Patent Publication Number: US-2023157738-A1

Title: Fixation assemblies for medical implants

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional No. 63/281,760 filed on Nov. 22, 2021 and entitled “Fixation Assemblies for Medical Implants,” the contents of which are incorporated in their entirety by reference. 
    
    
     FIELD 
     The systems, methods and devices described herein generally relate to medical implants and fixation assemblies for medical implants, and more specifically, to screw and washer assemblies for orthopedic implants that are attached to bone, such as spinal implants that facilitate the fusion of bone segments. 
     BACKGROUND 
     Medical implants have been used in the field of spine, orthopedics and dentistry for over a century, including trauma, fracture repair, reconstructive surgery and repairing or replacing damaged bone. These implants are typically used to replace a missing biological structure, support a damaged biological structure and/or to enhance an existing biological structure. 
     One example of such medical implants are spinal implants. The integrity of the spine, including its subcomponents like the vertebral bodies and intervertebral discs that are well known structural body parts forming the spine, are key to a patient&#39;s health. These parts may become crushed or damaged as a result of trauma or injury, or damaged by disease (e.g., by tumor, auto-immune disease) or as a result of wear over time or degeneration caused by the normal aging process. 
     In many instances, one or more damaged structural body parts can be repaired or replaced with a prosthesis or implant. For example, specific to the spine, one method of repair is to remove the damaged vertebra (in whole or in part) and/or the damaged disc (in whole or in part) and replace it with an implant or prosthesis. In some cases, it is necessary to stabilize a weakened or damaged spinal region by reducing or inhibiting mobility in the area to avoid further progression of the damage and/or to reduce or alleviate pain caused by the damage or injury. In other cases, it is desirable to join together the damaged vertebrae and/or induce healing of the vertebrae. Accordingly, an implant or prosthesis may be configured to facilitate fusion between two adjacent vertebrae. The implant or prosthesis may be placed without attachment means or fastened in position between adjacent structural body parts (e.g., adjacent vertebral bodies). 
     Typically, an implant or prosthesis is secured directly to a bone structure by mechanical or biological means. One manner of spine repair involves attaching a fusion implant or prosthesis to adjacent vertebral bodies using a fixation element, such as a screw. Most implants and their attachment means are configured to provide an immediate, rigid fixation of the implant to the implantation site. 
     Unfortunately, in some cases, after implantation the implants tend to subside, or settle, into the surrounding environment as the patient&#39;s weight is exerted upon the implant. In some cases, this subsidence may cause the rigidly fixed attachment means to either loosen, dislodge or potentially damage one or more of the vertebral bodies. In addition, compressive forces on the bone screws may cause the screws to move or toggle relative to the implant. If the screws move or toggle too much or for too long a period of time, this undesired movement can cause the screws to fracture, potentially causing pieces of the screw to break off within the patient and/or causing the implant to dislodge from the bone. 
     Furthermore, after insertion into the vertebral body, the fixation element or bone screw has a tendency to work itself loose and/or completely “back out” of the underlying bone. This problem is particularly relevant in areas of high stress, such as the spine. The consequence of fixation elements backing out or loosening includes improper or incomplete fusion, loss of stability, potential risk to the patient, and a separate costly and often painful revision surgery. 
     It is therefore desirable to provide an implant and associated fixation elements that can account for subsidence and/or compression forces that occurs to implants subsequent to implantation to inhibit the screws from toggling relative to the implant. In addition it is desirable to provide fixation elements that avoid the problem of screw loosening or backing out over time and with use. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of some aspects of the present systems, methods and devices. This summary is not an extensive overview of the claimed subject matter. It is intended to neither identify key or critical elements of the claimed subject matter nor delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts of the present systems, methods and devices in a simplified form as a prelude to the more detailed description that is presented later. 
     Systems, methods and devices are provided for implanting medical devices into patients. The systems and devices may include fixation elements, such as screw and washer assemblies, and implants that include such fixation elements. The systems, devices and methods are particularly useful for orthopedic implants that are attached to bone, such as spinal implants that facilitate fusion of bone segments. Although the following discussion focuses on spinal implants or prostheses, it will be appreciated that many of the principles may equally be applied to other structural body parts within a human or animal body. 
     The fixation assemblies described herein generally comprise a screw having a head and a shaft, and a washer having at least one outer surface with one or more frictional elements for engaging a surface of the implant. The shaft includes one or more projections that cooperate with the washer to prevent the washer from sliding back down the screw, while allowing for easy application of the washer to ensure optimal placement and desired contact with the screw head. The washer and/or the screw head may also include interlocking cams that create a wedge effect that inhibits the screw from backing out or loosening from a hole in bone or other tissue. 
     In one aspect, a fixation assembly for an implant comprises a screw having a head, a shaft with a threaded portion and a shank portion between the threaded portion and the head. The shaft comprises one or more projections extending from the shank portion. The fixation assembly further includes a washer having at least one outer surface with one or more frictional elements for engaging a surface of the implant. The washer defines upper and lower surfaces and a central opening. The central opening of the washer has a cross-sectional area at the upper surface that is smaller than the combined cross-sectional area of the projections and the shank, while the central opening at the lower surface has a cross-sectional area that is greater than the combined cross-sectional area of the projections and the shank. This allows the operator to manipulate the washer to advance it proximally over the projections to the screw head, while preventing the washer from sliding back down the screw. 
     In one embodiment, the opening at the upper surface of the washer has a diameter and the opening at the lower surface has at least one dimension greater than the diameter. This dimension may, for example, be a distance from one side of the opening to the other side of the opening. 
     In certain embodiments, the internal perimeter of the washer opening is substantially circular at the upper surface, whereas the internal perimeter of the washer opening is non-circular at the lower surface. The lower surface may have a first portion with a first width and a second portion with a second width smaller than the first width. 
     The projections on the shank may comprise extrusions, bumps, threads, or any other type of projection that extends from the shank. In some embodiments, the projections are frictional elements, such as surface roughening and the like. In an exemplary embodiment, the projections comprise extrusions that have been formed into a spiral formation around shank. 
     In one embodiment, the screw head includes a plurality of cams or other locking elements disposed on a lower surface facing the shaft. The washer includes a plurality of cams or other locking elements configured to interlock with the cams of the screw. The cams may have an angle of inclination that is greater than the pitch angle of the threaded portion of the shaft. This creates a wedge effect that inhibits the screw from backing out or loosening from a hole in bone or other tissue. The interlocked cams make it extremely difficult for dynamic loads of physiological forces to release the cams, which ensures that the washer remains fixed to the implant and the screw head after the implant has been secured within a patient. 
     The angle of inclination of the cams will depend on the pitch angle of the threads on the screw shaft. In certain applications, this cam inclination angle may be greater than 5 degrees. This angle may be greater than 10 degrees. The cam inclination angle may be about 11 degrees. 
