Patent Description:
The present disclosure relates to spinal fixation devices and, more particularly, to modular pedicle fixation assemblies.

The spinal column is a complex system of bones and connective tissues that provides support for the body while protecting the spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked on top of one another, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column, as well as maintains proper spacing of the bodies with respect to each other. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.

There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine) and spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine), for example, that are caused by abnormalities, such as disease or trauma, and that are characterized by misalignment of the spinal column. When the spinal column is misaligned, one or more of the misaligned vertebral bodies can "pinch" or apply pressure to the underlying spinal cord and nerves, which often results in debilitating pain and diminished nerve function. For this reason, the forgoing conditions regularly require the imposition and/or maintenance of corrective forces on the spine in order to return the spine to its normal alignment.

A surgical technique, commonly referred to as spinal fixation, utilizes surgical implants for fusing together and/or mechanically immobilizing two or more vertebral bodies of the spinal column. Spinal fixation may also be used to alter the alignment of adjacent vertebral bodies relative to one another so as to change the overall alignment of the spinal column.

One common type of spinal fixation device utilizes spinal rods placed generally parallel to the spine and fixation devices, such as pedicle screw assemblies, interconnected between the spinal rods and selected portions of the spine. In some instances, the spinal rods can then be connected to each other via cross-connecting members to provide a more rigid support and alignment system.

Pedicle screw assemblies typically include a bone screw and a housing or coupling element for coupling the bone screw to the spinal rod. Conventional pedicle screws are "top loaded" meaning that assembly of the pedicle screw requires inserting a shank of the bone screw into a proximal end of the housing until the head of the bone screw is retained within the housing and the shank extends from a distal end of the housing. Thus, when securing a conventional pedicle screw to bone, the surgeon must thread the screw into bone while the head of the screw is positioned within the housing.

Despite the improvements that have been made to spinal fixation devices, various drawbacks remain. For example, the housing of a conventional "top loaded" pedicle screw assembly can obstruct a surgeon's vision and/or access while performing operative tasks such as decortication and decompression. This problem is exacerbated by the fact that the housing is subject to "flop" (e.g., unwanted movement) around the head of the screw, which can complicate handling of the pedicle screw assembly, alignment of the housing and fastening of the pedicle screw assembly to bone.

<CIT> relates to a spinal fixation device including a modular head assembly and a bone screw.

A "bottom loaded" or "modular" pedicle screw assembly is provided herein. Among other advantages, the distal end of the modular head assembly is configured to receive the head of the bone screw after the screw has been secured to bone. As a result, the surgeon's vision and access is not impaired while performing necessary operative tasks. Moreover, the modular head assembly includes a biasing member, such as a wave spring, that provides a constant biasing force to the head of the bone screw after the bone screw has been loaded through the bottom of the modular head assembly. The biasing force prevents the housing from "flopping" about the head of the screw, which improves intraoperative handling of the modular pedicle screw and alignment of the pedicle screw relative to the spinal rods and other components of the spinal fixation device.

According to the invention, the spinal fixation device includes a modular head assembly and a bone screw including a head and a shank extending from the head. The modular head assembly includes a housing defining a proximal surface, a distal surface and a throughhole formed therethrough; an anvil slidable within a portion of the throughhole; a biasing member circumferentially surrounding the anvil; an assembly cap secured to the housing 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; a retaining ring positioned at least partially within the cavity and 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 screw. Movement of the retaining ring from the first portion to the second portion compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing.

A method (not claimed) of assembling a spinal fixation device is provided. The method includes: providing a modular head assembly including a housing having a throughhole formed through the housing from a proximal surface of the housing to a distal surface of the housing, an anvil slidable within a portion of the throughhole, a biasing member circumferentially surrounding the anvil, an assembly cap secured to the housing 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 a retaining ring positioned at least partially within the cavity of the assembly cap and transitionable between a first configuration in which the retaining ring is sized to receive a head of a bone screw and a second configuration in which the retaining ring is compressed about the bone screw; securing the bone screw within bone; positioning a bore defined through a distal surface of the assembly cap adjacent the head of the bone screw; advancing the modular head assembly over the head of the bone screw such that the head of the bone screw is received within the bore; moving the retaining ring from the second portion of the cavity to the first portion of the cavity; inserting the head of the bone screw through a lumen defined through distal and proximal surfaces of the retaining ring; and allowing the biasing member to apply a biasing force to the anvil which, in turn, applies a biasing force to the bone screw.

