Abstract:
A system for a dynamic facet replacement includes first and second inferior facets, first and second pars and first and second superior facets. The first and second inferior facets are configured to replace left and right natural inferior facets and each facet comprises an articulating surface. The first and second superior facets are configured to replace left and right natural superior facets and each facet comprises an articulating surface. Each of the first and second pars includes first and second articulating surfaces and is configured to articulately connect the first and second articulating surfaces with the first articulating surfaces of the inferior and superior facets, respectively.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 60/946,422 filed Jun. 27, 2007 and entitled “DYNAMIC FACET REPLACEMENT SYSTEM”, the contents of which are expressly incorporated herein by reference. 
     This application is also a continuation in part of U.S. application Ser. No. 11/852,379 filed on Sep. 10, 2007 and entitled “APPARATUS AND METHOD FOR CONNECTING SPINAL VERTEBRAE” the contents of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system and method for dynamic facet replacement, and in particular to a facet joint replacement that connects adjacent spinal vertebrae while preserving spinal stability and mobility. 
     BACKGROUND OF THE INVENTION 
     The human spine  29  comprises individual vertebrae  30  that interlock with each other to form a spinal column, shown in  FIG. 1A . Referring to  FIGS. 1B ,  1 C,  2 A,  2 B,  3  and  4 , each vertebra  30  has a cylindrical bony body (vertebral body)  32 , two pedicles  48   a ,  48   b  extending from the vertebral body  32 , a lamina  47  extending from the pedicles  48   a ,  48   b , three winglike projections (two transverse processes  33 ,  35  extending from the pedicles  48   a ,  48   b , respectively, and one spinous process  34  extending from the lamina  47 ), pars interarticularis  36   a ,  36   b , two superior facets  46   a ,  46   b  extending from the pedicles  48   a ,  48   b , respectively, and two inferior facets  45   a ,  45   b  extending from the lamina  47 . The pars interarticularis  36   a ,  36   b  connect the superior  46   a ,  46   b  and inferior  45   a ,  45   b  facets of the vertebra, respectively, on either side of the spinous process  34 . The bodies of the vertebrae  32  are stacked one on top of the other and form the strong but flexible spinal column. The spinous process  34 , lamina  47 , pars interarticularis  36   a ,  36   b , superior facets  46   a ,  46   b , inferior facets  45   a ,  45   b , transverse processes  33 ,  35 , and pedicles  48   a ,  48   b  are positioned so that the space they enclose forms a tube, i.e., the spinal canal  37 . The spinal canal  37  houses and protects the spinal cord and other neural elements. A fluid filled protective membrane, the dura  38 , covers the contents of the spinal canal. The spinal column is flexible enough to allow the body to twist and bend, but sturdy enough to support and protect the spinal cord and the other neural elements. The inferior facets  45   a ,  45   b  of one vertebra fit perfectly into the superior facets  46   a ,  46   b  of the vertebra below it, thereby forming left and right facet joints  50   a ,  50   b . The facet joints  50   a ,  50   b  provide stability and guide motion in the spine. Like the bones that form other joints in the human body, such as the hip, knee, or elbow, the articular surfaces of the facet joints are covered by a layer of smooth cartilage, surrounded by a strong capsule of ligaments, and lubricated by synovial fluid. 
     The vertebrae  30  are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs  40 . Inter-vertebral discs  40  provide flexibility to the spine and act as shock absorbers during activity. There is a small opening (foramen)  42  between each vertebra  30 , through which nerves  44  pass and go to different body parts. When the vertebrae are properly aligned the nerves  44  pass through without a problem. However, when the vertebrae are misaligned or a constriction  15  is formed in the spinal canal, the nerves get compressed  44   a  and may cause back pain, leg pain or other neurological disorders. Disorders of the spine that may cause misalignment of the vertebrae or constriction of the spinal canal include spinal injuries, infections, tumor formation, herniation of the inter-vertebral discs (i.e., slippage or protrusion), arthritic disorders, and scoliosis. In these pathologic circumstances, surgery may be tried to either decompress the neural elements and/or fuse adjacent vertebral segments. Decompression may involve laminectomy, discectomy, or corpectomy. Laminectomy involves the removal of part of the lamina  47 , i.e., the bony roof of the spinal canal. Discectomy involves removal of the inter-vertebral discs  40 . Corpectomy involves removal of the vertebral body  32  as well as the adjacent disc spaces  40 . Laminectomy and corpectomy result in central exposure of the dura  38  and its contents. An exposed dura  38  puts the neural elements and spinal cord at risk from direct mechanical injury or scarring from overlying soft tissues. Scarring is considered a major cause for failed back syndrome in which patients continue to have back and leg pain after spinal surgery. Current methods to decrease the risk of developing this syndrome include covering the dura with fat harvested from the patient&#39;s subcutaneous tissues or using a synthetic material. However, no material as yet has been used that completely or significantly prevents scarring of the dura and nerve roots after spine surgery in humans. 
