Patent Publication Number: US-8118869-B2

Title: Dynamic interbody device

Description:
BACKGROUND 
     1. Field of the Invention 
     Embodiments of the invention generally relate to functional spinal implant assemblies for insertion into an intervertebral space between adjacent vertebrae of a human spine, and reconstruction of the posterior elements to provide stability, flexibility, and proper biomechanical motion. More specifically, embodiments of the invention relate to artificial functional spinal units including an artificial intervertebral implant that can be inserted via a posterior surgical approach and used in conjunction with one or more dynamic posterior stabilization systems to approach an anatomically correct range of motion and segmental stiffness. Embodiments of the invention may also be inserted via an anterior surgical approach. 
     2. Description of Related Art 
     The human spine is a complex mechanical structure including alternating bony vertebrae and fibrocartilaginous discs that are connected by strong ligaments and supported by musculature that extends from the skull to the pelvis and provides axial support to the body. The intervertebral discs provide mechanical cushion between adjacent vertebral segments of the spinal column and generally include two basic components: the nucleus pulposus and the annulus fibrosis. The intervertebral discs are positioned between two vertebral end plates. The annulus fibrosis forms the perimeter of the disc and is a tough outer ring that binds adjacent vertebrae together. The end plates are made of thin cartilage overlying a thin layer of hard cortical bone that attaches to the spongy, cancellous bone of a vertebra. The vertebrae generally include a vertebral foramen bounded by the anterior vertebral body and the neural arch, which consists of two pedicles that are united posteriorly by the laminae. The spinous and transverse processes protrude from the neural arch. The superior and inferior articular facets lie at the root of the transverse process. 
     The human spine is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. However, genetic or developmental irregularities, trauma, chronic stress, and degenerative wear can result in spinal pathologies for which surgical intervention may be necessary. In cases of deterioration, disease, or injury, an intervertebral disc, or a portion of the intervertebral disc may be removed from the human spine during a discectomy. 
     After some discectomies, an intervertebral device may be placed in the disc space to fuse or promote fusion of the adjacent vertebrae. During some procedures, fusion may be combined with posterior fixation to address intervertebral disc and/or facet problems. The fusion procedure (e.g., posterior lumbar interbody fusion) and the posterior fixation procedure may be performed using a posterior approach. The posterior fixation may inhibit motion and promote bone healing. Fusing two vertebrae together may result in some loss of motion. Fusing two vertebrae together may also result in the placement of additional stress on one or more adjacent functional spinal units. The additional stress may cause deterioration of an adjacent functional spinal unit that may result in the need for an additional surgical procedure or procedures. 
     After some discectomies, an intervertebral dynamic device may be placed in the disc space. The dynamic device may allow for movement of the vertebrae coupled to the disc dynamic device relative to each other. U.S. Pat. No. 4,863,477 to Monson, which is incorporated herein by reference, discloses a resilient dynamic device intended to replace the resilience of a natural human spinal disc. U.S. Pat. No. 5,192,326 to Bao et al., which is incorporated herein by reference, describes a prosthetic nucleus for replacing just the nucleus portion of a human spinal disc. U.S. Patent Application Publication No. 2005/0021144 to Malberg et al., which is incorporated herein by reference, describes an expandable spinal implant. Allowing for movement of the vertebrae coupled to the disc prosthesis may promote the distribution of stress that reduces or eliminates the deterioration of adjacent functional spinal units. 
     An intervertebral device may be positioned between vertebrae using a posterior approach, an anterior approach, a lateral approach, or other type of approach. A challenge of positioning a device between adjacent vertebrae using a posterior approach is that a device large enough to contact the end plates and slightly expand the space must be inserted through a limited space. This challenge is often further heightened by the presence of posterior osteophytes, which may cause “fish mouthing” of the posterior vertebral end plates and result in very limited access to the disc. A further challenge in degenerative disc spaces is the tendency of the disc space to assume a lenticular shape, which may require a larger implant than can be easily introduced without causing trauma to adjacent nerve roots. The size of rigid devices that may safely be introduced into the disc space is thereby limited. During some spinal fusion procedures using a posterior approach, two implants are inserted between the vertebrae. During some posterior procedures, one or both facet joints between the vertebrae may be removed to provide additional room for the insertion of a fusion device. 
     The anterior approach poses significant challenges as well. Though the surgeon may gain very wide access to the interbody space from the anterior approach, this approach has its own set of complications. The retroperitoneal approach usually requires the assistance of a surgeon skilled in dealing with the visceral contents and the great vessels, and the spine surgeon has extremely limited access to the nerve roots. Complications of the anterior approach that are approach specific include retrograde ejaculation, ureteral injury, and great vessel injury. Injury to the great vessels may result in massive blood loss, postoperative venous stasis, limb loss, or death. The anterior approach is more difficult in patients with significant obesity and may be virtually impossible in the face of previous retroperitoneal surgery. 
     Furthermore, disc degeneration is often coupled with facet degeneration. Facet degeneration is addressed using a posterior approach. Thus a second surgical approach may be required if the disc degeneration is treated using an anterior approach. The need to address facet degeneration has led to the development of facet replacement devices. Some facet replacement devices are shown in U.S. Pat. No. 6,419,703 to Fallin et al.; U.S. Pat. No. 6,902,580 to Fallin et al.; U.S. Pat. No. 6,610,091 to Reiley; U.S. Pat. No. 6,811,567 to Reiley; and U.S. Pat. No. 6,974,478 to Reiley et al, each of which is incorporated herein by reference. The facet replacement devices may be used in conjunction with anterior disc replacement devices, but the facet replacement devices are not designed to provide a common center of rotation with the anterior disc replacement devices. The use of an anterior disc replacement device that has a fixed center of rotation contrary to the fixed center of rotation of the facet replacement device may restrict or diminish motion and be counterproductive to the intent of the operation. 
     Despite the difficulties of the anterior approach, the anterior approach does allow for the wide exposure needed to place a large device. In accessing the spine anteriorly, one of the major structural ligaments, the anterior longitudinal ligament, must be completely divided. A large amount of anterior annulus must also be removed along with the entire nucleus. Once these structures have been resected, the vertebral bodies may need to be over distracted to place the device within the disc space and restore disc space height. Failure to adequately tension the posterior annulus and ligaments increases the risk of device failure and/or migration. Yet in the process of placing these devices, the ligaments are overstretched while the devices are forced into the disc space under tension. Over distraction can damage the ligaments and the nerve roots. The anterior disc replacement devices currently available or in clinical trials may be too large to be placed posteriorly, and may require over distraction during insertion to allow the ligaments to hold them in position. 
     During some spinal stabilization procedures a posterior fixation system may be coupled to the spine. During some procedures, posterior fixation systems may be coupled to each side of the spine. The posterior fixation systems may include elongated members that are coupled to vertebrae by fasteners (e.g., hooks and screws). In some embodiments, one or more transverse connectors may be connected to the posterior fixation systems to join and stabilize the posterior fixation systems. 
     During some spinal stabilization procedures, dynamic posterior stabilization systems may be used. U.S. Patent Application Nos. 2005/0182409 to Callahan et al.; 2005/0245930 to Timm et al.; and 2006/0009768 to Ritland, each of which is incorporated herein by reference, disclose dynamic posterior stabilization systems. 
     SUMMARY 
     One or more dynamic interbody devices for a human spine may be inserted in a disc space between vertebrae. In some embodiments, a dynamic interbody device of a human spine may include a first member configured to couple to a first vertebra. The first member may have a curved surface. The dynamic interbody device may include a second member configured to couple to a second vertebra and the first member. The second member may have a curved surface complementary to the curved surface of the first member to allow for movement of the first vertebra relative to the second vertebra. The dynamic interbody device may include a bridge to couple the dynamic interbody device to a dynamic posterior stabilization system. 
     In some embodiments, the curved surface of the first member is a convex surface. In some embodiments, the width of the first dynamic interbody device is less than half the width of the first vertebra. In some embodiments, the curved surface of the first portion is a spherical portion. 
     In some embodiments, the bridge includes an angled surface. The angles surface may be configured to orient at least a portion of the posterior stabilization system so that the posterior stabilization system works in conjunction with the dynamic interbody device. The bridge may include an opening configured to couple to a bone fastener of the posterior stabilization system. 
     In some embodiments, a dynamic interbody device for a human spine may include a first member, a second member and a third member. The first member and the third member may be configured to couple to vertebrae. The first member may include a curved surface. The second member may be coupled to the first member. The second member may have a first curved surface that is complementary to the curved surface of the first member. The third member may couple to the second member. The third member may include a curved surface. The curved surface of the third member may be complementary to a second curved surface of the second member. In some embodiments, a width of the dynamic interbody device may be less than half the width of the vertebrae the dynamic interbody device is to be positioned between. In some embodiments, a bridge may be coupled to the dynamic interbody device. The bridge may couple the dynamic interbody device to a posterior stabilization system. 