As used herein, when referring to the modular pedicle screw assembly, the term "proximal" means the portion of the assembly or a component thereof that is closer to the clinician and the term "distal" means the portion of the assembly or a component thereof that is furthest from the clinician. Also, as used herein, the terms "substantially," "generally," and "about" are intended to mean that slight deviations from absolute are included within the scope of the term so modified.

<FIG> illustrate a modular pedicle screw assembly <NUM> in accordance with an embodiment of the present disclosure. Pedicle screw <NUM> includes a modular head assembly <NUM> and a bone screw <NUM>. Modular head assembly <NUM> is designed such that bone screw <NUM> can be "bottom loaded" or passed through a distal end of the modular head assembly and fastened to the bone screw after the screw has been implanted in bone.

With specific reference to <FIG>, modular head assembly <NUM> includes a housing <NUM>, an anvil <NUM> slidable within a portion of the housing, a biasing member <NUM> circumferentially coupled about the anvil, a retaining ring <NUM> for fixing the rotational and angular position of the bone screw relative to the housing, and an assembly cap <NUM> for securing the anvil, the biasing member and the retaining ring within the housing. As is explained in further detail herein, after bone screw <NUM> has been loaded through the bottom of modular head assembly <NUM>, biasing member <NUM> provides a constant biasing force to the head of the bone screw and prevents the modular head assembly from "flopping" relative to the bone screw.

Referring to <FIG>, bone screw <NUM> may be a polyaxial bone screw 14a (<FIG>) or a uniplanar bone screw 14b (<FIG>). Bone screw <NUM> includes a head <NUM> provided at a proximal end thereof and a shank <NUM> extending from the head along an axis. Shank <NUM> is formed as an elongated body and extends from a distal tip <NUM> to a proximal end that is coupled (e.g., monolithically formed) to head <NUM>. Distal tip <NUM> is generally conically-shaped to facilitate insertion of the screw <NUM> into bone and, in some embodiments, may be self-starting. The elongated body of shank <NUM> may have a substantially uniform outer diameter upon which a helical thread <NUM> is provided that allows bone screw <NUM> to be threadably inserted and retained within bone. Helical thread <NUM> may be continuous or discontinuous, of uniform or non-uniform pitch, single threaded or double threaded and self-tapping or non-self-tapping depending upon the needs of the procedure being performed. It is also contemplated to include cutting flutes to facilitate implantation into bone. In some embodiments, bone screw <NUM> may be cannulated to permit the passage of a guide wire (not shown) or other instrumentation therethrough. In embodiments in which bone screw <NUM> defines a cannulation, it is contemplated that fenestrations (not shown) may be formed through an outer surface of shank <NUM> and into communication with the cannulation. Such a design may permit the introduction of bone cement or the like after implantation of the screw within bone.

As shown in <FIG> and <FIG>, the head <NUM> of bone screw <NUM> defines a tool engaging recess <NUM> at a proximal portion thereof configured to receive a driving tool (not shown). Tool engaging recess <NUM> may be any suitable shape capable of transmitting a rotational motion of the tool to the head <NUM> of bone screw <NUM>. In one non-limiting embodiment, tool engaging recess <NUM> may be a hexalobe, as described in <CIT>.

When the bone screw is a polyaxial bone screw 14a, the head <NUM> of the screw is generally spherical in shape which assists modular head assembly <NUM> in rotating in multiple axis relative to the bone screw. On the other hand, when the bone screw is a uniplanar bone screw 14b, the head <NUM> of the screw may define at least cutout <NUM> arranged to receive a corresponding feature(s), such as a protrusion(s), provided on a uniplanar anvil 18b (<FIG>) to restrict relative movement between modular head assembly <NUM> and the bone screw to a single plane. For example, the head <NUM> of uniplanar bone screw 14b may define a pair of cutouts <NUM> annularly spaced about the head of the bone screw. As shown in <FIG>, each cutout may form a surface that extends substantially parallel with a longitudinal axis of shaft <NUM>. Thus, when the protrusions of uniplanar anvil 18b are positioned within the cutouts <NUM> of uniplanar bone screw 14b, the coaction between the protrusions and the corresponding flat surfaces restricts relative movement between modular head assembly <NUM> and the bone screw to a single plane.

