Abstract:
A dynamic bone fixation plate assembly includes a bone plate with at least one fastener-receiving aperture, and at least one self-locking fastener. Each fastener includes a threaded shaft or shank for secure engagement with patient bone, and a head for engaging the bone plate in a manner providing a low profile orthopedic device. The fastener shank includes features lock the fastener to the bone plate to prevent the fastener from backing out of the bone plate while still allowing rotational movement between the fastener and the plate. Utilizing the features of the present invention, the bone plate controllably subsides and settles into a position of stability.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to orthopedic bone fixation devices for stabilizing a plurality of bone segments, and more particularly, but not necessarily entirely, to a bone plate and a bone screw assembly for stabilizing the cervical spine and blocking movement of grafts, and otherwise maintaining the cervical vertebrae in a desired relationship.  
         [0002]     The spine is a flexible, multi-segmented column that supports the upright posture in a human while providing mobility to the axial skeleton. The spine serves the dual functions of encasing and protecting vital neural elements while providing structural support for the body by transmitting the weight of the body through the pelvis to the lower extremities. The cervical spine, because of the orientation of its facets and due to the lack of supporting structures, exhibits a wide range of motion. The thoracic and lumbar regions of the spine also have a significant range of motion, but are limited by other factors.  
         [0003]     The spine is made up primarily of bone and intervertebral discs, which are surrounded by supporting ligaments, muscle, fascia, blood vessels, nerves, and skin. As in other areas of the body, these elements are subject to a variety of pathological disturbances: inflammation, trauma, neoplasm, congenital anomalies, disease, etc. In fulfilling its role in the body, the spine can be subjected to significant trauma which can play a large role in the etiology of neck and low back pain. Trauma frequently results in damage at the upper end of the lumbar spine, where the mobile lumbar segments join the less mobile dorsal spine. Excessive forces on the spine not only produce life-threatening traumatic injuries, but may contribute to an increased rate of degenerative change.  
         [0004]     The cervical region of the spine comprises the seven most superior vertebrae of the spine, which begin at the base of the skull and end at the upper torso. Because the neck has a wide range of motion and is the main support for the head, the neck is extremely vulnerable to injury and degeneration.  
         [0005]     Spinal fixation has become a common method of treating spinal disorders, fractures, and degeneration. One common device used for spinal fixation is the bone fixation plate. Generally, there are two types of spinal plates available, (i) constrained plates and (ii) semiconstrained plates. These plates are usually used in conjunction with a graft device placed between the vertebral bodies. Generally, a constrained plate completely immobilizes the vertebrae and does not allow for graft settling. In this instance, the plate itself carries a significant portion of the loading. Constrained plates are useful in patients with highly unstable anatomy, such as with a vertebrectomy, or in patients with little chance of bone growth, such as cancer patients. In contrast, a semiconstrained plate is dynamic and allows for a limited degree of graft settling through micro-adjustments made between the plate and bone screws attaching the plate to the spine. The operation of the semiconstrained plate stimulates bone growth because the loading is transferred through the graft. Each type of plate has its own advantages depending upon the anatomy and age of the patient, and the results desired by the surgeon.  
         [0006]     A typical bone fixation plate includes a relatively flat, rectangular plate having a plurality of apertures formed therein. A corresponding plurality of bone screws may be provided to secure the bone fixation plate to the vertebrae of the spine.  
         [0007]     A common problem associated with the use of bone fixation plates is the tendency for bone screws to become dislodged and “back out” from the bone, thereby causing the plate to loosen. Some attempts to provide a screw with polyaxial capabilities to help avoid screw “back out” are known throughout the prior art. However, many of these attempts have resulted in a bone fixation plate having a very large profile size that can cause irritation and discomfort of the patient&#39;s esophagus and surrounding tissues. Additionally, fixed angle screws require more precision in drilling in order to properly align the plate with the screw. Another problem with a multi-component device is that it must be assembled prior to implantation, or even worse, while the device is in the wound, which can be laborious and time consuming for surgeons.  
