Modular polyaxial pedicle screw system

There is provided a modular polyaxial pedicle screw assembly that includes various components which may be configured in various manners so as to provide different functionalities to the pedicle screw assembly. This advantageously decreases surgery time, reduces repetitive and tedious surgical steps, and allows for streamlining inventory of expensive medical equipment. In one embodiment, a pedicle screw assembly includes a pedicle screw, a rod holding element, a polyaxial insert, a rod, and a set screw. The pedicle screw includes a threaded shaft and a cap. The rod holding element includes a screw hole, an insert bearing area, a chamber, chamber walls, a saddle area, and a threading area. The insert is disposed within the chamber, and the insert defines a bearing surface, an upper surface, side walls and a receiving area. The insert is positioned within the chamber such that the insert bearing surface contacts the rod holding element insert bearing area; additionally the side walls of the insert can bear against the chamber walls. The pedicle screw is positioned so that the screw shaft passes through the screw hole of the rod holding element and the screw cap rests within the receiving area of the insert. The rod is disposed so as to rest on the upper surface of the insert; and the set screw, joined to the threading area of the rod holding element, secures the rod to the upper surface of the insert.

FIELD OF THE INVENTION

The present invention relates to medical devices. More particularly, the invention relates to pedicle screw systems and methods for use in spinal fixation surgeries and therapies.

BACKGROUND OF THE INVENTION

Spinal surgery procedures often require securing various implants to vertebrae of the spine. One such implant is the pedicle screw and its related components. Other components, such as rods, are then secured to individual pedicle screw implants in order to provide a support or fixation function between and among neighboring vertebrae. Both the rods and screws may have varying diameters and dimensions depending on patient and therapeutic needs. Due to the complex curvature and anatomy of the spine it is difficult to align the bone screw and rod holder assembly with the rod, particularly when spanning multiple segments. Traditionally, this required extensive bending and test fitting of rods to correctly approximate the rod holding portion of the pedicle screw. More recently, the polyaxial screw type has become widely available, which allows the rod receiving portion of the screw to pivot about the screw head. The pivoting head allows the rod holder to interface with the rod with only minimal rod contouring. These polyaxial screws are now the most common type of pedicle screw used today.

Current pedicle screw designs consist of several component parts including: 1) a threaded bone screw shaft which is anchored into the pedicle bone of the vertebrae 2) a rod holding member which is attached to the head of the bone screw to receive a rod for stabilization of the spine, and 3) a set screw which interfaces with the top of the rod holder to secure the rod into the holder to form a stable construct. These components are assembled into two primary design types: monoaxial and polyaxial screw systems. The monoaxial screws typically have a fixed angular relationship between the bone screw and rod holder. These screw designs dictate that the rod is held perpendicular to the direction of the bone screw. While these designs are strong and stable, they make it difficult to position the screw and rod properly and require a lot of rod bending to correctly approximate the rod holder and rod. Polyaxial designs allow the rod holding member to pivot on the bone screw head such that the rod holder can properly interface with a rod that is not perfectly perpendicular to the direction that the bone screw is inserted. After the set screw is used to secure the rod into the rod holder, the polyaxial design will lock into place and no longer allow the rod holder to pivot on the screw head.

Current designs suffer from some limitation in their functionality and the manufacturing requirements to encompass the myriad variations of particular surgical approaches. For example, many companies must offer both a monoaxial and polyaxial screw set, each with applications for specific surgical procedures. Similarly, current designs rely on rod holders that are designed for one specific rod diameter. Thus, if a manufacturer wants to offer a system that can use both 5.5 mm and 6.0 mm rods, they must manufacture two different sized rod holders, and they must manufacture both monoaxial and polyaxial variants of each size holder. This necessitates holding an inventory of parts in surgical centers, hospitals, and supply houses, many of which are rarely used.

Another limitation of current pedicle screw systems is a tendency for the set screw threads to cross-thread when approximated with the rod holding element. This may complicate assembly during operative procedures and increase procedure time to the detriment of the patient. Additionally, set screws that are cross-threaded or over-torqued can cause head splay, whereby the top portion of the rod holding element is deformed outward by the set screw. This causes poor rod stability and often necessitates the removal and replacement of the pedicle screw, again, increasing operative time, damage to the pedicle bone and the potential for surgical complications. Undetected head splay at the time of surgery could also lead to premature failure of the construct and necessitate additional surgical procedures at a later time. It is therefore advantageous to eliminate cross threading of the set screw and prevent head splay of the rod holding element to any extent possible.

An additional shortcoming in current systems is the inability to create a uniplanar pedicle screw configuration that is able to pivot in only one plane of motion rather than the combined motions of the polyaxial design. A uniplanar design is useful in complex spinal reconstructive cases where multiple segments are spanned with stabilizing rods and when lateral forces must be applied to a vertebrae to bring it into alignment with neighboring segments. With the traditional polyaxial designs, a lateral force applied to the rod holding element will cause the rod holder to pivot on the bone screw head rather than rotate the vertebral body into a desired alignment.

Hence there has been identified a need to provide an improved pedicle screw device as well as systems and methods of employing and utilizing pedicle screw assemblies. For example, it is desired that an improved pedicle screw assembly allow for a reduction of the necessary inventory of expensive medical components. The present invention addresses one or more of these long felt but unmet needs.

SUMMARY OF THE INVENTION

The system proposed herein allows for a multipurpose pedicle screw assembly having multiple modular inserts. The combination of modular inserts allows the screw assembly to perform with different functionalities that would be applicable with different surgical procedures. The selection of a particular kind of modular insert can determine the assembly functionality. The modular pedicle screw assembly may additionally incorporate a rod adapter that allows multiple sized rods to be used with the same rod holding element. The differing screw configurations allow for a variety of screw functions that can all be achieved while using the same basic rod holding element, which is often the most machining intensive component of any pedicle screw.

