Modular head assembly for spinal fixation

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

BACKGROUND OF THE INVENTION

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 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.

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.

BRIEF SUMMARY OF THE INVENTION

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.

One embodiment of 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 modular head assembly may include 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.

In another embodiment, a spinal fixation kit is 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.

In yet another embodiment, a variant spinal fixation kit is 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.

DETAILED DESCRIPTION

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.

FIGS.1A-1Cillustrate a pedicle screw assembly10in accordance with an embodiment of the present disclosure. Pedicle screw10includes a modular head assembly12and a bone screw14. Modular head assembly12is designed such that bone screw14can 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 toFIG.1C, modular head assembly12includes a housing16, an anvil18, a cap20and a retaining ring22. Anvil18may be a polyaxial anvil18a, a uniplanar anvil18b, or a transverse uniplanar anvil18c(FIGS.6A-8D). Put differently, the polyaxial anvil18a, the uniplanar anvil18b, or the transverse uniplanar anvil18cmay be used in conjunction with housing16, cap20and retaining ring22to form modular head assembly12. When modular head assembly12includes polyaxial anvil18a, the modular head assembly will be permitted to freely rotate (e.g., move in multiple axis) relative to bone screw14. On the other hand, when uniplanar anvil18b, or transverse uniplanar anvil18care utilized, the movement of modular head assembly12will be restricted to a single axis relative to bone screw14and, more specifically, an axis in a front-back direction and an axis in a lateral-lateral direction, respectively.

Turning now toFIGS.2A and2B, bone screw14includes a head24provided at a proximal end thereof and a shank26extending distally from the head along an axis A. Shank26is formed as an elongated body and extends from a distal tip28to a proximal end that is coupled (e.g., monolithically formed) to head24. Distal tip28is generally conically-shaped to facilitate insertion of the screw14into bone and, in some embodiments, may be self-starting. The elongated body may have a substantially uniform outer diameter upon which a helical thread30is provided that allows bone screw14to be threadably inserted and retained within bone. Helical thread30may 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. In some embodiments, it is contemplated that the bone screw14may 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 head24of bone screw14is generally spherical in shape and defines a groove32extending in a length direction of the screw between a neck of the screw head and a proximal end of the screw head. Groove32is generally aligned with the longitudinal axis A of shank26and defines a first lateral wall and a second lateral wall. Thus, when a corresponding feature (e.g., a protrusion) provided on uniplanar anvil18bor transverse uniplanar anvil18c(FIGS.7A-8D) is disposed within groove32(e.g., between the first and second lateral walls), the coaction of the corresponding features is designed to restrict movement of modular head assembly12to a single plane when the modular head assembly is secured to screw12. The head24of bone screw14also defines a tool engaging recess34at a proximal portion thereof configured to receive a driving tool (not shown). Tool engaging recess34may be any suitable shape capable of transmitting a rotational motion of the tool to the head24of bone screw12. In one non-limiting embodiment, tool engaging recess34may be a hexalobe.

Housing16, as shown inFIGS.3A-3D, includes a body having a generally cylindrical profile with a proximal surface36and an opposite, distal surface38. Housing16defines a throughhole40extending along a longitudinal axis L of the body and between the proximal surface36and the distal surface38of the housing. An inner surface of the proximal portion of the through-hole40includes a thread42configured to threadably engage a set screw44(FIG.9A) for securing a spinal rod to modular head assembly12.

A counterbore46is formed in the distal surface38of housing16. Counterbore46extends towards the proximal surface36of housing16and terminates at an annular face48located 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 counterbore46is formed to have a greater diameter than an inner surface of the proximal portion of throughhole40such that anvil18can be received therein. An inner surface of throughhole40defines a pair of longitudinally extending slots50formed in juxtaposed relation to one another. Each slot50terminates at a stop51(FIG.1C) and is sized to receive a correspondingly shaped feature on anvil18to enable the anvil to slidably translate along the slot and to inhibit rotation of the anvil18within throughhole40.

