US20050131406A1 - Polyaxial adjustment of facet joint prostheses - Google Patents

Abstract

Prostheses, systems, and methods are provided for replacement of natural facet joints between adjacent vertebrae using polyaxial attachment mechanisms for securing the prostheses to the vertebrae. A cephalad prosthesis attached to a superior adjacent vertebra replaces the inferior half of a natural facet joint. A caudal prosthesis attached to an inferior adjacent vertebra replaces the superior half of a natural facet joint. Both the cephalad and caudal prostheses are configured with artificial facet joint structures that include articulating surfaces that cooperate and form an artificial articular configuration. The polyaxial attachment mechanism permits adjustment of the position of the artificial facet joint structure along more than one axis at or after the time the cephalad or caudal prosthesis is attached to a vertebra.

Description

FIELD OF THE INVENTION
• This invention relates to prostheses, systems, and methods for treating various types of spinal pathologies, and in particular relates to attachment of prostheses to spinal vertebrae.
BACKGROUND OF THE INVENTION
• The humanspinal column10, as shown inFIG. 1, is comprised of a series of thirty-three stackedvertebrae12 divided into five regions. The cervical region includes seven vertebrae, known as C1-C7. The thoracic region includes twelve vertebrae, known as T1-T12. The lumbar region contains five vertebrae, known as L1-L5. The sacral region is comprised of five vertebrae, known as S1-S5, while the coccygeal region contains four vertebrae, known as Co1-Co4.
• FIG. 2 depicts a superior plan view of a normalhuman lumbar vertebra12. Although human lumbar vertebrae vary somewhat according to location, they share many common features. Eachvertebra12 includes avertebral body14. Two short bones, thepedicles16, extend backward from each side of thevertebral body14 to form avertebral arch18.
• At the posterior end of eachpedicle16, thevertebral arch18 flares out into broad plates of bone known as thelaminae20. Thelaminae20 fuse with each other to form aspinous process22. Thespinous process22 serves for muscle and ligamentous attachment. A smooth transition from thepedicles16 to thelaminae20 is interrupted by the formation of a series of processes.
• Twotransverse processes24 thrust out laterally on each side from the junction of thepedicle16 with thelamina20. Thetransverse processes24 serve as levers for the attachment of muscles to thevertebrae12. Four articular processes, two superior26 and two inferior28, also rise from the junctions of thepedicles16 and thelaminae20. The superiorarticular processes26 are sharp oval plates of bone rising upward on each side of the vertebrae, while theinferior processes28 are oval plates of bone that jut downward on each side.
• The superior and inferiorarticular processes26 and28 each have a natural bony structure known as a facet. The superiorarticular facet30 faces upward, while the inferior articular facet31 (seeFIG. 3) faces downward. Whenadjacent vertebrae12 are aligned, thefacets30 and31, capped with a smooth articular cartilage, interlock to form afacet joint32, also known as a zygapophyseal joint.
• Thefacet joint32 is composed of a superior half and an inferior half. The superior half is formed by the vertebral level below thejoint32, and the inferior half is formed by the vertebral level above thejoint32. For example, in the L4-L5 facet joint, the superior half of thejoint32 is formed by bony structure on the L5 vertebra (i.e., a superior articular surface and supportingbone26 on the L5 vertebra), and the inferior half of thejoint32 is formed by bony structure on the L4 vertebra (i.e., an inferior articular surface and supportingbone28 on the L4 vertebra).
• Anintervertebral disc34 between eachadjacent vertebrae12 permits gliding movement between thevertebrae12. The structure and alignment of thevertebrae12 thus permit a range of movement of thevertebrae12 relative to each other.
• Back pain, particularly in the “small of the back” or lumbosacral (L4-S1) region, is a common ailment. In many cases, the pain severely limits a person's functional ability and quality of life. Such pain can result from a variety of spinal pathologies.
• Through disease or injury, the laminae, spinous process, articular processes, or facets of one or more vertebral bodies can become damaged, such that the vertebrae no longer articulate or properly align with each other. This can result in an undesired anatomy, loss of mobility, and pain or discomfort.
• For example, the vertebral facet joints can be damaged by either traumatic injury or by various disease processes. These disease processes include osteoarthritis, ankylosing spondylolysis, and degenerative spondylolisthesis. The damage to the facet joints often results in pressure on nerves, also called “pinched” nerves, or nerve compression or impingement. The result is pain, misaligned anatomy, and a corresponding loss of mobility. Pressure on nerves can also occur without facet joint pathology, e.g., a herniated disc.
• One type of conventional treatment of facet joint pathology is spinal stabilization, also known as intervertebral stabilization. Intervertebral stabilization prevents relative motion between the vertebrae. By preventing movement, pain can be reduced. Stabilization can be accomplished by various methods.
• One method of stabilization is spinal fusion. Another method of stabilization is fixation of any number of vertebrae to stabilize and prevent movement of the vertebrae.
• Another type of conventional treatment is decompressive laminectomy. This procedure involves excision of the laminae to relieve compression of nerves.
• These traditional treatments are subject to a variety of limitations and varying success rates. None of the described treatments, however, puts the spine in proper alignment or returns the spine to a desired anatomy. In addition, stabilization techniques, by holding the vertebrae in a fixed position, permanently limit a person's mobility.
• There is, therefore, a need for prostheses, systems, and methods that overcome the problems and disadvantages associated with current strategies and designs in various treatments for spine pathologies, and, particularly, a need for spinal prostheses with attachment mechanisms that facilitate positioning of the prostheses when attached to the vertebrae.
SUMMARY OF THE INVENTION
• The present invention provides prostheses, systems, and methods designed to replace natural facet joints and possibly part of the lamina at virtually all spinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L5-S1, T11-T12, and T12-L1, using polyaxial attachment mechanisms for securing the prostheses to the vertebrae. The prostheses, systems, and methods help establish a desired anatomy to a spine and return a desired range of mobility to an individual. The prostheses, systems, and methods also help lessen or alleviate spinal pain by relieving the source nerve compression or impingement.
• For the sake of description herein, the prostheses that embody features of the invention are identified as either “cephalad” or “caudal” with relation to the portion of a given natural facet joint they replace. As previously described, a natural facet joint, such as facet joint32 (FIG. 3), has a superior half and an inferior half. In anatomical terms, the superior half of the joint is formed by the vertebral level below the joint, which can thus be called the “caudal” portion of the facet joint because it is closer to the feet of the person. The inferior half of the facet joint is formed by the vertebral level above the joint, which can thus be called the “cephalad” portion of the facet joint because it is closer to the head of the person. Thus, a prosthesis that, in use, replaces the caudal portion of a natural facet joint (i.e., the superior half) will be called a “caudal” prosthesis. Likewise, a prosthesis that, in use, replaces the cephalad portion of a natural facet joint (i.e., the inferior half) will be called a “cephalad” prosthesis.
• In one aspect, the present invention provides a facet joint prosthesis including an artificial facet joint element connected to a fixation element by a polyaxially adjustable connection. In some embodiments the polyaxially adjustable connection is adapted and configured to permit the artificial facet joint element to be rotated with respect to the fixation element around more than one axis in order to tailor the prosthesis to the needs of the patient. The polyaxially adjustable connection may be further adapted and configured to permit the position of the artificial facet joint element to be continuously adjustable within an adjustability range with respect to the fixation element and may include a limit stop.
