Variable angle bone fixation device

A bone fixation element includes a threaded head and a shaft extending along a longitudinal axis from a proximal end to a distal end, an outer surface of the head being one of carburized and nitrided and including a first groove extending into an outer surface of the head along a path interrupting the threading and extending along an angle counter to an angle of the threading.

BACKGROUND

Bone fixation plates are often positioned over a fractured or otherwise damaged portion of bone and secured thereto using bone screws inserted through screw holes of the bone fixation plate. The screw holes extend transversely through the bone plate and are sometimes formed with threads to lockingly engage a threaded head of the bone screw. Variable angle screws are often employed which permit a user to insert the screw through the plate at a user-selected angle relative to an axis of the plate hole. However, the engagement threads of the head of such variable angle screw heads with the threading of the plate hole may burr threads of one or both of the bone screw and the bone plate, causing a loss in bone fixation strength. Damage to the bone plate or bone screw in this manner may cause the bone fixation procedure to lose efficacy. Those skilled in the art continue to search for ways to increase the strength of the screw-plate interface in variable angle systems.

SUMMARY OF THE INVENTION

The present invention is directed to a bone fixation element comprising a threaded head and a shaft extending along a longitudinal axis from a proximal end to a distal end, an outer surface of the head being one of carburized and nitrided and including a first groove extending into an outer surface of the head along a path interrupting the threading and extending along an angle counter to an angle of the threading.

DETAILED DESCRIPTION

The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The present invention relates to the stabilization of bones and, in particular, to the stabilization of a fractured or otherwise damaged bone using a bone screw inserted through a bone fixation device (e.g., a bone plate). Exemplary embodiments of the present invention describe a variable angle bone screw having a threaded head and a threaded shaft and having a carburized or nitrided outer surface configured to increase a surface hardness thereof to a desired level. The threaded head comprises one or more grooves extending into an outer surface thereof at an angle relative to a longitudinal axis of the bone screw to aid in alignment of the threads of the head with threads of a variable angle screw hole of the bone fixation device. The shaft comprises one or more notches extending into an outer surface thereof at any angle relative to the longitudinal axis within a permitted range of angulation, as will be described in greater detail later on. In one embodiment, the bone plate may be formed of a metallic alloy exhibiting a hardness within a predetermined range. The bone screw may be carburized or nitrided such that an outer surface of the bone screw has a hardness greater than a hardness of the bone plate. Thus, the exemplary bone screw according to the invention minimizes burring of the screw during insertion into the bone plate while providing a consistent connection strength to the bone and bone plate. Furthermore, the exemplary system according to the invention reduces galling during use while also providing an increased overall strength when compared to standard screws including increased yield strength, ultimate tensile strength and fatigue strength, as those skilled in the art will understand. It should be noted that the terms “proximal” and “distal” as used herein, are intended to refer to a direction toward (proximal) and away from (distal) a user of the device.

Embodiments of the present invention are formed of an implant-grade material having a carburized or nitrided outer surface. Implant-grade materials are those suitable for permanent implantation in the body—i.e., materials which will not have adverse health effects if left within the body for extended periods of time. The carburized or nitrided outer surface is selected to have a hardness greater than that of a bone being treated. In contrast to bone fixation devices which are formed of non-surface treated implant-grade material and often buckle or break when subjected to drilling, chiseling or reaming forces, exemplary bone fixation devices according to the invention are able to withstand increased levels of force without buckling or otherwise deforming. An exemplary bone fixation device according to the invention is formed with a carburized or nitrided outer surface which minimizes the burring of threads or the dulling of sharpened surfaces during insertion into the bone, permitting the continued use of the same bone fixation device without sharpening or replacement. Furthermore, the exemplary implant-grade material of the invention provides tactile feedback to prevent or inhibit breakage thereof. Specifically, the material is formed such that, when excessive force is applied thereto, the device will undergo a degree of bending instead of shattering. Thus, a surgeon or other user may react to the bending and eliminate/reduce a force being applied thereto to prevent breakage. The exemplary carburized or nitrided implant-grade material according to the invention offers the additional advantage that even if a small fragment thereof were separate from the device and inadvertently enter the body, removal would not be necessary, as will be described in greater detail hereinafter. If a device were to fracture, the exemplary material treatment according to the invention renders edges of fractured portions smoother and more rounded as compared to non-treated materials reducing trauma to tissue. Thus, exemplary bone fixation devices according to the invention exhibit increased overall strength when compared to non-surface treated bone fixation devices formed of implant-grade material, including increased yield strength, ultimate tensile strength and fatigue strength, as those skilled in the art will understand.