An outer surface of housing16defines a U-shaped opening52extending through the proximal surface36of the body and transverse to throughhole40. U-shaped opening52is configured to receive spinal rod54(FIGS.9A and9B). Two reliefs55are formed in the outer surface of housing16. The reliefs55are configured to receive a suitable tool (not shown) and enable a clinician to grasp and manipulate housing16during a surgical procedure. An outer surface of the distal portion of housing16includes a thread56or flange for coupling the housing to cap20. Housing16may 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 cap20, shown inFIGS.4A-4D, includes a corresponding discontinuous thread58or flange for engaging the thread56on housing16in order to facilitate assembly of modular head assembly12. Assembly cap20forms a retaining ring receiving portion defined by a proximal portion60and a distal portion62. An interior surface of the distal portion62is tapered inwardly from the proximal portion60to a distal end of assembly cap20. As a result, the retaining ring receiving portion is configured to slidably receive retaining ring22as will be described in further detail hereinbelow.

Referring toFIGS.5A-5D, retaining ring22has a ring shaped body with two arms64extending proximally from an upper surface of the cylindrical body. The arms64of retaining ring22are sized to fit within a gap of discontinuous thread58allowing the retaining ring to slidably translate within the retaining ring receiving portion of cap20and to inhibit rotation of the retaining ring within the retaining ring receiving portion.

Retaining ring22is formed of an elastic material, such as an elastic metal, and defines a slit65extending therethrough from an outer surface of the cylindrical body to the inner surface of the cylindrical body. In this manner, retaining ring22is 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 ring22is designed to transition between an expanded configuration in which the retaining ring is sized to receive the head24of bone screw14and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring.

When the head24of bone screw14is passed through the distal end of cap20(e.g., bottom loaded) and into engagement with retaining ring22, the head of the bone screw forces the retaining ring to slide proximally within the retaining ring receiving portion. When retaining ring22is disposed within the larger proximal portion60of the retaining ring receiving portion, an internal force applied by the head24of bone screw14forces 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 head24of bone screw12has passed through retaining ring22, the retaining ring will elastically transition back to its natural configuration around the neck (e.g., the junction of the proximal portion of shank26and the head) of the bone screw. Conversely, when set screw44is threaded distally into housing16, the set screw applies a distally directed force which causes retaining ring22to translate from the proximal portion60of the retaining ring receiving portion into the distal portion62of the retaining ring receiving portion. As a result, the tapered surface of the distal portion62of the retaining ring receiving portion applies a compressive force on the outer surface of retaining ring22and causes the retaining ring to compress around the neck of bone screw14to a diameter less than then the diameter of the retaining ring in its natural configuration. The reduced diameter of retaining ring22prevents the head24of screw14from passing distally through modular head assembly12.

Referring toFIGS.6A-6D, polyaxial anvil18ahas a body that is sized to be slidably received within the throughbore40of housing16. The body of polyaxial anvil18ahas a proximal surface68that defines a concave profile (e.g., extending toward a distal surface70of the body) configured to receive a portion of the spinal rod54(FIGS.9A and9B). An outer surface of polyaxial anvil18aincludes a pair of lugs66diametrically opposed from one another about the body. Each one of the lugs66extends in the longitudinal direction and is sized to be is received within a corresponding slot50of housing16to guide the sliding movement of the polyaxial anvil within throughhole40and to inhibit rotation of the anvil relative to the housing. In this manner, engagement between the lugs66of anvil18aand the slots50of housing16ensure that the concave proximal surface of the anvil remains aligned with the U-shaped opening52of the housing in order to receive spinal rod54. The distal surface70of polyaxial anvil18adefines a concave profile (e.g., extending toward the proximal surface68of the anvil). The concave profile of the distal surface70of polyaxial anvil18agenerally corresponds in shape to the spherical head24of bone screw14thus allowing modular head assembly12to freely rotate in multiple directions about the head of the screw. For this reason, the distal surface70of polyaxial anvil18ais sometimes referred to herein as the “contact surface.”

A plurality of leaf springs72are attached to and at least partially circumscribe the outer surface of polyaxial anvil18a. Leaf springs72are thus positioned to engage the annular face48of counterbore46. As a result, when the head24of bone screw14is bottom loaded through retaining ring22and into engagement with the contact surface of the bone screw, leaf springs72impart a biasing force to the bone screw. This biasing force ensures that the contact surface of polyaxial anvil18aapplies a constant distally directed force against the head of the bone screw and prevents modular head assembly12from “flopping” loosely about the head of the bone screw. As a result, the biasing force affords the clinician greater control while securing modular head assembly12to bone screw24. The fact that leaf springs72are attached to (or monolithically formed with) polyaxial anvil18afacilitates efficient assembly of modular head assembly12as the springs do not need to be aligned and loaded within housing16separately from the anvil. As shown inFIG.6D, a bore is defined along a longitudinal axis of polyaxial anvil18aand between the proximal and distal ends of the anvil.