In one embodiment, and by way of example only, there is provided a polyaxial pedicle screw assembly that includes a polyaxial pedicle screw, a rod holding element, a polyaxial insert, a rod, and a set screw. The polyaxial pedicle screw has a shaft and head. The rod holding element defines a screw hole, an insert bearing area, a chamber, a saddle area, a cutaway area, a head seating surface, and a threading area. The polyaxial insert is disposed within the chamber of the rod holding element, and the insert defines a bearing surface, a screw head bearing surface, and an upper surface; further the polyaxial insert is positioned within the chamber of the rod holding element such that the insert bearing surface of the rod holding element contacts the rod holding element insert bearing area. The pedicle screw is disposed so that the shaft passes through the screw hole, and the pedicle screw head contacts the screw head bearing surface of the polyaxial insert. The rod is disposed to rest on the upper surface of the polyaxial insert; and the set screw is joined to the threading area of the rod holding element so as to secure the rod to the upper surface of the insert and to lock the polyaxial screw and rod holding element into a desired position. The rod holding element may further define a notch, and the polyaxial insert may further comprise guide pins positioned such that the guide pins fit within the notch when the polyaxial insert is disposed in the chamber. The guide pins may be substantially cylindrical in shape, such that the polyaxial insert has freedom of movement in a plane by rotating around the guide pins. The rod holding element may also include a head seating surface disposed proximate to the screw hole, and wherein the pedicle screw head contacts the head seating surface. The insert bearing area of the rod holding element and the bearing surface of the polyaxial insert may have substantially the same circular shape. The polyaxial screw head may include surface structures such as ridges.

In a further embodiment, also by way of example only, there is provided a medical kit that includes as components of the kit: a polyaxial screw, a rod holding element, a polyaxial insert, and a set screw. The medical kit may also include additional elements such as a rod, a uniplanar insert, a monoaxial insert, a rod adapter, and at least two rods having different diameters.

In still a further embodiment, and still by way of example only, there is provided a method for forming a polyaxial screw assembly comprising the steps of: positioning a polyaxial screw with a shaft relative to a rod holding element having a screw hole such that the shaft of the polyaxial screw passes through the screw hole of the rod holding element; positioning a polyaxial insert such that a screw head bearing surface of the polyaxial insert contacts the head of the polyaxial screw; positioning a rod within the rod holding element; and joining a set screw to the rod holding element. The step of positioning a polyaxial insert mentioned above may further include positioning the polyaxial insert such that guide pins of the polyaxial insert fit within notches of the rod holding element. The method may also include the step of rotating the insert relative to the rod holding element by rotating the polyaxial insert around the axis passing through the guide pins. And the method may also include the step of adjusting the position of the polyaxial screw relative to the rod holding element by moving the screw head at the screw head bearing surface of the polyaxial insert. A further step of positioning a rod adapter around the rod may also be included in the method. Finally, the method may further include the step of fixing the polyaxial screw, the polyaxial insert, and the rod holding element into a desired substantially immovable configuration by way of the set screw.

Other independent features and advantages of the polyaxial pedicle screw assembly will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring initially toFIGS. 1,2, and3there is illustrated a first embodiment of rod holding element10. As further explained herein, rod holding element10functions as a structure with which various components cooperate in order to create a pedicle screw assembly. Thus, in one embodiment, rod holding element10includes various features which are designed to cooperate with other pieces, and these features of rod holding element10include rod cradle11, insert notch12, bearing surface13, and polyaxial head seating surface14. The rod holding element10is versatile in that this single element can be used to assemble different kinds of pedicle screws including static monoaxial screw, uniplanar screw, and polyaxial screw assemblies.

Rod holding element10is characterized by an opening16. Opening16is sized such that a tip and shaft of a bone screw (not shown) can pass from an interior chamber area15of rod holding element10to the exterior of rod holding element10. Rod holding element10further defines chamber walls17which also define and limit chamber area15. An insert, as explained further herein, can be placed within chamber area15.

Referring next toFIGS. 4,5, and6, there is illustrated an embodiment of a monoaxial insert40. Monoaxial insert40includes bearing surface41, locking tabs43, and in some embodiments, threading44. Bearing surface41is preferably designed so that bearing surface contacts rod holding element bearing surface13when monoaxial insert40is fully placed within rod holding element10. Further, locking tabs43are designed so as to fit within the space defined by insert notch12of rod holding element10. Preferably, locking tabs43are designed to fit within notch12so that when monoaxial insert40is fully assembled with rod holding element10the contact between locking tabs43and notch12substantially restricts any movement of monoaxial insert40. As explained further herein, locking tabs43and notch12may be configured with a mutually fitting dovetail shape. Thus, in effect, monoaxial insert40is held to a single or static position within rod holding element10.

FIGS. 4 and 5illustrate embodiments of monoaxial insert40that do not include threading and that do include threading44respectively. As will be understood by those skilled in the art, threading44such as illustrated inFIG. 5can be used to secure a threaded bone screw head to the insert. However, the threading aspect of monoaxial insert40does not affect the above-described position locking of monoaxial insert40with respect to rod holding element10.

Referring now toFIGS. 7,8, and9, there is illustrated an embodiment of a uniplanar insert70. It will be apparent that the overall shape and configuration of uniplanar insert70is similar, but not identical, to monoaxial insert40. For example, both monoaxial insert40and uniplanar insert70have an external shape that allows them to slide into opening16defined by rod holding element10. However, in a point of difference, uniplanar insert70does not include locking tabs43. Thus, for example, when disposed within opening16, uniplanar insert70is not completely restricted from movement. Rather, when seated so that bearing surface71of uniplanar insert contacts bearing surface41of rod holding element10, uniplanar insert70has two degrees of freedom so that it can have a movement when these bearing surfaces71and41move relative to one another. However, uniplanar insert70does not have a third degree of freedom in that side walls75of uniplanar insert are in substantial contact with chamber wall17of rod holding element10and so restricted in movement. Thus, with two degrees of freedom, uniplanar insert70can move within a plane of motion relative to rod holding element10.