         [0008]     In a typical anterior cervical fusion surgery, the carotid sheath and sternocleidomastoid muscles are moved laterally and the trachea and esophagus are moved medially in order to expose the cervical spine. The cervical plate is designed to lie on the anterior face of the spine, posterior to the esophagus. Due to its relative location to the esophagus and other connective tissue, if the bone screw securing the plate to the cervical spine backs out, the bone screw could pierce the esophagus, causing not only pain and infection, but also posing a serious risk of death to the patient. It is not only important that the screw securing mechanism avoid piercing of the esophagus, but it also must maintain a small anterior-posterior profile. This will help alleviate post-operative difficulty in swallowing is experienced by the patient.  
         [0009]     There are several spinal fixation devices known in the prior art. U.S. Pat. No. 6,193,721 (granted Feb. 27, 2001 to Michelson) describes a multi-locking anterior cervical plate system. In this patent, Michelson discusses at length the problems with many locking plates due to their complexity or delicate “watchmaker” parts to achieve interlocking. One issue with these plates is that the intricate locking mechanisms are quite fragile and require extra steps and special tools to engage the features. Additionally, they may cause sharp or jagged shavings to be created, which can lead to patient injury.  
         [0010]     U.S. Pat. No. 6,193,720 (granted Feb. 27, 2001 to Yuan et al.) discloses a cervical spine stabilization device. This cervical spine fixation device requires multiple component parts to provide fixation between a plurality of vertebrae. This device is complex in operation because it requires multiple parts, each of which must be adjusted by the surgeon during surgery, causing extra unnecessary and unwanted labor and time.  
         [0011]     U.S. Pat. No. 6,022,350 (granted Feb. 8, 2000 to Ganem) discloses a bone fixation device comprising an elongate link for receiving at least one bone-fastening screw containing a semi-spherical head, which bone-fastening screw passes through an orifice created in the elongate link. The bottom of the elongate link contains a bearing surface that essentially has a circular cross section, allowing the semispherical head to be seated therein. The device further includes a plug having a thread suitable for coming into clamping contact against the screw head to hold the head in a desired angular position. This device is characterized by several disadvantages, including the need for a larger profile fixation device in order to allow the semi-spherical bone-fastening screw head and the accompanying plug to fit within the bearing surface. Ganem&#39;s larger profile design reduces the effectiveness of the device because of the potential for increased discomfort for the patient.  
         [0012]     In U.S. Pat. No. 6,679,883 (granted Jan. 20, 2004 to Hawkes, et. al.), a screw securing mechanism is disclosed. This mechanism requires a tertiary component be introduced to interact between the bone plate and the fastener. This third piece increases the complexity of the device during manufacture and implantation.  
         [0013]     It is noteworthy that none of the prior art known to applicants provides a spinal fixation device which has a low profile size, utilizes only plate and fastener components, provides the surgeon with the ability to manipulate and micro-adjust the fixation device, and still provides a screw locking method. There is a long felt, but unmet, need for a spinal fixation device which is relatively inexpensive to make, simple in operation and provides a secure interlock between the head of a fastener without superfluous components, that also has a low profile.  
         [0014]     Additionally, all of the prior art known to the applicants are manufactured from metallic materials. Since common methods of analyzing the new bone growth are generally radiographs, X-rays, or magnetic resonance imaging (MRI), the metal materials can interfere with these evaluations. A secondary, unmet need for a spinal fixation device is imaging compatibility.  
         [0015]     The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.  
         [0016]     The features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the drawings, subsequent detailed description and appended claims.  
       SUMMARY OF THE INVENTION  
       [0017]     In accordance with the invention, an improved bone plate with screw locking mechanism is provided for human implantation adjacent, e.g., to the cervical vertebrae. The bone plate has a plurality of apertures each specifically designed to interact a corresponding bone screw. This interaction allows each bone screw to pass through the bone plate, until a head of the bone screw fastener engages an anterior face of the bone plate. At this point the features of the apertures prevent the fasteners from backing out of the bone plate. In all of the presented embodiments, there are no tertiary locking components, nor extra procedural steps needed to lock the bone screw fasteners with the bone plate.  
