Set screw and set screw driving tool for improved rod alignment

A set screw for threadable engagement with a head of a pedicle screw for holding a spinal rod, the set screw having a spinal rod facing side and a set screw driver facing side, the set screw including an opening at the set screw driver facing side for engaging with the set screw driver, and a convex surface at the rod facing side, an apex of the convex surface substantially corresponding with a rotational central axis of the set screw.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a United States national stage application of International patent application PCT/IB2020/052815 filed on Mar. 25, 2020 designating the United States, and claims foreign priority to International patent application PCT/IB2019/052451 filed on Mar. 26, 2019, the contents of both documents being herewith incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to orthopedics and more precisely to orthopedic tools and systems including pedicle screws, rods and corresponding set screws. The invention also relates to instruments which are used for manipulating these elements, and methods of using these elements.

BACKGROUND

In the field of orthopedics and implant tools and systems for orthopedic surgery, more specifically spinal fusion surgery for a spinal column, set screws are used to push down and attach a rod-type or bar-type device to a head of a pedicle screw. The process of pushing down the spinal rod towards and into the head of pedicle screw is also called rod reduction. Before attaching the rod to the head of the pedicle screw, the pedicle screw is attached to a vertebrae with a bone anchor, threaded bone-engaging part or bone screw for fastening the pedicle screw to the vertebrae of a patient or living being. For this purpose, as an example, for several adjacent vertebrae for vertebrae fusion, for each vertebra a pedicle screw is attached thereto, and thereafter, several pedicle screws are mechanically fastened towards each other by the use of the rod that is placed in a groove or U-shaped opening that is formed by the pedicle screw head, forming a row of pedicle screws in the spine. This allows to provide for the mechanical support needed for spinal stabilization for spinal fusion in a patient or living being.

For example, U.S. Pat. No. 10,058,355, this reference herewith incorporated by reference in its entirety, describes an orthopedic implant kit that provides for a pedicle screw, a corresponding set screw, a rod, and the tools to operate these, including a screw extender for holding the pedicle screw, and a set screw driver for tightening the set screw to the head of the pedicle screw. As another example, U.S. Pat. No. 8,795,283, this reference herewith incorporated by reference in its entirety, describes another type of kit orthopedic surgery system for surgical intervention for spinal stabilization, including pedicle screw with a head for receiving a rod, and tools necessary for the surgical intervention. In yet another example, U.S. Pat. No. 8,262,662, this reference herewith incorporated by reference in its entirety, provides for a system and method for delivering a spinal connector spinal anchor sites in a spinal column. In one embodiment, a spinal implant and access device is provided that includes a U-shaped receiver member, a bone-engaging member, an extension member, a spinal rod, and a set screw.

Similar orthopedic spinal surgery concepts, tools and devices have been proposed as discussed above, for attaching a rod to a pedicle screw via a set screw, for example U.S. Pat. Nos. 5,129,388, 5,147,360, 5,520,689, 5,536,268, 5,720,751, 5,817,094, 5,882,350, 5,984,923, 6,056,753, 6,113,601, 6,183,472, 6,258,090, 6,454,768, 6,648,888, 6,740,086, 7,618,442, 8,308,782, 8,876,868, U.S. Patent Publication No. 2006/0025771, and U.S. Patent Publication No. 2018/0289397, all of these references herewith incorporated by reference in their entirety.

However, the state of the art tools still present specific problems when a surgeon or operator of the spinal surgery tools needs to attach the rod to the pedicle screw by means of the set screw, that are due to misalignment of the spinal rod relative to the pedicle screw head. Therefore, despite all of the solutions currently proposed in the state of the art related spinal surgery tools, strongly improved methods, systems and devices for spinal surgery are strongly desired.

SUMMARY

According to one aspect of the present invention, a set screw for threadable engagement with a head of a pedicle screw for holding a spinal rod is provided. Preferably, the set screw has a spinal rod facing side and a set screw driver facing side, and includes an opening at the set screw driver facing side for engaging with the set screw driver; and a convex surface at the rod facing side, an apex of the convex surface substantially corresponding with a rotational central axis of the set screw.

