When Preston Nafz was 12, he asked his dad for permission to play lacrosse. “First practice, he came back, he said, ‘Dad, I love it,’” recalled his father, Lothar Nafz, of Hoover, Alabama. “He lives for lacrosse.”But years of youth sports took a toll on Preston’s body. By the time the teenager limped off the field during a lacrosse tournament last year, the pain in his left hip had become so intense that he had trouble with simple activities, su…