An apparatus for use in retaining a longitudinally extending member in position relative to a spinal column includes a top-entry hook assembly having a hook portion which is engageable with a spinal column. A body extending upwardly from the hook portion and has an open ended recess which extends through the body and is open along one side to enable a rod or longitudinal member to be moved into the recess through the open side. An end cap is engageable with the body to close the upper side opening. A clamp member or set screw is disposed at the center of the end cap to clamp the rod in the recess in the body. The end cap and body are interconnectable by different types of connectors including a bayonet connector, a linear cam connector, or a threaded connector.