The Boston Bruins secured a building block on defense by signing Brandon Carlo to an extension, but they lost blueline depth with the retirement of Kevan Miller.

Carlo, 24, received a six-year contract extension with an annual cap hit of $4.1 million. After his entry-level deal, he had signed a bridge contract in 2019 that carried a $2.85 million cap hit.

While Charlie McAvoy is now the face of the Bruins blueline after they traded Zdeno Chara, Carlo has been the anchor of the second pairing.

Carlo had four goals this season, which was limited to 27 games in large part due to a concussion suffered on a hit from Tom Wilson. It was Carlo's fourth major head injury since he broke into the league in 2016.

In fact, it could be argued that the Bruins' exit from the playoffs this year coincided with Carlo taking a head hit from the Islanders' Cal Clutterbuck in Game 3 of the teams' second-round series. The Bruins were up two games to one, but Carlo didn't play the rest of the series and Boston lost in six games.

After the season, Carlo said he's not worried about his health. And Bruins general manager Don Sweeney said in a statement that the team believes he's a big part of the future.

"The Bruins are very pleased to have extended Brandon on a long-term deal," Sweeney said. "Brandon is a player who has grown into a foundational defenseman with our team while also emerging as an important leader on and off the ice."

Also Wednesday, defenseman Kevan Miller, who has played in just 28 games since breaking his kneecap in 2019, announced his retirement in an Instagram post that he signed "Forever a Bruin."

"Although my spirit for the game is there, unfortunately my body isn't," he wrote. "My overall health and my family are now the priority. This was not an easy decision to make but it's time to hang up my skates."

Miller, 33, played in 352 games over eight seasons with the Bruins, scoring 13 goals with 58 assists. After going undrafted out of Vermont, he signed with Providence of the AHL in 2010, earned a promotion to Boston in 2013 and was in the regular defensive rotation by 2015.

He was injured in April 2019, and was rehabbing the knee during the Eastern Conference finals when he broke it again. He missed all of the 2019-20 season.

"It's hard to put into words what it means to wear the spoked 'B,'" he wrote. "It was an honor to put on that jersey on each night."

Information from The Associated Press was used in this report.