Both a monoaxial insert40and a uniplanar insert70may share certain common features. For example both inserts40and70may include side walls45and75. Further both inserts40,70may also include an upper surface46and76. In a preferred embodiment, side walls45,75and upper surface46,76are substantially planar in configuration. Inserts40and70may be configured such that when insert is placed within chamber area, side walls45and75substantially contact chamber walls17thereby restricting the movement of insert within rod holding element10. Additionally, both inserts40and70may include a hole (not shown) through which a bone screw (not shown) may pass; and they may also include a receiving area47and77shaped to receive the head of a bone screw as explained further herein. The shape of this receiving surface and congruent surface of the bone screw is not limited in scope by the preferred embodiment. Those skilled in the art will recognize that a conical tapered press fit could also be utilized to mate and secure the insert and bone screw head. Finally, both inserts40and70may also include threading for joining with a bone screw.

It will here be appreciated that the movement heretofore described for the monoaxial insert40and uniplanar insert70assumes that both inserts stay in a fully bottomed contact with rod holding element10. Until final assembly of a pedicle screw assembly, both monoaxial insert40and uniplanar insert70could be removed from opening16of rod holding element, a generally vertical movement (relative to the figures). However, it will be appreciated that the placement of monoaxial insert40and uniplanar insert70within chamber16accomplishes a preassembly which will ultimately lead to a final assembly of a pedicle screw assembly. Thus, the potential movement of removing the inserts40and70has been ignored.

In a preferred embodiment, the insert40and70are convex and cylindrical in shape on their bearing surface41,71where the insert contacts the reciprocal bearing surface13of the rod holding element10. Thus the curvature of bearing surface41,71of insert40,70preferably closely matches the curvature of rod holding element bearing surface13. The matching of these two surfaces forms, in one embodiment, a partial cylindrical bearing on which the insert can glide. If desired, bearing surfaces41,71of inserts40,70can be machined (or otherwise formed) with corrugations or ridges78as shown specifically inFIG. 9. Ridge-like structures78can aide in locking the assembly into a desired configuration when bearing surfaces are compressed together.

Referring now toFIGS. 10 and 11there is illustrated an embodiment of a final modular pedicle screw assembly, withFIG. 10showing a semi-exploded view andFIG. 11showing a fully assembled view. Pedicle screw assembly100includes bone screw101, rod holding element10, insert (uniplanar70or monoaxial40), rod105, and set screw104. In the final assembly ofFIG. 11, rod105is firmly held in position.

ComparingFIG. 10toFIG. 11illustrates further aspects of the modular pedicle screw embodiment100. For example, it is noted that at the point of assembly inFIG. 10, rod105has not yet come to a final position of rest. Screw head109projects through insert40,70, and rod holding element10is still free to move generally along the length of screw101. Likewise, at this point of assembly inserts40,70are free to move relative to rod holding element10, as previously described. That is, a uniplanar insert70can move in its plane of freedom; however the monoaxial insert40would be restricted, because of the tab/notch fit, except to move generally upward, with respect to the orientation ofFIG. 10. Rod105is also free to move relative to rod holding element10. Thus, at this point of the assembly, a surgeon would typically bring the elements into alignment, as shown inFIG. 10; and then, using the freedom of movement permitted, the surgeon could make what adjustments to that alignment the surgeon desires. And once the elements are suitably aligned, the surgeon can begin to make the final assembly as follows.

A surgeon would begin to press downward, relative to the orientation ofFIG. 10, for example on set screw104. In a related fashion, the surgeon could pull upward (again relative to the orientation ofFIG. 10) on rod holding element10. As a result of these movements, screw head109passes into the receiving area77of insert70, rod105passes into the saddle area11, and set screw109approaches threading area19of rod holding element10. It is noted that the profile dimension of screw head109is such that it can pass into receiving area77of insert70without significant resistance. As set screw104approaches threading area19, set screw104may be rotated so that set screw threads110engage threading area19. The further rotation of set screw104continues to move set screw104in a generally downward direction (relative to the orientation ofFIG. 10), which movement brings bottom surface14of set screw109into further contact with rod105. The movement of rod105likewise tends to move screw head109so that head109tends to move towards receiving area77. In this manner the further rotation of set screw109continues to move the elements into an assembled configuration until the point of final assembly is reached as shown inFIG. 11.

It will also be appreciated by those skilled in the art that a modular pedicle screw assembly, preferably partially assembled, can also be secured in a pedicle bone prior to final assembly. For example, in one common practice, bone screw101can be assembled with rod holding element10and insert40,70. Bone screw101can then be inserted into the patient's pedicle bone. Once bone screw101is anchored to the patient's pedicle bone, the bottom of rod holding element10contacts the bone which exerts an upward pressure on the rod holding element10. However, the pressure exerted upwardly by the bone against rod holding element10is not sufficient to prevent a uniplanar insert70from translating within rod holding element10. The final locking of the assembly occurs when rod105is added to the assembly and is pressed downward by set screw104. It is further to be appreciated in the general positioning and placement that surgeons are careful not to pull upwardly on these assemblies because in a degraded or diseased pedicle bone the screws can be uprooted with such an upward force.

In a further embodiment, the placement of a modular pedicle screw can be assisted with the use of an assembly tool. Referring now toFIG. 21there is shown an exploded view of an exemplary embodiment of assembly tool290positioned with respect to rod holding element10and set screw104. In a preferred embodiment, assembly tool290includes tabs291which can align and join with reciprocal groove structures (not shown inFIG. 21) on rod holding element10as previously described. Assembly tool290further includes bit292, which as illustrated can be hex shaped (or otherwise shaped) in order to fit within the reciprocal space on bone screw127. A surgeon or surgical assistant may place assembly tool290into rod holding element10by aligning pins291with grooves. Further, the hex bit292is aligned with the bone screw101, and set screw104is then joined with rod holding element10. Assembly tool290preferably has a flat surface or top (not shown) so that as set screw104is screwed into rod holding element10, set screw104contacts the flat surface and thereby exerts a downward force on assembly tool290. Additionally, assembly tool290preferably includes shoulder surface293. In one embodiment, shoulder surface293is substantially planar so that as assembly tool290is pressed against insert70and bone screw head127, shoulder293tends to force bone screw head127to recede within the receiving area of insert70. A surgeon can use a hex tool (or other kind of tool) to tighten set screw104. As set screw104is further tightened, it ultimately pushes assembly tool290into a bottomed out position. In this position, assembly tool290holds bone screw101into a final assembled position relative to insert40,70and rod holding element10. Taking advantage of the now positioned assembly tool296, the surgeon can further apply torque to set screw104which thereby serves to rotate the entire assembly, including bone screw101. In this manner bone screw101along with the entire assembly can be placed in a desired position relative to the patient's pedicle bone.