     The threaded portion of the shaft may include an upper part near the head and a lower part near the tip. The upper and lower parts may have a different thread pitch. In certain embodiments, the upper part has a lower thread pitch than the threads of the lower part. The higher thread pitch towards the tip of the screw allows for greater purchase into, for example, cancellous bone, while the lower thread pitch towards the head of the screw increases the holding power of the screw. In addition, this lower thread pitch provides a smaller pitch angle that works in tandem with the larger cam inclination angle of the interlocked cams. 
     The upper surface of the washer may have an outer dimension substantially equal to, or greater than, the outer dimension of the screw head. This increases the surface area of contact between the washer and the implant to distribute the load against the screw over a larger area, which increases the fixation between the screw and bone or other tissue in the patient. 
     The screw head may have any shape suitable for the intended procedure, such as cylindrical, conical, square, rectangular and the like. The screw head may include a mating feature for receiving a driving instrument to advance the screw into bone or other tissue. In some embodiments, the mating feature will be configured to resist stripping and may be star-shaped (e.g., torx) or the like. 
     The screw may be cannulated or non-cannulated, self-tapping and self-drilling and may be of a bone-screw type, such as those well known to skilled artisans. In some embodiments, the screw may include cutting notches near, or at, the distal tip, or cement holes along the shaft, to facilitate self-tapping. The implant may, for example, comprise an orthopedic implant, such as a spinal implant configured to facilitate fusion of bone segments. 
     In another aspect, a medical implant assembly comprises an implantable body having an upper surface, a lower surface and one or more through holes. The assembly further includes one or more bone screws, each having a head and a shaft with a threaded portion configured to pass through the through holes to attach the implantable body to bone. The shaft comprises one or more projections extending from a shank portion between the head and shaft. The assembly further includes a washer having at least one outer surface with one or more frictional elements for engaging a surface of the implant. The washer defines upper and lower surfaces and a central opening. The opening at the upper surface has a cross-sectional area smaller than the combined cross-sectional area of the projections and the shank and the opening at the lower surface has a cross-sectional area greater than the combined cross-sectional area of the projections and the shank. This prevents the washer from sliding back down the screw and allows for the easy application or removal of the washer while ensuring optimal placement and desired contact with the screw head. 
     The implantable body may be configured for insertion between vertebral bodies of a patient&#39;s spine. The spinal implant may have an upper surface, a lower surface, an anterior portion, a posterior portion and one or more apertures within the posterior portion for receiving at least one bone screw. 
     In another aspect, a method of securing a fixation assembly to an implantable body comprises providing a screw having a head, a shaft with a threaded portion, and a shank between the head and the shaft having one or more projections extending therefrom. A washer is advanced over the shaft of the screw to the shank portion. The washer is then toggled, shifted, rotated, adjusted or otherwise manipulated to allow the operator to advance the washer distally past the projections to the screw head. The washer may then be manipulated such that it will not slip proximally past the projections proximally. 
     In certain embodiments, the washer is manipulated by shifting, adjusting or rotating it relative to the shaft. In one such embodiment, the washer is rotated around the longitudinal axis of the shaft. In another embodiment, the washer is shifted, toggled and/or rotated around an axis substantially perpendicular to the shaft (e.g. by moving one side of the washer proximally while maintaining the position of the other side of the washer). In yet another embodiment, the washer is adjusted, manipulated and/or rotated about multiple axes. 
     In certain embodiments, the washer comprises an upper surface with a plurality of cams disposed thereon. The method further comprises aligning the cams of the washer with cams of the screw head and rotating the washer relative to the screw head to interlock the cams of the washer with the cams of the screw head. 
     The method may further comprise aligning visual indicators on the cams of the washer and the screw head with each other to ensure that the appropriate patterned threads are engaged to lock the washer to the bone screw. The visual indicators may comprise markings, colors, letters, surface features or the like. 
     The washer may be rotated until one or more frictional elements on an outer surface of the washer engage a lower surface of the head facing the shaft. The frictional elements provide additional frictional resistance between the washer and the screw head. 
     The method may further comprise inserting the threaded portion of the shaft through a through hole in the implant and driving the screw into bone to secure the implant to the bone. The shaft may be rotated until one or more frictional elements on a lower surface of the washer contact the implant. 
     The implant may, for example, comprise a spinal implant, such as an interbody fusion device. The method may further comprise inserting the implantable body between vertebral bodies of a patient&#39;s spine and driving at least the threaded portion of the screw through the through hole to attach the implantable body to the vertebral bodies. 
     In another aspect, a fixation assembly for an implant comprises a screw having a head and a shaft with a threaded portion, and a washer having an inner surface with internal threads and at least one outer surface with one or more frictional elements, such as serrations or other surface features, for contacting a surface of the implant. The shaft comprises external threads on a shank portion between the threaded portion and the head. The external threads mate with the internal threads of the washer to secure the washer to the screw. This prevents the washer from sliding back down the screw and allows for the easy application or removal of the washer while ensuring optimal placement and desired contact with the screw head. 
     In one embodiment, the screw head includes a plurality of cams or other locking elements disposed on a lower surface facing the shaft. The washer includes a plurality of cams or other locking elements configured to interlock with the cams of the screw. The cams may have an angle of inclination that is greater than the pitch angle of the threaded portion of the shaft. This creates a wedge effect that inhibits the screw from backing out or loosening from a hole in bone or other tissue. The interlocked cams make it extremely difficult for dynamic loads of physiological forces to release the cams, which ensures that the washer remains fixed to the implant and the screw head after the implant has been secured within a patient. 
     The angle of inclination of the cams will depend on the pitch angle of the threads on the screw shaft. In certain applications, this cam inclination angle may be greater than 5 degrees. This angle may be greater than 10 degrees. The cam inclination angle may be about 11 degrees. 
     In another embodiment, the washer comprises first and second washer portions configured to align with each other. The washer portions each have an inner surface with a plurality of cams disposed thereon. The cams of the first washer portion are configured to interlock with the cams of the second washer portion. The first and second washer cams have an angle of inclination that is greater than the pitch angle of the threaded portion of the shaft. 
     The first or upper washer portions may comprise one or more frictional elements, such as serrations or other surface features, on an outer surface. The serrations are positioned to contact and engage a lower surface of the screw head after the washer has been threaded onto the shaft. The serrations provide resistance to movement between the washer and the screw head. 
     The threaded portion of the shaft may include an upper part near the head and a lower part near the tip. The upper and lower parts may have a different thread pitch. In certain embodiments, the upper part has a lower thread pitch than the threads of the lower part. The higher thread pitch towards the tip of the screw allows for greater purchase into, for example, cancellous bone, while the lower thread pitch towards the head of the screw increases the holding power of the screw. In addition, this lower thread pitch provides a smaller pitch angle that works in tandem with the larger cam inclination angle of the interlocked cams. 
