Patent Description:
Conventionally, locking screws were inserted through the screw hole along the central screw hole axis in order to ensure that the threaded screw head mates with the plate in the threaded fixation hole. Locking screws can include standard-type locking screws and variable-angle (VA) screws. Standard-type locking screws are configured to lock within a bone fixation hole substantially only at a "nominal" orientation whereby the central screw axis is substantially coincident with the central hole axis. Variable-angle locking screws are configured to threadedly mate with a threaded bone plate in a variable angle hole of the bone plate so as to selectively lock to the bone plate at an angle within a range of permissible angles with respect to the central axis of the variable angle hole. <CIT> relates to dynamic bone fixation elements and a surgical method to stabilize bone or bone fragments. The dynamic bone fixation elements preferably include a bone engaging component and a load carrier engaging component. The bone engaging component preferably includes a plurality of threads for engaging a patient's bone and a lumen. The load carrier engaging component preferably includes a head portion for engaging a load carrier and a shaft portion. The shaft portion preferably at least partially extends into the lumen. Preferably at least a portion of an outer surface of the shaft portion is spaced away from at least a portion of an inner surface of the lumen via a gap so that the head portion can move with respect to the bone engaging component. The distal end of the shaft portion is preferably coupled to the lumen. <CIT> teaches a dynamic bone fixation element, which can include a sleeve elongate along a first direction and a fixation member. The sleeve can define a channel that extends from a proximal end through to a distal end along the first direction. The channel has a first cross-sectional dimension. The fixation member has a head, a shaft extending from the head and elongate along a second direction, and an abutment member extending from the shaft and integral with the shaft, wherein the shaft extends through the channel such that the sleeve is captured between the abutment member and the head. At least a portion of the shaft that is within the channel has a second cross-sectional dimension that is less than the first cross-sectional dimension such that the fixation member is moveable with respect to the sleeve along a direction that has a directional component transverse to the first direction.

According to one example of the present disclosure, a variable angle bone screw includes a first portion that defines at least a portion of a head having at least one external thread configured to mate with a thread of a variable angle locking hole of a bone plate. The variable angle bone screw further includes a second portion that is separate from the first portion and attached to the first portion. The second portion defines at least a portion of a shaft having at least one thread configured to purchase with bone. The at least one thread of the first portion has a first material hardness, and the at least one thread of the second portion has a second material hardness that can be less than the first material hardness, wherein the first portion comprises a material that is hardened, and the second portion comprises the material that is non-hardened.

The foregoing summary, as well as the following detailed description of illustrative embodiments of the present application, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the locking structures of the present application, there is shown in the drawings illustrative embodiments. It should be understood, however, that the application is not limited to the precise arrangements and instrumentalities shown. In the drawings:.

The present disclosure can be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this disclosure is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the scope of the present disclosure. Also, as used in the specification including the appended claims, the singular forms "a," "an," and "the" include "at least one" and the plural. Further, reference to a plural as used in the specification including the appended claims includes the singular "a," "an," and "one," and further includes "at least one. " Further still, reference to a particular numerical value in the specification including the appended claims includes at least that particular value, unless the context clearly dictates otherwise.

The term "plurality", as used herein, means more than one. When a range of values is expressed, another example includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another example. All ranges are inclusive and combinable.

The terms "approximately" and "substantially" as used herein with respect to various sizes, shapes, dimensions, or other parameter can include the states size, shape, dimension, or other parameter and <NUM>% greater than or less than the stated size, shape, dimension, or other parameter. Further, the terms "approximately" and "substantially" can equally apply to the specific value, shape, or direction of the structure as stated.

When variable angle (VA) locking screws are angulated and threadedly locked to a bone plate, the VA locking screws can achieve high cantilever strengths when the plate thread undergoes a smooth deformation. Further, it can be desirable to reduce or prevent the thread of the VA screw from undergoing deformation when the VA screw is threadedly mated to the bone plate. Both objectives can be met by providing a VA screw whose threaded head has a material hardness greater than the shaft. However, increasing the hardness of conventional VA screws has been found to reduce the fatigue strength of the shaft, particularly in the case of titanium screws. While a masking process could be used to selectively harden the head of the VA screws, this is a time consuming manual process. Accordingly, two-piece VA screws are described herein having a threaded VA head having a greater hardness than the BA shaft.

Referring initially to <FIG>, a bone fixation system <NUM> is configured to be implanted onto bone <NUM> so as to stabilize a first bone segment <NUM> with respect to a second bone segment <NUM> that is separated from the first bone segment <NUM> by a defect <NUM>. In one example, the first bone segment <NUM> can be defined by the diaphysis of the bone, while the second bone segment <NUM> can be defined by the epiphysis of the bone. It should be appreciated, however, that the first and second bone segments <NUM> and <NUM> can be defined by any region of the bone <NUM> as desired. Further, the bone <NUM> can be any bone in the human or animal anatomy suitable for bone plate fixation. Further still, while the bone <NUM> is illustrated having first and second bone segments <NUM> and <NUM>, it is appreciated that the bone <NUM> can include any number of defects or bone fragments as desired that are configured for fixation using the bone fixation system <NUM>. For instance, the diaphysis of the bone can include a plurality of bone fragments.

The bone fixation system <NUM> can include a bone plate <NUM> and a plurality of bone anchors <NUM> that are configured to fix the bone plate <NUM> to the underlying bone <NUM>, and in particular to each of the first and second bone segments <NUM> and <NUM>. The bone anchors <NUM> include a head <NUM> and a shaft <NUM> that extends out with respect to the head <NUM> in a distal direction along a central anchor axis <NUM>. For instance, the shaft <NUM> can extend out directly from the head <NUM>. Alternatively, the shaft <NUM> can extend out from a neck that extends between the head <NUM> and the shaft <NUM>. The shaft <NUM> can extend directly from the head <NUM>, or can extend from a neck that is disposed between the head <NUM> and the shaft <NUM>. The shaft <NUM> can be threaded, such that the bone anchor <NUM> is configured as a bone screw <NUM> whose shaft <NUM> extends out relative to the head <NUM> along the central anchor axis <NUM>, which can also be referred to as a central screw axis <NUM> when the bone anchor is configured as a bone screw. The threaded shaft <NUM> can be configured to threadedly purchase in the underlying bone <NUM>. For instance, one or more up to all of the bone screws <NUM> can be configured as a cortical screw whose shaft <NUM> is threaded and designed and configured to threadedly mate to cortical bone. Alternatively or additionally, one or more of the bone screws <NUM> can be configured as a cancellous screw whose threaded shaft <NUM> is designed and configured to threadedly mate to cancellous bone. It is appreciated that the shafts of cancellous bone screws typically have threads with a greater pitch than the threads of the shafts of cortical bone screws. Further, the threads of cancellous bone screws typically extend out from the shaft of the bone screw a greater distance than the threads of cortical bone screws.