The assembly tool290illustrated inFIG. 21provides advantages in the placement and formation of pedicle screw assemblies over other known methods. In a first instance, assembly tool290prevents bone screw101from moving out of axis with respect to rod holding element10when being inserted in the patient's bone. This phenomenon of movement, sometimes referred to as toggling, is an undesirable feature of other systems. Further, assembly tool290ensures that bone screw101remains in a desired alignment with the driver being used to insert the screw. The driver/tool is inserted into the set screw and thereby rotates the entire assembly. Thus the torsional force applied by the surgeon to the driver is advantageously aligned with bone screw101which allows the surgeon to confidently proceed with bone screw101placement.

In one embodiment it is desired that the configuration of screw head109closely matches the related configuration of receiving area77of insert70(as receiving area47of insert40). In this manner once screw head109is fully assembled so as to drop fully into receiving area77, the closely matching configuration restricts the movement of screw head109relative to insert70. Thus, rod holding element10, rod105, and screw101are also held into a desired position.

Referring now toFIG. 11it is noted that the various elements have come into a final position and are significantly secured in that position. Rod105rests in rod cradle/saddle area11, and the saddle arms20that partially define saddle area11are in contact with rod105. Preferably the width defined by saddle area11closely matches the diameter of rod105such that rod105is not unduly restricted in moving to a bottomed position within saddle area11, and once in the bottomed position, as shown inFIG. 11, rod105is substantially restricted from side-to-side movement relative to the orientation ofFIG. 11.

FIG. 11also illustrates the point of interface108between rod105and upper surface46or76of insert40or70. The point of interface108indicates that rod105comes to final rest against surface46,76rather than the bottom point18of saddle area11in the illustrated embodiment. (In an alternative embodiment discussed further herein, rod105can rest in whole or part on bottom point18of saddle area11). It is further noted that the force exerted by rod105against insert40,70holds insert40,70firmly against bearing surface13of rod holding element10, and thus insert40,70is positioned with respect to rod holding element10.

Referring now toFIG. 20, there is shown a further illustration of rod holding element10.FIG. 20shows a bottom view of rod holding element10, and in particular the figure illustrates screw hole opening16. As previously explained, screw hole opening16is configured so as to allow shaft128of a pedicle screw to pass therethrough.FIG. 20also illustrates the further preferred embodiment in which screw hole opening16is slightly oblong or elliptical in its shape, rather than substantially round, such that the dimension x24is greater than dimension y26, each of which is substantially perpendicular to the other. This arrangement allows shaft128to have greater linear movement in the direction that aligns with dimension x24.

The rod holding element with elliptical screw hole opening16may be advantageously used with uniplanar insert70. It is noted that dimension x24is preferably aligned so as to present a particular configuration with uniplanar insert70that would be positioned within rod holding element10. The plane of movement that uniplanar insert70would move in is a plane of movement that is also generally aligned with dimension x24. Thus, when a bone screw passes through uniplanar insert70, and shaft128of bone screw passes through screw hole opening16, the planar movement of uniplanar insert70will not be unduly impeded as shaft128is also allowed some freedom of movement within the space of screw hole opening16. As a further detail, it will be appreciated that dimension y26should be configured with a length at least as large as the diameter of bone screw shaft128.

Finally, while the preferred embodiment of screw hole opening16has been illustrated inFIG. 20as generally elliptical in shape, opening16could take other configurations, such as, for example a generally rectangular shape, or a generally rectangular shape with curved corners. It is also contemplated that the shape of opening16could follow a generally rectangular shape, however a rectangle with somewhat curved sides.

Referring now toFIG. 12there are shown two varieties of screws, and at least these two screw varieties may be used with embodiments of the modular pedicle screw system. Screw121includes a head122that has threads123, and screw121may thus also be referred to as a locking threaded screw. Screw125includes a head126with no threads, and screw125may be referred to as a non-locking screw or a flange-top screw. Screw125includes flange127positioned on its head126. Flange127is preferably shaped to match the configuration and area defined by receiving area47,77of insert40,70. The flange and receiving area could be tapered to form a press fit when assembled, held together by friction between mutual mating conical surfaces. The bone screws for use with the pedicle screw assembly also typically include a shaft128, and shaft128may include shaft threads130. All threads on both the bone screws and set screw are exemplary only and do not represent the actual thread pitch and taper.

Referring now toFIGS. 13 and 14there is illustrated an embodiment of a threaded screw121configured so as to be attached to a uniplanar insert70. Thus in the embodiment ofFIG. 13uniplanar insert70includes a reciprocal threading44for bonding with the matching threads123of threaded screw121. In the illustrated preferred embodiment, the threaded head123screws into a uniplanar or monoaxial insert on the underside of the insert, rather than a flanged head being inserted into the receiving area from above. Essentially, the insert40,70when screwed together with the pedicle screw121acts as a modular screw head. The function remains unchanged from the monoaxial and uniplanar designs. Insert70can be joined to threaded screw121by rotating insert70and/or threaded screw121so as to engage the reciprocal threading44with threads123.FIG. 13illustrates threaded screw121and insert70in an exploded view.FIG. 14illustrates threaded screw121joined with insert70.FIG. 14further illustrates how, in a preferred embodiment, the top124of threaded screw does not project beyond upper surface76of insert70to any significant extent, which allows a rod (not shown) to lay substantially flush against upper surface70. Additionally,FIG. 14illustrates how threaded screw121can include recessed area129for receiving a tool; and as known in the art, recessed area129may be configured as a socket with hexagonal walls (by way of example only) for receiving an Allen wrench. Although not illustrated, a nonthreaded screw may similarly include a recessed area.

