My son, who was 14 at the time, had a similar diagnosis. The
acl was completely absent -- no trace of it on
MRI. It appeared to have imploded, and there were signs of mensicus injury. PCL was OK.
He underwent acl reconstruct (irradiated iliotibial allograft), during which the surgeon "cleaned up" the knee. He rehabed for a year, went back to playing high school ball, and blew the new acl with 12 months.