Uniplanar anvil18b, as shown inFIGS.7A-7D, is substantially similar to polyaxial anvil18aexcept that the uniplanar anvil additionally includes a pair of longitudinally aligned protrusions74bprovided on the contact surface of the uniplanar anvil. Each protrusion74bis diametrically opposed about the bore from the other protrusion and spaced 90 degrees from each one of the lugs66. Protrusions74bare sized and shaped to be positioned within the groove32defined in the head24of bone screw14. In this regard, when the groove32of bone screw14receives the protrusion74bof uniplanar anvil18b, movement of modular head assembly12relative to the bone screw is restricted to a single plane (e.g., the plane along which the protrusion extends).

Transverse uniplanar anvil18cis shown inFIGS.8A-8D. Transverse uniplanar anvil18cis formed substantially similar to uniplanar anvil18bbut for the location of protrusions74c. That is, the protrusions74cof transverse uniplanar anvil18care rotated 90 degrees about the contact surface relative to the protrusions74bof uniplanar anvil18b. The protrusions74cof anvil18care thus configured to restrict relative movement between the modular head assembly12and bone screw14a single plane extending orthogonal to the U-shaped opening52of housing16.

A spinal fixation kit is also provided herein. The spinal fixation kit includes, inter alia, one or more modular head assemblies12, one or more bone screws14, one or more spinal rods54and one or more set screws44. Each of the one or more modular head assemblies12may 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 anvil18a, at least one modular head assembly utilizing a uniplanar anvil18aand at least one modular head assembly utilizing a transverse uniplanar anvil18c. In this regard, the clinician can determine whether to attach a polyaxial, uniplanar or transverse uniplanar modular head to a previously implanted bone screw14based upon intraoperative considerations.

FIGS.9A and9Billustrate modular pedicle screw10in an assembled state. Irrespective of whether modular head12is formed with a polyaxial anvil18a, a uniplanar anvil18b, or a transverse uniplanar anvil18c, 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 anvil18.

Modular head12is assembled by aligning the lugs66of anvil18with the corresponding slots50of housing16. Once lugs66are in alignment with slots50, the anvil is proximally advanced into counterbore46until leaf springs72engage the annular face48of the counterbore. Next, retaining ring22is inserted into the retaining ring receiving portion of assembly cap20such that the arms64of the retaining ring are positioned within a gap of the discontinuous thread58on assembly cap20and a lower surface of the body of the retaining ring is positioned within the distal portion62of the assembly cap. The thread58on assembly cap20is then threaded to the thread56disposed on the distal end of housing16to threadably secure the cap to the housing. In some embodiments, as shown inFIG.1C, pedicle screw10may also include assembly pins76which are inserted through apertures in the distal portion of housing16and welded to the housing to secure assembly cap20to the housing. It will be appreciated, however, that assembly of modular head12need not include the step of welding assembly pins76to housing16. Instead, cap20may be secured to housing16only by coupling the threads58on the cap to the thread56on the housing or via another known coupling mechanisms. In this regard, it is contemplated that a manufacturer can assemble modular head12before shipping the modular head to the end user, or alternatively, an end user could assemble the modular head before use.

Use of pedicle screw10to fixate spinal rod54will now be described. Bone screw14is first driven into bone using a driving tool (not shown) by inserting a working end of the driving tool into the tool engaging recess34of the screw and rotating the driving tool to thread the screw into bone. With the bone screw14secured 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 rod54. Because each one of the polyaxial, uniplanar and transverse uniplanar modular head assemblies are secured to screw14in 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 assembly12in hand, the modular head assembly may be placed adjacent the head24of screw14and advanced in a distal direction over the head of the bone screw. As the head24of bone screw14is advanced proximally within throughbore40, the head of the bone screw contacts retaining ring22and forces the retaining ring and anvil18to translate in a proximal direction and from the distal portion62of cap20into a proximal portion60of the cap. The interaction of the lugs66of anvil18and the slots50of housing16guides movement of the anvil within throughbore40as the anvil slides in the proximal direction until the proximal end68of the anvil engages the stop51of the slot. With anvil18pressed against stop51, continued application of a distally directed force on modular head12, will force the head24of bone screw14through retaining ring22and into contact with the concave, distal surface70of anvil18. Specifically, the spherical shape of the head24of bone screw14will place an outwardly directed force on an interior surface of retaining ring22and 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 ring22is permitted to expand to the expanded configuration, in part, because the retaining ring is disposed within the larger, proximal portion60of the retaining ring receiving portion of cap20. Once the head24of bone screw14has completely passed through retaining ring22, the retaining ring will elastically return to its natural size about the neck of bone screw14.