         [0018]     In one preferred embodiment of the disclosed device, each bone screw fastener has a ‘collar’ of flexible tangs encircling a threaded screw shaft or shank. The flexible tangs are positioned between the screw head and the screw threads. As the flexible tangs pass through the narrow portion of the associated bone plate aperture, they flex or bend in order to pass. Once through the associate aperture, the flexible tangs spring or flex back substantially to their original nondeformed position, thus preventing the bone screw from exiting or backing out of the bone plate. The flexible tangs are desirably positioned far enough away from the screw head to allow some relative movement between the bone screw and the bone plate.  
         [0019]     In a second preferred embodiment of the device, the bone screw fastener features two thread forms formed along its shaft or shank. A first thread form consists of threads used to interface with or engage vertebral bone upon device implantation. This first thread form is located at a distal end of the screw shaft, opposite the screw head. A second thread form, located more proximal to the screw head, has a greater or larger major diameter than the first thread form. This second thread form is designed to engage a similar or mating female thread form formed in the associated aperture of the bone plate. With this construction, the female thread form on the bone plate is sized to allows the first thread form on the bone screw to pass relatively freely, while threadably engaging the larger second thread form proximal to the bone screw head. The first and second bone screw thread forms are of generally the same pitch to allow continuous advancing of the bone screw, i.e., thread-in engagement of the first thread form with patient bone concurrently with thread-in engagement of the second thread form with the bone plate female thread. At this point, the bone screw is captured by the bone plate. In the preferred form, the second thread form on the bone screw is spaced sufficiently from the associated head, so permit the second thread form to be advanced past the threaded bone plate aperture for disengagement of the second thread form from the bone plate. This construction enables the bone screw to articulate within the associated aperture of the bone plate, allowing for various bone screw trajectories as well as settling between the bone plate and the adjacent patient bone structure such as spinal vertebrae. A further value of the second thread form disengaging from the bone plate is that it allows the bone screws to have a lag screw effect. If the threads do not disengage, it is impossible for the bone screws to pull the bone plate against the vertebral bodies.  
         [0020]     This second embodiment, with the two thread forms on the bone screw, also allows for constrained screws to be placed. Utilizing the same bone plate, both semi-constrained and constrained screws may be implanted. By making the second thread form on the bone screw a more intimate fit with the female threads within the associated bone plate aperture, the bone screw becomes constrained within the aperture. This can be useful if the surgeon needs only superior bone screws to articulate, but also needs inferior bone screws to be constrained.  
         [0021]     Additionally, both of the previous embodiments are able to be manufactured from a variety of materials. One such preferred material is a high strength ceramic. These high strength ceramics are both radiolucent and MRI compatible. They allow the surgeons to better assess the new bone growth in and around the plate using standard techniques. The bone plates, as well as the bone screws, are able to be manufactured from these ceramics. Another preferred material is high strength polymer. Although not as strong as the ceramics, the polymers offer similar benefits of radiolucency and MRI compatibility.  
         [0022]     Furthermore, the devices of the previous embodiments may be coated with a bio-active surface coating material selected for relatively high osteoconductive and bio-active properties, such as a hydroxyapatite or a calcium phosphate material. In accordance with a further aspect of the invention, the device may additionally carry one or more therapeutic agents for achieving further enhanced bone fusion and ingrowth. Such therapeutic agents may include natural or synthetic therapeutic agents such as bone morphogenic proteins (BMPs), growth factors, bone marrow aspirate, stem cells, progenitor cells, antibiotics, or other osteoconductive, osteoinductive, osteogenic, bio-active, or any other fusion enhancing material or beneficial therapeutic agent. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The accompanying drawings illustrate the invention. In such drawings:  
         [0024]      FIG. 1  is a front or anterior side perspective view depicting a dynamic bone plate fixation assembly including a bone plate with a plurality of bone screws attached, in accordance with one preferred form of the invention;  
