A bone plate assembly (10) is provided for fixation of spaced vertebra. Thebone plate assembly (10) has at least one through passage (15, 25) forsecuring the plate to bone with a bone fixation screw (23). The threaded shaftof a bone fixation screw (23) is inserted through a bushing (30) located inthe through passage (15, 22) of the bone plate assembly (10) and the screw(23) is thereby threadably secured to the underlying bone and the bushing (30)is then compressed inward against the head (25) of the screw (23) with cams(37) that are actuated by rotating the bushing in the through passage (15, 22)whereby the screw (23) is locked relative to the bone plate assembly (10). Thebushing (30) is not only compressed inwardly against the head (25) of thescrew (23) but is also compressed downwardly by the cams (37) into a seat (39)to clamp separate elements (11, 12) of the bone plate assembly (10) together.An access passage (51) in upper surface (41, 42) of bushing (30) isdimensioned (d) for receiving screw head (25) therethrough for access to asocket bore (35) within the bushing (30).
