Abstract:
The present invention provides a Posterior Dynamic Stabilization (PDS) device that allows elongation, which is a critical requirement for a PDS device as it allows pedicles to travel naturally in flexion, extension, and lateral bending of the spine. This interpedicular travel preserves a more natural center of rotation unlike other PDS devices that simply allow bending. In particular, the invention involves a PDS spring rod, wherein the helix is created with composite flow molding (CFM) technology and comprises a polymer matrix reinforced with continuous carbon fibers, wherein the fibers are oriented substantially parallel to the centerline of the helix, thereby creating a high strength spring.

Description:
BACKGROUND OF THE INVENTION 
     The vertebrae in a patient&#39;s spinal column are linked to one another by the disc and the facet joints, which control movement of the vertebrae relative to one another. Each vertebra has a pair of articulating surfaces located on the left side, and a pair of articulating surfaces located on the right side, and each pair includes a superior articular surface, which faces upward, and an inferior articular surface, which faces downward. Together the superior and inferior articular surfaces of adjacent vertebra form a facet joint. Facet joints are synovial joints, which means that each joint is surrounded by a capsule of connective tissue and produces a fluid to nourish and lubricate the joint. The joint surfaces are coated with cartilage that allow the joints to move or articulate relative to one another. 
     Diseased, degenerated, impaired, or otherwise painful facet joints and/or discs can require surgery to restore function to the three joint complex. Damaged, diseased levels in the spine were traditionally fused to one another. While such a technique may relieve pain, it effectively prevents motion between at least two vertebrae. As a result, additional stress may be applied to the adjoining levels, thereby potentially leading to further damage. 
     More recently, techniques have been developed to restore normal function to the facet joints. One such technique involves covering the facet joint with a cap to preserve the bony and articular structure. Capping techniques, however, are limited in use as they will not remove the source of the pain in osteoarthritic joints. Caps are also disadvantageous as they must be available in a variety of sizes and shapes to accommodate the wide variability in the anatomical morphology of the facets. Caps also have a tendency to loosen over time, potentially resulting in additional damage to the joint and/or the bone support structure containing the cap. 
     Other techniques for restoring the normal function to the posterior element involve arch replacement, in which superior and inferior prosthetic arches are implanted to extend across the vertebra typically between the spinous process. These arches can articulate relative to one another to replace the articulating function of the facet joints. One drawback to current articulating facet replacement devices, however, is that they require the facet joints to be resected, which entails an invasive surgery. Moreover, alignment of the articulating surfaces with one another can be challenging. 
     Accordingly, there remains a need for improved systems and methods that are adapted to mimic the natural function of the facet joints. 
     Traditional spine fusion may result in early degeneration at adjacent spine levels due to increased loading and compensation. This may result in subsequent surgeries to fuse additional levels. Stabilization using more dynamic rods with traditional pedicle screw instrumentation may improve surgical outcomes and reduce additional surgeries for adjacent level degeneration. 
     U.S. Pat. No. 7,419,714 (Magerl) discloses a composite of polymer or ceramic material having reinforcing fibers. The composite is used to manufacture medical implants such as osteosynthesis plates, endoprostheses, screw coupling elements, surgical instruments, and similar components. The reinforcing fibers and fibrous parts are made from a material that absorbs X-rays so that it can be seen during X-ray examination. Magerl proposes that the composite comprises a polymer or ceramic material with a high fiber percentage, primarily using continuous, long or short fibers, wherein at least a small percentage of fibers or fibrous parts consist of a material with a high X-ray absorption. Despite a very high percentage of continuous fibers, the volume percentage of residual material can be retained, and the existing strength characteristics can be retained or even enhanced through the sole replacement of otherwise present fibers with fibers consisting of a material with a high X-ray absorption. In one embodiment, it is proposed that the composite consist of carbon fiber-reinforced PAEK (poly-aryl-ether-ketone) and a percentage of fibers made out of a material with a high X-ray absorption. This makes it a material with a special compatibility, high strength and the visibility necessary for X-ray diagnostics. Optimal strength levels can be achieved by designing the carbon fibers and fibers made out of a material with a higher X-ray absorption as continuous fibers and/or fibers with a length exceeding 3 mm. To enable a transfer of force between the fibers and the other material of the composite, i.e., to also ensure an optimal strength at a high volume density of fibers, the selected fibers should be enveloped on the surface by the matrix material both in the preform and in the finished component. 