     Furthermore, laminectomy predisposes the patient to instability through the facet joints and may lead to post-laminectomy kyphosis (abnormal forward curvature of the spine), pain, and neurological dysfunction. Therefore the surgeon needs to stabilize the spine after laminectomy procedures and after corpectomy. One spine stabilization method is fusion. Fusion involves the fixation of two or more vertebrae. Fusion works well because it stops pain due to movement of the intervertebral discs  40  or facets  45   a ,  45   b ,  46   a ,  46   b , immobilizes the spine, and prevents instability and or deformity of the spine after laminectomy or corpectomy. However, spinal fusion limits spinal mobility. Maintaining spinal mobility may be preferred over fusion in some cases to allow more flexibility of the spine and to decrease the risk of junction problems above and below the level of the fixation due to increased stress. 
     An arthritic facet joint may also cause back pain (facet arthropathy). Since the majority of the motion along the spine occurs at the facet joints, fusing the diseased facet would often relieve pain but again at a high cost of fusing across at least one spinal segment thus preventing motion and effectively increasing stresses at the adjacent facet joints. Increased stresses predispose facet joints to accelerated arthritis, pain, and instability requiring additional surgery to fuse these levels. This cyclic process results in an overall decreased mobility of the spine. Therefore, it is an attractive alternative to attempt to replace the diseased facet without resorting to fusion, thus avoiding significant limitation in mobility of the spine. The obvious solution would be to replace the opposing surfaces of each facet to preserve motion between the surfaces. However, any efforts to replace the facets at their natural location necessitate destroying the facet capsule and risks producing an unstable joint. Therefore, it is desirable to achieve spine stabilization that preserves mobility, and does not cause tissue scarring or destroy the facet capsule. It is also desirable to be able to implant the stabilization device percutaneously utilizing minimally invasive surgery. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method for dynamic facet replacement, and in particular to a facet joint replacement that connects adjacent spinal vertebrae while preserving spinal stability and mobility. 
     In general in one aspect the invention features a dynamic facet replacement system articulately connecting a first spinal vertebra to an adjacent second spinal vertebra and the second spinal vertebra to an adjacent third spinal vertebra, along the natural facet joints. The facet replacement system includes first and second inferior facet components configured to replace left and right natural inferior facets of the first vertebra, respectively, first and second par components configured to replace left and right natural pars of a second vertebra, respectively, and first and second superior facet components configured to replace left and right natural facets of a third vertebra, respectively. Each of the inferior and superior facet components comprises a facet articulating surface and each of the par components comprises first and second par articulating surfaces. The first and second par components are shaped and dimensioned to be inserted between the first and second inferior and superior facet components, respectively, and to articulately connect the first and second inferior facet components to the first and second superior facet components, respectively, by connecting the first and second par articulating surfaces to the facet articulating surfaces of the inferior and superior facet components, respectively. 
