Shoulder injury forced Justin Smith to retire from NFL

Justin Smith announced his retirement from the NFL on Monday, becoming the third prominent member of the San Francisco 49ers' defense to walk away from the game this offseason.

Like his former teammates, linebackers Chris Borland and Patrick Willis, Smith's decision was motivated by health. He revealed during a conference call Monday that the left shoulder injury he sustained during training camp in 2013 never fully recovered.

"A piece of bone detached," Smith said, according to ESPN's Paul Gutierrez. "I just knew it hurt like hell."

Because of where he played, the majority of contact Smith endured was on his left side. Returning for a 15th season just wasn't in the books for the 35-year-old defensive end.

"It wasn't responding," Smith said of his shoulder. "If you don’t have the tools, you can't do the job. It's time to go. There's arthritis build up, wear and tear, yada, yada, yada ... it's a young man's game."

Smith finished his career with 613 tackles, 87 sacks, 17 forced fumbles and three interceptions.