Patent Publication Number: US-11026725-B2

Title: Hybrid spinal plates

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 14/562,804 filed on Dec. 8, 2014 and entitled “Hybrid Spinal Plates,” which is a continuation of U.S. application Ser. No. 13/049,147 filed on Mar. 16, 2011 and entitled “Hybrid Spinal Plates,” now U.S. Pat. No. 8,940,025, which is a continuation of U.S. application Ser. No. 10/904,984 filed on Dec. 8, 2004 and entitled “Hybrid Spinal Plates,” now U.S. Pat. No. 7,931,678, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     For a number of known reasons, bone fixation devices are useful for promoting proper healing of injured or damaged vertebral bone segments caused by trauma, tumor growth, or degenerative disc disease. The fixation devices immobilize the injured bone segments to ensure the proper growth of new osseous tissue between the damaged segments. These types of bone fixation devices often include internal bracing and instrumentation to stabilize the spinal column to facilitate the efficient healing of the damaged area without deformity or instability, while minimizing any immobilization and post-operative care of the patient. 
     One such device is an osteosynthesis plate, more commonly referred to as a bone fixation plate, that can be used to immobilize adjacent skeletal parts such as bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require immobilization with respect to one another. The plate is fastened to the respective bones, usually with bone screws, so that the plate remains in contact with the bones and fixes them in a desired position. Bone plates can be useful in providing the mechanical support necessary to keep vertebral bodies in proper position and bridge a weakened or diseased area such as when a disc, vertebral body or fragment has been removed. 
     Such plates have been used to immobilize a variety of bones, including vertebral bodies of the spine. These bone plate systems usually include a rigid bone plate having a plurality of screw openings. The bone plate is placed against the damaged vertebral bodies and bone screws are used to secure the bone plate to the spine, usually with the bone screws being driven into the vertebral bodies. 
     Bone screws can be supported in a spinal plate in either a rigid or a semi-rigid fashion. In a rigid fashion, the bone screws are not permitted to move angularly relative to the plate. Conversely, in a semi-rigid fashion, the bone screws can move relative to the plate. The use of rigid and semi-rigid bone screws allow the surgeon to select the appropriate bone screw based on the particular treatment. While current plating systems can be effective, they typically require the use of different plates to obtain the desired bone screw fixation. 
     Accordingly, there remains a need for an improved plating system that allows the surgeon to use a single plate and to select between various types of bone screw fixation. 
     SUMMARY 
     Disclosed herein are various exemplary spinal plating systems for use in treating spinal pathologies. The spinal plating systems can be configured to allow a surgeon to select a bone screw construct having a particular range of motion for attaching a spinal plate to bone as needed based on the intended use. In one exemplary embodiment, the spinal plating system includes a first bone screw that is polyaxially movable relative to the spinal plate, and a second bone screw that has a range of motion that is substantially limited to a single plane. 
     While the exemplary spinal plating systems can include a spinal fixation plate having virtually any configuration, in one exemplary embodiment the spinal plate includes a thru-bore formed therein that is adapted to interchangeably receive a first bone engaging fastener such that a shank of the first bone engaging fastener is movable in more than one plane of motion relative to the spinal plate, and a second bone engaging fastener such that movement of a shank of the second bone engaging fastener relative to the spinal plate is substantially limited to a single plane of motion. 
     While the thru-bore in the spinal plate can have a variety of configurations, one exemplary thru-bore includes a proximal inner wall and a distal inner wall that differ in shape relative to one another. The proximal inner wall can, for example, be substantially symmetrical about a common axis of the thru-bore, and the distal inner wall can, for example, be substantially asymmetrical about the common axis. In another exemplary embodiment, at least a portion of the distal inner wall can extend at an angle relative to a central axis of the thru-bore. One exemplary angle is in the range of approximately 1° to approximately 10°. In another exemplary embodiment, the proximal inner wall of the thru-bore can be substantially spherical, and the distal inner wall of the thru-bore can be oblong. The oblong inner wall can have a maximum extent and a minimum extent that is less than the maximum extent. Where the spinal fixation plate includes opposed proximal and distal ends, and opposed lateral sides extending between the opposed proximal and distal ends, in one embodiment the minimum extent can extend in a proximal-distal direction, and the maximum extent can extend in a medial-lateral direction. In another embodiment, the maximum extent can extend in a proximal-distal direction, and the minimum extent can extend in a medial-lateral direction. 