     The washer may have an outer dimension substantially equal to, or greater than, the outer dimension of the screw head. This increases the surface area of contact between the washer and the implant to distribute the load against the screw over a larger area, which increases the fixation between the screw and bone or other tissue in the patient. 
     The screw head may have any shape suitable for the intended procedure, such as cylindrical, conical, square, rectangular and the like. The screw head may include a mating feature for receiving a driving instrument to advance the screw into bone or other tissue. In some embodiments, the mating feature will be configured to resist stripping and may be star-shaped (e.g., torx) or the like. 
     The screw may be self-tapping and self-drilling and may be of a bone-screw type, such as those well known to skilled artisans. The implant may, for example, comprise an orthopedic implant, such as a spinal implant configured to facilitate fusion of bone segments. 
     In another aspect, a medical implant assembly comprises an implantable body having an upper surface, a lower surface and one or more through holes. The assembly further includes one or more bone screws, each having a head and a shaft with a threaded portion configured to pass through the through holes to attach the implantable body to bone. The assembly further includes a washer having an inner surface with internal threads and at least one outer surface with one or more frictional elements for engaging a surface of the implantable body. The screw shaft comprises external threads on a shank portion between the threaded portion and the head. The external threads are configured to mate with the internal threads of the washer to secure the washer to the screw. 
     In one embodiment, the screw head includes a plurality of cams or other locking elements disposed on a lower surface facing the shaft. The washer includes a plurality of cams or other locking elements configured to interlock with the cams of the screw. The cams may have an angle of inclination that is greater than the pitch angle of the threaded portion of the shaft. This creates a wedge effect that inhibits the screw from backing out or loosening from a hole in bone or other tissue. The interlocked cams make it extremely difficult for dynamic loads of physiological forces to release the cams, which ensures that the washer remains fixed to the implant and the screw head after the implant has been secured within a patient. 
     The angle of inclination of the cams will depend on the pitch angle of the threads on the screw shaft. This cam inclination angle may be greater than 5 degrees. This angle may be greater than 10 degrees. The cam inclination angle may be about 11 degrees. 
     In another embodiment, the washer comprises first and second washer portions configured to align with each other. The washer portions each have an inner surface with a plurality of cams disposed thereon. The cams of the first washer portion are configured to interlock with the cams of the second washer portion. The first and second washer cams have an angle of inclination that is greater than the pitch angle of the threaded portion of the shaft. 
     The first or upper washer portion may comprise one or more frictional elements, such as serrations, on an outer surface. The serrations are positioned to contact and engage a lower surface of the screw head after the washer has been threaded onto the shaft. The serrations provide resistance to movement between the washer and the screw head. 
     The threaded portion of the shaft may include an upper part near the head and a lower part near the tip. The upper and lower parts may have a different thread pitch. In certain embodiments, the upper part has a lower thread pitch than the threads of the lower part. The higher thread pitch towards the tip of the screw allows for greater purchase into, for example, cancellous bone, while the lower thread pitch towards the head of the screw increases the holding power of the screw. In addition, this lower thread pitch provides a smaller pitch angle that works in tandem with the larger cam inclination angle of the interlocked cams. 
     The implantable body may be configured for insertion between vertebral bodies of a patient&#39;s spine. The spinal implant may have an upper surface, a lower surface, an anterior portion, a posterior portion and one or more apertures within the posterior portion for receiving at least one bone screw. 
     In another aspect, a method of securing a fixation assembly to an implantable body comprises providing a screw having a head and a shaft with a threaded portion, and a washer having an inner surface with internal threads. The washer is advanced over the shaft of the screw to a shank portion of the screw between the threaded portion and the head. The internal threads of the washer are aligned with external threads on the shank portion of the screw shaft. The washer is rotated relative to the screw such that the internal threads of the washer engage the external threads of the screw shaft and translate the washer towards the head of the screw. 
     In certain embodiments, the washer comprises first and second washer portions each having an inner surface with a plurality of cams disposed thereon. The method further comprises aligning the cams of the first washer portion with the cams of the second washer portion and rotating the first washer relative to the second washer to interlock the cams of the first washer with the cams of the second washer. 
     The method may further comprise aligning visual indicators on the cams of the first and second washer portions with each other to ensure that the appropriate patterned threads are engaged to lock the washer to the bone screw. The visual indicators may comprise markings, colors, letters, surface features or the like. 
     The washer may be rotated until one or more frictional elements on an outer surface of the washer engage a lower surface of the head facing the shaft. The frictional elements provide additional frictional resistance between the washer and the screw head. 
     The method may further comprise inserting the threaded portion of the shaft through a through hole in the implant and driving the screw into bone to secure the implant to the bone. The shaft may be rotated until one or more frictional elements on a lower surface of the washer contact the implant. 
     The implant may, for example, comprise a spinal implant, such as an interbody fusion device. The method may further comprise inserting the implantable body between vertebral bodies of a patient&#39;s spine and driving at least the threaded portion of the screw through the through hole to attach the implantable body to the vertebral bodies. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. Additional features of the present systems, methods and devices will be set forth in part in the description which follows or may be learned by practice of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the present systems, methods and devices, and together with the description serve to explain the principles of the disclosure. 
         FIG.  1    is a side view of a fixation assembly; 
         FIG.  2    is a side view of a screw of the fixation assembly of  FIG.  1   ; 
         FIG.  3    is a close-up view of an upper portion of the screw of  FIG.  2   ; 
         FIG.  4    is a top view of the screw of  FIG.  2   ; 
         FIG.  5    is a cross-sectional view of the screw of  FIG.  2   ; 
         FIG.  6    is a perspective view of a washer of the fixation assembly of  FIG.  1   ; 
         FIG.  7    is a side view of another embodiment of a fixation assembly; 
         FIG.  8    is a side view of a screw of the fixation assembly of  FIG.  7   ; 
         FIG.  9    is a close-up view of an upper portion of the screw of  FIG.  8   ; 
         FIG.  10    is a cross-sectional view of the upper portion of the screw shown in  FIG.  8   ; 
         FIG.  11    is a perspective view of a washer of the fixation assembly of  FIG.  7   ; 
         FIG.  12    is a side view of the washer of  FIG.  11   ; 
         FIG.  13    is a cross-sectional view of the washer of  FIG.  11   ; 
         FIG.  14    is a close-up cross-sectional view of the washer of  FIG.  11   : 
         FIG.  15    is a side view of another embodiment of a screw for a fixation assembly; 
         FIGS.  16 A- 16 C  illustrate a method of assembling a fixation assembly to a medical implant; 
         FIG.  17    is side view of another embodiment of a fixation assembly; 
         FIG.  18    is an exploded view of the fixation assembly of  FIG.  17   ; 
         FIG.  19    is a close-up view of a screw head of the fixation assembly of  FIG.  17   ; 
         FIG.  20    is a top view of the screw head of  FIG.  19   ; 
         FIG.  21    is a cross-sectional view of a screw of the fixation assembly of  FIG.  17   ; 
         FIGS.  22 A- 22 C  are perspective views of a washer of the fixation assembly of  FIG.  17   ; 
         FIGS.  23 A- 23 D  are further perspective views of a washer of the fixation assembly of  FIG.  17   ; 
         FIG.  24 A  is a bottom view of the washer; 
         FIG.  24 B  is a top view of the washer; and 
         FIGS.  25 A- 25 D  are perspective views of a spinal implant with the fixation assembly of  FIG.  17   . 
