a prosthesis for use in maintaining an interspinal space is disclosed . the prosthesis may be provided as first and second substantially symmetrical halves . the first and second halves may each have a coupling portion and a process portion . the coupling portions allow the separate halves to be separately fit within the interspinal space and then connected to form a single prosthesis , eliminating the need for cutting connecting ligaments to allow insertion of the prosthesis . various connection and locking arrangements are disclosed to maintain the halves engaged with each other and the interspinal space .