     In some embodiments, the curved surface of the first member may be a spherical portion. In some embodiments, the curved surface of the first member allows for axial rotation of vertebrae coupled to the dynamic interbody device. In some embodiments, the curved surface of the first member allows for lateral bending of vertebrae coupled to the dynamic interbody device. 
     In some embodiments, the curved surface of the third member may allow for flexion of vertebrae coupled to the dynamic interbody device. In some embodiments, the curved surface of the third member may allow for extension of vertebrae coupled to the dynamic interbody device. In some embodiments, the third member may have a lead surface. The lead surface may facilitate placement of the dynamic interbody device in a disc space between vertebrae. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1  depicts a schematic view of a portion of an embodiment of a spinal stabilization system. 
         FIG. 2  depicts a perspective view of an embodiment of a dynamic interbody device. 
         FIG. 3  depicts a perspective view of a first member of the dynamic interbody device depicted in  FIG. 2 . 
         FIG. 4  depicts a side view of an embodiment of a separate component bridge. 
         FIG. 5  depicts a perspective view of a second member of the dynamic interbody device depicted in  FIG. 2 . 
         FIG. 6  depicts a perspective view of a third member of the dynamic interbody device depicted in  FIG. 2 . 
         FIG. 7  depicts a cross-sectional view of a third member of a dynamic interbody device. 
         FIG. 8  depicts a perspective view of embodiments of dynamic interbody devices. 
         FIG. 9  depicts a perspective view of an embodiment of a posterior stabilization system. 
         FIG. 10  depicts a perspective view of an embodiment of a closure member. 
         FIG. 11  depicts a side view of an embodiment of an elongated member. 
         FIG. 12  depicts a plot of applied moment versus rotation. 
         FIG. 13  depicts the components of an embodiment of a first bone fastener of a dynamic posterior stabilization system. 
         FIG. 14  depicts a top view of an embodiment of a fastener and collar combination for a bone fastener. 
         FIG. 15  depicts the components of an embodiment of a second bone fastener of a dynamic posterior stabilization system. 
         FIG. 16  depicts an embodiment of a dynamic posterior stabilization system with a laterally positioned elongated member. 
         FIG. 17  depicts a front view representation of the second bone fastener depicted in  FIG. 16 . 
         FIG. 18  depicts an embodiment of a multi-level dynamic posterior stabilization system. 
         FIG. 19  depicts an embodiment of a dynamic posterior stabilization system. 
         FIG. 20  depicts top view representation of an embodiment of a dynamic posterior stabilization system. 
         FIG. 21  depicts a front view representation of a portion of an embodiment of a second bone fastener of a dynamic posterior stabilization system. 
         FIG. 22  depicts a side view representation of a portion of an embodiment of a dynamic posterior stabilization system with a bridge, wherein a portion of the first bone fastener is depicted in cutaway to emphasize the interior of the first bone fastener. 
         FIG. 23  depicts a perspective view of an embodiment of a dynamic posterior stabilization system. 
         FIG. 24  depicts a perspective view of the elongated member of the dynamic posterior stabilization system depicted in  FIG. 23 . 
         FIG. 25  depicts the first bone fastener of the dynamic posterior stabilization system after insertion of the elongated member. 
         FIG. 26  depicts a representation of a dynamic interbody device and a posterior stabilization system coupled to vertebrae. 
         FIG. 27  depicts a representation of a transverse connector that couples a first dynamic posterior stabilization system to a second dynamic posterior stabilization system. 
         FIG. 28  depicts a representation of pins placed in vertebrae to be stabilized. 
         FIG. 29  depicts a representation of a distractor plug and a keel cutter during a spinal stabilization procedure. 
         FIG. 30  depicts a representation of dynamic interbody devices inserted in a disc space between vertebrae. 
         FIG. 31  depicts a representation of an embodiment of a dynamic interbody device and an embodiment of a dynamic posterior stabilization system of a spinal stabilization system. 
     
    
    
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     A “functional spinal unit” generally refers to a motion segment of a spine. The functional spinal unit may include two vertebrae, an intervertebral disc between the vertebrae, and the two facet joints between the vertebrae. An “artificial functional spinal unit” refers to a functional spinal unit where one or more of the components of the functional spinal unit are replaced by implants or devices that permit at least some motion of the spine. At least a portion of the intervertebral disc and/or one or both of the facet joints may be replaced by implants or devices during a spinal stabilization procedure. 
     As used herein, “coupled” includes a direct or indirect joining or touching unless expressly stated otherwise. For example, a first member is coupled to a second member if the first member contacts the second member, or if a third member is positioned between the first member and the second member. 
     A “dynamic interbody device” generally refers to an artificial intervertebral implant that allows for flexion/extension, lateral bending and/or axial rotation of vertebrae coupled to the device. The dynamic interbody device may replace a portion or all of an intervertebral disc. In some embodiments, a pair of dynamic interbody implants are installed during a spinal stabilization procedure. In some embodiments, a dynamic interbody device is installed using a posterior approach. In other embodiments, a dynamic interbody device may be installed using an anterior approach or other type of approach. In some embodiments, one or more dynamic interbody devices are placed in a disc space between vertebrae, and at least one posterior stabilization system is coupled to the vertebrae. In some embodiments, one or more dynamic interbody devices are placed in the disc space without coupling a posterior stabilization system to the vertebrae. 
     Dynamic interbody devices may have surfaces that contact vertebrae. In some embodiments, a surface of the dynamic interbody device that contacts a vertebra may include one or more keels, protrusions, and/or osteoconductive/osteoinductive layers or coatings. A keel of the dynamic interbody device may be positioned in a groove formed in a vertebra. The groove may be formed in the vertebra so that the dynamic interbody device will be positioned at a desired location when inserted into the patient. Protrusions of the dynamic interbody device may penetrate an endplate of the vertebra to secure the dynamic interbody device to the vertebra. An osteoconductive/osteoinductive layer may promote bone growth that secures the dynamic interbody device to the vertebra. The osteoconductive/osteoinductive layer may include, but is not limited to a scaffold, a roughened surface, a surface treated with a titanium plasma spray, bone morphogenic proteins, and/or hydroxyapatite. A roughened surface may be formed by chemical etching, by surface abrading, by shot peening, by an electrical discharge process, and/or by embedding particles in the surface. 
     A dynamic interbody device may include one or more angled surfaces so that the dynamic interbody device provides the patient with a desired amount of lordosis. Dynamic interbody devices that provide different amounts of lordosis may be provided in an instrument kit supplied for a spinal stabilization procedure. For example, the instrument kit for a spinal stabilization procedure may include pairs of dynamic interbody devices that establish 0°, 3°, 6°, 9°, 12° or 15° of lordosis. Pairs of dynamic interbody devices that provide other amounts of lordosis may be provided. The amount of lordosis provided by a dynamic interbody device may be printed or etched on a visible surface of the dynamic interbody device. Other information may also be printed or etched on the visible surface of the dynamic interbody device. Such information may include dimension information (e.g., length, width, and/or height) and whether the dynamic interbody device is to be installed on the left side of the patient or the right side of the patient. 
     In some embodiments, one or more dynamic interbody devices are installed in a disc space formed in the lumbar region of the spine during a spinal stabilization procedure. The shape and/or size of a dynamic interbody device may depend on a number of factors including surgical approach employed for insertion, intended position in the spine (e.g., cervical or lumbar), and patient anatomy. A dynamic interbody device for the lumbar spine may have a height that is less than about 22 mm. Several sizes of interbody devices may be provided in the instrument kit for the spinal stabilization procedure. In an embodiment, dynamic interbody devices having heights of 6 mm, 8 mm, 10 mm, 12, mm, 14 mm, 16 mm, 18 mm, and 20 mm are provided in the instrument kit for the spinal stabilization procedure. The dynamic interbody devices may include indicia indicating the height of the spinal stabilization devices. 
     In some embodiments, a single dynamic interbody device may be positioned in a disc space between vertebrae. The dynamic interbody device may be installed using an anterior approach, a posterior approach, or a different type of approach. In some embodiments, the height of the dynamic interbody device may be adjustable during the installation procedure. U.S. Patent Application Publication Nos. 2005/0278026 to Gordon et al., and 2005/0033439 to Gordon et al. which are incorporated herein by reference, describe expandable dynamic spinal implants. In some embodiments, insertion instruments may change the separation distance between endplates of the dynamic interbody device. One or more shims may be coupled to the dynamic interbody device to maintain the selected separation distance. 
     In some embodiments, a pair of dynamic interbody devices may be installed between a pair of vertebrae to establish a stabilization system. In some embodiments, the dynamic interbody device is a bimodal device. Bimodal refers to a device that has two separate curved surfaces to accommodate flexion/extension, lateral bending and/or axial rotation. 
       FIG. 1  depicts an anterior view of an embodiment of a portion of a stabilization system positioned between vertebrae. Dynamic interbody device  50 ′ may be positioned on a first side of the patient. Dynamic interbody device  50 ″ may be positioned on the contralateral side of the patient. Dynamic interbody device  50 ′ may be the mirror image of dynamic interbody device  50 ″. Dynamic interbody devices  50 ′,  50 ″ may be bimodal devices. Dynamic interbody devices  50 ′,  50 ″ may provide sufficient contact area against end plates of vertebrae  52 ,  54  to support the spinal column and inhibit subsidence of the vertebrae. 
