Patent Description:
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 often "pinches" or applies pressure to the underlying spinal cord and nerves, which can result 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 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. Pedicle screws generally come in two forms: a polyaxial pedicle screw (which allows the housing to freely rotate relative to the screw head) and a uniplanar pedicle screw (which restricts movement of the housing relative to the screw head to a single plane).

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. Document <CIT> discloses a spinal fixation device including a modular head assembly and a bone screw. The modular head assembly includes a housing, an anvil, an insert, and a snap ring. The housing defines a proximal surface and an opposite, distal surface, and the proximal and distal surfaces define a throughbore therethrough. The anvil is configured to be slidably received within a portion of the throughbore. The insert defines a proximal surface and an opposite, distal surface, and the distal surface defines a first counterbore therein that terminates at a first annular surface. The first annular surface defines a second counterbore that terminates at a second annular surface. The snap ring is configured to be disposed within the first counterbore of the insert when in a first configuration, and within the second counterbore of the insert when in a second configuration.

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" 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 to bone. Moreover, a surgeon may find it desirable to select between a polyaxial and a uniplanar pedicle screw, based upon various intraoperative considerations, after the screw has been secured to bone. However, switching from a conventional "top loaded" polyaxial pedicle screw to a conventional "top loaded" uniplanar pedicle screw (or vice-versa), after implantation, is not desirable because it requires removal of the previously implanted pedicle screw which can weaken the bone.

The invention is set out in claim <NUM>, dependent claims defining preferred embodiments. Various "bottom loaded" or "modular" pedicle screw assemblies are provided herein. Among other advantages, the distal end of each one of the modular heads is configured to receive the head of the bone screw after the bone 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 bone screw defines a feature, such as a groove, designed to selectively engage a corresponding feature, such as a protrusion, provided on select modular head assemblies to restrict movement of the modular head assembly to a single plane relative to the bone screw. Put another way, a kit of differently configured modular head assemblies can include at least one first modular head assembly without the corresponding feature (e.g., the protrusion) and at least one second modular head assembly provided with the corresponding feature. In this manner, a surgeon can select and secure one of the first or second modular head assemblies to the bone screw to create a polyaxial or uniplanar pedicle screw after the bone screw has been implanted into bone and without necessitating removal of the screw from bone. Furthermore, the first and second modular housings may include a biasing member, such as a leaf spring, that provides a constant biasing force to the head of the bone screw and prevents the housing from "flopping" on the screw head which improves handling and alignment of the modular pedicle screw.

The pedicle screw assembly includes a bone screw and a modular head assembly. The bone screw has a head and a shank extending from the head, the head defining at least one groove. The modular head assembly includes a housing and an anvil. The housing has a proximal end and a distal end, and defines a throughbore extending along a longitudinal axis of the housing between the proximal and distal ends. The anvil is disposed within the throughbore. The anvil includes at least one protrusion sized and shaped to be received by the at least one groove for restricting movement of the bone screw to a single plane.

A spinal fixation kit may be provided. The spinal fixation kit includes: a bone screw having a head and a shank extending from the head, the head defining at least one groove; a first modular head assembly and a second modular head assembly. The first modular head assembly including a first housing having a first proximal end and a first distal end, and defining a first throughbore extending between the first proximal and first distal ends; and a first anvil disposed within the first throughbore, wherein the first housing is configured to receive the bone screw and allow the first housing to move in multiple axis relative to the bone screw. The second modular head assembly including a second housing having a second proximal end and a second distal end, and defining a second throughbore extending between the second proximal and second distal ends; and a second anvil disposed within the second throughbore, the second anvil including a protrusion sized to be received by the at least one groove of the bone screw to restrict movement of the second housing relative to the bone screw to a single plane.

A variant spinal fixation kit may be provided. The variant spinal fixation kit includes: a bone screw having a head and a shank extending from the head, the head defining at least one groove; a first modular head assembly; and a second modular head assembly. The first modular head assembly includes a first housing having a first proximal end and a first distal end, and defining a first throughbore extending between the first proximal and first distal ends; and a first anvil disposed within the first throughbore, wherein the first housing is configured to receive the bone screw and allow the first housing to move in multiple axis relative to the bone screw. The second modular head assembly includes a second housing having a second proximal end and a second distal end, and defining a second throughbore extending between the second proximal and distal ends; and a second anvil disposed within the second throughbore, the second modular head assembly including a protrusion sized to be received by the at least one groove of the bone screw to restrict movement of the second housing relative to the bone screw to a single plane.

