POLYAXIAL LOCKING SCREW PLATE ASSEMBLY

摘要

A bone fixation assembly (10) which includes a fixation plate (11) having a through passage (12) and a fastening screw (13) having a threaded shaft (14) for insertion through the passage (12) and thereafter threadable insertion into underlying bone. The screw head (15) has substantially frustospherical shaped side surfaces. A bushing (16) is provided and has an annular cup shaped side wall (17) defining an upwardly open interior bowl (20) for receiving the screw head (15). The bushing (16) has annularly shaped slots (21) in its side wall (17) which depend downwardly from its upper end (18) for allowing inward compression of the side wall (17) at the upper end (18). A centrally located screw shank passage (22) is provided in the lower end (19) of the bushing (16) to prevent passage of the screw shank (13). The bushing bowl (20) is further configured and dimensioned for polyaxial rotation of the screw head (15) in the bowl (20) and for preventing back out of the screw head (15) when the side wall (17) is compressed inwardly. The through passage (12) of the bone plate (11) is further configured and dimensioned to downwardly receive the bushing (16) therein for inwardly compressing the bushing (16) about the screw head (15) and an annular recess (23) is provided in the through passage (12) which receives the upper annular end (18) of the bushing (16) under compression with a snap fit whereby the screw head (15) is retained in the bowl (20) and is thereby prevented from backing out. The screw head (15) is expandable for thereby locking the screw (13) relative to the bone fixation plate (11).