Patent Description:
Orthopedic medicine has evolved implant systems including implants, such as bone plates, nails, and cages that are used with fasteners such as pegs, and screws, for internal fixation of bones. These orthopedic systems include a bone plate, configured to be attached to one or more bones or bone segments, for example, spanning a fracture or osteotomy line. These bone plates generally include a plurality of apertures or holes through which bone screws and/or bone pegs are inserted for engaging the underlying bone and to assemble a "construct" which includes the plate, screws and the bone. The engagement mechanisms for securing the plate to the bones include both screws which lock relative to the plate, (i.e. "locking" screws) and screws which do not lock relative to the plate, (i.e. "non-locking" screws). Both types of screws typically have a head and a shaft including cortical bone threads to permit the screw shaft to be screwed into and fixed relative to the bone which underlays the plate surrounding the plate aperture. The type of fixation will affect the forces that are applied to the bone by the implant system, which in turn will influence the physiological response of the biological system.

Typically, in the case of locking screws, the plate aperture includes a set of internal or female threads and the screw head includes a set of external or male threads that mate with the female threads in a cooperation at a fixed orientation to accommodate typical fracture patterns. In another example, the bone plate systems can include locking screws, which can be configured for insertion at an orientation selected by the acting surgeon, for example, to capture a bone fragment that does not follow a typical fracture pattern. These systems are known as poly-axial or variable locking systems. An advantage of "locked plating" systems is that they do not require the bone plate to be compressed to the bone. Rather, in this construct, the interface formed between the bone plate and the locking screws can eliminate the need for compression between the plate and the bone by acting to stabilize the plate to the screw.

In other instances, it may be preferable to use a bone/implant construct in which the fasteners (i.e. screws) are only fixed in the bone, but which include a head that resides in an aperture in the implant so as to draw the plate toward the bone. Unlike "locked plating" systems, "non-locked plating" systems rely on the axial force toward the bone of the bone screw head in the plate aperture when the screw is inserted through the bone plate and tightened, and the plate typically spans at least two screws to support the bone to bone interface between them.

The present invention provides an improved implant fixation system which balances the stability afforded by the implant system with ease of use for the surgeon, and a construct with a wide variety of fixation, orientation and compression characteristics in a single system set.

Document <CIT> discloses a bone fixation element including 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 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.

Document <CIT> discloses a system including a bone plate and a plurality of variable angle screws, the bone plate comprising titanium and the variable angle screws comprise a TiMo alloy.

The present invention relates to an orthopedic implant system as defined by the claims.

The invention relates to an orthopedic implant system comprising an implant including a first surface and an opposing second surface with at least one compound threaded aperture there between, and a first bone fastener which is a non-locking fastener and a second fastener which is a variable axis locking fastener, and the at least one threaded aperture can accept either the first or the second fastener to secure the implant to a bone. The aperture is a compound recess having a first and/or second counterbore section joined to an annulus that includes internal threads and which includes a plurality of recesses or thread interruptions so as to form interrupted threaded areas. The head of the non-locking fastener includes a tapered rounded portion that rides on the threads of the aperture. The variable axis locking fastener has a tapered head which includes male threads, and the female threads of the aperture and the male threads of the fastener are configured to have a specifically engineered mismatch which causes the male threads to wedge against the female threads as the fastener is tightened into the aperture in an off-axis orientation.

More specifically, in a first embodiment, the invention relates to poly-axial fastener locking systems comprising an orthopedic implant, such as an orthopedic plate, having at least one compound fastener aperture with a cylindrical annulus having corresponding threads having a thread crest that describes a cylindrical helical shape, and that receives either <NUM>) a tapered non-locking fastener, such as a screw, having a smoothly rounded head including a lowered tapered portion which rides on the threads or <NUM>) a locking fastener, such as a screw, having a tapered head with multi-lead tapered threads having threads crests that describe a conical helical shape, and that have a mis-matched thread configuration relative to the internal threads of the aperture.

In a second embodiment, the annulus portion of the aperture has a diameter that decreases in diameter from the top surface toward the bottom surface of the plate. This configuration can comprise a taper including a conical configuration, or a hemispherical configuration, or a concave configuration that is more generally inwardly rounded.