According to another aspect of the present invention, a set screw for threadable engagement with a head of a pedicle screw for holding a spinal rod is provided. Preferably, the set screw has a spinal rod facing side and a set screw driver facing side, and includes an opening at the set screw driver traversing the set screw from the spinal rod facing side to the set screw driver facing side, and an annular surface at the rod facing side surrounding the opening, the annular surface being beveled or having a curved surface.

According to still another aspect of the present invention, an orthopedic tool kit is provided, including a set screw and a set screw driver. Preferably, the set screw configured for threadable engagement with a head of a pedicle screw for holding a spinal rod, the set screw having a spinal rod facing side and a set screw driver facing side. In addition, the set screw includes an opening at the set screw driver traversing the set screw from the spinal rod facing side to the set screw driver facing side, and an annular surface at the rod facing side surrounding the opening. Furthermore, preferably, the set screw driver includes a shaft, and an engagement part for engaging with the set screw via the opening, wherein in an engaged position, a frontal portion of the engagement part is protruding from a spinal rod facing side of the set screw.

According to yet another aspect of the present invention, a pedicle screw is provided, the pedicle screw having screw head with a U-shaped groove for accommodating a spinal rod. Preferably, a lower surface facing a lower surface of the spinal rod is forming a bottom of the U-shaped groove is semi-cylindrical, and has a curvature along a radial direction, the radial direction extending away from a center axis of the screw head, to provide for a smooth surface with less sharp edges towards the spinal rod.

BRIEF DESCRIPTION OF THE SEVERAL EMBODIMENTS

FIG.1Ashows a cross-sectional view of several pedicle screws11,12, or other types of back surgery screws, that are engaged with individual vertebrae V1, V2of a spine of a patient or living being, and interconnected with a spinal rod7or support rod, and are held by corresponding set screws31,32, or fastening devices. As shown on the left side of this representation, an axis of longitudinal extension RA1of spinal rod7is substantially perpendicular to a center axis HA1of screw head21of pedicle screw11. Moreover, the bone anchor or threaded part41of pedicle screw11also defines a center axis SA1, that can coincide with center axis HA1of screw head21in case of a mono-axial configuration, or can be different from center axis HA1of screw head21in case of a poly-axial configuration. For illustration purposes, pedicle screw11is in a mono-axial configuration, but it is also possible to use poly-axial screws.FIG.1Bshows a cross-sectional view along axis of longitudinal extension RA1of spinal rod7, depicting the U-shaped groove25or other type of cavity, opening, trench, depression, or mechanism in screw head21for accommodating spinal rod7. Screw head21has an internal threading27that is complementary or corresponding to an external threading of set screw31,32, so that the set screw31,32can threadably engage and be tightened relative to screw head21.

As shown on the right side ofFIG.1A, at least at some sections along the spinal rod7, it is possible that orientation of spinal rod7relative to screw head21is not perpendicular, for example angle α2between axis of longitudinal extension RA2of spinal rod7is not 90° degrees or perpendicular to center axis HA2or the thread axis of screw head22of pedicle screw12, but there is an inclination angle α2. Generally, as rod7is pre-bent before insertion and attachment to pedicle screws11, as an example, this misorientation can be a result of the bending of spinal rod7at one end, for example in case where the last pedicle screw12of a row or series of pedicle screws has the end of rod7is protruding upwards away from the spine. It is also possible that this deviation from the perpendicular arrangement is simply a misalignment between screw head22and rod7that cannot be easily corrected by the surgeon or operator, as the incision at the surgical location is not easy accessible or viewable, for example due to the minimal invasive surgical approach. Another reason for the misalignment is caused by the user, operator, or surgeon itself, when he attempts to tighten the set screw31to screw head21, via screw extender6and set screw driver20, as shown inFIG.1C. While the alignment between spinal rod7and pedicle screw head21may initially be correct, the surgeon may have moved or titled the screw extender6laterally by a certain angle, and therefore has bent or moved the screw head21relative to rod7, to cause the misalignment. This is common in surgeries where the incision for the surgery is small and does not allow for an easy view of the pedicle screws21,22and rod7.