The bone plate <NUM> defines a bone plate body <NUM>. The bone plate body <NUM>, and thus the bone plate <NUM>, defines a bone-facing inner surface <NUM> configured to face the underlying bone <NUM>, and an outer surface <NUM> that is opposite the inner surface <NUM> along a transverse direction T. The bone plate <NUM> further defines a plurality of fixation holes <NUM> that extend through the bone plate body <NUM> from the inner surface <NUM> to the outer surface <NUM>. In particular, the bone plate <NUM> can define a plurality of internal surfaces <NUM> that define the plurality of fixation holes <NUM>, respectively. Each of the fixation holes <NUM> can extend through the bone plate body <NUM>, and thus through the bone plate <NUM>, along a respective central hole axis <NUM>. The central hole axis <NUM> can be oriented along the transverse direction T. Thus, the central hole axis <NUM> can be oriented normal to each of the inner surface <NUM> and the outer surface <NUM>. It should be appreciated, of course, that the central hole axis <NUM> can be oriented in any suitable direction as desired, including a direction oblique to the transverse direction T.

The fixation holes <NUM> are sized to receive the shaft <NUM> of a respective one of the bone screws <NUM>. Thus, the bone screws <NUM> that extend through fixation holes <NUM> are permanent bone screws, meaning that they remain after completion of the surgical procedure. This is distinguished from temporary fixation holes that, for instance, can be configured to receive temporary fixation members, such as Kirschner wires that are removed prior to completion of the surgical procedure. In this regard, the fixation holes <NUM> can be referred to as permanent fixation holes. Accordingly, during operation, the shaft <NUM> of the bone screw <NUM> can be inserted through a respective one of the fixation holes <NUM> and into the underlying bone <NUM>. The bone screw <NUM> can then be rotated so as to cause the threaded shaft <NUM> to be driven into the underlying bone <NUM> as the threaded shaft <NUM> threadedly purchases with the underlying bone. The threaded shaft <NUM> can be driven into the underlying bone <NUM> until the head <NUM> engages the bone plate <NUM>, thereby securing the bone plate <NUM> to the underlying bone <NUM>.

Certain ones of the fixation holes <NUM> can be unthreaded compression holes <NUM>, while certain others of the fixation holes <NUM> can be threaded locking holes <NUM> and some holes can be a combination thereof, where a threaded locking hole <NUM> and an unthreaded compression hole <NUM> intersect each other to define a combination hole <NUM> (see <FIG>). The bone plate <NUM> defines an internal compression surface <NUM> that can extend between the outer surface <NUM> and the inner surface <NUM> so as to at least partially define the compression hole <NUM>. The internal surface <NUM> can extend from the outer surface <NUM> to the inner surface <NUM>, and can be threaded so as to define the locking hole <NUM>.

Thus, one or more of the bone screws <NUM> can be configured as a compression screw <NUM> whose head <NUM> defines a compression head <NUM> that is configured to bear against the bone plate <NUM> in the compression hole <NUM> so as to apply a compressive force against the bone plate <NUM> toward the underlying bone <NUM>. In particular, during operation, the shaft <NUM> of the compression screw <NUM> can be threaded and inserted through the compression hole <NUM> and driven into the underlying bone <NUM> as described above. In particular, rotation of the bone screw <NUM> causes the shaft <NUM> to advance distally into the underlying bone, which thereby causes the compression head <NUM> to compress against the internal compression surface <NUM>. As a result, the compression head <NUM> causes the bone plate <NUM> to apply a compressive force against the underlying bone. At least a portion of the internal compression surface <NUM> is typically spherical or tapered as it extends from the outer surface <NUM> toward the inner surface <NUM> so as to prevent the compression head <NUM> from passing completely through the compression hole <NUM>. The compression head <NUM> typically has an unthreaded external surface. Similarly, at least a portion up to an entirety of the internal compression surface <NUM> that abuts the unthreaded external surface of the compression head <NUM> is typically unthreaded. Thus, it is common to drive compression screws <NUM> into the unthreaded compression holes <NUM>. However, as is described in more detail below with respect to <FIG>, the compression screws <NUM> can also be driven into the combination hole <NUM>.

Alternatively, one or more up to all of the bone screws <NUM> can be configured as locking screws <NUM> that are configured to threadedly purchase with the bone plate <NUM> inside the threaded locking holes <NUM>. In particular, the head <NUM> of the locking screws <NUM> be a locking head <NUM> that carries at least one external thread <NUM>. The thread <NUM> can be a helical thread. The shaft <NUM> of the locking screw <NUM> can similarly be threaded. During operation, the shaft <NUM> of the locking screw <NUM> can be inserted through the fixation hole <NUM> and driven into the underlying bone <NUM> as described above. In particular, rotation of the screw <NUM> causes the shaft <NUM> to advance distally into the underlying bone <NUM>, which causes the locking head <NUM> to advance toward the threaded locking hole <NUM> of the bone plate. As the screw <NUM> is continuously rotated, the locking head <NUM> can threadedly mate with the threaded internal surface <NUM> of the bone plate that defines the threaded locking hole <NUM>. In particular, the at least one external thread <NUM> of the locking screw <NUM> can threadedly mate with a corresponding at least one thread <NUM> that extends out from the internal surface <NUM>. As a result, the locking screw <NUM> can secure the bone plate <NUM> to the underlying bone <NUM> without applying a compressive force onto the bone plate <NUM> against the underlying bone <NUM>. The bone plate <NUM> can be spaced from the underlying bone <NUM> when locked to the locking head <NUM>. Alternatively, the bone plate <NUM> can abut the underlying bone <NUM> when locked to the locking head <NUM>. At least a portion of the internal surface <NUM> can be tapered as it extends from the outer surface <NUM> toward the inner surface <NUM>. Thus, the internal surface <NUM> is configured to prevent the locking head <NUM> from passing completely through the threaded locking hole <NUM>.

One or more of the locking screws <NUM> can be configured as a standard-type locking screw <NUM>. The locking head <NUM> of a standard-type locking screw <NUM> can be referred to as a standard-type locking head <NUM> having an external thread <NUM>. The external thread <NUM> is configured to threadedly purchase to the bone plate <NUM> in the locking hole <NUM> only when the standard-type locking screw <NUM> is at predetermined orientation with respect to the central hole axis <NUM>. For instance, the predetermined orientation can be the nominal orientation whereby the central screw axis <NUM> is coincident with the central hole axis <NUM>. Further, the external thread <NUM> can be misaligned with the internal surface <NUM> of the locking hole <NUM> with respect to threaded mating when the central screw axis <NUM> is not in the predetermined orientation with the central hole axis <NUM>.

Further, one more of the locking screws <NUM> are configured as a variable angle (VA) locking bone screw <NUM>. The locking head <NUM> of the VA locking bone screw <NUM> are configured as a VA threaded head <NUM>, and the shaft <NUM> of the VA locking bone screw <NUM> is referred to as a VA threaded shaft <NUM>. As will be appreciated from the description below, VA locking bone screw <NUM> can include a plurality of portions that are separate from each other and attached to each other. For instance, a first portion of the VA locking bone screw <NUM> can include the locking head, and a second portion of the VA locking bone screw <NUM> can include at least a portion up to an entirety of the VA threaded shaft <NUM>. As will be appreciated from the description below, the first portion can have a material hardness that is greater than that of the second portion.