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 pedicle screw assembly <NUM>. 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 reference to <FIG>, modular head assembly <NUM> includes a housing <NUM>, an anvil <NUM>, a cap <NUM> and a retaining ring <NUM>. Anvil <NUM> may be a polyaxial anvil 18a, a uniplanar anvil 18b, or a transverse uniplanar anvil 18c (<FIG>). Put differently, the polyaxial anvil 18a, the uniplanar anvil 18b, or the transverse uniplanar anvil 18c may be used in conjunction with housing <NUM>, cap <NUM> and retaining ring <NUM> to form modular head assembly <NUM>. When modular head assembly <NUM> includes polyaxial anvil 18a, the modular head assembly will be permitted to freely rotate (e.g., move in multiple axis) relative to bone screw <NUM>. On the other hand, when uniplanar anvil 18b, or transverse uniplanar anvil 18c are utilized, the movement of modular head assembly <NUM> will be restricted to a single axis relative to bone screw <NUM> and, more specifically, an axis in a front-back direction and an axis in a lateral-lateral direction, respectively.

Turning now to <FIG>, bone screw <NUM> includes a head <NUM> provided at a proximal end thereof and a shank <NUM> extending distally from the head along an axis A. 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 may be self-starting. The elongated body 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 contemplated that the bone screw <NUM> may be cannulated to permit the passage of a guide wire (not shown) or other instrumentation therethrough. It is also contemplated to provide fenestrations that are fluidly connected to any included cannulation. Such a design may permit the introduction of bone cement or the like after the implantation of the screw within the bone.

The head <NUM> of bone screw <NUM> is generally spherical in shape and defines a groove <NUM> extending in a length direction of the screw between a neck of the screw head and a proximal end of the screw head. Groove <NUM> is generally aligned with the longitudinal axis A of shank <NUM> and defines a first lateral wall and a second lateral wall. Thus, when a corresponding feature (e.g., a protrusion) provided on uniplanar anvil 18b or transverse uniplanar anvil 18c (<FIG>) is disposed within groove <NUM> (e.g., between the first and second lateral walls), the coaction of the corresponding features is designed to restrict movement of modular head assembly <NUM> to a single plane when the modular head assembly is secured to screw <NUM>. The head <NUM> of bone screw <NUM> also 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>. Tool engaging recess <NUM> may be a hexalobe.

Housing <NUM>, as shown in <FIG>, includes a body having a generally cylindrical profile 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 surface <NUM> and the distal surface <NUM> of the housing. An inner surface of the proximal portion of the through-hole <NUM> includes a thread <NUM> configured to threadably engage a set screw <NUM> (<FIG>) for securing a spinal rod to modular head assembly <NUM>.

A counterbore <NUM> is formed in the distal surface <NUM> of housing <NUM>. Counterbore <NUM> extends towards the proximal surface <NUM> of housing <NUM> and terminates at an annular face <NUM> located at a middle portion of the housing, although it is contemplated that the counterbore may extend any suitable distance from the distal surface. An inner surface of counterbore <NUM> is formed to have a greater diameter than an inner surface of the proximal portion of throughhole <NUM> such that anvil <NUM> can be received therein. An inner surface of throughhole <NUM> defines a pair of longitudinally extending slots <NUM> formed in juxtaposed relation to one another. Each slot <NUM> terminates at a stop <NUM> (<FIG>) and is sized to receive a correspondingly shaped feature on anvil <NUM> to enable the anvil to slidably translate along the slot and to inhibit rotation of the anvil <NUM> within throughhole <NUM>.

An outer surface of housing <NUM> defines a U-shaped opening <NUM> extending through the proximal surface <NUM> of the body and transverse to throughhole <NUM>. U-shaped opening <NUM> is configured to receive spinal rod <NUM> (<FIG>). 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. An outer surface of the distal portion of housing <NUM> includes a thread <NUM> or flange for coupling the housing to cap <NUM>. 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 a corresponding discontinuous thread <NUM> or flange for engaging the thread <NUM> on housing <NUM> in order to facilitate assembly of modular head assembly <NUM>. Assembly cap <NUM> forms a retaining ring receiving portion defined by a proximal portion <NUM> and a distal portion <NUM>. An interior surface of the distal portion <NUM> is tapered inwardly from the proximal portion <NUM> to a distal end of assembly cap <NUM>. As a result, the retaining ring receiving portion is configured to slidably receive retaining ring <NUM> as will be described in further detail hereinbelow.

Referring to <FIG>, retaining ring <NUM> has a ring shaped body with two arms <NUM> extending proximally from an upper surface of the cylindrical body. The arms <NUM> of retaining ring <NUM> are sized to fit within a gap of discontinuous thread <NUM> allowing the retaining ring to slidably translate within the retaining ring receiving portion of cap <NUM> and to inhibit rotation of the retaining ring within the retaining ring receiving portion.

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 cylindrical body to the inner surface of the cylindrical body. In this manner, retaining ring <NUM> is configured to expand and compress upon the application of an external force (e.g., a compressive force applied to an outer surface of the cylindrical body) or 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 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.

