Patent Publication Number: US-2020289285-A1

Title: Arthroplasty implant for a facet joint

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Patent Application No. 62/587,089 filed on Nov. 16, 2017, titled Arthroplasty Implant for a Facet Joint, the content of which is incorporated herein in its entirety for any purpose. 
    
    
     FIELD 
     This disclosure relates generally to medical devices and methods, and more specifically to devices and methods related to spinal surgery and implants for placement in a facet joint. 
     BACKGROUND 
     The human spine is a segmented series of bones, or vertebrae, separated by cervical discs. Each vertebrae has two facet joints which function to provide stability and guidance during spinal motion. The facet joint comprises two bony surfaces separated by cartilage and surrounded by a capsule of ligaments. Facet joints are susceptible to chronic degenerative diseases, such as facet arthrosis, which result from the degeneration and breaking down of cartilage and bone in the joint structure. Facet arthrosis may affect different areas of the spine, such as cervical facet arthropathy or lumbar facet arthropathy which can result from osteoarthritis and rheumatoid arthritis. 
     Traditional methods of treatment include non-invasive methods such as rest, changes in sleep position, proper exercise supervised by a physical therapist, and posture correction. More invasive treatments, including spinal fusion, may also be used. 
     One procedure is spinal arthroplasty, which includes the implantation of a spinal prosthesis to restore posterior element structure and function, as an adjunct to neural decompression. The objective of spinal arthroplasty is to replace all or part of the joint with a prosthesis that stabilizes and decompresses the spine while retaining normal intervertebral motion and mitigating disc degeneration risk. 
     It is therefore desirable to provide an improved prosthesis that more generally offers improvements or an alternative to existing arrangements. 
     SUMMARY 
     According to an embodiment, a spinal implant, which may also be referred to as an arthroplasty implant, and which may be used for implantation in a spinal facet joint is disclosed. The spinal implant may include a first plate having a first surface, a second plate having a second surface, and a biasing element having a first end and a second end. The biasing element may be coupled to the first surface of first the plate at the first end and the second surface of the second plate at the second end. 
     In some embodiments, the spinal implant may include a first plurality of teeth coupled to a third surface opposite the first surface of the first plate, and a second plurality of teeth coupled to a fourth surface opposite the second surface of the second plate. 
     In some embodiments, the first plate may include a first rounded surface opposite to the first surface, and the second plate may include a second rounded surface opposite to the second surface. The first rounded surface and the second rounded surface may be covered in a compliant and high-friction material. 
     In some embodiments, the spinal implant may include one or more attachment tabs coupled to at least one of the first and second plates. 
     In some embodiments, the biasing element, may be at least one of a waveform spring, a coil spring, and a flexible core. The flexible core may include at least one of silicone, polyethylene, and hydrogel. 
     In some embodiments, the spinal implant may include first and second hollow cylinders. The first hollow cylinder may be coupled to the first plate and housing at least a portion of the biasing element. The second hollow cylinder may be coupled to the second plate and housing at least a portion of the biasing element. 
     In some embodiments, the first plate and the second plate may include a leading edge and a trailing edge, the leading edge configured for insertion into a facet joint. The biasing element may be positioned adjacent to the trailing edge. The spinal implant may include a second biasing element having a third end and a fourth end, the biasing element coupled to the first surface of the first plate at the third end and the second surface of the second plate at the fourth end. The second biasing element may be positioned adjacent to a trailing edge. The leading edge may be tapered. 
     In some embodiments, the spinal implant may include a central core fixed to one of the first plate and the second plate. The central core may be cylindrical or hemispherical. 
     In some embodiments, the spinal facet joint may be a cervical spinal facet joint. 
     According to another embodiment, a spinal implant for implantation in a spinal facet joint is disclosed. The spinal implant may include a top cylindrical portion having a first diameter, a bottom cylindrical portion having a second diameter that is larger than the first diameter and a recess formed therein to receive the top cylindrical portion, a biasing element positioned within the recess between the top cylindrical portion and the bottom cylindrical portion, and a fastener configured to hold the top cylindrical portion within the bottom cylindrical portion and compress the biasing element. 
     In some embodiments, the fastener may include first and second threaded portions. The first threaded portion may be integrally formed with and extending from the top cylindrical portion. The second threaded portion may be formed in the recess of the bottom cylindrical portion and configured to receive the first threaded portion. The spinal implant may include an engagement feature, such as a notch formed in a side wall of the top cylindrical portion and configured to rotate the top cylindrical portion relative to the bottom cylindrical portion. 
     In some embodiments, the fastener may include a first hole formed in a side wall of the top cylindrical portion, a second hole formed in a side wall of the bottom cylindrical portion and configured to align with the first hole when the biasing element is compressed, and a set screw configured to be removably inserted into the aligned first and second holes. 
     In some embodiments, the biasing element may be aligned along a longitudinal axis of the top cylindrical portion and the bottom cylindrical portion. 
     In some embodiments, the spinal facet joint may be a cervical spinal facet joint. 
     An arthroplasty implant is disclosed. The implant may include a first facet joint engagement plate having a first surface, a second facet joint engagement plate having a second surface, and a biasing element having a first portion and a second portion, the biasing element coupled to the first surface of the first engagement plate at or near the first portion and the second surface of the second engagement plate at or near the second portion. 
     In some aspects, the first and second facet joint engagement plates are cervical facet joint engagement plates. In some aspects, the biasing element may be at least one of a waveform spring, a coil spring, and a flexible core. In some aspects, the flexible core includes at least one of silicone, polyethylene, and hydrogel. 
     An arthroplasty implant is disclosed. In some aspects, the implant includes a first articulating subchondral engagement plate having a first surface, a second articulating subchondral engagement plate having a second surface, and a biasing element having a first portion and a second portion, the biasing element coupled to the first surface of the first engagement plate at or near the first portion and the second surface of the second engagement plate at or near the second portion. In some aspects, the first and second articulating subchondral engagement plates are configured for a cervical facet joint. In some aspects, the biasing element is at least one of a waveform spring, a coil spring, and a flexible core. In some aspects, the flexible core includes at least one of silicone, polyethylene, and hydrogel. 
     Additional embodiments and features are set forth in part in the description that follows, and will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and drawings, which form part of the disclosure. One of skill in the art will understand that each of the various aspects and features of the disclosure may advantageously be used separately in some instances, or in combination with other aspects and features of the disclosure in other instances. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate embodiments of the disclosure and, together with the general description above and the detailed description below, serve to explain the principles of these embodiments. 