Turning now to <FIG>, housing <NUM> has a generally cylindrical body with a proximal surface <NUM> and an opposite distal surface <NUM>. Housing <NUM> defines a throughhole <NUM> extending along a longitudinal axis L of the body and between the proximal and distal surfaces of the housing.

The distal surface <NUM> of housing <NUM> defines a counterbore <NUM> that extends towards the proximal surface <NUM> of the housing and terminates at an annular face <NUM> located at a middle portion of the body, although it is contemplated that the counterbore may extend any suitable distance from the distal surface. As shown in <FIG>, the sidewall delineating throughhole <NUM> defines a pair of longitudinally extending slots <NUM> in juxtaposed relationship to one another. Each slot <NUM> terminates at a stop <NUM> and is sized to receive a portion of anvil <NUM>, thereby enabling the anvil to slidably translate along the length of the slot and inhibiting the anvil from rotating within throughhole <NUM>. An inner sidewall, forming a distal portion of counterbore <NUM>, defines an internal threading <NUM> for securing the assembly cap <NUM> to housing <NUM>.

An outer surface of housing <NUM> defines a U-shaped opening <NUM> extending through the proximal surface <NUM> of the body in a transverse direction to throughhole <NUM>. U-shaped opening <NUM> is sized and shaped to receive a spinal rod <NUM> (<FIG>). As shown in <FIG>, an inner sidewall delineating a proximal portion of throughhole <NUM> defines an internal threading <NUM> for threadably receiving a set screw <NUM> (<FIG>) and securing spinal rod <NUM> within the U-shaped opening <NUM> of housing <NUM>. Two reliefs <NUM> are formed in the outer surface of housing <NUM>. The reliefs <NUM> are configured to receive a suitable tool (not shown) and enable a clinician to grasp and manipulate housing <NUM> during a surgical procedure. Housing <NUM> may be formed from any biocompatible material suitable for use in surgical procedures, such as metallic materials including titanium, titanium alloys, stainless steels, cobalt chrome alloys, etc., or non-metallic materials such as ceramics, polyetheretherketone (PEEK), etc..

Assembly cap <NUM>, shown in <FIG>, includes an external threading <NUM> configured to threadably engage the internal threading <NUM> of housing <NUM> to assemble modular head assembly <NUM>. With specific reference to <FIG>, an interior surface of assembly cap <NUM> defines a cavity <NUM> for receiving retaining ring <NUM>. The cavity <NUM> of assembly cap <NUM> is formed by an inwardly tapered sidewall extending from a proximal portion of the assembly cap to a distal portion of the assembly cap. Put differently, the cavity <NUM> of assembly cap <NUM> defines a first portion <NUM> (proximal portion) having a first diameter and a second portion <NUM> (distal portion) having a second diameter smaller than the first portion. As will be described in further detail hereinbelow, the cavity <NUM> of assembly cap <NUM> is thus sized to allow retaining ring <NUM> to expand as the retaining ring translates proximally from the second portion <NUM> of assembly cap <NUM> to the first portion <NUM> of the assembly cap. In contrast, as retaining ring <NUM> translates distally from the first portion <NUM> of assembly cap <NUM> to the second portion <NUM> of the assembly cap, the cavity <NUM> is sized to compress retaining ring <NUM>. The distal end of assembly cap <NUM> may be provided with an inwardly projecting ledge <NUM> to limit the distal movement of retaining ring <NUM> and to prevent bone screw <NUM> from passing distally through modular head assembly <NUM> after the screw has been received therein.

Referring to <FIG>, retaining ring <NUM> has a substantially ring shaped body sized to be slidably received within the cavity <NUM> of assembly cap <NUM> and a shelf <NUM> extending radially outward from a proximal end of the body. Shelf <NUM> has a relatively flat upper surface for engaging anvil <NUM> as retaining ring <NUM> translates within the cavity <NUM> of assembly cap <NUM>. Retaining ring <NUM> is formed of an elastic material, such as an elastic metal, and defines a slit <NUM> extending therethrough from an outer surface of the ring shaped body to the inner surface of the body. In this manner, retaining ring <NUM> is configured to compress upon the application of an external force (e.g., a compressive force applied to an outer surface of the body) and to expand upon the application of an internal force (e.g., an expansion force applied to an inner surface of the cylindrical body). In this regard, retaining ring <NUM> is designed to transition between a neutral (or unexpanded configuration), an expanded configuration in which the retaining ring is sized to receive the head <NUM> of bone screw <NUM> and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring.