     
    
    
     DETAILED DESCRIPTION 
     Systems, devices and methods are provided for implanting medical devices into patients. The systems and devices may include fixation elements, such as screw and washer assemblies, and implants that include such fixation elements. The systems, devices and methods are particularly useful for orthopedic implants that are attached to bone, such as spinal implants that facilitate fusion of bone segments. 
     While the following disclosure is presented with respect to spinal implants for fusion procedures, it should be understood that the features of the presently described devices may be readily adapted for use in any type of medical implant that must be attached to bone, such as an orthopedic implant, a dental implant, an artificial spinal disc, a total or partial joint replacement or repair device, trauma repair device, bone fracture repair device, reconstructive surgical device, alveolar ridge reconstruction device, veterinary implant or the like. 
     Referring now to  FIGS.  1 - 6   , a fixation assembly  10  comprises a washer  20  coupled to a fixation screw  30 . Fixation screw  30  comprises a head  32  and a shaft  34  with a distal tip  36 . Shaft  34  includes a threaded portion  38  and a shank portion  40  between threaded portion  38  and head  32  (see  FIG.  2   ). Fixation screw  30  may be self-tapping and self-drilling and may be of a bone-screw type, such as those well known to skilled artisans. As shown in  FIG.  3   , head  32  may further include a plurality of cams  70  on a lower surface  62  of head  32  that are configured to interlock with corresponding cams  80  on washer  20  (see  FIG.  6   ). 
     Fixation assembly  10  and its components may be formed of any suitable medical grade material, such as biocompatible metals like stainless steel, titanium, titanium alloys, etc. or a medical grade plastic such as polyetheretherketone (PEEK) or another radiolucent material, ultra high molecular weight polyethylene (UHMWPE), etc. If so desired, the fixation assembly  10  may also be formed of a bioresorbable material. The bioresorbable material may preferably be osteoconductive or osteoinductive (or both). 
     Referring again to  FIG.  2   , threaded portion  38  may include an upper portion  42  near head  32  and a lower portion  44  near tip  36 . Upper threaded portion  42  preferably comprises about 20% to about 75% of the total length of threaded portion  38 , preferably about 25% to about 50%. The threads in upper and lower portions  42 ,  44  preferably extend outward about the same distance from shaft  34 , although their outer dimensions may differ depending on the application. In one embodiment, the threads extend outward from shaft  34  by about 0.5 mm to 1.5 mm, or about 0.75 mm to about 0.85 mm (see  FIG.  5   ). 
     Upper portion  42  preferably has a lower thread pitch (i.e., the distance between threads) than lower portion  44 . The ratio of lower thread pitch to higher thread pitch may be about 1:2 to about 1:5, preferably about 1:3 to 1:4 (e.g., about 1:3.5). In certain embodiments, the distance between threads in upper portion  42  is about 0.5 mm to about 1.0 mm, preferably about 0.8 mm and the distance between threads in lower portion  44  is about 2.0 mm to about 4.0 mm, preferably about 2.5 mm to about 3.0 mm (e.g., 2.75 mm). The higher thread pitch towards tip  36  of the screw allows for a greater purchase into cancellous bone, while the lower thread pitch towards head  32  increases the holding power of the screw. In addition, the lower thread pitch of upper portion  42  provides for a smaller wedge angle for the threads (i.e., the angle that the thread make with a horizontal plane passing perpendicular to the longitudinal axis of the screw). This provides additional advantages in tandem with the larger cam angle created by the interlocking cams  70 ,  80  of washer  20  and screw head  32  (discussed below). 
     Of course, the devices disclosed herein are not limited to the thread pitch configuration described above. For example threaded portion  38  may also have a single, substantially uniform thread pitch throughout its length. Alternatively, threaded portion  38  may have more than two different thread pitches. In some embodiments, the thread pitches of upper portion  42  and lower portion  44  may be reversed such that upper portion  42  has a higher thread pitch than lower portion  44 . 
     Referring now to  FIGS.  3  and  5   , shank portion  40  extends from the upper part of threaded portion  42  and head  32  of fixation screw  20 . Shank portion  40  includes external threads  60  that extend adjacent to, or near, a lower surface  62  of head  30 . Threads  60  are configured to mate with internal threads  75  of washer  20  (shown in  FIG.  6    and discussed below). Threads  60  preferably have a lower thread pitch than upper threaded portion  42 . The ratio of thread pitch between threads  60  and the threads of upper portion may be about 1:2 to about 1:10, preferably about 1:3 to about 1:5 (e.g., 1:4). In certain embodiments, the distance between threads  60  is about 0.1 mm to about 0.4 mm, preferably between about 0.15 mm to about 0.25 mm (e.g., 0.2 mm). 
     In other embodiments, external threads  60  has substantially the same thread pitch as threaded portion  38  of screw shaft  32 . Alternatively, external threads  60  may have a higher thread pitch. 
     Referring now to  FIG.  6   , washer  20  may have a substantially annular shape with an inner diameter sized to fit over shaft  34  and fit against head  32  to minimize loosening of screw  30 . In certain embodiments, washer  20  may have an enlarged outer diameter, thereby increasing the surface area of contact to distribute the load from screw  30  over a larger area. In an exemplary embodiment, washer  20  has an outer diameter substantially equal to the outer diameter of head  32 . In other embodiments, washer  20  have may have a larger outer diameter than head  32 . 
     Washer  20  includes an inner surface having internal threads  75 , an outer surface  74  and upper and lower surfaces  76 ,  78 . In some embodiments, upper surface  76  is an anterior surface and lower surface  78  is a posterior surface. Washer  20  may have a sufficient thickness from upper surface  76  to lower surface  78  that optimizes strength and resistance to deformation, preferably about 1.5 to about 2.0 mm, more preferably about 1.6 mm (see, for example,  FIG.  13   ). 
     Lower surface  78  includes frictional elements, such as serrations  82 , to provide additional resistance between washer  20  and a surface of an implant (discussed below). Alternatively, frictional elements may include dimples, pits, depressions, projections, or other surface features, such as a roughened lower surface. 