As shown inFIGS. 1-6, a bone screw100according to an exemplary embodiment of the invention extends from a proximal end102comprising a head104along an elongated shaft106to a distal end108. In an exemplary embodiment, an outer surface of the head104is substantially spherical to permit variable angle insertion of the bone screw100into a bone fixation device200, as will be described in greater detail later on. It is noted, however, that the head104may be formed in any other shape without deviating from the scope of the invention (e.g., to permit a single-angle insertion of the bone screw100into the bone fixation device200). The outer surface of the head104is provided with threading110having a pitch configured to lockingly engage threading212formed on a walls of an opening202extending through the bone fixation device200, as will also be described in greater detail later on. One or more grooves112may be provided on the head104, each groove112extending at least partially into the threads110and extending along an axis substantially angled with respect to a longitudinal axis114of the bone screw100. The grooves112are configured to interrupt the thread110, thus creating a plurality of thread starts which aid in alignment of the thread110with the threads212of the hole202in an operative configuration especially when the bone screw100is inserted into a bone plate hole angled with respect to an axis of the bone plate hole (i.e., when the threading of the head104is misaligned with the threading of the bone plate hole). The grooves112further permit the bone screw100to advance distally into the bone when rotated via a driving mechanism (not shown).

Each of the grooves112may be angled, for example, at an angle of approximately 8.5±1° relative to the line B-B, although any other angle may be used without deviating from the scope of the invention. In an exemplary embodiment, the grooves112are angled counter to a direction of the threading110. For example, as seen inFIG. 1, the line B-B is perpendicular to the path of the threading110and the groove112is angled relative to the line B-B so that, traveling along the threading110from a proximal end110A thereof toward a distal end110B, the angle between the threading110and the groove110is greater than 90° on the proximal side of the thread and less than 90° on the distal side of the thread. In another embodiment, the grooves112extend at an angle of approximately 5-85° relative to the line B-B (i.e., 95° to 175° relative to the threading110). In yet another embodiment, the grooves112may extend substantially parallel to the line B-B. The grooves112according to this embodiment extend along substantially a complete length of the threading110. In another embodiment (not shown), the grooves112may extend for only a partial length of the threading110.

In a first exemplary embodiment of the invention, the bone screw100may be formed with five grooves112disposed evenly circumferentially about the head104and equidistant from one another, as shown inFIG. 5. Specifically, each of the grooves112in this embodiment is separated from adjacent grooves112by approximately 72°. In another embodiment (not shown), the bone screw100comprises six grooves112separated from one another by approximately 60°. In yet another embodiment, as shown inFIG. 6, the bone screw100may comprise eight grooves112separated from one another by approximately 45°.

The head104may further comprises a recess116extending thereinto from the proximal end102. The recess116is configured to permit engagement with a distal end of a driving mechanism (not shown) for applying torque to the bone screw100as would be understood by those skilled in the art. The embodiment ofFIGS. 1-6is depicted with a torx-shaped recess116. It is noted, however, that any other shape may be employed without deviating from the scope of the invention (e.g., slotted, phillips, square, hexagonal, etc.), as those skilled in the art would understand.

The shaft106is provided with threading118having a pitch substantially the same as the pitch of the threads110. In another embodiment of the invention (not shown), the pitch of the threading118may be greater than or smaller than the pitch of the threads110. The threading118of the shaft106may be formed with two leads, as those skilled in the art will understand. The multi-lead configuration of the threading118aids in linear advancement of the bone screw100into the bone, as those skilled in the art will understand. As would be understood by those skilled in the art, the length of the shaft106is generally selected to conform to requirements of a target procedure. A distal portion of the shaft106may comprise one or more notches120configured to create a gap in the continuity of the threads110and permit self-tapping of the bone screw100, as those skilled in the art will understand. The distal portion of the shaft106may taper to a smaller diameter at the distal end106to, for example, aid in insertion. The distal end106may be sharpened or blunt as desired.

The bone screw100may be formed of a material selected to have a greater hardness that a material of a bone fixation device200with which it is to be employed. Specifically, the bone screw100may be formed of one of stainless steel and CCM (Co-28Cr-6Mo Alloy). The bone screw100may then be carburized or nitrided to further increase a surface hardness thereof to approximately 68 HRC or more, as those skilled in the art will understand. In an exemplary embodiment, the hardness of the bone screw100may be approximately 67-74 HRC and, more particularly, 67.5-70.3 HRC. In contrast, the bone fixation device200may be formed of commercially pure Titanium grades 1, 2, 3 and 4, Ti-6Al-7Nb, Ti-6Al-4V, Ti-6Al-4V ELI, Ti-15Mo, CCM (Co-28Cr-6Mo Alloy), stainless steel or another material different than the material of the bone screw100. As those skilled in the art will understand, a hardness of the bone fixation device200may be between approximately 75 HRB (e.g., for a CP1 material) and approximately 45 HRC (e.g., for a CCM material). This configuration minimizes burring of the threads110of the bone screw100as they are inserted into the bone fixation device100while also increasing a holding strength of the bone fixation system in the bone.

