A method for implanting a prosthetic spinal disc nucleus in an intervertebral disc nucleus space generally includes: (a) making a hole in the annulus of a disc, with the hole having an undilated size that is smaller than the cross-sectional size of a folded prosthetic disc nucleus; (b) using an implant instrument to dilate the hole in the disc annulus; (c) using the implant instrument to pass a straightened prosthetic disc nucleus through the dilated hole and into the disc nucleus space; (d) withdrawing the implant instrument and allowing the hole in the disc annulus to return to a size smaller that its dilated size; and (e) causing or allowing the prosthetic disc nucleus to assume a folded configuration with a cross-sectional size that is larger than its straightened size. A device for use in the method is also disclosed.