Superelastic spinal stabilization system and method

A device for stabilizing at least a portion of the spinal column, including a longitudinal member sized to span a distance between at least two vertebral bodies and being at least partially formed of a shape-memory material exhibiting pseudoelastic characteristics at about human body temperature. A number of bone anchors are used to secure the longitudinal member to each of the vertebral bodies. The longitudinal member is reformed from an initial configuration to a different configuration in response to the imposition of stress caused by relative displacement between the vertebral bodies, and recovers toward the initial configuration when the stress is removed to thereby provide flexible stabilization to the spinal column. During reformation of the longitudinal member, at least a portion of the shape-memory material transforms into stress-induced martensite. In a particular aspect of the invention, the longitudinal member is a plate having a central portion at least partially formed of the shape-memory material, and a pair of connection portions disposed at opposite ends of the central portion for connection to each of the vertebral bodies. The central portion of the plate defines a number of alternating ridges and grooves along a length thereof having an initial amplitude corresponding to the initial configuration and a different amplitude corresponding to the different configuration.

FIELD OF THE INVENTION
 The present invention relates generally to the field of instrumentation and
 systems for treatment of the spine, and more particularly to a device for
 flexibly stabilizing the cervical spine.
 BACKGROUND OF THE INVENTION
 As with any bony structure, the spine is subject to various pathologies
 that compromise its load bearing and support capabilities. Such
 pathologies of the spine include, for example, degenerative diseases, the
 effects of tumors and, of course, fractures and dislocations attributable
 to physical trauma. In the treatment of diseases, malformations or
 injuries affecting spinal motion segments (which include two adjacent
 vertebrae and the disc tissue or disc space therebetween), and especially
 those affecting disc tissue, it has long been known to remove some or all
 of a degenerated, ruptured or otherwise failing disc. In cases in which
 intervertebral disc tissue is removed or is otherwise absent from a spinal
 motion segment, corrective measures are indicated to insure the proper
 spacing of adjacent vertebrae formerly separated by the removed disc
 tissue.
 Commonly, the adjacent vertebrae are fused together using a graft structure
 formed of transplanted bone tissue, an artificial fusion element, or other
 suitable compositions. Elongated rigid plates have been helpful in the
 stabilization and fixation of the spine when used alone or in conjunction
 with a grafting procedure, especially in the thoracic and lumbar regions
 of the spine. These plating systems also have the potential advantage of
 increasing union rates, decreasing graft collapse, minimizing subsequent
 kyphotic deformity, and decreasing the need for bulky or rigid
 postoperative immobilization. Additionally, rigid internal fixation
 systems may improve the overall quality of life of the patient and may
 provide the opportunity for earlier rehabilitation.
 The plating techniques described above have also found some level of
 acceptance by surgeons specializing in the treatment of the cervical
 spine. The cervical spine can be approached either anteriorly or
 posteriorly, depending upon the spinal disorder or pathology to be
 treated. Many well-known surgical exposure and fusion techniques of the
 cervical spine are described in the publication entitled Spinal
 Instrumentation, edited by Drs. Howard An and Jerome Cotler. The primary
 focus of cervical plating systems has been to restore stability and
 increase the stiffness of an unstable spinal motion segment. During the
 development of cervical plating systems, various needs have been
 recognized. For example, the system should provide strong mechanical
 fixation that can control movement of the vertebral segments. The system
 should also be able to maintain stress levels below the endurance limits
 of the plate material, while at the same time exceeding the strength of
 the anatomic structures or vertebrae to which the plating system is
 engaged. Additionally, the system should preferably be capable of
 accommodating for the natural movement of the vertebrae relative to one
 another, including torsional movement during rotation of the spine and
 translational movement during flexion or extension of the spine.
