A method of determining the size and/or placement of a prosthetic disc in a disc space between adjacent endplates of two vertebrae in a selected spinal area, comprising: using a computer to identify the disc space parameters of height, width, depth and lordosis between the adjacent vertebral endplates; using a computer to create an artificial volume corresponding to an actual prosthetic disc that can be positioned in the disc space in accordance with a manufacturer's size and lordosis specifications for prosthesis; using a computer to determine the center of the disc space and the center of rotation of the prosthetic disc volume; using a computer to position the prosthetic disc volume in the disc space such that the center of rotation of the prosthetic disc volume is positioned posterior to the center of the disc space; and using a computer to determine the prosthetic disc volume that fits within the disc space in accordance with a surgeon's or manufacturer's specification for prosthesis.