     SUMMARY OF THE INVENTION 
     The present invention provides a posterior dynamic stabilization (PDS) device that allows elongation, which is a critical requirement for a PDS device, as it allows pedicles to travel naturally during the flexion, extension, and lateral bending of the spine. This interpedicular travel preserves a more natural center of rotation, and so differentiates the PDS device of the present invention from other PDS devices that simply allow bending. In preferred embodiments, the invention involves a PDS spring rod having a helix created with composite flow molding (CFM) technology. The helix is preferably made of a polymer matrix reinforced with continuous carbon fibers, wherein the fibers are oriented substantially parallel to the centerline of the helix, thereby creating a high strength spring. 
     The device of the present invention provides superior yield strength, ultimate strength and fatigue properties. The device will allow greater elongation than PDS devices made from traditional materials. The composite material provides the PDS device of the present invention with high flexibility and strength, whereas conventional PDS systems that use metallic springs do not allow much elongation prior to yielding. It is further believed that the composite spring rod of the present invention will have better imaging capabilities than conventional metallic rods because it is an inherently radiolucent polymer. 
     Therefore, in accordance with the present invention, there is provided a posterior dynamic spinal stabilization system, comprising:
         a) a rod having a first end portion, an intermediate portion having a substantially helical portion having a centerline, and a second end portion,   b) a first bone anchor having i) a shank and ii) a rod-receiving portion mated to the first end portion of the rod,   c) a second bone anchor having i) a shank and ii) a rod-receiving portion mated to the second end portion of the rod,   wherein the substantially helical portion of the rod comprises a polymer matrix reinforced with continuous fibers, wherein the fibers are oriented substantially parallel to the centerline of the substantially helical portion.       

    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1 a    discloses a perspective view of an intermediate portion of the rod of the present invention, wherein the intermediate portion comprises threaded ends. 
         FIG. 1 b    discloses a semi-cross-section of the rod of  FIG. 1 a    taken across a thread. 
         FIG. 2  discloses a rod of the present invention, comprising the intermediate portion of  FIG. 1  threaded onto a pair of cylindrical end portions. 
         FIG. 3  discloses a unitary rod of the present invention. 
         FIG. 4  discloses a pair of PDS devices of the present invention implanted in a human spine. 
         FIG. 5  discloses a unitary rod of the present invention having an elongated end portion suitable for topping off. 
         FIG. 6  discloses a centerline of a helix. 
         FIGS. 7 a -7 c    disclose the construction of a mold of the present invention from multiple components. 
         FIG. 8 a    presents a table of estimated stresses for various embodiments of the present invention. 
         FIG. 8 b    discloses the major principle stress contour of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of the present invention, and now referring to  FIG. 6 , the “centerline” of a helix is the dotted line shown in the helix of  FIG. 6 . It is the line that is the sum of the centerpoints created by the sequential transverse cross-sections of the helix. 
     The present invention involves a spinal implant device for posterior dynamic stabilization that includes a helical spring device constructed from a composite material. In a preferred embodiment of the invention, the helical spring is molded from a cylinder of an endless fiber reinforced thermoplastic, which will provide the device with excellent fatigue, flexural, shear, and tensile strengths. The cylinder of endless fiber reinforced thermoplastic is preferably made by a pultrusion process that creates endless fibers embedded in a polymer matrix and running parallel to the longitudinal axis of the cylinder. This molding process combined with the endless fibers results in a spring with high strength, since the fibers are oriented to run substantially parallel to the centerline of the helix and never protrude from its surface. 
