Selectively locking polyaxial screw
The present disclosure generally relates to pedicle screws that can selectively lock polyaxial motion of a receiver head with a locking cap. The locking cap can allow locking of the polyaxial motion of the receiver member relative to the spherical head of the bone shank without having a rod placed in the rod slot of the receiver member. This can enable placement of the head of the pedicle screw in a desired position and orientation, locking the head in the desired position and orientation, and performing a maneuver, such as derotation, without a rod in place or any extra motion between the receiver member and the bone shank. In some embodiments, actuation of the locking cap can be achieved by rotating the locking cap relative to the receiver member, where rotation of the locking cap causes corresponding axial translation of the locking cap relative to the receiver member.
1 . A bone screw assembly, comprising:
a threaded shank having a proximal head portion and a distal portion;
a receiver member disposed around the proximal head portion such that the distal portion of the threaded shank extends along a proximal distal-axis of the receiver member through a hole formed in a distal end of the receiver member, the receiver member being defined by a U-shaped recess formed between a pair of arms, the pair of arms having a proximal first thread form and a distal second thread form separated by a non-threaded portion formed along an interior surface thereof;
a rigid locking cap configured to be disposed in the receiver member, the locking cap being defined by a cylindrical body having a channel that extends along a central longitudinal axis of the cylindrical body, and having a pair of projections extending from an outer surface of the cylindrical body in opposite directions;
wherein the locking cap is configured to move between an unlocked configuration, in which the receiver member can move polyaxially relative to the threaded shank, and a locked configuration, in which the receiver member is prevented from moving relative to the threaded shank; and
wherein the pair of projections are configured to ride along a surface of the distal second thread form to control a position of the locking cap along the proximal distal-axis.
2 . The assembly of claim 1 , wherein at least one of the first thread form or the second thread form extends across a lateral midpoint of the interior surface of the pair of arms.
3 . The assembly of claim 1 , wherein each of the first thread form and the second thread form extends across a lateral midpoint of the interior surface of the pair of arms.
4 . The assembly of claim 1 , wherein the locking cap is configured to be disposed in the receiver member proximal to the head portion of the threaded shank.
5 . The assembly of claim 1 , further comprising one or more keyways that intersect the distal second thread form and extend axially along the interior surface of the receiver member to facilitate movement of the one or more projections into the distal second thread form.
6 . The assembly of claim 1 , wherein the distal second thread form is adjacent to a distal end of the U-shaped recess.
7 . The assembly of claim 1 , wherein the distal end of the receiver member further comprises a polyaxial seat and a groove configured to receive a spring clip therein that engages the proximal head when the proximal head is disposed in the polyaxial seat.
8 . The assembly of claim 1 , wherein the outer surface extends proximal and distal to the pair of projections.
9 . The assembly of claim 1 , wherein the locking cap is configured to be translated distally from a proximal end of the receiver member to the distal second thread form without rotating the locking cap.
10 . The assembly of claim 1 , wherein the locking cap is unitary.
11 . A surgical method, comprising:
disposing a proximal head of a threaded shank within a receiver member such that the threaded shank and the receiver member can move polyaxially relative to one another;
distally advancing a locking cap having an uninterrupted circumference into the receiver member through a proximal-facing opening past a proximal first thread form and a non-threaded portion formed along an interior surface of the receiver member until one or more projections formed on an exterior surface of the locking cap pass into a distal second thread form formed along the interior surface of the receiver member; and
rotating the locking cap relative to the receiver member to selectively lock out polyaxial motion of the receiver member relative to the threaded shank.
12 . The method of claim 11 , wherein the locking cap is rotated a full-turn or less.
13 . The method of claim 11 , wherein the locking cap is rotated a half-turn or less.
14 . The method of claim 11 , wherein the locking cap is rotated by a driver received in a channel formed in the locking cap.
15 . The method of claim 11 , wherein rotating the locking cap moves the locking cap from an unlocked configuration, in which relative movement between the receiver member and the threaded shank is permitted, to a locked configuration, in which polyaxial motion between the receiver member and the threaded shank is prevented by friction between the locking cap and the proximal head of the bone shank.
16 . The method of claim 11 , wherein the locking cap translates without rotating during advancement through the receiver member into the distal second thread form.
17 . The method of claim 11 , further comprising performing any of a derotation maneuver or a distraction maneuver after rotating the locking cap.
18 . The method of claim 11 , further comprising placing a spinal fixation rod within a U-shaped recess of the receiver member.
19 . The method of claim 18 , further comprising threading a set screw into the proximal first thread such that the set screw contacts the spinal fixation rod and the spinal fixation rod contacts the locking cap.
20 . A surgical method, comprising:
disposing a proximal head of a threaded shank within a receiver member such that the threaded shank and the receiver member can move polyaxially relative to one another;
distally advancing a rigid locking cap into the receiver member through a proximal-facing opening past a proximal first thread form and a non-threaded portion formed along an interior surface of the receiver member until one or more projections formed on an exterior surface of the locking cap pass into a distal second thread form formed along the interior surface of the receiver member; and
rotating the locking cap relative to the receiver member to selectively lock out polyaxial motion of the receiver member relative to the threaded shank.