An Artificial Disc Replacement (also called a disc replacement or spine arthroplasty device) is a device that is implanted into the spine to imitate the functions of a normal disc (carry load and allow motion). With a disc nucleus replacement, only the center of the disc (the nucleus) is removed and replaced with an implant. The outer part of the disc (the annulus) is not removed. Artificial discs are usually made of metal or plastic-like (biopolymer) materials, or a combination of the two. The indications for disc replacement may vary for each type of implant.