In all cases, the outer thread crest of the screw head describes a configuration that differs from the configuration of the aperture, such as differing shapes and/or differing degrees of angulation on a tapered conical surface. In addition, in all cases, it is preferable that the internal threads on the annular portion of the aperture have a sharper angled thread crest than the thread recess of the external screw head threads. Thus, the plate's female thread form is <NUM>° +/-<NUM>° in cross-section as compared to the screw head's male thread cut-out, which is <NUM>°, +/- <NUM>°, so as to facilitate off-axis threading without damaging the thread. The locking screw head threads are on a decreasing diameter, such as a conical taper to facilitate variable angle engagement and to prevent the screw from being advanced all the way through the plate. The locking screw locks in at a predetermined orientation which aligns with the central axis of the aperture or at an orientation that is off-axis by up to <NUM>° each way in a conical distribution. The threaded aperture includes multiple radially symmetrically distributed lobed thread interruptions so that the threads of the screw head will pick up at one of these interruptions. In at least the first and the second embodiment of the invention, the interruptions are recesses formed in the vertical threaded surfaces of the threaded annulus which form complex three-dimensional non-discrete sections that include a thread segment. As used herein "non-discrete" refers to the fact that the threaded annulus has a pre-existing topography that varies in the direction of the central axis of the aperture and the interruption has the effect of interrupting the thread valley and the thread crest to differing degrees along a vertical line taken along the annular surface. The aperture is a compound recess including a narrow necked portion that forms the annulus and the underside of the aperture includes a widened mouth area such as a counterbore or chamfer, to accommodate the neck of the screw in wide angle insertions. The underside of the locking screw head includes a cutting feature to counterbore any bone that might inhibit the full engagement of the screw.

The top surface of the screw includes a torque driving recess that tapers as it descends into the screw and the top surfaces that extend away from this recess are smoothly rounded downward into the head side portions that include the thread cut outs.

In a further embodiment, the bone facing surface of the implant includes a recess that is concentric to the aperture and the interruptions in the threads connect so as to form flexible tines of threads between the interruptions.

The present invention relates to an orthopedic implant system <NUM> which includes an implant <NUM>, such as a member having two major opposing, and typically co-extending, or concentric surfaces, which will be referred to as a "top surface" <NUM>, meaning the surface intended to face out relative to the affected bone, and a "bottom surface" <NUM>, meaning the surface that is configured to contact, or face toward the affected bone and a side edge <NUM> including a long edge <NUM>. Plates have typically been configured in a curved blade-like configuration where the curve is meant to accommodate the long radiused surfaces of bone they are meant to support, such as the femur, fibula, radius or humerus. However, as orthopedic implants have become increasingly sophisticated, the outlines and profiles have evolved in more complex shapes with generalized curves or bends to accomplish certain medical goals. In addition, sometimes implants will also include portions that are intended to project into the bone, rather than sitting against it, and these areas tend to be more cylindrical and less "plate-like" in order to support the bone from the intramedullary channel.

The implant, or plate <NUM>, has at least one threaded compound aperture <NUM> which includes a necked area or annulus <NUM> having internal or female threads <NUM>. More typically the implant has multiple apertures <NUM>, which in a first embodiment, has a cylindrical configuration in the annular portion, meaning that throughout at least <NUM>%, and preferably <NUM>%, and most preferably <NUM>% of the depth of the annulus <NUM> as measured from one surface of the plate to the opposing surface there is a constant major diameter in the annulus portion of the aperture and a constant minor diameter in the annulus portion of the aperture (as defined by a line at which intersections the edges of the major diameter and the minor diameter shown in cross-section or alternatively, the threads, or more specifically, the thread crest follow a cylindrical helical path for at least <NUM>° radially, and preferably <NUM>°, and preferably <NUM>° radially, depending on whether the threads are single or multiple leads, and preferably double lead). This means that the thread edges at the major diameter subscribes a portion of a cylinder for at least a portion of the height of the aperture, likewise, so may the thread root. The aperture <NUM> includes one or more counterbores or chamfers <NUM> joined to the annulus <NUM>, to allow for a wider angulation or to accommodate the screw head member <NUM>. The internal thread <NUM> is preferably a double lead thread which will lock with less thread deformation since the locking head does not seat fully in the aperture as the bottom threads are not in the way try to force the screw back on-axis (and therefore causes less need for thread deformation in that bottom-left corner).

