Polyaxial locking fastener with negative threads
A polyaxial ball-and-socket fastener is disclosed that provides enhanced locking performance through intersecting negative thread profiles formed on complementary hemispherical surfaces. The fastener includes a saddle assembly having a hemispherical socket with a right-hand negative thread profile and an anchor member having a spherical head with a left-hand negative thread profile. When the saddle assembly is advanced toward the anchor member, the oppositely oriented negative thread profiles intersect and counter-rotate relative to one another to generate multidirectional frictional interference along the curved ball-and-socket interface. This interaction produces a strong locking effect without requiring proportionally increased torque or compressive force, while still permitting wide-range polyaxial articulation prior to tightening. The configuration increases resistance to micro-motion and cyclic loading, maintains a substantially spherical geometry for smooth pre-lock articulation, and enables controlled deformation of the thread profiles during final tightening to further enhance fixation stability.
1 . A polyaxial ball-and-socket fastener comprising:
a saddle assembly including a spherical shaped socket with a lower hemispherical surface with a right hand thread profile that is negative to the inside radius surface of the socket;
an anchor member having a shank with bone threads at a proximal end and a spherical head integrally formed at a distal end having an upper hemispherical surface with a left hand thread profile that is negative to the outside surface of the spherical head; and
wherein the left-hand thread profile of the upper hemispherical surface of the spherical head and the right-hand thread profile of the lower hemispherical surface of the socket intersect and counter-rotate relative to one another as the saddle member is advanced toward the anchor member, such that the oppositely oriented negative thread profiles generate an increased frictional interference at the ball-and-socket interface without requiring a proportionally increased torque or compressive clamping load.
2 . The polyaxial ball-and-socket fastener of claim 1 , wherein the left-hand negative thread profile on the spherical head and the right-hand negative thread profile on the socket each exhibit a substantially constant pitch along their respective hemispherical circumferences.
3 . The polyaxial ball-and-socket fastener of claim 1 , wherein the right-hand and left-hand negative thread profiles are formed with matching pitch and thread height to promote efficient intersecting engagement during advancement of the saddle member toward the anchor member.
4 . The polyaxial ball-and-socket fastener of claim 1 , wherein the negative left-hand thread profile extends over at least fifty percent of the upper hemispherical surface of the spherical head.
5 . The polyaxial ball-and-socket fastener of claim 1 , wherein the intersecting negative thread profiles permit a polyaxial range of motion of at least twenty degrees in any direction prior to initiation of locking engagement.
6 . The polyaxial ball-and-socket fastener of claim 1 , wherein the intersecting thread engagement increases the normal force between the socket and the spherical head as compressive load is applied, thereby enhancing resistance to slippage under cyclic loading.
7 . The polyaxial ball-and-socket fastener of claim 1 , wherein at least one of the negative thread profiles is configured to undergo controlled deformation during final locking to further increase frictional retention without requiring additional torque.
8 . The polyaxial ball-and-socket fastener of claim 1 , wherein the left-hand negative thread profile on the spherical head is produced by subtractive machining while maintaining a substantially spherical exterior geometry.
9 . The polyaxial ball-and-socket fastener of claim 1 , wherein the right-hand negative thread profile of the socket is formed as an internal subtractive thread pattern that preserves the hemispherical curvature of the socket.
10 . The polyaxial ball-and-socket fastener of claim 1 , wherein the intersecting thread profiles generate multidirectional micro-interferences that supplement locking performance even when the compressive load is reduced by at least thirty percent.
11 . The polyaxial ball-and-socket fastener of claim 1 , wherein the hemispherical socket is dimensioned to permit substantially unrestricted polyaxial articulation until the saddle member is torqued to initiate intersecting thread engagement.
12 . The polyaxial ball-and-socket fastener of claim 1 , wherein said lower hermispherical surface is a left hand thread and said upper hermispherical surface is a right hand thread.