If modular head assembly12includes a polyaxial anvil18a, the spherically shaped head24of bone screw14will be permitted to freely rotate about the concave distal surface70of the anvil in multiple directions relative to the bone screw. In contrast, if modular head assembly12includes a uniplanar anvil18bor a transverse uniplanar anvil18c, engagement between the groove32of screw14and the protrusion74b,74cof 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 head12is coupled to bone screw14, the leaf springs72of anvil18contact the annular face48of counterbore46and impart a biasing force on the bone screw which ensures that the contact surface of the anvil applies a constant distal pressure to the head24of bone screw14. The biasing force prevents modular head assembly12from “flopping” loosely about the head of the screw. In this regard, modular head assembly12is held in place relative to screw14such 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 toFIGS.9A and9B, spinal rod54may then be interconnected between adjacent modular head assemblies by inserting the spinal rod within the U-shaped openings52of each housing16and within the concave relief of the proximal surface68of anvil18. Using a driving tool, the surgeon may then thread set screw44into the threads42of housing16, which in turn, forces spinal rod54, anvil18and retaining ring22in a distal direction within the counterbore40of the housing. Distal translation of anvil18urges retaining ring22, along with the bone screw14captured therewithin, to translate into the distal portion62of the retaining ring receiving portion of cap20. The tapered sidewall and smaller diameter of the distal portions62of cap20imparts an inwardly directed force on an outer surface of retaining ring22and causes the retaining ring to compress and clamp around the neck of bone screw14, thereby fixing the rotational and angular position of the bone screw relative to housing16and preventing the bone screw from passing through a distal end of the housing.

FIGS.10and11illustrate a modular pedicle screw assembly100in accordance with another embodiment of the present disclosure. Pedicle screw100includes a bottom loading modular head assembly112and a bone screw114.

Bone screw114is substantially similar to previously described bone screw14(FIGS.2A and2B) and, for brevity, is not described again in detail hereinafter. Instead, when like features are referenced in connection with bone screw114, the previously described features will be renumbered with sequential one hundred series numerals. In some embodiments, however, bone screw114may additionally define a series of friction enhancing notches133that extend about the head124of the bone screw in a circumferential direction as illustrated inFIGS.15A-15C. Circumferential notches133are designed to cooperate with features on the anvil to create friction so that modular head assembly112is poseable and to also assist in retaining screw114within the modular head assembly.

Modular head assembly112includes a housing116and an anvil118. Unlike modular head assembly12(which relies on the absence or presence of a protrusion on the anvil), modular head assembly112relies on the absence or presence of a protrusion extending through or from on a polyaxial housing116a, a uniplanar housing116bor a transverse uniplanar housing116cfor performing the same function.

As shown inFIGS.11,13,14and15A-15C, polyaxial housing116a, uniplanar housing116b, and transverse uniplanar housing116care 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 numeral116. Housing116includes a body having a generally cylindrical profile with a proximal surface136and an opposite, distal surface138. Housing116defines a throughhole140extending along a longitudinal axis L′ of the body and between the proximal surface136and the distal surface138of the housing. An inner surface of the proximal portion of the through-hole140includes threading142configured to threadably engage a set screw for securing a spinal rod to modular head assembly112.

The distal surface138of housing116defines a counterbore146having a greater diameter than an inner surface of the proximal portion of throughhole140such that anvil118can be received therein. Counterbore146extends towards the proximal surface136of housing116and terminates at a middle portion of the housing. A pair of lugs166extend through an aperture formed through the sidewall of housing116and into counterbore146. Each one of lugs166are spaced 180 degrees about the inner surface of throughbore140from the other lug and configured to engage a corresponding feature, such as a slot on anvil118, to allow the anvil to slidably translate within throughbore140and to inhibit rotation of the anvil within the throughbore. It will be appreciated, however, that lugs166may extend from an inner sidewall of housing116instead of extending therethrough.

