CLOSURE HAVING STRONG BALANCED SPLAY LIMITING THREAD FORMS FOR OPEN RECEIVER HOUSINGS
Open implant closure structures include a helically wound thread form having splay control surfaces with reverse angle and horizontal linear load flanks. The thread form includes a crest height slightly less than or equal to a base height and a thread depth substantially equal to the crest height for a stronger more balanced thread form.
1 . A bone anchor assembly for securing an elongate rod to a bone of a patient, the bone anchor assembly comprising:
a receiver housing including a pair of arms, each of the pair of arms including an inner surface with a mating discontinuous helically wound thread form therein with a load flank having a downwardly facing linear surface portion, the pair of arms defining a channel configured to receive the elongate rod;
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 including a load flank having an upwardly facing linear surface portion defined between an inner end and an outer end thereof with a slope angle sloping radially outwardly and upwardly toward the top end of the cylindrical body or extending radially outward substantially perpendicular to the central axis, a stab flank having a downwardly facing linear clearance surface portion defined between an inner end and an outer end thereof with a slope angle sloping radially outwardly and upwardly toward the top end of the cylindrical body, 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 root surface defining an outer surface of the cylindrical body, the mating discontinuous helically wound thread having a base with a height adjacent to and integrally formed with the cylindrical body, a thread crest having a cylindrical surface extending parallel with respect to the central axis of the closure and defining a crest height along a vertical length thereof, and a thread depth defined as a horizontal distance between the root surface and the cylindrical surface on the thread crest, wherein the crest height equals to or less than a base height, and wherein the thread depth substantially equals to the crest height for a stronger more balanced thread form.
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 the mating discontinuous 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 an upper radiused surface communicating with the load flank and a lower radiused surface communicating with the stab flank.
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 portion of the load flank and the cylindrical surface on the thread crest, the toe angling upward from the upwardly facing linear surface portion of the 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 . A bone anchor assembly for securing an elongate rod to a bone of a patient, the bone anchor assembly comprising:
a receiver housing including a pair of arms, each of the pair of arms including an inner surface with a mating discontinuous helically wound thread form therein with a load flank having a downwardly facing linear surface portion, the pair of arms defining a channel configured to receive the elongate rod;
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 including a load flank having an upwardly facing linear surface portion defined between an inner end and an outer end thereof with a slope angle sloping radially outwardly and upwardly toward the top end of the cylindrical body or extending radially outward substantially perpendicular to the central axis, a stab flank having a downwardly facing linear clearance surface portion defined between an inner end and an outer end thereof with a slope angle sloping radially outwardly and upwardly toward the top end of the cylindrical body, 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 root surface defining an outer surface of the cylindrical body, the mating discontinuous helically wound thread having a base with a height adjacent to and integrally formed with the cylindrical body, a thread crest having a cylindrical surface extending parallel with respect to the central axis of the closure and defining a crest height along a vertical length thereof, and a thread depth defined as a horizontal distance between the root surface and the cylindrical surface on the thread crest, wherein the crest height equals to or less than a base height, and wherein the thread depth substantially equals to the crest height so as to provide a balance for the mating discontinuous helically wound thread on either side of the load flank and result in a stronger thread form for controlling a tendency of the arms to splay when the bone anchor assembly is locked by the closure.
10 . The bone anchor assembly of claim 9 , wherein the thread pitch ranges between about 1.0 mm to about 1.6 mm and the crest height is about half or less than half of the thread pitch.