• In various embodiments the artificial facet joint element is adapted to be movable medially, laterally, superiorly and/or inferiorly with respect to the fixation element. In some embodiments the polyaxially adjustable connection may be adapted and configured to permit the artificial facet joint element to be moved with respect to the fixation element after installation of the facet joint prosthesis in a patient. The invention may also include a fastener adapted and configured to prevent movement between the artificial facet joint element and the fixation element.
• The artificial facet joint element in some embodiments may include a cephalad facet joint bearing surface and a cephalad facet joint bearing surface support. The cephalad facet joint bearing surface and the cephalad facet joint bearing surface support may be adapted and configured to be disposed laterally from a midline, or approximately at a midline, of a vertebra when the facet joint prosthesis is installed in a patient.
• In some other embodiments the artificial facet joint element may include a caudal facet joint bearing surface and a caudal facet joint bearing surface support. The caudal facet joint bearing surface and the caudal facet joint bearing surface support may be adapted and configured to be disposed laterally from a midline of a vertebra when the facet joint prosthesis is installed in a patient.
• The facet joint prosthesis may also include a longitudinally adjustable connection between the artificial facet joint element and the fixation element. For example, in embodiments where the artificial facet joint element includes a facet joint bearing surface and a facet joint bearing surface support, the facet joint bearing surface support may be adapted and configured to be longitudinally movable with respect to the fixation element. The facet joint prosthesis may also include a fastener adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element. The facet joint bearing surface support may also be further adapted and configured to be rotatable about a support longitudinal axis.
• In some embodiments the polyaxially adjustable connection may include a facet joint element connection surface and a fixation element connection surface, with the facet joint element connection surface and fixation element connection surface being adapted and configured to be movable with respect to each other. In some embodiments the polyaxially adjustable connection may include a base member attached to the artificial facet joint element and movable with respect to the fixation element, and the base member may be further adapted and configured to be movable with respect to the artificial facet joint element.
• Another aspect of the invention is a facet joint prosthesis including first and second artificial facet joint elements; a fixation element; and a polyaxially adjustable connection between at least one of the first or second facet joint elements and the fixation element. In some embodiments the first artificial facet joint element may include an artificial cephalad facet joint element, and the second artificial facet joint element may include an artificial caudal facet joint element. The fixation element may be a cephalad fixation element, and the polyaxially adjustable connection may include a cephalad polyaxially adjustable connection, with the facet joint prosthesis further including a caudal fixation element and a caudal polyaxially adjustable connection, between the artificial caudal facet joint bearing element and the caudal fixation element. In some embodiments the artificial cephalad facet joint element includes a cephalad bearing surface and a cephalad bearing surface support, and in some embodiments the artificial caudal facet joint element comprises a caudal bearing surface and a caudal bearing surface support.
• Under this aspect of the invention the first and second artificial facet joint elements may also include first and second artificial cephalad facet joint elements, respectively. The fixation element may be a first cephalad fixation element and the polyaxially adjustable connection may include a first cephalad polyaxially adjustable connection, with the facet joint prosthesis further including a second cephalad fixation element and a second cephalad polyaxially adjustable connection between the second artificial cephalad facet joint element and the second cephalad fixation element. In some embodiments the first and second artificial cephalad facet joint elements may include first and second cephalad bearing surfaces, respectively. In some embodiments the first and second artificial facet joint elements may include first and second support arms, respectively, and at least one cephalad bearing surface supported by at least one of the first and second support arms. In those embodiments the cephalad bearing surface may be disposed approximately at a midline of a vertebra when the prosthesis is installed in a patient, and the prosthesis may also include an artificial caudal facet joint element comprising a caudal bearing surface adapted and configured to mate with the cephalad bearing surface. The cephalad bearing surface may also be adapted to rotate about the support arm or arms by which it is supported.
• Also according to this aspect of the invention the polyaxially adjustable connection may be adapted and configured to permit at least one of the artificial facet joint elements to be rotated with respect to the fixation element around more than one axis. The polyaxially adjustable connection may also be further adapted and configured to permit the position of at least one of the artificial facet joint elements to be continuously adjustable within an adjustability range with respect to the fixation element and may include a limit stop. The polyaxially adjustable connection may also be adapted and configured to permit at least one of the artificial facet joint elements to be moved with respect to the fixation element after installation of the facet joint prosthesis in a patient. In some embodiments the facet joint prosthesis further includes a fastener adapted and configured to prevent relative movement between at least one of the artificial facet joint elements and the fixation element.
• Also according to this aspect of the invention the facet joint prosthesis may further include a longitudinally adjustable connection between at least one of the artificial facet joint elements and the fixation element. In some embodiments, at least one of the artificial facet joint elements includes a facet joint bearing surface and a facet joint bearing surface support, with the facet joint bearing surface support being adapted and configured to be longitudinally movable with respect to the fixation element. The facet joint prosthesis may further include a fastener adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element. In some embodiments the facet joint bearing surface support may be further adapted and configured to be rotatable about a support longitudinal axis.
• Further according to this aspect of the invention the polyaxially adjustable connection may include a facet joint element connection surface and a fixation element connection surface, with the facet joint element connection surface and fixation element connection surface being adapted and configured to be movable with respect to each other. The polyaxially adjustable connection may include a base member attached to at least one of the artificial facet joint elements and movable with respect to the fixation element, with the base member possibly being further adapted and configured to be movable with respect to at least one of the artificial facet joint elements.
• Yet another aspect of the invention provides a facet joint prosthesis including first and second fixation elements; an artificial facet joint bearing surface (such as a cephalad bearing surface) adapted and configured to be disposed approximately at a midline of a vertebra when the facet joint prosthesis is installed in a patient; and first and second polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively. In some embodiments the first and second polyaxially adjustable connections may each be adapted and configured to permit the first and second fixation elements to be rotated with respect to the artificial facet joint bearing surface around more than one axis. The facet joint prosthesis may also further include first and second fasteners adapted and configured to prevent relative movement between the artificial facet joint bearing surface and the first and second fixation elements, respectively.
