Somewhere, Lee Majors must be smiling. Best known for his starring role as Colonel Steve Austin, an ex-astronaut outfitted with bionic implants in the 1970s TV series “The Six Million Dollar Man,” Majors made us all believe that one day humans could be rebuilt joint by joint. Today, most of the key joints in the body can indeed be replaced—hips, knees, ankles, wrists and fingers—all for a lot less than $6 million.