When the head <NUM> of bone screw <NUM> is passed through the distal end of cap <NUM> (e.g., bottom loaded) and into engagement with retaining ring <NUM>, the head of the bone screw forces the retaining ring to slide proximally within the retaining ring receiving portion. When retaining ring <NUM> is disposed within the larger proximal portion <NUM> of the retaining ring receiving portion, an internal force applied by the head <NUM> of bone screw <NUM> forces the retaining ring to expand to a diameter greater than its natural configuration and allows the head of the screw to pass proximally through the retaining ring. After the head <NUM> of bone screw <NUM> has passed through retaining ring <NUM>, the retaining ring will elastically transition back to its natural configuration around the neck (e.g., the junction of the proximal portion of shank <NUM> and the head) of the bone screw. Conversely, when set screw <NUM> is threaded distally into housing <NUM>, the set screw applies a distally directed force which causes retaining ring <NUM> to translate from the proximal portion <NUM> of the retaining ring receiving portion into the distal portion <NUM> of the retaining ring receiving portion. As a result, the tapered surface of the distal portion <NUM> of the retaining ring receiving portion applies a compressive force on the outer surface of retaining ring <NUM> and causes the retaining ring to compress around the neck of bone screw <NUM> to a diameter less than then the diameter of the retaining ring in its natural configuration. The reduced diameter of retaining ring <NUM> prevents the head <NUM> of screw <NUM> from passing distally through modular head assembly <NUM>.

Referring to <FIG>, polyaxial anvil 18a has a body that is sized to be slidably received within the throughbore <NUM> of housing <NUM>. The body of polyaxial anvil 18a has a proximal surface <NUM> that defines a concave profile (e.g., extending toward a distal surface <NUM> of the body) configured to receive a portion of the spinal rod <NUM> (<FIG>). An 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 the polyaxial anvil 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 in order to receive spinal rod <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 bone screw <NUM> thus allowing modular head assembly <NUM> to freely rotate in multiple directions about the head of the screw. For this reason, the distal surface <NUM> of polyaxial anvil 18a is sometimes referred to herein as the "contact surface.

A plurality of leaf springs <NUM> are attached to and at least partially circumscribe the outer surface of polyaxial anvil 18a. Leaf springs <NUM> are thus positioned to engage the annular face <NUM> of counterbore <NUM>. As a result, when the head <NUM> of bone screw <NUM> is bottom loaded through retaining ring <NUM> and into engagement with the contact surface of the bone screw, leaf springs <NUM> impart a biasing force to the bone screw. This biasing force ensures that the contact surface of polyaxial anvil 18a 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. As a result, the biasing force affords the clinician greater control while securing modular head assembly <NUM> to bone screw <NUM>. The fact that leaf springs <NUM> are attached to (or monolithically formed with) polyaxial anvil 18a facilitates efficient assembly of modular head assembly <NUM> as the springs do not need to be aligned and loaded within housing <NUM> separately from the anvil. As shown in <FIG>, a bore is defined along a longitudinal axis of polyaxial anvil 18a and between the proximal and distal ends of the anvil.

Uniplanar anvil 18b, as shown in <FIG>, is substantially similar to polyaxial anvil 18a except that the uniplanar anvil additionally includes a pair of longitudinally aligned protrusions 74b provided on the contact surface of the uniplanar anvil. Each protrusion 74b is diametrically opposed about the bore from the other protrusion and spaced <NUM> degrees from each one of the lugs <NUM>. Protrusions 74b are sized and shaped to be positioned within the groove <NUM> defined in the head <NUM> of bone screw <NUM>. In this regard, when the groove <NUM> of bone screw <NUM> receives the protrusion 74b of uniplanar anvil 18b, movement of modular head assembly <NUM> relative to the bone screw is restricted to a single plane (e.g., the plane along which the protrusion extends).

Transverse uniplanar anvil 18c is shown in <FIG>. Transverse uniplanar anvil 18c is formed substantially similar to uniplanar anvil 18b but for the location of protrusions 74c. That is, the protrusions 74c of transverse uniplanar anvil 18c are rotated <NUM> degrees about the contact surface relative to the protrusions 74b of uniplanar anvil 18b. The protrusions 74c of anvil 18c are thus configured to restrict relative movement between the modular head assembly <NUM> and bone screw <NUM> a single plane extending orthogonal to the U-shaped opening <NUM> of housing <NUM>.

A spinal fixation kit is also provided herein. The spinal fixation kit includes, inter alia, one or more modular head assemblies <NUM>, one or more bone screws <NUM>, one or more spinal rods <NUM> and one or more set screws <NUM>. Each of the one or more modular head assemblies <NUM> may be similarly configured, such that they contain the same anvil, or differently configured, such that the anvils have different configurations. For example, the spinal fixation kit may include at least one modular head assembly utilizing a polyaxial anvil 18a, at least one modular head assembly utilizing a uniplanar anvil 18a and at least one modular head assembly utilizing a transverse uniplanar anvil 18c. In this regard, the clinician can determine whether to attach a polyaxial, uniplanar or transverse uniplanar modular head to a previously implanted bone screw <NUM> based upon intraoperative considerations.

