A spinal implant assembly is provided having a top loading anchor which is fixed to a spinal member by means of a hook or screw. The anchor includes a seat having a channel to receive a stabilizer rod and a sliding interconnecting closure member which slides in a longitudinal direction relative to the anchor seat to form an integral closed anchor seat assembly. The closure member includes retaining flanges which dove tail relative to undercut retaining flanges of the seat member so as to prohibit the sliding closure member from being dislodged from the closure from the seat channel, as well as inhibit the splaying of the seat member in response to forces which may be imposed upon the seat member during derotation of the spine. In addition, the seat member includes a boss and/or hollow to form a biased lock and compression screw to lock the rod in position relative to the anchor. An instrument is provided which holds the slide for assembly in a spring loaded ball plunger.