Abstract:
An article including a facet prosthesis at least partially mounted in a lumen artificially formed between superior and inferior halves of a facet joint, and an elastomeric cushioning element disposed in the facet prosthesis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 USC §119 to U.S. Provisional Patent Application Ser. No. 60/512,193, filed on Oct. 20, 2003, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to human prostheses, and more particularly to prostheses for a spinal facet joint. 
   BACKGROUND OF THE INVENTION 
   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 has been shown 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. Also, 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 that 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 facet joints help to support axial, torsional, and shear loads that act on the spinal column. Furthermore, facet joints are diarthroidal joints that provide both sliding articulation and load transmission features. The effects of their absence as a result of facetectomy has been observed 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. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to facet joint prostheses, as is described more in detail hereinbelow. 
   There is thus provided in accordance with an embodiment of the present invention an article including a facet prosthesis at least partially mounted in a lumen artificially formed between superior and inferior halves of a facet joint, and an elastomeric cushioning element disposed in the facet prosthesis. 
   The facet prosthesis can include one or more of the following features. For example, the elastomeric cushioning element can include first and second mechanical fasteners at opposite ends thereof, which are attachable to superior and inferior halves of the facet joint. The elastomeric cushioning element can expand radially outward upon axial movement of the first and second mechanical fasteners towards each other. The first and second mechanical fasteners can include an expandable sleeve member in which is disposed a wedge, wherein insertion of the wedge into the expandable sleeve member expands the expandable sleeve member radially outwards. The expandable sleeve member can include an axial slit formed therein. The elastomeric cushioning element can include a plurality of elastomeric balls disposed in the lumen and sealed in the lumen with end caps. The elastomeric cushioning element can include a wire coil coated with an elastomeric material disposed in the lumen and sealed in the lumen with end caps. The wire coil can be constructed of a shape memory alloy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIGS. 1A and 1B  are pictorial illustrations of part of a human spine including a facet joint; 
       FIGS. 2A and 2B  are pictorial illustrations of a facet prosthesis in an uncompressed and compressed from, respectively; 
       FIG. 3  is a pictorial illustration of lumens being formed through aligned superior and inferior processes of adjacent vertebrae; 
       FIG. 4  is a pictorial illustration of the facet prosthesis of  FIGS. 2A and 2B  implanted within a facet joint between two adjacent vertebrae; 
       FIG. 5  is a pictorial illustration of a facet prosthesis including an elastomeric cushioning element, constructed and operative in accordance with another embodiment of the present invention; 
       FIGS. 6A and 6B  are pictorial illustrations of a facet prosthesis including an elastomeric cushioning element, constructed and operative in accordance with yet another embodiment of the present invention, respectively showing the prosthesis installed in a facet joint and showing a wire coil coated with an elastomeric material that forms part of the prosthesis; and 
       FIGS. 7A and 7B  are pictorial illustrations of a facet prosthesis including an elastomeric cushioning element, constructed and operative in accordance with still another embodiment of the present invention, respectively showing the prosthesis installed in a facet joint and showing a wire coil coated with an elastomeric material that forms part of the prosthesis. 
   

   DESCRIPTION OF EMBODIMENTS 
   Reference is now made to  FIGS. 1A and 1B , which illustrate a structure of a human spine, in particular the lumbar vertebrae including a facet joint. 
     FIGS. 1A and 1B  illustrate the fourth and fifth lumbar vertebrae L 4  and L 5 , respectively, in a lateral view (while in anatomic association) and in a superior view (separately). The lumbar vertebrae (of which there are a total of five) are in the lower back, also called the “small of the back.” 
   As is typical with vertebrae, the vertebrae L 4  and L 5  are separated by an intervertebral disk  25 . The configuration of the vertebrae L 4  and L 5  differ somewhat, but each (like vertebrae in general) includes a vertebral body  10 , which is the anterior, massive part of bone that gives strength to the vertebral column and supports body weight. The vertebral arch  12  is posterior to the vertebral body  10  and is formed by the right and left pedicles  14  and lamina  16 . The pedicles  14  are short, stout processes that join the vertebral arch  12  to the vertebral body  10 . The pedicles  14  project posteriorly to meet two broad flat plates of bone, called the lamina  16 . 
   Seven other processes arise from the vertebral arch. Three processes, called the spinous process  18  and two transverse  20  processes, project from the vertebral arch  12  and afford attachments for back muscles, forming levers that help the muscles move the vertebrae. The remaining four processes, called articular processes, project superiorly from the vertebral arch (and are thus called the superior articular processes  22 ) and inferiorly from the vertebral arch (and are thus called the inferior articular processes  24 ). The superior and inferior articular processes  22  and  24  are in opposition with corresponding opposite processes of vertebrae superior and inferior adjacent to them, forming joints, called zygapophysial joints or, in short hand, the facet joints or facets. Facet joints are found between adjacent superior and inferior articular processes along the spinal column and the facet joints permit gliding movement between the vertebrae L 4  and L 5 . 
   The facet joints can deteriorate or otherwise become injured or diseased, causing lack of support for the spinal column, pain, and/or difficulty in movement. 
   As described herein, a facet joint has a superior half and an inferior half, with cartilage therebetween. The superior half of the joint is formed by the vertebral level below the joint, and the inferior half of the joint is formed by the vertebral level above the joint. For example, in the L 4 –L 5  facet joint, the superior half of the joint is formed by structure on the L- 5  vertebra, and the inferior half of the joint is formed by structure on the L- 4  vertebra. 