     In yet another exemplary embodiment of the present invention, first and second bone engaging fasteners are provided having a shank with a head formed thereon and adapted to be received within a thru-bore in the spinal plate. The head of the second bone engaging fastener can be different from the head of the first bone engaging fastener such that the fasteners interact with a thru-bore in a spinal plate in two different orientations. While each bone engaging fastener can have a variety of configurations, in one exemplary embodiment the head of the first bone engaging fastener can have a distal portion with an extent that is substantially less than the maximum and minimum extents of a distal inner wall of the thru-bore formed in a spinal plate, and the head of the second bone engaging fastener can have a distal portion with an extent that is adapted to engage the minimum extent of the distal inner wall of the thru-bore. 
     In another embodiment, the spinal plate can include opposed proximal and distal ends and lateral sides extending between the proximal and distal ends. When a first bone engaging fastener is disposed within a thru-bore in the plate, a shank of the first bone engaging fastener can be movable in a proximal direction, a distal direction, a medial direction, a lateral direction, and combinations thereof. When a second bone engaging fastener is disposed within the thru-bore in the plate, a shank of the second bone engaging fastener can be substantially limited to movement in only one of a proximal direction, a distal direction, a medial direction, a lateral direction, a medial-lateral direction, and a proximal-distal direction. 
     An exemplary spinal plate having an insert disposed therein for receiving a first bone screw in a variable angle construct and a second bone screw in a limited angle construct is also provided. In another embodiment, the insert can be a ring-shaped member disposed within a thru-bore in the plate. The ring-shaped member can have a variety of configurations, for example it can include a split formed therein such that an extent of the ring-shaped member is adjustable. In one exemplary embodiment, the ring-shaped member can include an outer surface having a shape that complements a shape of an inner surface of the thru-bore, and an inner surface having at least a portion that is asymmetrical about an axis of the thru-bore in the insert. By way of non-limiting example, at least a portion of the inner surface of the thru-bore can have an oblong shape. In another embodiment, the ring-shaped member can be adapted to be disposed within the thru-bore in the spinal plate in a plurality of positions. The ring-shaped member can include an alignment mechanism adapted to align the ring-shaped member in one of the plurality of positions in the thru-bore in the spinal plate. By way of non-limiting example, the alignment mechanism can be at least one protrusion formed on an external surface of the ring-shaped member. The thru-bore in the spinal plate can include at least one corresponding detent formed therein for receiving the protrusion(s) on the ring-shaped member. 
     An exemplary spinal plating kit is also provided. In one embodiment, the spinal plating kit includes a first bone engaging fastener having a shank with a head formed thereon, a second bone engaging fastener having a shank with a head that differs from the head of the first bone engaging fastener, and a spinal plate having a thru-bore formed therein and adapted to selectively seat the head of the first and second bone engaging fasteners. At least a portion of the thru-bore can be substantially asymmetrical about an axis of the thru-bore such that the thru-bore is adapted to allow polyaxial movement of the shank of the first bone engaging fastener, and it is adapted to substantially limit movement of the shank of second bone engaging fastener to within a single plane of motion. In one exemplary embodiment, the thru-bore in the spinal plate can include a proximal portion that is adapted to selectively seat a proximal portion of the head of the first and second bone engaging fasteners, and a distal portion that is adapted to selectively seat a distal portion of the head of the first and second bone engaging fasteners. By way of non-limiting example, the proximal portion of the thru-bore can be substantially spherical and the distal portion of the thru-bore can be substantially oblong. In another exemplary embodiment, the head of the first bone engaging fastener can include a substantially spherical proximal portion and a distal portion, and the head of the distal portion of the second bone engaging fastener can include a substantially spherical proximal portion and a substantially cylindrical distal portion having a size that is greater than a size of the distal portion of the first bone engaging fastener such that the distal portion of the head of the second bone engaging fastener is adapted to engage at least a portion of the distal portion of the thru-bore. 
