These days, some surgeons have four arms and are made of metal and plastic. Use of a robotic assistant called the Da Vinci Surgical System has quadrupled in the last four years, and the machine now helps with incisions and sutures in 2,000 hospitals around the world. Da Vinci is a multi-purpose robot — the only one of its kind — that can scrub in on heart bypass and valve repair operations, hysterectomies, prostate removal surgeries and other procedures. The Da Vinci robot is not actually performing operations; it only mirrors the movements of the surgeon's hands on two joystick-like controllers. Hospitals with the robot proudly proclaim its modern capabilities. Some patients insist on being treated by the mechanical surgeon. But despite the Da Vinci's popularity, its surgical talents may not surpass those of flesh-and-blood physicians. "There's never been a study showing clinical superiority," says Dr. Marty Makary, a surgeon at the Johns Hopkins University School of Medicine in Baltimore. "For the patient, there's clearly no difference."