Pivotal bone anchor assembly with bottom loaded shank head and resiliently expandable retainer

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

• a receiver including a body and a pair of arms extending upwardly from the body, the body defining a cavity with a bottom opening in a bottom exterior of the body, the cavity including an expansion region and a locking region below the expansion region, the pair of arms including opposed interior surfaces that define an open channel above the cavity to receive the elongate rod, the open channel communicating with the cavity to define a central bore extending from the bottom opening to top surfaces of the pair of arms, the interior surfaces of the open channel including a closure-mating structure formed therein;
• a shank including a head and an anchor portion extending distally from the head to attach to the bone, the head being partially spherical, having a maximum diameter perpendicular to a longitudinal axis of the shank, and being up-loadable into the cavity of the receiver through the bottom opening;
• a retaining structure that is resiliently expandable and at least partially positionable within the expansion region prior to the shank and operable to capture the head of the shank, the retaining structure maintaining the head in spaced relationship from an internal surface of the cavity of the receiver upon the uploading of the head through the bottom opening and into the retaining structure, the retaining structure having an discontinuous internal surface to engage the head of the shank and an inwardly facing surface above the discontinuous internal surface, the discontinuous internal surface being partially spherical;
• and
• a compression insert having an upwardly facing surface, an upper exterior surface, a lower outwardly facing surface, and a downwardly facing partial spherical surface, the upwardly facing surface sized and shaped to engage the elongate rod, the upper exterior surface to slidably engage an internal surface of the central bore of the receiver, the lower outwardly facing surface configured for side-to-side engagement with the inwardly facing surface of the retaining structure above the discontinuous internal surface, and the downwardly facing partial spherical surface to frictionally engage the head of the shank,
• wherein the compression insert is positionable within the central bore of the receiver prior to the shank and in overlapping engagement with the retaining structure, the compression insert being prevented from moving upwardly within the central bore by at least partial overlapping engagement with at least one downwardly facing planar surface formed in the receiver;
• and
• wherein after the head of the shank is captured within the retaining structure, the discontinuous internal surface extends above and below the maximum diameter of the head of the shank when the longitudinal axis of the shank is coaxial with a longitudinal axis of the receiver.

A method of assembling a pivotal bone anchor assembly configured to secure an elongate rod to a bone of a patient, the method comprising:

• position a retaining structure that is resiliently expandable at least partially within an upper expansion region of a receiver, the retaining structure having a discontinuous partial spherical internal surface and an inwardly facing surface above the discontinuous partial spherical internal surface, the receiver having a longitudinal axis, a lower body defining a cavity with a bottom opening, the upper expansion region, and a lower locking region, and a pair of upwardly extending arms with opposed interior surfaces that define an open channel to receive the elongate rod, the open channel communicating with the cavity to define a central bore, the interior surfaces of the open channel having a closure-mating structure formed therein, the central bore extending to a top of the receiver along the longitudinal axis;
• position a compression insert within the central bore of the receiver in a side-to-side overlapping engagement with the retaining structure, with the compression insert being prevented from moving upwardly within the central bore by at least partial overlapping engagement with at least one downwardly facing planar surface formed in the receiver, the compression insert including an upwardly facing surface sized and shaped to engage the elongate rod, an upper exterior surface to slidably engage an internal surface of the central bore of the receiver, a lower outwardly facing surface configured for side-to-side engagement with the inwardly facing surface of the retaining structure above the discontinuous partial spherical internal surface, and a downwardly facing partial spherical surface;
• after the retaining structure is at least partially within the upper expansion region of the receiver and the compression insert is positioned in the central bore of the receiver, upload a head of a shank into the cavity of the receiver through the bottom opening of the receiver, the head of the shank being partially spherical with a hemisphere, the shank including an anchor portion extending distally from the head to attach to bone;
• and
• after the uploading of the head through the bottom opening, capture the head of the shank within the discontinuous partial spherical internal surface of the retaining structure and in spaced relationship from an internal surface of the cavity of the receiver, with the discontinuous partial spherical internal surface extending above and below the hemisphere of the head of the shank.