     Implementations of this aspect of the invention may include one or more of the following features. The connection of the par components to the inferior and superior facet components comprises a surface-to-surface articulation mechanism or a constrained articulation mechanism. The constrained articulation mechanism comprises a male articulation component engaging a female articulation component. Each of the inferior facet components comprises a first extension member protruding from the inferior facet articulating surface and each of the par components comprises a first groove formed in the first par articulating surface and wherein the first groove is shaped and dimensioned to receive the first extension member and thereby to articulately connect the inferior facet component to the par component. Each of the par components further comprises a second extension member protruding from the second par articulating surface and wherein the superior facet component comprises a second groove formed in the superior facet articulating surface and wherein the second groove is shaped and dimensioned to receive the second extension member and thereby to articulately connect the superior facet component to the par component. Each of the inferior facet components comprises an elongated curved body and the body comprises a first cylindrical shaped end, configured to be attached to a location of the first vertebra and a second cylindrical shaped end comprising the inferior facet articulating surface. In each of the inferior facet components, the first cylindrically shaped end&#39;s axis is oriented perpendicular to the second cylindrical shaped end&#39;s axis. Each of the par components comprises an elongated curved body and the body comprises a first cylindrical shaped end, configured to be attached to a location of the second vertebra, a second cylindrical shaped end comprising the second par articulating surface, and wherein the first cylindrically shaped end further comprises a wing extension comprising the first par articulating surface. In each of the par components the first cylindrically shaped end&#39;s axis is oriented perpendicular to the second cylindrical shaped end&#39;s axis. Each of the superior facet components comprises a cylindrically shaped end, configured to be attached to a location of the third vertebra and wherein the cylindrically shaped end further comprises a wing extension comprising the superior facet articulating surface. Any of the cylindrically shaped ends is attached to the vertebral locations via a poly-axial screw. Any of the vertebral locations comprise one of a pedicle, transverse processes, facets, pars interarticularis, intervertebral disc, lamina, or vertebral body. The dynamic facet replacement system comprises at least one of metal, plastic, ceramic, bone, polymers, composites, absorbable material, biodegradable material, or combinations thereof. The vertebras comprise one of cervical, thoracic, lumbar or sacrum vertebras. The male articulation component comprises an extension member and the female articulation member comprises a slot shaped and dimensioned to receive the extension member. The constrained articulation mechanism further comprises a locking member configured to fit over the slot and to lock the extension member within the slot. 
     In general in one aspect the invention features a method for articulately connecting a first spinal vertebra to an adjacent second spinal vertebra and the second spinal vertebra to an adjacent third spinal vertebra, along the natural facet joints. The method includes providing first and second inferior facet components configured to replace left and right natural inferior facets of a first vertebra, respectively, and attaching the first and second inferior facet components to first and second locations of the first vertebra, respectively. Each of the inferior facet components comprises an articulating surface. Next, providing first and second par components configured to replace left and right natural pars of a second vertebra, respectively, and attaching the first and second par components to first and second locations of the second vertebra, respectively. Each of the par components comprises first and second par articulating surfaces. Next, providing first and second superior facet components configured to replace left and right natural facets of a third vertebra, respectively, and attaching the first and second superior facet components to first and second locations of the third vertebra, respectively. Each of the superior facet components comprises an articulating surface. Finally, articulately connecting the first and second par articulating surfaces to the articulating surfaces of the inferior and superior facet components, respectively. The connection of the par components to the inferior and superior facet components comprises a constrained articulation mechanism and the constrained articulation mechanism comprises a male articulation component engaging a female articulation component. The male articulation component comprises an extension member and the female articulation member comprises a slot shaped and dimensioned to receive the extension member, and wherein the constrained articulation mechanism further comprises a locking member configured to fit over the slot and to lock the extension member within the slot. 