As shown in <FIG>, polyaxial anvil 18a has a body defining a proximal surface <NUM>, a distal surface <NUM> and an outer sidewall. The body of polyaxial anvil 18a is sized to slide within a portion of the throughhole <NUM> of housing <NUM>. The proximal surface <NUM> of polyaxial anvil 18a defines a concave profile (e.g., extending toward the distal surface <NUM> of the body) configured to receive a portion of spinal rod <NUM> (<FIG>). The outer surface of polyaxial anvil 18a includes a pair of lugs <NUM> diametrically opposed from one another about the body. Each one of the lugs <NUM> extends in the longitudinal direction and is sized to be received within a corresponding slot <NUM> of housing <NUM> to guide the sliding movement of polyaxial anvil 18a within throughhole <NUM> and to inhibit rotation of the anvil relative to the housing. In this manner, engagement between the lugs <NUM> of anvil 18a and the slots <NUM> of housing <NUM> ensure that the concave proximal surface of the anvil remains aligned with the U-shaped opening <NUM> of the housing to assist in properly aligning spinal rod <NUM> relative to modular head assembly <NUM>.

The distal surface <NUM> of polyaxial anvil 18a defines a concave profile (e.g., extending toward the proximal surface <NUM> of the anvil). The concave profile of the distal surface <NUM> of polyaxial anvil 18a generally corresponds in shape to the spherical head <NUM> of polyaxial bone screw 14a thus allowing modular head assembly <NUM> to freely rotate in multiple directions about the head of the screw. Polyaxial anvil 18a includes an outwardly extending flange <NUM> circumscribing a distal end of the body. The combination of lugs <NUM> and flange <NUM> defines and annular seat <NUM> circumscribing the outer surface of polyaxial anvil 18a. The annular seat is arranged to receive biasing member <NUM> and to couple the biasing member to polyaxial anvil 18a. The bottom surface of flange <NUM> may be substantially flat and configured to engage the shelf <NUM> of retaining ring <NUM>.

Uniplanar anvil 18b, as shown in <FIG>, is substantially similar to polyaxial anvil 18a except that the uniplanar anvil additionally includes at least one protrusion <NUM>. For example, uniplanar anvil 18b may include a pair of protrusions <NUM> extending from the distal surface <NUM> of the anvil in a longitudinal direction and to a location distal of flange <NUM>. As shown in <FIG>, protrusion <NUM> may be circumferentially spaced about the anvil <NUM> degrees from one another and aligned with a respective one of the lugs <NUM>. Protrusions <NUM> are sized and shaped to be positioned within the cutout(s) <NUM> of uniplanar bone screw 14b and to engage with the substantially flat surface of the screw head. The cooperation between the protrusions <NUM> of uniplanar anvil 18b and the head <NUM> of uniplanar bone screw 14b restricts movement of modular head assembly <NUM> relative to the bone screw to a single plane (e.g., the midplane between the protrusions).

As shown in <FIG>, biasing member <NUM> may be a low profiled spring, such as a wave spring, configured to sit within the annular seat <NUM> of anvil <NUM> and to entirely circumscribe the outer surface of the anvil. In this regard, when modular head assembly <NUM> is assembled, the wave spring is positioned to engage the annular face <NUM> of counterbore <NUM> (<FIG>). As a result, when the head <NUM> of bone screw <NUM> is bottom loaded through retaining ring <NUM> and into engagement with anvil <NUM>, the wave spring will impart a biasing force to the bone screw. This biasing force ensures that the distal surface <NUM> of anvil <NUM> applies a constant distally directed force against the head of the bone screw and prevents modular head assembly <NUM> from "flopping" loosely about the head of the bone screw. In this manner, the biasing force affords the clinician greater control while securing modular head assembly <NUM> to bone screw <NUM>. Furthermore, because the wave spring is seated within the annular seat <NUM> of anvil <NUM> prior to inserting the anvil into housing <NUM>, modular head assembly <NUM> can be quickly and, more accurately, assembled without requiring a user to align and load individual springs within a pocket of the housing and/or pocket of the anvil prior to inserting the anvil into the housing in separate and consecutive steps.

