Abstract:
A prosthesis for the replacement of a diseased or traumatized facet of a mammalian vertebra includes a surface that articulates with another prosthetic facet or a natural facet, a portion that replaces at least a bony portion of the diseased or traumatized spine facet which is to be replaced, and an element to attach the prosthesis to the vertebra in a manner that does not require attachment to or abutment against the posterior arch. A method of installing the prosthesis includes the steps of resecting at least a portion of a facet and attaching the prosthesis in a manner that does not require attachment or abutment against the posterior arch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of:
         U.S. application Ser. No. 10/421,078, filed Apr. 23, 2003 now U.S. Pat. No. 7,041,136 and entitled FACET JOINT REPLACEMENT,       

     Which is a continuation of:
         U.S. application Ser. No. 09/726,169, filed Nov. 29, 2000 and entitled FACET JOINT REPLACEMENT, which is now issued as U.S. Pat. No. 6,579,319.       

     The disclosures listed above are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to surgical devices and methods to replace a damaged, diseased, or otherwise painful spinal facet joint. 
     2. The Relevant Technology 
     Traumatic, inflammatory, metabolic, synovial, neoplastic, and degenerative disorders of the spine can produce debilitating pain that can have severe socioeconomic and psychological effects. One of the most common surgical interventions today is arthrodesis, or spine fusion, of one or more motion segments, with approximately 300,000 procedures performed annually in the United States. Clinical success varies considerably, depending upon technique and indications, and consideration must be given to the concomitant risks and complications. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, it is believed that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused motion segments. Additionally, pseudoarthrosis, as a result of an incomplete or ineffective fusion, may reduce or even eliminate pain relief for the patient. Finally, the fusion device, whether artificial or biological, may migrate out of the fusion site. 
     Recently, several attempts have been made to recreate the natural biomechanics of the spine by use of an artificial disc. Artificial discs provide for articulation between vertebral bodies to recreate the full range of motion allowed by the elastic properties of the natural intervertebral disc which directly connects two opposed vertebral bodies. 
     However, the artificial discs proposed to date do not fully address the mechanics of motion of the spinal column. In addition to the intervertebral disc, posterior elements called the facet joints help to support axial, torsional and shear loads that act on the spinal column. Furthermore, the facet joints are diarthroidal joints that provide both sliding articulation and load transmission features. The effects of their absence as a result of facetectomy is believed to produce significant decreases in the stiffness of the spinal column in all planes of motion: flexion and extension, lateral bending, and rotation. Furthermore, contraindications for artificial discs include arthritic facet joints, absent facet joints, severe facet joint tropism or otherwise deformed facet joints. 
     U.S. Pat. No. Re. 36,758 to Fitz discloses an artificial facet joint where the inferior facet, the mating superior facet, or both, are covered with a cap. The cap requires no preparation of the bone or articular surfaces; it covers and, therefore, preserves the bony and articular structure. 
     The capping of the facet has several potential disadvantages. If the facet joint is osteoarthritic, a cap will not remove the source of the pain. Additionally, at least in the case of surface replacements for osteoarthritis femoral heads, the capping of articular bone ends has proven to lead to clinical failure by means of mechanical loosening. The clinical failure is hypothesized to be a sequela of disrupting the periosteum and ligamentum teres femoris, both serving a nutrition delivery role to the femoral head, thereby leading to avascular necrosis of the bony support structure for the surface replacement. Another potential disadvantage is that in order to accommodate the wide variability in anatomical morphology of the facets, not only between individuals but also between levels within the spinal column, a very wide variety of sizes and shapes would be required. 
     U.S. Pat. No. 6,132,464 to Martin discloses a spinal facet joint prosthesis that is supported on the lamina, or the posterior arch of the vertebra. Extending from this support structure are inferior and/or superior blades that replace the cartilage at the facet joint. Like the Fitz design, the Martin prosthesis generally preserves existing bony structures and therefore does not address pathologies which affect the bone of the facets in addition to affecting the associated cartilage. Furthermore, the Martin invention requires a mating condition between the prosthesis and the lamina, or the posterior arch, that is a thin base of curved bone that carries all four facets and the spinous process. Since the posterior arch is a very complex and highly variable anatomic surface, it would be very difficult to design a prosthesis that provides reproducible positioning to correctly locate the cartilage-replacing blades for the facet joints. 
     Another approach to surgical intervention for spinal facets is provided in WO9848717A1 to Villaret. While Villaret teaches the replacement of spine facets, the replacement is interlocked in a manner to immobilize the joint. 
     Facet joint replacement in conjunction with artificial disc replacements represent a unique solution to recreating a fully functional motion segment that is compromised due to disease or trauma. Together, facet joint and disc replacement can eliminate all sources of pain, return full function and range of motion, and completely restore the natural biomechanics of the spinal column. Additionally, degenerative or traumatized facet joints may be replaced in the absence of disc replacement when the natural intervertebral disc is unaffected by the disease or trauma. 
     It would therefore be an improvement in the art to provide a vertebral facet replacement device and method that replaces a bony portion of the facets so as to remove the source of arthritic, traumatic, or other disease mediated pain. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an artificial vertebral facet that replaces the cartilage and a portion of the bone of a facet. 
     It is a further object of the invention to provide a method for preparing a facet for the installation of an artificial vertebral facet. 
     It is another object to provide a method for replacing a spinal facet. 
     It is yet another object of the invention to provide a total vertebral facet joint replacement. 
     In the preferred embodiment, an inferior facet of a superior vertebra is resected at the base of the facet where it connects to the posterior arch. The fin of a prosthetic inferior facet is pressed into the interior bone space of the posterior arch. Alternatively, a tool, such as a broach or punch, may be used to first prepare a space for the fin within the posterior arch. 
     Alternatively, or in addition, a superior facet of an inferior vertebra that articulates with the inferior facet is resected at the base of the facet where it connects to the pedicle. The post of a prosthetic superior facet is pressed into the interior bone space of the pedicle. Alternatively, a tool, such as a broach or punch, may be used to first prepare a space for the post within the pedicle. 
     The post and the fin may be porous coated to promote bone ingrowth in order to achieve long term fixation. Acute fixation is provided by a press fit between the post or fin and the internal surface of the bone. The porous coating may carry osteoconductive agents, such as hydroxylapatite, calcium sulfate, or demineralized bone matrix. Alternatively, the porous coating may carry osteoinductive agents, such as bone morphogenic proteins, including rhBMP-2 and rhBMP-7. 
     Another embodiment of the present invention provides a flange extending from the prosthetic facet. The flange is oriented relative to the body of the prosthesis such that when the flange is placed against the pedicle and in a manner such that the planar surface of the flange is perpendicular to the axis of the pedicle interior bone canal, the articulating surface of the prosthesis will be properly positioned to match the articulating surface of the natural facet. The flange includes a hole for the passage of a fastener to securely attach the prosthesis to the pedicle. The fastener can be a screw, spike, tack, staple, or the like. 
     Because the present invention allows for the individual replacements of facets, only compromised facets need be replaced. For example, if only one facet is affected by disease or trauma, it can be resected and replaced with a facet prosthesis that articulates with an opposing natural facet. 
     The present invention has numerous advantages over the prior art. One advantage is that the quality of attachment of the prosthesis is improved. The present invention provides a precise and tight press fit into bones, as opposed to relying on prosthetic surfaces mating with highly complex and variable external surfaces of the vertebra, such as the posterior arch or facet. Another advantage is that the optional porous coating is placed into interior bone spaces where porous coatings have proven to achieve bone ingrowth for excellent long term fixation strength. This ability to achieve bone ingrowth is uncertain for the prior art devices that engage the external bone surfaces of the vertebra. Yet another advantage lies in the removal of the facet bone structure; where the facet bone is involved in the disease pathology or the trauma that compromised the articular or cartilaginous surface of the facet, resection provides a means for ensuring that all pain associated with the disease or trauma is removed and the true joint line is restored. With prior art devices, the bony structure of the facets was generally left intact. 
     The above, and other objects, features and advantages of the present invention, will become apparent from the following description which is to be read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
         FIG. 1  is a perspective view of a portion of the spine; 
         FIG. 1A  is a dorsal view of the portion of the spine shown in  FIG. 1 ; 
         FIG. 2  is a lateral view of a facet joint reconstructed in accordance with the present invention; 
         FIG. 3  is a dorsal view of the facet joint shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of the implanted left inferior facet prosthesis shown in  FIGS. 2 and 3 ; 
         FIG. 5  is a perspective view of the left inferior facet prosthesis shown in  FIGS. 2 and 3 ; 
         FIG. 6  is a cranial view of the implanted left superior facet prosthesis shown in  FIGS. 2 and 3 ; 
         FIG. 7  is a perspective view of the left superior facet prosthesis shown in  FIGS. 2 and 3 ; 
         FIG. 8  is a perspective view of an alternate implanted left superior facet prosthesis; 
         FIG. 9  is a perspective view of an alternate left superior facet prosthesis; 
         FIG. 10  is a lateral view of an alternative reconstructed facet joint; 
         FIG. 11  is a dorsal view of an alternative reconstructed facet joint; 
         FIG. 12  is a perspective view of the implanted left inferior facet prosthesis shown in  FIGS. 10 and 11 ; 
         FIG. 13  is a perspective view of the alternative left inferior facet prosthesis shown in  FIGS. 10 and 11 ; 
         FIG. 14  is a cranial view of the alternative implanted left superior facet prosthesis  FIGS. 10 and 11   
         FIG. 15  is a perspective view of the alternative left superior facet prosthesis shown in  FIGS. 10 and 11 ; and 
         FIG. 16  is a perspective view of an alternate bearing surface for the superior facet is shown in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1 and 1A , there is shown a superior vertebra  1  and an inferior vertebra  3 , with an intervertebral disc  2  located in between. Vertebra  1  has superior facets  43 , inferior facets  6 , posterior arch  35  and spinous process  46 . Vertebra  3  has superior facets  7 , inferior facets  44 , posterior arch  36  and spinous process  45 . 
     Referring now to  FIG. 2 , the left inferior facet  6  of vertebra  1  has been resected and an inferior facet prosthesis  4  has been attached to vertebra  1 . Similarly, the left superior facet of vertebra  3  has been resected and a superior facet prosthesis  5  has been attached to vertebra  3 . 
       FIG. 3  illustrates a dorsal view of the elements shown in  2 . It can be appreciated that inferior facet prosthesis  4  replicates the natural anatomy when compared to the contralateral inferior facet  6  of vertebra  1 . Similarly, it can be appreciated that superior facet prosthesis  5  replicates the natural anatomy when compared to the contralateral superior facet  7  of vertebra  3 . 
     Turning now to  FIG. 4 , a perspective view of vertebra  1  with implanted inferior facet prosthesis  4  is provided. Resection at  31  has removed the natural inferior facet  6  at the bony junction between the inferior facet  6  and the posterior arch  35 . In this manner, any bone pain associated with a disease, such as osteoarthritis, or trauma will be eliminated as the involved bony tissue has been osteotomized 
       FIG. 5  illustrates a perspective view of inferior facet prosthesis  4 . Surface  8  replicates the natural articular surface of the replaced inferior facet  6 . Post  9  provides a means to affix inferior facet prosthesis  4  to vertebra  1 . Post  9  is implanted into the interior bone space of the left pedicle P ( FIG. 6 ) on vertebra  1  and may or may not extend into the vertebral body of vertebra  1  to provide additional stability. 
       FIG. 6  illustrates a cranial view of vertebra  3  with implanted superior facet prosthesis  5 . Resection surface  32  represents the bony junction between the natural superior facet and the posterior arch  35 . 
       FIG. 7  illustrates a perspective view of superior facet prosthesis  5 . Surface  36  replicates the natural articular surface of the replaced superior facet  7 . Post  37  provides a means for affixing superior facet prosthesis  5  to vertebra  3 . Post  37  is implanted into the interior bone space of the left pedicle P ( FIG. 6 ) on vertebra  3  and may or may not extend into the vertebral body of vertebra  3  to provide additional stability. 
