http://www.mlive.com/lions/index.ssf...rological.html
Failed his neurological tests, unable to play for the 2012 season. The Lions wanted to keep Best around so he could get onto the the NFLs pension plan.
I feel bad for the guy, but I really think it's in his best interest to retire. He had a lot of potential, but with how many bad concussions he has had, it's just not worth it.
Sad day.