Use of the threaded screw head embodiment can assist in locking the bone screw into the insert. The joinder achieved by the mutual threading of the bone screw and the insert can increase the rigidity of the screw/insert assembly and help to prevent movement of the screw relative to the insert. The locking threaded screw head can be useful in both the uniplanar and monoaxial configurations. It is also preferred that the locking threaded screw head embodiment be used with relatively larger diameter bone screws, i.e. those with a diameter greater than opening26.

It is here noted that the screw ofFIGS. 13 and 14may be advantageously used in certain surgical procedures. For example, some surgeons prefer to affix a bone screw to a pedicle bone before any additional pedicle screw structure is attached to the bone screw. In such a technique the surgeon generally has a clearer view of the pedicle bone while placing the bone screw; additionally the technique allows the surgeon to shave bone material away from the area of the bone against which the rod holding element may ultimately be seated. Consistent with this technique, certain prior art systems, such as the Blackstone Icon system, are adapted so as to allow pedicle screw elements to snap onto the bone screw head after the bone screw has been placed in the pedicle bone. The above-noted embodiments of the pedicle screw system can also be assembled to the bone screw after the bone screw has been attached to the patient. For example, insert70can be placed in rod holding element10. An assembly tool290and set screw104, if desired, can hold these items in a desired relationship. Thereupon, this assembly can be screwed onto the threaded head123of bone screw121.

Referring now toFIG. 15there is illustrated an embodiment of rod adapter200. As previously discussed, orthopedic therapies can call for rods having different qualities of strength and flexibility (among other criteria), and this is achieved in one respect by specifying rods with different diameters. To take two common examples in the industry, rods can be used that have a diameter of either 5.5 mm or 6.0 mm. The modular pedicle screw assembly can accommodate rods with different diameters, from as small as 3.0 mm up to and including 6.5 mm, by use of rod adapter200.

A preferred embodiment of rod adapter200generally includes a rod cradling portion202, side wall204, and rod adapter tab205. In another embodiment, rod adapter200can further include a post206with an inset207for use with a locking ring device209as shown inFIGS. 16 and 17. Rod cradling portion202defines a generally semicircular surface with a radius adapted to substantially conform to the radius of the rod desired to be used. For example, in one embodiment, it is desired to use the rod adapter200to secure rods with a 5.5 mm diameter. In such a situation, rod cradling portion202would have a substantially similar diameter. Further, it is preferred that the surface of rod cradling portion202extend from a first end to the opposite end of rod adapter200, as shown inFIG. 15, in order to provide a maximum surface area with which to secure a rod.

As with the monoaxial insert40, the tab205on rod adapter200is configured to fit within the reciprocal space defined by notch12on rod holding element10.FIG. 10illustrates in particular that embodiment in which tab205assumes a dovetail configuration, and in such a situation, notch12would likewise have a reciprocal dovetail shape in order to receive tab205. The function of this dovetail shape is to prevent the top portion of the rod holding element from spreading apart laterally when the set screw is tightened against the rod, thereby imparting a force between screw threads19and104. Further, the improved alignment of threads19and104created by the tabs205prevent cross-threading and further mitigate the risk of rod holder head splay when the set screw is tightened. Walls204are generally planar structures configured to contact chamber walls17of rod holding element10. In general, rod adapter200is configured to rest firmly within rod holding element10.

Referring now toFIGS. 16 and 17, there is illustrated a subassembly of rod adapter200and set screw210. In the illustrated preferred embodiment, set screw210includes a recessed area212. Within recessed area212is an opening (not illustrated) which allows post206to pass therethrough. Recessed area212is generally configured so that when set screw210is fully positioned on rod adapter200; i.e., when the bottom surface213of set screw210contacts upper surface208of rod adapter200, post206of rod adapter200projects through set screw opening sufficiently such that inset207of post206is exposed within the recessed area212. In this manner locking ring209can be disposed so as to lock with inset207and around post206. This positioning of locking ring209with post206then acts to restrict the movement of set screw210away from rod adapter200. However, locking ring209does not generally restrict the rotational movement of set screw210relative to rod adapter200.

It is also noted that recessed area212can define, in whole or part, a configuration for receiving a torsional tool such as a screw driver or a hex wrench. In the embodiment illustrated inFIG. 12, for example, recessed area212defines walls within the body of the set screw212itself. The walls may define a hexagonal shape that can accommodate a hex wrench. A surgeon can thus dispose a tool within recessed area212in order to forcefully manipulate set screw210. Optionally, a spacer (not shown), such as a washer structure, can be disposed between locking ring209and the set screw210.

The above subassembly of set screw210with rod adapter200is useful during surgical procedures in the following manner. Reference toFIG. 18is useful in understanding the following description. Using elements earlier discussed, a surgeon has disposed a rod105within the chamber area15defined by rod holding element10. Further, rod105may be resting against an upper surface46of an insert such as a monoaxial insert40or uniplanar insert70. Portions of rod105may also rest against the saddle bottom18defined by rod holding element10. However, if rod105, such as a 5.5 mm rod, is somewhat undersized for the given rod holding element10, rod105will experience some lateral movement within the rod holding element10. It is generally desired to restrict this rod movement as the pedicle screw reaches its final assembly, and here is where the rod adapter subassembly comes into play. Rod adapter200can now be positioned so that rod cradling portion202surrounds the upper portion of rod105. As the rod adapter200is so positioned, tabs205are aligned with notches12. Then, as rod adapter200is moved in a generally vertical downward position so as to fully engage the rod105, set screw210(previously assembled with rod adapter200) comes into contact with threaded area19of rod holding element10. Locking ring209does not allow set screw210to dislodge from rod adapter200; however, it does permit set screw210to rotate. Thus, the surgeon can manipulate set screw210to rotate such that threads of set screw210engage with the reciprocal threaded area19of rod holding element10. This manipulation may be a hand manipulation or a tool-assisted manipulation. Further, the surgeon can ultimately exert a desired amount of torque or tortional force on set screw210so that rod adapter200forcefully engages with rod105and thereby securely holds rod105in a desired position. Further, the degree of contact between the surfaces of rod105and the closely matching rod cradling portion202provide a sufficient level of contact so that rod105cannot easily move laterally relative to rod adapter200.