The bone fixation device100is formed of an implant grade material selected from a group including, but not limited to, implant quality austenitic stainless steel (e.g., 316L, 22-13-5, Biodur 108), cobalt alloys such as CCM (Co-28Cr-6Mo Alloy), MP35N, L605, ASTM-F-1058 and Elgiloy and Titanium and its alloys such as Ti-6Al-4V, Ti-6Al-7Nb and Ti-15Mo. The selected material is preferably non-magnetic so that, if fragmented and left within the body, the patient may undergo magnetic resonance imaging (“MRI”) without suffering adverse effects, as those skilled in the art will understand. Furthermore, the carburized/nitrided treatment of the selected material results in fragmented portions that do not contain sharp edges, preventing trauma to surrounding tissue. While the selected material of the bone fixation device100is substantially soft as compared to conventional devices, the addition of a carburized or nitrided outer surface increases a rigidity thereof to a level greater than that of a bone within which it is to be employed and substantially greater than conventional bone fixation devices. Specifically, the bone fixation device100may have a surface hardness of approximately 68 HRC or more, as those skilled in the art will understand. In an exemplary embodiment, the hardness of the bone fixation device100may be approximately 67-74 HRC and, more particularly, 67.5-70.3 HRC. As those skilled in the art will understand, this configuration minimizes dulling of the threading110,118after prolonged use while also easing insertion of the bone fixation device100into the bone in accordance with an exemplary reaming procedure. During operation, the carburized or nitrided outer surface of the bone fixation device100aids in cutting through bone and/or metal without seizing or losing sharpness. The exemplary carburized or nitrided outer surface of the bone fixation device100permits use thereof in bone without the risk of excessive burring or warranting replacement due to said burring. Furthermore, the carburized or nitrided material of the present invention provides an increased rigidity to the bone fixation device without having to enlarge or otherwise change a geometry of the device.

In one embodiment, the bone fixation device may be formed of a Biodur 108 alloy which is an essentially nickel-free austenitic stainless alloy. The alloy contains a high nitrogen content to maintain its austenitic structure. As a result, BioDur 108 alloy has improved levels of tensile and fatigue strength, as compared to nickel-containing alloys such as Type 316L (ASTM F138), 22Cr-13Ni-5Mn alloy (ASTM F1314), and 734 alloy (ASTM F1586). The resistance of BioDur 108 alloy to pitting and crevice corrosion is superior to Type 316L alloy and equivalent to the 22Cr-13Ni-5Mn and 734 alloys. BioDur 108 alloy is produced by the Electro-Slag Remelting (ESR) process to assure its microstructural integrity and cleanness. The alloy is non-magnetic and essentially free of ferrite phase. BioDur 108 alloy possesses a high resistance to corrosion due to its high levels of chromium and nitrogen and its molybdenum content. The alloy exhibits excellent resistance to pitting and crevice corrosion. BioDur 108 alloy was designed to have corrosion resistance equivalent to or greater than the nickel-containing alloys, 22Cr-13Ni-5Mn (ASTM F1314) and 734 (ASTM F1586). The corrosion resistance levels of these alloys are superior to Type 316L alloy (ASTM F138). Critical crevice temperatures of 50° F. (10° C.) were measured (per ASTM G48, Method D) in BioDur 108 alloy specimens. Critical temperatures of 41° F. (5° C.) were measured in identically prepared specimens of the 22Cr-13Ni-5Mn alloy. Under these test conditions, the critical temperature of the Type 316L alloy would be below 32° F. (0° C.). The relative corrosion resistances of BioDur 108 alloy and the comparative alloys were confirmed with anodic polarization testing in Ringer's solution at 98.6° F. (37° C.). The BioDur 108 alloy test article was concluded to be non-cytotoxic, non-toxic, non-hemolytic, negligibly irritant, exhibits no signs of toxicity, were observed and the test article was concluded to meet the requirements of ISO 10993-11, contains no pyrogens, is non-mutagenic based on the methods employed.

An exemplary material according to the invention is treated using low-temperature carburization which, in contrast with other treatment methods, minimizes the formation of carbides. U.S. Pat. No. 6,464,448 entitled “Low Temperature Case Hardening Process,” the entire disclosure of which is incorporated herein by reference, describes low temperature carburization of a ferrous based material for industrial parts and assemblies. These processes were not previously applied to implant grade medical devices or implants perhaps because the presence of surface imperfections which, while not problematic in industrial settings, made the materials susceptible to corrosion when deployed in the body. The present application applies low-temperature carburization of steel or other materials to provide a corrosion resistant material sufficient for use in surgical instruments. That is, the exemplary system and method according to the invention adapts a novel technique of carburizing/nitriding an implant-grade, ferrite free material to form devices having increased corrosion resistance as compared to other materials known in the art in which corrosion may be caused, for example, in part by the binding of chromium to carbide instead of being available to form an oxide. Higher levels of molybdenum in the material according to the invention further increase the corrosion-resistance thereof. As those skilled in the art will understand, a combination of annealing and cold-working may be used to form any of the devices described herein. The resultant material includes a diffusion zone in which carbon has supersaturated the matrix in the form of an interstitial carbon. The effect of this supersaturation is improved hardness, wear resistance and corrosion resistance. The exemplary material of the invention is described in greater detail below.