• The facet joint prosthesis according to this aspect of the invention may also include first and second longitudinally adjustable connections between the artificial facet joint bearing surface and the first and second fixation elements, respectively. In some embodiments the facet joint prosthesis may include a first support arm disposed between the first fixation element and the artificial facet joint bearing surface and a second support arm disposed between the second fixation element and the artificial facet joint bearing surface, with the first and second support arms each being adapted and configured to support the artificial facet joint bearing surface and to be longitudinally movable with respect to the first and second fixation elements, respectively. The facet joint prosthesis may also further include first and second fasteners each adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element. In some embodiments, the first support arm may be further adapted and configured to be rotatable about a first support arm longitudinal axis and the second support arm is further adapted and configured to be rotatable about a second support arm longitudinal axis. The first and second support arms may be two separate pieces, or they may be one integral piece. In some embodiments, third and fourth polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively, may be employed.
• Yet another aspect of the invention provides a facet joint prosthesis including an artificial facet joint bearing surface supported by first and second support arms adapted and configured to dispose the artificial facet joint bearing surface approximately at a midline of a vertebra when the facet joint prosthesis is installed in a patient; and first and second fixation elements adapted and configured to attach the first and second support arms, respectively, to a vertebra. The first and second support arms and the artificial facet joint bearing surface may be one integral piece. In some embodiments the facet joint prosthesis further includes first and second polyaxially adjustable connections between the first and second arms and the first and second fixation elements, respectively. In some embodiments, third and fourth polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively, may be employed.
• Another aspect of the invention provides a method of installing an artificial facet joint prosthesis, where the prosthesis includes a facet joint element and a fixation element, the method including the steps of attaching the prosthesis to a vertebra with the fixation element; and adjusting positions of the facet joint element and the fixation element to a relative orientation. The adjusting step could include the step of moving the facet joint element medially, laterally, superiorly and/or inferiorly with respect to the fixation element. In some embodiments the attaching step is performed prior to the adjusting step, and in some embodiments the attaching step is performed after the adjusting step.
• In some embodiments the adjusting step includes rotating the facet joint element with respect to the fixation element around more than one axis. The method may also include the step of preventing further rotation of the facet joint element after the rotating step.
• In embodiments of the method where the facet joint element includes a facet joint bearing surface, the adjusting step may include the step of positioning the facet joint bearing surface laterally from a vertebra midline or approximately at a vertebra midline, and may include the step of positioning the facet joint bearing surface to face caudad or to face cephalad.
• The adjusting step may also include the step of moving the facet joint element longitudinally with respect to the fixation element. In some embodiments the method may also include the step of preventing further longitudinal movement of the facet joint element after the moving step. The adjusting step may also include the step of rotating the facet joint element about a facet joint element longitudinal axis.
• Other features and advantages of the invention are set forth in the following description and drawings, as well as in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a lateral elevation view of a normal human spinal column;
• FIG. 2 is a superior plan view of a normal human lumbar vertebra;
• FIG. 3 is a lateral elevation view of adjoining normal human lumbar vertebrae L4 and L5;
• FIG. 4 is a perspective view of one embodiment of a cephalad prosthesis constructed in accordance with the present invention for replacing the inferior half of a natural facet joint on a superior vertebral body;
• FIG. 5 is a top plan view of the cephalad prosthesis shown inFIG. 4;
• FIG. 6 is a front elevation view of the cephalad prosthesis shown inFIG. 4;
• FIG. 7 is a side elevation view of the cephalad prosthesis shown inFIG. 4;
• FIG. 8 is a front section view of the cephalad prosthesis shown inFIG. 4;
• FIG. 9 is an exploded perspective view depicting various components of the cephalad prosthesis shown inFIG. 4, including a set screw, a fixing nut, an artificial facet joint structure, a fixing insert, a base member, and a polyaxial screw member;
• FIGS. 10A and 10B provide a top plan view and side section view, respectively, of the set screw shown inFIG. 9;
• FIGS. 11A and 11B provide a top plan view and side elevation view, respectively, of the fixing nut shown inFIG. 9;
• FIGS. 12A through 12D provide a top plan view, a front elevation view, a front section view, and a side elevation view, respectively, of the fixing insert shown inFIG. 9;
• FIGS. 13A through 13C provide a top plan view, a front elevation view, and a front section view, respectively, of the base member shown inFIG. 9;
• FIGS. 14A through 14D provide a side elevation view, a top plan view, a bottom plan view, and a side section view, respectively, of the polyaxial screw member shown inFIG. 9;
• FIG. 15 is a perspective view of one embodiment of a caudal prosthesis constructed in accordance with the present invention for replacing the superior half of a natural facet joint on an inferior vertebral body;
• FIG. 16 is a top plan view of the caudal prosthesis shown inFIG. 15;
• FIG. 17 is a side elevation view of the caudal prosthesis shown inFIG. 15;
• FIG. 18 is a front elevation view of the caudal prosthesis shown inFIG. 15;
• FIG. 19 is a front section view of the caudal prosthesis shown inFIG. 15;
• FIG. 20 is a posterior elevation view of two pairs of cephalad and caudal prostheses of this invention as installed in a patient;
• FIG. 21 is a posterior perspective view of the two pairs of installed cephalad and caudal prostheses ofFIG. 20;
• FIG. 22 is a posterior perspective view of installed cephalad and caudal prostheses according to another embodiment of the invention;
• FIG. 23 is a posterior elevation view of the prostheses shown inFIG. 22;
• FIG. 24 is a side elevation view of the prostheses shown inFIGS. 22 and 23;
• FIG. 25 is a posterior elevation view of installed cephalad and caudal prostheses according to yet another embodiment of the invention;
• FIG. 26 is a side elevation view of the prostheses ofFIG. 25;
• FIG. 27 is a perspective view of the prostheses ofFIGS. 25 and 26;
• FIG. 28 is a partial sectional view taken along the line A-A shown inFIG. 25;
• FIG. 29 is a perspective view of a portion of the cephalad prosthesis ofFIGS. 25-27;
• FIG. 30 is a partial sectional view taken along the line A-A shown inFIG. 29;
• FIG. 31 is a perspective view of the caudal prosthesis ofFIGS. 25-27; and
• FIG. 32 is a partial sectional view taken along the line A-A shown inFIG. 31.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• Although the disclosure presented herein provides details to enable those skilled in the art to practice various embodiments of the invention, the physical embodiments disclosed herein merely exemplify the invention which may be embodied in other specific structure. Accordingly, while preferred embodiments of the invention are described below, details of the preferred embodiments may be altered without departing from the invention. All embodiments that fall within the meaning and scope of the appended claims, and equivalents thereto, are intended to be embraced by the claims.
• FIGS. 4-14 show one embodiment of an artificial cephalad facetjoint prosthesis40 configured to replace the inferior portion of a natural facet joint, such as after the surgical removal of the articulating process forming the inferior portion of the facet joint. When thecephalad prosthesis40 is attached to a vertebra, the artificial facetjoint structure44 articulates with the superior half of the facet joint32. The superior half of the facet joint32 can comprise the natural superior half of the facet joint (i.e., the natural superior articulatingsurface30 and supporting bony structure26) on the vertebral body below the facet joint32. Alternatively, the superior half of the facet joint32 may be comprised of an artificial facet joint prosthesis, such as thecaudal prosthesis100 as shown inFIGS. 15-19.
• Prosthesis40 includes an artificial facetjoint element44 connected to afixation element52 via apolyaxial connection41 that permits facetjoint element44 andfixation element52 to be rotated with respect to each other around more than one axis. As will be seen from a detailed discussion of its structure, the embodiment ofFIGS. 4-14 permits continuous adjustment through relative rotation of the facet joint element and the fixation element around many different axes through an adjustability range, up to a motion limit provided by a limit stop. In other embodiments, however, the number of axes of rotation may be limited, and the movement may be permitted only in discrete increments. In various embodiments the facet joint element may be moved medially, laterally, superiorly and/or inferiorly with respect to the fixation element.
