It's wrong. It's not clear exactly what the meaning is. It could be that the surgery in question is the 8th or the 18th most common. Without knowing the intention of the original writer I cannot say for sure. But correct ways to write it would be: Coronary artery bypass graft is the 8th most common surgery in the US. Coronary artery bypass graft is the eighth most common surgery in the US. Or if it's a typo: Coronary artery bypass graft is the 18th most common surgery in the US. Coronary artery bypass graft is the eighteenth most common surgery in the US.

It looks like a mistake. It should be either "the eighth most common surgery" (writing it as 1/8th - "one-eighth" - is simply wrong) or "the eighteenth most common surgery", if it's a typing error.