Abstract: An elongated, capsule shaped prosthetic spinal disc nucleus body for implantation into a human inter vertebral spinal disc, made of a hydrogel core and a constraining jacket surrounding the hydrogel core that permits the hydrogel core to deform and reform. The constraining jacket is configured to allow the hydrogel core to hydrate to a predetermined volume and will deform and reform in a desired fashion in response to various loads placed upon the spinal tract. The prosthetic spinal disc nucleus assists in restoring the natural physiology of the human intervertebral disc. By implanting two prosthetic spinal disc nuclei side-by-side into a damaged disc of a human spine, both height and motion can be maintained.