. Be sure to take your child to the doctor for a full examination to identify any damage to the knee joint and surrounding soft tissues.

 
kneecap function

(Left) The patella normally rests in a small groove at the end of the femur called the trochlear groove. (Right) As you bend and straighten your knee, the patella slides up and down within the groove.

Reproduced with permission from The Body Almanac. (c) American Academy of Orthopaedic Surgeons, 2003.

Cause

There are a several ways in which the kneecap can become unstable or dislocate. In many cases, the patella dislocates with very little force because of an abnormality in the structure of a child’s knee.

  • A shallow or uneven groove in the femur can make dislocation more likely.
  • Some children’s ligaments are looser, making their joints extremely flexible and more prone to patellar dislocation. This occurs more often in girls, and the problem may affect both knees.
  • Children with cerebral palsy and Down syndrome may have kneecaps that dislocate frequently due to imbalance and muscle weakness.
  • Rarely, children are born with unstable kneecaps causing dislocations at a very early age, often without pain.
 
patellar dislocation

Many things can cause patellar dislocation, such as a shallow groove in the femur or direct force on the knee joint.

Reproduced with permission from The Body Almanac. (c) American Academy of Orthopaedic Surgeons, 2003.

In children with normal knee structure, patellar dislocations are often the result of a direct blow or a fall onto the knee. This incidence is more common in high-impact sports, such as football.

Dislocations can occur without contact, as well. A common example is that of a right-handed baseball player who dislocates the right patella while swinging the bat. When the right foot is planted on the ground and the torso rotates during the swing, the patella lags behind, resulting in dislocation.

Symptoms

The symptoms associated with a patellar dislocation depend on how far out of place the patella has moved and how much damage occurred when it happened.

Some general symptoms your child may experience include:

  • Pain
  • Feeling the kneecap shift or slide out of the groove
  • Feeling the knee buckle or give way
  • Hearing a popping sound when the patella dislocates
  • Swelling
  • A change in the knee’s appearance — the knee may appear misshapen or deformed
  • Apprehension or fear when running or changing direction.

Doctor Examination

If your child’s patella has slid back into place, you should see your doctor as soon as possible. If your child’s patella is still out of place, go to the emergency room.

Medical History and Physical Examination

During the examination, your doctor will ask you and your child about how the injury occurred and specific symptoms. Your doctor also will evaluate the range of motion, tenderness, and appearance of the knee.

Tests

Imaging tests can help your doctor diagnose patellar instability, as well as determine a treatment plan.

  • X-rays. These tests create clear pictures of bone. Your doctor may order x-rays to look for skeletal abnormalities in the knee, such as a shallow groove in the femur.
  • Magnetic resonance imaging (MRI) scans. These scans create better pictures of the soft structures surrounding the knee, like ligaments. An MRI is seldom necessary because the doctor can usually diagnose a dislocated patella through an examination and x-rays. However, if your doctor needs additional, more detailed images, he or she may order an MRI.

Sometimes a piece of bone or cartilage can dislodge or loosen when the patella dislocates. This can be seen on an x-ray or MRI scan.

 
x-ray of knee dislocation
In this x-ray of a bent knee taken from above, the patella is clearly out of alignment within the groove in the femur.
Reproduced with permission from Schepsis AA Patellar instability. Orthopaedic Knowledge Online Journal, 12/18/2003. Accessed February 2014.

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