As those skilled in the art will understand, there are three main cubic forms of iron: austenite (FCC), Martensite (BCT) and ferrite (BCC). Both Martensite and ferrite are magnetic, while austenite is not. Thus, conventional implant quality 316L stainless steel is intentionally balanced to be fully austenitic even in the as-cast condition to minimize or eliminate the interaction of the medical device with. MRI magnetic fields. This is done by balancing ferrite stabilizing elements with austenite stabilizing elements in such a way as to ensure that the as-cast balance is in the austenite region. It is well known that certain elements stabilize either austenite or ferrite. Since many of the ferrite stabilizing elements, such as molybdenum and chromium, also promote corrosion resistance, they must be balanced by increasing the austenite forming elements or the alloy will contain ferrite along with the austenite. Specification ranges may seem overly broad. However, when one balances the need to create certain phase balances with the need to maximize corrosion resistance while minimizing cost, it becomes evident that actual chemistries will vary in a much smaller range than the specifications imply. In conventional industrial versions of 316L, a certain amount of ferrite is purposely present in the alloy to improve welding characteristics of the alloy as ferrite is known to reduce hot cracking in welds. For the steelmaker, this provides a similar reduction of hot cracking during melting and casting, especially during continuous casting. The typical commercial 316L material is an alloy that contains a majority austenite with a small percentage of ferrite. This is true in the as-cast condition and also as finished wrought products such as bar, wire, sheet and plate.

On the other hand, implant quality 316L is chemically balanced so that no ferrite is present in the alloy. Although the chemical ranges given in specifications such as ASTM F 138 are capable of producing ferrite, the specifications require that the end product contain no ferrite. To accomplish this, producers balance the actual chemistry into the 100% austenite region. There are many methods for predicting the austenite-ferrite balance in stainless steels. Two of the most common are the Schaeffler and the WRC-1992 diagrams. In each of these techniques, a correlation has been made between the chrome equivalent, the nickel equivalent and the phase balance. The chrome and nickel equivalents relate the total amount of ferrite or austenite forming elements present to their stabilizing effect in relation to the base elements of chrome and nickel. Carburization is a diffusion controlled process wherein only a small region near an exterior surface layer of a device on the order of 20 μm -35 μm thick is carburized. If a ferrite grain remains present in this region, it will not be carburized forming an uncarburized area that will not be as corrosion resistant as the carburized layer. Corrosion tunneling effects can occur in these areas allowing corrosion to penetrate to the core of the item potentially resulting in catastrophic failure.

The exemplary material according to the invention utilizes implant quality 316L for carburizing as it does not contain any ferrite, thus mitigating the risk of the presence of ferrite particles disrupting the carburized layer. To show the propensity for formation of ferrite, the following were compared: (1) implant quality 316L received at Synthes that meets the requirements of ASTM F138, ASTM F 139 and ISO 5832-1. Sample size—1366 samples and (2) industrial quality 316L produced by a supplier to the requirements of ASTM A 276. —Samples size—3,556 samples. The average chemistry of each was plotted to determine the ferrite content using the Schaeffler and the WRC-1992 methods, as shown in the following tables:

As depicted inFIGS. 11-18, the comparison of the above disclosed materials show that the implant quality 316L is balance into the 100% austenite region, while the industrial quality 316L is balanced at approximately 7-8% ferrite. Each of the techniques also shows the possible % ferrite band based on the specification ranges. Specifically,FIG. 12provides information on the welding properties of various types of conventional industrial strength microstructures ofFIG. 11as a function of the alloying elements they contain. The chart ofFIG. 12corresponds to a Schaeffler diagram for a range of standard analysis for the following material compositions.FIG. 14provides information on welding properties of the various types of conventional industrial strength microstructures ofFIG. 13as a function of the alloying elements they contain. The chart ofFIG. 14corresponds to a WRC-1992 diagram for a range of standard analysis for the following material compositions.FIGS. 15-18provide the same data for the exemplary implant grade material according to the invention, whereinFIG. 16corresponds to a Schaeffler diagram of the data ofFIG. 15andFIG. 18corresponds to a WRC-1992 of the data ofFIG. 17. In light of the above, it is evident that the exemplary material according to the invention provides an implant grade material that is balanced in a 100% austenite region, eliminating the ferrite commonly produced in conventional materials.

Although the exemplary construction depicted herein is directed to a bone fixation device 100 such as a bone screw, the inventive concept may be employed with any other bone fixation device/implant without deviating from the scope of the invention. Such bone fixation devices include, but are not limited to bone pins, buttress pins, bone plates, intramedullary nails, trochanteric nails, etc.