     Exemplary methods for implanting a spinal fixation plate are also provided. One exemplary methods includes positioning a spinal fixation plate against bone. The spinal fixation plate includes a thru-bore with an insert disposed therein. The insert can have a central opening formed therethrough and defining a single plane of motion of a bone engaging fastener to be received therein. The insert can then be rotated to orient the single plane of motion in a desired direction, and a bone engaging fastener can then be inserted through the insert to attach the spinal fixation plate to bone, wherein movement of a shank of the bone engaging fastener is limited to the desired direction of the single plane of motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an exemplary embodiment of a spinal fixation plate having a bone screw disposed within a thru-bore formed therein and showing an exemplary range of motion of the bone screw; 
         FIG. 1B  is an end view of the spinal plate and bone screw shown in  FIG. 1A ; 
         FIG. 1C  is a side view of the spinal plate and bone screw shown in  FIG. 1A ; 
         FIG. 2A  is a perspective view of the spinal plate shown in  FIG. 1A  having another exemplary embodiment of a bone screw disposed within a thru-bore formed therein and showing an exemplary range of motion of the bone screw; 
         FIG. 2B  is an end view of the spinal plate and bone screw shown in  FIG. 2A ; 
         FIG. 2C  is a side view of the spinal plate and bone screw shown in  FIG. 2A ; 
         FIG. 3A  is a superior perspective view of another exemplary embodiment of a spinal fixation plate; 
         FIG. 3B  is a side view of the spinal fixation plate shown in  FIG. 3A ; 
         FIG. 3C  is a cross-sectional view of the spinal fixation plate shown in  FIG. 3A  taken across line C-C; 
         FIG. 3D  is a cross-sectional view of the spinal fixation plate shown in  FIG. 3A  taken across line D-D; 
         FIG. 4A  is a perspective view of one exemplary embodiment of a bone screw adapted to be disposed within one of the thru-bores shown in the spinal fixation plate of  FIGS. 3A-3D ; 
         FIG. 4B  is an enlarged view of the head of the bone screw shown in  FIG. 4A ; 
         FIG. 5A  is a perspective view of another exemplary embodiment of a bone screw adapted to be disposed within one of the thru-bores shown in the spinal fixation plate of  FIGS. 3A-3D ; 
         FIG. 5B  is an enlarged view of the head of the bone screw shown in  FIG. 5A ; 
         FIG. 6A  is a perspective view of an exemplary embodiment of an insert that is adapted to be disposed within a thru-bore in a spinal fixation plate; 
         FIG. 6B  is a superior view of one embodiment of a spinal fixation plate showing the insert of  FIG. 6B  disposed within two thru-bores formed therein; 
         FIG. 6C  is a superior view of the spinal fixation plate shown in  FIG. 6B  showing the insert of  FIG. 6B  disposed within two thru-bores formed therein and having two bone screws disposed therethrough; 
         FIG. 7  is a perspective view of another exemplary embodiment of a spinal fixation plate; and 
         FIG. 8  is a perspective view of yet another embodiment of a spinal fixation plate. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those of ordinary skill in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. 
     In one exemplary embodiment, a spinal plating system is provided having a spinal plate with at least one thru-bore formed therein for selectively receiving at least two types of bone screws, thus allowing a surgeon to select an appropriate construct depending on the intended use. While various techniques can be used to achieve such a spinal plating system, and certain exemplary embodiments will be discussed in more detail below,  FIGS. 1A-2C  generally illustrate the functionality of one such exemplary spinal plating system having a spinal plate  10 , a variable angle bone screw  20 , and a limited angle bone screw  30 . At the outset, one skilled in the art will understand that the spinal plate  10  and bone screws  20 ,  30  shown in  FIGS. 1A-2C  are merely shown for illustration purposes, and that the spinal plate  10  and bone screws  20 ,  30  can have virtually any configuration. By way of non-limiting example,  FIG. 7  illustrates another exemplary embodiment of a spinal fixation plate that can include various features disclosed herein. A person skilled in the art will also appreciate that a variety of other fastening devices can be used in place of the bone screws  20 ,  30  to attach the spinal plate  10  to bone. While not shown or particularly described, the exemplary spinal plating systems disclosed herein can also include a rigid bone screw that is adapted to be disposed through a thru-bore in the plate at a fixed angle. 
     Referring first to  FIGS. 1A-1C , one exemplary embodiment of a variable angle bone screw  20  is shown disposed within a thru-bore  12  in a spinal plate  10 . The bone screw  20 , various exemplary embodiments of which will be discussed in more detail below, generally includes a head  22  and a shank  24  extending from the head  22 . In this exemplary embodiment, when the shank  24  of the bone screw  20  is disposed through the thru-bore  12  in the plate  10  and the head  22  of the bone screw  20  is seated within the thru-bore  12 , the shank  24  of the bone screw  20  can move polyaxially relative to the plate  10 . In particular, the head  22  of the bone screw  20  can pivot within the thru-bore  12  such that the shank  24  can move freely within multiple planes of motion, as indicated by the cone-shaped shaded area M f . The polyaxial range of motion of the bone screw  20  can vary depending on the particular configuration of the bone screw  20  and the plate  10 , for example on the size and shape of the screw head  22  relative to the size and shape of the thru-bore  12 , but in the illustrated exemplary embodiment the shank  24  of the bone screw  20  can move approximately 15° in all directions from a neutral axis A s  of the screw  20 , such that the cone-shaped shaded area M f  has a cone angle α f  of about 30°. A person skilled in the art will appreciate that the range of motion can be less than or substantially greater than 15° depending on the intended use. For example, the shank  24  of the bone screw  20  can move approximately 10°-20°, and in some cases greater than 25°. 