The subassembly of rod adapter200to set screw210is additionally important to reduce the time needed during surgery. The use of such a subassembly, which can be constructed prior to surgery, eliminates the amount of time needed during surgery that would otherwise be required to put together these parts. Furthermore, it is generally desired to reduce the number of repetitive or fatiguing steps that must take place during surgery, and moving the subassembly to a time period outside the surgical theater achieves that advantage. This advantage particularly increases in importance where multiple such screw assemblies will be employed in the overall surgical procedure.

Referring now toFIG. 19there is shown a further embodiment of a fully configured pedicle screw assembly.FIG. 19is also useful in illustrating one significant aspect of the function of rod adapter200. As previously mentioned, rod adapter200is employed in those situations where the rod to be used during the surgical procedure is smaller than would normally fit in the saddle area11of rod holding element10. Thus,FIG. 19shows how the diameter of the saddle area11is indeed larger than the diameter of rod105disposed within saddle area11. Were it not for rod adapter200, rod105could more easily experience a side-to-side motion (relative toFIG. 19) within the saddle area11nor would the smaller diameter rod exert the necessary downward force onto the insert when the set screw is tightened. However, rod adapter200, shown with a diameter that closely matches rod105, a diameter that is less than saddle area11diameter, closely surrounds rod105and acts to prevent that movement. The rod adapter200also increases the surface area of contact with the superior aspect of rod105as compared to the flat bottom profile of the set screw alone. It should also be noted that a preferred embodiment would use a rod adapter200for all rod sizes, even those that fill the entire width of the saddle, to take advantage of the antisplay properties and increased area of contact discussed previously.

In a further embodiment, the modular pedicle screw system can be configured to allow for a dynamic stabilizing system. Referring again toFIG. 11andFIG. 19(the assembly with a rod adapter), it was noted that in the final assembly of one preferred embodiment, rod105rests against upper surface46or76of insert40,70rather than resting against saddle area11of the rod holding element110. In such a scenario, once rod105is inserted and the top set screw104is tightened, there will be no motion of the screw104relative to rod holding element10due to the compression created at the rod—insert interface108. In the same manner as a traditional polyaxial screw, the rod compresses the bearing surface, insert, and screw head together to form a stable construct. However, with a simple modification of insert40and70(for example, by reducing the height dimension of insert slightly so that rod105does not compress insert40,70but lies on the bottom point of saddle11) the insert70will remain able to pivot within the rod holding element10. In a preferred embodiment of such dynamic stabilization, in the final assembly rod105is also held at its upper position by the rod adapter. This can be useful in creating a dynamic system that allows for some motion between the vertebrae that are spanned by rod105, which more typically creates a rigid system.

There can be disadvantages to a rigid system in some instances that can be ameliorated through a “dynamic stabilization.” For example, the use of rods that have flexible sections has been attempted. However, weakness of the flexible rod structure, movement in multiple degrees of freedom, and repeated stresses may lead to failure of the system and other complications. The modular system described herein allows the rod holder to be dynamic, rather than the rod itself. From a biomechanical perspective, it is advantageous to dynamize the rod holding element because the degrees of freedom in the dynamic system can be limited by choosing the appropriate insert and the allowed motion can be aligned with the natural motion of the spinal segment being stabilized.

In a further embodiment, an insert may be eliminated altogether as a separate piece in the assembly. In this embodiment, the head of the bone screw is machined in the same shape, or substantially the same shape, as either the uniplanar insert70or monoaxial insert40. Essentially two pieces are replaced with a single piece, which may further lead to time savings during surgery procedures and cost savings from machining fewer parts in total. Functionally the combined screw insert assembly is similar to the previously enumerated monoaxial and uniplanar designs but the machining process and parts are different.

As has been mentioned, embodiments of the modular pedicle screw system can be used with various surgical techniques and procedures as practiced in orthopedic surgery. Referring toFIG. 22, there are listed some exemplary steps that may be followed in one such method of usage. In step321, a surgeon may begin the process by creating a subassembly comprising a bone screw, insert, assembly tool, rod holding element, and set screw. This advantageously secures the bone screw in a desired relationship relative to the rod holding element. Further, the bone screw can then be set in the patient's pedicle bone, step322, by applying a hand tool to the set screw, which, because of the assembly tool's linkage with the bone screw, applies torsional force to the bone screw. It will be appreciated that in other embodiments, such as the embodiment described previously in which the bone screw includes top threading, the bone screw may be applied directly to the pedicle bone. Step323calls for positioning an insert within the rod holding element. This step may occur as part of the preassembly construction, step321or after the bone screw has been positioned, depending on the option selected. Step324which calls for aligning a tab on the insert with a notch on the rod holding element is an optional step in the overall step of placing the insert if the insert is equipped with such a structure. In step325, the head of the bone screw is positioned such that the head of the bone screw is disposed within the rod holding element chamber. Again, depending on the surgical option, this may occur in constructing the preassembly of step321. Optionally, this step may occur after the bone screw has been placed in the patient.

In step326a rod is placed within the rod holding element. It will here be appreciated that in those embodiments that use an assembly tool, the subassembly that includes the assembly tool must be disassembled (the set screw must be removed) and the assembly tool must be removed. Once the assembly tool has been removed, the rod may be placed within the rod holding element. Thus, placement of the rod within the overall assembly typically takes place after placement of the bone screw. Once the rod has been positioned, the final assembly can be secured; i.e., the set screw can be joined to the rod holding element. And, as previously described, tightening of the set screw pushes against the rod, which in turn pushes against the insert until a desired tension is obtained. Note, that if dynamic tensioning is desired, the rod may also contact the lower saddle portion of the rod holding element.

