under the knife
Undergoing a surgical procedure, especially an invasive one in which actual blades are used. Typically used in the phrase "go under the knife." My dad is going under the knife on Thursday to clear the blockage in his heart.
under the knife
Undergoing surgery, as in He was awake the entire time he was under the knife. The phrase is often put as go under the knife meaning "be operated on," as in When do you go under the knife? Knife standing for "surgery" was first recorded in 1880.
under the knife
Informal Undergoing surgery.