     Now referring to  FIGS. 2A-2C , the spinal plate  10  is shown having a limited angle bone screw  30  disposed within thru-bore  12 . Again, the bone screw  30 , various exemplary embodiments of which will be discussed in more detail below, generally includes a head  32  and a shank  34  extending from the head  32 . In this exemplary embodiment, when the shank  34  of the bone screw  30  is disposed through the thru-bore  12  in the plate  10  and the head  32  of the bone screw  30  is seated within the thru-bore  12 , the shank  34  of the bone screw  30  can be substantially limited to movement within a single plane of motion relative to the plate  10 . In particular, the head  32  of the bone screw  30  can be configured to pivot within the thru-bore  12  such that the shank  34  has a limited range of motion that can be substantially within a single plane, as indicated by the shaded area M f . The limited range of motion of the bone screw  30  can vary depending on the particular configuration of the bone screw  30  and the plate  10 , for example on the size and shape of the screw head  32  relative to the size and shape of the thru-bore  12 , but in the illustrated exemplary embodiment the shank  34  of the bone screw  30  can move up to approximately 5° in one direction, i.e., a total of 10° in opposed directions, substantially within a single plane from a neutral axis A l  of the screw  30 . A person skilled in the art will appreciate that the range of motion can be less than or substantially greater than 5° depending on the intended use. For example, the range of motion of the shank  34  from the neutral axis of the bone screw  30  can be approximately 5° to approximately 15°. Moreover, while the shank  34  of the bone screw  30  can be substantially limited to movement within a single plane of motion, the bone screw  30  may toggle slightly or have some micro-motion that is outside of the plane of motion, for example, as a result of manufacturing tolerances. It will also be understood that the term “single plane of motion” is intended to generally refer to a direction of movement. 
     The exemplary spinal plating system shown in  FIGS. 1A-3C  can be achieved using a variety of techniques.  FIGS. 3A-6C  illustrate certain exemplary embodiments. A person skilled in the art will appreciate that the exemplary techniques used to achieve a system having two interchangeable fastening elements can be incorporated into a variety of other surgical devices, and that the exemplary spinal plating system disclosed can include a variety of other features known in the art. 
     Referring first to  FIGS. 3A-5B , one exemplary spinal plating system is shown having a fixation plate  40  (shown in  FIGS. 3A-3D ), a limited angle bone screw  50  (shown in  FIGS. 4A-4B ), and a variable angle bone screw  60  (shown in  FIGS. 5A-5B ). While the spinal fixation plate  40  can have virtually any configuration and the illustrated exemplary plate  40  is merely shown for reference purposes only, the exemplary plate  40  has a generally elongate shape with opposed proximal and distal ends  40   p ,  40   d , opposed lateral sides  40   a    40   b  extending between the proximal and distal ends  40   p ,  40   d , a superior non-bone contacting surface  40   s , and an inferior bone contacting surface  40   i . The plate  40  also includes four thru-bores  42   a ,  42   b ,  42   c ,  42   d  formed therein and extending between the superior and inferior surfaces  40   a ,  40   b . The plate  40  can, however, include any number of thru-bores. The bone screws  50 ,  60  can also have a variety of configurations, but in the illustrated exemplary embodiment the bone screws  50 ,  60  generally include a head  52 ,  62  and a shank  54 ,  64  extending distally from the head  52 ,  62 . 
     In this exemplary embodiment, one or more of the thru-bores  42   a ,  42   b ,  42   c ,  42   d  in the spinal plate  40  can be adapted interchangeably receive the limited angle bone screw  50  and the variable angle bone screw  60  such that the variable angle bone screw  60  can move polyaxially, as described with respect to  FIGS. 1A-1C , while the limited angle bone screw  50  can be substantially limited to movement within a single plane of motion, as described with respect to  FIGS. 2A-2C . In one exemplary embodiment, as shown in more detail in  FIGS. 3C and 3D , one or more of the thru-bores, e.g., thru-bore  42   c , can have a proximal inner wall  43   a  and a distal inner wall  43   b , and the shape of each portion of the inner wall  43   a ,  43   b  of the thru-bore  42   c  can be adapted to interact differently with each bone screw  50 ,  60 . In particular, in the illustrated exemplary embodiment the proximal inner wall  43   a  of the thru-bore  42   c  can have a shape that is complementary to the shape of at least a proximal portion of the head  52 ,  62  of each bone screw  50 ,  60 , while the distal inner wall  43   b  of the thru-bore  42   c  can have a shape that differs from the proximal inner wall  43   a  and that allows free angular movement of the variable angle bone screw  60  while limiting movement of the limited angle bone screw  50 . 