     Among the advantages of this invention may be one or more of the following. The implantable spinal stabilization device stabilizes the spine along the facet joints, while allowing the patient to retain spinal flexibility by preserving motion between adjacent vertebras. This spinal stabilization device may be implanted using minimally invasive surgery along lines left and right of the midline of the spinal column. The spinal stabilization device may be used for the treatment of a multitude of spinal disorders including facet arthritis and spinal stenosis. The implantable device has a compact structure and low profile. The implant can be inserted percutaneously along the sides of the spine without the need to make a large midline incision and stripping the erector spinal muscles laterally. There is no need to remove the posterior elements of the vertebras such as the spinous processes and lamina. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the figures, wherein like numerals represent like parts throughout the several views: 
         FIG. 1A  is a side view of the human spinal column; 
         FIG. 1B  is an enlarged view of area A of  FIG. 1A ; 
         FIG. 1C  is an axial cross-sectional view of a lumbar vertebra; 
         FIG. 2A  illustrates the facet joints of two adjacent vertebras; 
         FIG. 2B  illustrates another axial cross-sectional view of a lumbar vertebra and a side view of adjacent lumbar vertebras; 
         FIG. 3  is a front (posterior) perspective view of the lumbar section of the human spinal column; 
         FIG. 4  is a side view of the lumbar section of  FIG. 3 ; 
         FIG. 5  is a front (posterior) perspective view of an embodiment of the dynamic facet replacement system; 
         FIG. 6  is a side view of the dynamic facet replacement system of  FIG. 5   
         FIG. 7  is another front perspective view of the dynamic facet replacement system of  FIG. 5 ; 
         FIG. 8  is a front (posterior) perspective view of the inferior facet replacement assembly; 
         FIG. 9  is a side view of the inferior facet replacement assembly of  FIG. 8 ; 
         FIG. 10A  is a front perspective view of a facet replacement component of  FIG. 8 ; 
         FIG. 10B  is a side view of the facet replacement component of  FIG. 10A ; 
         FIG. 11A  is another front perspective view of the facet replacement component of  FIG. 8 ; 
         FIG. 11B  is a side perspective view of the facet replacement component of  FIG. 11A ; 
         FIG. 12  is a cross-sectional view of the facet replacement component of  FIG. 11B  along the axis A-A; 
         FIG. 13  is a partially exploded anterior view of the inferior facet replacement assembly of  FIG. 8 ; 
         FIG. 14  is a partially exploded posterior view of the inferior facet replacement assembly of  FIG. 8 ; 
         FIG. 15  is a front (posterior) perspective view of the pars replacement assembly; 
         FIG. 16  is a side perspective view of the pars replacement assembly of  FIG. 15 ; 
         FIG. 17  is front perspective view of a pars replacement component of  FIG. 15 ; 
         FIG. 18  is side perspective view of the pars replacement component of  FIG. 17 ; 
         FIG. 19  is a cross-sectional view of the pars replacement component of  FIG. 18  along the A-A axis; 
         FIG. 20  is a partially exploded posterior view of the pars replacement assembly of  FIG. 15 ; 
         FIG. 21  is a front (posterior) perspective view of the superior facet replacement assembly; 
         FIG. 22  is a side perspective view of the superior facet replacement assembly of  FIG. 21 ; 
         FIG. 23  is a front perspective view of a superior facet replacement component of  FIG. 21 ; 
         FIG. 24  is a top view of the superior facet replacement component of  FIG. 23 ; 
         FIG. 25  is a cross-sectional view of the superior facet replacement component of  FIG. 24  along the A-A axis; 
         FIG. 26  is a partially exploded view of the superior facet replacement assembly of  FIG. 21 ; 
         FIG. 27  is a top view of the partially exploded superior facet replacement assembly of  FIG. 26 ; 
         FIG. 28  is a front (posterior) perspective view of the interfaces (joints) between the pars assembly with the superior facet replacement assembly and the pars assembly with the inferior facet replacement assembly; 
         FIG. 29  is a side perspective view of  FIG. 28 ; 
         FIG. 30  is detailed view of the facet articulation mechanism; and 
         FIG. 31  is another embodiment of a facet articulation mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a system and method for dynamic facet replacement, and in particular to a facet joint replacement that connects adjacent spinal vertebrae while preserving spinal stability and mobility. 