• The relative positions of facetjoint element44 andfixation element52 may be set prior to implant, after implant, or both before and after implant. After implant and adjustment, the facetjoint bearing surface74 of facetjoint element44 may be in an anatomically correct position within the patient's body or in an non-anatomically correct position, depending on the requirements of the situation. For example,FIG. 20 shows facetjoint prosthesis40 implanted and adjusted to place the facetjoint bearing surface74 laterally from themidline119 ofvertebrae115 and117.
• In alternative embodiments, other convex or concave shapes may be used for the facet joint bearing surface. Bearingsurface74 may be formed from biocompatible metals (such as cobalt chromium steel, surgical steels, titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.), ceramics, polyethylene, biocompatible polymers, and other materials known in the prosthetic arts.
• Fixation element52 may be a screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. In the embodiment shown inFIGS. 4-14, fixation element is a screw with ahead54 and a securingportion56. A well58 with an outer periphery shaped to mate with a driver tool is formed inhead54. Thebottom tip57 offixation element52 may include cuttingedges59 that facilitate insertion offixation element52 into the pedicle or other portion of a vertebra.
• The invention may employ any suitable polyaxial connection structure, such as the structure disclosed in U.S. Pat. No. 5,360,431, the disclosure of which is incorporated by reference. In the embodiment ofFIGS. 4-14, thepolyaxial connection41 ofcephalad prosthesis40 includes a base42 connected to asupport arm72 of facetjoint element44. In alternative embodiments,base42 may be integral with facetjoint element44. As shown inFIGS. 8, 13C and14A,base42 has a concaveinside surface91 that corresponds to and mates with a correspondingconvex surface53 formed on thehead54 offixation element52. A fixinginsert50 withinbase42 also has aconcave surface78 corresponding to and mating withfixation element head54 as shown.Concave surfaces91 and78 slide alongconvex surface53 to permit continuous adjustment through relative rotation between facetjoint element44 andfixation element52 along any axis desired. Alimit stop surface93 formed on the inside ofbase42 interacts withfixation element52 to limit the range of relative rotational motion betweenbase42 andfixation element52 to be within an adjustability range.
• In some embodiments, the facet joint prosthesis may also provide for longitudinal adjustment of the facet joint bearing surface location. For example, in the embodiment shown inFIGS. 4-14, a longitudinallyadjustable connection71 permits supportarm72 to be moved along its longitudinal axis with respect tofixation element52. In this embodiment, longitudinallyadjustable connection71 is formed from the interaction between the rod-like support arm72 and aU-shaped channel76 formed on fixinginsert50. The relative positions ofsupport arm76 andfixation element52 may be set prior to implant, after implant, or both before and after implant to adjust the position of the facet joint bearing surface.
• Support arm72 may also be rotated about its longitudinal axis if desired. While the embodiment shown inFIGS. 4-14 has a substantially spherical facetjoint bearing surface74, other facet joint bearing surface shapes may be used as well, and rotation ofsupport arm72 can help adjust the orientation of the facet joint bearing surface. Once again, the relative positions ofsupport arm72, facetjoint bearing surface74 andfixation element52 may be set prior to implant, after implant, or both before and after implant.
• One preferred method of installation, assembly and use of the cephalad facetjoint prosthesis40 is as follows.Fixation element52 is inserted through the interior bore ofbase42 until itshead54 rests againstsurface91 ofbase42. Aneck92 formed withinbase42 has a diameter smaller than the diameter ofhead54 to preventfixation element52 from passing throughbase42. A driver tool (not shown) is then placed in well58 to screwfixation element52 into the pedicle or other portion of the vertebra. In some embodiments, an insertion hole may be formed in the vertebra prior to screwing infixation element52. Bone cement may also be used to holdfixation element52 in place after insertion.
• Next, the fixinginsert50 is placed withinbase42 to rest onfixation element head54. The U-shaped channel defining theupper surface76 of the fixinginsert50 is aligned with aU-shaped opening84 of thebase member42.Support arm72 of the artificial facetjoint element44 is placed within theU-shaped opening84 ofbase42 and fixinginsert50.
• A fixingnut48 is threadably engaged with the outer periphery ofbase42 above the artificial facetjoint element44. Similarly, aset screw46 is threadably engaged with the inner periphery ofbase42 above the artificial facetjoint element44. The center ofset screw46 is defined by anaperture64 having a circumference shaped to mate with a corresponding driver tool (not shown).
• Before theset screw46 and fixingnut48 are tightened, the position ofbase42 is adjusted by rotating the artificial facetjoint element44, fixinginsert50 andbase42 around the outer surface offixation element head54. As discussed above, the sliding interaction of fixinginsert50 andbase42 withhead54 permit relative rotation of facetjoint element54 around more than one axis with respect tofixation element52. The artificial facetjoint element44 is also adjustable along and about its longitudinal axis withinbase42 to place facetjoint bearing surface74 at its desired location. When tightened, fixingnut48 and setscrew46 act as fasteners to prevent further relative movement between artificial facetjoint element44 andfixation element52. Other fasteners may be used, of course, as known in the art.
• FIGS. 15-19 show one embodiment of an artificial caudal facetjoint prosthesis100 configured to replace the superior portion of a natural facet joint, such as after the surgical removal of the articulating process forming the superior portion of the facet joint. When thecaudal prosthesis100 is attached to a vertebra, the artificial facetjoint structure104 articulates with the inferior half of the facet joint. The inferior half of the facet joint can comprise the natural inferior half of the facet joint (i.e., the natural inferior articulatingsurface31 and supportingbony structure28 shown inFIGS. 2 and 3) on the vertebral body below the facet joint. Alternatively, the inferior half of the facet joint may be comprised of an artificial facetjoint structure44, such as the cephalad prosthesis shown inFIGS. 4-14.
• Prosthesis100 includes an artificial facetjoint element104 connected to afixation element116 via apolyaxial connection115 that permits facetjoint element104 andfixation element116 to be rotated with respect to each other around more than one axis. Like the embodiment shown inFIGS. 4-14, the embodiment ofFIGS. 15-19 permits continuous adjustment through relative rotation of the facet joint element and the fixation element around many different axes through an adjustability range, up to a motion limit provided by a limit stop. In other embodiments, however, the number of axes of rotation may be limited, and the movement may be permitted only in discreet increments.
• The relative positions of facetjoint element104 andfixation element116 may be set prior to implant, after implant, or both before and after implant. After implant and adjustment, the facetjoint bearing surface118 of facetjoint element104 may be in an anatomically correct position within the patient's body or in an non-anatomically correct position, depending on the requirements of the situation. For example,FIG. 20 shows facetjoint prosthesis104 implanted and adjusted to place the facetjoint bearing surface118 laterally from themidline119 ofvertebrae115 and117.
• In alternative embodiments, other convex or concave shapes may be used for the facet joint bearing surface.Bearing surface118 may be formed from biocompatible metals (such as cobalt chromium steel, surgical steels, titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.), ceramics, polyethylene, biocompatible polymers, and other materials known in the prosthetic arts.
• Fixation element116 may be a screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. As in the embodiment shown inFIGS. 4-14,fixation element116 in this embodiment is a screw with ahead114 and a securing portion. A well with an outer periphery shaped to mate with a driver tool is formed inhead114, and the bottom tip offixation element116 may include cutting edges that facilitate insertion offixation element116 into the pedicle or other portion of a vertebra.