     Each dynamic interbody device  50 ′,  50 ″ of the pair of dynamic interbody devices may have a width that is significantly less than the width of a single dynamic interbody device that can be installed in the disc space between first vertebra  52  and second vertebra  54 . Using two interbody devices  50 ′,  50 ″ may limit the amount of bone and tissue removal needed for insertion using a posterior approach. Each of the dynamic interbody devices may have a width that is less than 17 mm. In some embodiments, the dynamic interbody devices may be provided in pairs having small, medium or large widths. The instrument kit for the spinal stabilization procedure may include one or more sizes of dynamic interbody devices. In an embodiment, the instrument kit includes dynamic interbody device pairs having medium and large widths. 
     As used herein a “dynamic posterior stabilization system” generally refers to an apparatus used to replace or supplement a facet joint while allowing for both dynamic resistance and at least some motion of the first vertebra to be stabilized relative to the second vertebra to be stabilized. The first vertebra and the second vertebra may be vertebrae of a functional spinal unit. In some embodiments, bone fasteners of the dynamic posterior stabilization system are secured to the first vertebra and the second vertebra. In some embodiments, a bone fastener of the dynamic posterior stabilization system may be coupled to a vertebra adjacent to the vertebrae of the functional spinal unit being stabilized. The bone fasteners may be coupled to lamina, pedicles, and/or vertebral bodies of the vertebrae. In some embodiments, dynamic posterior stabilization systems may be positioned in three or more vertebrae to form a multi-level stabilization system. 
     The dynamic posterior stabilization system may replace or supplement a normal, damaged, deteriorated, defective or removed facet joint. The dynamic posterior stabilization system may include bone fasteners, an elongated member, and at least one bias member. The bias member may provide little initial resistance to movement of a first vertebra coupled to the system relative to a second vertebra coupled to the system. Resistance to additional movement of the first vertebra relative to the second vertebra may increase. The increasing resistance provided by the bias member may mimic the behavior of a normal functional spinal unit. The dynamic posterior stabilization system may stabilize the vertebrae, limit the range of motion of the first vertebra relative to the second vertebra, and/or share a portion of the load applied to the vertebrae. 
     The dynamic posterior stabilization systems disclosed herein may allow for rotational and/or translational motion of an elongated member (e.g., a rod or plate) relative to one or more bone fasteners. The bone fasteners may include threading, barbs, rings or other protrusions that secure the bone fasteners to vertebrae. In some embodiments, the bone fasteners may be cemented or glued to the vertebrae. Bone fasteners may include collars. In some embodiments, a collar of a bone fastener is an integral portion of the bone fastener. In some embodiments, the collar is a separate component that is coupled to at least one other component of the bone fastener. The collar of the bone fastener is the portion of the bone fastener that couples to an elongated member of the dynamic posterior stabilization system. In some embodiments, the bone fasteners are polyaxial pedicle screws and the collars are the upper portions of the polyaxial pedicle screws. In some embodiments, the bone fasteners are bone screws and the collars are plates or other structures that are coupled to the bone screws. 
     During installation of dynamic interbody devices of a spinal stabilization system, or during installation of a single dynamic interbody device, one or both facet joints of the vertebrae may be removed. A dynamic posterior stabilization system may be installed to replace a removed facet joint. One or both of the dynamic interbody devices of the spinal stabilization system, or the single dynamic interbody device, may be coupled to a dynamic posterior stabilization system. Coupling a dynamic interbody device to the dynamic posterior stabilization system may inhibit backout of the dynamic interbody device from the disc space. Coupling the dynamic interbody device to the dynamic posterior stabilization system may facilitate operation of the dynamic interbody device with the dynamic posterior stabilization system. 
     In some embodiments, a dynamic posterior stabilization system may be installed without removal of a facet joint. The dynamic posterior stabilization system may be installed after a discectomy, laminectomy, or other procedure. The dynamic posterior stabilization system may change the dynamic resistance that is not normal due to degeneration, disease, loss of a portion of the intervertebral disc and/or tissue damage as a result of the surgery. 
     A dynamic interbody device and a dynamic posterior stabilization system may include one or more biocompatible metals having a non-porous quality and a smooth finish (e.g., surgical grade stainless steel, titanium and/or titanium alloys). In some embodiments, a dynamic interbody device or dynamic posterior stabilization system may include ceramic and/or one or more other suitable biocompatible materials, such as biocompatible polymers and/or biocompatible metals. Biocompatible polymers may include, but are not limited to, polyetheretherketone resins (“PEEK”), carbon reinforced PEEK, ultra high molecular weight polyethylenes, polyethylenes, polyanhydrides, and alpha polyesters. For example, an implant may be constructed of a combination of biocompatible materials including cobalt chromium alloy, ultra high molecular weight polyethylene, and polycarbonate-urethane or silicone blend. 
     In some embodiments, dynamic interbody devices and dynamic posterior stabilization systems may be made of non-magnetic, radiolucent materials to allow unrestricted post-operative imaging. Certain material may interfere with x-ray and/or magnetic imaging. Magnetic materials may interfere with magnetic imaging techniques. Most non-magnetic stainless steels and cobalt chrome contain enough iron and/or nickel so that both magnetic imaging and x-ray imaging techniques are adversely affected. Other materials, such as titanium and some titanium alloys, are substantially iron free. Such materials may be used when magnetic imaging techniques are to be used, but such materials are often radio-opaque and sub-optimal for x-ray imagining techniques. Many ceramics and polymers are radiolucent and may be used with both magnetic imaging techniques and x-ray imaging techniques. The dynamic interbody devices and/or the dynamic posterior stabilization systems may include coatings and/or markers that indicate the positions of the devices and/or systems during operative and/or post-operative imaging. 
       FIG. 2  depicts a portion of an embodiment of dynamic interbody device  50 ′ that may be positioned on a first side of a patient. Dynamic interbody device  50 ′ may include first member  56 , second member  58 , and third member  60 . Dynamic interbody device  50 ′ may be positioned in a disc space between two vertebrae. First member  56 , second member  58 , and third member  60  may work together to allow for flexion/extension, lateral bending, and/or axial rotation of a vertebra coupled to the first member relative to a vertebra coupled to the third member. 
       FIG. 3  depicts an embodiment of first member  56 . First member  56  may include keel  62 , slot  64 , arms  66 , arcuate surface  68  and bridge  70 . During an implant insertion procedure, keel  62  may be positioned in a channel formed in a vertebra. Keel  62  may couple first member  56  to the vertebra. The bottom surfaces of first member and keel may be osteoconductive/osteoinductive to promote bone growth that secures the first member to the vertebra. 
     Slot  64  may accommodate a first pin of the second member. In some embodiments, the first member includes a pin that fits in a slot of the second member. The first pin may allow the second member to rotate relative to first member  56  to accommodate axial rotation of the vertebra coupled to the first member relative to the vertebra coupled to the third member of the dynamic interbody device. 
     Arms  66  may interact with the second member to limit an amount of rotation of the second member relative to first member  56 . In some embodiments, the position of arms  66  and/or the size of the second member allows for up to about 4° of axial rotation (e.g. ±2° of rotation from a neutral position). The position of arms  66  and/or the size of the second member may be designed to allow for more or less rotation. In some embodiments, the position of arms  66  and/or the size of the second member allows for about ±1.5° of axial rotation from a neutral position. Arms  66  may also inhibit separation of the second member from first member  56 . Gaps may be present between arms  66  and the second member to allow movement of the second member relative to first member  56  that accommodates axial rotation and/or lateral bending of vertebrae coupled to the dynamic interbody device. 
     A first pin of the second member may slide along a length of slot  64  to accommodate lateral bending of the vertebra coupled to first member  56  relative to the vertebra coupled to the third member of the dynamic interbody device. The curvature of arcuate surface  68  and the length of slot  64  may allow the dynamic interbody device to accommodate about 10° of lateral bending (e.g. ±5° of lateral bending from a neutral position). The length of slot  64  and/or the curvature of arcuate surface  68  may be designed to allow for more or less lateral bending. In some embodiments, the curvature of arcuate surface  68  and the length of slot  64  allows the dynamic interbody device to accommodate about ±3° of lateral bending from a neutral position. In some embodiments, arcuate surface  68  may be a spherical portion. 
     Bridge  70  may couple the dynamic interbody device to a dynamic posterior stabilization system. In some embodiments, bridge  70  is an integral part of first member  56 . Bridge  70  may be used to help position keel  62  of first member  56  in the groove formed in the vertebra. In some embodiments, bridge  70  is attached to the dynamic interbody device during the insertion procedure (e.g., by threading, a sliding connection, a snap lock connection or other type of connection). Coupling the bridge to the dynamic interbody device and to the dynamic posterior stabilization system connects the two apparatus together so that one or more centers of rotation that allow for movement of the dynamic interbody device align or substantially align with the curvature of the elongated member of the dynamic posterior stabilization system so that the dynamic posterior stabilization system works in conjunction with the dynamic interbody device to guide motion of the vertebrae coupled to the dynamic interbody device and to the dynamic posterior stabilization system. 
