The second is a replacement just for the inner core of the disc called the nucleus. The nuclear implants are designed to cushion the spine and absorb water to restore disc height. They also keep their shape even when the spine is overloaded.
Another type of nuclear implant is a balloon-shaped device that is inserted inside the disc. It is injected with a substance that has an elastic memory. This allows it to act as a shock absorber some of the time. And finally, motion-limiting devices called posterior stabilization devices (PSDs) are a possibility. These are used more often in Europe where research has been done for years using these systems. PSDs may be soft or hard and rigid in design. They can restrict spine flexion or extension depending on the patient's problem.
Your surgeon would have to advise you which (if any) of these options is best for you. Right now because these are fairly new treatment alternatives to fusion, patients are being selected carefully. The long-term effects remain unknown. Safety is always a primary issue of concern.