FIGS. 7-10depict the exemplary bone fixation device200according to the invention. Although the device200shown is a bone plate, it is submitted that any other bone fixation device may be used without deviating from the scope of the invention (e.g., an intramedullary nail, etc.). The bone plate200may, for example, be a 4 5 mm broad variable angle compression plate including eight holes202extending through a body204. Any or all of the holes202may be formed as variable angle combination holes comprising a first variable angle hole portion206and a second compression hole portion208open to the first hole portion. The first hole portion206may comprise a first relief cut210formed adjacent a first surface203, a second cylindrical threaded portion212extending distally therefrom and a third relief cut214formed adjacent a second surface205configured to contact the bone in an operative configuration. The relief cut210may extend at an angle of approximately 15° relative to a longitudinal axis of the hole202, although other angles may be used without deviating from the scope of the invention. The first hole portion206further comprises one or more slots207provided on an outer wall thereof, the slots207extending substantially perpendicular to a screw hole axis. As those skilled in the art will understand, the slots207interrupt the threads of the threaded portion212to provide multiple thread starts which aid in alignment of the threaded portion212with the bone screw100. The second hole portion208may comprise a first tapered hole portion216and a second tapered hole portion218extending distally therefrom. It is noted that although the bone fixation device200is depicted with eight holes, any other number of holes may be used without deviating from the scope of the invention and these holes may include any variety of know bone screw mounting holes. The bone fixation device200may also comprise any number and combination of variable angle holes, single holes and combination holes without deviating from the scope of the invention. The second surface205may further comprise a plurality of undercuts220configured to reduce a contacting surface area between the bone fixation device200and the bone to, for example, reduce impairment of blood supply after implantation, as those skilled in the art will understand.

In an operative configuration, the bone screw100is inserted through the bone fixation device200and into the bone. As those skilled in the art will understand, a physician or other user may select a desired angle of insertion to conform to the requirements of a particular procedure. Multiple thread starts provided by the grooves112provided on the head104and the slots207provided in the hole202aid in alignment of the threads110of the head with the threaded portion212of the hole202. As the bone screw100is screwed through the bone fixation device200and into the bone, the carburized or nitrided outer surface of the bone screw100minimizes burring of the threads110. The increased rigidity of the bone screw100relative to the bone fixation device200also permits removal and reinsertion of the bone screw100into the bone (e.g., to correct a position thereof within the bone) without causing a burring thereof.

The exemplary carburized or nitrided outer surface according to the invention is not limited to the bone screw100. In another embodiment, the carburized or nitrided outer surface and the ferrite-free construction may be applied to any conventional bone screw including, but not limited to, a variable angle bone screw, locking screw, compression screw or any other bone screw known in the art. The exemplary bone screw may be formed without any grooves112on the head104. An outer profile of the head104may be one of tapered, rounded, spherical and cylindrical.

In one embodiment, the bone screw may be formed similar to the bone screw10disclosed in U.S. Pat. No. 8,343,196 entitled “Bone Plate”, the entire disclosure of which is incorporated herein by reference. Specifically, as shown inFIGS. 19 and 21, a bone plate1has an underside2on a bone-contacting side thereof, an upper side8and a plurality of holes3in the plate connecting the underside2with the upper side8, the holes having a central hole axis5. The holes3in the plate have an internal jacket surface4that tapers towards the underside2. Furthermore, the internal jacket surface4has three recesses6which extend radially away from the hole axis5of the hole at a uniform distance of 120 degrees from one another. Their peripheral expansion is approximately 40 degrees and they extend exclusively within the internal jacket surface4. The recesses6extend tapered over the entire height of the bone plate1from the upper side8to the underside2. In addition, the internal jacket surface4is provided with a three-dimensional structure7in the form of a thread.

FIG. 22illustrates a variation of the execution according toFIG. 21, wherein the recesses extend radially away from the axis of the hole past the internal jacket surface.

FIGS. 20 and 23illustrate a further alternative embodiment, wherein the holes3in the plate are constructed as oblong holes. The bone plate is made basically from a plastic material (PEEK) with embedded metallic thread inserts9from titanium, forming the holes3in the plate. In the case of this embodiment the holes3in the plate have four recesses6, which extend radially away from the axis5of the hole past the internal jacket surface4. The internal jacket surface4is divided into four sections of the jacket surface. The recesses extend tapered over the entire height of the bone plate1from the upper side8to the underside2. In addition, the internal jacket surface4is provided with a three-dimensional structure7in the form of a multi-start thread. As far as material is concerned, this embodiment may also be inverted, whereby the bone plate is basically made from metal (titanium) and the embedded therein thread inserts9are made from plastic material (PEEK), forming the holes3in the plate. The bone plate1may alternatively be formed of any other material known in the art exhibiting a hardness as described in greater detail with respect to earlier embodiments. The bone screws10may be formed of a ferrite-free biocompatible material having a carburized or nitrided outer surface, as also described in greater detail earlier.

FIG. 24illustrates the bone plate according toFIG. 19, with bone screws10inserted from above, the head portions11of which are spherical.FIG. 25shows the same bone plate1from below.

InFIG. 26, a bone plate1is illustrated with bone screws10inserted therein without angular misalignment. The internal jacket surface4of the hole of the bone plate1and the head portion11of the bone screw10have matching threads13.FIG. 27illustrates the same variation asFIG. 26, while the bone screw10is angularly misaligned.

In another embodiment, the bone screw may be similar to the bone screw 100, 200, 300, 500, 600, 702, 1360, 14100, 14200, 31200 of U.S Publication No. 2008/0140130, the disclosure of which is also incorporated herein. Specifically,FIG. 28shows a typical non-locking bone screw150, also known as a cortex screw. Generally, any surgical bone screw having a non-threaded head152with a generally smooth surface and of an appropriate size and geometry for a selected plate hole can be formed with the exemplary material and carburized or nitrided outer surface according to the invention. The shape of head152may be, for example, conically tapered, straight-sided, spherical, hemispherical, etc. Non-locking screw150has a shaft154that is at least partially threaded for attachment to bone. The length of shaft154and the thread configuration (e.g., pitch, profile, etc.) of shaft thread157can vary depending on the application. As is known in the art, tip156and shaft threads157may be self-tapping and/or self-drilling to facilitate implantation into bone. Head152and shaft154may also have a cannula158for receiving a guide wire to aid in proper placement.

