Polyaxial bone screw with multi-part shank retainer

A polyaxial bone screw assembly comprising:

• a) a shank having an elongate body and a capture structure, the body configured for fixation to a bone and having an axis, the capture structure extending from the body substantially along the axis, the capture structure having a surface spaced from the body and disposed perpendicular to the axis, the surface having an edge and at least one face disposed between the edge and the body, the face sloping toward the body;
• b) a head having a top portion and a base, the head top portion defining an open channel, the base having a 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;
• and
• c) a retainer structure having at least two parts, each part having an inner surface and an outer surface, each inner surface configured to be in frictional engagement with the face and each outer surface configured to be in slidable mating engagement with the seating surface.

A polyaxial bone screw assembly comprising:

• a) a shank having an elongate body, a driving tip and a capture structure, the body disposed between the driving tip and the capture structure, the body configured for fixation to a bone, the capture structure having an oblique surface, the oblique surface sloping in a direction toward the driving tip;
• b) a head having a top portion and a base, the head top portion defining an open channel, the base having a 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;
• and
• c) a retainer structure having first and second pieces, the first piece having a first inner surface and a first outer surface, the second piece having a second inner surface and a second outer surface, at least one of the first and second inner surfaces configured for frictional engagement with the oblique surface and the first and second outer surfaces configured for slidable engagement with the seating surface.

A polyaxial bone screw assembly method comprising:

• (a) inserting up to a plurality of retainer structure pieces into a head cavity through one of an upper rod-receiving channel and a lower shank receiving opening;
• (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, the capture structure having an edge spaced from the body and an oblique surface disposed between the edge and the body;
• and
• (c) frictionally engaging the capture structure to the retainer structure within the cavity by moving the capture structure toward the upper rod-receiving channel and pivoting the retainer structure pieces about the capture structure while moving the retainer structure pieces toward the lower shank receiving opening until at least one of the retainer structure pieces fully contacts the oblique surface.

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

• (a) providing a bone screw shank, a head, and a retainer structure having up to a plurality of discrete pieces;
• (b) providing the shank with an upper structure having at least one oblique surface;
• (c) providing the head with a rod receiving channel communicating with a central cavity and a shank receiving bore connecting the cavity with an underside of the head;
• (d) loading each retainer structure piece into the cavity;
• (e) uploading the shank capture structure into the cavity through the shank receiving bore;
• and
• (f) moving the capture structure toward the rod receiving channel while pivoting each retainer structure piece about the shank upper structure within the head cavity until each piece contacts an oblique surface and is captured between the contacted oblique surface and the head.