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

Modular head <NUM> is assembled by aligning the lugs <NUM> of anvil <NUM> with the corresponding slots <NUM> of housing <NUM>. Once lugs <NUM> are in alignment with slots <NUM>, the anvil is proximally advanced into counterbore <NUM> until leaf springs <NUM> engage the annular face <NUM> of the counterbore. Next, retaining ring <NUM> is inserted into the retaining ring receiving portion of assembly cap <NUM> such that the arms <NUM> of the retaining ring are positioned within a gap of the discontinuous thread <NUM> on assembly cap <NUM> and a lower surface of the body of the retaining ring is positioned within the distal portion <NUM> of the assembly cap. The thread <NUM> on assembly cap <NUM> is then threaded to the thread <NUM> disposed on the distal end of housing <NUM> to threadably secure the cap to the housing. As shown in <FIG>, pedicle screw <NUM> may also include assembly pins <NUM> which are inserted through apertures in the distal portion of housing <NUM> and welded to the housing to secure assembly cap <NUM> to the housing. It will be appreciated, however, that assembly of modular head <NUM> need not include the step of welding assembly pins <NUM> to housing <NUM>. Instead, cap <NUM> may be secured to housing <NUM> only by coupling the threads <NUM> on the cap to the thread <NUM> on the housing or via another known coupling mechanisms. In this regard, it is contemplated that 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 use.

Use of pedicle screw <NUM> to fixate spinal rod <NUM> will now be described. 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 screw and rotating the driving tool to thread the screw into bone. With the bone screw <NUM> secured at a desired location, the surgeon may select one of the polyaxial, uniplanar or transverse uniplanar head assemblies to couple to the screw in view of intraoperative considerations including the exact placement and orientation of the screw(s) and the desired placement of the spinal rod <NUM>. Because each one of the polyaxial, uniplanar and transverse uniplanar modular head assemblies are secured to screw <NUM> in a substantially similar manner, a single generic description of the coupling will be described hereinafter such that specific descriptions of the polyaxial, uniplanar and transverse uniplanar modular head assemblies are only set forth when describing contrasting features between the modular assemblies.

With the selected modular head assembly <NUM> in hand, the modular head assembly 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 throughbore <NUM>, the head of the bone screw contacts retaining ring <NUM> and forces the retaining ring and anvil <NUM> to translate in a proximal direction and from the distal portion <NUM> of cap <NUM> into a proximal portion <NUM> of the cap. The interaction of the lugs <NUM> of anvil <NUM> and the slots <NUM> of housing <NUM> guides movement of the anvil within throughbore <NUM> as the anvil slides in the proximal direction until the proximal end <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 <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 spherical shape of the head <NUM> of bone screw <NUM> will place an outwardly directed force on an interior surface of retaining ring <NUM> and causes 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 retaining ring receiving portion of 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 of bone screw <NUM>.

If modular head assembly <NUM> includes a polyaxial anvil 18a, the spherically shaped head <NUM> of bone screw <NUM> will be permitted to freely rotate about the concave distal surface <NUM> of the anvil in multiple directions relative to the bone screw. In contrast, if modular head assembly <NUM> includes a uniplanar anvil 18b or a transverse uniplanar anvil 18c, engagement between the groove <NUM> of screw <NUM> and the protrusion 74b, 74c of uniplanar anvil or transverse uniplanar anvil, respectively, will restrict movement of the modular head assembly to a single axis relative to the bone screw. More particularly, the uniplanar anvil will restrict movement of the housing in a front-back direction while the transverse uniplanar anvil will restrict movement of the housing in a lateral-lateral direction (e.g., orthogonal to the front-back direction).

When modular head <NUM> is coupled to bone screw <NUM>, the leaf springs <NUM> of anvil <NUM> contact the annular face <NUM> of counterbore <NUM> and impart a biasing force on the bone screw which ensures that the contact surface of the anvil applies a constant distal pressure to the head <NUM> of bone screw <NUM>. The biasing force prevents modular head assembly <NUM> from "flopping" loosely about the head of the screw. In this regard, modular head assembly <NUM> is held in place relative to screw <NUM> such that the surgeon has to apply a rotational force to the modular head assembly in order to reposition the modular head assembly relative to the bone screw. 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.

