An extension for use in a procedure to implant a stabilization system for bony structures such as vertebrae is described. The extension is formed by a tubular body with an engagement mechanism at one end to removably engage with a poly-axial head of a bone anchor assembly. A locking ring working with a slide are used to temporarily lock the poly-axial head and extension in the desired relationship for performing the procedure. The locking ring includes a slot in which resides a pin on the slider. Twisting the locking ring causes the pin to move in the slot and thereby move the slider either up or down. In one position a locking body on the slider engages with the poly-axial assembly locking the extension in place. In the other position, the locking body is disengaged from the poly-axial head allowing the extension to be decoupled from the bone anchor assembly.