• Many aspects of the caudal prosthesis of this embodiment are similar to aspects of the cephalad prosthesis described above. As shown inFIGS. 15-19, thepolyaxial connection115 ofcaudal prosthesis100 includes a base112 connected to asupport arm102 of facetjoint element104. In alternative embodiments,base112 may be integral with facetjoint element104.Base112 has a concave inside surface that corresponds to and mates with a corresponding convex surface formed on thehead114 offixation element116. A fixinginsert110 withinbase112 also has a concave surface corresponding to and mating withfixation element head114 as shown. The concave surfaces of the base and fixing insert slide along the convex surface of thefixation element head114 to permit continuous adjustment through relative rotation between facetjoint element104 andfixation element116 along any axis-desired. Alimit stop surface120 formed on the inside ofbase112 interacts withfixation element116 to limit the range of relative rotational motion betweenbase112 andfixation element116 to be within an adjustability range.
• In some embodiments, the facet joint prosthesis may also provide for longitudinal adjustment of the facet joint bearing surface location. For example, in the embodiment shown inFIGS. 15-19, a longitudinallyadjustable connection111 permits supportarm102 to be moved along its longitudinal axis with respect tofixation element116. In this embodiment, longitudinallyadjustable connection111 is formed from the interaction between the rod-like support arm102 and a U-shaped channel formed on fixinginsert110. The relative positions ofsupport arm102 andfixation element116 may be set prior to implant, after implant, or both before and after implant to adjust the position of the facet joint bearing surface.
• Caudal facetjoint bearing surface118 is shaped to mate with a corresponding artificial cephalad facet joint bearing surface that is spherically shaped. In this embodiment, bearingsurface118 is generally concave with a length along one axis126 greater than the length along a perpendicular axis128. Other bearing surface shapes may be used, of course.Support arm102 may be rotated about its longitudinal axis to adjust the orientation of bearingsurface118, and the relative positions ofsupport arm102, facetjoint bearing surface118 andfixation element116 may be set prior to implant, after implant, or both before and after implant.
• One preferred method of installation, assembly and use of the caudal facetjoint prosthesis100 is as follows.Fixation element116 is inserted through the interior bore ofbase112 until itshead114 rests against theinterior surface91 ofbase112. Aneck122 formed withinbase112 has a diameter smaller than the diameter offixation element head114 to preventfixation element116 from passing throughbase112. A driver tool (not shown) is then used to screwfixation element116 into the pedicle or other portion of the vertebra. In some embodiments, an insertion hole may be formed in the vertebra prior to screwing infixation element116. Bone cement may also be used to holdfixation element116 in place after insertion.
• Next, the fixinginsert100 is placed withinbase112 to rest onfixation element head114. A U-shaped channel in the upper surface of the fixinginsert100 is aligned with a U-shaped opening of thebase member112.Support arm102 of the artificial facetjoint element104 is placed within the U-shaped opening ofbase112 and fixinginsert100.
• A fixingnut108 is threadably engaged with the outer periphery ofbase112 above the artificial facetjoint element104. Similarly, aset screw106 is threadably engaged with the inner periphery ofbase112 above the artificial facetjoint element104. The center ofset screw106 is defined by an aperture having a circumference shaped to mate with a corresponding driver tool (not shown).
• Before theset screw106 and fixingnut108 are tightened, the position ofbase112 is adjusted by rotating the artificial facetjoint element104, fixinginsert100 andbase112 around the outer surface offixation element head114. As discussed above, the sliding interaction of fixinginsert100 andbase112 withhead114 permit relative rotation of facetjoint element104 around more than one axis with respect tofixation element116. The artificial facetjoint element104 is also adjustable along and about its longitudinal axis withinbase112 to place facetjoint bearing surface118 at its desired location. When tightened, fixingnut108 and setscrew106 act as fasteners to prevent further relative movement between artificial facetjoint element104 andfixation element116. Other fasteners may be used, of course, as known in the art.
• From the description herein, it should be understood that either thecephalad prosthesis40 or thecaudal prosthesis100 may be used for unilateral facet joint replacement (one side of a given vertebral body). Theprostheses40 and100 may also be used to provide bilateral facet joint replacement (i.e., on both the left and right sides of a given vertebral body).
• Furthermore, as shown inFIGS. 20 and 21, a system comprised of bothprostheses40 and100 may be used to provide complete (i.e., superior and inferior) facet joint replacement of one or more natural facet joints. The cephalad prostheses40 andcaudal prosthesis100 are desirably positioned to form an articulated system that replaces a natural facet joint.Cephalad prostheses40 are attached to the superior adjoining vertebra and replace the articulating function of the cephalad portion of the natural facet joints. Likewise,caudal prostheses100 are attached to the inferior adjoining vertebra and replace the articulating function of the caudal portion of the natural facet joints. The facet joint bearing surfaces ofprostheses40 and100 thus cooperate to provide an artificial articular configuration. The complete facet joint replacement can be unilateral or bilateral, as desired. A bilateral facet joint replacement is shown inFIGS. 20 and 21.
• FIG. 20 provides a posterior elevation view of two adjacent lumbar vertebrae after removal of the inferior and superior halves of the natural facet joints on both the left and right sides of the adjacent vertebrae. Since the inferior and superior halves of the natural facet joint are removed, the artificial articular configuration need not be constrained by, and can be unlike, the preexisting articulation of the natural facet joint prior to its removal.
• Preferably, a polyaxial attachment mechanism is configured to attach either thecephalad prostheses40 to the superior adjoining vertebra, or thecaudal prostheses100 to the inferior adjoining vertebra, or both. As described earlier, the polyaxial attachment mechanism permits the position of the artificial facet joint structure of the cephalad and/orcaudal prosthesis40,100 (as the case may be) to be adjusted along more than one axis after the polyaxial attachment mechanism has been attached to the superior and/or inferior vertebra. Thesystems130 depicted inFIGS. 20 and 21 include polyaxial attachment mechanisms for both the cephalad prostheses and the caudal prostheses. Accordingly, a physician is provided maximum flexibility to position the articulating surfaces of theprostheses40 and100 when the prostheses are attached to the vertebrae. The position of the artificial facet joint structures can be fully adjusted to obtain an optimal articulation between the adjoining vertebrae, and together create a desired lordotic angle between the vertebral bodies of the vertebrae.
• FIGS. 22-24 show cephalad and caudal facet joint prostheses according to another embodiment of the invention providing an artificial facet joint located approximately at the center or midline of the vertebrae to which the prostheses are attached.Cephalad prosthesis200 has a facet joint element201 including two support arms orrods202 and204 supporting a cephalad bearingsurface element206. While other materials may be used, supportarms202 and204 are preferably formed from titanium, and bearingsurface element206 is preferably formed from biocompatible metals (such as cobalt chromium steel, surgical steels, titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.), ceramics, polyethylene, biocompatible polymers, and other materials known in the prosthetic arts. In this embodiment, supportarms202 and204 and bearingsurface element206 are separate elements and may be selected from a kit containing different size elements to meet the needs of the patient. For example, support arms ranging in size from 10 mm to 25 mm in length may be used for cephalad prostheses attached to the L4 vertebra.
• In alternative embodiments, supportarms202 and204 may be formed as a single element and may also be integral with bearingsurface206. In other alternative embodiments, one or more bushings (not shown) are disposed between bearingsurface element206 and thesupport arms202 and204 to enable bearingsurface element206 to rotate aboutsupport arms202 and204.
• Support arms202 and204 are each attached tosuperior vertebra208 via fixation elements such as screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. As shown, the fixation elements attachprosthesis200 to pedicle portions ofvertebra208. Attachment to other portions ofvertebra208 are possible as well.
• Polyaxial connections210 and212 betweensupport arms202 and204 and their respective fixation elements permitsupport arms202 and204, and thereby cephalad facet joint element201, to be rotated about the fixation elements around more than one axis. In fact,polyaxial connections210 and212 permit continuous adjustment betweensupport arms202 and204 and their fixation elements around many axes, up to a motion limit provided by a limit stop. In this embodiment,polyaxial connections210 and212 are substantially the same as those shown in the embodiments ofFIGS. 4-19. In other embodiments, however, other polyaxial connections may be used; the number of axes of rotation may also be limited, and movement may be permitted only in discrete increments.
• The relative positions of cephalad facet joint element201 and the fixation elements may be set prior to implant, after implant, or both before and after implant. In addition, this embodiment provides for longitudinal adjustment between the facet joint element201 and the fixation elements. The longitudinallyadjustable connections214 and216permits arms202 and204, respectively, to be moved along longitudinal axes with respect to their fixation elements. Once again, the longitudinally adjustable connections of this embodiment are substantially similar to the longitudinally adjustable connections described above with respect to the embodiments shown inFIGS. 4-19. In this embodiment, supportarms202 and204 may also be rotated about longitudinal axes, if desired. The relative longitudinal and rotational positions ofsupport arms202 and204 and their fixation elements may be set prior to implant, after implant, or both before and after implant to adjust the position of the cephalad facetjoint bearing surface206.
• FIGS. 22-24 also show a caudal facetjoint prosthesis300 with an artificial facetjoint bearing surface302 supported bysupport arms304 and306. In this embodiment, artificialfacet bearing surface302 is disposed substantially at the midline ofvertebra308 so as to meet and interact with thefacet bearing surface206 ofcephalad prosthesis200, as shown.Fixation elements310 and312 attachsupport arms304 and306, respectively, tovertebra308, such as at pedicle portions of the vertebra.Fixation elements310 and312 may be a screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. In this embodiment, supportarms304 and306 andfacet bearing surface302 are formed as a integral piece. In other embodiments, these elements may be formed as separate pieces. In use,caudal prosthesis300 may be selected from a kit containing caudal prostheses of different sizes, such as between 32 mm and 60 mm wide prostheses for the L5 vertebra.
• FIGS. 25-30 show yet another embodiment of the invention providing another artificial facet joint located approximately at the midline of the vertebrae. Cephalad prosthesis400 has a facet joint element including two support arms orrods402 and404 supporting acephalad bearing element406. In this embodiment, facetjoint element406 includes a facet jointbearing surface element414 with a facetjoint bearing surface416 formed on one side thereof. As shown, facet jointbearing surface element414 has aplate418 extending from its posterior side. In this embodiment,plate418 attaches to facet jointbearing surface element414 via a force fit connection at the crossbar'scenterpoint420.Plate418 connects to supportarms402 and404 viapolyaxial connections422 and424, respectively. The appropriate size of the prosthesis, such as the width ofplate418 and the size of bearingsurface416, may be determined by a sizing tool and selected appropriately.
• Support arms402 and404 are each attached tosuperior vertebra408 via fixation elements such as screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. As shown, the fixation elements arescrews403 which attach prosthesis400 to pedicle portions ofvertebra408. Attachment to other portions ofvertebra408 are possible as well.
• Polyaxial connections410 and412 betweensupport arms402 and404 and their respective fixation elements permitsupport arms402 and404, and thereby cephalad facetjoint element406, to be rotated about the pedicle fixation elements around more than one axis. In fact,polyaxial connections410 and412 permit continuous adjustment betweensupport arms402 and404 and their fixation elements around many axes, up to a motion limit provided by a limit stop. In this embodiment,polyaxial connections410 and412 are substantially the same as those shown in the embodiments ofFIGS. 4-19. In other embodiments, however, other polyaxial connections may be used; the number of axes of rotation may also be limited, and movement may be permitted only in discrete increments.
• The structure ofpolyaxial connections422 and424 in this embodiment is shown inFIG. 30. As shown,support arm404 has ahead426 with a substantiallyspherical surface428 disposed in aspherical pocket430 formed inplate418.Support404 extends through anopening432 inplate418 that is wider than the arm portion ofsupport arm404 but narrower than thesupport arm head426. As can be seen fromFIG. 30, the connection betweensupport arm404 andplate418 permits supportarm404 andplate418 to be rotated with respect to each other about more than one axis. Once the proper relative orientation has been achieved, anut434 mounted on a threadedportion436 ofsupport arm404 is tightened againstplate418. The concave face ofnut434 matches the convex shape of the underside ofplate418 to enablenut434 to be tightened againstplate418 to maintain the relative orientation ofsupport arm404 andplate418.Polyaxial connection422 has a substantially similar construction.
• While other materials may be used, supportarms402 and404 andplate418 are preferably formed from titanium, and facetjoint element406 is preferably formed from biocompatible metals (such as cobalt chromium steel, surgical steels, titanium, titanium alloys, tantalum, tantalum alloys, aluminum, etc.), ceramics, polyethylene, biocompatible polymers, and other materials known in the prosthetic arts.
• Caudal prosthesis500 has an artificial facetjoint bearing surface502 supported bysupport arms504 and506. In this embodiment, artificial facetjoint bearing surface502 is disposed substantially at the midline ofvertebra508 so as to meet and interact with the facetjoint bearing surface416 of the cephalad prosthesis400, as shown inFIG. 28. When installed in the patient, supportarms504 and506 elevate facetjoint bearing surface502 above the dura portion of the vertebra.
• Fixation elements510 and512 attachcaudal prosthesis500 tovertebra508, such as at pedicle portions of the vertebra as shown.Fixation elements510 and512 may be a screw, stem, corkscrew, wire, staple, adhesive, bone, and other materials known in the prosthetic arts. In the embodiment shown inFIGS. 25-30,fixation elements510 and512 are self-tapping screws.
• The appropriate size ofcaudal prosthesis500 may be determined by using a sizing tool. To attachcaudal prosthesis500 tovertebra508, a probe is used to determine the appropriate screw length to be used, as known in the art. The probe makes a pilot hole for the screw. As shown inFIG. 30, thescrews510 and512 are inserted intocaudal screw housings514 and516, respectively, which have substantially conical inside surfaces, such assurface518 ofhousing514. The screws have substantially spherical heads, such as screw head520 ofscrew510. The interaction of the spherical screw heads with the conical screw housing surfaces enables the screws to be inserted at angles other than 90°. After insertion, setscrews522 and524 with substantially spherically concave bottom surfaces are inserted intoscrew housings514 and516, respectively, to holdsupport arms504 and506 against their respective screws.
• The interaction between the cephalad and caudal bearing surfaces governs how the vertebrae to which they are attached move with respect to each other. The bearing surface shapes can provide motion limit stops and can help replace the function of removed ligaments. For example, the cephalad and caudal bearing surfaces of some preferred embodiments of the invention provide for ranges of motion of 0 to 20 degrees in flexion, 0 to 10 degrees in extension, 0 to 15 degrees in axial rotation, and 0 to 4 degrees in lateral bending.
• The relative shapes of the cephalad and caudal bearing surfaces also govern the quality of the relative movement between the vertebrae to which they are attached. For example, the quality of the relative movement provided by the movable cephalad bearing surface described above with reference toFIGS. 22-24 is different than the sliding point of contact connection between the concave cephalad bearing surface and convex caudal bearing surface of the embodiment shown inFIGS. 25-30.
• While preferred embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims (77)

1. A facet joint prosthesis comprising an artificial facet joint element connected to a fixation element by a polyaxially adjustable connection.

2. The facet joint prosthesis ofclaim 1 wherein the polyaxially adjustable connection is adapted and configured to permit the artificial facet joint element to be rotated with respect to the fixation element around more than one axis.

3. The facet joint prosthesis ofclaim 2 wherein the polyaxially adjustable connection is further adapted and configured to permit the position of the artificial facet joint element to be continuously adjustable within an adjustability range with respect to the fixation element.

4. The facet joint prosthesis ofclaim 2 wherein the polyaxially adjustable connection comprises a limit stop.

5. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint element is adapted to be movable medially with respect to the fixation element.

6. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint lement is adapted to be movable laterally with respect to the fixation element.

7. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint element is adapted to be movable superiorly with respect to the fixation element.

8. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint element is adapted to be movable inferiorly with respect to the fixation element.

9. The facet joint prosthesis ofclaim 1 wherein the polyaxially adjustable connection is adapted and configured to permit the artificial facet joint element to be moved with respect to the fixation element after installation of the facet joint prosthesis in a patient.

10. The facet joint prosthesis ofclaim 1 further comprising a fastener adapted and configured to prevent movement between the artificial facet joint element and the fixation element.

11. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint element comprises a cephalad facet joint bearing surface and a cephalad facet joint bearing surface support.

12. The facet joint prosthesis ofclaim 11 wherein the cephalad facet joint bearing surface and the cephalad facet joint bearing surface support are adapted and configured to be disposed laterally from a midline of a vertebra when the facet joint prosthesis is installed in a patient.

13. The facet joint prosthesis ofclaim 11 wherein the cephalad facet joint bearing surface and the cephalad facet joint bearing surface support are adapted and configured to be disposed approximately at a midline of a vertebra when the facet joint prosthesis is installed in a patient.

14. The facet joint prosthesis ofclaim 1 wherein the artificial facet joint element comprises a caudal facet joint bearing surface and a caudal facet joint bearing surface support.

15. The facet joint prosthesis ofclaim 14 wherein the caudal facet joint bearing surface and the caudal facet joint bearing surface support are adapted and configured to be disposed laterally from a midline of a vertebra when the facet joint prosthesis is installed in a patient.

16. The facet joint prosthesis ofclaim 1 further comprising a longitudinally adjustable connection between the artificial facet joint element and the fixation element.

17. The facet joint prosthesis ofclaim 16 wherein the artificial facet joint element comprises a facet joint bearing surface and a facet joint bearing surface support, the facet joint bearing surface support being adapted and configured to be longitudinally movable with respect to the fixation element.

18. The facet joint prosthesis ofclaim 17 further comprising a fastener adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element.

19. The facet joint prosthesis ofclaim 17 wherein the facet joint bearing surface support is further adapted and configured to be rotatable about a support longitudinal axis.

20. The facet joint prosthesis ofclaim 1 wherein the polyaxially adjustable connection comprises a facet joint element connection surface and a fixation element connection surface, the facet joint element connection surface and fixation element connection surface being adapted and configured to be movable with respect to each other.

21. The facet joint prosthesis ofclaim 1 wherein the polyaxially adjustable connection comprises a base member attached to the artificial facet joint element and movable with respect to the fixation element.

22. The facet joint prosthesis ofclaim 21 wherein the base member is further adapted and configured to be movable with respect to the artificial facet joint element.

23. A facet joint prosthesis comprising:

first and second artificial facet joint elements;

a fixation element; and

a polyaxially adjustable connection between at least one of the first or second facet joint elements and the fixation element.

24. The facet joint prosthesis ofclaim 23 wherein the first artificial facet joint element comprises an artificial cephalad facet joint element and the second artificial facet joint element comprises an artificial caudal facet joint element.

25. The facet joint prosthesis ofclaim 24 wherein the fixation element comprises a cephalad fixation element and the polyaxially adjustable connection comprises a cephalad polyaxially adjustable connection, the facet joint prosthesis further comprising a caudal fixation element and a caudal polyaxially adjustable connection between the artificial caudal facet joint bearing element and the caudal fixation element.

26. The facet joint prosthesis ofclaim 25 wherein the artificial cephalad facet joint element comprises a cephalad bearing surface and a cephalad bearing surface support.

27. The facet joint prosthesis ofclaim 25 wherein the artificial caudal facet joint element comprises a caudal bearing surface and a caudal bearing surface support.

28. The facet joint prosthesis ofclaim 23 wherein the first and second artificial facet joint elements comprise first and second artificial cephalad facet joint elements, respectively.

29. The facet joint prosthesis ofclaim 28 wherein the fixation element comprises a first cephalad fixation element and the polyaxially adjustable connection comprises a first cephalad polyaxially adjustable connection, the facet joint prosthesis further comprising a second cephalad fixation element and a second cephalad polyaxially adjustable connection between the second artificial cephalad facet joint element and the second cephalad fixation element.

30. The facet joint prosthesis ofclaim 28 wherein the first and second artificial cephalad facet joint elements comprise first and second cephalad bearing surfaces, respectively.

31. The facet joint prosthesis ofclaim 28 wherein the first and second artificial facet joint elements comprise first and second support arms, respectively, and at least one cephalad bearing surface supported by at least one of the first and second support arms.

32. The facet joint prosthesis ofclaim 31 wherein the cephalad bearing surface is disposed approximately at a midline of a vertebra when the prosthesis is installed in a patient.

33. The facet joint prosthesis ofclaim 32 wherein the cephalad bearing surface is adapted to rotate about the support arm or arms by which it is supported.

34. The facet joint prosthesis ofclaim 32 further comprising an artificial caudal facet joint element comprising a caudal bearing surface adapted and configured to mate with the cephalad bearing surface.