     Referring to  FIG. 5 ,  FIG. 6  and  FIG. 7  a dynamic facet replacement system  100  includes an inferior facet replacement assembly  110 , pars replacement assembly  120  and a superior facet replacement assembly  130 . The inferior facet replacement assembly  110  includes first and second inferior facet replacement components  110   a ,  110   b . The pars replacement assembly  120  includes first and second pars replacement components  120   a ,  120   b . The superior facet replacement assembly includes first and second superior facet replacement components  130 ,  130   b . In the embodiment of  FIG. 5  the inferior facet replacement components  110   a ,  110   b  articulate with the pars replacement components  120   a ,  120   b , respectively, and the pars replacement components  120   a ,  120   b , articulate with the superior facet replacement components  130   a ,  130   b , respectively, to dynamically stabilize the facet joints  50   a ,  50   b  and  51   a ,  51   b  between the adjacent vertebras  30   a ,  30   b  and  30   b ,  30   c , respectively. In this embodiment the articulation between the inferior facet replacement components or the superior facet replacement components and the pars replacement components is surface to surface articulation. In other embodiments the articulation is a constrained articulation. In one embodiment the constrained articulation is by engaging a male articulation component with a female articulation component, as described in the co-pending patent application Ser. No. 10/660,927 entitled “Apparatus and method for connecting spinal vertebrae”, the contents of which are incorporated herein by reference. In the embodiment of  FIG. 30  the articulation mechanism is a tongue and groove type articulation. As shown in  FIG. 30 , one surface includes a groove  240  and the opposite articulating surface includes an extension  210  that fits into the opposite groove to form a tongue and groove type connection. Extension  210  may be snapped in or slidably inserted into the groove  200 . In other embodiments, extension  210  is placed in a slot and then rotated to engage and lock into a groove communicating with the slot. Furthermore, in other embodiments any of the described replacement components articulate with the surfaces of the natural facets. In another embodiment of the articulation mechanism, shown in  FIG. 31 , a first articulating member  250   b  includes and extension member  210  that fits into a slot  220  of an opposite second articulating member  240   b . An elongated locking member  230  fits over the open end of the slot  220  and locks the extension member  210  in the slot  220 . The locking member  230  includes an elongated body having a portion  234  that fits over the slot  220  and a portion that includes a through opening  236 . Opening  236  is aligned with an opening  238  in the second articulating member  240   b  and a bolt or screw  235  is inserted into openings  236  and  238  to lock locking member  230  onto articulating member  240   b  and extension member  210  in the slot  220 . 
     Referring to  FIG. 8  and  FIG. 9 , the inferior facet replacement assembly  110  includes facet replacement components  110   a ,  110   b  that articulate with facet-like surfaces of the pars  120   a ,  120   b , respectively, as shown in  FIG. 5 , or with the natural superior facets  46   a ,  46   b  of the adjacent vertebra  30   b . Each inferior facet replacement component  110   a ,  110   b , includes an elongated curved body  112   a ,  112   b  having a cylindrical shaped first end  114   a ,  114   b  and cylindrical shaped second end  116   a ,  116   b . The axis  113  of the cylindrical shaped second end  116   b  is oriented perpendicular to the axis  115  of the cylindrical shaped first end  114   a , as shown in  FIG. 10A . The cylindrical second end  116   b  extends away from the main body  112   b  and has a portion that overhangs in the direction of  113   b . The cylindrical second end  116   b  has an elliptical first surface  118   b  configured to articulate with the natural superior facet or any other facet-like surface. The cylindrical shaped first end  114   b  has a through opening  119  dimensioned to receive a fixation element  150   b . In the example of  FIG. 8  the fixation element  150  is an elongated poly-axial screw and is used to anchor the facet replacement component  110   b  to pedicle  48   b  of the vertebra  30   a . The poly-axial screw  150  includes a spherical head  156  and an elongated body  152  having outer threads  154 . The spherical head  156  includes a cutout  158  on the top dimensioned and configured to receive a screwdriver, shown in  FIG. 12 . Once the screw is anchored in the desired vertebral location, the cylindrical shaped first end  114   b  is rotated and oriented to position the main body  112   b  so that the elliptical surface  118   b  of the second cylindrical end  116  articulates with a facet-like surface of the replacement par or a natural facet. Once the desired orientation of the main body  112   b  is set, the first cylindrical shaped end  114   b  is secured onto the spherical screw head  156  with a set screw  160 , as shown in  FIG. 12  and  FIG. 13 . Other vertebral locations where the fixation element  150  may be anchored include the vertebral body, lamina or the processes. 