     Now referring to  FIG. 1 a   , there is provided a component that represents an intermediate portion  1  of the rod of the present invention, wherein the intermediate portion comprises a helical mid portion  3  and threaded ends  5 . The entire intermediate portion of the rod comprises (and preferably consists essentially of) a polymer matrix reinforced with continuous carbon fibers. In the helical mid portion thereof, the continuous carbon fibers are oriented parallel to the centerline of the helix substantially throughout the helix, thereby creating a high strength spring. 
     Therefore, in accordance with the present invention, there is provided a posterior dynamic spinal stabilization rod having a first end portion, an intermediate portion having a substantially helical portion having a centerline, and a second end portion, wherein the substantially helical portion of the rod comprises a polymer matrix reinforced with continuous fibers, wherein the continuous fibers are oriented substantially parallel to the centerline of the helical portion. 
     As shown in  FIG. 1 a   , the ends of the intermediate portion may have threads. If threads are present, the fibers in the thread region can advantageously be oriented along the contour of the thread (i.e., parallel to the contour surface) such that the threads possess a high strength. In the threaded end regions, the deeper continuous carbon fibers are oriented parallel to the longitudinal axis of this region, while the more superficial continuous carbon fibers are oriented as waves along the profile of the thread, thereby creating threads of high strength. Now referring to  FIG. 1 b   , there is provided a diagram of such fiber preferred orientation. This image illustrates deep axial fibers (AF): transition zone fibers (TZF); and then superficial fibers oriented along the contour (CF) to give the threads high strength. The present invention takes advantage of this graded orientation for PDS spring rod applications. 
     Now referring to  FIG. 2 , there is provided a rod  11  of the present invention, comprising the intermediate portion  1  of  FIG. 1  threaded onto a pair of cylindrical end portions  13 . The intermediate portion  1  of the modular rod comprises a helical mid portion  3  and threaded ends  5 . The first and second cylindrical end portions  13  of the modular rod each have threaded inner ends  15 . The threaded inner ends of the first and second cylindrical end portions of the modular rod threadably mate with respective threaded ends of the intermediate portion of the modular rod. 
     Therefore, in accordance with the present invention, there is provided a posterior dynamic spinal stabilization system comprising:
         a) a modular rod having a first end portion, an intermediate portion, and a second end portion,   b) a first bone anchor having i) a shank and ii) a rod-receiving portion mated to the first end portion of the rod,   c) a second bone anchor having i) a shank and ii) a rod-receiving portion mated to the second end portion of the rod,
 
wherein the intermediate portion of the modular rod comprises a helical mid portion and first and second threaded ends, wherein the first and second end portions of the modular rod each have threaded inner ends, wherein the threaded inner ends of the first and second end portions of the modular rod threadably mate with the first and second threaded ends of the intermediate portion of the modular rod.
       

     In some embodiments, the molded threads  5  at the ends of the intermediate portion are threadably attached to threaded metallic inner ends  15 . Preferably, these metallic inner ends are titanium. In some embodiments, a tapered thread or other interference feature can be used to prevent the mated end components from loosening. 
     Now referring to  FIG. 3 , in this embodiment, there is provided a unitary rod  21  of the present invention. The rod has first and second cylindrical end portions  23 , and an intermediate portion  25  therebetween that comprises a substantially helical mid portion  27 . 
     Preferably, the end portions of the rod are molded into a substantially cylindrical shape. These cylindrical end portions can then be attached to a pair of conventional pedicle screws. 
     When, as here, the intermediate and end portions are integral, they are made of the same composite material. Alternatively, they can be made from different materials, e.g., from metallic cylindrical sections and composite spring sections, as in  FIG. 2 . 
     Now referring to  FIG. 4 , there is disclosed an implanted PDS device  31  of the present invention (comprising the rod attached to a pair of pedicle screws) providing dynamic stabilization to the spine. Although the device is shown in a single level application, it may also be used in multi-level procedures as well. Conventional techniques for implanting PDS rods and screws may be used. In  FIG. 4 , the pedicle screws attach to the end portions of the device of the present invention. In other embodiments (not shown), at least one of the pedicle screws may attach to a helical portion of the device. 