         FIG. 1  is a side view of an arthroplasty implant with engagement plates and a biasing element in accordance with aspects of the present disclosure. 
         FIG. 2  is a side view of the arthroplasty implant of  FIG. 1 , except the engagement plates are a different size. 
         FIG. 3  is a perspective view of the arthroplasty implant of  FIG. 1  positioned within a facet joint. 
         FIG. 4  is a side view of an arthroplasty implant with engagement plates and a biasing element positioned within a housing. 
         FIG. 5  is a sectional view of the arthroplasty implant of  FIG. 4 . 
         FIG. 6  is another sectional view of the arthroplasty implant of  FIG. 4 . 
         FIG. 7  is a side view of an arthroplasty implant having tapered engagement plates and two biasing elements. 
         FIG. 8  is a side view of the arthroplasty implant of  FIG. 7 , except the implant has one biasing element. 
         FIG. 9  is a perspective view of the arthroplasty implant of  FIG. 7  positioned within a facet joint. 
         FIG. 10A  is a side view of an arthroplasty implant having engagement plates and a core of compliant material. 
         FIG. 10B  is a side view of an arthroplasty implant having engagement plates and a core of compliant material. 
         FIG. 11  is a perspective view of the arthroplasty implant of  FIG. 10A  positioned within a facet joint. 
         FIG. 12  is a side view of an arthroplasty implant having engagement plates and a waveform biasing element and a cylindrical central core. 
         FIG. 13  is a side view of an arthroplasty implant having engagement plates and a waveform biasing element and a cylindrical central core. 
         FIG. 14  is a side view of an arthroplasty implant having engagement plates and a hemispherical central core. 
         FIG. 15  is a side view of an arthroplasty implant having engagement plates and a hemispherical central core. 
         FIG. 16  is a sectional view of the arthroplasty implant of  FIG. 15 . 
         FIG. 17A  is a perspective view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 17B  is a perspective view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 17C  is a perspective view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 18A  is a side view of the arthroplasty implant of  FIG. 17A . 
         FIG. 18B  is a side view of the arthroplasty implant of  FIG. 17B . 
         FIG. 19A  is an exploded view of the arthroplasty implant of  FIG. 17A . 
         FIG. 19B  is an exploded view of the arthroplasty implant of  FIG. 17B . 
         FIG. 20  is a sectional view of the arthroplasty implant of  FIG. 17A , taken along line  20 A- 20 A. 
         FIG. 21A  is a sectional view of the arthroplasty implant of  FIG. 17A , taken along line  20 A- 20 A, in a compressed position. 
         FIG. 21B  is a sectional view of the arthroplasty implant of  FIG. 17B , taken along line  21 B- 21 B, in a compressed position. 
         FIG. 22A  is a sectional view of the arthroplasty implant of  FIG. 17A , taken along line  20 A- 20 A, in an expanded position. 
         FIG. 22B  is a sectional view of the arthroplasty implant of  FIG. 17B , taken along line  21 B- 21 B, in an expanded position 
         FIGS. 23-28  are perspective views of the insertion and release of the implant of  FIG. 17A , wherein some or all delivery tool(s) are hidden for clarity. 
         FIG. 29  is a perspective view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 30  is a sectional view of the arthroplasty implant of  FIG. 29  in an expanded position taken along the line  30 - 30 . 
         FIG. 31  is a perspective view of the arthroplasty implant of  FIG. 29  in an expanded position. 
         FIG. 32  is a sectional view of the arthroplasty implant of  FIG. 29  in a compressed position taken along the line  30 - 30 . 
         FIG. 33  is a perspective view of the arthroplasty implant of  FIG. 29  positioned in a facet joint in a compressed position. 
         FIG. 34  is a perspective view showing the set screw of the arthroplasty implant of  FIG. 29  being removed. 
         FIG. 35  is a perspective view showing the set screw of the arthroplasty implant of  FIG. 29  being removed. 
         FIG. 36  is a side view of an arthroplasty implant having facet joint surface engaging plates and attachment tabs according to aspects of the present disclosure. 
         FIG. 37  is a sectional view of the arthroplasty implant of  FIG. 36 . 
         FIG. 38  is a side view of the arthroplasty implant of  FIG. 36 , except the engagement plates are a different size. 
         FIG. 39  is a side view of the arthroplasty implant of  FIG. 36  with the attachment tabs positioned at an angle. 
         FIG. 40  is a sectional view of the arthroplasty implant of  FIG. 39 . 
         FIG. 41  is a side view of the arthroplasty implant of  FIG. 39 , except the engagement plates are a different size. 
         FIG. 42  is a side view of the arthroplasty implant of  FIG. 36  with the attachment tabs positioned at right angles relative to the plates. 
         FIG. 43  is a sectional view of the arthroplasty implant of  FIG. 42 . 
         FIG. 44  is a side view of the arthroplasty implant of  FIG. 42 , except the engagement plates are a different size. 
         FIG. 45  is a side view of the arthroplasty implant of  FIG. 38  with fasteners. 
         FIG. 46  is a side view of the arthroplasty implant of  FIG. 38  with fasteners, except the central core is removed. 
         FIG. 47  is a perspective view of the arthroplasty implant of  FIG. 46  positioned in a facet joint and secured with fasteners. 
         FIG. 48  is another perspective view of the arthroplasty implant of  FIG. 46  positioned in a facet joint and secured with fasteners. 
         FIG. 49  is a perspective view of an arthroplasty implant having rounded surfaces according to aspects of the present disclosure. 
         FIG. 50  is a side view of the arthroplasty implant of  FIG. 49 . 
         FIG. 51  is a perspective view of the arthroplasty implant of  FIG. 49  positioned in a facet joint. 
         FIG. 52  is a side view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 53  is a sectional view of the arthroplasty implant of  FIG. 52  along line  53 - 53 . 
         FIG. 54  is a side view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 55  is a side view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 56  is an exploded view of the arthroplasty implant of  FIG. 55 . 
         FIG. 57  is a perspective view of an arthroplasty implant according to aspects of the present disclosure. 
         FIG. 58  is a rear view of the arthroplasty implant of  FIG. 57  shown in a first compressed position. 