Counterbore146forms an anvil retaining space having a proximal portion160and a distal portion162. An inner surface of distal portion162is tapered inwardly from proximal portion160to the distal surface162of housing116. As a result, anvil118is configured to slide within counterbore146and to receive and clamp bone screw114as described below. The inner surface of proximal portion160defines a series of detents147annularly spaced about the inner surface of counterbore146for temporarily securing anvil118thereby preventing the anvil from sliding within the counterbore until the anvil is forced from the detent.

The outer surface of housing116defines a U-shaped opening152extending through the proximal surface136of the body and transverse to throughhole140. As a result, U-shaped opening152is configured to receive a spinal rod. Two reliefs155are formed in the outer surface of housing. The reliefs155are configured to receive a suitable tool (not shown) and enable a clinician to grasp and manipulate housing116during a surgical procedure.

Anvil118, as shown inFIG.12, has a body that is sized to be slidably received within counterbore146of housing116. The body of118has a proximal surface168that defines a concave profile (e.g., extending toward a distal surface170of the body) such that it is configured to receive a spinal rod. A pair of diametrically opposed slots150are defined through the outer surface of the proximal portion of anvil118. Each slot150extends in the longitudinal direction and is sized to receive the lugs166of housing116to guide the sliding movement of anvil118within counterbore146and to inhibit rotation of the anvil relative to housing116. In this regard, the lugs166of housing116and the slots150of anvil118coact to ensure that the concave relief of the anvil remains aligned with the U-shaped opening152of the housing to receive a spinal rod. The outer surface of anvil118includes a series of ledges165annular spaced about the anvil. The ledges165are correspondingly shaped to be received by the detents147of housing116and allow the housing to temporarily secure anvil118within a proximal portion of counterbore146when the ledges of the anvil are disposed within the detents.

The distal surface170of anvil118defines a concave surface (e.g., extending toward the proximal surface168of the anvil). The concave profile of the distal surface170of anvil118generally corresponds in shape to the spherical head124of bone screw114thus allowing modular head assembly112to freely rotate in multiple directions about the head of the bone screw unless otherwise prevented by another feature.

Anvil118further includes a plurality of flanges178annularly spaced about a perimeter of the anvil and extending from the distal surface170in a distal direction. The plurality of flanges178collectively form a receiving cavity for receiving the head124of bone screw112. Each one of the plurality of flanges178is 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, flanges178are flexibly moveable between an outwardly flexed or expanded configuration in which the receiving cavity is sized to receive the head124of the bone screw114and 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 surface138of housing116.

The clamping of the head124of bone screw114is assisted by an inwardly extending lip180provided at a distal end of each one of the flanges178. The lips180are configured to engage the circumferential notches133of bone screw114to more securely clamp the screw. Adjacently spaced flanges form a gap182that allows protrusion174b,174cto extend therethrough and into a groove132of bone screw114.

With specific reference toFIGS.15A-15C, a polyaxial housing116a, a uniplanar housing116b, or a transverse uniplanar housing116cmay be used in conjunction with anvil118to form a polyaxial modular head assembly, a uniplanar modular head assembly, or a polyaxial modular head assembly, respectively. Polyaxial housing116ais devoid of protrusions capable of engaging the head124of bone screw114and, 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, protrusion174bextends through an aperture formed in the distal portion of uniplanar housing116b, through the gap182formed between adjacent flanges178and into the groove132of bone screw114to restrict rotation of the housing to a single axis (e.g., front-back relative to the U-shaped opening152of the housing) relative to the bone screw. Similarly, a protrusion174cextends through an aperture formed in the distal portion of housing116c, through the gap182formed between adjacent flanges178and into the groove132of bone screw114to restrict rotation of the housing to a single axis relative to the bone screw (e.g., lateral-lateral relative to the U-shaped opening152of the housing).

A spinal fixation kit formed of various modular head assemblies112is also contemplated. The spinal fixation kit includes one or more modular head assemblies112, one or more bone screws114, one or more spinal rods and one or more set screws. Each of the one or more modular head assemblies112may 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 housing116a, at least one modular head assembly including uniplanar housing116band at least one modular head assembly including transverse uniplanar housing116c. In this regard, the clinician can determine which modular head assembly to secure to a previously implanted bone screw114based upon intraoperative considerations and after the bone screw has been implanted.