         [0025]      FIG. 2  is a frontal or anterior or outboard side view of the bone plate fixation assembly of  FIG. 1 ;  
         [0026]      FIG. 3  is a right side elevation view of the bone plate fixation assembly of  FIG. 1 ;  
         [0027]      FIG. 4  is a perspective view showing one alternative preferred embodiment of the bone plate with attached bone screws, illustrating a multi-level device;  
         [0028]      FIG. 5  is an enlarged and partially fragmented rear side or posterior or inboard side perspective view of the bone plate fixation assembly, and showing bone screws having dual thread forms or dual thread sets formed thereon;  
         [0029]      FIG. 6  is an enlarged and fragmented side elevation view corresponding with a portion of  FIG. 5 , and showing a bone screw head not yet engaging an outboard side of the bone plate;  
         [0030]      FIG. 7  is an enlarged and fragmented side elevation view similar to  FIG. 6 , but showing the bone screw head engaging the outboard side of the bone plate;  
         [0031]      FIG. 8  is a front or anterior or outboard side perspective view showing the bone plate of  FIG. 5 , but omitting the bone screws;  
         [0032]      FIG. 9  is a top plan view of the bone plate of  FIG. 8 ;  
         [0033]      FIG. 10  is an enlarged and fragmented side elevation view similar to  FIG. 7 , but showing a bone screw with dual thread forms secured rigidly to the bone plate;  
         [0034]      FIG. 11  is a rear or posterior side or inboard side perspective view showing another alternative preferred form of the invention, comprising a bone screw with flexible tangs assembled with a bone plate; and  
         [0035]      FIG. 12  is an enlarged and fragmented side elevation view corresponding with a portion of  FIG. 11 , and depicting a bone screw with flexible tangs assembled with the bone plate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     For the purposes of promoting an understanding of the principles in accordance with the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications of the inventive features illustrated herein, and any additional applications of the principles of the invention as illustrated herein, which would normally occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention claimed.  
         [0037]     Before the present device and methods for implantation of said device are disclosed and described, it is to be understood that this invention is not limited to the particular configurations, process steps, and materials disclosed herein as such configurations, process steps, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention will be limited only by the appended claims and equivalents thereof.  
         [0038]      FIGS. 1-3  illustrate the bone fixation plate assembly  10  of the present invention, including the improved bone plate  12  with at least one and preferably multiple fasteners such as bone screws  20  attached. The illustrative bone plate  12  comprises of a pair of elongated struts  14  spanning between two landings  16 . Each landing  16  contains a set or pair of apertures  18  for respectively receiving the bone screws  20 . One aperture  18  is designed to accept only one bone screw  20 . The elongated struts  14 , together with the landings  16 , frame an opening  22  in the central portion of the bone plate  12 . This central opening  22  allows visualization of the intervertebral graft and aids in the placement of the bone plate. This particular embodiment of the bone plate  12  with bone screws  20  depicts a concave landing area  16  in order to lower the risk of esophageal irritation.  
         [0039]     Each bone screw  20  has a head  24  formed with a spherical or part-spherical underside or inboard side surface geometry. This spherical underside surface beneficially enables the bone screw  20  to toggle or articulate within of the associated bone plate aperture  18 . The outwardly presented surface or face of each bone screw head  24  includes an inserter feature  26  such as a recessed cavity of non-circular cross sectional shape, such as the illustrative hexagonal shape, for accepting a tool tip (not shown) of an appropriately sized and shaped driver tool (also not shown). The bone screw  20  further includes an elongated threaded shaft or shank extending from the head  24 , wherein this shaft or shank has notches  28  cut into threads  30  at a distal end thereof (opposite the head  24 ) to allow the threads  30  to be either self-tapping or self-drilling. These features aid in the implantation of the bone screws  20  by reducing the number of extraneous instruments required to implant the device. In a preferred embodiment, the threads  30  of the bone screws  20  may be coated with an osteoconductive material  32  in order to aid in the fixation of the bone screws  20  to patient bone. Examples of such osteoconductive material  32  include calcium phosphate, hydroxyapatite, bone morphogenic proteins, and stem cells.  
