SPLAY CONTROL CLOSURE FOR OPEN BONE ANCHOR
Open receiver implant and closure structures having helically wound guide and advancement splay control surfaces. Thread form heights, thicknesses, and other geometries are described.
1 . A bone anchor assembly comprising:
a receiver housing including a pair of arms and a channel between the arms for receiving an elongate rod, each of the pair of arms including an inner surface with a portion of a mating helically wound thread form therein with a receiver load flank having a downwardly facing linear surface;
a closure having a central axis, a cylindrical body including a top end having a top surface, a bottom end having a bottom surface opposite the top surface, a central internal drive structure extending into the top end, and an outer mating continuous helically wound thread form thereon and comprising a closure load flank having an upwardly facing linear loading surface extending radially outwardly from a curved corner surface at a root-loading juncture with a lower end of a root surface, the upwardly facing linear loading surface having a slope angle sloping upwardly toward the top end of the cylindrical body or extending radially outward substantially perpendicular to the central axis, the outer mating continuous helically wound thread form further comprising a stab flank having a downwardly facing linear clearance surface extending radially outwardly from another curved corner surface at a root-stab juncture with an upper end of the root surface, the downwardly facing linear clearance surface having a slope angle sloping upwardly toward the top end of the cylindrical body, the outer mating continuous helically wound thread form further comprising a thread height defined as an axial height of the outer mating continuous helically wound thread form between the root-loading juncture and the stab-loading juncture, a thread pitch defined as a distance from a location on the mating discontinuous helically wound thread to a same location on an adjacent thread, a thread crest having a cylindrical surface extending parallel with respect to the central axis of the closure, and a thread depth defined as a horizontal distance between a vertical line for the axial height between the junctures thereof and the cylindrical surface on the thread crest, wherein a ratio of the thread depth to the thread height is less than 1.
2 . The bone anchor assembly of claim 1 , wherein the thread depth ranges between about 0.4 mm to about 0.8 mm and the thread pitch ranges from about 1.0 mm to about 1.6 mm.
3 . The bone anchor assembly of claim 1 , wherein each of the portions of the mating helically wound thread form in the receiver housing includes an upwardly facing stab flank and an inwardly facing root surface spaced apart from the stab flank and the cylindrical surface on the thread crest of the closure, respectively, when the elongate rod is locked in the channel by the closure.
4 . The bone anchor assembly of claim 1 , wherein the root surface extends between the root-loading juncture and the stab-loading juncture.
5 . The bone anchor assembly of claim 1 , wherein the root surface is parallel with respect to the central axis of the closure and helically wound around the cylindrical body of the closure.
6 . The bone anchor assembly of claim 1 , wherein the outer mating continuous helically wound thread further comprises a toe defined between the upwardly facing linear surface of the closure load flank and the cylindrical surface on the thread crest, the toe angling upward from the upwardly facing linear surface of the closure load flank.
7 . The bone anchor assembly of claim 1 , wherein the bottom surface of the bottom end of the closure end includes a central nub protruding downward therefrom.
8 . The bone anchor assembly of claim 1 , further comprising a shank configured to be received in the receiver housing.
9 . The bone anchor assembly of claim 1 , wherein the curved corner surfaces of the root-loading juncture and the stab-loading juncture are radiused.