         FIG. 59  is a rear view of the arthroplasty implant of  FIG. 57  shown in an alternate compressed position. 
         FIG. 60  is a side view of an arthroplasty implant according to aspects of the present disclosure. 
     
    
    
     Additional embodiments and features are set forth in part in the description that follows, and will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and drawings, which form part of the disclosure. One of skill in the art will understand that each of the various aspects and features of the disclosure may advantageously be used separately in some instances, or in combination with other aspects and features of the disclosure in other instances. 
     DETAILED DESCRIPTION 
     Embodiments described in the present disclosure recognize that previous surgical treatments for degenerative diseases of the facet joint, such as facet arthrosis, often require intrusive surgery such as spinal fusion, or implantation of large prostheses such as a facet replacement system. Further, traditional treatments typically use anterior approaches, i.e., surgical procedures with an incision on the front of the body. Such procedures require surgical precision in order to avoid damaging nerves or arteries between the incision and the facet joint. Finally, most, if not all, arthroplasty implants are specially designed for use in the lumbar spine, which often requires invasive surgery. Disclosed herein are various arthroplasty implants, which may also be referred to as spinal implants, for use in cervical facet joints which are smaller in size and may have different requirements than an implant for the lumbar spine. Examples of the different requirements for lumbar applications include higher expected loads and more difficult anatomical access. The disclosed arthroplasty implants may be implanted using minimally invasive surgery. Additionally, the arthroplasty implants may be inserted using a posterior approach which allows for a more direct path to insertion of the implant in the facet joint. Once implanted, the disclosed embodiments allow the implant to articulate in directions that follow the natural motion of the facet joint. 
     In various aspects described herein, the arthroplasty implant includes facet joint surface engaging plates and at least one spring or spring-like material positioned between the joint surface engaging plates. In some embodiments, the spring or spring-like material may also be referred to as a biasing element. More specifically, the facet joint surface engaging plates include a superior facet joint surface engaging plate and an inferior facet joint surface engaging plate. The facet joint may be a cervical facet joint (e.g. a joint located between cervical vertebrae) in a human. 
     Referring now to  FIGS. 1-3 ,  FIG. 1  is a side view of an arthroplasty implant, generally designated  100  with a waveform or disc spring type of biasing element. The implant  100  includes facet joint surface engaging plates including a first or top plate  102 , a second or bottom plate  104 , and at least one biasing element  106 , such as a wave-form or disc spring, and may include a plurality of teeth  108 . The top plate  102  may be a superior facet joint surface engaging plate and the bottom plate  104  may be an inferior facet joint surface engaging plate. The plates  102 ,  104  may generally be made of any suitable rigid material. For example, the top plate  102  and bottom plate  104  may be made of metal (e.g., implant-grade titanium), plastic, a rigid polymer, a combination thereof, or any other suitable material. The biasing element  106 , such as a wave-form spring  106 , is a disc spring having a frusto-conical shape that provides the spring characteristics and may be made of any material (such as Nitinol) which allows for flexion that mimics the range and directions of motion typical of a facet joint. For example, in one embodiment, the biasing element  106 , such as a wave-form spring  106 , allows for about 19 degrees of flexion, 14 degrees of extension, 28 degrees of lateral bending, and 17 degrees of rotation. The biasing element  106  may be made, for example, from metal, plastic, rubber, or any other flexible material or polymer. In some embodiments, the biasing element  106  may include a plurality of individual wave-form springs stacked together between the top plate  102  and the bottom plate  104 . In various embodiments, the biasing element  106  may include a plurality of welded wave washers to form a generally cylindrical shape. By increasing the number of wave-form springs in the stack, the implant  100  may increase the separation between the upper and lower surfaces of the facet joint as may be desirable for increased movement and support during spine flexion, extension, lateral flexion, compression, rotation and translation. 
     Each of the top plate  102  and bottom plate  104  may have a plurality of engagement features, such as teeth  108 , for fixing or securing the implant  100  in place within the facet joint. The teeth  108  may be coupled or attached to or integrally formed with the top plate  102  and the bottom plate  104 . The teeth  108  may be formed of rigid materials such as those used to form the top plate  102  and the bottom plate  104 . In various embodiments, the teeth  108  may have a sloped leading edge to facilitate insertion of the implant  100  into the facet joint, and a trailing edge that is substantially perpendicular to the top plate  102  and/or the bottom plate  104 , to hold the implant  100  in place within the facet joint. 
     The top plate  102  and the bottom plate  104  may be positioned and attached to opposing ends or faces of the biasing element  106  or the structure created by welding a plurality of the disc springs  106  together. The top plate  102  and the bottom plate  104  may be substantially parallel when no force is applied to either plate. In the embodiment of  FIG. 1 , the top plate  102  and the bottom plate  104  may each be cylindrical and have diameters that are substantially the same as the biasing element  106 . The top plate  102  and the bottom plate  104  may move relative to each other when the biasing element  106  flexes, extends, compresses, and/or rotates. The pluralities of teeth  108  may be attached to the top plate  102  and the bottom plate  104  opposite to the biasing element  106 . By including a plurality of teeth  108  on each of the top plate  102  and the bottom plate  104 , the implant  100  may be securely positioned within the facet joint to prevent the implant  100  from disengaging with the facet joint surfaces or otherwise moving within coming out of position from within the joint once set in the desired location by a practitioner. Although shown as triangular engagement features in  FIGS. 1 and 2 , those skilled in the art will appreciate that the teeth  108  may have any shape suitable for securing or holding the implant  100  in place within the facet joint. 
     In some aspects, the surface engaging plates may have an increased diameter relative to the biasing element located therebetween.  FIG. 2  is a side view of the arthroplasty implant  100  with enlarged diameter top and bottom plates. As shown in the embodiment of  FIG. 2 , the top plate  102  and the bottom plate  104  may each have a diameter that is substantially greater than the diameter of the biasing element, such as a disc or wave-form spring  106 . By increasing the contact area between the top plate  102  and the bottom plate  104  and the respective surface of the facet joint, the implant  100  may be more securely positioned in the facet joint. Additionally, the increased contact area between the top plate  102  and the bottom plate  104  and the facet joint may allow for smaller engagement features  108  (e.g. teeth  108 ) to help secure the implant  100  in place within the facet joint. 
