Pivotal bone anchor assembly having a pre-lock friction fit retainer

A pivotal bone anchor assembly intended for securing an elongate rod to a bone of a patient with a closure top, the pivotal bone anchor assembly comprising:

• a receiver comprising a lower portion defining an internal chamber communicating with a bottom of the receiver through a bottom opening, and an upper portion defining a channel sized and shaped for receiving the elongate rod, the internal chamber communicating with the channel to define a central bore and including a lower region adjacent the bottom opening with a seating surface;
• a bone anchor comprising a longitudinal axis, a partial spherically shaped head having a hemisphere plane at a maximum width perpendicular to the longitudinal axis, and an anchor portion opposite the head configured for attachment to the bone, the head being positionable in the internal chamber of the lower portion of the receiver with the bone anchor extending downward through the bottom opening;
• an open retainer comprising a base portion configured to be positioned in the lower region of the internal chamber and in engagement with the seating surface to capture the head of the bone anchor in the receiver, and a plurality of flexible members extending upwards from the base portion;
• and
• a pressure insert positionable within the internal chamber above the open retainer, the pressure insert having an upwardly-open rod seat configured to downwardly receive the elongate rod through the channel and a downwardly-facing lower surface configured to engage the head of the bone anchor upon the head being uploaded through the bottom opening,
• wherein the plurality of flexible members are configured to extend upwards to grip the head of the bone anchor above the hemisphere plane so as to provide a pre-lock friction fit for a non-floppy arrangement between the receiver and the bone anchor prior to locking the assembly with the closure top.

A method of assembling a pivotal bone anchor assembly intended for securing an elongate rod to a bone of a patient with a closure top, the method comprising:

• positioning an open retainer in a receiver, the receiver comprising a lower portion defining an internal chamber communicating with a bottom of the receiver through a bottom opening and an upper portion defining a channel sized and shaped for receiving a portion of the elongate rod, the internal chamber communicating with the channel to define a central bore and including a lower region adjacent the bottom opening with a seating surface, the open retainer comprising a base portion configured to be positioned in the lower region of the internal chamber and a plurality of flexible members extending upwardly from the base portion;
• and
• uploading a bone anchor into the receiver, the bone anchor comprising a longitudinal axis, a partial spherically shaped head having a hemisphere plane at a maximum width perpendicular to the longitudinal axis, and an anchor portion opposite the head configured for attachment to the bone, the head being positionable in the internal chamber of the lower portion of the receiver with the bone anchor extending downward through the bottom opening,
• wherein the open retainer is configured to capture the head of the bone anchor with the base portion of the open retainer engaged with the seating surface, and
• wherein the plurality of flexible members are configured to extend upwards to grip the head of the bone anchor above the hemisphere plane so as to provide a pre-lock friction fit for a non-floppy arrangement between the receiver and the bone anchor prior to locking the assembly with the closure top.