One or more of the threaded locking holes <NUM> can be configured as variable angle (VA) locking holes <NUM> that is configured to receive a respective one or more of the VA locking bone screws <NUM>. In particular, the internal surface <NUM> of the bone plate <NUM> is configured to threadedly mate with the VA threaded head <NUM> in the VA locking holes <NUM> when the central screw axis <NUM> of the VA bone screw <NUM> is oriented at any one of a plurality of angles within a range of angles with respect to the central hole axis <NUM> at which the VA threaded head <NUM> is configured to threadedly mate with the bone plate <NUM> in the VA locking hole <NUM>. The bone plate <NUM> can be referred to as a locking compression plate, as its fixation holes are configured to engage compression screws and locking screws, respectively.

As illustrated in <FIG>, the bone plate <NUM> can be configured to be secured to the underlying bone <NUM> to promote healing of the defect <NUM>. In one example, the bone plate body <NUM>, and thus the bone plate <NUM>, can include a first plate portion that can define a plate head portion <NUM> that is configured to overlie the second bone segment <NUM>, and a second plate portion that can be referred to as a plate shaft portion <NUM> that is configured to overlie the first bone segment <NUM>. Each of the plate head portion <NUM> and the plate shaft portion <NUM> can include at least one up to a plurality of bone fixation holes <NUM>. Thus, bone anchors <NUM> that extend through respective fixation holes <NUM> of the plate head portion <NUM> can be driven into the epiphysis region of the underlying bone, and bone anchors <NUM> that extend through respective fixation holes <NUM> of the plate shaft portion <NUM> can be driven into the diaphysis region of the underlying bone. The epiphysis region can, for instance, be defined by the distal region of the radius bone. Any one or more up to all of the fixation holes <NUM> of the bone plate <NUM> can be compression holes, locking holes, VA locking holes or a combination thereof (also referred to as the "combination holes").

In one example, one or more up to all of the fixation holes <NUM> in the plate head portion <NUM> can be configured as VA locking holes <NUM>. Further, in one example, one or more up to all of the fixation holes <NUM> in the plate shaft portion <NUM> are compression holes <NUM> configured to receive cortical bone screws. Further, at least one or more up to all of the compression holes can be configured as slots that are elongate along a central longitudinal axis of the bone plate <NUM> to allow for positional flexibility of the bone screw <NUM> received therein. Alternatively or additionally, at least one or more up to all of the compression holes can have a circular cross-section so as to locate the position of the bone screw <NUM> received therein. While the bone plate <NUM> has been described in accordance with one specific example, it should be appreciated that the bone plate <NUM> can be configured in any suitable manner as desired. Further, bone plates <NUM> constructed in accordance with any of the examples described herein can be configured to attach to any region or region or regions of any suitable bone in the human or animal anatomy suitable for bone plate fixation.

The VA locking hole <NUM> will now be described in more detail with respect to <FIG>. As described above, the internal surface <NUM> of the bone plate <NUM> can be threaded, and can extend from the outer surface <NUM> to the inner surface <NUM> so as to define the VA locking hole <NUM> that extends from the outer surface <NUM> to the inner surface <NUM>. In particular, the VA locking hole <NUM> extends along the central hole axis <NUM>. The central hole axis <NUM> can be oriented along the transverse direction T. Thus, the central hole axis <NUM> can be oriented normal to either or each of the inner surface <NUM> and the outer surface <NUM>. It should be appreciated, of course, that the central hole axis <NUM> can be oriented in any suitable direction as desired, including a direction oblique to the transverse direction T.

As described above with respect to the threaded locking hole <NUM> of the bone plate <NUM>, the internal surface <NUM> of the VA locking hole <NUM> can be threaded. For instance, the internal surface <NUM>, and thus the bone plate <NUM>, can define a plurality of threaded regions <NUM> that each carries at least one thread <NUM>. Alternatively, the bone plate <NUM> can include a threaded insert in the VA locking hole <NUM>, whereby the threaded insert carries the at least one thread <NUM>. The internal surface <NUM>, and thus the bone plate <NUM>, can further define a plurality of recessed regions <NUM> that are disposed circumferentially between respective adjacent ones of the threaded regions <NUM>. Thus, the threaded regions <NUM> and the recessed regions <NUM> can be alternatingly arranged with each other circumferentially about the central hole axis <NUM>. The threaded regions <NUM> and the recessed regions <NUM> are configured such that the VA locking screws <NUM> are configured to threadedly purchase with the internal surface <NUM> at the threaded regions <NUM> without threadedly purchasing with the internal surface <NUM> at the at least one recessed regions <NUM>.

In one example, the at least one thread <NUM> projects out from the internal surface <NUM> at the threaded regions <NUM> into the VA locking hole <NUM> generally toward the central hole axis <NUM>. The at least one thread <NUM> can be monolithic with the internal surface <NUM>. The at least one thread <NUM> can extend along a thread path. The thread path can be a helical thread path. In one example, the at least one thread <NUM> can be a single lead thread, a double lead thread, or any suitably constructed thread as desired. The internal surface <NUM> can further define a recess <NUM> at each of the recessed regions <NUM>. The recesses <NUM> can circumferentially interrupt the at least one thread <NUM> so as to define a plurality of thread segments <NUM>. Axially aligned ones of the thread segments can combine to define a plurality of threaded columns <NUM>. Thus, it can be said that the threaded columns <NUM> are defined by thread segments <NUM>. Because the at least one thread <NUM> can extend along a helical thread path, the threaded columns <NUM> can have different numbers of thread segments <NUM>. The recesses <NUM> and the columns <NUM> can be alternatingly arranged with each other circumferentially about the central hole axis <NUM>. The at least one recess <NUM> is offset with respect to the columns <NUM> of thread segments <NUM> in a radially outward direction. The internal surface <NUM> can be said to define a recessed surface <NUM> at the recesses <NUM>.

The axial direction is defined as a direction between the outer surface <NUM> and the inner surface <NUM> of the bone plate <NUM>. Thus, the directional term "axially inward" and derivatives thereof as used herein refers to a direction from the outer surface <NUM> toward the inner surface <NUM>. Conversely, the terms "axially outward" and derivatives thereof as used herein refers to a direction from the inner surface <NUM> toward the outer surface <NUM>. The axially inward and axially outward directions can be oriented along the central hole axis <NUM>. Alternatively, the axially inward and axially outward directions can be oriented along a direction oblique to the central hole axis <NUM>, for instance when used with reference to the internal surface <NUM>.

The recesses <NUM> can have a radial depth sufficient such that the recessed surface <NUM> is recessed with respect to the internal surface <NUM> at the columns <NUM> along the radially outward direction. That is, the recessed surface <NUM> can define a radial distance from the central hole axis <NUM> that is greater than the radial distance from the central hole axis <NUM> to the major diameter of the at least one thread <NUM> of the columns <NUM>. Therefore, during operation, a threaded bone screw head <NUM> that threadedly purchases with the internal surface <NUM> at the columns <NUM> of thread segments <NUM> are spaced radially inward from the internal surface <NUM> at the recess <NUM>. The recessed surfaces <NUM> can be devoid of the thread <NUM>. For instance, the recessed surfaces <NUM> can be unthreaded and smooth.