As a result thereof, when the surgeon or operator attaches spinal rod7to pedicle screw12with the set screw32, there are several problems that can arise. This is illustrated in on the right side ofFIG.1A, and further detailed inFIG.1C, showing a schematic cross-section view of a screw extender6attached to pedicle screw12, where the set screw driver20is threadably engaging with inner threaded portion19of the screw extender6, the screw extender6attached to screw head22with engagement part21, to screw in the set screw32to lower the rod7into the U-shaped groove26provided by screw head22. As shown inFIG.1C, angle α2between axis of longitudinal extension RA2of spinal rod7is not 90° degrees or perpendicular to center axis HA2of screw head22of pedicle screw12, but for illustration purposes in a range between 60° and 80°. Upon tightening set screw32within screw head22by set screw engagement part21of set screw driver20, in the variant shown a shaft that is in threadable engagement with an internal thread19of the screw extender6, an edge of the set screw32will contact an upper, outer surface point of spinal rod7, forming a single contact point CP1. Would the alignment be perpendicular, i.e. α2=90°, a full front flat surface33,34of set screw31,32would come into contact with upper, round outer surface point of spinal rod7, forming a line of contact, and not a singular contact point CP1. This is shown on the left side ofFIG.1A, with pedicle screw11. This contact point CP1will also be off-axis of the center axis HA2of pedicle screw head22, by a distance OD1. The bending or misalignment of spinal rod7can be such that this contact point CP1may be the only substantial mechanical contact point that acts on rod7, other than a minor lateral support contact by side walls of U-shaped groove26. In addition, due to the misalignment, in addition or alternative to contact point CP1, at the opposite lateral side of screw head22, spinal rod7will contact an edge of one end of the U-shaped groove26, to form a semi-circular contact line or arc CP2, instead of rod7being embedded in the semi-cylindrical lower surface of U-shaped groove26. This contact line CP2will also be offset from the center axis HA2of head22of pedicle screw12, by a distance OD2.

As the spinal rod7is held in place by several other pedicle screws11,12and corresponding set screws as shown inFIG.1A, the tightening of set screw32with respect to screw head22may not provide enough compression or bending force to bend spinal rod7into a perpendicular arrangement with respect to screw head22, for example to realign it by pressing rod7against U-shaped groove26, as the spinal rod7is generally very stiff. In addition, this misalignment will causes stress onto one or more vertebra V2that are affected by the misalignment via pedicle screw12and its bone anchor, and create lateral forces to the vertebra V2, as these can only move relative to other vertebra by a limited range. Also, in case a poly-axial pedicle screw12is used, the angular range of screw head22by axis HA2relative to anchor42by axis SA2may be not sufficient and is mechanically blocked to be able to compensate for the misalignment between rod by axis RA2to screw head22by axis HA2, creating additional mechanical stress to affected vertebra V2.

The resulting undesired limited contact surface or region between rod7and screw head22, either by contact point CP1at an lower edge of flat surface34of pedicle screw32with a point of the cylindrical surface of rod7, or by contact point or arc CP2at the opposite lateral side of screw head22, for example by a semi-circular arc along an edge of U-shaped groove25, can lead to several problems.

First, due to the offset between the central axis of screw head HA2, for example by offset distance OD1or offset distance OD2, this attachment situation will lead to a lateral undesired torque that is applied to bone anchor or threaded part41, that in turn will create lateral tensions to the bone structure of vertebrae V2of patient or living being. Any upward or downward pressure or movement that is exerted on spinal rod7, for example in parallel or substantially in parallel to axis HA2, will lead to a lateral pressures of bone anchor41to vertebrae V2, as indicated by the arrows shown on the right side ofFIG.1Bshowing vertebrae V2. For example, post-surgery, bodily movements of the patient or living moving could cause such pressures or motions to spinal rod7, which will lead to strong lateral tensions to vertebrae V2via bone anchor41.