In a further embodiment of the variable locking system of the present invention, the annulus portion <NUM> of the plate aperture <NUM> as considered at the thread root or thread crest, has a diameter that decreases along the central axis of the aperture from the top of the plate to the bottom of the plate. In particular, the configuration of the plate aperture differs from the configuration of the screw head where both configurations are defined at the thread crests. This could mean tapering conical configurations at differing angles, such as the screw head having a taper of from <NUM>° to <NUM>°, and preferably from <NUM>° to <NUM>° (or <NUM>° +/- <NUM>° or <NUM>°) and the plate aperture has a taper that is from <NUM>° to <NUM>°, and preferably <NUM>° to <NUM>° (or <NUM>° +/- <NUM>° or <NUM>°), in difference as measured by the angle between a line at the thread crest and the central longitudinal axis of the relative configuration, with the thread of the aperture forming a greater angle of from <NUM>° to <NUM>°, and preferably from <NUM>° to <NUM>° (or <NUM>° +/- <NUM>° or <NUM>°). Alternatively, the differing configurations could be rounded or hemispherical shapes of differing diameters.

The invention further includes a first fastener, which is variable locking screw <NUM> which has a shaft <NUM> including a distal portion <NUM> that has a cortical bone thread <NUM> extending from the shaft <NUM> which defines the minor diameter of the cortical thread to the thread crest <NUM> which defines the major diameter of the cortical thread, and a proximal portion <NUM> that includes a head member <NUM> which is convexly rounded, tapered or conical in shape and which includes external or male threads <NUM>. The threads <NUM> run from a top surface <NUM> of the screw head member <NUM> to a necked portion <NUM> which joins the head member <NUM> to the shaft <NUM>. At the distal portion <NUM> of the head member <NUM>, the screw includes at least one, and preferably two or three cutting flutes <NUM>. The distal portion <NUM> of the screw can also include cutting flutes. The proximal portion <NUM> of the screw <NUM> includes a top surface <NUM> of the screw, which has a torque driving recess <NUM> which can be a hexalobe shape, and preferably can include an internal taper to enable a press fit with a mating torque driver head. The cutting flutes are provided to bore a countersink into the bone to accommodate a deeper screw head in a thinner plate. Thus, the countersink helps to minimize prominence above the bone by sinking part of the screw head below the bottom surface of the plate into the bone and to reduce the number of operative steps, the cutting flutes would help automatically countersink any bone that gets in the way.

The locking threads <NUM> of the head member <NUM> are preferably dual lead threads, with a start point <NUM>° apart, and have a major diameter <NUM> and a minor diameter <NUM> with a locking thread crest <NUM> at the junction of a bottom thread face <NUM> and a top thread face <NUM> which together define a thread cut-out angle <NUM> as shown in <FIG>. The screw head member <NUM> configuration includes a decreasing diameter, or more specifically, a taper, meaning that lines connecting the major diameters converge toward the screw axis in the distal direction and that lines connecting the minor diameters converge toward the screw axis in the distal direction as can be seen in <FIG>. A taper angle A can be defined between the longitudinal axis <NUM> of the screw and the outer diameter at the crest of the locking threads of the screw head member <NUM>. Although any suitable taper angle A can be used, suitable taper angles A can be between about <NUM>° and about <NUM>°, and more particularly between about <NUM>° and about <NUM>°, such as about <NUM>°+/-<NUM>°. Similarly to the corresponding definition of the cylindrical annulus, the screw head is considered tapered as marked by the following concepts: the edge of the thread or threads, i.e., the thread crest, or the edge of the minor diameter, i.e., the thread root, in the case of multiple lead threads at the major diameter subscribe a portion of a cone, and define for at least <NUM>°, and preferably at least <NUM>°, or <NUM>° or <NUM>° or for an at least <NUM>% of an entire diameter or preferably for at least a fully turn of the screw head, the threads follow the path of a helical cone.