FIG. 29shows a typical locking screw160. Generally, any surgical bone screw having a threaded head162can be used with the invention provided that head162is of an appropriate size and geometry for a selected plate hole and that threads163mate with the columns of thread segments in the plate hole. The shape of head162is typically conically tapered, but also may be, for example, straight-sided. Locking screw160has a shaft164that is at least partially threaded for attachment to bone. The length of shaft164and the thread configuration (e.g. pitch, profile, etc.) of shaft thread167can vary depending on the application. As is known in the art, tip166and shaft threads167may be self-tapping and/or self-drilling to facilitate implantation into bone. Head162and shaft164may also be cannular for receiving a guide wire to aid in proper placement.

FIGS. 30 and 31show head302of a typical locking screw300. The profile of thread303on head302includes thread peaks310and troughs312connected to each other by flanks311, two adjoining flanks311forming a thread angle317, as shown inFIG. 32. Head302, which is conically shaped as is usual on known locking screws, is typically oriented such that thread peaks310lie on a straight line, such as lines313or315, and thread troughs312lie on another straight line, such as lines314or316, wherein the pairs of lines (313,314) and (315,316) are parallel to each other. Furthermore, the thread profile lines of each thread peak310and each thread trough312extend parallel to each other and perpendicular or normal to the central axis319of the screw, as represented by trough profile lines318a-eshown inFIG. 31. Profile lines318a-eare formed by extending the longitudinal axis301of a cutting bit305of a thread cutter as the cutting bit contacts the outer surface of head302to cut thread303. A typical locking screw also has a constant thread pitch (the distance from peak to peak, trough to trough, or profile line to profile line) as measured along the central axis (e.g.,319).

A variable-angle locking screw according to the invention has a screwhead that is at least partially spherical. The spherically-shaped portion of the head has a thread on an outer surface thereof which is preferably a double lead thread. The thread has a profile that follows the arc-shaped (i.e., non-linear) radius of curvature of the spherically-shaped portion of the head. Note that the thread pitch is constant as measured along the radius of curvature, but varies from narrow-to-wide-to-narrow as measured along the central axis of the screw from one end (e.g. the top) of the spherically-shaped portion of the head to the other end (e.g. the bottom) (see, e.g.FIGS. 57-60and the description thereof further below). This thread profile allows the variable-angle locking screw to engage a bone plate hole of the invention at a selectable angle within a range of angles while advantageously maintaining the same degree of contact with the bone plate regardless of the angle chosen. That is, the angle of the screw with respect to the central axis of the bone plate hole within the permissible range of angles does not affect the engagement of the screwhead thread with respect to the inner surface of the plate hole. A tight lock is advantageously obtained between the screw and the bone plate regardless of the angle (within the range of angles) at which the screw is inserted into the bone plate hole, because the threads on the spherically-shaped portion of the screwhead engage the columns of thread segments in precisely the same manner, ensuring a good fit.

FIGS. 33-35show an embodiment of a variable-angle locking screw according to the invention. Variable-angle locking screw500has a partially-spherical head502and a shaft504. Head502has a thread503, and shaft504has a thread507. Head502preferably has a recess509for receiving a tool to drive and extract the screw into and out of bone and into and out of a bone plate hole. Preferably, tip506and shaft thread507are self-tapping and/or self-drilling to facilitate implantation into bone. Head502and shaft504may be cannular for receiving a guide wire to aid in proper placement.FIGS. 34 and 35show the profile of thread503, which advantageously follows the radius of curvature525. In one embodiment, the radius is about 2 mm. Respective peaks510and troughs512of thread503as seen in profile are preferably separated by equal angular increments. Peaks510and troughs512are connected by flanks511at thread angles517, which in this embodiment, are preferably about 60 degrees. The thread profile lines518a-fextend through troughs512and result in a series of lines that intersect the center526of the radius of curvature525. Profile lines518a-fare formed by extending the longitudinal axis501of a cutting bit505of a thread cutter as the cutting bit contacts the outer spherical surface of head502to cut thread503. In this embodiment, cutting bit505is always normal to the outer spherical surface of head502as thread503is cut. Also in this embodiment, the radius of curvature is such that the radius center526lies on the central axis519of screw500. Depending on the length of the radius and the dimensions of the screw, center526may or may not lie on the central axis of the screw. Moreover, as the radius increases while the dimensions of the screw remain constant, the radius center will move outside the screwhead, as shown, for example, inFIG. 36.

