A poly-axial bone fastener assembly having a collar and a bone fastener can be coaxially locked to prevent poly-axial movements of the collar relative to the bone fastener while permitting the collar to rotate about an axis of the bone fastener, thereby combining the functions and advantages of a poly-axial bone screw and a fixed angle bone screw. Some embodiments of a coaxial locking mechanism may include a c-clip with a locking pin, a c-clip with hooks, a split ring with square corners, a pin that spins inside the collar, pins that travel about a neck of the bone fastener, a coaxially locking top that screws into the collar over a head of the bone fastener, and a top nut that threads onto the head of the bone fastener inside the collar to trap a flange of the collar between a shoulder of the bone fastener and the top nut.