     The “topping off” indication for early degenerative disc disease and stenosis may be a commercially valuable application for PDS devices. It is believed that the device of the present invention would also be suitable as a topping off solution to prevent overloading of the adjacent disc. Now referring to  FIG. 5 , the composite rod may be made with one or more elongated end portions, wherein the length L E  of the elongated end portion is at least 150% of the length L I  of the intermediate portion. Such a rod may be advantageously used in topping off applications, such as in DDD or stenosis applications. As shown in  FIG. 5 , the intermediate portion may be connected to the end portion by a transition region, in this case a frustoconical transition region. 
     In some embodiments (as in  FIG. 5 ), a preferred “topping off” device comprises first and second end portions, wherein the first end portion is longer than the second end portion. Preferably, the first end portion is at least twice as long as the second end portion. 
     In some embodiments (not shown), the rod may possess more than one helical sections. 
     The CFM process can be used to create components with varying levels of carbon fiber. Carbon fiber content may typically be between 10 vol % and 70 vol %. The thermoplastic material may be polyarylethyl ketone (PAEK) or any other biocompatible polymer. The carbon content and/or thermoplastic content may be altered to create springs with varying stiffnesses for different clinical applications. 
     In some embodiments, at least 50 vol % of the continuous fibers are at least 3 mm in length. More preferably, at least 50 vol % of the continuous fibers are at least 10 mm in length. More preferably, at least 50 vol % of the continuous fibers run from one end of the intermediate portion of the rod to the other end. 
     When a polymer is chosen as a material of construction, the polymer is preferably selected from the group consisting of polycarbonates, polyesters, (particularly aromatic esters such as polyalkylene terephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE; polyarylethyl ketone (PAEK); and mixtures thereof. 
     In some embodiments, at least a portion of the rod component is made from a composite comprising PAEK and carbon fiber. Composites comprising carbon fiber are advantageous in that they typically have a strength and stiffness that is superior to neat polymer materials such as a polyarylethyl ketone PAEK. In some embodiments, the rod is made from a polymer composite such as a polyether ketone ketone (PEKK)-carbon fiber composite. 
     Preferably, the composite comprising carbon fiber further comprises a polymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). More preferably, the PAEK is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK). In preferred embodiments, the PAEK is PEEK. 
     In some embodiments, the carbon fiber comprises between 40 vol % and 80 vol % (more preferably, between 50 vol % and 70 vol %) of the composite. In some embodiments, the polymer and carbon fibers are homogeneously extruded, with the fibers running parallel to the longitudinal axis of the extrudate. In others, the material is a laminate. In other embodiments, the longitudinal fibers are covered with a layer of braided fibers, with a braid angle between about 10 degrees and about 80 degrees, preferably about 45 degrees. 
     In especially preferred embodiments, the composite comprises: 
     a) 30-50% (more preferably, 40 vol %) polyarylethyl ketone (PAEK), and 
     b) 50-70% (more preferably, about 60 vol %) carbon fiber, 
     wherein the polyarylethyl ketone (PAEK) is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK). 
     In some embodiments, the composite consists essentially of PAEK and carbon fiber. More preferably, the composite comprises 30-50 vol % PAEK and 50-70 vol % carbon fiber. Still more preferably the composite comprises about 40 vol % PAEK and 60 vol % carbon fiber. 
     Preferred composite materials of the present invention can be obtained from Icotec AG, Industriestrasse 12, CH-9450 Altstatten, Switzerland. 
     Preferably, the substantially helical portion of the rod comprises a polymer matrix reinforced with continuous fibers, wherein the fibers are oriented substantially parallel to the centerline of the helix. Accordingly, the continuous fibers should never break the surface of the helix. 
     In some embodiments, a first portion of the fibers are carbon fibers and a second portion of the fibers are made of a material that is more x-ray opaque than carbon. In some embodiments, such fiber mixtures are selected from the disclosure in U.S. Pat. No. 7,419,714, the specification of which is incorporated by reference in its entirety. In some embodiments, the x-ray opaque fibers are selected from the group consisting of tantalum, tungsten, gold, platinum, and their oxides. 