     Upper surface  76  includes a plurality of cams  80  configured to interlock with cams  70  on head  32  of screw  30 . Cams  80  have an inclination angle greater than the pitch of the threads of upper threaded portion  42 . The inclination angle is defined as the angle that the cams  80  make with a horizontal plane extending through washer  20  that is perpendicular to the longitudinal axis of screw  30  (see for example,  FIG.  12   ). The “thread pitch” also refers to the angle each thread makes with the horizontal plane extending through shaft  30  perpendicular to the longitudinal axis of screw  30 . 
     Providing an inclination angle of the interlocked cams  70 ,  80  that is greater than the wedge or pitch angle of the threads of threaded portion  34  creates a wedge effect to prevent screw  30  from backing out or loosening from the implant, making it nearly impossible for the dynamic load of physiological forces to release the interlocked cams. Only a driver turning the bone screw counterclockwise with a specific torque parameter provides enough force to disengage the cams  70 ,  80  from each other. This results in a significant reduction or complete elimination of the screw  30  backing out from the implant. 
     The specific inclination angle of cams  80  will depend on the application and particularly on the pitch angle of threaded portion  34 . In certain embodiments, the inclination angle of cams  80  is greater than 5 degrees, preferably greater than 10 degrees. In an exemplary embodiment, the cam inclination angle is about 11 degrees. 
     Of course, it should be recognized that fixation assembly  10  is not limited to the cams  70 ,  80  described above and shown in  FIGS.  1 - 6   . For example, cams  70 ,  80  may comprise other types of interlocking members, such as interference fitting members, or other frictional elements that lock or otherwise make it difficult to pull washer  20  apart from screw head  32 . In addition, although cams  70 ,  80  extend around the entire circumference of washer  20  and head  32  the devices disclosed herein are not limited to this configuration. Cams  70 ,  80  may have a semicircular shape (i.e., only extending around a portion of the circumference of washer  20  and head  32 ), or cams  70 ,  80  may be formed at discrete locations around the circumference of washer  20  and head  32  (e.g. two cams formed on opposite sides, four cams spaced from each other and formed around washer  20 , and head, etc.). 
     Internal threads  75  are configured to engage exterior threads  60  of the shank portion  40  of screw  30 . Once washer  20  has been moved over shaft  30  and onto shank portion  40 , rotation of washer  20  will causes threads  60 ,  75  to engage each other and displace washer  20  towards head  32  of screw  30 . Threads  60 ,  75  are positioned such that washer  20  can be tightened against lower surface  62  of screw head  32 , thereby allowing cams  70 ,  80  to interlock with each other. This reduces toggle of the screw that may result from compressive forces, providing additional rigidity to minimize breakage or fracture of the screw and washer assembly. Internal threads  70  may, for example, be threaded with a metric die, while external threads  60  may, for example, be formed with a metric tap. 
     As shown in  FIG.  4   , screw head  32  includes a mating feature  50  for receiving an instrument designed to tighten screw  30  against a medical implant (discussed below). Mating feature is preferably configured to resist stripping from the use of force to allow the operator to provide more torque to tighten the screw. This feature may be particularly advantageous given that the presence of washer  20  requires significant tension to secure screw  20  to the implant. In certain embodiments, mating feature  50  comprises a shape that orients the flats more perpendicular to the axis of the applied force, such as a star-shaped or torx configuration. 
     Referring now to  FIGS.  7 - 14   , another embodiment of a fixation assembly  100  will now be described. Similar to the previous embodiment, assembly  100  comprises a washer  120  coupled to a fixation screw  130 . Fixation screw  130  comprises a head  132  and a shaft  134  with a distal tip  136 . In this embodiment, head  132  includes a substantially smooth lower surface  162  facing shaft  134 , although it is recognized that head  132  may have a variety of different shapes and configurations, such as the substantially conical shape shown in  FIG.  15   . Alternatively, lower surface  162  may include serrations or other frictional elements that provides resistance to washer  120  (discussed below). 
     Shaft  134  includes a threaded portion  138  and a shank portion  140  between threaded portion  138  and head  132 . In certain embodiments, threaded portion  138  may include an upper portion  142  near head  132  and a lower portion  144  near tip  136 . Upper portion  142  preferably has a lower thread pitch (i.e. distance between threads) than lower portion  144  such that threaded portion  138  has a dual lead cortical cancellous pitch. The ratio of lower thread pitch to higher thread pitch may be similar to the embodiment described in  FIGS.  1 - 6   . 
     As shown in  FIGS.  9  and  10   , shank portion  140  extends from the upper part of threaded portion  142  and head  132  of fixation screw  130 . Shank portion  140  includes external threads  160  that extends adjacent to, or near, a lower surface  162  of head  130 . Threads  160  are configured to mate with internal threads  186  of washer  120  (shown in  FIG.  11    and discussed below). Threads  160  preferably have a lower thread pitch than upper threaded portion  142 . The ratio of thread pitch between threads  160  and the threads of upper portion  142  may be about 1:2 to about 1:10, preferably about 1:3 to about 1:5 (e.g.,  1 : 4 ). In certain embodiments, the distance between threads  160  is about 0.1 mm to about 0.4 mm, preferably between about 0.15 mm to about 0.25 mm (e.g., 0.2 mm). 
     As shown in  FIGS.  11 - 14   , washer  120  includes a first, upper washer portion  180  and a second, lower washer portion  182 . In certain applications, first washer portion  180  is an anterior portion and second washer portion  182  is a posterior portion. First and second washer portions  180 ,  182  each include an inner surface having internal threads  186  and an outer surface  188 . Internal threads  184  are configured to engage exterior threads  160  of the shank portion  140  of screw  130 . Once washer  120  has been moved over shaft  130  and onto shank portion  140 , rotation of washer  120  will causes threads  160 ,  186  to engage each other and displace washer  120  towards head  132  of screw  130 . Threads  160 ,  186  are positioned such that washer  120  can be tightened against lower surface  162  of screw head  132 . Similar to the previous embodiment, this reduces toggle of the screw that may result from compressive forces, providing additional rigidity to minimize breakage or fracture of the screw and washer assembly. 
     Internal threads  186  may have a thickness of about 0.1 mm to about 0.2 mm, preferably about 0.15 mm (see  FIG.  14   ). Threads  186  may extend outward from washer  120  by about 0.1 to about 0.3 mm, preferably about 0.2 mm. 
     First washer portion  180  includes an upper surface with one or more serrations or other frictional elements  190  extending therefrom and second washer portion  182  includes a lower surface with one or more serrations or other frictional elements  192  extending therefrom. Serrations  190  function to provide additional resistance between washer  120  and lower surface  162  of head  132 , while serrations  192  provide additional resistance between washer  120  and a surface of an implant (discussed below). Alternatively, lower surface  162  of head  132  may include serrations or frictional elements, or both head and washer portion  180  may include frictional elements that interact with each other. As in the previous embodiment, frictional elements  190 ,  192  may comprise dimples, pits, depressions, projections, or other surface features, such as a roughened lower surface. 