     Referring to  FIG. 15  and  FIG. 16 , the pars replacement assembly  120  includes pars replacement components  120   a ,  120   b  that articulate with facet-like surfaces of the superior and inferior facet replacement components  110   a ,  110   b , and  130   a ,  130   b , respectively, as shown in  FIG. 5 , or with the natural superior facets  46   a ,  46   b  and inferior facets  45   a ,  45   b  of the adjacent vertebras  30   a ,  30   c , respectively. Each par replacement component  120   a ,  120   b , includes an elongated curved body  122   a ,  122   b  having a cylindrical shaped first end  124   a ,  124   b  and cylindrical shaped second end  128   a ,  128   b . The axis  123  of the cylindrical shaped second end  128   b  is oriented perpendicular to the axis  125  of the cylindrical shaped first end  124   b , as shown in  FIG. 17 . The cylindrical first end  124   b  also includes a wing-like extension  126   b  having an elliptical surface  129   a  configured to articulate with a facet-like surface of an inferior facet replacement component or a natural inferior facet. The cylindrical second end  128   b  extends away from the main body  122   b  and has a portion that overhangs in the direction of  123   b . The cylindrical second end  128   b  has an elliptical first surface  129   b  configured to articulate with a facet-like surface of the superior facet replacement component, shown in  FIG. 5  or a natural superior facet. The cylindrical shaped first end  124   b  has a through opening  89  dimensioned to receive a fixation element  150 . In the example of  FIG. 17  the fixation element  150  is an elongated poly-axial screw and is used to anchor the par replacement component  110   b  to pedicle  48   b  of the vertebra  30   b . The poly-axial screw  150  includes a spherical head  156  and an elongated body  152  having outer threads  154 . The spherical head  156  includes a cutout on the top  158  dimensioned and configured to receive a screwdriver. Once the screw is anchored in the desired vertebral location, the cylindrical shaped first end  124   b  is rotated and oriented to position the main body  122   b  so that the elliptical surfaces  129   a  of the wing-like extension of the first end and  129   b  of the second cylindrical end  128   b  articulate with facet-like surfaces of the inferior facet replacement component and the superior facet replacement component, respectively, or the inferior and superior natural facets. Once the desired orientation of the main body  122   b  is set, the first cylindrical end is secured onto the spherical screw head  150  with a setscrew  160 , as shown in  FIG. 20 . 
     Referring to  FIG. 21  and  FIG. 22 , the superior facet replacement assembly  130  includes superior facet replacement components  130   a ,  130   b  that articulate with facet-like surfaces of the pars  120   a ,  120   b , respectively, as shown in  FIG. 5  and  FIG. 28 , or with the natural inferior facets  45   a ,  45   b  of the adjacent vertebra  30   b . Each superior facet replacement component  130   a ,  130   b , includes a cylindrical shaped first end  134   a ,  134   b  and a wing-like extension  132   a ,  132   b , extending from the cylindrical shaped first end  134   a ,  134   b , respectively. The wing-like extension  132   a  has an elliptical surface  133   a  configured to articulate with a facet-like surface of an inferior facet replacement component, or par replacement component or a natural inferior facet. The cylindrical shaped first end  134   a  has a through opening  139  dimensioned to receive a fixation element  150 . In the example of  FIG. 23  the fixation element  150  is an elongated poly-axial screw and is used to anchor the facet replacement component  130   a  to pedicle  48   a  of the vertebra  30   c . The poly-axial screw  150  includes a spherical head  156  and an elongated body  152  having outer threads  154 . The spherical head  156  includes a cutout on the top  158  dimensioned and configured to receive a screwdriver. Once the screw is anchored in the desired vertebral location, the cylindrical shaped first end  134   a  is rotated and oriented so that the elliptical surface  133   a  of the wing-like structure  132   a  articulates with a facet-like surface of the replacement par or a natural facet. Once the desired orientation of the elliptical surface  133   a  is set, the first cylindrical end is secured onto the spherical screw head  156  with a setscrew  160 , as shown in  FIG. 26 . 
     The superior replacement components  130   a ,  130   b  articulate with the par replacement components  120   a ,  120   b , respectively, to form facet joint  51   a ,  51   b  and the par replacement components  120   a ,  120   b  articulate with the inferior replacement components  110   a ,  11   b , respectively, to form facet joints  50   a ,  50   b , as shown in  FIG. 5 ,  FIG. 28  and  FIG. 29 . 
     Other embodiments are within the scope of the following claims. The facet replacement components and the par replacement components are made of metal, plastic, ceramic, bone, polymers, composites, absorbable material, biodegradable material, or combinations thereof. The articulating surfaces may be flat or slightly curved. The articulation may be a constrained articulation, as was described above. The facet replacement components may have adjustable lengths. The facet replacement system may be extended in either caudad  272  or cephalad  270  directions 
     Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.