     While the shape of the proximal inner wall  43   a  of the thru-bore  42   c  can vary, in one exemplary embodiment the proximal inner wall  43   a  of the thru-bore  43   a  can be substantially symmetrical about a common or central axis A of the thru-bore  42   c . For example, the proximal inner wall  43   a  can have a substantially spherical shape. At least a proximal portion  52   a ,  62   a  of the head  52 ,  62  of each bone screw  50 ,  60  can also have a symmetrical shape, such as a spherical shape as shown in  FIGS. 4A-5B , that complements the spherical shape of the proximal inner wall  43   a  of the thru-bore  42   c . Thus, in use, the spherical proximal inner wall  43   a  of the thru-bore  42   c  can interchangeably seat the spherical proximal portion  52   a ,  62   a  of the head  52 ,  62  of each bone screw  50 ,  60 , and in an exemplary embodiment the proximal inner wall  43   a  does not impinge on or otherwise present movement of the proximal portion  52   a ,  62   a  of each bone screw  50 ,  60 . A person skilled in the art will appreciate that while the exemplary proximal inner wall  43   a  is described as having a substantially spherical shape, that the proximal inner wall  43   a  can have some interruptions in the shape. For example, the proximal inner wall  43   a  can include a cut-out portion to facilitate use of a locking mechanism with the plate  40 , as will be described in more detail below. 
     The distal inner wall  43   b  of the thru-bore  42   c  can also have a variety of shapes and sizes, but in one exemplary embodiment the distal inner wall  43   b  of the thru-bore  42   c  is substantially asymmetrical about a common or central axis A of the thru-bore  42   c . For example, the distal inner walls  43   b  of the thru-bore  42   c  can have an oblong shape, as shown. As a result of the oblong shape of the distal inner wall  43   b , the distal inner wall  43   b  can include a minimum extent D t1  and a maximum extent D t2  that is greater that minimum extent D t1 . The minimum and maximum extents D t1 , D t2  can be adapted to control movement of each bone screw  50 ,  60 . 
     As shown in  FIGS. 5A and 5B , the exemplary variable angle bone screw  60  has a head  62  with a distal portion  62   b  that is adapted to be received within the distal portion  43   b  of the thru-bore  42   c . While the shape of the distal portion  62   b  of the head  62  can vary, in the illustrated exemplary embodiment the distal portion  62   b  is substantially cylindrical. The distal portion  62   b  can have an extent, e.g., a diameter D v , that is substantially less than the minimum and maximum extents D t1 , D t2  of the distal portion  43   b  of the thru-bore  42   c . As a result, the distal portion  62   b  of the head  62  of the variable angle bone screw  60  can move in multiple directions, e.g., proximal, distal, medial, lateral, and combinations thereof, such that the shank  64  is polyaxial relative to the plate  40 . A person skilled in the art will appreciate that the head  62  of the variable angle bone screw  60  does not necessarily need to include a distal portion  62   b , and that the head  62  can merely taper into the shank  64 . 
     As shown  FIGS. 4A and 4B , the limited angle bone screw  50  can also have a head  52   b  with a distal portion  52   b  that is also adapted to be received within the distal portion  43   b  of the thru-bore  42   c . However, in an exemplary embodiment, the distal portion  52   b  of the head  52  of the limited angle bone screw  50  can differ in size relative to the distal portion  62   b  of the head  62  of the variable angle bone screw  60 . In an exemplary embodiment, the distal portion  52   b  of the head  52  of the limited angle bone screw  50  has a substantially cylindrical shape with an extent, e.g., a diameter D L , that is greater than an extent, e.g., a diameter D v , of the distal portion  62   b  of the variable angle bone screw  60 , that is substantially less than the maximum extent D t2  of the oblong distal inner wall  43   b  of the thru-bore  42   c , and that is only slightly less than the minimum extent D t1  of the oblong distal inner wall  43   b  of the thru-bore  42   c . As a result, when the head  52  of the limited angle bone screw  50  is seated within the thru-bore  42   c , the portion of the distal inner wall  43   b  of the thru-bore  42   c  having a minimum extent D t1  can engage the distal portion  52   b  of the head  52  of the limited angle bone screw  50 , thereby preventing movement of the bone screw  50  in the direction of the minimum extent D t1 . The bone screw  50  can move in the direction of the maximum extent D t2  of the distal inner wall  43   b  of the thru-bore  42   c  as the maximum extent D t2  is greater than the extent, e.g., diameter D L , of the distal portion  52   b  of the limited angle bone screw  50 . 
     The direction of movement of the limited angle bone screw  50  can vary depending on the positioning of the oblong distal inner wall  43   b  of the thru-bore  42   c . In other words, the minimum and maximum extents D t1 , D t2  of the oblong distal inner wall  43   b  of the thru-bore  42   c  can extend in any direction relative to the plate  40  depending on the intended plane of motion of the limited angle bone screw  50 . In one exemplary embodiment, the minimum extent D t1  extends in a proximal-distal direction, as shown in  FIG. 3D , and the maximum extent D t2  extends in a side-to-side direction, also referred to as a medial-lateral direction, as shown in  FIG. 3C . The limited angle bone screw  50  can thus move freely in a medial-lateral direction, but it can be substantially prevented from moving in a proximal-distal direction. 