     In some embodiments, two dynamic interbody devices are placed in the disc space between the vertebrae, and two dynamic posterior stabilization systems are coupled to the vertebrae. Bridges may couple the dynamic interbody devices to the dynamic posterior stabilization systems. The bridges may fix the position of the dynamic interbody devices in working relation to each other so that each of the dynamic interbody devices move during flexion/extension, lateral bending, and/or axial rotation. The bridges may ensure that the dynamic interbody devices work in unison to allow movement of the vertebrae coupled to the dynamic interbody devices. 
     In some embodiments, a first dynamic interbody device is placed in a prepared disc space between vertebrae. A second dynamic interbody device is also placed in the disc space between the vertebrae. A connector may be coupled to the first dynamic interbody device and the second dynamic interbody device. The connector may link the first dynamic interbody device to the second dynamic interbody device and provide stabilization to the dynamic interbody devices. The connector may inhibit migration of the dynamic interbody devices. The connector may ensure that the dynamic interbody devices work in unison to allow movement of the vertebrae coupled to the dynamic interbody devices. In some embodiments, the connector may slide into the dynamic interbody devices. Detents, adhesive, setscrews or other fastening systems may inhibits separation of the connector from the dynamic interbody devices. 
     Bridge  70  may include member  72  and connector  74 . In some embodiments, a length of the member is adjustable. A first portion of the member may slide relative to a second portion of the member. A setscrew or other fastening system may set the position of the first portion of the member relative to the second portion of the member when the desired length of the bridge is set. Connector  74  may couple the dynamic interbody device to a posterior stabilization system. Connector  74  may include slot  76 . A bone fastener of the posterior stabilization system may be positioned in slot  76  to secure the bridge to a vertebra and to the dynamic posterior stabilization system. In some embodiments, the connector may be a plate or other structure with an opening (e.g., a ring). A shaft of a bone fastener of a dynamic posterior stabilization system may be positioned through the opening to secure the bridge to a vertebra and to the dynamic posterior stabilization system. 
     In some embodiments, the bridge may be a separate component from the first member.  FIG. 4  depicts an embodiment of separate component bridge  70 . Connector  74  of bridge  70  may be coupled to the dynamic posterior stabilization system. End  78  of bridge  70  may contact the dynamic interbody device during use to couple the bridge to the dynamic posterior stabilization system and inhibit posterior migration and/or backout of the dynamic interbody device from the disc space. 
     The connector of a separate component bridge or the connector of a bridge that is coupled to a dynamic interbody device may include an angled surface. For example, bridge  70  may include angled surface  80 . Angled surface  80  may facilitate alignment of the center of a curved elongated member of the dynamic posterior stabilization system with at least one rotation center of the dynamic interbody device (e.g., the rotation centers that control flexion/extension and lateral bending of the dynamic interbody device) so that the dynamic posterior stabilization system works in conjunction with the dynamic interbody device to allow for motion of the vertebra coupled to the dynamic interbody device. 
     In some embodiments where bridge  70  is a separate component, the dynamic interbody device includes an opening that couples to an inserter. The inserter may couple to the opening of the dynamic interbody device by a detent, threading and/or other reversible coupling system. 
     The bridge may couple the dynamic interbody device to the dynamic posterior stabilization system. In some embodiments, the bridge may be coupled to the second bone fastener of the dynamic posterior stabilization system. The second bone fastener may be secured to the more caudal of the vertebrae being stabilized. A portion of the bridge may be positioned near the end plate of the more caudal vertebra. The position of the bridge may inhibit contact of the bridge with neural structures (e.g., spinal ganglion) exiting the vertebrae: 
       FIG. 5  depicts an embodiment of second member  58 . Second member  58  may include ledges  82 , first pin  84 , first arcuate surface  86 , second pins  88  (one on each side of the second member), convex surface  90 , and upper anterior surface  92 . In some embodiments, the second member includes a slot instead of first pin  84 . In some embodiments, the second member may includes recesses that accommodate pins of the third member to couple the third member to the second member and allow for flexion/extension of vertebrae coupled to the dynamic interbody device. The recesses may take the place of second pins  88 . 
     Ledges  82  may interact with the arms of the first member to inhibit separation of the first member from second member  58 . First pin  84  may be positioned in the slot of the first member to allow for lateral bending and/or axial rotation of the vertebra coupled to the third member relative to the vertebra coupled to the first member. First arcuate surface  86  may have a curvature that complements the curvature of the arcuate surface of the first member. 
     Second pin  88  may couple second member  58  to the third member. Curved surface  90  may interact with a curved surface of the third member to allow for flexion/extension of the vertebra coupled to the third member relative to the vertebra coupled to the first member. In some embodiments, curved surface  90  is convex and the curved surface of the third member is concave. In some embodiments, the curved surface of the second member is concave and the curved surface of the third member is convex. 
     The radius of curved surface  90  and the radius of the curved surface of the third member may be small so that translational movement of the vertebra coupled to the third member relative to the vertebra coupled to the first member is kept within a proper range during flexion or extension of the vertebrae. In some embodiments, the radial center of curved surface  90 , and the corresponding radial center of the curved surface of the third member, may be located towards posterior end  94  of second member  58  to limit the amount of posterior translation during extension. 
     In some embodiments, upper anterior surface  92  is a contact surface. A lower anterior surface of the third member may contact upper anterior surface  92  when the dynamic interbody device is at maximum flexion. 
       FIG. 6  depicts an embodiment of third member  60 .  FIG. 7  depicts a cross-sectional view of an embodiment of third member  60 . Third member  60  may include outer surface  96 , lead surface  98 , angled lower posterior surface  100 , angled lower anterior surface  102 , slots  104 , and curved surface  106 . One slot  104  may be formed on each side of third member  60 . At least a portion of outer surface  96  may be sloped to provide a desired amount of lordosis between the vertebrae coupled to the first member and third member  60 . Curved surface  106  may be complementary to the curved surface of the second member to allow for flexion and/or extension of the vertebra coupled to the first member relative to the vertebra coupled to the third member. 
     During insertion of the dynamic interbody device in a disc space, the vertebrae may be distracted so that the disc space has a height that is less than the height of the dynamic interbody device. Lead surface  98  may contact one of the vertebrae and provide force against the vertebra that distracts the vertebrae to allow for insertion of the dynamic interbody device. 
     In some embodiments, angled lower posterior surface  100  and angled lower anterior surface  102  are contact surfaces. In some embodiments, only one side of the third member includes an angled lower posterior surface and an angled lower anterior surface. In some embodiments, both sides of the third member include angled lower posterior surfaces and angled lower anterior surfaces. Having two angled lower posterior surfaces and two angled lower anterior surfaces may allow the third member to be used in a right dynamic interbody device or a left dynamic interbody device, thus eliminating the need for different left and right third members. 
     Angled posterior surface  100  may contact the arcuate surface of the first member when the dynamic interbody device is at maximum extension. The dynamic interbody device may allow for a maximum of about 15° of extension from the neutral position. Angled posterior surface  100 , the geometry of one or both slots  104 , and/or, the geometry of the second member may be designed so that the dynamic interbody device has a smaller or a larger maximum angle of extension from the neutral position. In some embodiments, the dynamic interbody device allows for a maximum of about 7° of extension from the neutral position. 
     Angled lower anterior surface  102  may contact the arcuate surface of the first member when the dynamic interbody device is at maximum flexion. The dynamic interbody device may allow for a maximum of about 20° of flexion from the neutral position. Angled lower anterior surface  102 , the geometry of one or both slots  104 , and/or the geometry of the second member may be designed so that the dynamic interbody device has a smaller or a larger maximum angle of flexion from the neutral position. In some embodiments, the dynamic interbody device allows for a maximum of about 7° of flexion from the neutral position. In some embodiments, the maximum amount of flexion allowed by the dynamic interbody device is different from the maximum amount of extension. For example, an embodiment of a dynamic interbody device allows for a maximum of about 150 of flexion and a maximum of about 100 of extension. 
     The second pins of the second member may be positioned in slots  104  to couple third member  60  to the second member. Slots  104  may accommodate rotational movement and translational movement of third member  60  relative to the second member during flexion and extension. 
       FIG. 8  depicts an embodiment of dynamic interbody devices  50 ′,  50 ″. Each dynamic interbody device  50 ′,  50 ″ may include bridge  70 , convex member  108  and concave member  110 . Convex member  108  and concave member  110  may be coupled to vertebrae. One or more removable members of a dynamic interbody device may initially hold the position of convex member  108  constant relative to concave member  110 . After the dynamic interbody device is positioned in a disc space between vertebrae, the removable member or removable members may be removed to allow concave member  110  the ability to move relative to convex member  108 . 
