Karns has been on the 60-day disabled list since May 21 with right forearm soreness. As Karns attempted to rehab from that injury, he continued to have setbacks as the soreness would not fade entirely.
"We tried everything," manager Ned Yost said. "We tried rest, we tried injections. This was the only other possible solution, so best to take care of it now and hopefully he will be ready to go for Spring Training."
Several former Royals have had the surgery, including Dillon Gee, Chris Young and Luke Hochevar. Royals prospect Kyle Zimmer also had the surgery last season, and he is presently pitching at Triple-A Omaha.
The syndrome is caused by compression of nerves, arteries or veins in the area between the lower neck and armpit area.
"I don't know the rehab process, but basically it's a rib removal," Karns said. "Take a rib out, free up some space in there, give me a little more room around my nerve."
Karns said he plans to reach out to Young to get some advice about the procedure and the rehab.
"Definitely going to reach out and get his wisdom, his experience and what he found was beneficial throughout rehab to focus on," Karns said. "The more information I can have to get through this, the better off I am.
"Everything I've been told is I'll be ready before Spring Training. If we waited a little longer to find out more, it would jeopardize our confidence in being ready for Spring Training. I feel like this is our best option at the moment. ... My expectation is to be ready for next year, no ifs, ands or buts."
Karns, acquired in the offseason in a trade with the Mariners in exchange for outfielder Jarrod Dyson, made eight starts (nine total appearances) in his first season for the Royals, going 2-2 with a 4.17 ERA and a career-best 27.1 percent strikeout rate.
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.