US Patent 7311712 Polyaxial locking screw plate assembly

A bone fixation assembly includes a fixation device, a cup shaped bushing, and a fastening screw. The fixation device has a through passage with an annular recess. The a fastening screw has a threaded shaft for insertion through the through passage and for threadable insertion into bone. The screw has a head with substantially frustospherical shaped side surfaces and mates with the cup shaped bushing. The annular recess in the through passage of the fixation device is configured and dimensioned to downwardly, directly, and annularly receive an upper end of the bushing under compression with a snap fit to prevent the screw from backing out of the bone fixation assembly.