Second, the limited surface area of the contact points CP1, CP2, or both, can lead to undesired cold welding, that could even happen during the surgical operation. Such cold welding attachment could block further attachment or threading of set screw32relative to screw head22, and would prevent from properly attaching rod7to pedicle screw12.

Third, the pressure caused offset distance OD1or offset distance OD2that is not in axis with central axis of screw head HA2could lead to a blocking of set screw32inside screw head22, when an user, operator, or surgeon is tightening the set screw32with set screw driver. The generation of a pressure that is off-axis of HA2will lead to different and non-asymmetrical lateral pressures to set screw32to the threaded part of screw head22, that could lead to a blocking of set screw32inside screw head22. This in turn can lead to damage of the threads that can further block the set screw32to the screw head22, and/or create additional undesired friction between the rotational threadable engagement between set screw32and screw head22that could lead to potential cold welding. A similar cold welding problem can arise with some of the state of the art set screws that show pointed tips that permit additional grip to the rod, for example as shown in U.S. Patent Publication No. 2018/0289397 or U.S. Pat. No. 5,129,388, these references herewith incorporated by reference in their entirety.

FIGS.2A-2Dshow different exemplary cross-sectional views of a set screw130according to an aspect of the present invention, to alleviate the problems described above. As shown inFIG.2A, showing a cross-sectional and frontal view of set screw130, set screw130does not have a flat surface34that faces the rod7, but has a frontal surface or face FF forming a convex shape and circular-symmetrical around axis HA2. In the variant shown, and as illustrated in more detail inFIG.2D, a central area or inner area IA of frontal face FF forms a first surface135with a curvature having a first radius R1, the curvature being defined in a radial direction of the cylindrical shape of the set screw130, and an outer area or annular surface ring OA of frontal face FF forms a second surface134having a second curvature having a second radius R2, and an edge area133or annular surface ring edge area EA having a flat but beveled edge. Moreover, the rod7is shown to have a cylindrical smooth surface with no indentations or notches. Preferably, the inner area IA has more curvature than the outer area OA, e.g. the first radius is smaller than the second radius, but it is also possible that IA and OA form a continuous spherical surface with the same radius R1=R2. In a variant, starting from a central point or center of the front surface, for both inner area IA and outer area OA, the radius change of frontal face FF is progressive, i.e. it is the smallest at the center, and progressively changes to a larger radius towards the edge, for example by following an elliptic shape when seen from a cross-sectional view, in other words a shape of an ellipsoid, with the vertex of the ellipsoid located at the rotational center of frontal face FF. In another variant, the entire frontal face FF including areas IA, OA, and EA are spherical, and the slope of the edge area EA is chosen to fit the flanks or thread angle of threading137of the set screw130. In another variant, for both the inner area IA and outer area OA form a spherical surface for frontal face FF, with a constant radius of curvature, but still having the beveled circular outer edge EA for the threading137. With this arrangement of frontal face FF of set screw130, showing a continuous convex or protruding bulge along an entire diameter of set screw130, covering circular inner area IA, annular outer area OA, and annular edge area EA, it is also possible to avoid any sharp edges, for example edges that are formed by two surfaces arranged perpendicularly which lead to an edge angle of 90°, that could cut in or otherwise damage a surface of the spinal rod7, even if spinal rod7is arranged at an oblique angle relative to screw head22. In this respect, an entire surface of the spinal rod facing side of the set screw130forms a convex volume that protrudes from a surface that is perpendicular to the thread axis of set screw. The only sharp edges of set screw130of 90° or less are arranged on the side walls of set screw130with the crests of the threading137, but these are such that they do cannot come into contact with spinal rod7when set screw130is threadably engaged with head22.