     Curved surface  112  of convex member  108  may have the contour of an outer surface of a portion of a sphere. Concave member  110  may have a curved surface that is complementary to curved surface  112  of convex member  108  to allow for flexion/extension, lateral bending, and axial rotation of the vertebra coupled to convex member  108  relative to the vertebra coupled to concave member  110 . In an embodiment, convex member  108  includes a protrusion that extends into a recess of the concave member  110 . In an embodiment, convex member  108  includes a recess and concave member  110  includes a protrusion that extends into the recess. The size of the recess may be designed to limit the available range for flexion, extension, lateral bending and/or axial rotation. 
     Bridge  70  may include elongated portion  72  and connector  74 . A section of elongated portion  72  may be a keel for the dynamic interbody device that is positionable in a channel formed in a vertebra. The channels formed in the vertebra for the two dynamic interbody devices may be formed to the same depth and the same distance away from a center line of the vertebra so that the spherical portions of the right dynamic interbody device is in working relation to the spherical portion of the left dynamic interbody device. 
     In some embodiments, a section of elongated portion  72  may be bendable. Bending elongated portion  72  may allow the dynamic interbody device to be conformed to the patient so that connector  74  can be coupled to a dynamic posterior stabilization system. 
     Connector  74  may include slot  76  and channel  114  between arms  116 . A portion of a bone fastener may be positioned in slot  76  to couple the dynamic interbody device to a bone fastener. Coupling the bone fastener to the dynamic interbody device may inhibit backout of the dynamic interbody device from the disc space between the vertebrae Coupling the bone fastener to the dynamic interbody device may allow for alignment of the center of rotation of the dynamic interbody device with the center of rotation of a curved elongated member of the dynamic posterior stabilization system so that the dynamic interbody device works in conjunction with the dynamic posterior stabilization system to allow for movement of the vertebrae coupled to the dynamic interbody device. 
     An elongated member of a posterior stabilization system or dynamic posterior stabilization system may be positioned in channel  114  between arms  116 . The elongated member may be coupled to connector  74  to inhibit removal of the elongated member from channel  114 . The elongated member of a dynamic posterior stabilization system may be coupled to connector  74  such that translational and/or rotational movement of the elongated member relative to arms  116  is not inhibited. 
       FIG. 9  depicts an embodiment of dynamic posterior stabilization system  118 . Dynamic posterior stabilization system  118  may include closure members  120 ; first bone fastener  122 ; second bone fastener  124 ; elongated member  126 ; and bias members  128 ,  130 . In some embodiments, first bone fastener  122  is positioned in the more upper of the vertebrae to be stabilized. In other embodiments, first bone fastener is positioned in the lower of the vertebrae to be stabilized. 
     When closure member  120  couples elongated member  126  to first bone fastener  122 , movement of the elongated member relative to the first bone fastener may be inhibited. When closure member  120  couples elongated member  126  to second bone fastener  124 , translational and/or rotational movement of the elongated member relative to the second bone fastener may be possible. The ability to have translational movement of elongated member  126  relative to second bone fastener  124  may allow dynamic posterior stabilization system  118  to accommodate flexion, extension and lateral bending of a first vertebra coupled to the dynamic posterior stabilization system relative to a second vertebra coupled to the dynamic posterior stabilization system. The ability to have rotational movement of elongated member  126  relative to second bone fastener  124  may allow dynamic posterior stabilization system  118  to accommodate axial rotation of the first vertebra coupled to the dynamic posterior stabilization system relative to the second vertebra coupled to the dynamic posterior stabilization system. 
       FIG. 10  depicts an embodiment of closure member  120 . Closure member  120  may couple the elongated member of the dynamic posterior stabilization system to the first bone fastener or to the second bone fastener. Closure member  120  may include threading  134  or other structure that secures the closure member to a collar of the first bone fastener  122  or to a collar of the second bone fastener. Closure member  120  may include tool opening  136 . A portion of a driver may be inserted into tool opening  136  to facilitate attaching closure member  120  to the collar of the first bone fastener or to the collar of the second bone fastener. 
     Closure members may be other types of fasteners, including but not limited to clips and snap on connectors. In some embodiments, the closure member coupled to the first bone fastener may be different from the closure member coupled to the second bone fastener. For example, the closure member used to secure the elongated member to the first bone screw may be a closure member as depicted in  FIG. 10 , while a closure member used to couple the elongated member to the second bone fastener may be a clip that allows the elongated member to move relative to the second bone fastener. 
     As shown in  FIG. 9 , dynamic posterior stabilization system  118  includes elongated member  126 . Elongated member  126  may be a rod, bar, plate, combination thereof, or other type of member coupled to first bone fastener  122  and second bone fastener  124 . In some embodiments where the dynamic posterior stabilization system is to be used with a dynamic interbody device, elongated member  126  may be bent so that the elongated member has a curvature that facilitates the use of the dynamic posterior stabilization system in conjunction with the dynamic interbody device. In embodiments where the dynamic posterior stabilization system is not used in conjunction with a dynamic interbody device, the elongated member may be straight or curved. Elongated members with appropriate curvature may be included in the instrument kit for the spinal stabilization procedure. 
       FIG. 11  depicts an embodiment of bent elongated member  126 . In an embodiment, a portion of elongated member  126  near first end  138  is secured to the first bone fastener of the dynamic posterior stabilization system so that movement of the elongated member relative to the first bone fastener is inhibited. A portion of elongated member  126  near second end  140  may be coupled to the second bone fastener of the dynamic posterior stabilization system so that translational movement and or rotational movement of the elongated member relative to the second bone fastener is allowed. In some embodiments, concave portion  142  of elongated member  126  may be oriented to face the vertebrae coupled to the dynamic posterior stabilization system so that a center of the curve aligns or substantially aligns with the center or centers of rotation of the dynamic interbody device that allow for flexion/extension and/or lateral bending. The alignment or substantial alignment allows the dynamic posterior stabilization system and the dynamic interbody device to simultaneously accommodate flexion/extension and/or lateral bending of the vertebrae being stabilized. In some embodiments, a portion of elongated member  126  near second end  140  may be bent so that the elongated member does not contact or approach a vertebra during patient movement. 
     As shown in  FIG. 9 , an end of elongated member  126  near second bone fastener  124  may include stop  132 . Stop  132  may retain bias member  130  on elongated member  126 . In some embodiments, the position of the stop may be adjustable along the length of the elongated member. A fixed position stop or an adjustable position stop may be used in conjunction with bias member  128  instead of using the collar of first bone fastener  122  as the stop for bias member  128 . In some embodiments, a removable stop may initially maintain bias member  128  in compression. In some embodiments, a removable stop may initially maintain bias member  128  in compression. The removable stops may facilitate coupling elongated member  126  to second bone fastener  124 . After elongated member  126  is coupled to second bone fastener  124 , the removable stops may be removed so that the bias members can accommodate movement of the elongated member relative to the second bone fastener caused by flexion/extension and/or lateral bending. In some embodiments, an insertion instrument may hold bias members  128 ,  130  in compression when elongated member  126  is being coupled to first bone fastener  122  and second bone fastener  124 . 
     Bias members  128 ,  130  may surround or partially surround elongated member  126 . Bias members  128 ,  130  may be stacks of elastic washers, elastic tubes, springs, or other systems that provide resistance to compression. In some embodiments, bias members  128 ,  130  may be formed of biocompatible polymeric material. For example, bias members  128 ,  130  may be formed of silicone-urethane co-polymer. 
     Bias members  128 ,  130  may transmit little or no force to second bone fastener  124  when dynamic posterior stabilization system  118  is in a neutral position. If second bone fastener  124  is coupled to the more caudal vertebra of the vertebrae to be stabilized, compression of bias member  128  may accommodate translational movement of elongated member  126  caused by extension and/or lateral bending of the vertebrae coupled to dynamic posterior stabilization system  118 . If second bone fastener  124  is coupled to the more caudal vertebra of the vertebrae to be stabilized, compression of bias member  130  may accommodate translational movement of elongated member  126  caused by flexion and/or lateral bending of the vertebrae coupled to dynamic posterior stabilization system  118 . 
     Bias member  128  may accommodate up to about 3 mm of travel of second bone fastener  124  towards first bone fastener  122 . Bias member  130  may accommodate up to about 2 mm of travel of second bone fastener  124  away from first bone fastener  122 . 
     In some embodiments, bias member  128  and bias member  130  are the same. For example, bias members  128 ,  130  may be stacks of washers. In some embodiments, bias member  128  is different than bias member  130 . For example, bias member  128  is a spring, and bias member  130  is an elastic tube. 