The thread segments <NUM> of each of the columns <NUM> are spaced from each other in the axial direction so as to define interstices that receive corresponding external threads of a locking screw head <NUM>. The axially aligned thread segments <NUM> of at least a portion of the axial lengths of the columns <NUM> can each have a common circumferential length. In one example, all thread segments <NUM> of each of the columns <NUM> can have the same circumferential length. Accordingly, lines along circumferentially ends of the columns <NUM> can lie in respective planes that also include the central hole axis <NUM>.

Thus, the thread segments <NUM> of each of the columns <NUM> can be circumferentially offset from the thread segments <NUM> of the other ones of the columns <NUM>. Further, adjacent ones of the circumferentially spaced thread segments <NUM> can be separated by a respective common one of the recesses <NUM>. Thus the thread segments <NUM> of each column <NUM> can be aligned with the thread segments <NUM> of one or both adjacent column <NUM> along the thread path. Because the thread path can be helical, the thread segments <NUM> can be aligned with the thread segments <NUM> of an adjacent one of the columns <NUM> along a helical path. In one example, each of the thread segments <NUM> of a respective one of the columns <NUM> is aligned along the thread path with <NUM>) one the thread segments <NUM> a first adjacent column <NUM>, and <NUM>) one the thread segments <NUM> of a second adjacent column <NUM>. Thus, the respective one of the columns <NUM> is disposed circumferentially between the first adjacent column <NUM> and the second adjacent column <NUM>. Further, the thread segments <NUM> of the respective one of the columns <NUM> is disposed between the first one of the thread segments <NUM> and the second one of the thread segments <NUM> with respect to the axial direction.

In one example, the bone plate <NUM> can include three recesses <NUM>. The recesses <NUM> are circumferentially spaced apart from each other. However, it is appreciated that the bone plate <NUM> can include any number of recesses <NUM>, greater than one, as desired, so as to define the variable angle locking hole <NUM> of the type described herein. Further, the respective radial distance of the recessed surfaces <NUM> of each of the recesses <NUM> can be the same as each other. In this regard, each of the recesses <NUM> can be substantially (within manufacturing tolerance) identical to each other. Further, the recesses <NUM> can be circumferentially equidistantly spaced from each other about the central hole axis <NUM>. Alternatively, the recesses <NUM> can be circumferentially spaced from each other at a variable distance about the central hole axis <NUM>.

Each of the recesses <NUM> can define an adjacent pair of the columns <NUM>. Further, the bone plate <NUM> can define an equal number of recesses <NUM> and columns <NUM>. Thus, in one example, the bone plate <NUM> can include three columns 50a, 50b, and 50c of thread segments <NUM> in one example. The columns <NUM> are circumferentially spaced apart from each other. However, it is appreciated that the bone plate <NUM> can include any number of columns <NUM>, greater than one, as desired, so as to define the variable angle locking hole <NUM> of the type described herein. Each adjacent pair of columns <NUM> can be separated from each other by a respective recess <NUM>. The columns <NUM> can be substantially (within manufacturing tolerance) identical to each other. Further, the columns <NUM> can be circumferentially equidistantly spaced from each other about the central hole axis <NUM>. Alternatively, the columns <NUM> can be circumferentially spaced from each other at a variable distance about the central hole axis <NUM>.

With additional reference to <FIG>, the columns <NUM> allows for the bone plate <NUM> to mate with the VA threaded head <NUM> of the VA locking screw <NUM> in the VA locking hole <NUM> at an angle within a range of angles, such that the VA threaded head <NUM> threadedly purchases with a plurality of the columns <NUM> of thread segments <NUM> simultaneously. The angle and the range of angles are defined by the central screw axis <NUM> and the central hole axis <NUM>. In particular, at least one external thread <NUM> of the VA threaded head <NUM> can threadedly mate with the at least one thread <NUM> of the bone plate <NUM>, and in particular with respective ones of the thread segments <NUM> at the angle. In one example, the threaded head can threadedly purchase with all of the columns <NUM> of thread segments <NUM> simultaneously at the angle. The angle can be one of any angle within the range of angles when the central screw axis <NUM> is oblique to the central hole axis <NUM>, and at which the VA threaded head <NUM> can mate with the bone plate <NUM> in the VA locking hole <NUM>.

Referring again to <FIG>, while the threaded regions <NUM> include respective columns <NUM> of threaded segments <NUM>, it should be appreciated that the internal surface <NUM> need not be threaded along its entirety at locations axially aligned with the columns <NUM>. For instance, the internal surface <NUM> can include a tapered lead-in surface <NUM> at the axially outer end of the VA locking hole <NUM>. Thus, the lead-in surface <NUM> can extend from the outer surface of the bone plate <NUM> to the columns <NUM>. Further, the internal surface <NUM> can include a tapered undercut surface <NUM> at the axially inner end of the VA locking hole <NUM>. Thus, the tapered undercut surface <NUM> can extend from the inner surface of the bone plate <NUM> to the columns <NUM>. Either or both of the lead-in surface <NUM> and the undercut surface <NUM> can be unthreaded in one example.

It should be appreciated that the columns <NUM> can extend from the lead-in surface <NUM> to the undercut surface <NUM>. Further, the columns <NUM> can taper radially inward toward the central hole axis <NUM> as they extend axially inward. In one example, the columns <NUM> can extend linearly along the axial direction from the lead-in surface <NUM> to the undercut surface <NUM>. Further, the VA locking hole <NUM> can be constructed such that no portion of the internal surface <NUM> extends radially inward of the columns <NUM>. Therefore, the VA locking screw <NUM> described herein can threadedly purchase within the columns <NUM> without contacting any other surface except for the columns <NUM> and the undercut surface <NUM>.

Referring now to <FIG> in particular, the VA locking holes <NUM> can be configured to threadedly receive the VA locking screw <NUM>. When the locking screw <NUM> is configured as a VA locking screw <NUM>, the locking head <NUM> can be configured as a VA locking head <NUM>. Further, as the locking screw <NUM> is configured as the VA locking screw <NUM>, the at least one external thread <NUM> of the VA locking head <NUM> can be configured as at least one external VA thread <NUM>. The VA threaded shaft <NUM> can define an outer surface <NUM> and at least one thread <NUM> that projects out from the outer surface <NUM>. The at least one thread <NUM> can be a helical thread. The VA locking screw <NUM> is configured to threadedly mate with the bone plate <NUM> in the VA locking hole <NUM> at different orientations of the VA locking screw <NUM> with respect to the central hole axis <NUM>.

The VA threaded head <NUM> can be constructed in accordance with any suitable example, for instance as described in <CIT>. Thus, the VA threaded head <NUM> can define an outer surface <NUM>, and the at least one external thread <NUM> can extend out from the outer surface <NUM>. The outer surface <NUM> can be curved. For instance, the outer surface <NUM> can be convex. In particular, the outer surface <NUM> can be spherical. The at least one external thread <NUM> can define a thread peak and a thread trough (or crest and root in thread terminology, respectively). Each of the thread peak and the thread trough can like on respective first and second curved or spherical paths. Thus, each of the thread peak and the thread trough can be defined by respective radii, wherein a first radius of the thread peak is greater than a second radius of the thread trough. Further, the center of the first and second spherical paths can coincide with the center of the spherical outer surface <NUM>. In one example, the center of the outer surface <NUM> and the center of the first and second spherical paths can define a geometrical center of the VA threaded head <NUM>.

