Polyaxial bone screw

In a polyaxial bone screw assembly for surgical implantation and including a shank having an upper end and a threaded body for inserting into a bone and a head having an outward opening channel adapted to receive a rod within the channel, the head having a shank receiving opening, the improvement comprising:

• (a) a capture structure disposed on the shank upper end sized and configured to be uploaded through the shank receiving opening, the capture structure having an outer surface with a first helically wound guide and advancement structure and a smooth cylindrical surface disposed adjacent the first helically wound advancement structure;
• (b) a retaining ring having an external surface and a smooth inner cylindrical surface defining a bore, the smooth inner cylindrical surface configured to slidably mate with the smooth cylindrical surface of the capture structure;
• and
• (c) a nut having a faceted external surface and an internal surface with a second helically wound guide and advancement structure, the first helically wound guide and advancement structure configured to rotatably mate with the second helically wound guide and advancement structure to secure the retaining ring to the capture structure within the head, when assembled and in a non-locked configuration, the retaining ring external surface enabling selective angular positioning of the shank with respect to the head.

A polyaxial bone screw assembly comprising:

• (a) a shank having a body for fixation to a bone and a capture structure extending from the body, the capture structure having an annular seating surface and a cylindrical surface with a first helically wound guide and advancement structure and a smooth surface disposed between the first guide and advancement structure and the annular seating surface;
• (b) a head having a top portion and a base, the head top portion defining an open channel, the base having a substantially hemispherical seating surface partially defining a cavity, the channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening sized and shaped to receive the capture structure therethrough;
• (c) a retaining ring having a substantially hemispherical external surface configured to slidingly mate with the substantially hemispherical seating surface of the head, and an internal surface defining a central bore configured to slidably mate with the smooth surface of the capture structure, the retaining ring external surface configured to be in slidable mating engagement with the seating surface of the head so as to enable selective angular positioning of the shank body with respect to the head;
• and
• (d) a nut having a faceted outer surface and an internal surface having a second helically wound guide and advancement structure thereon, the first helically wound guide and advancement structure configured to rotatably mate with the second helically wound guide and advancement structure to frictionally fix the retaining ring between the nut and the annular seating surface of the capture structure, at least one facet of the nut having a weakened area compressible by an outside pressing force directed at the weakened area, at least one of the first and second guide and advancement structures being deformable by the outside force, thereby interlocking the first and second guide and advancement structures and locking the retaining ring between the nut and the annular seating surface within the head.

A polyaxial bone screw assembly method comprising:

• (a) inserting a retaining ring through a channel and into a cavity of a head, the head channel opening outwardly and adapted to receive a rod within the channel, the head cavity being disposed between and communicating with both the channel and a shank receiving opening, the retaining ring having an inner cylindrical surface defining a bore;
• (b) inserting a capture structure of a bone screw shank through the shank receiving opening of the head and into the cavity thereof, the capture structure being integral with an elongate threaded shank body and having an outer surface with a first helically wound guide and advancement structure thereon adjacent to a smooth cylindrical surface;
• and
• (c) attaching the capture structure to the retaining ring within the cavity by inserting the capture structure through the bore until the inner cylindrical surface of the retaining ring is in sliding engagement with the smooth cylindrical surface of the capture structure, and inserting a nut having a second helically wound guide and advancement structure onto the capture structure and rotating the nut, mating the first and second helically wound guide and advancement structures.

A method of assembling a polyaxial bone screw comprising the steps of:

• (a) providing a bone screw shank, head, retaining ring and nut;
• (b) providing the shank with an upper threaded capture structure;
• (c) providing the retaining ring with an inner cylindrical surface defining a bore;
• (d) providing the head with a central cavity and a shank receiving bore connecting the cavity with an underside of the head;
• (e) loading the retaining ring into the cavity;
• (f) uploading the shank capture structure into the cavity through the shank receiving bore and the retaining ring bore;
• and
• (g) screwing the nut onto the shank capture structure while within the head.