     In use, the implant engages the upper and lower surfaces of the facet joint as may be desirable for increased movement and support during spine flexion, extension, lateral flexion, compression, rotation and translation.  FIG. 3  is a perspective view of the arthroplasty implant  100  positioned within a facet joint. As shown in  FIG. 3 , the top plate  102  engages the inferior facet of the superior vertebra  110  of the facet joint, and the bottom plate  104  engages the superior facet of the inferior vertebra  112  of the facet joint. The biasing element  106 , such as the wave-form spring  106 , applies an expansion force to the top plate  102  and the bottom plate  104  to push against the upper vertebra  110  and the lower vertebra  112 , respectively, and hold the implant  100  in place. The teeth  108  (not shown in  FIG. 3 ) provide additional engagement features for holding the implant  100  in place. 
     In some aspects, the biasing element may include or be enclosed by a housing for additional structural support.  FIG. 4  is a side view of an arthroplasty implant, generally designated  200 , with a biasing element, such as a central coil spring enclosed by a housing. The implant  200  generally includes a first or top cylindrical housing  202  having a first or top joint surface engaging plate  210 , a second or bottom cylindrical housing  204  having a second or bottom joint surface engaging plate  212 , a biasing element  206  (see  FIGS. 5 and 6 ), and a plurality of teeth  208  on at least one of the top or bottom plate. The top housing  202  and the bottom housing  204  may each be made from any suitable rigid material, similar to those listed above with respect to  FIG. 1 . The top housing  202  and the bottom housing  204  may be aligned along a central axis and separated by a distance, D, which allows for the top housing  202  to flex, extend, compress, and/or rotate relative to the bottom housing  204 . Depending on the particular application, the distance, D, may vary between about 0.5 mm and about 2 mm, though other suitable distances are contemplated. The teeth  208  may be made of the same or similar rigid material as the top housing  202  and the bottom housing  204  and may be integrally formed or attached to the top or bottom plate, respectively. The first or top plate may be an engaging plate to the inferior facet of the superior vertebra. The second or bottom plate may be an engaging plate to the superior facet of the inferior vertebra. 
       FIG. 5  is a sectional view of the arthroplasty implant  200  of  FIG. 4  to show the biasing element in more detail. As shown in  FIG. 5 , the biasing element  206  may be positioned within the top cylinder  202  and the bottom cylinder  204  and attached to the top plate  210  and the bottom plate  212 .  FIG. 6  is another sectional view of the arthroplasty implant of  FIG. 4  and also showing the biasing element within the housing. 
     The biasing element  206  may generally be any type of biasing element capable of imitating the flexion, extension, contraction, and rotation of a facet joint. In one embodiment, the biasing element  206  is a coil spring or a metal coil spring. In other embodiments, other types of biasing elements and other types of materials may be used. The biasing element  206  is coupled to the top cylinder  202  via the top plate and the bottom cylinder  204  via the bottom plate. For example, one end of the biasing element  206  may be attached to an inside surface of a top plate  210  of the top cylinder  202 . The other end of the biasing element  206  may be attached to an inside surface of a bottom plate  212  of the bottom cylinder  204 . In use (i.e. when implanted), the biasing element  206  may hold the top cylinder  202  and the bottom cylinder  204  in place relative to each other and separated by the distance, D. Although shown and described as a single, central spring in some embodiments, those skilled in the art will appreciate that the biasing element  206  may comprise more than one biasing element and may be arranged in any suitable configuration to simulate the motion of the facet joint. For example, two, three or four biasing elements may be arranged at regular intervals near the circumference of the inner surfaces of the top cylinder  202  and the bottom cylinder  206 . By using multiple biasing elements  206 , the motion (e.g., stiffness) of the implant  200  may be tuned by using biasing elements  206  with differing spring constants. 
     In some aspects, the facet joint surface engaging plates as described herein may have a tapered profile to provide easier entry of the implant into the facet joint. For example,  FIG. 7  is a side view of an arthroplasty implant, generally designated  300 , having tapered facet joint surface engaging plates and two biasing elements. In the embodiment of  FIG. 7 , the implant  300  includes a first or top plate  302 , a second or bottom plate  304 , a first biasing element  312 , and a second biasing element  314 . Each of the top plate  302  and the bottom plate  304  have a leading end  306  and a trailing end  308 . Each of the top plate  302  and the bottom plate  304  may also have a plurality of teeth  310  to fix or secure the implant  300  in position within the facet joint. As discussed above with respect to  FIG. 1 , the top plate  302 , bottom plate  304 , and teeth  310  may be formed of any suitable rigid material. The teeth  310  may be integrally formed with or attached or coupled to the top plate  302  and the bottom plate  304 . 
     The top plate  302  and the bottom plate  304  may be coupled together via the first biasing element  312  and the second biasing element  314 . The first biasing element  312  may be positioned adjacent to the trailing ends  308 , while the second biasing element may be positioned adjacent to the leading ends  306 . The first biasing element  312  and the second biasing element  314  may provide an expansion force to the first plate  302  and the second plate  304  to distract the facet joint and also allow for movement of the first plate  302  and second plate  304  that simulates the natural motion of the facet joint. The leading edges  306  of the top plate  302  and the bottom plate  304  may be tapered to facilitate easier insertion of the implant  300  into the facet joint. Additionally, the second biasing element  314  may be compressed during insertion of the implant  300  to make the insertion easier. Each of the trailing ends  308  of the top plate  302  and the bottom plate  304  may include structure (such as a protrusion  320  or recess) configured to engage (e.g., receive) a delivery device, which may be used to insert the implant  300  into the facet joint. For example, a delivery device may engage the protrusions  320  of the top and bottom plates  302 ,  304  to compress the first biasing element  312  during insertion of the implant  300 . 
     In another embodiment where the facet joint surface engaging plates are tapered, the plates may be separated by a single biasing element.  FIG. 8  is a side view of the arthroplasty implant  300  having one biasing element. As shown in  FIG. 8 , one or both of the biasing elements  312  and  314 , may be removed from the implant  300 .  FIG. 8  shows the implant  300  with the first biasing element  312  located adjacent to the trailing ends  308 , but does not include the second biasing element (shown in  FIG. 7  as biasing element  314 ). The embodiment of  FIG. 8  may provide for easier insertion of the implant  300  because the leading ends  306  have an increased taper by virtue of the single biasing element configuration and specifically the location of the biasing element near the trailing end of the implant. In use and as shown in  FIG. 9 , the arthroplasty implant  300  is positioned within a facet joint. The implant  300  may be positioned between an upper vertebra  316  of the facet joint and a lower vertebra  318  of the facet joint. The top plate  302  engages the superior joint surface at the upper vertebra  316  and the bottom plate  304  engages the inferior joint surface at the lower vertebra  318 . 