FIG. 36shows another embodiment of a variable-angle locking screw of invention. In this embodiment, screwhead602of variable-angle locking screw600has a larger radius of curvature625than screw500. This results in trough profile lines618a-fintersecting radius of curvature center626, which is a distance630(measured perpendicularly) from central axis619of screw600. If, for example, radius624is 10 mm, distance630may be about 8.2 mm for a 2.4 mm screw (the 2.4 mm refers to the major diameter of shaft604). Note, however, that as the radius of curvature increases, the screwhead becomes less and less spherical in shape, causing the thread profile to become more and more aligned with a straight line (such as, e.g., lines313-316) as in known locking screwheads.

FIG. 37shows still another embodiment of a variable-angle locking screwhead in accordance with the invention. Screwhead702has a central axis719, thread703, and a recess709for receiving a driving/extracting tool. As in previous embodiments, the profile of thread703advantageously follows the arc-shaped (i.e., non-linear) radius of curvature725and includes thread peaks710, troughs712, and flanks711. However, unlike previous embodiments, the thread profile lines do not intersect the center of the radius of curvature. Instead, the thread profile lines, represented by trough profile lines718a-f, extend parallel to each other and perpendicular to central axis719. These lines extend in this manner because of the way in which cutting bit705of a thread cutter contacts the outer spherical surface of head702to cut thread703, lines718a-frepresenting extensions of longitudinal axis701of cutting bit705. Functionally, this difference results in a less ideal screwhead/hole thread engagement. However, screwhead702is currently easier to fabricate than screwhead502.

FIGS. 38 and 39show bone plate900having bone plate holes940in accordance with the invention. Instead of a helical thread around the inner surface935of the plate holes as in conventional locking screw bone plate holes, bone plate holes of the invention have discrete, vertical columns942of preferably thread segments arranged around the inner surface of the hole. The thread segment columns, if expanded to join each other (i.e. if extended completely around inner surface935), would form a helical thread. The columns extend in a direction from upper surface937to lower surface939and are spaced preferably equidistantly apart around the inner surface of the hole. The number of thread segments921per column can vary depending on the surgical application and the dimensions of the bone plate and bone screw (e.g., plate thickness and thread pitch). However, each column should have at least two thread segments and preferably more to ensure a fixed angular relationship between the screw and the plate.

Note that instead of thread segments, columns942alternatively may have a plurality of teeth formed thereon. The columns of teeth, if expanded to join each other (i.e., if extended completely around inner surface935), will not form a helical thread, but a series of concentric ridges and grooves perpendicular to the central axis of the bone plate hole. While such columns of teeth can also receive non-locking, locking, and variable-angle locking bone screws, the engagement of the teeth with the screwhead threads of the locking and variable-angle locking bone screws is less ideal than the engagement of thread segments with the screwhead threads of the locking and variable-angle locking bone screws.

Bone plate holes of the invention preferably have four columns942of thread segments, as shown inFIGS. 38 and 39. However, bone plate holes of the invention alternatively may have other numbers of columns of thread segments.

For example, as illustrated in the two embodiments ofFIGS. 40-42 and 43-45, respectively, bone plate holes1040A and1040D of respective bone plates1000A and1000D each have six columns of thread segments (note that because of the perspective shown, only three columns are visible inFIGS. 42 and 45). The difference between thread segment columns1042A and thread segment columns1042D is that the column width1041A of thread segments1042A is about twice that of column width1041D of thread segments1042D. In one exemplary embodiment, 3 to 6 thread columns may be provided. It is noted, however, that any number of thread columns may be used without deviating from the scope of the invention.

FIG. 46shows a cross-section of a bone plate hole according to the invention. Bone plate hole1140is formed in and extends completely through a bone plate1100from an upper surface1137to a lower bone-engaging surface1139. Hole1040has an inner surface1135comprising a top portion1144, a middle portion1146, and a bottom portion1148. Top portion1144extends from upper surface1137to middle portion1146. Middle portion1146extends from top portion1144to bottom portion1148and preferably has the smallest diameter of the hole. And bottom portion1148extends from middle portion1146to lower surface1139. Top portion1144is unthreaded, has a preferably smooth inner surface1143, and is preferably conically tapered inward toward the lower surface. Bone plate hole1140has a shoulder1145at the intersection of top portion1144and middle portion1146(which is the top of the first thread segment in each column). Shoulder1145may serve as a stop for the screwhead of a non-locking bone screw inserted through hole1140and, in one embodiment, is angled such that it forms an angle of about 60 degrees with the central axis of the hole. Note that inner surface1143or upper surface1137may serve as a stop for the screwhead of a non-locking bone screw depending on the size and shape of the head. Bottom portion1148also has a preferably smooth inner surface1149and is preferably tapered inward toward the upper surface in the form of an undercut sphere. In one embodiment of the invention, the radius of the undercut sphere is about 1.75 mm. For a bone plate thickness of about 2 mm, for example, the top portion may extend about 1 mm and the middle and bottom portions each may extend about 0.5 mm.

In this embodiment, middle portion1146of bone plate hole1140has four discrete columns of thread segments1142on inner surface1135. Each column1142is preferably inclined inward toward lower surface1139at an angle1150measured with respect to the central axis1119. In one embodiment, angle1150is preferably about 15 degrees. Each column1142also preferably has four or five thread segments1121. Other embodiments may have more or less thread segments as described above. For a bone plate hole accommodating a 2.4 mm variable-angle locking screw, the column width1141of each thread segment is preferably about 0.35 mm. Other embodiments may have other column widths, depending on the application.