Preferably, the radii R1, R2of the curvature is more than a radius of the set screw of pedicle screw12, and preferably, about the half of the diameter of an outer cylindrical surface of screw head22of pedicle screw12or the diameter of set screw130, ±25%, more preferably at least two (2) times the radius of screw head22or more, or at least two (2) times the diameter of set screw130. Smaller radii are possible, but would lead to set screws that are thicker and would require more space inside the body of the patient or living being. The edge area EA preferably does not have a curvature, but is beveled or otherwise shaped to match the flank of the threading137of the set screw130, to be able to keep the set screw130as thin as possible with respect to thickness TH, when viewed along the axis HA2, for example by an angle of 45°. Moreover, set screw130has an opening132or other type of releasable attachment or engagement mechanism for engaging with corresponding or complementary engagement part21of set screw driver20. In the variant shown, a hexagonal socket head for opening132is provided, but different types of releasable attachment or engagement mechanism between set screw130and screw driver are also possible, for example but not limited to filister heads, torx heads, spanner head with two or more drills, square heads, clutch heats, multiple slot heads, and with corresponding complementary engagement tools or parts21by set screw driver20. Also, set screw130is designed to keep the overall profile low, and for the set screw130this means that the thickness TH is minimized to avoid needing screw heads22that have are higher build, to minimize the space requires inside the body of the patient or living being that protrudes from the vertebrae, so that there is reduced protrusion of pedicle screw from the spine. Therefore, according to an aspect, set screw130is designed as one piece, such that the front face FF is an integral part of the set screw130that is made of one piece of material, without any moving parts attached or otherwise integrated thereto, having the convex front face FF. Also, a distance OS between a minor diameter of the threading137of set screw130and a diameter of outer annular area OA can be zero, such that a width of the edge annular area EA matches with the distance between the root and the crest of the thread137, but in a variant, it is also possible that the distance OS is positive such that the diameter of outer annular area OA is smaller than the minor diameter of threading137.

As shown inFIG.2Bdepicting a side cross-sectional view with frontal surface FF set screw130engaging with an obliquely arranged spine rod7relative to screw head22, in other words an angle α2between axis RA2and HA2is not 90°. This lead to a contact point CP3that is much approached to the central axis HA2of the screw head2, to an offset distance OD3. Comparing to the contact point CP1shown inFIG.1C, the distance could be reduced by a factor 3 (three), given that misalignment angle α2is substantially the same. This will lead to a strong reduction by a factor 3 (three) of unwanted lateral torques that are applied to bone anchor41to vertebra V2, upon movement of spinal rod7along axis HA2.

Moreover, as a surface of frontal face FF of set screw130has a curvature that will engage or abut against spinal rod7, which has an outer surface that is also curved and has the shape of a cylinder, a contact surface at contact point CP3will be substantially larger than a contact surface formed by contact point CP1the sharp edge of set screw32, in the variant shown by an edge angle of 90° or less, as there will be certain amount of deformation of an area of contact of spinal rod7and frontal face FF of set screw130. This can reduce or even entirely eliminate the problems of cold welding at the contact point CP3. In addition, as shown inFIG.1C, an edge of set screw32will be turned or rotated against the cylindrical surface of spinal rod7, when a user, operator, or surgeon turns the set screw32via set screw driver20relative to screw head22. Thereby, the edge will act like a knife and can provide for a cutting action by cutting a groove into spinal rod7with edge, to thereby damage an outer surface of spinal rod7. As in the variant shown inFIG.2C, a curved surface, frontal face FF of set screw130, is turned against another curved surface, cylindrical surface of rod7, this cutting can be entirely avoided. Frontal face FF of set screw32can also be hardened or coated with a hard surface, to provide for additional hardness relative to its body, for example but not limited to the use of a chrome-cobalt alloy or anodization. Materials used to manufacture set screw32can be the usual materials used for implant devices, for example but not limited to titanium, different types of titanium alloys with different grades, stainless steel, CrCoMo.