     Referring now to  FIGS. 10A-11 , in some aspects, the facet joint surface engaging plates are coupled together via a spring-like or biased compliant material.  FIG. 10A  is a side view of an arthroplasty implant, generally designated  400   a,  having a biasing element of a flexible core of compliant material. The implant  400   a  generally includes a first or top facet joint surface engaging plate  402   a,  a second or bottom facet joint surface engaging plate  404   a,  a biasing element  406   a,  such as flexible core  406   a,  and a plurality of teeth  410   a.  The top plate  402   a,  bottom plate  404   a,  and teeth  410   a  may each be made of similar materials and made in similar ways as the top plate  102 , bottom plate  104 , and teeth  110  as described above with respect to  FIG. 1 . The biasing element  406   a  may be made of a compliant material, such as silicone, polyethylene, or hydrogel. The compliant nature of the biasing element  406   a  may allow for relative motion between the top plate  402   a  and the bottom plate  404   a  in order to simulate the natural motion of the facet joint. 
       FIG. 10B  is a side view of an arthroplasty implant  400   b  having engagement plates and a biasing element, such as a flexible core of compliant material. The implant  400   b  of  FIG. 10B  may be similar to the implant  400   a  of  FIG. 10A  with similar components, but with the top plate  402   b  and the bottom plate  404   b  being larger or greater in size than the plates  402   a,    404   a  of  FIG. 10A . The top plate  402   b  and the bottom plate  404   b  may be larger than or have a greater width, length, perimeter and/or other corresponding dimension of the biasing element  406   b.  When the biasing element is compressed, the top plate  402   b  and the bottom plate  404   b  may still be greater in a width, length, perimeter, and/or other corresponding dimension of the compressed biasing element. In some embodiments, the top plate  402   b  and bottom plate  404   b  may be similar or dissimilar in size or shape. 
     In use, and as shown in  FIG. 11 , the arthroplasty implant  400  of  FIG. 10A  is positioned within a facet joint. The implant  400  may be positioned between the inferior facet of the superior vertebra  412  of the facet joint and the superior facet of the inferior vertebra  414  of the facet joint. As discussed above, as the facet joint flexes, extends, contracts, and/or rotates, the compliant material may deform while maintaining separation between the upper vertebra  412  and the lower vertebra  414 . 
     In some aspects, the facet joint surface engaging plates are coupled together via biasing element that may include a spring and a core.  FIG. 12  is a perspective view of an arthroplasty implant, generally designated  500 , having a biasing element that includes a wave-form spring and a central core. The implant  500  generally includes a first or top plate  502 , a second or bottom plate  504 , a biasing element  506 , a plurality of teeth  508 , and a central core  510 . Each of the top plate  502 , the bottom plate  504 , the biasing element  506 , and the plurality of teeth  508  may be implemented similarly to the top plate  102 , the bottom plate  104 , the biasing element  106 , and the plurality of teeth  108  as described above with respect to  FIG. 1 . The central core  510  may be made of a rigid or semi-rigid material such as metal, plastic, hard rubber, or other suitable polymer. The central core  510  may be cylindrical in shape, as shown in  FIGS. 12 and 13 . Alternatively, the central core  510  may be hemispherical in shape as shown in  FIGS. 14 and 15 . Other shapes may also be used. The central core  510  may be positioned in the center of and attached or coupled to the bottom plate  504 . Alternatively, the central core  510  may be attached or coupled to the top plate  502 , such as via mechanical means (e.g., thread, press-fit, fastener) or by adhesive. The central core  510  may limit the flexion, compression, and/or rotation of the top plate  502  relative to the bottom plate  504 . For example, as the biasing element is compressed, the top plate may contact the central core  510  before the top plate  502  reaches the bottom plate  504 . Accordingly, the amount by which the implant  500  may deform during movement may be limited by the central core  510 . The size and the shape of the central core  510  may be determined based on the desired amount of movement. For example, a biasing element that uses a smaller central core, as shown in  FIG. 14  may allow for greater range of motion than a comparatively larger central core  510  as shown in  FIG. 15 . Furthermore, a flat-topped cylindrical core, as in  FIGS. 12 and 13 , can limit the relative angular motion of the plates  502 ,  504  better than a spherical or hemi-spherical core, as in  FIGS. 14 and 15 and 16 . A biasing element that is spherical shaped, such as in  FIGS. 52-53 , or includes aspherical core, on the other hand, may allow for more complex movement of the plates  502 ,  504 , such as via a combined sliding and tilting motion. 
     In some aspects, the core or the implant in general may be compressed for insertion into the facet joint.  FIG. 17A  is a perspective view of an arthroplasty implant, generally designated  600   a.    FIG. 18A  is a side view of the implant  600   a.  The implant  600   a  generally includes a first cylinder body  602   a,  a second cylinder body  604   a,  and a core  608   a.  The first cylinder  602   a  may include an engagement feature  606   a,  such as a notch, for releasing the first cylinder, as discussed in further detail below with respect to  FIG. 25-28 . The core  608   a  may be positioned at least partially within the first cylinder. As shown in  FIG. 17A , at least a portion of the first cylinder  602   a  may be housed within the second cylinder  604   a  until implanted in the facet joint and allowed to transition from the compressed state to the expanded state. 
       FIG. 19A  is an exploded view of the implant  600   a.  The first cylinder  602   a  may include a threaded portion  612   a  at its distal end for securing the first cylinder  602   a  to the second cylinder  604   a,  as discussed in further detail with respect to  FIGS. 20A, 21A, and 22A . As shown in  FIG. 19A , the core  608   a  may include a cap  614   a,  which has a diameter that is less than a diameter of the main core body  608   a.  The cap  614   a  and core  608   a  may define a ledge  616   a  which may be complementary to and configured for receipt at a surface (shown in  FIG. 20 ) within the first cylinder  602   a.    
     The implant  600   a  may further include a biasing element  610   a.  The biasing element  610   a  may be, for example, a coil spring. Other types of biasing elements may also be used. The biasing element  610   a  may be positioned between the core  608   a  and the second cylinder  604   a.  When the implant  600   a  is compressed, the biasing element  610   a  may provide an expansion force against the core  608   a  and the second cylinder  604   a.  Because the ledge  616   a  is also in contact with a complementary surface within the first cylinder  602   a,  the expansion force may also be transferred to the first cylinder  602   a.    