35. The facet joint prosthesis ofclaim 34 wherein the polyaxially adjustable connection is adapted and configured to permit at least one of the artificial facet joint elements to be rotated with respect to the fixation element around more than one axis.

36. The facet joint prosthesis ofclaim 35 wherein the polyaxially adjustable connection is further adapted and configured to permit the position of at least one of the artificial facet joint elements to be continuously adjustable within an adjustability range with respect to the fixation element.

37. The facet joint prosthesis ofclaim 35 wherein the polyaxially adjustable connection comprises a limit stop.

38. The facet joint prosthesis ofclaim 23 wherein the polyaxially adjustable connection is adapted and configured to permit at least one of the artificial facet joint elements to be moved with respect to the fixation element after installation of the facet joint prosthesis in a patient.

39. The facet joint prosthesis ofclaim 23 further comprising a fastener adapted and configured to prevent relative movement between at least one of the artificial facet joint elements and the fixation element.

40. The facet joint prosthesis ofclaim 23 further comprising a longitudinally adjustable connection between at least one of the artificial facet joint elements and the fixation element.

41. The facet joint prosthesis ofclaim 40 wherein at least one of the artificial facet joint elements comprises a facet joint bearing surface and a facet joint bearing surface support, the facet joint bearing surface support being adapted and configured to be longitudinally movable with respect to the fixation element.

42. The facet joint prosthesis ofclaim 41 further comprising a fastener adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element.

43. The facet joint prosthesis ofclaim 41 wherein the facet joint bearing surface support is further adapted and configured to be rotatable about a support longitudinal axis.

44. The facet joint prosthesis ofclaim 23 wherein the polyaxially adjustable connection comprises a facet joint element connection surface and a fixation element connection surface, the facet joint element connection surface and fixation element connection surface being adapted and configured to be movable with respect to each other.

45. The facet joint prosthesis ofclaim 23 wherein the polyaxially adjustable connection comprises a base member attached to at least one of the artificial facet joint elements and movable with respect to the fixation element.

46. The facet joint prosthesis ofclaim 45 wherein the base member is further adapted and configured to be movable with respect to at least one of the artificial facet joint elements.

47. A facet joint prosthesis comprising:

first and second fixation elements;

an artificial facet joint bearing surface adapted and configured to be disposed approximately at a midline of a vertebra when the facet joint prosthesis is installed in a patient; and

first and second polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively.

48. The facet joint prosthesis ofclaim 47 wherein the first and second polyaxially adjustable connections are each adapted and configured to permit the first and second fixation elements to be rotated with respect to the artificial facet joint bearing surface around more than one axis.

49. The facet joint prosthesis ofclaim 47 further comprising first and second fasteners adapted and configured to prevent relative movement between the artificial facet joint bearing surface and the first and second fixation elements, respectively.

50. The facet joint prosthesis ofclaim 47 further comprising first and second longitudinally adjustable connections between the artificial facet joint bearing surface and the first and second fixation elements, respectively.

51. The facet joint prosthesis ofclaim 50 further comprising a first support arm disposed between the first fixation element and the artificial facet joint bearing surface and a second support arm disposed between the second fixation element and the artificial facet joint bearing surface, the first and second support arms each being adapted and configured to support the artificial facet joint bearing surface and to be longitudinally movable with respect to the first and second fixation elements, respectively.

52. The facet joint prosthesis ofclaim 51 further comprising first and second fasteners each adapted and configured to prevent relative movement between the facet joint bearing surface support and the fixation element.

53. The facet joint prosthesis ofclaim 51 wherein the first support arm is further adapted and configured to be rotatable about a first support arm longitudinal axis and the second support arm is further adapted and configured to be rotatable about a second support arm longitudinal axis.

54. The facet joint prosthesis ofclaim 51 wherein the first and second support arms comprise two separate pieces.

55. The facet joint prosthesis ofclaim 51 wherein the first and second support arms comprise one integral piece.

56. The facet joint prosthesis ofclaim 47 wherein the artificial facet joint bearing surface is adapted and configured to be a cephalad facet joint bearing surface.

57. The facet joint prosthesis ofclaim 47 further comprising third and fourth polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively.

58. A facet joint prosthesis comprising:

an artificial facet joint bearing surface supported by first and second support arms adapted and configured to dispose the artificial facet joint bearing surface approximately at a midline of a vertebra when the facet joint prosthesis is installed in a patient; and

first and second fixation elements adapted and configured to attach the first and second support arms, respectively, to a vertebra.

59. The facet joint prosthesis ofclaim 58 wherein the first and second support arms and the artificial facet joint bearing surface comprise one integral piece.

60. The facet joint prosthesis ofclaim 58 further comprising first and second polyaxially adjustable connections between the first and second arms and the first and second fixation elements, respectively.

61. The facet joint prosthesis ofclaim 60 further comprising third and fourth polyaxially adjustable connections between the facet joint bearing surface and the first and second fixation elements, respectively.

62. A method of installing an artificial facet joint prosthesis, the prosthesis comprising a facet joint element and a fixation element, the method comprising:

attaching the prosthesis to a vertebra with the fixation element; and

adjusting positions of the facet joint element and the fixation element to a relative orientation.

63. The method ofclaim 62 wherein the adjusting step comprises moving the facet joint element medially with respect to the fixation element.

64. The method ofclaim 62 wherein the adjusting step comprises moving the facet joint element laterally with respect to the fixation element.

65. The method ofclaim 62 wherein the adjusting step comprises moving the facet joint element superiorly with respect to the fixation element.

66. The method ofclaim 62 wherein the adjusting step comprises moving the facet joint element inferiorly with respect to the fixation element.

67. The method ofclaim 62 wherein the attaching step is performed prior to the adjusting step.

68. The method ofclaim 62 wherein the attaching step is performed after the adjusting step.

69. The method ofclaim 62 wherein the adjusting step comprises rotating the facet joint element with respect to the fixation element around more than one axis.

70. The method ofclaim 69 further comprising preventing further rotation of the facet joint element after the rotating step.

71. The method ofclaim 62 wherein the facet joint element comprises a facet joint bearing surface, the adjusting step comprising positioning the facet joint bearing surface laterally from a vertebra midline.

72. The method ofclaim 62 wherein the facet joint element comprises a facet joint bearing surface, the adjusting step comprising positioning the facet joint bearing surface approximately at a vertebra midline.

73. The method ofclaim 62 wherein the facet joint element comprises a facet joint bearing surface, the adjusting step comprising positioning the facet joint bearing surface to face caudad.

74. The method ofclaim 62 wherein the facet joint element comprises a facet joint bearing surface, the adjusting step comprising positioning the facet joint bearing surface to face cephalad.

75. The method ofclaim 62 wherein the adjusting step comprises moving the facet joint element longitudinally with respect to the fixation element.

76. The method ofclaim 75 further comprising preventing further longitudinal movement of the facet joint element after the moving step.

77. The method ofclaim 75 wherein the adjusting step further comprises rotating the facet joint element about a facet joint element longitudinal axis.

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