     Bias members  128 ,  130  may allow dynamic posterior stabilization system  118  to provide stability while still allowing for anatomical motion and dynamic resistance that mimics normal segmental stiffness of the spine. Knowledge of the elastic properties (e.g., the amount of compression per degree of rotation) of the material chosen for bias members  128 ,  130  allows the length of the bias members placed on the elongated member to be selected so that the dynamic posterior stabilization system provides a desired amount of resistance.  FIG. 12  depicts a plot of the applied moment versus the amount of rotation for an intact (normal) functional spinal unit (plot  144 ), for an unconstrained functional spinal unit (plot  146 ), and for a functional spinal unit with a dynamic posterior stabilization system (plot  148 ). The slope of the curves at each point represents spinal stiffness. The neutral zone is the low stiffness region of the range of motion. The dynamic posterior stabilization system may allow for stabilization of the spine while providing substantially unconstrained motion within the neutral zone and increasing resistance to rotation within the elastic zone. The stiffness of vertebrae supported by the dynamic posterior stabilization system may closely mimic the stiffness of a normal functional spinal unit. The behavior of the dynamic posterior stabilization system may closely mimic the normal kinematics of the functional spinal unit. 
       FIG. 13  depicts the components of an embodiment of first bone fastener  122 . First bone fastener  122  may include fastener  150 , collar  152 , and saddle  154 . Fastener  150  may include shaft  156  and head  158 . Shaft  156  may secure first bone fastener  122  to bone (e.g. a vertebra). Shaft  156  may include threading  160  that secures the shaft to the bone. 
     A portion of outer surface  162  of head  158  may have a spherical contour complementary to a portion of spherically contoured inner surface  172  of collar  152 . The shape of outer surface  162  and inner surface  172  of collar  152  may allow for polyaxial positioning of the collar relative to fastener  150 . Inner surface  164  of head  158  may be spherically contoured. The spherical contour of inner surface  164  may allow saddle  154  to be positioned in fastener  150  at a desired angle to accommodate the position of collar  152  relative to the fastener. 
     Collar  152  may include arms  166  and lower body  168 . A portion of the elongated member may be positioned in the slot between arms  166 . A portion of the inner surfaces of arms  166  may include threading  170  that is complementary to threading of the closure member used to secure the elongated member to first bone fastener  122 . Portion  172  of the inner surface of lower body  168  may have a spherically contoured section that complements the spherical contour of outer surface  162  of fastener head  158  to allow for polyaxial positioning of collar  152  relative to fastener  150 . 
     Head  158  of fastener  150  may be positioned in collar  152  to form a fastener and collar combination.  FIG. 14  depicts a top view of fastener and collar combination  174 . When head  158  is positioned in collar  152 , separation of the fastener from the collar may be difficult. Several fastener and collar combinations  174  may be provided in an instrument kit for a dynamic spinal stabilization procedure. The instrument kit may include several combinations  174  with fasteners  150  of varying lengths. For example, the kit may include fastener and collar combinations with fastener having lengths from about 30 mm to about 75 mm in 5 mm increments. In some embodiments, collar  152  of each combination  174  is stamped, printed or etched with the length of fastener  150 . Fasteners  150  and/or collars  152  of combinations  174  in the instrument kit may be color coded to indicate the length of the fasteners. For example, the collars of all combinations in the instrument kit with fasteners  150  that are about 30 mm in length have an orange color, the collars of all combinations in the instrument kit with fasteners that are about 35 mm in length have a yellow color, and the collars of all combinations with fasteners that are about 40 mm in length have a green color. Additional colors may be used for additional sizes. 
     Fastener  150  may include tool opening  176 . Tool opening  176  may complement a head of a driver. The driver may be used to insert fastener  150  into bone. The driver may be included in the instrument kit for the spinal stabilization procedure. In some embodiments, arms  166  may include flats, recesses or openings that engage insertion tools or guides. 
     Referring to  FIG. 13 , saddle  154  may have post  178  and support  180 . Saddle  154  may be positioned in fastener  150  after the fastener and collar combination has been inserted into a vertebra. Post  178  may be positioned in fastener  150 . Post  178  may be angled within head  158  of fastener  150  so that saddle  154  can accommodate polyaxial positioning of collar  152  relative to the fastener. In some embodiments, a retaining ring inhibits separation of saddle  154  from fastener  150 . 
     Support  180  may include groove  182 . A portion of the elongated member of the dynamic posterior stabilization system may be positioned in groove  182 . In some embodiments, saddle  154  and/or collar  152  are shaped so that groove  182  aligns with the slot formed between arms  166  of collar  152  when the saddle is placed in the collar. 
     A portion of the elongated member may be positioned in groove  182 . The closure member for first bone fastener  122  may be threaded on collar  152  and tightened against elongated member  126 . In some embodiments, the closure member may include one or more points or edges that bite into the elongated member when the closure member is tightened against the elongated member. When the closure member is tightened against the elongated member, the position of collar  152  relative to fastener  150  may become fixed, and movement of the elongated member relative to first bone fastener may be inhibited. 
       FIG. 15  depicts an embodiment of second bone fastener  124 . Second bone fastener  124  may include fastener  150 , collar  152 , saddle  154 , and cover  184 . Fastener  150 , collar  152 , and saddle  154  of second bone fastener  124  may be substantially the same as the fastener, collar and saddle of the first bone fastener. Cover  184  may include groove  186 . 
     Saddle  154  may be positioned in collar  152  after the fastener and collar combination are inserted into a vertebra. A portion of the elongated member may be positioned in groove  182  of saddle  154 . Cover  184  may be positioned on top of the elongated member. The radius of groove  186  may be larger than the radius of the portion of the elongated member positioned in the groove. The closure member for second bone fastener  124  may be threaded on collar  152  and tightened against cover  184 . In some embodiments, the closure member may include one or more points or edges that bite into cover  184  when the closure member is tightened against the cover. The position of collar  152  relative to fastener  150  may become fixed when the closure member is tightened against cover  184 . Having the radius of groove  186  larger than the radius of the portion of the elongated member positioned in the groove may allow translational movement and/or rotational movement of the elongated member relative to second bone fastener  124  when the closure member couples the elongated member to the second bone fastener. 
     When a closure member secures the elongated member between saddle  154  and cover  184 , significant change in height of the elongated member relative to second bone fastener  124  may be inhibited. Inhibiting height change of the elongated member relative to second bone fastener may allow the dynamic posterior stabilization system to share a portion of the shear load applied to a dynamic interbody device or intervertebral disc between the vertebrae being stabilized. 
     In some dynamic posterior stabilization system embodiments, the elongated member may be positioned lateral to the first bone fastener and/or the second bone fastener.  FIG. 16  depicts a top view representation of an embodiment of dynamic posterior stabilization system  118  where elongated member  126  is positioned lateral to second bone fastener  124 . A closure member may secure elongated member  126  to first bone fastener  122  so that movement of the elongated member relative to the first bone fastener is inhibited. 
     Second bone fastener  124  may include member  188 . A portion of member may slide over or into a portion of collar  152  of second bone fastener  124 . The connection between the collar and member may inhibit rotation of member  188  relative to collar  152 . A closure member may secure member  188  to collar  152  and second bone fastener  124 . When the closure member secures member  188  to collar  152  movement of second bone fastener  124  relative to elongated member  126  is allowed. Second bone fastener  124  may be able to move axially relative to elongated member  126  to accommodate flexion/extension and/or lateral bending of vertebrae coupled to the dynamic posterior stabilization system. Second bone fastener  124  may also be able to rotate relative to elongated member  126  to accommodate axial rotation of vertebrae coupled to the dynamic posterior stabilization system. 
       FIG. 17  depicts a front view of a portion of second bone fastener  124  of  FIG. 16  with member  188  coupled to collar  152  of the second bone fastener. Member  188  may include slot  190 . A portion of elongated member  126  may pass through slot  190 . Slot  190  and/or the portion of elongated member  126  that can pass through slot may have cross sectional shapes that accommodate rotation of second bone fastener  124  relative to the elongated member so that the dynamic posterior stabilization system is able to accommodate axial rotation of vertebrae being stabilized. Second bone fastener  124  may also be able to move axially along elongated member  126  so that the dynamic posterior stabilization system can accommodate flexion/extension and/or lateral bending of vertebrae being stabilized. 
     Placement of the elongated member adjacent to the second bone fastener may allow for construction of a multi-level dynamic posterior stabilization system.  FIG. 18  depicts a multi-level dynamic posterior stabilization system that includes dynamic posterior stabilization system  118 ′ and dynamic posterior stabilization system  118 ″. Elongated member  126 ″ of dynamic posterior stabilization system  118 ″ may be positioned in and secured to the collar of second bone fastener  124 ′ of dynamic posterior stabilization system  118 ′. A mirror image dynamic posterior stabilization system construction may be installed on the contralateral side of the spine. 
     During lateral bending of a first vertebra relative to a second vertebra, the geometry of the facet joints may cause some axial rotation to occur. In some embodiments, the dynamic posterior stabilization system may be designed to cause some axial rotation during lateral bending.  FIG. 19  depicts a top view representation of dynamic posterior stabilization system  118  that may introduce some axial rotation when lateral bending occurs. All of elongated member  126  or a portion of the elongated member that passes through member  188  may have a non-circular cross section. The passage of the opening for the elongated member through member  188  may be at an angle. In some embodiments, the orientation of the slot for elongated member  126  at the entrance into member  188  may be different than the orientation of the slot for the elongated member at the exit of the member. Interaction of elongated member  126  with member  188  may allow for some automatic rotation of second bone fastener  124  relative to the elongated member when the second bone fastener moves laterally relative to first bone fastener  122 . 