Use of pedicle screw100to fixate a spinal rod will now be described. Bone screw114is 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 assembly112may then be advanced in a distal direction over the head124of bone screw114such that the head of the bone screw is advanced proximally within throughbore140and into contact with the lips180of flanges178. The interaction between the ledges165of anvil118and the detents147of counterbore146prevents the anvil from translating relative to housing116. Continued application of a distally directed force on modular head112will forces the head124of bone screw114into the receiving cavity of anvil118and into contact with the concave distal surface170of the anvil. More particularly, the spherically shaped bone screw head places an outwardly directed force on the lips180of flanges178and causes each of the flanges to flex outwardly such that the receiving cavity expands in diameter and allows the head124of bone screw114to be received therein. Once the largest diameter of the head124of bone screw114has passed over the lips180of flanges178, the flanges will elastically return to their natural configuration and the lips will engage the circumferential notches133formed on the bone screw head to secure the head within the cavity. It will be appreciated that flanges178are permitted to flex, in part, because the flanges are disposed within the larger, proximal portion160of the counterbore146of housing116.

If modular head assembly112includes a polyaxial housing116a, the spherically shaped head124of bone screw114will be permitted to freely rotate about the concave distal surface170of anvil118in multiple directions relative to the bone screw. In contrast, if modular head assembly112includes a uniplanar housing116bor a transverse uniplanar housing116c, the interaction between the protrusions174b,174cof 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 screw114. More particularly, uniplanar housing116bwill restrict movement of the modular head assembly to a front-back direction while transverse uniplanar housing116cwill 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 openings152of each housing116and within the concave relief of the proximal surface168of anvil118. Using a driving tool, the surgeon may then thread a set screw (not shown) to threads142of housing116. As the set screw is threaded into housing116in the distal direction, the set screw exerts a distal directed force on anvil118and urges the ledges165from detents147. Further threading of the set screw causes the spinal rod, anvil118and bone screw114to slide from a proximal portion160of the counterbore146into a distal portion162of the counterbore. The smaller diameter of distal portions162imparts an inwardly directed force on flanges178and causes the flanges to flex inwardly such that the lips180of the flanges clamp around the head124of bone screw114, thereby fixing the rotational and angular position of the bone screw relative to housing116and preventing the bone screw from passing through a distal end of the housing.

FIG.16illustrates a modular pedicle screw assembly200in accordance with yet another embodiment of the present disclosure. Modular pedicle screw assembly200is similar to modular pedicle screw assembly10but for the differences described hereinafter. When like features are referenced in connection with modular pedicle screw assembly200, the previously described features are renumbered with sequential two hundred series numerals.

Pedicle screw assembly200includes a bottom loading modular head assembly212and a bone screw214. Modular head assembly212includes a housing216, an anvil218, a cap220, a retaining ring222and a biasing member223. Housing216is formed generally similar to housing16but instead of defining longitudinal slots50, a pair of guide posts250are formed on an inner surface of the wall defining the throughhole240of housing216in juxtaposed relation to one another. Each guide post250terminates at a stop251and is sized to cooperate with a correspondingly shaped recess on anvil218to enable the anvil to slidably translate along the guide post and to inhibit rotation of the anvil218within throughhole240.

Assembly cap220includes an external threading258configured to threadably engage the threading256of housing216to assemble modular head assembly212. Of course, other connection mechanisms such as a press fit connection may be used. Like assembly cap20, an interior surface of assembly cap220defines a cavity for receiving retaining ring222. The cavity of assembly cap220defines a first portion260(proximal portion) having a first diameter and a second portion262(distal portion) having a second diameter smaller than the first portion to allow retaining ring222to 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 ring222translates distally from the first portion260of assembly cap220to the second portion262of the assembly cap, the cavity is sized to compress retaining ring222.

Retaining ring222has a substantially ring shaped body sized to be slidably received within the cavity of assembly cap220and a shelf264extending radially outward from a proximal end of the body. Shelf264has a relatively flat upper surface for engaging anvil218as retaining ring222translates within the cavity of assembly cap220. Retaining ring222is formed of an elastic material, such as an elastic metal, or plastic, and defines a slit265extending therethrough from an outer surface of the ring-shaped body to the inner surface of the body. In this manner, retaining ring222is 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 ring222is designed to transition between a neutral (or unexpanded configuration), an expanded configuration in which the retaining ring is sized to receive the head224of bone screw214and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring.