         [0040]      FIG. 2  depicts the anterior or front side view of the embodiment shown in  FIG. 1 . The central opening  22  is better illustrated to depict visualization through the center of the bone plate  12 . The elongated struts  14  span directly between the bone plate apertures  18 , and thus also between the bone screws  20 , in order to transfer the loading from a first patient bone segment to an adjacent or second patient bone segment to which the bone screws  20  are respectively attached. In one preferred application, each bone segment would represent a single vertebral body. Each landing  16 , and corresponding bone plate apertures  18 , is placed adjacent to a first vertebral body, while the bone screws  20  penetrate into that vertebral body. This fixates that particular portion of the bone plate  12  to the vertebral body, or more generically, bone segment. The opposite landing  34  ( FIG. 2 ) and aperture set are therefore placed adjacent to a second patient bone segment or vertebral body for securement thereto by means of the bone screws  20 , Accordingly, the second bone segment is fixated or constrained relative to the first bone segment, by means of the assembly  10  of the present invention.  
         [0041]      FIG. 3  shows a side elevation view of the assembly or construct  10  discussed previously in  FIGS. 1 and 2 . In  FIG. 3 , the curvature of both the anterior or outboard face  36  as well as the posterior or inboard, or bone contacting, face  38  of the bone plate  12  is illustrated. Although it is not necessary for the bone plate  12  to have a curvature associated with it, in cervical spinal applications it can be beneficial. The cervical spine of the human body presents a generally lordotic curvature. To aid in positioning of the bone plate  12  to the cervical spine, it is often advantageous for the bone plate  12  to have a lordotic curvature as well. The anterior face  36  is contoured and rounded in such a manner as to reduce irritation of the espophagus and the surrounding tissues. In a further preferred embodiment, the posterior bone contacting face  38  may be made porous or roughened in natured to promote or encourage bone ingrowth into the bone plate  12 . Bone growth into the posterior face  38  of the bone plate  12  would aid in the fixation of the device to the host bone.  FIG. 3  also illustrates the low anterior-posterior profile of the bone plate  12 . The head  24  of each bone screw  20  is preferably recessed within a matingly shaped counterbore or countersink formed in the bone plate at the anterior or outboard side of each bone plate aperture  18  in order to maintain an overall low profile for the entire assembled device  10 .  
         [0042]      FIG. 4  depicts one alternative preferred embodiment of the invention, wherein a modified bone fixation plate assembly or device  410  is intended to span multiple bone segments (not shown) such as multiple vertebral bodies. The device  410  comprises a modified bone plate  412  and multiple bone screws  20 , wherein these bone screws  20  are of the same design as those shown and described with respect to  FIGS. 1-3 . The modified bone plate  412  comprises multiple pairs of elongated struts  414  for transferring load from one bone segment, or vertebral body, to the adjacent bone segment. Each set of struts  414  span between two landings  416 . These landings each have a pair of apertures  418  intended to house one bone screw  20  each. Due to the multiple strut sets  414  and landings  416 , the modified bone plate  412  has multiple central openings  422  which, like the device  10  depicted in  FIGS. 1-3 , aid in the placement and positioning of the bone plate  412  in the course of implantation.  
         [0043]      FIGS. 5-8  depict a preferred embodiment of present invention, namely, an assembly or construct  510  comprising a bone plate  512  and related set of bone screws  514 , wherein these bone screws  514  have a preferred dual thread form configuration for use in the bone screws  20  shown in  FIGS. 1-3 .  
         [0044]     In  FIGS. 5-7 , a portion of the bone plate  512  is removed in order to better display the relationship between each bone screw  514  and the associated bone plate aperture  524 . The bone screws  514  each have a head  516  with a substantially spherical or part-spherical underside surface geometry. This spherical geometry of the head  516  is somewhat larger than a spherical or part-spherical seat portion at the anterior or outboard side of the associated bone plate aperture  524 , and thus prevents the bone screw  514  from passing completely through the bone plate  512 . The mating of these two spherical or part-spherical surfaces allows each bone screw  514  to articulate within of the bone plate aperture  524  relative to the bone plate  512 , and thereby maintain the dynamic loading nature of the construct relative to patient bone to which the bone screws  514  are attached.  