     In some dynamic posterior stabilization system embodiments, the elongated member may be at a substantially fixed height relative to the second bone fastener. In some dynamic posterior stabilization system embodiments, the elongated member may angulate so that the height of the elongated member relative to the second bone fastener is variable. Allowing the height of the elongated member relative to the second bone fastener to vary may allow for the use of a straight elongated member with a dynamic interbody device. 
       FIG. 20  depicts a top view representation of an embodiment of dynamic posterior stabilization system  118 . Dynamic posterior stabilization system  118  may include first bone fastener  122 , second bone fastener  124 , elongated member  126 , and bias members  128 ,  130 . Elongated member  126  may include threaded portion  192 . Second bone fastener  124  may include member  188 . Member  188  may allow elongated member  126  to be positioned lateral to the fastener of second bone fastener  124 . Lateral placement of the elongated member may allow for the establishment of multi-level stabilization systems. The elongated member of a second dynamic posterior stabilization system may couple to the collar of the second bone fastener of the first dynamic posterior stabilization system. In some embodiments, the member may position the elongated member through the collar of the second bone fastener. 
       FIG. 21  depicts a front view representation of a portion of second bone fastener  124 . Member  188  may include slot. Slot  190  may allow for change in vertical position of elongated member  126  relative to second bone fastener  124 . Change in vertical position of elongated member  126  relative to second bone fastener  124 , along with the compression of one the bias members, may allow the dynamic posterior stabilization system to accommodate flexion or extension of vertebrae coupled to the dynamic posterior stabilization system. 
     The portion of elongated member  126  positioned in slot  190  may have one or more flats. For example, elongated member  126  may have a hexagonal portion. The flats may interact with member  188  to inhibit rotation of elongated member  126  relative to second bone fastener  124  while still allowing for changes in vertical position of the elongated member relative to the second bone fastener. Elongated member  126  may be able to rotate relative to the first bone fastener so that the dynamic posterior stabilization system is able to accommodate axial rotation of a first vertebra coupled to the first bone fastener relative to a second vertebra coupled to the second bone fastener. 
       FIG. 22  depicts a side view representation of a portion of dynamic posterior stabilization system  118  with a portion of first bone fastener  122  depicted in cutaway to emphasize the interior of the first bone fastener. Ball  194  may be threaded on threaded portion  192  of elongated member  126 . Ball  194  may be positioned in collar  152  of first bone fastener  122 . Ball  194  may allow elongated member  126  to be pivotably coupled to first bone fastener  122 . Closure member  196  for first bone fastener  122  may include a spherically shaped portion that complements a portion of the outer surface of ball  194 . In some embodiments, the collar of the second bone fastener may accept a closure member that is identical to closure member  196  for first bone fastener  122  to avoid the need for different types of closure members for the first bone fastener and the second bone fastener. 
     In some embodiments, one or more lock rings may be placed on the threaded end of the elongated member. After the position of the ball is adjusted so that the elongated member will fit in the first bone fastener and the second bone fastener, one or more lock rings may be positioned against the ball to inhibit rotation of the ball relative to the elongated member. In some embodiments, an adhesive may be used to inhibit change in position of the ball relative to the elongated member after the position of the ball is set. Other systems may also be used to inhibit change in position of the ball relative to the elongated member after the position of the ball is set. In some embodiments, a portion of the end of the elongated member may be removed after the position of the ball is set so that there is little or no extension of the end of the elongated member beyond the collar of the first bone fastener when the dynamic posterior stabilization system is assembled. 
     In some embodiments, the ball may be at a fixed position on the elongated member. The length of the elongated member may be adjustable to allow the elongated member to be positioned in the first bone fastener and the second bone fastener. In an embodiment, a first portion of the elongated member may move relative to a second portion of the elongated member. A setscrew or other fastener may fix the position of the first portion relative to the second portion. Having a fixed position of the ball allows little or no extension of the end of the elongated member beyond the collar of the first bone fastener. 
     When closure member  196  is secured to collar  152  of first bone fastener  122 , the closure member and the collar may allow rotation of ball  194  relative to the first bone fastener. Rotation of ball  194  allows for rotation and/or angulation of elongated member  126  relative to first bone fastener  122 . 
     Closure member  196 , collar  152  and ball  194  may allow for angulation of elongated member  126  relative to first bone fastener  122 . The angular movement of elongated member  126 , along with compression of bias member  128  or bias member  130 , allows dynamic posterior stabilization system  118  to accommodate flexion/extension and/or lateral bending of the vertebrae coupled to the dynamic posterior stabilization system. 
     Elongated member assemblies may be provided in the instrument kit for the spinal stabilization procedure. The elongated member assemblies may include elongated member  126 ; ball  194  threaded on the elongated member; member  188 ; bias members  128 ,  130 ; and stops  132 . During an installation procedure, the fastener of the first bone fastener  122  and the fastener of second bone fastener  124  may be positioned in the vertebrae to be stabilized. Bridge  70  may be positioned between the collar of second bone fastener  124  and the vertebra to which the second bone fastener is attached. Bridge  70  may be secured to the vertebra by the collar of the second bone fastener. 
     The position of ball  194  on elongated member  126  may be adjusted by rotating the ball relative to the elongated member until the position of the ball on the elongated member allows the ball to be positioned in collar  152  of first bone fastener  122  when member  188  is positioned in the collar of second bone fastener  124 . Member  188  may be coupled to the collar of the second bone fastener and ball  194  may be positioned in collar  152  of first bone fastener  122 . Closure member  196  may be used to secure member  188  to second bone fastener  124 . Closure member  196  may be used to couple ball  194  to collar  152  of first bone fastener  122 . 
     In some dynamic posterior stabilization system embodiments, the passage through the second bone fastener for the elongated member may inhibit rotation of the elongated member and may also inhibit angulation of the elongated member relative to the second bone fastener. The second bone fastener may be configured to move axially relative to the elongated member when the second bone fastener is coupled to a vertebra. The first bone fastener may inhibit axial movement of the elongated member relative to the first bone fastener, but the first bone fastener may allow for rotation of the elongated member relative to the first bone fastener. At least a portion of the elongated member may be curved so that the assembled dynamic posterior stabilization system allows for flexion/extension and/or lateral bending of vertebrae coupled to the dynamic posterior stabilization system. The ability of the elongated member to rotate relative to the first bone fastener but not the second bone fastener may allow for accommodation of axial rotation movement of vertebrae coupled to the dynamic posterior stabilization system. 
       FIG. 23  depicts an alternate embodiment of a dynamic posterior stabilization system that allows elongated member to angulate so that the height of the elongated member relative to the second bone fastener is variable. Dynamic posterior stabilization system  118  may include first bone fastener  122 , second bone fastener  124 , elongated member  126 , and bias members  128 ,  130 . Elongated member may be coupled to collar  198  of first bone fastener  122  so that rotation of the elongated member relative to the first bone fastener about a central axis of the elongated member is inhibited. When elongated member  126  is positioned in collar  198 , the elongated member may angulate so that a height of the elongated member relative to second bone fastener  124  is variable. 
       FIG. 24  depicts a perspective view of an embodiment of elongated member  126 . Elongated member may include end portion  200 . End portion  200  may fit in a keyway in the collar of the first bone fastener to inhibit removal of the elongated member from the collar. When end portion  200  is positioned in the keyway, rotation of the rotation of elongated member  126  about the longitudinal axis of the elongated member is inhibited, but the elongated member may be angulated relative to the first bone fastener.  FIG. 25  depicts elongated member  126  after insertion into collar  198  of first bone fastener  122 . After insertion, elongated member may be rotated about the longitudinal axis of the elongated member to seat end portion  200  in the keyway of collar  198 . 
       FIG. 23  depicts dynamic posterior stabilization system after elongated member  126  is angulated downward to position a portion of the elongated member in collar  202  of second bone fastener  124 . When the portion of elongated member  126  is positioned in collar  202  of second bone fastener  124 , stop  204  may be coupled to the collar. Stop  204  may inhibit removal of elongated member from collar  202 . Bias member  128  may be positioned against collar  202 , and stop  132 ′ may be used to fix the position of the bias member. Bias member  130  may be positioned on elongated member  126  and stop  132 ″ may be used to inhibit removal of the bias member from the elongated member. 
       FIG. 26  depicts a representation of dynamic interbody device  50  and posterior stabilization system  118  positioned between vertebrae  52 ,  54 . Bridge  70  of dynamic interbody device  50  may be coupled to second bone fastener  124  of dynamic posterior stabilization system  118 . Coupling dynamic interbody device  50  to dynamic posterior stabilization system  118  may inhibit undesired migration of the dynamic interbody device relative to vertebrae  52 ,  54  while still allowing for flexion, extension, lateral bending, and/or axial rotation of the vertebrae. 