Referring now toFIGS. 23 and 24there is shown an embodiment of a polyaxial insert400. Polyaxial insert400is also configured to fit within chamber15of rod holding element10. Polyaxial insert400thus includes bearing surface441, side walls445, upper surface446, and screw hole448. Polyaxial insert400further includes screw head bearing surface449, as shown inFIG. 24. In a preferred embodiment, polyaxial insert400also includes guide pins443positioned to project from side walls445.

The overall shape and configuration of polyaxial insert400allows it to fit within rod holding element10. Side walls445preferably align closely with chamber walls17so as to restrict lateral movement of polyaxial insert400. Polyaxial bearing surface441is preferably shaped with a curvature that substantially matches bearing surface13of rod holding element10; and thus when polyaxial insert400is fully positioned within rod holding element10bearing surface441of polyaxial insert400makes substantial contact with bearing surface13of rod holding element10. In the illustrated embodiment the matching of these bearing surfaces, the polyaxial bearing surface441and the rod holding element bearing surface13, allows polyaxial insert400to rotate within chamber, on a plane of freedom, in a manner similar to uniplanar insert70. In a preferred embodiment both the polyaxial bearing surface441and the rod holding element bearing surface13define a substantially circular curvature; however other curvatures are acceptable.

The preferred embodiment of polyaxial insert400illustrated inFIGS. 23 and 24shows the insert having guide pins443. However, polyaxial insert400can be configured without guide pins443. Guide pins443are preferably configured as rounded, substantially circular, cylindrical projections. Guide pins443preferably have a diameter that allows them to pass through notch12of rod holding element10; however, guide pins443do not project so far from side walls445so as to limit the movement of polyaxial insert400within chamber15. Guide pins443are advantageously positioned when they are located on side walls445at a point that substantially corresponds to the center point of the geometric circle that is defined by bearing surface441. Having that center point location, and given their rounded, cylindrical profile, guide pins443can rotate within notch12as insert400rotates. Further, by having such freedom to rotate, guide pins443do not restrict the rotational motion of polyaxial insert400. Thus guide pins443facilitate the positioning and firm placement of polyaxial insert400while not restricting the insert's intended freedom of movement.

Alternatively, polyaxial insert400can be configured with tabs (not shown) that are substantially similar to locking tabs43positioned on monoaxial insert40. And, as with the monoaxial insert40, a polyaxial insert400that includes locking tabs43would have its freedom of movement restricted by the tabs43.

It is additionally noted that guide pins443such as those illustrated inFIGS. 23 and 24can also be placed on the uniplanar insert70. Further, the embodiment of uniplanar insert70having generally cylindrical guide pins443would also have the advantages described above with respect to polyaxial insert400.

Referring again toFIG. 24it has been noted that polyaxial insert400includes screw head surface449. This surface449is configured so as to receive a polyaxial screw head, transfer support to and from the screw, and ultimately lock the pedicle screw assembly into a desired position relative to the bone screw. Preferably surface449defines a partial hemisphere or other curvature that matches the geometry of a polyaxial screw head. One exemplary embodiment of the partially spherical head on a pedicle screw is shown inFIG. 25.

Referring again toFIG. 25there is illustrated an embodiment of a polyaxial screw460that may be advantageously used in pedicle screw assemblies. Polyaxial screw460includes shaft468, neck464, and head463. Shaft468may include any of the known threading structures, and shaft468is generally configured as known in the art to function as a pedicle screw. Head463is characterized by radial ridges465. Head463may additionally have a recess or recessed area466which, as shown in the preferred embodiment, may itself be configured so as to receive a torsional tool. In the illustrated embodiment, recessed area466has a female hex shape so as to receive an Allen wrench.

In an alternative embodiment, rather than having radial ridges465, head463may include other structures (not shown) such as bumps, grooves, non-radial ridges, or other surface anomalies which serve to increase the frictional lock between head463and surface449once a final assembly is created. As an additional alternative, surface449may also be configured with surface structures such as ridges, bumps, and the like so as to create the desired frictional lock. Further, ridges465or other friction-inducing structures may be positioned on both the upper surfaces and lower surfaces or even side surfaces of head463. In this manner, screw head463can achieve frictional fixation or contact where it contacts structures both above and below the head463in the pedicle screw assembly.

Polyaxial screw460preferably includes neck464which can be a neck with a generally reduced diameter. The preferred reduced diameter neck464allows shaft468and thus screw460to move more freely within screw hole16. The reduced diameter neck464includes any size neck diameter which allows head463to pivot against surface449. In a further preferred embodiment, reduced neck464and hole16operate together so as to allow head to pivot with approximately a 60° range of motion. In such an embodiment, screw460can pivot up to approximately 30° from the vertical position. In other embodiments, screw460can pivot up to approximately 60° from the vertical position. The maximum range of motion is a factor of screw diameter and aperture size26with larger screws tending to have a more limited range of motion.

In a preferred embodiment, screw460and rod holding element10are mutually configured so that when screw head463is assembled so as to rest within chamber15, but prior to final tightening, screw head463still has a freedom of motion. The freedom of motion allows the surgeon to position screw460; then, upon final tightening of the modular assembly, screw460remains in the desired position. Referring again toFIG. 3it is noted that rod holding element10includes a cut away area27. Preferably this cut away area27defines an area in which screw head463is allowed to rotate and move. Cut away area27can comprise mutually opposing sections in rod holding element10. Where screw head463defines a partial sphere, cut away area27can likewise be partially spherical in shape.