<FIG> illustrate modular pedicle screw assembly <NUM> in an assembled state. Irrespective of whether modular head <NUM> encompasses a polyaxial bone screw 14a and a polyaxial anvil 18a, or a uniplanar bone screw 14b and a uniplanar anvil 18b, the modular head is assembled in the same manner. For this reason, the following description of the assembly process refers to the bone screw and the anvil generically as bone screw <NUM> and anvil <NUM>.

Modular head <NUM> may be assembled by a manufacturer or an end user. To assembly modular head <NUM>, spring <NUM> is first snapped over lugs <NUM>, in a proximal to distal direction, and seated within the annular seat <NUM> of anvil <NUM>. In this position, the spring may be slightly biased between lugs <NUM> and flange <NUM> which aids in securing the spring circumferentially about the outer surface of anvil <NUM>. A user may then insert anvil <NUM> through the distal end <NUM> of housing <NUM> and position lugs <NUM> into the corresponding slots <NUM> of the before sliding the anvil proximally within throughhole <NUM> until spring <NUM> engages the annular face <NUM> of counterbore <NUM>. Next, retaining ring <NUM> may be inserted into the cavity <NUM> of assembly cap <NUM>. The external threading <NUM> of assembly cap <NUM> may then be threaded into the internal threading <NUM> of housing <NUM> to threadably secure the assembly cap to the housing and, in turn, to secure anvil <NUM>, spring <NUM> and retaining ring <NUM> within the housing. It will be appreciated, however, that assembly cap <NUM> may be secured to housing <NUM> via welding or any other known coupling mechanism. In this regard, a manufacturer can assemble modular head <NUM> before shipping the modular head to the end user, or alternatively, an end user could assemble the modular head before surgery.

Use of pedicle screw assembly <NUM> to fixate spinal rod <NUM> will now be described. The surgeon may first evaluate the desired placement of spinal rod <NUM> and determine the desired type(s) of bone screws best suited for the operation. Because polyaxial anvil 18a is secured to polyaxial bone screw 14a in substantially the same manner in which uniplanar anvil 18b is secured to uniplanar bone screw 14b, a single generic description of the coupling will be described hereinafter such that specific descriptions pertaining to the polyaxial and uniplanar components are only set forth when describing contrasting features between the modular assemblies.

Bone screw <NUM> is first driven into bone using a driving tool (not shown) by inserting a working end of the driving tool into the tool engaging recess <NUM> of the head <NUM> and rotating the driving tool to thread the screw into bone. With the bone screw <NUM> secured at a desired location, modular head assembly <NUM> may be placed adjacent the head <NUM> of screw <NUM> and advanced in a distal direction over the head of the bone screw. As the head <NUM> of bone screw <NUM> is advanced proximally within throughhole <NUM>, the head of the bone screw contacts retaining ring <NUM> and forces the retaining ring and anvil <NUM> in a proximal direction. More particularly, retaining ring <NUM> translates in a proximal direction from the distal portion <NUM> of assembly cap <NUM> into the proximal portion <NUM> of the assembly cap. The interaction of the lugs <NUM> of anvil <NUM> and the slots <NUM> of housing <NUM> guides proximal movement of the anvil within throughhole <NUM> until the proximal surface <NUM> of the anvil engages the stop <NUM> of the slot. With anvil <NUM> pressed against stop <NUM>, continued application of a distally directed force on modular head assembly <NUM>, will force the head <NUM> of bone screw <NUM> through retaining ring <NUM> and into contact with the concave, distal surface <NUM> of anvil <NUM>. Specifically, the head <NUM> of bone screw <NUM> will place an outwardly directed force on an interior surface of retaining ring <NUM> and cause the elastic retaining ring to transition from a natural configuration to an expanded (e.g., larger diameter) configuration allowing the head of the bone screw to pass completely though the aperture of the retaining ring. It will be appreciated that retaining ring <NUM> is permitted to expand to the expanded configuration, in part, because the retaining ring is disposed within the larger, proximal portion <NUM> of the cavity <NUM> of assembly cap <NUM>. Once the head <NUM> of bone screw <NUM> has completely passed through retaining ring <NUM>, the retaining ring will elastically return to its natural size about the neck (e.g., the junction of the proximal portion of shank <NUM> and the head) of bone screw <NUM>.

