a posterior spinal prosthesis comprising a unitary body with at least three attachment points attachable to a spinal structure, said unitary body comprising a single flexure assembly centrally positioned between first and second attachment members and attached to central portions of both said first and second attachment members, wherein flexure of said flexure assembly permits movement of the first attachment member relative to the second attachment member, and further comprising a stopper that limits flexure of said flexure assembly and limits relative movement of said first and second attachment members with respect to one another, wherein said flexure assembly comprises two joint members that glide over one another and wherein one of said joint members comprises two portions spaced from one another by a gap.