A fixation device for use in association with ALIF procedures includes a couple of spaced plates having integral spikes on facing surfaces thereof for pressing into spinal processes of adjacent vertebrae. One of the plates has a spherical socket which captures a spherical head end of a post whose other end is received through an aperture in the other plate. The socket mounting is arranged to enable the post to pivot therein for at least two degrees of freedom to a limited extent, enabling angulation between the two plates so as to accommodate the different thicknesses and orientations of the spinal processes on adjacent vertebrae. The reception of the post in the second plate enables adjustment of spacing between the plates to accommodate effective installation of the assembly on the spinal processes by a compression instrument, and permanent reliable maintenance of that spacing following removal of the compression instrument. The cross-sectional configuration of the post and a receiver aperture in the plate inhibits rotation of the plate relative to the post about the post axis, and a set screw in the apertured plate engages a flat on the post fixing the inter-plate space as adjusted.