Still referring toFIG. 3, and also toFIG. 2, it is again noted that rod holding element10defines screw hole16. Further, screw hole16is defined at its outer perimeter by head seating surface14. Preferably hole16and head seating surface14are selected of a size such that screw shaft468can pass through; however, screw head463cannot pass completely out of chamber area15by passing completely past head seating surface14. However, as further explained with respect toFIGS. 26 and 27, head463can partially, but not fully, extend past head seating surface14. Thus, in one embodiment, head seating surface14acts as a restrictor to the movement of screw head463. It is also noted that head seating surface14can exhibit different types of surfaces that mate with screw head463. For example, the surface14may have a generally sharp edge, or a flat surface, or a generally rounded edge.

Referring now toFIGS. 26 and 27the joining of a polyaxial modular pedicle screw assembly is described. Polyaxial screw460is positioned so that shaft468passes through screw hole16of rod holding element10. Polyaxial screw460may be positioned such that a bottom portion of head463contacts head seating surface14of rod holding element10. Polyaxial insert400is then positioned within chamber area15of rod holding element10. When polyaxial insert400is initially positioned bearing surface441may contact an upper portion of screw head463. Further, if polyaxial insert400includes guide pins443, the guide pins443may be positioned within notch12. Rod105(not shown inFIG. 26or27) can next be positioned within rod holding element10such that rod105contacts upper surface446of insert400. Finally, set screw104can also be positioned so as to engage threaded area19of rod holding element10.

It will be appreciated that in creating a polyaxial screw assembly, it is preferred to first assemble the elements into a preassembly configuration, adjust the elements, and then create a final assembly. The preassembly configuration holds the elements in their generally desired arrangement; however the elements are still free to move relative to one another and have not yet been locked into a final position. For example, in the preassembly configuration, set screw104has only been partially tightened so that the elements can not disassemble. Polyaxial screw460can be adjusted as can polyaxial insert400, with respect to rod holding element10and rod105. Thus in the preassembled configuration a surgeon can move and adjust the elements in the polyaxial assembly until a desired configuration is achieved. At that point, the surgeon can further tighten set screw104until the final assembled configuration is achieved. It will further be appreciated that set screw104can be gradually tightened so that the mobility of the elements becomes gradually lessened. Further, set screw104can be “backed off” or partially unscrewed so as to lessen the tightness of the assembly, and once the assembly has been loosened the elements therein can again be adjusted. In this manner, a surgeon can manipulate the polyaxial assembly until a desired configuration is achieved.

It is here recalled that a rod adapter200could be used with a uniplanar insert70or a monoaxial insert40as a means to use different sized rods with the rod holding element10. Such a rod adapter200can also be used with a polyaxial insert400. Again, rod adapter200allows different sized rods to be used in creating polyaxial assemblies while keeping the same rod holding element10. Thus, the previous discussion regarding rod adapter200and its use in various assemblies is incorporated herein with respect to the polyaxial assembly. It is further noted that the steps shown inFIG. 22, steps for the assembly of a pedicle screw, can also be applied to embodiments of the polyaxial pedicle screw. Accordingly the discussion relating toFIG. 22is also incorporated with respect to polyaxial pedicle screw embodiments.

The following discussion points refer generally to all pedicle screw embodiments unless specifically noted. Referring again toFIG. 1, it is to be noted that in the preferred embodiment, rod holding element10includes flats21on an outer surface of rod holding element10. Flats21are typically a generally flat area, on mutually opposing sides of rod holding element10, configured for receiving any number of surgical tools. By way of example, a wrench could be applied to the flats21to rotate the rod holding element directly. Similarly, as would be familiar to those skilled in the art of spinal fixation devices, a rod reducing tool could be placed on the flats as a secure anchoring point for a tool to forcefully guide the rod into the rod cradle when alignment by hand is difficult. Such a manipulation of rod holding element10may be useful at various stages of surgery rather than manipulating some other element of the system.

As previously mentioned, a dovetail configuration of mutually assembled elements can be useful in assembling and locking pieces into a desired arrangement. Thus, rod holding element10can be configured with a dovetailed notch12and rod adapter200can be configured with a dovetailed tab205where both tab205and notch12are mutually configured to match each other. Additionally, the monoaxial insert40can be configured with a dovetailed locking tab43that also reciprocally matches the dovetailed configuration of notch12. This arrangement of elements is useful not only for firmly engaging pieces in a desired configuration; it can also help to prevent the head of the rod holder10from tending to splay outward when, as the set screw210is tightened, it exerts a force on the elements that tend to push outwardly against chamber walls17of rod holding element. The illustrated configuration of tabs and notches, where rod holding element notch12has a closed rather than open configuration, helps to resist that outward force. It is thus to be noted that the dovetail configuration described above with respect to the rod adapter200can be applied to other elements in the various pedicle screw embodiments, and is not limited to that single application.

Attachment means other than the described set screw210may also be used with the various pedicle screw embodiments described herein. For example an external nut can be applied as the fastening and tightening mechanism. Similarly, helically cut threads, which reduce cross-threading and head splay, may also be employed as a fastening and tightening mechanism.

It will be appreciated by those skilled in the art that a variety of thread patterns are known for use with pedicle screws. Thus, where a certain kind of thread pattern has been illustrated with a particular element, such as the threads by which the set screw is joined to the rod holding element or the thread pattern by which the pedicle screw itself is lodged in the bone, the illustrated thread pattern is presented for illustrative purposes only. The embodiments of the modular pedicle screw system can be used with a variety of different screw patterns and designs.

Additionally, bone screws may also vary beyond their thread pattern, and the embodiments described herein are generally useful with other varieties of bone screw than the particular examples described. More specifically, certain bone screws may have a cannulated design to assist in surgical placement and navigation to the correct location and angulation. Such cannulated bone screws may also be used with the described embodiments of the invention.

The materials that may comprise the various elements of the modular pedicle screw assembly are now discussed. Generally, any suitable material used for orthopedic implants may be employed. Particularly, those materials known to a practitioner skilled in the art that have been used for prior art pedicle screw designs may also be used in constructing the elements of the invention embodiments described herein. The rod holding element, the rod adapter element, and the insert element may likewise be fabricated of these same materials. Thus, by way of illustrative example only, titanium and stainless steel alloys may be used in fabrication.