       FIG. 20  is a sectional view of the arthroplasty implant of  FIG. 17A , taken along line  20 A- 20 A. As shown in  FIG. 20  and discussed above, the first cylinder  602   a  may have a surface  624   a  which is complementary to the ledge  616   a  such that any expansion force exerted by the biasing element  610   a  on the core  608   a  is transferred to the first cylinder  602   a.    
       FIG. 20  also shows a first channel  620   a  defined in the core  608   a  and configured to receive at least a portion of the biasing element  610   a  and a second channel  622   a  defined in the second cylinder  604   a  and configured to receive at least a portion of the biasing element  610   a.  The channels may maintain the biasing element in place along a central axis of the core  608   a  and the second cylinder  622   a.    
     The second cylinder  604   a  may further include a complementary threaded portion  618   a  configured to engage the threaded portion  612   a  of the first cylinder  602   a  and hold the implant  600   a  in a compressed state until released after insertion in the facet joint.  FIG. 21A  is a side view of the arthroplasty implant of  FIG. 17A  in a compressed position. As shown in  FIG. 21A , the threaded portion  612   a  of the first cylinder  602   a  engages the threaded portion  618   a  of the second cylinder  604   a  to hold the implant  600   a  in a compressed position. When the implant is in the compressed position, the biasing element  610   a  may also be in a compressed position.  FIG. 22A  is a side view of the arthroplasty implant of  FIG. 17A  in an expanded position. The first cylinder  602   a  may be rotated relative to the second cylinder  604   a  to disengage the threaded portions  612   a  and  618   a.  Once disengaged, the biasing element  610   a  may release, applying biasing against the first cylinder  602   a  and the second cylinder  604   a  to place the implant  600   a  in an expanded position. 
     An additional embodiment of arthroplasty implant  600   b  is shown in  FIGS. 17B, 18B, 19B, 21B, and 22B . Implant  600   b  may be similar to the implant  600   a.  An additional embodiment of an arthroplasty implant  600   c  similar to implant  600   b  is shown in  FIG. 17C . The implants  600   b,    600   c  generally include a first cylinder body  602   b,    602   c  and a second cylinder body  604   b,    604   c.  The first cylinder  602   b,    602   c  may include a feature  606   b,    606   c  for releasing the first cylinder  602   b,    602   c  from the second cylinder  604   b,    604   c.  A difference between the implants  600   b,    600   c  is the feature  606   b  of  FIG. 17B  is a raised engagement feature that may include a keyway or notch or other mating device. The feature  606   c  of  FIG. 17C  is a recessed engagement feature that may include a keyway or notch or other mating device. In some examples, the keyway or notch may be shaped to receive a hex bit, torx bit, star bit, Philips head, flat head, or other type of keyed tool. In use, the engagement feature may be used to release the first cylinder body from the second cylinder body. 
       FIG. 19B  is an exploded view of the arthroplasty implant  600   b  of  FIG. 17B .  FIG. 21B  is a sectional view of the arthroplasty implant  600   b,  taken along line  21 B- 21 B, in a compressed position.  FIG. 22B  is a sectional view of the arthroplasty implant  600   b,  taken along line  21 B- 21 B, in an expanded position. 
     As shown in  FIG. 19B , the first cylinder  602   b  of the implant  600   b  may include a threaded portion  612   b  at or adjacent a distal end. The threaded portion  612   b  may include externally formed or male threads. As shown in  FIG. 20B , the second cylinder  604   b  may also include a threaded portion  618   b  extending around a portion of the inner circumference of the second cylinder. The threaded portion  618   b  may include internally formed or female threads. The threaded portions  612   b  and  618   b  may be complementary to help releasably secure the first cylinder  602   b  to the second cylinder  604   b.    
     The implant  600   b  also includes a biasing element  610   b  to help adjust the position of the first cylinder  602   b  with the second cylinder  604   b.  In some examples, at least one or both of the first cylinder  602   b  and the second cylinder  604   b  may include a groove to help position or align the biasing element  610   b  with respect to the cylinder. This groove does not interfere with the rotation of the first cylinder  602   b  with the second cylinder  604   b.    
       FIG. 21B  shows the implant  600   b  with the biasing element  610   b  compressed between the first cylinder  602   b  and the second cylinder  604   b  as the threads  612   b  and  618   b  are engaged.  FIG. 22B  shows the implant  600   b  with the biasing element expanded between the first cylinder  602   b  and the second cylinder  604   b,  and were the threads  612   b  and  618   b  are no longer engaged. Once the threads  612   b,  and  618   b  are disengaged, the first cylinder  602   b  may be directly disengaged from second cylinder  604   b,  but still indirectly engaged with the second cylinder  604   b  via the biasing element  610   b.    
       FIGS. 23-28  are perspective views of the insertion and release of the implant  600   a  of  FIG. 17A  into a facet joint where some or all of the delivery tools are hidden for clarity. The insertion and release of the implants  600   b  and  600   c  into a facet joint may be similar to the insertion and release of the implant  600   a.  As shown in  FIGS. 23 and 24 , the implant  600   a  may be inserted into a facet joint between an upper vertebra  626  and a lower vertebra  628  while in a compressed configuration. For example, the threaded portion  612   a  of the first cylinder  602   a  may be engaged with the threaded portion  618   a  of the second cylinder  604   a  to hold the biasing element  610   a  in a compressed position. As shown in  FIGS. 25 and 26 , a release tool  630 , such as a rod or probe, may be used to engage the engagement feature  606   a,  such as notch  606   a,  of the implant  600   a.  The release tool  630  engages the engagement feature causing the first cylinder  602   a  to rotate relative to the second cylinder  604   a  and disengage the threaded portion  612   a  of the first cylinder  602   a  from the threaded portion  618   a  of the second cylinder  604   a.  As shown in  FIGS. 27 and 28 , once the first cylinder  602   a  has rotated a sufficient distance relative to the second cylinder  604   a,  the threaded portion  612   a  is completely disengaged from the threaded portion  618   a,  allowing the biasing element  610   a  to expand and push the first cylinder  602   a  away from the second cylinder. The implant  600   a  may then expand to hold it in place within the facet joint between the upper vertebra  626  and the lower vertebra  628 . Once the implant  600   a  is in place and expanded, the release tool  630  may be removed as shown in  FIG. 28 . 