Anvil218is formed similarly to anvil18and may be a polyaxial anvil, a uniplanar anvil, or a transverse uniplanar anvil (as such structures are detailed above). Put differently, anvil218may be a polyaxial anvil and have a smooth, concave, distal surface that, when used in conjunction with modular head assembly212, will allow the modular head assembly to freely rotate (e.g., move in multiple axis) relative to bone screw214. On the other hand, anvil218may be uniplanar anvil or a transverse uniplanar anvil that includes protrusions similar to protrusions74bof uniplanar anvil18bor protrusion74cof transverse uniplanar anvil18cthat cooperate with groove232to restrict movement of modular head assembly212to a single axis relative to bone screw14and, more specifically, an axis in a front-back direction and an axis in a lateral-lateral direction, respectively.

With reference toFIG.17, unlike anvil18, anvil218does not include leaf springs that circumscribe the outer surface of the anvil. Instead, anvil218, as shown, include leaf springs272that extend from a distal end of the anvil. In this regard, leaf springs272are positioned to engage the shelf264of retaining ring222. As a result, when the head224of bone screw214is bottom loaded through retaining ring222and into engagement with the distal surface of the anvil, leaf springs272impart a biasing force on the retaining ring222to bias the retaining ring toward the distal portion262of cap220and prevent the head224of bone screw212from accidentally passing through the retaining ring as the modular head assembly is being manipulated.

Biasing member223is a low profiled spring, such as a wave spring, configured to sit around anvil218and to entirely circumscribe the outer surface of the anvil. When modular head assembly212is assembled, the wave spring is positioned to engage the annular face of the counterbore of the housing. As a result, when the head224of bone screw214is bottom loaded through retaining ring222and into engagement with anvil218, the wave spring will impart a biasing force to the bone screw. This biasing force ensures that the distal surface of anvil18applies a constant distally directed force against the head of the bone screw and prevents modular head assembly212from “flopping” loosely about the head of the bone screw. In this manner, the biasing force affords the clinician greater control while securing modular head assembly212to bone screw224. Furthermore, because the wave spring can be seated about anvil218prior to inserting the anvil into housing216, modular head assembly212can 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 assembly200is assembled and used largely as described above with respect to modular pedicle screw assembly10. For brevity, only the differences are described hereinafter. First, the surgeon (or other user such as a manufacturer), seats biasing member223around anvil218and slides the anvil into housing216. Specifically, the recesses of anvil218are aligned about the guide posts250of housing216and the anvil is slid in a proximal direction. Retaining ring222may then be inserted into the220and before the cap is threaded to housing216, thereby securing the anvil, the retaining ring, and the biasing member within the housing.

Next, with bone screw214secured at a desired location (e.g., implanted in a pedicle of a patient), modular head assembly212may be placed adjacent the head224of the bone screw and advanced in a distal direction over the head of the screw. As the head224of bone screw214is advanced proximally within throughhole240, the head of the bone screw contacts retaining ring222and forces the retaining ring and anvil218in a proximal direction which, in turn, compresses biasing member223between the annular face of the counterbore and a flange of anvil218. Distal movement of modular head assembly212relative to bone screw214will cause the head224of the bone screw to apply an outwardly directed force on retaining ring222and result in the resilient retaining ring transitioning from the neutral (or unexpanded) configuration to the expanded configuration. Retaining ring222is permitted to expand to the expanded configuration, in part, because the retaining ring is disposed within the larger proximal portion260of assembly cap220.

Further movement of the modular head assembly212in the distal direction relative to bone screw214, causes the head224of the bone screw to pass completely through the lumen of the retaining ring and into the contact with the distal surface of anvil218. Again, due to biasing member223, the distal surface of anvil18applies a constant distally directed force against the head224of bone screw214and prevents modular head assembly212from “flopping” loosely about the head of the bone screw.

Once the head224of bone screw214has passed completely through retaining ring222, the retaining ring will elastically return to its neutral or unexpanded size. Leaf springs272will then immediately force retaining ring222toward the distal portion262of housing216and cause the retaining ring to transition from the neutral (unexpanded) configuration to a slightly compressed configuration, thereby preventing the head224of bone screw214from passing distally through the retaining ring as the surgeon manipulates housing216. Put differently, if leaf spring272did not bias retaining ring222toward the distal portion262of retaining cap220, certain manipulation techniques may cause the retaining ring to move proximally within the cap (via contact with the bone screw) thereby transitioning the retaining ring back to the neutral configuration and making the head224of bone screw214susceptible to incidentally passing distally through the retaining ring.