According to another aspect of the present invention, as shown inFIGS.3A to3D, a set screw230is provided that has a traversing opening or through hole232that fully traverses or crosses through a center axis of set screw230, at least a part of the opening232serving as an engagement mechanism to engage with set screw driver200. In addition, according to another aspect of the present invention, the engaging tool or mechanism220of set screw driver200is dimensioned to slightly protrude outside of front face FF of set screw230by engaging tool220traversing the opening232, when fully engaged with set screw230, by a distance DD from the outermost point of the set screw230. For example, the distance DD can be in a range between 0.05 mm and 0.5 mm, more preferably between 0.1 mm and 0.4 mm. For example, as shown inFIGS.3A to3C, the engagement tool220of set screw driver200is shown to be fully engaged with set screw230, which means engagement tool220is blocked or otherwise mechanically prevented from being further introduced or traversing the set screw230, for example with an abutment surface231of set screw230, and a corresponding surface or abutment ledge or other mechanical means on set screw driver200. Other types of mechanical arrangement can be used to limit the penetration of set screw driver200relative to set screw230. In this engagement position, a front tip surface222of engagement tool220slightly protrudes outside of set screw230by a distance DD, taken the highest or most protruding point of the front face FF of set screw230, being the circular edge236, as illustrated inFIG.3B. In addition, in this fully engaged position between set screw230and tool200, a distance between set screw230and tool200is set such that an outer threading237of set screw230and an outer threading of set screw driver200are aligned with each other to form a common virtual threading spiral line, in other words both threads or threadings will be aligned such that the threading pitch matches, as indicated inFIG.1C, so that set screw230and tool200can be both together be moved by rotative threadable engagement through inner thread19of screw extender6without adjusting a position between set screw230and tool200. Analogously, which the full insertion of screw extender6with head22, inner thread of screw head22will match a distance to inner thread19of screw extender to provide for a continuous virtual threading spiral line.

In this embodiment, front face FF of set screw230, as shown inFIGS.3C and4C, defined as the front face FF that will be facing and engaging with spinal rod7includes an annular ring section234that is not flat, in other words, not perpendicular to a central or rotational axis of set screw230, but forms a slope towards an outer edge of set screw230in a radial direction away from the central axis, and preferably also has a curved or spherical surface with a curve or sphere radius, also when seen in a radial direction, as can be seen inFIG.4A. In this variant, front face FF includes a circular edges236that forms the portion of set screw230that is the most forwardly protruding or outermost part of set screw230, and a radially outermost beveled annular edge233that forms part of the threading of set screw230. A slope or inclination angle of annular edge233that forms part of the threading is larger or steeper, for example 45°, than a slope or inclination angle of a tangent that is placed on annular ring section234, for example in a range between 15° and 30°, as indicated by the dashed lines ofFIG.4A. In the variant where annular ring section234is annularly curved or has a spherical shape, a curvature or sphere radius is preferably more than half of the outer cylindrical diameter of screw head22or the outer diameter of set screw230, ±25%, more preferably at least two (2) times the radius of screw head22or more, or at least two (2) times the radius of set screw230.