     In some aspects, the core and/or the implant may be compressed via a set screw for insertion into the facet joint.  FIG. 29  is a perspective view of a arthroplasty implant, generally designated  700 . The implant  700  generally includes a bottom cylindrical body  702 , a top cylindrical body  704 , and a set screw  706 . The top cylindrical body and the bottom cylindrical body may each be made of any suitable rigid material as discussed above with respect to  FIG. 1 . The top cylindrical body  702  may have a diameter that is larger than a diameter of the top cylindrical body  704 . The bottom cylindrical body  702  may also have a cavity defined therein and configured to receive the top cylindrical body  704 . For example, the cavity may be substantially cylindrical in shape and have a diameter that is sufficiently large to receive the top cylindrical body  704 . The set screw  706  may be any type of fastener that is capable of holding the top cylindrical body  704  and the bottom cylindrical body  702  in a compressed position. 
       FIG. 30  is a sectional view of the arthroplasty implant of  FIG. 29  in an expanded position taken along the line  30 - 30 . As shown in  FIG. 30 , the implant  700  may further include a biasing element  712 . The biasing element  712  may be a wave-form spring, as shown in  FIG. 30 , or any other suitable biasing element, such as coil spring, etc. The biasing element  712  may be positioned within the cavity of the bottom cylindrical body  702  and between a base of the bottom cylindrical body  722  and a base of the top cylindrical body  724 . By placing the biasing element  712  between the bottom cylindrical body  702  and the top cylindrical body  704 , the biasing element may apply an expansionary force to the top cylindrical body  704  and the bottom cylindrical body  702  causing the implant  700  to expand, which may aid in holding the implant  700  in place within the facet joint. 
     The bottom cylindrical body  702  may have a first hole or aperture  710  formed therethrough and configured to receive the set screw  706 . The top cylindrical body  704  may also have a second hole or aperture  714  formed therein and configured to receive the set screw  706 . In various embodiments, when the implant  700  is in a compressed position, the first hole  710  and the second hole  714  may align and the set screw  706  may be inserted into both the first hole  710  and the second hole  714  to hold the implant  700  in the compressed position. In some embodiments, the set screw may have a threaded portion  708  to engage a complementary threaded portion in the hole  710  and/or the hole  714 . As shown in  FIG. 31 , when the implant  700  is in an expanded position, the hole  710  and the hole  714  may be misaligned such that the set screw  706  cannot be inserted. When compressed, the hole  710  and the hole  714  align, allowing for insertion of the set screw  706 , as shown in  FIG. 32 . 
       FIG. 33  is a perspective view of the arthroplasty implant  700  positioned in a facet joint in a compressed position. As shown in  FIG. 33 , the implant  700  may be positioned within a facet joint between an upper vertebra  716  and a lower vertebra  718 .  FIG. 34  is a perspective view showing the set screw being removed. A release tool  720  may be used to remove the set screw  706  from the implant  700 . For example, the set screw  706  may be configured to receive an end of the release tool  720 , such as a standard hex key, Philips head screw, or flat head screw. The release tool  720  may engage and rotate the set screw  706  until the screw is completely removed from the implant  700  (see  FIG. 35 ). Once the set screw  706  is disengaged from the implant  700 , the release tool  720  and the set screw  706  may be removed and the implant  700  may expand within the facet joint as the biasing element  712  expands. 
     In some aspects, the facet joint engagement plates may include features that contact other surfaces of the vertebrae for fixation of the implant. For example,  FIG. 36  is a side view and  FIG. 37  is a sectional view of an arthroplasty implant, generally designated  800 , having attachment tabs. The implant  800  may include features as described previously such as facet joint surface engagement plates and a biasing element, such as a spring, flexible material or spring-like member as described elsewhere herein positioned therebetween. The implant  800  may include any number of attachment tabs configured to secure the implant to the posterior or lateral edges of the superior and inferior vertebral lateral masses. In the embodiment of  FIG. 36 , the implant  800  includes a first attachment tab  802  coupled to a first or top facet joint engagement plate  804  and a second attachment tab  802  coupled to a second or bottom facet joint engagement plate  806 . The attachment tabs  802  may be made of any suitable material, such as metal, plastic, or other material. The attachment tabs  802  may be fixed in place with respect to the implant  800  or may be hinged, so as to allow for adjustment of the attachment tabs  802  to best secure the implant  800  within the facet joint. As shown in  FIG. 36 , the attachment tabs  802  may extend substantially parallel to the top plate  804  and the bottom plate  806 .  FIG. 38  is similar to the embodiment of  FIG. 36  except that the tabs  802  and engagement plates  804 ,  806  are longer and have a smaller width. In  FIGS. 39-40 , the attachment tabs  802  may be positioned at an angle with respect to the top plate  804  and the bottom plate  806 . The angle may be selected so as to make the attachment tabs  802  engage the posterior or lateral edges of the superior and inferior vertebral lateral masses.  FIG. 41  is similar to the embodiment of  FIGS. 39-40  except that the tabs  802  and engagement plates  804 ,  806  are longer and have a smaller width. 
     As shown in  FIGS. 42-43 , the attachment tabs  802  may be positioned substantially perpendicular to the top plate  804  and the bottom plate  806 .  FIG. 44  is similar to the embodiment of  FIGS. 42-43  except that the tabs  802  and engagement plates  804 ,  806  are longer and have a smaller width. 
     In some aspects, the attachments tabs  802  include openings configured to receive fasteners.  FIG. 45  is a side view of the arthroplasty implant  800  with fasteners, such as attachment screws,  808 .  FIG. 46  is similar to the implant of  FIG. 45  except the central or flexible core is removed. The attachment screws  808  may be inserted through openings formed through the attachment tabs  802 . The attachment screws  808  may be threaded to secure the attachment screws  808  to the vertebrae.  FIGS. 47 and 48  illustrate the implant positioned in a facet joint and fastened with attachment screws. As shown in  FIGS. 47-48 , one of the attachment screws  808  may secure the implant  800  to the posterior or lateral edges of an upper vertebra  810  of the facet joint, and another attachment screw  808  may secure the implant  800  to the posterior or lateral edges of lower vertebra  812 . 
