Polyaxial bone screw with helically wound capture connection
View Patent ↗A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a head and a closed retainer ring. The external capture structure surface and retainer ring internal bore surface are both threaded for rotatable attachment within a cavity of the head. The head has a U-shaped cradle defining a channel for receiving a spinal fixation rod. The head channel communicates with the cavity and further with a restrictive opening that allows for loading the capture structure into the head but prevents passage of the closed retainer ring out of the head. The retainer ring has an external substantially spherical surface that mates with an internal surface of the head, providing a ball joint, enabling the head to be disposed at an angle relative to the shank body. The threaded capture structure or the closed retainer structure includes a tool engagement formation and gripping surfaces for non-slip engagement by a tool for driving the shank body into bone.
1. In a polyaxial bone screw assembly for surgical implantation and including a shank having a capture end and an elongate threaded body having an axis of rotation for being driven by rotation into a bone, and a head having an outward opening channel adapted to receive a rod within the channel, the head further having an inner cavity and a shank receiving opening, the improvement comprising:
(a) a capture structure disposed on the shank capture end sized and configured to be receivable through the shank receiving opening, the capture structure having a tool engagement structure and an upper engagement surface; and
(b) a retainer structure configured to mate with the capture structure within the head cavity so as to polyaxially rotate with the capture structure during positioning of the shank relative to the head, when the retainer structure is mated with the capture structure, the capture structure upper engagement surface being located above and in spaced relationship with respect to the retainer structure, and the capture structure upper engagement surface being positioned to continuously engage an upper structure when fully assembled, so as to apply a downward force against the shank for locking the articulation of the shank and the retainer relative to the head in a locked configuration.
2. The improvement of claim 1 wherein:
(a) the capture structure has an outer cylindrical surface with a first raised helically wound rib thereon; and
(b) the retainer structure has an external surface and an inner cylindrical surface with a second raised helically wound rib thereon, the first helically wound rib configured to rotatably mate with the second helically wound rib to secure the retainer structure to the capture structure within the head cavity, and the retainer structure external surface enabling selective angular positioning of the shank with respect to the head.
3. The improvement of claim 2 wherein:
(a) the head has an inner substantially partially spherical seating surface partially defining the cavity; and
(b) the retainer structure external surface is substantially partially spherical and in slidable mating engagement with the spherical seating surface of the head.
4. In a polyaxial bone screw assembly for surgical implantation and including a shank and a head, the shank having a capture end and an elongate threaded body having an axis of rotation for being driven by rotation into a bone, the head having an outward opening channel adapted to receive a rod within the channel, the head further having an inner cavity and a shank receiving opening, the improvement comprising:
(a) a capture structure disposed on the shank capture end sized and configured to be receivable through the shank receiving opening, the capture structure having a projection with a tool engagement structure and an upper engagement surface;
(b) a retainer structure configured to mate with the capture structure within the head cavity so as to polyaxially rotate with the capture structure during positioning of the shank relative to the head, when the retainer structure is mated with the capture structure, and the capture structure upper engagement surface being located above and in spaced relationship with respect to the retainer structure; the capture structure upper engagement surface adapted to engage the rod and receive a downward force from the rod to lock the position of the shank and retaining structure relative to the head; and wherein a tool seating surface is disposed on the capture structure, the projection and the capture structure tool seating surface partially defining a recess adapted for receiving a driving tool engaged with the projection and wherein the driving tool is adapted to be in contact with the capture structure tool seating surface when driving the shank body into bone.
5. The improvement of claim 4 wherein the tool seating surface is a first tool seating surface on the capture structure and includes a second tool seating surface on the retainer structure, the projection and the first and second tool seating surfaces partially defining a recess adapted for receiving a driving tool engaged with the projection and wherein the driving tool is adapted to be in contact with both the first and second tool seating surfaces when driving the shank body into bone.