An artificial disc is the soft cushioning structure located between the individual bones of the spine, called “vertebra.” It is made of cartilage-like tissue and consists of an outer portion, called the annulus, and an inner portion, called the nucleus (Figure 1). In most cases, the disc is flexible enough to allow the spine to bend.
An artificial disc (also called a disc replacement, disc prosthesis 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).
There are many artificial disc designs classified into two general types: total disc replacement and disc nucleus replacement. As the names imply, with a total disc replacement, all or most of the disc tissue is removed and a replacement device is implanted into the space between the vertebra. 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. These materials have been used in the body for many years. Total disc replacements have been used in Europe since the late 1980s. The most commonly used total disc replacement designs have two plates. One attaches to the vertebrae above the disc being replaced and the other to the vertebrae below. Some devices have a soft, compressible plastic-like piece between these plates. The devices allow motion by smooth, usually curved, surfaces sliding across each other.
Most nucleus replacement devices are made of plastic-like (biopolymer) materials. One such material is called hydrogel. This material expands as it absorbs water. The device is placed into the nuclear cavity of the disc and hydrates to expand and fill the cavity. The device is compressible and by this means, allows motion, much like a normal disc nucleus. Another design consists of a piece of a plastic-like material that coils around to fill the nuclear cavity. No nuclear replacement devices are available for use in the United States at this time, even as a part of an FDA-approved study.
There are also disc replacements designed for use in the cervical spine (the neck). These devices have only been used a relatively short time, and several are currently undergoing evaluation in FDA-approved trials in the United States.