     Washer portions  180 ,  182  together may have a sufficient thickness that optimizes strength and resistance to deformation, preferably about 1.5 to about 2.0 mm, more preferably about 1.6 mm (see  FIG.  13   ). 
     Upper washer portion  180  includes a plurality of cams  194  on its lower surface configured to interlock with cams  196  on an upper surface of lower washer portion  182 . Cams  194 ,  196  have an inclination angle that is greater than the pitch of the threads of upper threaded portion  142  of shaft  134 . Similar to the previous embodiments, this creates a wedge effect to prevent screw  130  from backing out or loosening from the implant, making it nearly impossible for the dynamic load of physiological forces to release the interlocked cams 
     In certain embodiments, the inclination angle of cams  194 ,  196  is greater than 5 degrees, preferably greater than 10 degrees. In an exemplary embodiment, the cam inclination angle is about 11 degrees. 
     As in the previous embodiment, cams  194 ,  196  may comprise other types of interlocking members, such as such as interference fitting members, or other frictional elements that lock or otherwise make it difficult to pull washer  120  apart from screw head  132 . Cams  194 ,  196  may also have a semicircular shape, or cams  194 ,  196  may be formed at discrete locations around the circumference of washer  120  and head  132 . 
       FIG.  15    illustrates another embodiment of a screw  200  that includes a head  220  and a shaft  222 . Head  220  generally tapers in the direction towards shaft  222  to form a substantially conical shape. Shaft  222  includes external threads  224  for mating with the internal threads of a washer (not shown) similar to the embodiments described above. In this embodiment, the washer preferably has a design similar to washer  120  as the conical shape of screw head  220  precludes the use of cams on head  220 . The washer, however, may have a tapered upper surface with serrations thereon to provide more engagement with head  220 . 
       FIGS.  16 A- 16 C  conceptually depict a method of assembling the fixation assembly  100  described in  FIGS.  7 - 14   . For spinal implants, such as an interbody fusion device, the surgeon will complete the discectomy and trialing and then begin to assembly fixation assembly  100 . As shown in  FIGS.  16 A and  16 B , first and second washer portions  180 ,  182  each include a visual indicator  197 ,  198 , which may be located on one of the cams  194 ,  196 . Visual indicators  197 ,  198  may comprise any suitable indicator for the physician to easily locate them, such as colors, markings, letters, surface features and the like. In one embodiment, visual indicators  197 ,  198  are colored (e.g., red). 
     Once the operator has located visual indicators  197 ,  198 , they are aligned with each other to ensure that the appropriate patterned threads match to lock washer  120  to bone screw  130 . Washer portions  180 ,  182  are locked together and then advanced up shaft  132  of screw  130  until they are positioned opposite external threads  160  of shank portion  140  (see  FIG.  7   ). Internal threads  186  are aligned with external threads  160  and washer portions  180 ,  182  are rotated relative to shaft  132 , preferably in a direction that secures cams  194 ,  196  to each other (i.e., in the direction that locks the cams). In one embodiment, this direction is clockwise, although those skilled in the art will recognize that the configuration of cams  194   196  can be reversed to enable a counterclockwise rotation. 
     Washer portions  180 ,  182  are preferably rotated until serrations  190  on upper washer portion  180  come in contact with, and engage, lower surface  162  of screw head  132  (see  FIG.  16 C ). The operator should ensure that washer portions  180   182  are secured to screw  130  and serrations  190  have solid contact with head  132 . 
     The operator may then attach mating feature  150  of screw head  132  to a suitable instrument driver (not shown) and place fixation assembly  100  into an appropriate hole of an implant. The screw  130  is driven into the bone of the patient (e.g., a vertebral body) until serrations  192  on lower washer portion  182  come in contact with, and engage, a surface of the implant. The operator should use sufficient torque when placing screw  130  to ensure tension is placed on washer portions  180 ,  182 , which activates the wedge-lock component and prevents screw  130  from backing out of the hole. 
     A method of assembling the fixation assembly  10  described above and illustrated in  FIGS.  1 - 6    is similar except that cams  80  of washer  20  and cams  70  on head  32  of screw  30  are interlocked with each other after washer  20  has been rotated relative to shaft  32 , until cams  80  come in contact with, and engage, cams  70  of screw head  32 . 
     Referring now to  FIGS.  17 - 24   , another embodiment of a fixation assembly  300  will now be described. Fixation assembly  300  comprises a washer  320  coupled to a fixation screw  330 . Fixation screw  330  comprises a head  332  and a shaft  334  with a distal tip  336 . Shaft  334  includes a threaded portion  338  and a shank portion  340  between threaded portion  338  and head  332 . Fixation screw  330  may be cannulated or non-cannulated. In some embodiments, fixation screw  330  may be cannulated and comprise a central hole or lumen extending through its longitudinal axis. Screw  330  may be self-tapping and self-drilling and may be of a bone-screw type, such as those well known to skilled artisans. In some embodiments, screw  330  may, for example, include notches or cutouts in the distal tip and/or cement holes in the shaft to facilitate self-tapping. 
     Fixation assembly  300  and its components may be formed of any suitable medical grade material, such as biocompatible metals like stainless steel, titanium, titanium alloys, etc. or a medical grade plastic such as polyetheretherketone (PEEK) or another radiolucent material, ultra high molecular weight polyethylene (UHMWPE), etc. If so desired, the fixation assembly  300  may also be formed of a bioresorbable material. The bioresorbable material may preferably be osteoconductive or osteoinductive (or both). 
     Referring to  FIG.  18   , threaded portion  338  may include an upper portion  342  near head  332  and a lower portion  344  near tip  336 . Upper threaded portion  342  preferably comprises about 20% to about 75% of the total length of threaded portion  338 , preferably about 25% to about 50%. The threads in upper and lower portions  342 ,  344  preferably extend outward about the same distance from shaft  334 , although their outer dimensions may differ depending on the application. In one embodiment, the threads extend outward from shaft  334  by about 0.5 mm to 1.5 mm, or about 0.75 mm to about 0.85 mm. 