Referring now to <FIG>, spinal rod <NUM> may then be interconnected between adjacent modular head assemblies 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>. Using a driving tool, the surgeon may then thread set screw <NUM> into the threads <NUM> of housing <NUM>, which in turn, forces spinal rod <NUM>, anvil <NUM> and retaining ring <NUM> in a distal direction within the counterbore <NUM> of the housing. Distal translation of anvil <NUM> urges retaining ring <NUM>, along with the bone screw <NUM> captured therewithin, to translate into the distal portion <NUM> of the retaining ring receiving portion of cap <NUM>. The tapered sidewall and smaller diameter of the distal portions <NUM> of cap <NUM> imparts an inwardly directed force on an outer surface of retaining ring <NUM> and causes the retaining ring to compress 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.

<FIG> illustrate a modular pedicle screw assembly <NUM>. Pedicle screw <NUM> includes a bottom loading modular head assembly <NUM> and a bone screw <NUM>.

Bone screw <NUM> is substantially similar to previously described bone screw <NUM> (<FIG>) and, for brevity, is not described again in detail hereinafter. Instead, when like features are referenced in connection with bone screw <NUM>, the previously described features will be renumbered with sequential one hundred series numerals. However, bone screw <NUM> may additionally define a series of friction enhancing notches <NUM> that extend about the head <NUM> of the bone screw in a circumferential direction as illustrated in <FIG>. Circumferential notches <NUM> are designed to cooperate with features on the anvil to create friction so that modular head assembly <NUM> is poseable and to also assist in retaining screw <NUM> within the modular head assembly.

Modular head assembly <NUM> includes a housing <NUM> and an anvil <NUM>. Unlike modular head assembly <NUM> (which relies on the absence or presence of a protrusion on the anvil), modular head assembly <NUM> relies on the absence or presence of a protrusion extending through or from on a polyaxial housing 116a, a uniplanar housing 116b or a transverse uniplanar housing 116c for performing the same function.

As shown in <FIG>, <FIG>, <FIG>, polyaxial housing 116a, uniplanar housing 116b, and transverse uniplanar housing 116c are substantially the same as one another but for the presence and/or location of the protrusion. For this reason, a single description of the housing is set forth below with the housing generically referenced by numeral <NUM>. Housing <NUM> includes a body having a generally cylindrical profile 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 surface <NUM> and the distal surface <NUM> of the housing. An inner surface of the proximal portion of the through-hole <NUM> includes threading <NUM> configured to threadably engage a set screw for securing a spinal rod to modular head assembly <NUM>.

The distal surface <NUM> of housing <NUM> defines a counterbore <NUM> having a greater diameter than an inner surface of the proximal portion of throughhole <NUM> such that anvil <NUM> can be received therein. Counterbore <NUM> extends towards the proximal surface <NUM> of housing <NUM> and terminates at a middle portion of the housing. A pair of lugs <NUM> extend through an aperture formed through the sidewall of housing <NUM> and into counterbore <NUM>. Each one of lugs <NUM> are spaced <NUM> degrees about the inner surface of throughbore <NUM> from the other lug and configured to engage a corresponding feature, such as a slot on anvil <NUM>, to allow the anvil to slidably translate within throughbore <NUM> and to inhibit rotation of the anvil within the throughbore. It will be appreciated, however, that lugs <NUM> may extend from an inner sidewall of housing <NUM> instead of extending therethrough.

Counterbore <NUM> forms an anvil retaining space having a proximal portion <NUM> and a distal portion <NUM>. An inner surface of distal portion <NUM> is tapered inwardly from proximal portion <NUM> to the distal surface <NUM> of housing <NUM>. As a result, anvil <NUM> is configured to slide within counterbore <NUM> and to receive and clamp bone screw <NUM> as described below. The inner surface of proximal portion <NUM> defines a series of detents <NUM> annularly spaced about the inner surface of counterbore <NUM> for temporarily securing anvil <NUM> thereby preventing the anvil from sliding within the counterbore until the anvil is forced from the detent.

The outer surface of housing <NUM> defines a U-shaped opening <NUM> extending through the proximal surface <NUM> of the body and transverse to throughhole <NUM>. As a result, U-shaped opening <NUM> is configured to receive a spinal rod. Two reliefs <NUM> are formed in the outer surface of housing. 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.

Anvil <NUM>, as shown in <FIG>, has a body that is sized to be slidably received within counterbore <NUM> of housing <NUM>. The body of <NUM> has a proximal surface <NUM> that defines a concave profile (e.g., extending toward a distal surface <NUM> of the body) such that it is configured to receive a spinal rod. A pair of diametrically opposed slots <NUM> are defined through the outer surface of the proximal portion of anvil <NUM>. Each slot <NUM> extends in the longitudinal direction and is sized to receive the lugs <NUM> of housing <NUM> to guide the sliding movement of anvil <NUM> within counterbore <NUM> and to inhibit rotation of the anvil relative to housing <NUM>. In this regard, the lugs <NUM> of housing <NUM> and the slots <NUM> of anvil <NUM> coact to ensure that the concave relief of the anvil remains aligned with the U-shaped opening <NUM> of the housing to receive a spinal rod. The outer surface of anvil <NUM> includes a series of ledges <NUM> annular spaced about the anvil. The ledges <NUM> are correspondingly shaped to be received by the detents <NUM> of housing <NUM> and allow the housing to temporarily secure anvil <NUM> within a proximal portion of counterbore <NUM> when the ledges of the anvil are disposed within the detents.