In contrast, the annulus <NUM> of the plate aperture <NUM> has a deliberately mis-matched configuration that differs from the configuration of the screw head, such as being generally cylindrical with a tapered screw head (aside from any chamfers or other reliefs), and has a diameter that is intermediate the widest major diameter of the screw head <NUM> and the narrowest major diameter <NUM>. In a first embodiment, the threads <NUM> of the plate <NUM> follow a cylindrical (non-tapered) helical path while the thread <NUM> of the screw head member <NUM> follows a tapered helical path that increases in diameter as it travels up proximally on the screw head <NUM>. Thus, in a mating cooperation the trailing (upward) surfaces of the plate threads support the screw while the crests of the screw threads press radially outward on the plate's thread major as the screw's advanced until it binds as is shown in <FIG>. The present invention utilizes a tapered locking screw head member and a cylindrical threaded locking aperture, which has the advantage over a system using a tapered locking aperture which tends to re-orient an off-axis screw orientation back to center.

In a further embodiment, the configuration of the annulus of the aperture varies from the configuration of the screw head in that both may have a decreasing diameter, but which decreases at a different rate in the direction of the central axis from the top to the bottom respectively. In addition, the thread angle of the aperture and of the screw head differ so as to present an intended mis-match of <NUM> degrees in either direction between the mating thread surfaces. This allows for <NUM> degrees of conical freedom in the angulation but still provides for locking by means of the tapered mismatch on the thread crest of the plates onto the thread valley of the screw.

As illustrated in <FIG>, the threaded apertures <NUM> can define a longitudinal axis <NUM> extending through the threaded aperture <NUM> between the first surface <NUM> and the second surface <NUM> of the bone plate <NUM>. The longitudinal axis <NUM> can represent the "centerline" of the threaded aperture <NUM> and is therefore dependent on the orientation in which the threaded aperture <NUM> is formed in the bone plate <NUM>. In various examples, the threaded aperture <NUM> can be formed in the bone plate <NUM> such that it extends generally perpendicular to the first surface <NUM> and the second surface <NUM>, or the threaded aperture <NUM> can be formed in the bone plate <NUM> such that it extends at an oblique or non-perpendicular angle relative to the first surface <NUM> and the second surface <NUM>. Regardless of the orientation of the threaded aperture <NUM>, the longitudinal axis <NUM> defines only one of the axes along which the variable locking fastener can be inserted. Thus, the threaded aperture <NUM> and the locking fastener <NUM> can define a "variable axis locking" or "polyaxial" fastening system wherein the locking fastener <NUM> can be inserted into the threaded aperture <NUM> in a plurality of different insertion angles relative to the longitudinal axis <NUM> of the aperture while achieving a locking engagement between the thread head <NUM> and the threaded aperture <NUM>. An example of this variable axis locking capability is illustrated in <FIG>, wherein the elongate shaft <NUM> of the locking fastener <NUM> does not extend along the longitudinal axis <NUM> of the corresponding threaded aperture14, but instead forms an angle with the longitudinal axis <NUM> while maintaining a locking connection between the externally threaded head member <NUM> and the internally threaded aperture <NUM>.

<FIG> illustrates the threads <NUM> of the plate aperture <NUM>. The threads can be thrust threads having a truncated V-shape defining a series of roots <NUM> and crests <NUM> and having an internal thread angle T1 at <NUM>. The internal thread angle T1 is <NUM>° +/- <NUM>°, and preferably +/- <NUM>° or <NUM>°. A thread height H1 of the threads of the internally threaded surface <NUM> can be defined between one of the roots <NUM> and an adjacent one of the crests <NUM>. The roots <NUM> and/or the crests ° can be truncated to avoid the formation of a "sharp" V-shape, or a trough. A perfectly sharp <NUM>° V-thread generally includes a thread height equal to about <NUM> of the pitch. However, with truncated threads, the thread height decreases. In an example, the thread height H1 can be between about <NUM> (<NUM> inches) and about <NUM>,<NUM> (<NUM> inches), such as about <NUM>,<NUM> (<NUM> inches). The screw threads have an external thread angle T2, <NUM>, at the thread crest which is about <NUM>°+/- <NUM>° preferably +/- <NUM>° or <NUM>°. T1 is different than T2, and is preferably <NUM>°. preferably +/- <NUM>° or <NUM>° inclusively (meaning including both sides of the thread crest or trough.

<FIG> illustrates that the angle T2 of the screw head has a differential on both sides of the crest (i.e. the leading and trailing surfaces) which are smaller than the corresponding thread trough on the male threads of the screw head. The difference on one of the two sides T3 is equal to the one half of the available conical angle of variability for the variable locking aspect of the screw in the aperture.