         [0045]     Each bone screw  514  has two different thread sets or thread forms formed along its shaft or shank. A first thread form most distal from the head  516  of the bone screw  514  is the bone thread  520 , being shaped in a manner as to secure the bone screw  514  to the host patient bone. More proximal to the head  516  of the bone screw  514  is a second thread form comprising a lock element in the form of a locking thread  518 . This second locking thread  518  has the same or substantially the same pitch as that of the first bone thread  520 . However, the major diameter of the second locking thread  518  is somewhat larger than that of the first bone thread  520 . The difference in major diameters between these two thread forms  518 ,  520  allows the first bone thread  520  to pass relatively freely through the associated bone plate aperture  524  with substantially an axial sliding motion, and more specifically, to pass or slide freely through internal female threads  526  formed within the bone plate aperture  524 . The major diameter of the first bone thread  520  is smaller than the minor diameter of the bone plate aperture female threads  526 . This difference in diameters also aids in allowing the bone screw  514  to be inserted at various angles relative to the bone plate  512 , thereby affording the surgeon greater flexibility during implantation.  
         [0046]     The female threads  526  within the bone plate aperture  524  have same pitch as that of the second locking threads  518  on the bone screw  514 . In addition, the diameters of the second locking threads  518  and the aperture female threads  526  are similar, with the aperture threads  526  being slightly larger. Moreover, in the preferred form, the thread geometry of the second locking threads  518  differs from that of the aperture female threads  526 . Specifically, the geometry of the second locking threads  518  is of a generally trapezoidal or triangular nature, whereas the aperture female threads  526  are of a more rectangular or truncated conical form. These differing thread geometries, combined with the slight difference in diameters, allows the bone screw  514  to engage the bone plate  512  at differing degrees of angulation. This allows the surgeon greatly flexibility for bone screw placement. With the aperture female threads  526  and the second locking threads  518  being of the same pitch and similar diameter, the locking threads are able to engage and advance past the bone plate  512 . As the first bone threads  520  are of the same pitch as both the second locking threads  518  and aperture female threads  526 , as the bone screw  514  advances into the host bone, the bone screw  514  advances through the bone plate  512  at the same rate.  
         [0047]     The aperture female threads  526  are of short enough length as to allow the second locking threads  518  to pass completely through and beyond the female threads  526 , thereby disengaging therefrom at the posterior or inboard side thereof. Posterior to the aperture female threads  526  is a radially enlarged posterior-side cavity  528  into which the second locking threads  518  enter upon advancing beyond the female threads  526 . Once the locking threads  518  advance into this posterior-side cavity  528 , disengaged from the aperature female threads  526 , the bone screw  514  is granted a significantly greater freedom of motion relative to the bone plate  512 , being constrained by the mate of the part-spherical underside surface of the screw head  516  with the part-spherical seat at the anterior side of the aperture  524 , and limited by the walls of the cavity  528  and the major diameter of the locking threads  518 . At this point, the bone screw  514  is captured within the bone plate  512  since the locking thread  518  is unable to back out through aperture threads  526 , unless timed properly. This is due in part to the timing of the threads, but also to the thread form geometries. The trailing edge of the second locking threads  518  is of a different form than that of the aperture female threads  526  trailing edge, adding to the difficulty of screw removal. The natural back-out tendencies of a bone screw would preclude the bone screw  514  from disengaging from the bone plate  512 . However, upon need for a surgeon to remove the screw  514 , it can be threaded out of the bone plate. This can be achieved by holding the bone plate  512  against the bone while rotating the bone screw  514  counterclockwise. This will force the locking threads  518  to re-engage the aperture female threads  526 , thereby allowing the bone screw  514  to more backwards through the bone plate  512 . The simplicity of the screw removal technique can be advantageous during revision surgeries.  