The distal surface <NUM> of anvil <NUM> defines a concave surface (e.g., extending toward the proximal surface <NUM> of the anvil). The concave profile of the distal surface <NUM> of anvil <NUM> generally corresponds in shape to the spherical head <NUM> of bone screw <NUM> thus allowing modular head assembly <NUM> to freely rotate in multiple directions about the head of the bone screw unless otherwise prevented by another feature.

Anvil <NUM> further includes a plurality of flanges <NUM> annularly spaced about a perimeter of the anvil and extending from the distal surface <NUM> in a distal direction. The plurality of flanges <NUM> collectively form a receiving cavity for receiving the head <NUM> of bone screw <NUM>. Each one of the plurality of flanges <NUM> is formed of a flexible and resilient material that allows the flange to flex outwardly and inwardly and then to return to its natural configuration (e.g. parallel to the longitudinal axis of the anvil). Thus, flanges <NUM> are flexibly moveable between an outwardly flexed or expanded configuration in which the receiving cavity is sized to receive the head <NUM> of the bone screw <NUM> and an inwardly flexed or compressed configuration in which the flanges clamp the head of the screw to lock the screw in position and prevent the screw from passing distally through the distal surface <NUM> of housing <NUM>.

The clamping of the head <NUM> of bone screw <NUM> is assisted by an inwardly extending lip <NUM> provided at a distal end of each one of the flanges <NUM>. The lips <NUM> are configured to engage the circumferential notches <NUM> of bone screw <NUM> to more securely clamp the screw. Adjacently spaced flanges form a gap <NUM> that allows protrusion 174b, 174c to extend therethrough and into a groove <NUM> of bone screw <NUM>.

With specific reference to <FIG>, a polyaxial housing 116a, a uniplanar housing 116b, or a transverse uniplanar housing 116c may be used in conjunction with anvil <NUM> to form a polyaxial modular head assembly, a uniplanar modular head assembly, or a polyaxial modular head assembly, respectively. Polyaxial housing 116a is devoid of protrusions capable of engaging the head <NUM> of bone screw <NUM> and, as a result, the polyaxial head is permitted to freely rotate (e.g., move in multiple axis) relative to the bone screw. On the other hand, protrusion 174b extends through an aperture formed in the distal portion of uniplanar housing 116b, through the gap <NUM> formed between adjacent flanges <NUM> and into the groove <NUM> of bone screw <NUM> to restrict rotation of the housing to a single axis (e.g., front-back relative to the U-shaped opening <NUM> of the housing) relative to the bone screw. Similarly, a protrusion 174c extends through an aperture formed in the distal portion of housing 116c, through the gap <NUM> formed between adjacent flanges <NUM> and into the groove <NUM> of bone screw <NUM> to restrict rotation of the housing to a single axis relative to the bone screw (e.g., lateral-lateral relative to the U-shaped opening <NUM> of the housing).

A spinal fixation kit formed of various modular head assemblies <NUM> is also contemplated. The spinal fixation kit includes one or more modular head assemblies <NUM>, one or more bone screws <NUM>, one or more spinal rods and one or more set screws. Each of the one or more modular head assemblies <NUM> may be similarly configured, such that they contain the same housing, or differently configured, such that the housings have different configurations. For example, the spinal fixation kit may include at least one modular head assembly including polyaxial housing 116a, at least one modular head assembly including uniplanar housing 116b and at least one modular head assembly including transverse uniplanar housing 116c. In this regard, the clinician can determine which modular head assembly to secure to a previously implanted bone screw <NUM> based upon intraoperative considerations and after the bone screw has been implanted.

Use of pedicle screw <NUM> to fixate a spinal rod will now be described. Bone screw <NUM> is first driven into bone at a desired location. The surgeon may then select one of the polyaxial, uniplanar or transverse uniplanar modular head assemblies to couple to the bone screw in view of intraoperative considerations including the exact placement and/or orientation of the bone screw(s) and the desired placement of the spinal rod.