FIG. 47shows a cross-sectional profile of a portion of a column1242of thread segments1221. (Note that a cross-sectional profile of an alternative column of teeth, as described above, appears the same as the thread segments.) InFIG. 47, two of the five thread segments1221of column1242are shown. Column1242of thread segments is preferably inclined toward the lower surface of the bone plate at angle1250. In one embodiment, angle1250is about 15 degrees. As seen in profile, column1242of thread segments1221includes peaks (or crests)1210and troughs (or roots)1212connected to each other by flanks1211at thread angles1217. Peaks1210preferably have a length1252, which in one embodiment is about 0.04 mm. Troughs1212preferably have a radius1254, which in one embodiment is about 0.03 mm. Angle1217is preferably about 60 degrees, and the bisection of troughs1212, as represented by trough profile line1218, occurs at an angle1256of preferably about 30 degrees as measured from a flank1211. Other embodiments of bone plate hole thread-segment columns alternatively may have other values of column incline angle, peak lengths, trough radiuses, thread angles, and bisection angles (which are a function of thread angle).

Advantageously, variable-angle locking bone screws of the invention can be driven into bone and secured to the bone plate at a selectable angle within a range of selectable angles.FIG. 48shows an embodiment of the invention in which bone plate1300has bone plates holes1340constructed in accordance with the invention. Each hole1340can advantageously receive a variable-angle locking screw1360, also constructed in accordance with the invention, at a selectable angle in any direction within a range of angles. The range of angles forms a cone having an angle1362, which in this embodiment is about 30 degrees. In other words, variable-angle locking screw1360can be inserted into a hole1340and secured to bone plate1300at a selectable angle ranging from 0 degrees to 15 degrees in any direction with respect to central axis1319of bone plate1340.

FIGS. 49-56show an advantageous feature of a bone plate hole constructed in accordance with the invention. Bone plate1400has at least three bone plate holes1440. Each hole1440has four columns of thread segments1542and can advantageously receive any one of a non-locking, locking, or variable-angle locking bone screw.

As shown inFIGS. 49, 50, 51 and 53, a conventional non-locking bone screw14100can be inserted through one of bone plate holes1440. Non-locking bone screw14100has a non-threaded screwhead14102and a threaded shank14104, each appropriately sized and configured for use with hole1440. Note that non-locking bone screw14100does not have to be inserted through hole1440coaxially with the central axis of the hole, but may instead be inserted through hole1440at a selectable angle, as shown inFIG. 50.FIG. 53shows that screwhead14102does not engage the columns of thread segments1542, but instead contacts shoulder1545of hole1440when fully seated therein.

FIGS. 49, 50, 52, and 54show conventional locking bone screw14200inserted though a second bone plate hole1440. Locking bone screw14200has a screwhead14202with a thread14203on an outer surface therefore. Both the screwhead and thread are appropriately sized and dimensioned such that thread14203can threadingly engage and mate with columns of thread segments1542. In order to properly engage and mate with columns of thread segments1542, locking bone screw14200should be inserted through hole1440coaxially with central axis1419of the hole. Screw14200also has a threaded shank14204for engaging bone. Shank14204is also appropriately sized and dimensioned for insertion through hole1440.

FIGS. 49, 50, 55 and 56show variable-angle locking bone screw1460inserted through a third bone plate hole1440. Variable-angle locking bone screw1460, constructed in accordance with the invention, has a threaded shank1404and a partially-spherical head1402with thread1403on an outer surface thereof. Screwhead thread1403has a profile that advantageously follows the arc-shaped (i.e., non-linear) radius of curvature of the spherically-shaped portion of head1402. Screw1460is shown inserted into the third hole1440non-coaxially with the central axis1719with thread1403securely engaging columns of thread segments1542.

Returning to the screwhead thread features of variable-angle locking bone screws constructed in accordance with the invention,FIGS. 57-59show three embodiments of a variable-angle locking screw screwhead that illustrate the varying thread pitches (e.g., the peak to peak distance) as measured along the central axis of each screw. The following table lists the size of the variable-angle screw to which the illustrated screwhead belongs and the varying pitches (all dimensions in millimeters).

Other embodiments of variable-angle locking bone screws of the invention may have other varying thread pitches.

Note that in each case, the angular distance between adjacent thread peaks (or adjacent thread troughs) as measured along the radius of curvature is constant, as illustrated inFIG. 60. That is, each angular distance35AD between adjacent thread peaks3510as measured along the radius of curvature3525is the same--in contrast to thread pitches35P01-35P05which, as illustrated inFIGS. 57-59, vary as measured along or parallel to central axis3519.

It will be apparent to those skilled in the art that various modifications and variations can be made in the structure and the methodology of the present invention without departing from the spirit or scope of the invention. Specifically, the features and illustrations described herein may be used singularly or in any combination with other features and embodiments. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.