The threads <NUM> of the annulus <NUM> of the aperture <NUM> include one or more recesses <NUM> (<NUM>',<NUM>") circumferentially spaced around the annulus <NUM> so as to define a plurality of threaded sections <NUM> (<NUM>', <NUM>") having at least one thread segment on a section. In an example, the one or more recesses <NUM> can be formed by creating radially symmetrical hemicylindrical cuts in the internally threaded surface of the threaded aperture <NUM>. Preferably, there are <NUM>-<NUM> recesses, and more preferably <NUM>, <NUM> or <NUM> recesses as is illustrated in <FIG>. The cuts <NUM>, <NUM>' and <NUM>" can have a depth sufficient to completely remove the threads in the area of the recesses such that a substantially smooth recessed surface is created to cause an interruption to the threads crest and at varying degrees to the thread trough such that the vertical edges of the interruptions are not aligned but spiral about the edge of the annulus <NUM>. The effect of the recesses is to form sections with a thread crest <NUM> and multiple thread troughs on the side wall of the aperture between the recesses. Alternatively, the recess <NUM> can be formed to a depth that retains the thread segment in the area of the recess <NUM>, albeit at a reduced height (between a root and a crest of the thread) but still sufficient to create the interruption to the thread.

The locking screw head member <NUM> has a <NUM>-start thread which mates with the female locking threads <NUM> of the aperture, but the plate's female thread form is sharper (i.e., a <NUM>°-<NUM>°, and preferably <NUM>°+/- <NUM>° triangular cross-section) than the screw head's male cutout (i.e., a <NUM>°-<NUM>°, and preferably <NUM>° +/- <NUM>° or <NUM>° triangular cross-section) to facilitate off-axis cross-threading without damaging the threads and so as to avoid generating tailings or metal debris. The locking screw head threads are on a conical taper (smallest diameter at bottom) to facilitate engagement with the plate at various angles and to prevent the screw from being advanced all the way through the plate. To minimize the plate thickness and overall height of the screw/plate assembly over the bone, the underside of the locking screw's head/neck area has a cutting feature which cuts into bone if the screw protrudes too far under the plate. The one or more external threads <NUM> on the elongate shaft <NUM> can have double the pitch and the same lead as the locking threads <NUM> of the head member <NUM>.

<FIG> illustrates a non-locking screw <NUM>' in the aperture <NUM>'. In this example, the non-locking screw <NUM>' has a head <NUM>' with a rounded proximal portion that extends into a tapered necked portion <NUM>' which enable the screw to have a wide angle at which it extends through the aperture <NUM>'.

A further embodiment of the implant of the present invention is shown having a trepanned aperture <NUM>". In this embodiment, the aperture includes a concentric outer annular groove <NUM> which extends vertically up from a surface, and preferably the bottom surface of the implant more that <NUM>%, and preferably more than <NUM>% of the thickness of the implant such that the columns of tabs <NUM>" of threads become independent from the greater plate material to create tines that can flex radially to better accommodate the off-axis external threads of the fastener.

Claim 1:
An orthopedic implant system (<NUM>), comprising:
a plate (<NUM>) including a first surface (<NUM>) and a second surface (<NUM>), the plate including at least one aperture (<NUM>), the aperture including one or more female threads (<NUM>) which define a thread crest angle T1, and a thread root and at least two recesses within the threads which interrupt the threads to define at least two sections having a thread crest segment, wherein the aperture is a compound opening including an annulus having an annulus configuration and joined to a mouth in at least one of the first surface and the second surface; and
at least one screw (<NUM>) including an elongate shaft (<NUM>) and a head member (<NUM>) having a head configuration including male locking threads (<NUM>), the male locking threads of the head member defining a thread trough angle, T2, and;
wherein the at least one screw is configured for insertion within at least one threaded aperture at a plurality of different insertion angles while achieving a locking engagement between the threaded head and the threaded aperture; and
characterized in that:
the male locking threads of the fastener head member and the female threads of the aperture having a differing thread configuration wherein T1 is different to T2; and
the annulus configuration is cylindrical and the head configuration decreases in diameter along a vertical axis; or
the annulus configuration decreases in diameter along a vertical axis and the head configuration decreases in diameter along a vertical axis at a different rate that the annulus configuration.