It should be appreciated that the outer surface <NUM> can be geometrically defined in any suitable manner as desired to achieve variable angle locking as described herein. For instance, the outer surface <NUM> can be conically shaped in one example. Thus, the thread peaks and thread troughs can similarly be conically shaped. Further, the external thread <NUM> of the VA threaded head <NUM> of the VA locking screw <NUM> can be circumferentially continuous about the central screw axis <NUM>. It should be appreciated, however, that the VA threaded head <NUM> can be alternatively constructed in any manner desired so as to threadedly mate with the at least one thread <NUM> in the manner described herein.

Otherwise stated, and referring to <FIG> and <FIG>, the VA locking screw <NUM> is configured to be inserted into the VA locking hole <NUM> such that the central screw axis <NUM> is at one of a plurality of angles with respect to the central hole axis <NUM> within a range of angles at which the VA threaded head <NUM> is configured to threadedly mate with the at least one thread <NUM> in the VA locking hole <NUM>. For instance, the VA locking screw <NUM> is configured to be inserted into the VA locking hole <NUM> such that the central screw axis <NUM> is at one of a plurality of angles within a range of angles defined by the central screw axis <NUM> and the central hole axis <NUM> at which the VA threaded head <NUM> is configured to threadedly mate with each of the columns 50a, 50b, and 50c. The range of angles can be disposed within a cone that is centered about the central hole axis <NUM>. In one example, the range of angles can be disposed within a cone of up to thirty degrees. The range of angles can be measured as <NUM> degrees with respect to the central hole axis <NUM>. The central hole axis <NUM> can define the center of the cone. Thus, the VA threaded head <NUM> of the VA locking screw <NUM> can mate with the bone plate <NUM> in the manner described herein both when central screw axis <NUM> of the VA locking screw <NUM> is coincident with the central hole axis <NUM> and when the central screw axis <NUM> of the VA locking screw <NUM> is at any other angle with respect to the central hole axis <NUM> within the range of angles.

Thus, it can be said that the at least one thread <NUM> of the bone plate is configured to threadedly mate with the VA threaded head <NUM> while the VA locking screw <NUM> is inserted into the VA locking hole <NUM> such that the central screw axis <NUM> is oriented at a first angle with respect to the central hole axis <NUM>, and the at least one thread <NUM> is further configured to threadedly mate with the VA threaded head <NUM> when the VA locking screw <NUM> is inserted into the VA locking hole <NUM> such that the central screw axis <NUM> is oriented at a second angle with respect to the central hole axis <NUM> that is different than the first angle. At least one or both of the first and second angles can be non-zero angles. Alternatively, the central screw axis <NUM> can be coincident with the central hole axis <NUM> in one of the orientations in the range of orientations. The at least one thread <NUM> and the threads of the VA threaded head <NUM> are defined prior to insertion of the VA locking screw <NUM> into the VA locking hole <NUM>. That is, the internal surface <NUM> is not designed or configured to cut or form threads into the VA threaded head <NUM>. Similarly, the VA threaded head <NUM> is not designed or configured to cut or form threads into the internal surface <NUM>. It is recognized, however, that after locking of the VA locking screw <NUM> in the VA locking hole <NUM>, the respective roots of the thread segments <NUM> that are mated with the VA threaded head <NUM> can undergo a small amount of elastic and/or plastic deformation.

The VA locking hole <NUM> is configured to receive the VA locking screw <NUM> such that more than one of the columns up to all of the columns <NUM> threadedly purchase with the externally threaded VA threaded head <NUM> of the VA locking screw <NUM> when the central screw axis <NUM> and the central hole axis <NUM> define any angle within the range of angles that are disposed within the range of angles. In particular, two or more of the columns <NUM> up to all of the columns 50a-50c can threadedly purchase with the externally threaded VA threaded head <NUM> of the VA locking screw <NUM> when the VA threaded head <NUM> is mated with the bone plate <NUM> in the VA locking hole <NUM> while the central screw axis <NUM> is within the range of angles with respect to the central hole axis <NUM>.

Referring now to <FIG>, and as described above, the VA locking screw <NUM> is configured as a two piece screw having a first portion <NUM> and a second portion <NUM> that is separate from the first portion <NUM>. The first portion <NUM> includes at least a portion of the VA threaded head <NUM>, and the second portion <NUM> includes at least a portion of the VA threaded shaft <NUM>. The second portion <NUM> can also include the distal tip of the VA locking screw <NUM>. The distal tip can be self-tapping in some examples. In one example, the first portion <NUM> can include an entirety of the VA threaded head <NUM>. Thus, the first portion <NUM> can include an entirety of the at least one external VA thread <NUM>. Alternatively, the first portion <NUM> can include a portion less than an entirety of the at least one external VA thread <NUM>. Further, the second portion <NUM> can include an entirety of the VA threaded shaft <NUM>. Thus, the second portion <NUM> can include an entirety of the at least one thread <NUM>. Alternatively, the second portion <NUM> can include a portion less than an entirety of the at least one thread <NUM>.

The first and second portions <NUM> and <NUM> can further include respective first and second portions of a neck <NUM> that extends from the threaded VA threaded shaft <NUM> to the VA threaded head <NUM>. The neck <NUM> can be unthreaded in one example, and can flare radially outward from the VA threaded shaft <NUM> to the VA threaded head <NUM>. In this regard, it should be appreciated that the VA threaded head <NUM> can define a greater diameter than the VA threaded shaft <NUM>. In one example, each of the first and second portions <NUM> and <NUM> can be a one-piece monolithic body.

Because the first and second portions <NUM> and <NUM> are separate from each other, at least a portion of the first portion <NUM> is fabricated having a material hardness greater than the second portion <NUM>. That is, either or both of the at least the outer surface <NUM> and the at least one external thread <NUM> of the VA threaded head <NUM> can have a material hardness greater than the material hardness of either or both of the outer surface <NUM> and at least one thread <NUM> of the VA threaded shaft <NUM>. Otherwise stated, either or both of the outer surface <NUM> and the at least one external thread <NUM> can have a first hardness, and either or both of the outer surface <NUM> and the at least one thread <NUM> can have a second hardness that is less than the first hardness. In one example, the first portion <NUM> and the second portion <NUM> can be made from the same material, but the first portion <NUM> can be treated with a hardening process. For instance, the first portion <NUM> is subjected to a hardening process to harden at least a portion of either or both of the outer surface <NUM> of the VA threaded head <NUM> and the at least one external thread <NUM> that is defined by the first portion <NUM>. The hardening process can include a nitrogen diffusion or nitriding process, an oxygen diffusion hardening, or any suitable alternative hardening process. As will be described in more detail below, the hardening process renders the at least one external thread <NUM> of the VA threaded head <NUM> harder than the internal surface <NUM> of the bone plate <NUM>. Thus, the at least one external thread <NUM> of the VA threaded head <NUM> can have a cantilever strength greater than that of the at least one thread <NUM> of the bone plate <NUM>. Further, the outer surface <NUM> of the VA threaded shaft <NUM> can be non-hardened. Alternatively, in some examples, the VA locking bone screw <NUM> can be a single monolithic structure that includes the VA threaded head <NUM> and the VA threaded shaft <NUM>. In this example, at least a portion up to an entirety of the VA locking bone screw <NUM> can be hardened using an oxygen diffusion hardening process. Thus, the VA locking bone screw <NUM>, including the VA threaded head <NUM>, can be oxygen diffusion hardened. As described above, a masking process can be used to selectively harden the VA threaded head <NUM> in an oxygen diffusion hardening process, without hardening the VA threaded shaft <NUM> if desired.