As of another variant, annular ring section234is flat, in other words a surface that defines the extension of the flat annular ring section234is perpendicular to a central axis of set screw230. This embodiment allows to avoid edge contact points CP5as shown inFIG.5B, and instead a linearly and radially extending portion of flat surface of ring section234can contact rod7, in a direction that is parallel to the extension of rod7. For example, upon making contact with tip of tool200with contact point CP4at rod7, as a result of the threading action of set screw230by tool200as a set screw driver as shown inFIG.5A, tool200is pushed out from set screw230, by the force P, as shown inFIG.5B, and with the additional threadable engagement of set screw230with internal threading27, instead of small contact points CP5, the flat surface of annular ring section234will make a contact line CL with rod7, given that rod7lies substantially parallel to an axis of lateral extension of U-shaped groove26of screw head322. Such set screw230with a flat annular ring section234could be used in the cases where the rod7does not lie obliquely inside U-shaped groove26but is somewhat perpendicularly arranged to the screw, as shown inFIGS.5A and5B. This can be in contrast to the use of a beveled, curved or spherically-shaped annular ring section234(non-flat) of set screw230, used in a case where rod7lies obliquely to the U-shaped groove26, as visualized inFIG.3D. These two different types of set screws230allow a surgeon or operator to selectively choose flat-faced or non-flat faced screws depending on an orientation of rod7inside U-shaped groove26of screw head322.

As can be seen inFIGS.3B,3D, and4B, when set screw driver200is fully engaged with set screw230, an apex point of tip222is protruding from outermost edge236of frontal face FF of set screw230by a distance DD. In addition, a front tip face222itself is curved, for example it can be spherical, can form part of ellipsoid, or can be progressively curved with a smaller curvature radius at an inner central section, and having a larger curvature angle at an outer annular section. As shown inFIGS.3B and3D, with this arrangement, an circular edge that is formed between front tip face222and side walls of engagement tool220lies within the opening232of set screw230, and this can serve to avoid the contact of a sharp edge with spinal rod7. As shown inFIGS.3D and5A, with this arrangement, an orientation of spinal rod7expressed by its longitudinal axis RA2with respect to screw head22and central axis HA2can vary within a certain angular range, and the contact point CP4made between set screw230and set screw driver assembly200will be between front face tip222and a surface point of spinal rod7. No contact will be made between front face FF of set screw230and spinal rod, while the user or operator is tightening set screw230with set screw driver200, given that RA2and HA2remain within a certain angular range, for example front face FF and front tip face222, as well as protrusion depth of engagement tool220of set screw driver200can be designed to allow for an angular variation between 70° and 110° of RA2and HA2. Front tip face222of set screw driver200can be made of hardened stainless steel.

The contact point CP4between rod7and set screw driver200allows to create a counter pressure or counter force P against engagement part220of set screw driver200, so that set screw driver200will be progressively pushed back and released from set screw230, upon the tightening of set screw230with set screw driver200, and with the progressive pushing back or release, the outermost surface of set screw, being edge236, will come into contact with spinal rod7, to form contact points CP5, as illustrated inFIG.5B. As illustrated byFIGS.5A and5B, an initial first contact that is made by front tip face222of set screw driver200with rod7with contact point CP4, while threadably tightening set screw230to screw head22. Thereafter, with the progressive release of set screw driver200from set screw230by a pushing back with force P, by the tightening action, contact between rod7and set screw230will be made with contact points CP5. This can be done as long as rod7is in initial contact with set screw driver200and not the set screw230.

This presents several advantages for an orthopedic tool kit or system that uses such set screw230with a traversing opening232and corresponding set screw driver200, as discussed above. First, it allows to make a first contact point CP4with spinal rod7that minimizes a distance of CP4from the central axis HA2of screw head22of pedicle screw21, even in a case where longitudinal axis RA2of rod7and central axis HA2of screw head22are oblique to each other. In addition, the counter force P against engagement part220of set screw driver200allows to substantially reduce a retention force that is created between a full engagement of engagement part220with set screw230, and allows to substantially reduce a force necessary to remove set screw driver200from set screw230.

FIGS.6A,6B, and6Cshow another aspect of the present invention, in which a screw head321for a pedicle screw311is presented, that allows to alleviate a sharp contact between spinal rod7and an edge at the groove25of head22of pedicle screw12, as illustrated inFIG.1C, where a sharp contact edge angle of 90° at the cylindrical outer surface of screw head can be created at contact point CP2, in a case where spinal rod7lies obliquely to head22. As shown inFIGS.6A and6B, a screw head321has a groove325that has less sharp edges327at a an interface between the outer cylindrical surface of screw head321and the semi-cylindrical surface formed by groove325, as a curvature along a radial extension of groove325is provided, for example by a radius R3. For example, the radius R3can correspond to a diameter of screw head321, or can be larger than the diameter. This allows to reduce edge angle of edge327, for example to form an edge angle of 100° or more.