     Upper portion  342  preferably has a lower thread pitch (i.e., the distance between threads) than lower portion  344 . The ratio of lower thread pitch to higher thread pitch may be about 1:2 to about 1:5, preferably about 1:3 to 1:4 (e.g., about 1:3.5). In certain embodiments, the distance between threads in upper portion  342  is about 0.5 mm to about 1.0 mm, preferably about 0.8 mm and the distance between threads in lower portion  344  is about 2.0 mm to about 4.0 mm, preferably about 2.5 mm to about 3.0 mm (e.g., 2.75 mm). The higher thread pitch towards tip  336  of the screw allows for a greater purchase into cancellous bone, while the lower thread pitch towards head  332  increases the holding power of the screw. In addition, the lower thread pitch of upper portion  342  provides for a smaller wedge angle for the threads (i.e., the angle that the thread make with a horizontal plane passing perpendicular to the longitudinal axis of the screw). This provides additional advantages in tandem with the larger cam angle created by the interlocking cams  370 ,  380  of washer  320  and screw head  332  (discussed below). 
     Of course, the devices disclosed herein are not limited to the thread pitch configuration described above. For example threaded portion  338  may also have a single, substantially uniform thread pitch throughout its length. Alternatively, threaded portion  338  may have more than two different thread pitches. In some embodiments, the thread pitches of upper portion  342  and lower portion  344  may be reversed such that upper portion  342  has a higher thread pitch than lower portion  344 . 
     As shown in  FIG.  20   , screw head  32  includes a mating feature  350  for receiving an instrument designed to tighten screw  330  against a medical implant (discussed below). Mating feature is preferably configured to resist stripping from the use of force to allow the operator to provide more torque to tighten the screw. This feature may be particularly advantageous given that the presence of washer  320  requires significant tension to secure screw  320  to the implant. In certain embodiments, mating feature  350  comprises a shape that orients the flats more perpendicular to the axis of the applied force, such as a star-shaped or torx configuration. 
     Referring now to  FIGS.  18  and  19   , shank portion  340  extends from the upper part of threaded portion  342  and head  332  of fixation screw  320 . Shank portion  340  includes a locking mechanism for ensuring that washer  320  does not translate distally after it has been attached to screw head  332  (discussed below). In one embodiment, the locking mechanism comprises one or more projections  360  that extend outward from the shaft between head  332  and upper threaded portion  342 . Projections  360  may comprise extrusions, bumps, threads, or any other type of projection that extends from shank  340 . In some embodiments, projections  360  are frictional elements, such as surface roughening and the like. 
     In an exemplary embodiment, projections  360  comprise extrusions that have been formed into a spiral formation around shank  340 . The extrusions preferably extend outward from shank  340  a sufficient distance to inhibit or prevent washer  320  from sliding back down the shaft  330  of screw after it has been advanced past the extrusions. In one such embodiment, projections  360  extend outward from shank  340  a distance of about 0.25 mm to about 0.5 mm, preferably about 0.375 mm. Projections  360  and shank  340  together form a combined cross-sectional area relative to the longitudinal axis of the shaft  338 . As discussed in further detail below, this combined cross-sectional area is sized to allow proximal translation of washer  320  over projections  360  to screw head  332 , while inhibiting or preventing distal translation of the washer  320  back over projections  360 . 
     Referring now to  FIGS.  22 A- 22 C,  23 A- 23 D and  24 A- 24 B , washer  320  may have a substantially annular shape with a central opening having an inner diameter or cross-sectional area sized to fit over shaft  334  and fit against head  332  to minimize loosening of screw  330 . In certain embodiments, washer  320  may have an at least an upper surface  376  with an enlarged outer diameter, thereby increasing the surface area of contact to distribute the load from screw  330  over a larger area. In an exemplary embodiment, the upper surface  376  of washer  320  has an outer diameter substantially equal to the outer diameter of head  332 . In other embodiments, washer  320  have may have a larger outer diameter than head  332 . 
     Washer  320  includes an inner surface  375 , an outer surface  374  and upper and lower surfaces  376 ,  378 . Upper surface  376  generally faces towards head  332  of screw  330  and lower surface  378  generally faces towards distal tip  336 . In some embodiments, upper surface  376  is an anterior surface and lower surface  378  is a posterior surface. Washer  320  may have a sufficient thickness from upper surface  376  to lower surface  378  that optimizes strength and resistance to deformation, preferably about 1.5 to about 2.0 mm, more preferably about 1.6 mm. 
     Lower surface  378  includes frictional elements, such as serrations  382 , to provide additional resistance between washer  320  and a surface of an implant (discussed below). Alternatively, frictional elements may include dimples, pits, depressions, projections, or other surface features, such as a roughened lower surface. 
     As shown in  FIGS.  19   , head  332  may further include a plurality of cams  370  on a lower surface  362  of head  332  that are configured to interlock with corresponding cams  380  on washer  320  (see  FIG.  22 A ). Cams  380  may have an inclination angle greater than the pitch of the threads of upper threaded portion  342 . The inclination angle is defined as the angle that the cams  380  make with a horizontal plane extending through washer  320  that is perpendicular to the longitudinal axis of screw  330 . The “thread pitch’ also refers to the angle each thread makes with the horizontal plane extending through shaft  330  perpendicular to the longitudinal axis of screw  330 . 
     Providing an inclination angle of the interlocked cams  370 ,  380  that is greater than the wedge or pitch angle of the threads of threaded portion  334  creates a wedge effect to prevent screw  330  from backing out or loosening from the implant, making it nearly impossible for the dynamic load of physiological forces to release the interlocked cams. Only a driver turning the bone screw counterclockwise with a specific torque parameter provides enough force to disengage the cams  370 ,  380  from each other. This results in a significant reduction or complete elimination of the screw  330  backing out from the implant. 
     The specific inclination angle of cams  380  will depend on the application and particularly on the pitch angle of threaded portion  334 . In certain embodiments, the inclination angle of cams  80  is greater than 5 degrees, preferably greater than 10 degrees. In an exemplary embodiment, the cam inclination angle is about 11 degrees. 
     Of course, it should be recognized that fixation assembly  300  is not limited to the cams  370 ,  380  described above and shown in  FIGS.  17 - 24   . For example, cams  370 ,  380  may comprise other types of interlocking members, such as interference fitting members, or other frictional elements that lock or otherwise make it difficult to pull washer  320  apart from screw head  332 . In addition, although cams  370 ,  380  extend around the entire circumference of washer  320  and head  332  the devices disclosed herein are not limited to this configuration. Cams  370 ,  380  may have a semicircular shape (i.e., only extending around a portion of the circumference of washer  320  and head  332 ), or cams  370 ,  380  may be formed at discrete locations around the circumference of washer  320  and head  332  (e.g. two cams formed on opposite sides, four cams spaced from each other and formed around washer  320 , and head, etc.). 