     In some embodiments, the composite material comprising the polymer matrix and continuous carbon fiber is extruded to form a long cylinder, in which the fiber runs parallel to the longitudinal axis of the cylinder. The extruded cylinder is then molded under elevated temperature and pressure to form the desired shape. 
     In one prophetic method of making the present invention, first, a rod is made by pultrusion using carbon fiber reinforced PEEK with carbon fiber volume fraction of 60 to 65%. The rod is cut into blank whose volume is precisely equal to the volume of the final spring. The blank is heated in a chamber where the PEEK melts and wet-out the continuous fibers, minimizing voids. The blank is then transferred to a compression mold cavity where it is pressed to form the final spring with fiber orientation along the coil to enhance strength. 
     Now referring to  FIG. 7 a   , in one preferred prophetic method of manufacturing the present invention, mold elements  51 , 52 , 53 , 54 , and  55  are first created. Now referring to  FIG. 7 b   , mold elements  52 , 53  and  54  are then assembled to form a subassembly  56 . Now referring to  FIG. 7 c   , lastly, mole elements  51  and  55  are combined with the subassembly  56  to form a final mold  57  that is useful for forming the implants of the present invention. 
     In an effort to optimize the implant design, a number of variables were modified on the implant design and qualities such as stiffness, stress and failure stress were estimated. Some of the variables modified include wire diameter, helix ID and OD, pitch and the number of coils.  FIG. 8 a    presents a table of estimated stresses for various embodiments of the present invention. The following Regression Equations were used to calculate stiffness and stress:
 
Stiffness=170.2+227.96*WD−101.95*CD+12.15*P−55.62*NC, and
 
Stress=1887.19+199.19*WD−369.85*CD+86.96*P−159.72*NC,
 
where WD is the wire diameter; CD is the nominal coil diameter ((OD+ID)/2); P is the pitch; and NC in the number of coils.
 
     These results demonstrate that a large wire diameter and a large OD/ID ratios produce favorable stress distributions in the implant. 
       FIG. 8 b    discloses the major principle stress contour of an embodiment of the present invention that was considered in the design optimization work presented in  FIG. 8   a.    
     One skilled in the art will appreciate that the rod of the device may be configured for use with any type of bone anchor, e.g., bone screw or hook; mono-axial or polyaxial. Typically, a bone anchor assembly includes a bone screw, such as a pedicle screw, having a proximal head and a distal bone-engaging portion, which may be an externally threaded screw shank. The bone screw assembly may also have a receiving member that is configured to receive and couple a spinal fixation element, such as a spinal rod or spinal plate, to the bone anchor assembly. 
     The receiving member may be coupled to the bone anchor in any well-known conventional manner. For example, the bone anchor assembly may be poly-axial, as in the present exemplary embodiment in which the bone anchor may be adjustable to multiple angles relative to the receiving member, or the bone anchor assembly may be mono-axial, e.g., the bone anchor is fixed relative to the receiving member. An exemplary poly-axial bone screw is described U.S. Pat. No. 5,672,176, the specification of which is incorporated herein by reference in its entirety. In mono-axial embodiments, the bone anchor and the receiving member may be coaxial or may be oriented at angle with respect to one another. In poly-axial embodiments, the bone anchor may biased to a particular angle or range of angles to provide a favored angle the bone anchor. Exemplary favored-angle bone screws are described in U.S. Patent Application Publication No. 2003/0055426 and U.S. Patent Application Publication No. 2002/0058942, the specifications of which are incorporated herein by reference in their entireties. 
     In some embodiments, the assembly may be implanted in accordance with the minimally invasive techniques and instruments disclosed in U.S. Pat. No. 7,179,261; and US Patent Publication Nos. US2005/0131421; US2005/0131422; US 2005/0215999; US2006/0149291; US2005/0154389; US2007/0233097; and US2005/0192589, the specifications of which are hereby incorporated by reference in their entireties.