Orpik was in a scrum with a few Penguins players with Boston's Gregory Campbell in the Pittsburgh defensive end of the ice with 8:54 left in the period.
Boston's Brad Marchand was injured on the other end and Thornton came skating up quickly from center ice and grabbed Orpik from behind, knocking him down backward before throwing a punch. Orpik appeared to hit his head on the ice when he fell. Thornton was given a match penalty.
Players immediately called for medical assistance for Orpik. He was surrounded by members of both team's medical staffs before he was taken off on a stretcher after about a 10-minute delay.
Pittsburgh's James Neal was assessed a kneeing penalty for his hit on Marchand.
In the game's opening minute, Orpik hit Boston's Loui Eriksson with his shoulder near the winger's head as he turned to move up the ice.
Eriksson was stunned and went to the bench dazed before heading to the dressing room with a trainer taking one arm to help his balance as he got up from the bench.
A few minutes later, the Bruins announced in an injury update that he "would not return."
A few seconds after Orpik's hit on Eriksson, Boston captain Zdeno Chara charged at the defenseman and drew a cross-checking penalty.
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