       FIGS. 49 and 50  illustrate another fixation mechanism for the implant, generally designated  900  and having rounded surfaces. The implant  900  generally includes a first or top plate  902 , a second or bottom plate  904 , a biasing element  906  positioned between the top plate  902  and the bottom plate  904 , and rounded surfaces  908 . Each of the top plate  902 , bottom plate  904 , and biasing element  906  may be implemented in a similar manner as the top plate  102 , bottom plate  104 , and biasing element  106  described above with respect to  FIG. 1  or elsewhere herein. In some aspects, the rounded surfaces  908  may be covered in a compliant, high-friction material to retain the implant  900  in the facet joint. The rounded surfaces  908  may be positioned on the top plate  902  and the bottom plate  904  opposite the biasing element  906 . In some aspects, the rounded surfaces  908  may be rigid and rounded to mimic the profile of the facet joint. As shown in  FIG. 51 , the implant  900  may be positioned within a facet joint having an upper vertebra  910  and a lower vertebra  912 . The rounded surfaces  908  may contact the upper vertebra  910  and the lower vertebra  912  to hold the implant  900  in place within the facet joint. 
       FIGS. 52 and 53  are views of an arthroplasty implant.  FIG. 52  is a side view of an arthroplasty implant  1000 .  FIG. 53  is a sectional view of the arthroplasty implant  1000  of  FIG. 52  along line  53 - 53 . The implant  1000  may be similar to the implant  400  of  FIG. 10A . The implant includes a top plate  1002  and a bottom plate  1004 , with a biasing element  1006 , such as an elastomer or a flexible core, positioned between the plates. Each plate  1002 ,  1004  may include teeth  1008  extending from an outer surface and that may be used to engage with a vertebra surface. As shown in  FIG. 53 , the inside surfaces  1010  and  1012  of the top and bottom plates  1002 ,  1004  may form a shaped surface, such as a concave or convex surface. The biasing element  1006  may be shaped to interact or complement the shaped surfaces  1010 ,  1012 . In some embodiments, the biasing element  1006  may be spherical shaped and the inner surfaces  1010 ,  1012  are concave shaped. This configuration may allow for the implant  1000  to include a tilting or rolling movement in addition to vertical compression when inserted or used within a facet joint. 
       FIG. 54  is a side view of an arthroplasty implant  1100 . The implant  1000  may be similar to the implant  200  of  FIG. 4 . The implant  1100  includes a top plate  1102 , a bottom plate  1104 , and a biasing element  1106 . The biasing element  1106  may be frustum or conical shape, in some examples a coil spring or elastomer, with one end being larger than the other. This shape of the spring may allow for the implant  1100  to include a tilting or rolling movement in addition to vertical compression when inserted or used within a facet joint. 
       FIG. 55  is a side view of an arthroplasty implant  1200 .  FIG. 56  is an exploded view of the arthroplasty implant  1200 . In some aspects, the implant  1200  may be similar to the implant  100  of  FIG. 1 . The implant  1200  includes a top plate  1202  with teeth  1208  extending from an outer surface or upper surface, a bottom plate  1204  with teeth  1208  extending from an outer surface or a lower surface, and a biasing element  1206  positioned between the top plate  1202  and the bottom plate  1204 . In addition, a sleeve  1210  may extend around an outer perimeter of the assembly of the plates  1202 ,  1204  and biasing element  1206 . The sleeve  1210  may be formed of an elastomer or flexible material with a stiffness that is similar to or greater than a stiffness of the biasing element  1206 . The sleeve  1210  may help encapsulate the assembly and provide stability to the implant. In some examples, the sleeve  1210  may help dampen the biasing element  1206  to aid in the implant insertion assembly and in use once the implant is implanted between vertebrae. This may be helpful when the loading on the plates is uneven. Uneven loading may cause the implant to undesirably move or shift positions. Dampening of the biasing element may provide stability for the implant to prevent undesirably movement or shifting. 
       FIGS. 57-59  are views of an arthroplasty implant  1300 .  FIG. 57  is a perspective view of an arthroplasty implant.  FIG. 58  is a rear view of the arthroplasty implant of  FIG. 57  shown in a first compressed position.  FIG. 59  is a rear view of the arthroplasty implant of  FIG. 57  shown in an alternate compressed position. In some aspects, the implant  1300  may be similar to the implant  1000  of  FIG. 52 . The implant  1300  includes a top plate  1302  and a bottom plate  1304 . Similar to implant  1000 , the inner surface  1310  and  1312  of the top and bottom plates  1302 ,  1304  is concave shaped. The implant  1300  also includes a bias element  1306 , such as an elastomer. In the implant  1300 , the bias element  1306  may be cylindrically shaped, with either solid or hollow central section. The bias element  1306  may be positioned between the top and bottom plates  1302 ,  1304 , with the round outer edges of the bias element  1306  adjacent the concave surfaces  1310 ,  1312 . In addition to vertical compression, the configuration of the implant  1300  may allow the long edges  1314 ,  1316  of the top and bottom plates  1302 ,  1304  to rotate towards and away from each other, for example in  FIG. 59 , about an axis of the bias element  1306 . The shape may also allow for the bias element  1306  to be unevenly compressed, for example, where the ends  1318  and  1320  are compressed towards each other, while the ends that are opposite ends  1318 ,  1320  are compressed to a lesser amount. 
       FIG. 60  is a side view of an arthroplasty implant  1400 . The implant  1400  of  FIG. 60  may be similar in aspect to the implant  1100  of  FIG. 54 . The implant  1400  includes a top plate  1402 , a bottom plate  1404 , and a biasing element  1406 . In the implant of  1400 , the basing element allows for both a vertical compression of the implant, in either an even application or uneven application, and may also allow for a controlled translational movement of the plates  1402 ,  1404 . In some embodiments, the biasing element  1406  may include a vertical wave-form spring, or other biasing element that allows the described controlled movement. 
     All relative and directional references (including: upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, side, above, below, front, middle, back, vertical, horizontal, and so forth) are given by way of example to aid the reader&#39;s understanding of the particular embodiments described herein. They should not be read to be requirements or limitations, particularly as to the position, orientation, or use unless specifically set forth in the claims. Connection references (e.g., attached, coupled, connected, joined, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other unless specifically set forth in the claims. 
     Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. Thus, it is intended that the scope of the present disclosure should not be limited by the particular embodiments described above.