With specific reference to <FIG>, when modular head <NUM> is coupled to bone screw <NUM>, the spring <NUM> of anvil <NUM> contact the annular face <NUM> of counterbore <NUM> and imparts a biasing force on the anvil which ensures that the distal surface <NUM> of the anvil applies a constant force to the head <NUM> of the bone screw. The constant force prevents modular head assembly <NUM> from "flopping" loosely about the head <NUM> of the screw <NUM>. In this regard, the orientation and position of modular head assembly <NUM>, relative to bone screw <NUM>, is not altered unless the surgeon intentionally applies a meaningful rotational force to the modular head assembly. As a result, the biasing force affords the clinician greater control and the ability to make minor adjustments in the position of the modular head assembly relative to the screw.

If pedicle screw assembly <NUM> includes a polyaxial bone screw 14a and a polyaxial anvil 18a, the spherically shaped head <NUM> of the polyaxial bone screw will permit the concave distal surface <NUM> of the anvil to rotate about the head in multiple directions, thereby allowing the surgeon to adjust the position of modular head assembly <NUM> relative the bone screw in multiple axis. In contrast, if pedicle screw assembly <NUM> includes a uniplanar bone screw 14b and a uniplanar anvil 18b, the protrusions <NUM> of the uniplanar anvil will be positioned within cutouts <NUM> and engaged with the opposing flat surface of the head <NUM> of the uniplanar screw, thereby restricting the surgeon's ability to adjust modular head assembly <NUM> relative to the bone screw to a single axis.

Referring now to <FIG>, spinal rod <NUM> may then be interconnected between adjacent modular head assemblies <NUM> by inserting the spinal rod within the U-shaped openings <NUM> of each housing <NUM> and within the concave relief of the proximal surface <NUM> of anvil <NUM>. Again, the biasing force imparted by spring <NUM> will prevent modular head assemblies <NUM> from rotating relative to their bone screws <NUM> during placement of the spinal rod <NUM> (e.g., as a result of gravitations forces and/or minor forces imparted by the rod itself). In this regard, spring <NUM> assists in more efficiently and more accurately assembling the fixation device.

With spinal rod <NUM> properly positioned between modular head assemblies <NUM>, the surgeon may then use a driving tool to thread set screw <NUM> into the threads <NUM> of housing <NUM>, which in turn, forces the spinal rod, anvil <NUM> and retaining ring <NUM> to translate in a distal direction within the throughhole <NUM> of the housing. As retaining ring <NUM> moves in the distal direction, from the proximal portion <NUM> of cavity <NUM> to the distal portion <NUM> of the cavity, the inwardly tapered sidewall forming the cavity will impart an inwardly directed force on an outer surface of the retaining ring and cause the retaining ring to transition to the compressed configuration and clamp around the neck of bone screw <NUM>, thereby fixing the rotational and angular position of the bone screw relative to housing <NUM> and preventing the bone screw from passing through a distal end of the housing.

To summarize the foregoing, a spinal fixation device includes a bone screw having a head and a shank extending from the head; and a modular head assembly that includes 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 biasing member circumferentially surrounding the anvil; an assembly cap secured to the housing and including 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; a retaining ring positioned at least partially within the cavity of the assembly cap and 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, whereby movement of the retaining ring from the first portion to the second portion compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing; and/or.

Claim 1:
A spinal fixation device (<NUM>), comprising:
a bone screw (<NUM>) including head (<NUM>) and a shank (<NUM>) extending from the head;
a modular head assembly (<NUM>), comprising:
a housing (<NUM>) having a proximal surface (<NUM>), a distal surface (<NUM>), and a throughhole (<NUM>) formed through the housing from the proximal surface to the distal surface;
an anvil (<NUM>) slidable within a portion of the throughhole;
an assembly cap (<NUM>) secured to the housing, the assembly cap including an inner surface defining a cavity (<NUM>) having a first portion (<NUM>) and a second portion (<NUM>), the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter;
a retaining ring (<NUM>) 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,
wherein movement of the retaining ring from the first portion to the second portion compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing,
characterized in that the modular head assembly further comprises a biasing member (<NUM>) circumferentially surrounding an outer surface of the anvil and arranged to impart a distally directed force on the head of the bone screw as the modular head assembly is passed over the head of the bone screw and the head of the bone screw is received within the throughole.