In one example, the second portion <NUM> can be a commercially pure titanium or titanium alloy, and the second portion can be a hardened commercially pure titanium or hardened titanium alloy. In one particular example, the second portion can be non-hardened Ti-6Al-7Nb (TAN), and the first portion <NUM> can be a hardened TAN. It should be appreciated, of course, that the first and second portions <NUM> and <NUM> can each be made of any suitable alternative biocompatible material as desired, such as other titanium alloys, commercially pure titanium or stainless steel. In this regard, the first portion <NUM> can be a hardened commercially pure titanium. The second portion <NUM> can be a non-hardened commercially pure titanium. Alternatively, the first and second portions <NUM> and <NUM> can be made from Ti-6Al-4V (TAV). Thus, the first portion <NUM> can be a hardened TAV, and the second portion <NUM> can be a non-hardened TAV. Alternatively or additionally still, the first and second portions <NUM> and <NUM> can be made of cobalt-chrome-molybdenum (CCM). Thus, the first portion <NUM> can be a hardened CCM, and the second portion <NUM> can be a non-hardened CCM. Alternatively still, the first portion <NUM> can be a hardened stainless steel. The second portion <NUM> can be a non-hardened stainless steel. In one example, the stainless steel can be <NUM> stainless steel. According to an embodiment not falling under the scope of the present invention, the first portion <NUM> can be made of a first material, and the second portion <NUM> can be made of a second different material having a material hardness less than that of the first material. Thus, the first material can be non-hardened but still have a hardness greater than that of the second material. According to an embodiment not falling under the the scope of the present invention, the first portion <NUM> can be made of a first material and the second portion <NUM> can be made of a second material having a material hardness greater than the first material. However, the first portion <NUM> can be hardened, such that the hardened first material has a material hardness greater than the second material. The first and second materials can be chosen from any one of the materials listed above, or any suitable alternative material.

The first and second portions <NUM> and <NUM> are configured to attach to each other at a joint <NUM> so as to form the VA bone screw <NUM>. Thus, the joint <NUM> can define an interface between the first and second portions <NUM> and <NUM>. For instance, the first and second portions <NUM> and <NUM> can be welded to each other so as to define a weld joint <NUM>, or attached to each other in accordance with any suitable alternative manner. In one example, one of the first portion <NUM> and the second portion <NUM> can define a projection <NUM>, and the other of the first portion <NUM> and the second portion <NUM> can define a recess <NUM> that is configured to receive the projection <NUM>. Thus, the joint <NUM> can include the projection <NUM> and the recess <NUM>.

In one example, the first portion <NUM> can include the projection <NUM>. The projection <NUM> can extend in a distal direction from the at least a portion of the head that is defined by the first portion <NUM>. The projection <NUM> can be recessed radially inward toward the central screw axis <NUM> with respect to the outer surface <NUM> of the VA threaded head <NUM>. In particular, the first portion <NUM> can define an instrument engagement member <NUM> that is configured to engage a driving instrument that drives the VA bone screw <NUM> to rotate about the central screw axis <NUM>. The instrument engagement member <NUM> can define a socket, a cruciform, or any suitable alternative structure as desired. The instrument engagement member <NUM> can be defined by a proximal surface <NUM> of the first portion <NUM>. The projection <NUM> can extend out in the distal direction from a distal surface <NUM> of the first portion <NUM> that is opposite the proximal surface <NUM>. The projection <NUM> can be spaced radially inward from the outer surface <NUM>. The projection <NUM> can define any cross section in a plane oriented normal to the central screw axis <NUM> as desired. For instance, the shape can be polygonal. Thus, the shape can be triangular, square, rectangular, or the like. Alternatively, the shape can be round as desired. It may be desirable for the shape to be non-circular in some examples.

The recess <NUM> can extend into a proximal surface <NUM> of the second portion <NUM>. The proximal surface <NUM> can be disposed opposite the distal tip. The second portion <NUM> can include an outer wall <NUM> that includes an internal surface <NUM>. The internal surface <NUM> can define the recess <NUM>. In this regard, the proximal surface <NUM> of the second portion <NUM> and the distal surface <NUM> of the first portion <NUM> can partially define the joint <NUM>. Further, the proximal surface <NUM> is configured to abut the distal surface <NUM> of the first portion <NUM> when the first and second portions <NUM> and <NUM> are attached to each other. The proximal surface <NUM> and the distal surface <NUM> can be disposed at the neck <NUM> of the VA locking bone screw <NUM> in one example. The projection <NUM> and the internal surface <NUM> can also partially define the joint <NUM>. The recess <NUM> can extend into the second portion <NUM> along the distal direction, and can terminate in the second portion <NUM>. Thus, in one example, the recess <NUM> does not extend entirely through the second portion <NUM>. The recess <NUM> can extend centrally along the central screw axis <NUM>. The recess <NUM> can define the same shape as the projection <NUM> in the plane oriented normal to the central screw axis <NUM>. Further, the recess <NUM> can be entirely enclosed by the second portion <NUM> along the plane. Thus, an entirety of the internal surface <NUM> can be radially inwardly spaced from the outer surface <NUM> of the VA threaded shaft <NUM> in a direction toward the central screw axis <NUM>.

During fabrication of the VA locking screw <NUM>, the projection <NUM> can be aligned with the recess <NUM>, and subsequently inserted into the recess <NUM>. For instance, the first portion <NUM> can be moved in the distal direction with respect to the second portion <NUM> so as to insert the projection <NUM> into the recess <NUM>. The projection <NUM> can thus be attached to the internal surface <NUM>, thereby attaching the first portion <NUM> to the second portion <NUM>. In one example, the recess <NUM> can be sized substantially equal to the projection, such that projection <NUM> can be press-fit to the internal surface <NUM> in the recess <NUM>. In another example, the internal surface <NUM> and the projection <NUM> can be threaded and configured to threadedly mate with each other. Thus, the first and second portions <NUM> and <NUM> can threadedly engage each other so as to attach the first portion <NUM> to the second portion <NUM>.