     As shown in  FIGS.  24 A and  24 B , central opening  322  of washer  320  has a smaller cross-sectional area at the upper surface  376  ( FIG.  24 B ) than the lower surface  378  ( FIG.  24 A ). More specifically, inner surface  375  around the perimeter of opening  322  has a substantially circular shape at upper surface  376  that generally corresponds to the size and dimension of head  332  of screw  330  so that cams  370 ,  380  may engage with each other to secure washer  320  to head  332 . By contrast, one portion of inner surface  375  includes a bulge or cutout  383  at lower surface  378  that increases the internal distance from a first portion of the inner surface  375  to a second portion opposite the first portion. This cutout  383  increases the overall cross-sectional area of the opening  322  within washer  320  at the lower surface  378  (relative to the overall cross-sectional area of the opening  322  within washer at the upper surface  376 ). 
     The cross-sectional area of opening  322  at upper surface  376  is also smaller than the combined cross-sectional area of projections  360  and shank  340 . This prevents washer  320  from sliding distally down shaft  338  after it has been attached to screw head  332 . At the same time, the cross-sectional area of opening  322  at lower surface  378  is larger than combined cross-sectional area of projections  360  and shank  340 . This allows the operator to advance washer  320  proximally over projections  360  to secure washer  320  to screw head  332 . 
     In an exemplary embodiment, at least one portion of the inner surface  375  at lower surface  378  has a larger diameter (or is further from the longitudinal axis of the shaft) than the outer diameter of projections  360  on shank  340  of screw  330 . This allows washer  320  to be passed proximally over projections  360  towards the screw head  332 . At the same time, the diameter of inner surface  375  at upper surface  376  is smaller than the diameter of projections  360  on shank  340  of screw  330 . This prevents the washer  320  from passing distally back over projections  360  once it has been advanced over shank  340  to screw head  332 . 
     In use, the operator advances washer  320  over distal tip  336  and shaft  334  of screw  330 . When the washer  320  engages projections  340 , the operator may toggle, shift, rotate, adjust or otherwise manipulate washer  320  so that cutout  383  aligns with projections  340  and allows the operator to advance washer  320  distally past the projections  340  to head  332 . Once washer  320  has been moved distally of projections  340 , it generally will not slip past projections proximally unless it is manipulated by the operator in a similar manner. 
     As shown in  FIG.  23 A , the inner surface  375  of washer  320  includes a shelf  390  that extends around a portion of the perimeter of central opening  322  between upper and lower surfaces  376 ,  378 . Shelf  390  has a height in the longitudinal direction that varies as shelf  390  extends around the perimeter of washer  320 . More specifically, the height of shelf  390  reduces around perimeter until it reaches a minimum height in the area  392  of cutout  383 . This creates a curved or C-shaped inner surface that extends around only a portion of the perimeter of washer  320 . The curved surface gradually reduces to a less-curved surface on the other side of the perimeter from cutout  383  (see also  FIG.  23 D ). 
     Inner surface  375  of washer  320  may have other configurations. For example, instead of shelf  390 , a portion of inner surface  375  may taper inwardly from upper surface  376  to lower surface  378 . In this configuration, inner surface  375  curves around the internal perimeter of washer  320  such that on one side of the washer  320 , inner surface is substantially parallel to the longitudinal axis of screw  330  and upper and lower surfaces  376 ,  378  extend the same distance laterally from the longitudinal axis. However, on the other side of the washer  320 , inner surface  375  tapers inwardly from upper surface  376  to lower surface  378  such that lower surface  378  extends further away from the longitudinal axis than upper surface  376 . 
     While the screw heads are shown as integrated with their shafts, it is contemplated that the screw heads may also be detachable from the shafts in some embodiments. Further, lower portions of the shank portions may also have various shapes such as a conical or rounded cup shape in some embodiments. 
       FIGS.  25 A- 25 D  illustrate a representative medical implant assembly  400  that includes a spinal implant  410  and one or more fixation assemblies which may have the configuration of any of the embodiments described above. Spinal implant  410  may include anterior and posterior portions  412 ,  414  and upper and lower surfaces  416 ,  418  profiled to correspond with the profile of any bone material to which they are to be secured. As shown, the upper and lower surfaces  416 ,  418  may further include surface enhancements, such as, teeth  440  to enhance bone attachment. It is understood, however, that alternative surface modifications such as surface roughenings, barbs, spikes, bumps, etc. may also be employed. In one embodiment, the spinal implant  410  defines a generally wedge shaped structure. The spinal implant  410 , however, may have other shapes depending on the desired implantation site. Further, it is to be understood that the medical implant assembly may include any type of implant having a screw hole for receiving the fixation assembly of the present disclosure, and is in no way limited to fusion cages as illustrated herein. 
     The spinal implant  410  and its components may be formed of any suitable medical grade material, such as biocompatible metals like stainless steel, titanium, titanium alloys, etc. or a medical grade plastic such as polyetheretherketone (PEEK) or another radiolucent material, ultra high molecular weight polyethylene (UEMWPE), etc. If so desired, the implant  410  may also be formed of a bioresorbable material. The bioresorbable material may preferably be osteoconductive or osteoinductive (or both). 
     As shown, the spinal implant  410  may include a central opening or lumen  450  extending between the upper and lower surfaces  416 ,  418  to facilitate bony ingrowth or fusion between adjacent bone segments, such as vertebral bodies. Opening  450  may extend through one or more of the surfaces of implant, such as one or more the side surfaces  452 . If so desired, the opening  450  may be used to receive and hold bone graft material. 
     The spinal implant  410  may include holes (not shown) for placement of the fixation assemblies  300  therethrough to secure the spinal implant  410  to adjacent bone tissue. The fixation assemblies may comprise one of the embodiments described above, or a combination of such embodiments. In the embodiment shown, the implant  410  includes four holes staggered from each other from one side of the implant to the other. One skilled in the art will appreciate that the implant  410  may comprise any number of holes in any location on the implant  410 . For instance, a two-hole or three-hole version of the spinal implant  410  may be envisioned. Optionally, the implant  410  may comprise holes for receiving features like a radiologic marker or other imaging marker. 
     Implant  410  may include a backout plate  490  positioned over the posterior portion  414  of the implant  410 . Backout plate  490  serves to prevent backing out of the fixation assembly  300  from the holes of the implant. Of course, one skilled in the art will appreciate that any number of known mechanisms may be used to inhibit or prevent backing out of the fixation assemblies. 
     This description and the accompanying drawings illustrate exemplary embodiments and should not be taken as limiting, with the claims defining the scope of the present disclosure, including equivalents. Various mechanical, compositional, structural, and operational changes may be made without departing from the scope of this description and the claims, including equivalents. Like numbers in two or more figures represent the same or similar elements. Furthermore, elements and their associated aspects that are described in detail with reference to one embodiment may, whenever practical, be included in other embodiments in which they are not specifically shown or described. For example, if an element is described in detail with reference to one embodiment and is not described with reference to a second embodiment, the element may nevertheless be claimed as included in the second embodiment. Moreover, the depictions herein are for illustrative purposes only and do not necessarily reflect the actual shape, size, or dimensions of the system or illustrated components.