Letowski was skating up ice from the Hurricanes' end when he made a drop pass to teammate Eric Belanger between the circles. Moments after he made the pass, and skating with his head turned in the opposite direction, Letowski was checked hard by Armstrong.
Armstrong's shoulder clipped Letowski in the head, causing Letowski to drop limply to the ice. The hit appeared to be clean and no penalty was called.
Letowski landed face down on the ice at the top of the slot and lay motionless for several minutes. After being attended to by the team's medical staff, he raised his head slightly and a pool of blood could be seen on the ice.
After he was lifted onto a backboard and then a stretcher, Letowski had his neck placed in a brace before he was taken to a nearby hospital.
Letowski's injury was the second by a Hurricanes forward in as many games in Pittsburgh. Star left wing Erik Cole broke his neck while being slammed into the boards by Pittsburgh defenceman Brooks Orpik in a March 4 game. Cole missed the rest of the season and did not return to the Stanley Cup champion Hurricanes until Games 6 and 7 of the finals against Edmonton.
Orpik drew a five-minute boarding penalty and was ejected for the hit on Cole and also was suspended for three games.
Cole played Saturday night, but Orpik has been out this season because of a broken hand.
The 29-year-old Letowski is playing in his eighth NHL season after signing with Carolina in July. He had 73 goals and 102 assists in 484 games, playing previously for Phoenix, Vancouver and Columbus. He had 10 goals and 18 assists last season with Columbus.