Modular Head For Pedicle Screw

1 . A spinal fixation device, comprising:

a bone screw including head and a shank extending from the head; and

a modular head assembly, comprising:

a housing having a proximal surface, a distal surface, and a throughhole formed through the housing from the proximal surface to the distal surface;

an anvil slidable within a portion of the throughhole;

a washer disposed within the throughhole of the housing, the washer having a proximal surface, a distal surface, and an aperture extending through the washer from the proximal surface to the distal surface, the aperture sized to receive the head of the bone screw therethrough;

an assembly cap secured to the housing, the assembly cap including an inner surface defining a cavity having a first portion and a second portion, the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter;

a first biasing member arranged to bias the anvil toward the proximal surface of the washer;

a retaining ring positioned at least partially within the cavity of the assembly cap, the retaining ring transitionable between a first configuration in which the retaining ring is sized to receive the head of the bone screw and a second configuration in which the retaining ring is compressed about the bone screw; and

a second biasing member arranged to bias the retaining ring toward the second portion of the assembly cap,

wherein movement of the retaining ring from the first portion of the assembly cap to the second portion of the assembly cap compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing.

2 . The spinal fixation device of claim 1 , wherein a distal portion of the housing defines a counterbore terminating at a first annular face.

3 . The spinal fixation device of claim 2 , wherein the anvil defines a proximal end, a distal end and a circumferential sidewall extending therebetween, the distal end of the anvil comprising an outwardly extending flange.

4 . The spinal fixation device of claim 3 , wherein the first biasing member surrounds the circumferential sidewall of the anvil and is biased between the first annular face and the outwardly extending flange of the anvil.

5 . The spinal fixation device of claim 1 , wherein the retaining ring defines a proximal end and a distal end, the proximal and distal ends of the retaining ring defining a lumen therethrough.

6 . The spinal fixation device of claim 5 , wherein the retaining ring is formed from a resilient material, and wherein an outer surface of the retaining ring defines a slit in communication with the lumen.

7 . The spinal fixation device of claim 5 , wherein the second biasing member is biased between the washer and the proximal surface of the retaining ring.

8 . The spinal fixation device according to claim 1 , wherein at least one of the first biasing member or the second biasing is a wave spring.

9 . The spinal fixation device of claim 1 , wherein the housing defines a threading and the assembly cap includes a corresponding threading configured to threadably secure the assembly cap to the housing.

10 . The spinal fixation device of claim 1 , wherein the inner surface of the assembly cap includes an inwardly extending ledge at a distal end of the assembly cap to limit distal movement of the retaining ring.

11 . The spinal fixation device of claim 1 , wherein the retaining ring and the second biasing member are formed as separate components.

12 . The spinal fixation device of claim 1 , wherein the retaining ring and the second biasing member are formed as a monolithic component.

13 . The spinal fixation device of claim 1 , wherein the head of the bone screw defines at least one notch.

14 . The spinal fixation device of claim 13 , wherein the at least one notch comprises first and second notches on opposite sides of the head of the bone screw.

15 . A method of assembling a modular head assembly, comprising:

providing a housing having a proximal portion adjacent a proximal surface, a distal portion adjacent a distal surface, and a throughhole formed through the housing from the proximal surface to the distal surface defining first and second annular faces;

sliding an anvil into the throughhole to bias a first biasing member between the first annular face and a portion of the anvil;

placing a washer within the throughhole such that the washer is located distal of the anvil;

inserting a resilient retaining ring at least partially into an assembly cap, the assembly including an inner surface defining a cavity having a first portion with a first diameter and a second portion with a second diameter smaller than the first diameter; and

securing the assembly cap to the housing, thereby fixing the washer against the second annular face and compressing a second biasing member to bias the retaining ring toward the second portion of the assembly cap.

16 . The method of claim 15 , further comprising positioning the first biasing member around the anvil prior to or during the sliding step.

17 . The method of claim 15 , wherein the securing step comprises threadably engaging threading on the assembly cap with corresponding threading on the housing.

18 . A spinal fixation kit, comprising:

a bone screw having a head and a shank, the head defining at least one notch;

a first modular head assembly comprising:

a first housing having a first proximal surface, a first distal surface, and a first throughhole formed through the housing from the first proximal surface to the first distal surface; and

a first anvil disposed within the first throughhole, the first anvil being shaped such that when the first housing receives the head of the bone screw, the first housing is permitted to move about the head of the bone screw in multiple axis; and

a second modular head assembly comprising:

a second housing having a second proximal surface, a second distal surface, and a second throughhole formed through the housing from the first proximal surface to the first distal surface; and

a second anvil disposed within the second throughhole, the second anvil including a protrusion sized to be received by the at least one notch of the bone screw to restrict relative movement between the second housing and the bone screw to a single plane.

19 . The kit of claim 18 , further comprising:

a spinal rod; and

a set screw for securing the spinal rod to one of the first housing or the second housing.

20 . The kit of claim 18 , wherein at least one of the first or second modular head assemblies further comprises:

a first biasing member sized to surround the anvil;

a washer;

a second biasing member;

a resilient retainer ring transitionable between a first configuration in which the retaining ring is sized to receive the head of the bone screw and a second configuration in which the retaining ring is compressed about the bone screw; and

an assembly cap securable to the housing to secure the anvil, the retaining ring, the first biasing member, the washer, and the second biasing member within the housing, the assembly cap including an inner surface defining a cavity having a first portion and a second portion, the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter.