PIVOTAL BONE ANCHOR ASSEMBLY WITH FRICTIONAL SHANK HEAD SEATING SURFACES

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

a shank having a longitudinal axis, a shank head having a partial spherical shape defined by a circumferentially-continuous and non-resilient outer spherical surface extending above and below a hemisphere plane perpendicular the longitudinal axis, and an anchor portion opposite the shank head configured for fixation to the bone;

a receiver having a vertical centerline axis, a base defining a lower portion of a central bore centered around the vertical centerline axis and in communication with a bottom surface of the base through a bottom opening, and a pair of upright arms extending upward from the base to define an open channel configured to receive the elongate rod, the central bore extending upward from the bottom opening through the open channel to tops of the upright arms and including a lower spherical seating surface proximate the bottom opening configured to receive the outer spherical surface of the shank head of the shank to provide for pivotal movement between the shank and the receiver; and

an insert positionable within the central bore above the shank head of the shank, the insert having a downwardly-facing surface engageable with the shank head above the hemisphere plane and an upwardly-facing surface engageable with the elongate rod,

wherein, prior to the engagement of the downwardly-facing surface of the insert with the shank head, the outer spherical surface of the shank head is frictionally engaged below the hemisphere plane by the lower spherical seating surface of the central bore and above the hemisphere plane by an upper spherical surface located in the central bore of the receiver below the insert, so as to provide a non-floppy pivotal frictional engagement between the shank and the receiver.

2 . The pivotal bone anchor assembly of claim 1 , wherein the lower spherical seating surface of the receiver continuously frictionally engages around an entire circumference of the outer spherical surface of the shank head below the hemisphere when the shank head is disposed within the receiver and the longitudinal axis of the shank and the vertical centerline axis of the receiver are co-aligned with respect to each other.

3 . The pivotal bone anchor assembly of claim 1 , wherein the upper spherical surface is integral with the central bore of the receiver.

4 . The pivotal bone anchor assembly of claim 3 , wherein the upper spherical surface is continuous with the lower spherical seating surface of the receiver.

5 . The pivotal bone anchor assembly of claim 3 , wherein the upper spherical surface and the lower spherical seating surface of the receiver have a common diameter that is substantially equal to a diameter of the outer spherical surface of the shank head.

6 . The pivotal bone anchor assembly of claim 1 , wherein the upper spherical surface is configured to continuously frictionally engage around an entire circumference of the outer spherical surface of the shank head above the hemisphere when the shank head is disposed within the receiver and the longitudinal axis of the shank and the vertical centerline axis of the receiver are co-aligned with respect to each other.

7 . The pivotal bone anchor assembly of claim 1 , wherein at least a portion of the receiver bottom surface is non-perpendicular to the receiver vertical centerline axis and acutely upwardly angled toward the lower spherical seating surface of the receiver to truncate a lower portion of the lower spherical seating surface, thereby defining an acute edge portion of the receiver bottom opening that provides for increased pivotal movement of the shank relative to the receiver in at least one direction when the shank head is received within the lower spherical seating surface.

8 . The pivotal bone anchor assembly of claim 1 , wherein the shank is top loaded into the central bore of the receiver and pushed down along the receiver vertical centerline axis until the hemisphere is moved past the upper spherical surface and the shank head is captured into the non-floppy pivotal frictional engagement with the receiver.

9 . The pivotal bone anchor assembly of claim 1 , wherein the insert downwardly-facing surface has a spherical shape complementary with the spherical shape of the shank head above and below the hemisphere.

10 . The pivotal bone anchor assembly of claim 1 , wherein the insert has a central opening.

11 . The pivotal bone anchor assembly of claim 1 , wherein the insert further comprises a cylindrically-shaped body with upstanding arms defining a second open channel for receiving the elongate rod.

12 . The pivotal bone anchor assembly of claim 11 and further comprising the elongate rod and the closure, and wherein the insert upstanding arms extend above the elongate rod to engage an underside surface of the closure when the elongate rod is fully reduced into the receiver open channel via the closure.

13 . The pivotal bone anchor assembly of claim 1 and further comprising the closure, wherein the closure is a single piece closure having an outer cylindrical surface configured to threadably engage inner sidewall surfaces of the upright arms and a bottom surface configured to engage the elongate rod.

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

a shank having a shank head with an at least partial spherical shape defined by a circumferentially-continuous and non-resilient outer spherical surface, and an anchor portion extending downwardly from the shank head for fixation to the patient bone;

a receiver having a vertical centerline axis, a base defining an inner cavity centered around the vertical centerline axis and in communication with a bottom surface of the base through a bottom opening, and a pair of upright arms extending upward from the base to define an open channel configured to receive the elongate rod, the inner cavity in communication with the open channel to define a central bore, the inner cavity configured to receive the shank head and to provide for pivotal movement between the shank and the receiver; and

an insert positionable with the central bore above the shank head and having a downwardly-facing surface configured to engage the shank head above its hemisphere and an upwardly-facing surface configured to engage the elongate rod to transfer a downwardly-directed force from the elongate rod onto the shank head,

wherein at least a portion of the receiver bottom surface is non-perpendicular to the longitudinal center axis and upwardly angled into the receiver inner cavity to enlarge the receiver bottom opening and provide for increased pivotal movement of the shank relative to the receiver in at least one direction when the shank head is received within the receiver inner cavity, and

wherein, prior to the engagement of the downwardly-facing surface of the insert with the shank head, the shank head is held in frictional contact within the inner cavity of the receiver between and against a lower spherical surface and an upper spherical surface located below and above the hemisphere of the shank head, respectively, to provide a non-floppy pivotal frictional engagement directly between the shank and the receiver.

15 . The pivotal bone anchor assembly of claim 14 , wherein the insert downwardly-facing surface has a spherical shape complementary with the spherical shape of the shank head above and below the hemisphere.

16 . The pivotal bone anchor assembly of claim 14 , wherein the lower spherical surface further comprises a spherical seating surface formed into the inner cavity proximate the bottom opening configured to receive the outer spherical surface of the shank head to provide for pivotal movement between the shank and the receiver.

17 . The pivotal bone anchor assembly of claim 16 ,

wherein the upper spherical surface is formed integral with the inner cavity of the receiver, and

wherein the upper spherical surface and the spherical seating surface of the inner cavity have a common diameter that is substantially equal to a diameter of the outer spherical surface of the shank head to provide the non-floppy pivotal frictional engagement directly between the shank and the receiver.

18 . The pivotal bone anchor assembly of claim 17 , wherein the upper spherical surface is continuous with the lower spherical seating surface of the receiver.

19 . The pivotal bone anchor assembly of claim 17 , wherein the shank is top loaded into the central bore of the receiver and pushed down along the vertical centerline axis until the hemisphere is moved past the upper spherical surface and the shank head is captured into the non-floppy pivotal frictional engagement with the receiver.