Modular head assembly <NUM> may then be advanced in a distal direction over the head <NUM> of bone screw <NUM> such that the head of the bone screw is advanced proximally within throughbore <NUM> and into contact with the lips <NUM> of flanges <NUM>. The interaction between the ledges <NUM> of anvil <NUM> and the detents <NUM> of counterbore <NUM> prevents the anvil from translating relative to housing <NUM>. Continued application of a distally directed force on modular head <NUM> will forces the head <NUM> of bone screw <NUM> into the receiving cavity of anvil <NUM> and into contact with the concave distal surface <NUM> of the anvil. More particularly, the spherically shaped bone screw head places an outwardly directed force on the lips <NUM> of flanges <NUM> and causes each of the flanges to flex outwardly such that the receiving cavity expands in diameter and allows the head <NUM> of bone screw <NUM> to be received therein. Once the largest diameter of the head <NUM> of bone screw <NUM> has passed over the lips <NUM> of flanges <NUM>, the flanges will elastically return to their natural configuration and the lips will engage the circumferential notches <NUM> formed on the bone screw head to secure the head within the cavity. It will be appreciated that flanges <NUM> are permitted to flex, in part, because the flanges are disposed within the larger, proximal portion <NUM> of the counterbore <NUM> of housing <NUM>.

If modular head assembly <NUM> includes a polyaxial housing 116a, the spherically shaped head <NUM> of bone screw <NUM> will be permitted to freely rotate about the concave distal surface <NUM> of anvil <NUM> in multiple directions relative to the bone screw. In contrast, if modular head assembly <NUM> includes a uniplanar housing 116b or a transverse uniplanar housing 116c, the interaction between the protrusions 174b, 174c of the uniplanar housing or the transverse uniplanar housing, respectively, will restrict movement of the modular head assembly to a single axis relative to the bone screw <NUM>. More particularly, uniplanar housing 116b will restrict movement of the modular head assembly to a front-back direction while transverse uniplanar housing 116c will restrict movement of the modular head assembly to a lateral-lateral direction (e.g., orthogonal to the front-back direction).

A spinal rod (not shown) may then be interconnected between adjacent modular head assemblies 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>. Using a driving tool, the surgeon may then thread a set screw (not shown) to threads <NUM> of housing <NUM>. As the set screw is threaded into housing <NUM> in the distal direction, the set screw exerts a distal directed force on anvil <NUM> and urges the ledges <NUM> from detents <NUM>. Further threading of the set screw causes the spinal rod, anvil <NUM> and bone screw <NUM> to slide from a proximal portion <NUM> of the counterbore <NUM> into a distal portion <NUM> of the counterbore. The smaller diameter of distal portions <NUM> imparts an inwardly directed force on flanges <NUM> and causes the flanges to flex inwardly such that the lips <NUM> of the flanges clamp around the head <NUM> 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.

<FIG> illustrates a modular pedicle screw assembly <NUM> in accordance with an embodiment. Modular pedicle screw assembly <NUM> is similar to modular pedicle screw assembly <NUM> but for the differences described hereinafter. When like features are referenced in connection with modular pedicle screw assembly <NUM>, the previously described features are renumbered with sequential two hundred series numerals.

Pedicle screw assembly <NUM> includes a bottom loading modular head assembly <NUM> and a bone screw <NUM>. Modular head assembly <NUM> includes a housing <NUM>, an anvil <NUM>, a cap <NUM>, a retaining ring <NUM> and a biasing member <NUM>. Housing <NUM> is formed generally similar to housing <NUM> but instead of defining longitudinal slots <NUM>, a pair of guide posts <NUM> are formed on an inner surface of the wall defining the throughhole <NUM> of housing <NUM> in juxtaposed relation to one another. Each guide post <NUM> terminates at a stop <NUM> and is sized to cooperate with a correspondingly shaped recess on anvil <NUM> to enable the anvil to slidably translate along the guide post and to inhibit rotation of the anvil <NUM> within throughhole <NUM>.

Assembly cap <NUM> includes an external threading <NUM> configured to threadably engage the threading <NUM> of housing <NUM> to assemble modular head assembly <NUM>. Of course, other connection mechanisms such as a press fit connection may be used. Like assembly cap <NUM>, an interior surface of assembly cap <NUM> defines a cavity for receiving retaining ring <NUM>. The cavity 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 to allow retaining ring <NUM> to expand as the retaining ring translates proximally from the second portion of assembly cap to the first portion 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 is sized to compress retaining ring <NUM>.

Retaining ring <NUM> has a substantially ring shaped body sized to be slidably received within the cavity 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 of assembly cap <NUM>. Retaining ring <NUM> is formed of an elastic material, such as an elastic metal, or plastic, 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.

Anvil <NUM> is formed similarly to anvil <NUM> and is a uniplanar anvil, or a transverse uniplanar anvil (as such structures are detailed above). Anvil <NUM> is a uniplanar anvil or a transverse uniplanar anvil that includes protrusions similar to protrusions 74b of uniplanar anvil 18b or protrusion 74c of transverse uniplanar anvil 18c that cooperate with groove <NUM> to restrict movement of modular head assembly <NUM> to a single axis relative to bone screw <NUM> and, more specifically, an axis in a front-back direction and an axis in a lateral-lateral direction, respectively.

