Last year, when Collin Link was 11 years old, he was tackled as he went in for a touchdown in pee-wee football.

"He didn't get up," his mother, Crystal Link, said. "He kept saying his knee hurt real bad." But Link was not overly concerned, thinking it was just a sprain.

But the next morning when the family was getting ready to go to church near their home in The Woodlands, Texas, Collin said he could not walk. That Monday, a doctor told the Links what was wrong.

Collin had an injury that doctors used to think almost never occurred in children. He had torn the anterior cruciate ligament, or ACL, in his left knee, the main ligament that stabilizes the joint.

The standard and effective treatment for such an injury in adults is surgery. But the operation poses a greater risk for children and adolescents who have not finished growing because it involves drilling into a growth plate, an area of still-developing tissue at the end of the leg bone.

Although there are no complete or official numbers, orthopedists at leading medical centers estimate that several thousand children and young adolescents are getting ACL tears each year, with the number being diagnosed soaring recently.