Additionally, the first and second portions <NUM> and <NUM> can be sealed to each other at the joint <NUM>, which can prevent leakage of anatomical fluids into the joint <NUM> between the first and second portions <NUM> and <NUM>, and can also enhance the strength of the joint. For instance, the projection <NUM> and the internal surface <NUM> can be sealed to each other. Further, the distal surface <NUM> of the first portion <NUM> and the proximal surface <NUM> of the second portion <NUM> can be sealed to each other. For instance, the projection can be glued to the internal surface <NUM>. Further, the distal surface <NUM> of the first portion <NUM> can be glued to the proximal surface <NUM> of the second portion <NUM>. Alternatively or additionally, the projection <NUM> can be welded to the internal surface <NUM>. For instance, the projection <NUM> can be laser welded or electron beam welded to the internal surface <NUM>. Further, the distal surface <NUM> of the first portion <NUM> can be welded to the proximal surface <NUM> of the second portion <NUM>. For instance, the distal surface <NUM> of the first portion <NUM> can be laser welded or electron beam welded to the proximal surface <NUM> of the second portion <NUM>. Alternatively or additionally still, the outer wall <NUM> can be radially crimped against the projection <NUM> to attach the first portion <NUM> to the second portion <NUM>. For instance, the respective one of the first and second portions <NUM> and <NUM> that includes the outer wall <NUM> can define a crimp zone at an outer surface of the outer well <NUM> that is radially opposite the inner surface <NUM>. The outer surface can, for instance, be unthreaded at the crimp zone.

With specific reference to <FIG>, it is recognized that the VA locking screw <NUM> can be constructed in a plurality of sizes. For instance, in one example, the VA locking screw can define any suitable outer diameter as desired, as defined by the outer diameter of the VA threaded shaft <NUM>. The core diameter CD can be defined at the root of the at least one thread <NUM> of the VA threaded shaft <NUM>. In one example, the outer diameter can range from approximately <NUM> to approximately <NUM>. For instance, in one example, the core diameter CD can be approximately <NUM>. In another example, the core diameter can be approximately <NUM>. It should be appreciated, of course, that the VA threaded shaft <NUM> can define any suitable alternative outer diameter as desired.

The projection <NUM> can further define any suitable size and shape as desired. In one example, the projection <NUM> can define any suitable maximum cross-sectional dimension DP along a plane that is oriented perpendicular to the central screw axis <NUM>. The maximum cross-sectional dimension DP of the projection <NUM> can be configured as a diameter when the projection <NUM> is cylindrical. It is recognized, however, that the projection <NUM> can define any suitable cross-sectional shape as desired. In one example, the maximum cross-sectional dimension DP of the projection <NUM> can range from approximately <NUM> to approximately <NUM>. For instance, when the outer diameter CD is approximately <NUM>, the maximum cross-sectional dimension DP of the projection <NUM> can range from approximately <NUM> to approximately <NUM>. When the outer diameter CD is approximately <NUM>, the maximum cross-sectional dimension DP of the projection <NUM> can range from approximately <NUM> to approximately <NUM>.

Thus, it should be appreciated that the core diameter CD of the VA threaded shaft <NUM> and the maximum cross-sectional dimension DP of the projection <NUM> can define any suitable ratio as desired. For instance, the ratio can range from approximately <NUM> to approximately <NUM>. For instance, when the outer diameter CD is approximately <NUM>, the ratio can range from approximately <NUM> to approximately <NUM>. When the outer diameter is approximately <NUM>, the ratio can range from approximately <NUM> to approximately <NUM>.

Further, the VA locking screw <NUM> can define any length L as desired. The length L can be measured along the central screw axis <NUM> from the proximal surface <NUM> of the first portion to the distal tip of the second portion <NUM>. In one example, the length can range from approximately <NUM> to approximately <NUM>. For instance, when the outer diameter OT is approximately <NUM>, the length can range from approximately <NUM> to approximately <NUM>. When the outer diameter CD is approximately <NUM>, the length L can range from <NUM> to <NUM>. Further, the distal end of the projection <NUM> can be spaced from the end of the distal tip by any suitable distance as desired. For instance, the distance can be greater than <NUM> in one example. The above dimensions are provide as example only, it being appreciated that the VA locking screw <NUM> can define other sizes and shapes in accordance with the present disclosure. It has been found, however, that the above described dimensions can allow for the first and second portions <NUM> and <NUM> to reliably attach to each other while maintaining the overall strength of the VA locking screw <NUM>.

Referring now to <FIG>, in another example, the second portion <NUM> can include the projection <NUM>. The projection <NUM> can extend in a proximal direction from the at least a portion of the shaft that is defined by the second portion <NUM>. In particular, the projection <NUM> can extend out from the proximal surface <NUM> of the second portion <NUM> in the proximal direction. The proximal direction can be opposite the distal direction. The projection <NUM> can be recessed radially inward toward the central screw axis <NUM> with respect to the outer surface <NUM> of the VA threaded shaft <NUM>.

The recess <NUM> can extend into the distal surface <NUM> of the first portion <NUM>. In particular, the first portion <NUM> can include the outer wall <NUM> and the internal surface <NUM> that defines the recess <NUM>. The recess <NUM> can extend into the first portion <NUM> along the proximal direction, and can terminate in the first portion <NUM>. Thus, in one example, the recess <NUM> does not extend entirely through the first portion <NUM>. The recess <NUM> can extend centrally along the central screw axis <NUM>. The recess <NUM> can be entirely enclosed by the first portion <NUM> along a plane oriented normal to the central screw axis <NUM>. Further, the recess <NUM> can be entirely enclosed by the first portion <NUM> along the plane. Thus, an entirety of the internal surface <NUM> can be radially inwardly spaced from the outer surface <NUM> of the VA threaded head <NUM> in a direction toward the central screw axis <NUM>. The first and second portions <NUM> and <NUM> can be attached to each other and sealed to each other in the manner described above.

Referring now to <FIG>, in another example, the distal surface <NUM> of the first portion <NUM> and the proximal surface <NUM> of the second portion <NUM> can define substantially flat abutment surfaces. Thus, neither the first portion <NUM> nor the in the second portion <NUM> includes a defined projection or recess that mate to attach the first and second portions <NUM> and <NUM> to each other. In this example, the distal surface <NUM> and the proximal surface <NUM> can be brought into abutment with each other so as to define the joint. Further, at least a portion of the distal surface <NUM> and the proximal surface <NUM> can be friction-welded to each other to attach the first and second portions <NUM> and <NUM> to each other. For instance, the distal and proximal surfaces <NUM> and <NUM>, respectively, can be friction welded to each other at the outer circumference of the bone screw. Thus, the friction weld can seal the first and second portions <NUM> and <NUM> to each other at the joint to prevent leakage of anatomical fluids between the first and second portions <NUM> and <NUM>, and can also enhance the strength of the joint. In one example, the distal and proximal surfaces <NUM> and <NUM>, respectively, can be friction welded to each other along their substantial entireties.

Referring now to <FIG> generally, the at least one thread <NUM> of the VA threaded shaft <NUM> can be formed in the outer surface <NUM> of the VA threaded shaft <NUM> after the first and second portions <NUM> and <NUM> have been attached to each other. Further, the at least one thread <NUM> of the VA threaded head <NUM> can be formed in the outer surface <NUM> after the first and second portions <NUM> and <NUM> have been attached to each other. Alternatively, the at least one thread <NUM> of the VA threaded shaft <NUM> can be formed in the outer surface <NUM> of the VA threaded shaft <NUM> prior to attaching the first and second portions <NUM> and <NUM> to each other. Further, the at least one thread <NUM> of the VA threaded head <NUM> can be formed in the outer surface <NUM> prior to attaching the first and second portions <NUM> and <NUM> to each other. The threads <NUM> and <NUM> can be formed by engaging any suitable cutting instrument with the outer surface <NUM> and <NUM>, respectively, so as to cut into the respective outer surface.