With reference to <FIG>, unlike anvil <NUM>, anvil <NUM> does not include leaf springs that circumscribe the outer surface of the anvil. Instead, anvil <NUM>, as shown, include leaf springs <NUM> that extend from a distal end of the anvil. In this regard, leaf springs <NUM> are positioned to engage the shelf <NUM> of retaining ring <NUM>. As a result, when the head <NUM> of bone screw <NUM> is bottom loaded through retaining ring <NUM> and into engagement with the distal surface of the anvil, leaf springs <NUM> impart a biasing force on the retaining ring <NUM> to bias the retaining ring toward the distal portion <NUM> of cap <NUM> and prevent the head <NUM> of bone screw <NUM> from accidentally passing through the retaining ring as the modular head assembly is being manipulated.

Biasing member <NUM> is a low profiled spring, such as a wave spring, configured to sit around anvil <NUM> and to entirely circumscribe the outer surface of the anvil. When modular head assembly <NUM> is assembled, the wave spring is positioned to engage the annular face of the counterbore of the housing. 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 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 can be seated about 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.

Modular pedicle screw assembly <NUM> is assembled and used largely as described above with respect to modular pedicle screw assembly <NUM>. For brevity, only the differences are described hereinafter. First, the surgeon (or other user such as a manufacturer), seats biasing member <NUM> around anvil <NUM> and slides the anvil into housing <NUM>. Specifically, the recesses of anvil <NUM> are aligned about the guide posts <NUM> of housing <NUM> and the anvil is slid in a proximal direction. Retaining ring <NUM> may then be inserted into the <NUM> and before the cap is threaded to housing <NUM>, thereby securing the anvil, the retaining ring, and the biasing member within the housing.

Next, with bone screw <NUM> secured at a desired location (e.g., implanted in a pedicle of a patient), modular head assembly <NUM> may be placed adjacent the head <NUM> of the bone screw and advanced in a distal direction over the head of the 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 which, in turn, compresses biasing member <NUM> between the annular face of the counterbore and a flange of anvil <NUM>. Distal movement of modular head assembly <NUM> relative to bone screw <NUM> will cause the head <NUM> of the bone screw to apply an outwardly directed force on retaining ring <NUM> and result in the resilient retaining ring transitioning from the neutral (or unexpanded) configuration to the expanded configuration. 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 assembly cap <NUM>.

Further movement of the modular head assembly <NUM> in the distal direction relative to bone screw <NUM>, causes the head <NUM> of the bone screw to pass completely through the lumen of the retaining ring and into the contact with the distal surface of anvil <NUM>. Again, due to biasing member <NUM>, the distal surface of anvil <NUM> applies a constant distally directed force against the head <NUM> of bone screw <NUM> and prevents modular head assembly <NUM> from "flopping" loosely about the head of the bone screw.

Claim 1:
A pedicle screw assembly (<NUM>), comprising:
a bone screw (<NUM>) having a head (<NUM>) and a shank, the head defining at least one groove (<NUM>) extending in a length direction of the bone screw (<NUM>) between a neck of the head (<NUM>) and a proximal end of the head (<NUM>); and
a modular head assembly (<NUM>) comprising:
a housing (<NUM>) having a proximal surface and a distal surface, the housing defining a through hole (<NUM>) extending along a longitudinal axis of the housing and between the proximal and distal surfaces of the housing (<NUM>), a counterbore being formed in the distal surface of the housing (<NUM>), extending towards the proximal surface of the housing (<NUM>) and terminating at an annular face, an inner surface of the counterbore being formed to have a greater diameter than an inner surface of the proximal portion of the throughole (<NUM>);
a cap (<NUM>) coupled to the housing (<NUM>), the cap (<NUM>) forming a retaining ring receiving portion having a proximal portion (<NUM>) defining a first diameter and a distal portion (<NUM>) defining a second diameter smaller than the first diameter;
a retaining ring (<NUM>) formed of a resilient material disposed within the retaining ring receiving portion, the retaining ring (<NUM>) being transitionable between an expanded configuration in which the retaining ring is sized to receive the head (<NUM>) of the bone screw (<NUM>) and a compressed configuration in which the retaining ring (<NUM>) prevents the head (<NUM>) of the bone screw (<NUM>) from passing distally through the retaining ring (<NUM>);
an anvil (<NUM>) received in the counterbore of the housing (<NUM>), the anvil including a leaf spring (<NUM>) extending from a distal end thereof and at least one protrusion sized and shaped to be received by the at least one groove (<NUM>) for restricting movement of the bone screw (<NUM>) to a single plane, wherein the leaf spring (<NUM>) is arranged to bias the retaining ring (<NUM>) toward the distal portion (<NUM>) of the retaining ring receiving portion after the head (<NUM>) of the bone screw (<NUM>) has been uploaded through the retaining ring (<NUM>); and
a biasing member (<NUM>) circumferentially disposed about the anvil and positioned to engage the annular face of the counterbore of the housing (<NUM>).