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.