A prosthesis assembly replaces a cephalad portion of a left natural facet joint on a vertebral body and a cephalad portion of a right natural facet joint on a vertebral body. The prosthesis assembly has a left prosthesis body accommodating fixation to the vertebral body at or near a left pedicle and without support by a lamina. An artificial left facet joint structure carried by the left prosthesis body and is adapted and configured to replace a cephalad portion of the left natural facet joint. The prosthesis assembly further has a right prosthesis body accommodating fixation to the vertebral body at or near a left pedicle and without support by a lamina. An artificial right facet joint structure is carried by the right prosthesis body and is adapted and configured to replace a cephalad portion of the right natural facet joint.
