Who is responsible for retired football players' medical costs?

The Washington Post: Do No Harm: Who Should Bear The Costs Of Retired NFL Players' Medical Bills?
Who should bear the costs of football-related medical problems that arise years after retirement? This question is at the heart of Williams's case -; and scores of others like it. The average NFL player's career lasts just 3.3 seasons. Studies show that one in four retirees will need a joint replacement, they suffer arthritis at five times the rate of their peers and are four times as likely to suffer neurodegenerative diseases, such as Alzheimer's or ALS (Jenkins and Maese, 5/9).