     The amount of movement of each bone screw  50 ,  60  relative to the plate  40  can also vary, and the size of the head  52 ,  62  of each bone screw  50 ,  60 , as well as the size of the thru-bore  42   c , can be used to control the amount of movement in a particular direction. By way of non-limiting example, at least a portion of the distal inner wall  43   b  of the thru-bore  42   c  can be positioned at an angle relative to the central axis A of the thru-bore  42   c , and the angle can be determinative of the amount of movement. In the embodiment shown in  FIG. 3C , the opposed sides of distal inner wall  43   b  of the thru-bore  42   c  that define the maximum extent D t2  each extend at angle α 1 , α 2  that is approximately 5° such that the limited angle bone screw  50  can move 5° in a medial direction and 5° in a lateral direction. A person skilled in the art will appreciate that each angle α 1 , α 2  can vary, and that only one or both sides of the distal inner wall  43   b  of the thru-bore  42   c  that define the maximum extent D t2  can extend at an angle to control movement of the limited angle bone screw  50 . Moreover, the distal inner wall  43   b  of the thru-bore  42   c  does not need to extend at an angle to control movement of the limited angle bone screw  50 . In other exemplary embodiments, some or all of the distal inner wall  43   b  can be substantially parallel to the central axis A. For example, the inner wall  43   b  can have a stepped configuration such that the extent of the inner wall  43   b  changes between the proximal inner wall  43   a  and the distal inner wall  43   b . In other embodiments, the inner wall  43   b  can include a series of steps to change the extent between the proximal and distal inner walls  43   a ,  43   b . A person skilled in the art will appreciate that a variety of other techniques can be used to control movement of a limited angle bone screw  50  relative to the plate  40 . 
       FIGS. 6A-6C  illustrate another exemplary embodiment of a spinal plating construct. In this embodiment, rather than having a spinal plate with at least one thru-bore that is adapted to control movement of a variable angle bone screw and a limited angle bone screw, an insert  70  is provided for use with a spinal fixation plate. In one exemplary embodiment, the insert  70  is used with the limited angle bone screw  50  shown in  FIGS. 4A-4B  and the variable angle bone screw  60  shown in  FIGS. 5A-5B . A person skilled in the art will appreciate that the insert  70  can be used with a variety of other fastening devices. 
     The insert  70  can have virtually any shape and size, but in certain exemplary embodiments the insert  70  can have a shape that is adapted to be received within a thru-bore in a spinal plate. As shown in  FIG. 6A , the exemplary insert  70  is substantially ring-shaped with an outer surface  70   a  and an inner surface  70   b  defining a bore  72  extending therethrough. As is further shown in  FIG. 6A , the exemplary insert  70  can include a split or gap  71  formed therein to allow an extent or size of the insert  70  to be adjusted as may be needed to position the insert within a thru-bore in a spinal plate. 
     The outer surface  70   a  of the insert  70  can vary depending on the shape and size of the thru-bore which the insert  70  is adapted to be received within. In the illustrated exemplary embodiment, the outer surface  70   a  of the insert  70  is substantially cylindrical, but it can have a stepped configuration as shown. The stepped configuration allows the insert  70  to be seated within a thru-bore having a corresponding stepped configuration, thus preventing the insert  70  from passing completely through the thru-bore. An exemplary embodiment of a spinal plate  80  having thru-bores  82   a ,  82   b ,  82   c ,  82   d  is shown in  FIG. 6B , and as shown two inserts  70 ,  70 ′ are disposed within two of the thru-bores, e.g., thru-bores  82   b  and  82   d . A person skilled in the art will appreciate that the insert  70  can be used with virtually any spinal plate, and plate  80  is merely shown for reference purposes. 
     The inner surface  70   b  of the insert  70  can also have a variety of configurations, but in one exemplary embodiment the inner surface  70   b  is adapted to receive and interact differently with a variable angle bone screw, such as bone screw  60  shown in  FIGS. 5A-5B , and a limited angle bone screw, such as bone screw  50  shown in  FIGS. 4A-4B . As shown in  FIG. 6A , at least a portion of the inner surface  70   b  of the exemplary insert  70  can be substantially asymmetrical about a common or central axis of the insert  70 . In an exemplary embodiment, the inner surface  70   b  is similar to thru-bore  42   c  previously described in  FIGS. 3A-3D  and it can include a proximal portion that is substantially symmetrical about a common axis of the thru-bore  72 , and a distal portion that is substantially asymmetrical about the common axis. By way of non-limiting example, the proximal portion can have an spherical shape and the distal portion can having an oblong shape such that the distal portions includes a minimum extent d i1  and maximum extent d i2  that is greater than the minimum extent d i1 . 