     When the total facet joint is replaced, as shown in  FIGS. 2 and 3 , then surface  8  ( FIG. 5 ) articulates with surface  36  ( FIG. 7 ) to recreate the natural biomechanics of the spine motion segment made up of vertebra  1 , vertebra  3 , and intervertebral disc  2 . The post  37  has an implantable portion  50  (i.e., the portion of the post  37  that is implanted in to the interior bone space of the left pedicle P in  FIG. 6 ). The implantable portion  50  has a longitudinal axis  52  extending along its length, and a largest cross section  54  perpendicular to the longitudinal axis  52 . A projection  56  of the largest cross section  54  extends posteriorly along the longitudinal axis  52 . As shown in  FIG. 7 , the entire surface  36  is positioned medially of the projection  56 . The surface  36  is also displaced from the longitudinal axis  52  by an offset  58 . 
       FIG. 8  illustrates an alternative inferior facet prosthesis  10  which is implanted into the interior bone space of posterior arch  35 . The interior bone space is accessed from the resection  31 . 
       FIG. 9  shows details of alternative inferior facet prosthesis  10 , including the fin  13  that extends into the interior bone space of posterior arch  35 . Surface  12  replicates the natural articular surface of the replaced facet. 
     If desired, a corresponding fin construction can be used to form a prosthetic superior facet. 
     The surfaces of post  9  ( FIG. 5 ), post  37  ( FIG. 7 ) and fin  13  ( FIG. 9 ) may or may not include porous coatings to facilitate bone ingrowth to enhance the long term fixation of the implant. Furthermore, such porous coatings may or may not include osteoinductive or osteoconductive substances to further enhance the bone remodeling into the porous coating. 
     Referring now to  FIG. 10 , there is shown a lateral view of a superior vertebra  14  and an inferior vertebra  16 , with an intervertebral disc  15  located in between is shown. The left inferior facet of vertebra  14  has been resected and an inferior facet prosthesis  18  has been attached to vertebra  14  by means of a screw fastener  17 , wherein the screw fastener is implanted into an interior bone space of the pedicle of the vertebra. Similarly, the left superior facet of vertebra  16  has been resected and a superior facet prosthesis  19  has been attached to vertebra  16  by means of a screw fastener  17 , wherein the screw fastener is implanted into an interior bone space of the pedicle of the vertebra. 
       FIG. 11  illustrates a dorsal view of the elements of  FIG. 10 . It can be appreciated that inferior facet prosthesis  18  replicates the natural anatomy when compared to the contralateral inferior facet  22  of vertebra  14 . Similarly, it can be appreciated that superior facet prosthesis  19  replicates the natural anatomy when compared to the contralateral superior facet  21  of vertebra  16 . In a preferred embodiment of the present invention, inferior facet prosthesis  18  and/or superior facet prosthesis  19  has a bone contacting surface which is porous coated to allow for bone ingrowth. 
     Turning now to  FIG. 12 , there is provided a perspective view of vertebra  14  with implanted inferior facet prosthesis  18 . Resection  34  has removed the natural inferior facet at the bony junction between the inferior facet and the posterior arch  37 . In this manner, any bone pain associated with a disease, such as osteoarthritis, or trauma will be eliminated inasmuch as the involved bony tissue has been osteotomized. 
       FIG. 13  illustrates a perspective view of inferior facet prosthesis  18 . Surface  23  replicates the natural articular surface of the replaced facet. Flange  25  contacts the pedicle and hole  24  receives a fastener to attach inferior facet prosthesis  18  to vertebra  14 . 
       FIG. 14  illustrates a cranial view of vertebra  16  with implanted superior facet prosthesis  19 . Resection surface  35  represents the bony junction between the natural superior facet and the posterior arch  38 . 
       FIG. 15  illustrates a perspective view of superior facet prosthesis  19 . Surface  27  replicates the natural articular surface of the replaced facet. Flange  39  contacts the pedicle and hole  26  receives a fastener to attach inferior facet prosthesis  19  to vertebra  16 . 
       FIG. 16  illustrates an alternative superior facet prosthesis  40  with an bearing surface  41  that mounts to substrate  42 . The bearing surface  41  is a biocompatible polymeric material, such as ultra high molecular weight polyethylene. Alternately, the bearing surface can be ceramic, such as zirconia or alumina, or metal. The substrate is a biocompatible metal alloy, such as an alloy of titanium, cobalt, or iron. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the embodiments shown herein are by way of example, and that various changes and modifications may be effected by one skilled in the art without departing from the scope or spirit of the invention as defined in the following claims.