Bell: Young retirees should draw NFL's concern
The Chris Borland Effect: He's Changed His Future, but Did He
NFL: Players call it quits in prime of careers
Chris Borland's Concussion Concerns, Retirement Are Latest Big
Chris Borland: NFL player quits over concussion fears - BBC Sport
49ers Chris Borland retires from football, citing concern over head
Health fears lead young San Francisco 49ers linebacker Chris
Is Chris Borland an outlier or the start of a trend? With
Expect more early NFL retirements, says former player