         [0048]     The bone screws  514  in  FIGS. 5-7  have features to aid in their insertion and fixation to the patient bone. One such feature is a star-type driver  530  indention. This enables a large amount of torque to be applied to the bone screw  514  through the screw driver tool (not shown). Another feature of the bone screw  514  is that of the notched leading edge  522  which allows the bone screw  514  to be self-tapping, self-drilling, or both. This eliminates the need for extra surgical steps and tools, thereby adding efficiency to the entire procedure. Additionally, the threads  520  of the bone screws  514  may be coated with an osteoconductive material  532  in order to aid in the fixation of the bone screws  520  to patient bone. Examples of this osteoconductive material  532  are calcium phosphate, hydroxyapatite, bone morphogenic proteins and stem cells. In an alternate embodiment, the threads  520  of the bone screws may have a plurality of pores loaded or coated with such osteoconductive material coating  532  in order to aid in the fixation of the screws  514  to the bone. In yet another alternate embodiment, the threads  520  of the bone screws may have a plurality of pores that can be coated with bone cement such as poly methyl methacrylate cement or the like, in order to aid in the fixation of the screws  514  to osteoporotic bone.  
         [0049]      FIGS. 7-9  display views the bone plate  512  of the embodiment as described in  FIG. 5-6 .  FIG. 8  is an anterior perspective view of the bone plate  512  depicting the spherical recessed portion  524  of the bone plate aperture at the anterior or outboard side thereof, as well as the aperture female threads  526 . These features enable the bone plate  512  to retain the bone screws  514  while still allowing relative articulatory motion between the two.  FIG. 9  shows a top view of the plate  512 , depicting the curvature of the anterior face  534  and the posterior face  536 . The posterior face  536  of the bone plate  512  is slightly concave, allowing to better mate with the host patient bone. Since the cervical vertebrae are cylindrical in nature, the concavity of the posterior face  536  lets the plate  512  wrap around the vertebrae. In a further preferred embodiment, this posterior or inboard side bone contacting face  536  may be made porous or roughened in natured, or otherwise coated with a porous bone ingrowth material, to encourage bone ingrowth into the bone plate  512 . Bone growth into the posterior face  536  of the implant  510  would aid in the fixation of the device to the host bone. The curvature of the anterior face  534  is a combination of both convex and concave curves. The lateral aspects of the face  534  are convex, conforming generally to the concave nature of the posterior face  536 . This convex anterior curvature has a similar effect, allowing the plate to wrap around the bone, and reducing the risk of irritating the surrounding tissue structures. The medial portion of the anterior face  534  has a concave curvature located between the two laterally opposed bone plate apertures  524 . This reduces the profile of the plate  512  along the midline, which is where the esophagus lies adjacent. This greatly reduces the risk of esophageal irritation related to the plate.  
         [0050]     A further embodiment  1010  is depicted in  FIG. 10 , showing the same bone plate  512  from  FIGS. 5-9 , but with a modified bone screw  1014 . The modified bone screw  1014  includes a second locking thread or thread form  1018  which has a similar thread geometry (including minor and major diameters) with respect to the aperture female threads  526  of the bone plate  512 . This geometry restricts the angulation of the bone screw  1014  relative to the bone plate  512  during insertion. Additionally, since the major diameter of the aperture female threads  526  is similar or the same as the internal diameter of the posterior-side or inboard-side aperture cavity  528 , the second locking threads  1018  are thereby constrained within the cavity  528  to limit or restrict articulation between the screw head  1016 , and the bone plate aperture  524 , thereby creating a constrained plate fixation system or assembly. Since this system  1010  utilizes the same bone plate  512  as the semi-constrained system  510  ( FIGS. 5-9 ), a hybrid system can be constructed with the bone plate  512  accepting both constrained  1014  bone screws ( FIG. 10 ) and semiconstrained  514  screws ( FIGS. 5-7 ), depending upon surgeon preference and patient need.  