     As previously described with respect to the thru-bore  42   c  in spinal fixation plate  40 , the minimum and maximum extent d i1 , d i2  portions can be adapted to control movement of the bone screws  50 ,  60 , which are shown in  FIG. 6C  disposed through the inserts  70 ,  70 ′ in the thru-bores  82   b ,  82   d  of plate  80 . In an exemplary embodiment, the extent, e.g., diameter D v , of the distal portion  62   b  of the exemplary variable angle bone screw  60  (shown in  FIGS. 5A and 5B ) can be substantially less than the minimum and maximum extents d i1 , d i2  of the oblong portion of the inner wall  70   b  of the insert  70 . As a result, the distal portion  62   b  of the head  62  of the variable angle bone screw  60  can move in multiple directions, e.g., proximal, distal, medial, lateral, and combinations thereof, such that the shank  64  is polyaxial relative to the plate  40 . In another exemplary embodiment, the extent, e.g., diameter D L , of the distal portion  52   b  of the head  52  of the limited angle bone screw  50  can be substantially less than the maximum extent d i2  of the oblong portion of the inner wall  72   b  of the insert  70  and only slightly less than the minimum extent d i1  of the oblong portion of the inner wall  72   b  of the insert  70 . As a result, when the head  52  of the limited angle bone screw  50  is seated within the insert  70 , the minimum extent d i1  portion of the inner wall  72 B of the insert  70  can engage the distal portion  52   b  of the head  52  of the limited angle bone screw  50 , thereby substantially preventing movement of the bone screw  50  in the direction of the minimum extent d i1 . The bone screw  50  can move in the direction of the maximum extent d i2  of the distal inner wall  72   b  of the insert  70  as the maximum extent d i2  can be greater than the extent, e.g., diameter D L , of the distal portion  52   b  of the limited angle bone screw  50 . 
     As was previously described with respect to thru-bore  42   c  in plate  40 , the minimum and maximum extents d i1 , d i2  of the oblong inner wall  72   b  of the insert  70  can be adapted to control the intended plane of motion of the limited angle bone screw  50 . For example, at least a portion of the oblong portion of the inner wall  72   b  of the insert  70  can be positioned at an angle to control the range of motion of the limited angle bone screw  50 . A person skilled in the art will appreciate that the shape of bore  72  in the insert  70  can have a variety of other configurations, and that the shape can be adapted in other ways to control the plane of motion of the limited angle bone screw  50  and/or the range of motion. 
     In another exemplary embodiment of the present invention, the insert  70  can be adapted to allow the direction of motion of the limited angle bone screw  50  to be selectively adjusted. While various techniques can be used to provide such a configuration, in one exemplary embodiment the direction in which the insert  70  is positioned within the thru-bore in the plate can be determinative of the plane of motion of the limited angle bone screw  50 . For example, the maximum extent d i2  of the inner wall  70   b  of the insert  70  can be positioned within a thru-bore  82   a - d  in the plate  80  in a direction of desired movement of the limited angle bone screw  50 , as the maximum extent d i2  portion of the inner wall can control the direction in which the limited angle bone screw  50  is allowed to move. As shown in  FIG. 6A , the maximum extent d i2  of the insert  70  is aligned with the slit  71 . Thus, when the insert  70  is disposed within one of the thru-bores  82   a - d  in the plate, the slit  70  can be positioned in the desired direction of movement. A person skilled in the art will appreciate that a slit  71  is not necessary and that a variety of other techniques can be used to indicate the orientation of the insert, including, for example, indicia formed on the insert  70 . Moreover, in use, the insert can be oriented as desired either before or after a bone screw is inserter therethrough. 
     In another embodiment, the insert  70  can include an alignment mechanism formed thereon and adapted to allow the insert  70  to be selectively aligned with the thru-bore in a desired direction of movement. By way of non-limiting example, the alignment mechanism can be one or more ridges, grooves, protrusions, detents, etc., or other features formed on the outer surface  70   a  of the insert  70 , and the inner surface of at least one of the thru-bores  82   a - 82   d  in the plate  80  can include corresponding ridges, grooves, protrusions, detents, etc., or other features formed on the inner surface thereof. The insert  70  can thus be inserted into one of the thru-bores  82   a - 82   d  in the plate  80  in a desired position, and the alignment mechanism can be effective to maintain the insert  70  in that position, i.e., to prevent rotation of the insert. 