   Reference is now made to  FIGS. 2A and 2B , which illustrate a facet prosthesis  30 , constructed and operative in accordance with an embodiment of the present invention. Facet prosthesis  30  may include an elastomeric cushioning element  32 , constructed of an elastomeric material, such as but not limited to, polyurethane or polyurethane containing materials, silicone or silicone containing materials, polyethylene based elastomers, hydrogels, and polypropylene containing materials, and the like, or any combination thereof. First and second mechanical fasteners  34  and  36  may be provided at opposite ends of the cushioning element  32 , which are attachable to the superior and inferior halves  22  and  24  ( FIGS. 1A and 1B ) of the facet joint, as described hereinbelow. 
   The elastomeric cushioning element  32  may initially have an oblong shape ( FIG. 2A ) prior to implantation in the facet joint. The elastomeric cushioning element  32  may then expand radially outwards (as indicated by arrows  38  in  FIG. 2B ) upon axial (e.g., compressive) movement of the first and second mechanical fasteners  34  and  36  towards each other (as indicated by arrows  40  in  FIG. 2B ). 
   In one non-limiting example, first and second mechanical fasteners  34  and  36  may be constructed as an expandable sleeve member  42  in which is disposed a wedge  44 . Insertion of the wedge  44  into expandable sleeve member  42  in the direction of arrow  40  expands expandable sleeve member  42  radially outwards in the direction of arrows  38 . Wedges  44  may be part of a threaded fastener, which may be screwed into expandable sleeve member  42 , similar to an expansion bolt (also called an anchor bolt or molly bolt) used to anchor objects on soft walls or ceilings. The expandable sleeve member  42  may have an axial slit  46  formed therein to facilitate outward expansion thereof. Wedges  44  may have a hexagonal socket  48  for turning with an Allen key and the like. The expandable sleeve member  42  may be made of a plastic, such as but not limited to, nylon, DELRIN or polyurethane, for example, or from a metal, such as but not limited to, stainless steel. Wedge  44  may be made of metal, such as but not limited to, stainless steel, titanium alloy, cobalt chromium alloys, ceramics, or other hard, rigid materials, or may be made of a plastic, such as but not limited to, nylon, DELRIN or polyurethane. 
   Reference is now made to  FIG. 3 , which shows a side view of a facet joint prior to placement of a facet prosthesis within the joint. 
   When adjacent vertebra  29  are aligned, and the superior articular facet  22  of one vertebra faces the inferior articular facet  24  of an adjacent vertebra facet, two lumens, or holes,  26  and  28  may be formed, such as by drilling with a drill  30 ′, through the aligned superior and inferior processes of the adjacent vertebrae  29 . 
   The facet prosthesis  30  may then be inserted in the lumens  26  and  28 . During insertion, the sleeve member  42  has not yet been expanded, so that the prosthesis  30  has a smaller diameter than the lumens  26  and  28  to aid in insertion of the prosthesis. Afterwards, the sleeve member  42  is expanded by the above-described action of wedges  44 , thereby anchoring facet prosthesis  30  in the lumens  26  and  28  formed in the facet joint, as seen in  FIG. 4 . In this installed orientation, the elastomeric cushioning element  32  is in its compressed form (expanded radially outward). 
   Reference is now made to  FIG. 5 , which illustrates a facet prosthesis  50 , constructed and operative in accordance with another embodiment of the present invention. In facet prosthesis  50 , the elastomeric cushioning element includes a plurality of elastomeric balls  52  disposed in a lumen  54 , which may be formed by drilling through cartilage between superior and inferior halves  22  and  24 . Access may be gained to lumen  54  alternatively by drilling an access hole  55  directly through superior half  22 . The elastomeric balls  52  may be sealed in lumen  54  with end caps  56 . Elastomeric balls  52  may be constructed of an elastomeric material, such as but not limited to, polyurethane or polyurethane containing materials, silicone or silicone containing materials, polyethylene based elastomers, hydrogels, and polypropylene containing materials, and the like, or any combination thereof, and may have any suitable size. The term “balls” encompasses not only spherical shaped objects, but also oblong, ellipsoidal, prismatic and other shapes that may be placed in lumen  54 . 
   Reference is now made to  FIGS. 6A and 6B , which illustrate a facet prosthesis  60 , constructed and operative in accordance with yet another embodiment of the present invention. In facet prosthesis  60 , the elastomeric cushioning element includes a wire coil  62  coated with an elastomeric material  64 , such as but not limited to, polyurethane or polyurethane containing materials, silicone or silicone containing materials, polyethylene based elastomers, hydrogels, and polypropylene containing materials, and the like, or any combination thereof. The wire coil  62  may be constructed of a shape memory alloy, such as but not limited to, NITINOL. 
   The elastomeric coated wire coil  62  may be disposed in lumen  54 , which may be formed by drilling through cartilage between superior and inferior halves  22  and  24 . The elastomeric coated wire coil  62  may be sealed in lumen  54  with end caps  66 . 
     FIGS. 7A and 7B  illustrate a facet prosthesis  70 , very similar to facet prosthesis  60 , except that facet prosthesis  70  may be introduced into lumen  54  through a passageway  72  formed by drilling directly through superior half  22 . Both facet prostheses  60  and  70  may have any shape or size, such as but not limited to, round, square, oblong, elliptical, triangular, etc. 
   Although various specific implementations have been described, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, other alternatives, modifications, and variations that fall within the scope of the following claims.