         [0051]      FIGS. 11-12  depict still another preferred embodiment of the present invention. This alternative assembly or construct  1110  comprises a bone plate  1112  and a set of self-locking bone screws  1114 . Each bone screw  1114  has a head  1130 , flexible locking tangs  1116  carried by an elongated screw shaft or shank at the underside or posterior side of the head  130 , and thread features  1120  formed on the elongated screw shaft or shank. The bone plate  1112  comprises a general body with a series of apertures  1118  formed therein. The threaded portion  1120  of each bone screw  1114  is sized to pass relatively freely and completely through the associated bone plate aperture  1118  to the inboard side thereof. The flexible locking tangs  1116  are also sized to be able to pass through a spherical or part-spherical seat  1128  formed at an anterior or outboard side of the bone plate aperture  1118 , and further through a narrow or neck portion  1132  of the bone plate aperture  1118 . Importantly, in order for the locking tangs  1116  to pass these features, the tangs  1116  must flex radially inwardly toward the screw shaft or shank, and also axially toward the screw head  1130 , thereby reducing their effective outer diameter. Once the locking tangs  1116  are displaced to a position axially beyond or to the inboard side of the narrow or neck portion  1132  of the aperture  1118 , the tangs  1116  resiliently or springably return substantially to their original non-deformed position assuming a diametric size greater than the neck portion  1132 . In this original position, the locking tangs  1116  are thus unable to return back through the narrow or neck portion  1132  of the bone plate aperture  1118 . This therefore locks the bone screw  1114  to the bone plate  1112 , not allowing the bone screw  1114  to back out or dislodge. The spherical or part-spherical underside surface of the bone screw head  1130  is sufficiently larger than the spherical anterior-side or outboard-side seat  1128  of the bone plate aperture  1118 , thereby preventing the bone screw  1114  from advancing past and through the bone plate  1112 . The mating of these two spherical surfaces allows the bone screw  1114  to articulate within the bone plate  1118 . Additionally, posterior to or inboard of the narrowed or neck portion  1132  of the aperture  1118 , a larger diameter posterior-side cavity  1134  is formed, enabling the bone screw  1114  to have a greater freedom of articulation relative to the bone plate  1112 .  
         [0052]     The bone screws  1114  in  FIGS. 11-12  have features to aid in their insertion and fixation to the bone. One such feature is a star-type driver  1136  indention. This enables a large amount of torque to be applied to the bone screw  1114  through the screw driver tool. Another feature of the bone screw  1114  is that of the notched leading edge  1122  which enables the bone screw  1114  to be self-tapping, self-drilling, or both. This eliminates the need for extra surgical steps and tools, thereby adding efficiency to the entire procedure. Additionally, the threads  1120  of the bone screws may be coated with an osteoconductive material  1124  in order to aid in the fixation of the bone screws  1114  to patient bone. Examples of this osteoconductive material  1124  are calcium phosphate, hydroxyapatite, bone morphogenic proteins and stem cells. In an alternate embodiment, the threads  1120  of the bone screws may have a plurality of pores loaded or coated with such osteoconductive material in order to aid in the fixation of the screws  1114  to patient bone. In yet another alternate embodiment, the threads  1120  of the bone screws may have a plurality of pores that can be coated with bone cement such as poly methyl methacrylate cement or the like, in order to aid in the fixation of the screws  1114  to osteoporotic bone.  
         [0053]     The devices presented in  FIGS. 1-12  are intended to be manufactured from a variety of materials. One such preferred material is that of a high strength ceramic or high strength polymer. These materials offer the benefit of radiolucency and MRI compatibility, features to aid in the evaluation of new bone growth around the implant. Another preferred material of construction is a biocompatible metal. While not being radiolucent or MRI compatible, metals offer advantages such as strength and ductility. In this regard, persons skilled in the art will appreciate that the flexible tangs  1116  as shown and described in  FIGS. 11-12  will be constructed from a suitable and typically non-ceramic material having the desired flex characteristics.  
         [0054]     The invention thus provides a substantial improvement in addressing clinical problems indicated for medical treatment of degenerative disc disease, cervical pain and traumatic injury.  
         [0055]     The bone plate and self-locking bone screws of the present invention provide at least the following benefits over the prior art:  
         [0056]     [a]a simple method of securing the bone screws to the bone plate with no tertiary components or technique steps;  
         [0057]     [b] a low profile, dynamic bone plate construct with self-retaining screws;  
         [0058]     [c] an easily revisable bone plate;  
         [0059]     [d] a radiolucent and MRI compatible cervical bone plate construct,  
         [0060]     [e] bone screws with osteoconductive or osteoinductive coatings;  
         [0061]     [f] a bone plate with osteoconductive or osteoinductive properties;  
         [0062]     [g] sterilizable; and  
         [0063]     [h] low manufacturing cost.