a polyaxial fastener is secured within an implant with a retaining ring . the ring is assembled onto a circumferential groove formed in the fastener head . the groove and ring are positionable , when the fastener is installed into an implant , at a wide portion of the mating polyaxial aperture in the implant . the ring is compressible into the groove to form a narrowed diameter , which is passable into a narrow diameter of the aperture , near an entrance to the aperture . the ring is released to expand and increase an overall diameter of the head and ring assembly , thereby rending the assembly too wide to pass by the narrow entrance to the aperture , thereby securing the assembly within the implant .