to minimize potential head injuries if they're struck by a liner. On Monday KSDK-TV cited the Cardinals' Matt Bowman, Zach Duke and Trevor Rosenthal in a story on head guards and Bowman said in a Fox Sports Midwest interview on Wednesday, "I'd say there are probably five of us now that have started to use it and there was a big wave of guys who ordered the insert after the Robbie Ray incident."

The insert is what Astros starter Collin McHugh has worn for years -- the Safer Sports Technologies Pro Performance head guard. And, as ESPN's Outside the Lines reported this spring, the Angels' Matt Shoemaker started wearing it in a comeback from a skull fracture and blood clot in his brain that he suffered when a linerRays, was also hit in the head by a liner and later started wearing the SST head guard, but he told OTL he stopped wearing it after getting into a pitching slump.

Arizona starter Robbie Ray dropped flat on the mound in a game last week after being struck by a line drive off the bat of St. Louis first baseman Luke Voit. Jeff Curry/USA TODAY Sports

MLB and the players' association have not tested or approved the SST product, but pitchers may select any protective headwear they wish, as long as it doesn't interfere with competition or licensing agreements. The only headwear that's mandatory is the official cap.

Two other companies' head safety devices have received MLB/MLBPA approval -- a hybrid cap-helmet resembling a visor that is produced by Boombang