     When closure member  120  is tightened in collar  152  of second bone fastener  124 , a bottom surface of the collar may align and be tightened against angled surface  80  of bridge  70 . Tightening closure member  120  may fix the position of bridge  70 . When closure member  120  is tightened so that the bottom of collar  152  is positioned against angled surface  80 , the center of curvature of elongated member  126  may align or substantially align with the center or centers of curvature of dynamic interbody device  50  that allow for flexion/extension and/or lateral bending. Aligning or substantially aligning the center of curvature of elongated member  126  with the center or centers of curvature of dynamic interbody device  50  allows the elongated member to move relative to second bone fastener  124  during flexion/extension and/or lateral bending so that dynamic posterior stabilization system  118  works in conjunction with the dynamic interbody device. 
     Dynamic posterior stabilization system  118  may share a portion of the load applied to the vertebrae  52 ,  54  while providing guidance and resistance to flexion/extension and/or lateral bending that is, or is approximate to, the resistance provided by a normal functional spinal unit. Allowing for movement of the dynamic interbody device and for movement of the dynamic posterior stabilization system may inhibit deterioration of adjacent functional spinal units. 
     In some embodiments, first bone fastener  122  of dynamic posterior stabilization system is placed in the more cephalad of the vertebrae to be stabilized. Bridge  70  may couple dynamic interbody device  50  to dynamic posterior stabilization system  118 . Bridge may be coupled to dynamic posterior stabilization system  118  at or near to second bone fastener  124 . Coupling bridge  70  to dynamic posterior stabilization system  118  at or near to second bone fastener  124  may inhibit or eliminate contact of the bridge with nerves extending from between the vertebrae. 
     In some embodiments, a first dynamic posterior stabilization system coupled to vertebrae may be unconnected to a second dynamic posterior stabilization system on a contralateral side of the vertebrae. In some embodiments, one or more transverse connectors may connect dynamic posterior stabilization systems placed on contralateral sides of vertebrae.  FIG. 27  depicts a schematic representation of transverse connector  206  coupled to dynamic posterior stabilization systems  118 ′,  118 ″. Transverse connector  206  may be coupled to the collars of second bone fasteners  124 ′,  124 ″. Transverse connector  206  may include first arm  208 , second arm  210 , and joint  212 . To attach transverse connector  206 , a fastener of joint  212  may be loosened. First arm  208  may be positioned in the collar of second bone fastener  124 ′. Second arm  210  may be positioned in the collar of second bone fastener  124 ″. Closure member  120 ′ may secure member  188 ′ and first arm  208  to second bone fastener  124 ′. Closure member  120 ″ may secure member  188 ″ and second arm  210  to second bone fastener  124 ″. The fastener of joint  212  may be tightened. Transverse connector may join dynamic posterior stabilization system  118 ′ to dynamic posterior stabilization system  118 ″ and increase the stiffness of the stabilization system. Attaching transverse connector  206  to the collars of second bone fasteners  124 ′,  124 ″ does not inhibit movement of elongated members  126 ′,  126 ″ relative to first bone fasteners  122 ′,  122 ″ and/or the second bone fasteners, 
     In some embodiments, a single dynamic interbody device may be positioned in a disc space between vertebrae. The dynamic interbody device may be positioned using an anterior, posterior, anterio-lateral, or other approach. A bridge may couple the dynamic interbody device to a dynamic posterior stabilization system. In some embodiments, a separate component bridge is coupled to the dynamic posterior stabilization system to inhibit posterior migration of the dynamic interbody device. 
     In some embodiments, a posterior approach may be used to install a stabilization system for a patient. The stabilization system may replace one or more parts of a functional spinal unit of the patient. The stabilization system may include one or more dynamic interbody devices, and one or more dynamic posterior stabilization systems. 
     During a posterior insertion procedure of a spinal stabilization system, a first dynamic interbody device may be installed on a first side of the patient, and then a second dynamic interbody device may be installed on a second side (contralateral side) of the patient. The dynamic interbody devices may be bimodal devices. A facet joint between the pair of vertebrae to be stabilized may be removed on the first side to provide access to the intervertebral disc between the vertebrae. Tissue may be retracted to provide access to the intervertebral disc between the vertebrae. A discectomy may be performed to remove all or a portion of the intervertebral disc. A channel may be formed in one or both of the vertebrae for the dynamic interbody device or for a keel of the dynamic interbody device. The dynamic interbody device may be installed in the formed space between the vertebrae. 
     In some embodiments, a bone fastener may be secured to each of the vertebrae to be stabilized. The bone fasteners may be used during insertion of the dynamic interbody device to provide distraction of the vertebrae. The bone fasteners may be components of a dynamic posterior stabilization system. In some embodiments, a bridge coupled to the dynamic interbody device may be coupled to one of the bone fasteners secured to the vertebrae. 
     After the dynamic interbody device on the first side is inserted between the vertebrae, the dynamic posterior stabilization system may be formed. After insertion of the bone fasteners in the vertebrae, the elongated member and bias member may be coupled to the bone fasteners. 
     A second dynamic interbody device may be installed on the contralateral side of the patient. In some embodiments, a transverse connector may be couple the dynamic posterior stabilization system on the first side to the dynamic posterior stabilization system on the second side. 
     During some spinal stabilization procedures, posterior elements of the vertebrae to be stabilized may be removed. Pins may be threaded in the vertebrae. The pins may be inserted parallel or inclined away from the endplates of the vertebrae to be stabilized. A discectomy may be performed to remove the intervertebral disc. The pins may be used to inhibit compression of the disc space between the vertebrae.  FIG. 28  depicts pins  214  positioned in vertebrae  52 ,  54  on a first side of the patient. Pins may also be positioned in vertebrae  52 ,  54  on the second side of the patient. 
     Pins  214  may be used to distract the disc space between vertebrae  52 ,  54 . In some embodiments, a distraction plug may be inserted in the disc space of the second side to hold the disc space open. A guide of a keel cutter may be placed over a pin. A head of the keel cutter may be impacted to form a channel in a surface of one of the vertebra.  FIG. 29  depicts distraction plug  216  positioned in the disc space between vertebrae  52 ,  54 . Guide  218  of keel cutter  220  is positioned over pin  214 , and the keel cutter has been used to form a channel for a dynamic interbody implant in vertebra  54 . After the channel has been formed, keel cutter may be removed from pin  214 . The endplates of vertebrae  52 ,  54  may be prepared for the dynamic interbody implant by removing any osteophytes and/or tissue. A distraction plug  216  may be inserted in the first side, and distraction plug  216  may be removed from the second side. The keel cutter may be placed over the appropriate pin on the second side. The keel cutter may be used to form a channel in the vertebra on the second side. The vertebrae on the second side may be prepared for receiving a dynamic interbody device. After the first side and second side are prepared, the distraction plug may be removed. The height of the disc space may be maintained by force applied to pins  214 . 
     Trial implants may be used to determine the heights of the dynamic interbody devices to be inserted into the prepared disc space. Appropriate dynamic interbody devices may be attached to inserters. The dynamic interbody devices may be inserted into the prepared disc space. The dynamic interbody devices may be inserted into the disc space and rotated to place keels of the dynamic interbody devices in the channels formed in vertebra.  FIG. 30  depicts a representation of dynamic interbody devices  50 ′,  50 ″ inserted in the disc space. Shafts  222  of the inserters  224  should be parallel to each other and dynamic interbody devices  50 ′,  50 ″ should be inserted to the same depth in the disc space. After insertion, the distraction force is removed and the spine is allowed to return to a neutral position. If necessary, pins  214  may be used to compress the vertebrae towards each other. Inserters  224  may be removed from the dynamic interbody devices. 
     Pins  214  may be removed from the vertebrae. Fasteners of a dynamic posterior stabilization system may be inserted into the openings in the vertebra where the pins were located. In some embodiments, a bridge may be coupled to the second bone fastener of the dynamic posterior stabilization system. Ball  194  (depicted in  FIG. 22 ) on elongated member  126  may be positioned on the elongated member so that member  188  couples to the second bone fastener when the ball is positioned in the first bone fastener. When ball is positioned in the first bone fastener and member  188  is coupled to the second bone fastener, closure members may be secured to the collars of the first bone fastener and the second bone fastener to complete the dynamic posterior stabilization system. A dynamic posterior stabilization system may be formed on each side of the patient.  FIG. 31  depicts dynamic interbody device  50  and dynamic posterior stabilization system  118  coupled to vertebrae  52 ,  54 . 
     In some embodiments, a dynamic interbody device may be installed between a pair of vertebrae using an anterior approach. After insertion of the dynamic interbody device, a dynamic posterior stabilization system can be attached to the vertebrae. In some embodiments, a bridge is coupled to the dynamic posterior stabilization system. In some embodiments, the bridge is coupled to both the dynamic interbody device and the dynamic posterior stabilization system. 
     In this patent, certain U.S. patents, and U.S. patent applications have been incorporated by reference. The text of such U.S. patents and U.S. patent applications is, however, only incorporated by reference to the extent that no conflict exists between such text and the other statements and drawings set forth herein. In the event of such conflict, then any such conflicting text in such incorporated by reference U.S. patents and U.S. patent applications is specifically not incorporated by reference in this patent. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.