     In certain exemplary embodiments, the insert  70  can include four protrusions (not shown) formed on the outer surface  70   a  thereof, and at least one of the thru-bores  82   a - d  in the plate  80  can include four corresponding detents (not shown) formed therein for receiving the protrusions. The detents or protrusions can be adapted to align the minimum and maximum extents d i1 , d i2  portions of the insert  70  in a particular direction, such as a proximal-distal direction or a medial-lateral direction. As a result, the insert  70  can be disposed within the thru-bore  82   a - d  in one of several orientations. In the first orientation, the slit  71 , which can function as an indicator for the maximum extent d i2  which can be aligned with the slit  71 , can be positioned toward the proximal end  80   p  of the plate  80  to allow movement of the limited angle bone screw  50  in a proximal direction, a distal direction, or both a proximal and distal direction. The slit  71  can likewise be positioned in a second, opposed orientation toward the distal end  80   d  of the plate  80  to likewise allow movement in a proximal direction, a distal direction, or both a proximal and distal direction. In a third orientation, the slit  71  can be positioned toward lateral side  80   a  of the plate  80  to allow movement of the limited angle bone screw  50  toward lateral side  80   a , toward the opposed lateral side  80   b , or in both directions, e.g., a medial-lateral or side-to-side direction. Likewise, in the fourth orientation, the slit  71  can be positioned toward lateral side  80   b  of the plate  80  to allow movement of the limited angle bone screw  50  toward lateral side  80   a , toward the opposed lateral side  80   b , or in both directions, e.g., a medial-lateral or side-to-side direction. A person skilled in the art will appreciate that a variety of other techniques can be used to allow the direction of movement of the limited angle bone screw  50  to be controlled. 
     While  FIGS. 1A-3D and 6B-6C  illustrate various embodiments of spinal fixation plates  10 ,  40 ,  50 ,  60 ,  80  having thru-bores  12 ,  42   a - d ,  82   a - d  with a generally circular configuration, the thru-bores can have a variety of other shapes. By way of non-limiting example,  FIG. 8  illustrates another exemplary embodiment of a spinal fixation plate  90  having a slotted thru-bore  92  formed therein. While not shown, the slotted thru-bore  92  can include features, as previously described, to allow a variable angle bone screw, such as screw  60 , to move polyaxially relative to the plate  90 , and to substantially limit movement of a limited angle bone screw, such as bone screw  50 , to a single plane of motion. The slotted thru-bore  92  can also allow the variable angle bone screw  60  and the limited angle bone screw  50  to translate within the thru-bore  92  to allow a position of the screw  50 ,  60  to be adjusted relative to the plate  90 . 
     In other exemplary embodiments, a spinal fixation plate can be provided having a thru-bore having a configuration that is substantially opposite to the configuration of the thru-bores  12 ,  42   a - d ,  82   a - d  described above with respect to spinal fixation plates  10 ,  40 ,  50 ,  60 ,  80 . In particular, while not illustrated, an exemplary thru-bore can include a proximal portion that is asymmetrical, e.g., oblong, about a central axis of the thru-bore, and a distal portion that is symmetrical, e.g., spherical shape, about the central axis. An exemplary variable angle bone screw and limited angle bone screw for use with such a thru-b ore can likewise have a reverse orientation, such that a head of the limited angle bone screw includes a proximal portion that is substantially cylindrical and a distal portion that is substantially spherical, and a head of the variable angle bone screw can be substantially spherical. The head of the variable angle bone screw does not necessarily need to include a proximal portion having any particular configuration. 
     While not illustrated, the various embodiments of the spinal plates disclosed herein can also include a locking or retaining mechanism for preventing bone screw backout. In one embodiment, the locking mechanism can be integrated into the screw head, as described in a U.S. Patent filed on even date herewith and entitled “Locking Bone Screw and Spinal Plate System” of Gorhan et al., which is incorporated by reference herein in its entirety. In another embodiment, the locking mechanism can be integrated onto the surface of the plate. The integrated locking mechanism can be, for example, a cam that is rotatable between an unlocked position and a locked position, in which the cam is forced against the head of the bone screw to provide bone screw backout resistance. An exemplary cam-type locking mechanism is described in U.S. Pat. No. 5,549,612 of Yapp et al. entitled “Osteosynthesis Plate System,” which is also incorporated by reference herein in its entirety. Other exemplary retaining or locking mechanisms include, by way of non-limiting example, locking washers, locking screws, and bone screw covers. One skilled in the art will appreciate that various combinations of locking mechanisms can be used as well. Other exemplary locking mechanisms are disclosed in U.S. Pat. No. 6,331,179 to Fried et al., U.S. Pat. No. 6,159,213 to Rogozinski; U.S. Pat. No. 6,017,345 to Richelsoph; U.S. Pat. No. 5,676,666 to Oxland et al.; U.S. Pat. No. 5,616,144 to Yapp et al.; U.S. Pat. No. 5,261,910 to Warden et al.; and U.S. Pat. No. 4,696,290 to Steffee. 
     One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.