With this arrangement, when spinal rod7lies obliquely to head321, a contact point between rod7and groove325can be such that it does not lie on edge327, but somewhere on outer area OA3that is radially curved with radius R3or on inner edge area EA3, outer area OA3defined as being the radially outer areas of groove325of head322at edge327, and inner edge area EA3defines as being the radially inner areas of groove325at inner edge329, as shown inFIG.6B. As defined inFIG.1C, this will also reduce an offset distance OD2between rod7and contact point CP2, which will now lie closer to axis HA2. The curvature by radius R3can be arranged such that at inner edge329, the edge angle is 90°, in other words a tangent to surface EA3at edge area329is perpendicular to center axis HA2of screw head322. In a variant, edge area EA3is flat when seen in the cross-sectional view, which means it forms a semi-cylindrical surface, and perpendicular to center axis HA2of screw head322, and the curvature with radius R3starts at outer area OA3. This allows to provide for a semi-cylindrical contact surface for contact with rod7when rod lies perpendicular to center axis HA2. As of another variant, radius R3progressively increases with an increased radial distance, for example such that radius R3at outer area OA3is larger than radius R3at edge area EA3. For example, radius R3at outer edge327of outer area OA3can be made such that edge angle at edge327is more than 120°, or more than 135°.

In the variant shown ofFIG.6A, the bone anchor42and head322are arranged as a blockable poly-axial screw as the pedicle screw312. Top connecting element or head44of bone anchor42and groove325are arranged such that in any possible angular position of bone anchor42relative to screw head322, bone anchor cannot touch spinal rod7. This is done by preserving a minimal distance OD5between head44of bone anchor42that is not zero. In the variant shown, a virtual spherical surface that is formed by partially spherical head44of poly-axial bone anchor42does not reach or otherwise protrude above a level that is defined by inner edge329of edge area EA3. In case a mono-axial screw is used, distance OD4should be bigger than zero.

It also possible that a lower section of screw head322includes additional elements, for example an additional seat, ring or compression element326for engaging with head44of bone anchor42, located within groove325, as shown inFIGS.3D,5A,5Bhaving a sharp edge, and as shown in a cross-sectional view inFIG.6C. For example, such pedicle screws that can block polyaxiality by such mechanism have been described in U.S. Pat. Nos. 5,882,350, 6,113,601, and 6,660,004, these references herewith incorporated by reference in their entirety. Another variant is the Medtronic™ CD Horizon Solera pedicle screw system that allows for such blocking, having a blocking ring or seat that urges against the spherical head of pedicle screw. With a downward pressure of set screw3to spinal rod7onto element326, the angular position between bone anchor42and head322will be blocked. In such variant, surfaces of screw head322that form a spinal-rod facing side of groove325, and surfaces of seat that face spinal rod7, can be made to be continuous, for example such that the surfaces are flush with each other, being substantially semi-cylindrical, and having curvature in a radial extension from central axis HA2, as discussed above, to make sure that no sharp edges are present that will face or otherwise come into contact with spinal rod7, even in a case there spinal rod7is oblique to axis HA2. For example, the face of element326can have a flat surface seen in the cross-sectional view ofFIG.6Cat edge329, and then is curved at an inner outer section towards edge327, such that in compressed state, when spinal rod7presses down on element326, a surface of U-shaped groove325facing rod7and the surface of element326facing rod7are continuous, for example such that the radii of curvature at the edge of element326is matching with the neighboring area of the surface of U-shaped groove, and the surfaces are aligned with each other.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments, and be given the broadest reasonable interpretation in accordance with the language of the appended claims.