When Jason Lalli tore his left anterior cruciate ligament at age 26, he thought he would be fine as soon as he had his knee repaired. As a soccer player who competed through college and then on recreational teams, he knew that A.C.L. injuries could be debilitating but also that orthopedists could fix them.

He figured that he would miss a season, but that he could play and coach the game he loved for the rest of his life.

Four years later, his knee began to ache, and the pain became more constant over time, nagging almost “like a toothache,” he said. Within about another year, Lalli’s doctor did more work on the knee and gave him bad news: He had arthritis.

And, Lalli eventually learned, it was almost predictable.

Orthopedists have believed for years that torn tendons or ligaments put patients, no matter how young, at risk for arthritis. But quantifying the long-term risk has been difficult because most orthopedic patients are not studied for extended periods after their injuries.