As described above, the at least one thread <NUM> of the VA threaded head <NUM> can have a hardness greater than the at least one thread <NUM> of the VA threaded shaft <NUM>. Further, the hardness of the at least one thread <NUM> of the VA threaded head <NUM> can be greater than the hardness of the at least one thread <NUM> of the bone plate <NUM>, and thus the thread segments <NUM> of the threaded columns <NUM> (see <FIG>). For instance, the hardness of the at least one thread <NUM> of the VA threaded head <NUM> can be greater than the hardness of the entirety of the bone plate <NUM>. Further, the hardness of the at least one thread <NUM> of the threaded shaft <NUM> can be less than or equal to the hardness of the at least one thread <NUM> of the bone plate <NUM>, and thus the thread segments <NUM> of the threaded columns <NUM>. For instance, the hardness of the at least one thread <NUM> of the threaded shaft <NUM> can be less than or equal to the hardness of the entirety of the bone plate <NUM>. In one example, the plate <NUM>, including the at least one thread <NUM>, can be made from stainless steel, commercially pure titanium, TAN, or any suitable alternative material.

It is recognized that the at least one thread <NUM> of the VA threaded head <NUM> can be forced against the at least one thread <NUM> of the bone plate <NUM> as the VA threaded head <NUM> mates with the bone plate <NUM> inside the VA locking hole <NUM> at the oblique angle within the range of angles as described above. In this regard, it will be appreciated that the at least one thread <NUM> of the VA threaded head <NUM> can have a cantilever strength that is greater than the cantilever strength of the at least one thread <NUM> of the bone plate <NUM>. This can be because the at least one thread <NUM> of the VA threaded head <NUM> has a hardness greater than the at least one thread <NUM> of the bone plate <NUM>. Alternatively or additionally, in some examples, the geometry of the at least one thread <NUM> and the at least one thread <NUM> can have different geometries that impart an increased cantilever strength to the at least one thread <NUM> that is greater than the cantilever strength of the at least one thread <NUM> even when the material hardness of the at least one thread <NUM> is substantially equal to the material hardness of the at least one thread <NUM>. Otherwise stated, the at least one thread <NUM> of the variable angle locking hole <NUM> can have a material hardness that is less than or equal to the hardness of the at least one thread <NUM>, such that the at least one thread <NUM> of the variable angle threaded head <NUM> is configured to deform the at least one thread <NUM> of the variable angle locking hole <NUM> of the bone plate as the at least one thread <NUM> of the VA threaded head <NUM> mates with the at least one thread <NUM> of the variable angle locking hole <NUM>.

Thus, when the VA locking screw <NUM> is mated to the bone plate <NUM> in the VA locking hole <NUM>, the at least one thread <NUM> of the VA threaded head <NUM> can apply a force to the at least one thread <NUM> of the bone plate <NUM> that causes the at least one thread <NUM> of the bone plate <NUM> to deform, while the at least one thread <NUM> of the VA threaded head <NUM> does not deform. Otherwise stated, the thread segments <NUM> that are purchased with the VA threaded head <NUM> can undergo a slight elastic and/or plastic deformation at their respective roots when the harder VA threaded head <NUM> is locked in the softer VA locking hole <NUM>. The form fit can increase the reliability of purchase between the VA locking screw <NUM> and the bone plate <NUM> in the VA locking hole <NUM>. Further, because the VA threaded shaft <NUM> is non-hardened, the VA threaded shaft <NUM> has a desirable fatigue strength that would be reduced if the threaded shaft was hardened.

Referring now to <FIG>, the bone plate <NUM> can include any suitable fixation hole that is configured to mate with the VA locking screw <NUM> at any of the angles within the range of angles described above. For instance in one example, the bone plate <NUM> can include a combination hole <NUM> that includes both the VA locking hole <NUM> and the compression hole <NUM>. Thus, the internal surface <NUM> of the combination hole <NUM> can define both the VA locking hole <NUM> and the compression hole <NUM>. The VA locking hole <NUM> and the compression hole <NUM> of the combination hole <NUM> can be open to each other along a direction that is perpendicular to one or both of the central hole axis <NUM> of the VA locking hole <NUM> and the central hole axis <NUM> of the compression hole <NUM>. The central hole axis <NUM> of the VA locking hole <NUM> and the compression hole <NUM> of the combination hole <NUM> can be aligned with each other along the longitudinal L and thus along the central axis of the bone plate <NUM>, or along any suitable alternative direction as desired.

The internal surface <NUM> of the bone plate <NUM> can thus also define the compression surface <NUM> of the compression hole <NUM> of the combination hole <NUM>. In one example, the compression surface <NUM> can be concave in the axial direction with respect to the central hole axis <NUM> of the compression hole <NUM>. For instance, the compression surface <NUM> can be dish shaped or spherical. Thus, the compression surface <NUM> can be configured to be placed in surface contact with the compression head <NUM> of the compression screw <NUM>. Alternatively, the compression surface <NUM> can be linear in the axial direction as it tapers radially inwardly toward the central hole axis <NUM>.

During operation, the shaft <NUM> of the compression screw <NUM> can be inserted through the compression hole <NUM> of the combination hole <NUM> and driven into the underlying bone <NUM> as described above. For instance, the compression screw <NUM> can be inserted into the compression hole <NUM> such that the central screw axis <NUM> is oriented at any angle with respect to the central hole axis <NUM> within the range of angles described above. Rotation of the compression screw <NUM> while the shaft <NUM> is threadedly purchased with the underlying bone <NUM> causes the compression head <NUM> to bear against the compression surface <NUM>, and thus the internal surface <NUM>, so as to apply a compression force to the bone plate <NUM> that, in turn, becomes compressed against the underlying bone <NUM>.

Claim 1:
A variable angle bone screw (<NUM>) that extends along a central screw axis, the variable angle bone screw (<NUM>) comprising:
a first portion (<NUM>) defining at least a portion of a head (<NUM>) having at least one external thread configured to mate with a thread of a variable angle locking hole (<NUM>) of a bone plate (<NUM>) when the central screw axis (<NUM>) is oriented at any one of a plurality of angles within a range of angles with respect to a central axis of the variable angle locking hole (<NUM>) at which the head (<NUM>) is configured to threadedly mate with the thread of the variable angle locking hole (<NUM>);
a second portion (<NUM>) defining at least a portion of a shaft (<NUM>) having at least one thread configured to purchase with bone, wherein the at least one thread of the first portion (<NUM>) has a first material hardness, the at least one thread of the second portion (<NUM>) has a second material hardness less than the first material hardness, and the second portion is separate from the first portion and attached to the first portion (<NUM>) at a joint (<NUM>), characterized in that
the first portion (<NUM>) comprises a material that is hardened, and the second portion (<NUM>) comprises the material that is non-hardened.