 There is increased concern in the spinal medical community that anterior or
 posterior plating systems may place excessive loads on the vertebrae or
 graft structure in response to small degrees of spinal motion. See, e.g.,
 K. T. Foley, D. J. DiAngelo, Y. R. Rampersaud, K. A. Vossel and T. H.
 Jansen, The In Vitro Effects of Instrumentation on Multi-level Cervical
 Strut-Graft Mechanics, 26th Proceeding of the Cervical Spine Research
 Society, 1998. If the plating system is used in conjunction with grafting,
 these loads may promote pistoning, which can ultimately lead to
 degradation or failure of the graft construct. Additionally, even small
 degrees of spinal motion can cause significant forces to be placed on the
 spinal plate and the bone anchor devices which attach the plate to the
 vertebrae, whether they be bone screws, hooks, etc. These forces may lead
 to failure of the plate or loosening of the points of attachment between
 the bone anchors and the vertebrae, thus resulting in the potential loss
 of support by the plate.
 Thus, there is a general need in the industry to provide a device for
 flexibly stabilizing the spine, and in particular the cervical region of
 the spine. The present invention meets this need and provides other
 benefits and advantages in a novel and unobvious manner.
 SUMMARY OF THE INVENTION
 The present invention relates generally to a system for flexibly
 stabilizing the spine, and more particularly the cervical region of the
 spine. While the actual nature of the invention covered herein can only be
 determined with reference to the claims appended hereto, certain forms of
 the invention that are characteristic of the preferred embodiments
 disclosed herein are described briefly as follows.
 In one form of the present invention, a device is provided for stabilizing
 at least a portion of the spinal column. The device includes a
 longitudinal member sized to span a distance between at least two
 vertebral bodies and being at least partially formed of a shape-memory
 material exhibiting pseudoelastic characteristics when implanted within
 the body. The device also includes bone anchors for securing the
 longitudinal member to each of the vertebral bodies. The longitudinal
 member is reformed from an initial configuration to a different
 configuration in response to the imposition of stress caused by relative
 displacement between the vertebral bodies and recovers toward the initial
 configuration when the stress is removed.
 In another form of the present invention, a device is provided for
 stabilizing at least a portion of the spine. The device includes a
 compliant element at least partially formed of a pseudoelastic
 shape-memory material displaying reversible stress-induced martensitic
 behavior at about human body temperature. The compliant element has a
 length sized to span a distance between at least two spinal motion
 segments and is secured to each of the spinal motion segments by at least
 two anchoring elements. The length of the compliant element is variable
 between an initial length and a different length through the imposition of
 stress caused by relative displacement between the spinal motion segments,
 with the different length occurring through the transformation of at least
 a portion of the pseudoelastic shape-memory material into reversible
 stress-induced martensite, and with the compliant element recovering or
 reforming toward the initial length when the stress is removed.
 In yet another form of the present invention, a spinal stabilization system
 is provided, comprising an elongate member for placement adjacent the
 cervical region of the spine and being at least partially formed of a
 pseudoelastic shape-memory material displaying reversible stress-induced
 martensitic behavior at about human body temperature. The system is
 further comprised of at least two bone engaging members, each adapted to
 engage a respective one of at least two cervical vertebrae to secure the
 elongate member thereto. The elongate member is deformed during relative
 displacement between the cervical vertebrae, thus transforming a portion
 of the shape-memory material into a stress-induced martensitic state. The
 elongate member exerts a substantially constant restorative force on the
 cervical vertebrae when the shape-memory material is in the stress-induced
 martensitic state to thereby flexibly stabilize the cervical region of the
 spine.
 In still another form of the present invention, a connector apparatus is
 provided for connecting a first member to a second member. The apparatus
 is comprised of a central portion having a longitudinal axis and being at
 least partially formed of a shape-memory material exhibiting pseudoelastic
 characteristics at about body temperature. The central portion includes a
 number of alternating ridges and grooves disposed along the longitudinal
 axis. The apparatus also includes at least two connection portions, each
 configured to engage a respective one of the first and second members. The
 ridges and grooves are transformed from an initial configuration to a
 different configuration in response to the imposition of stress caused by
 relative displacement between the first and second members, and are
 reformed toward the initial configuration when the stress is removed.
 In a further form of the present invention, a method is provided for
 stabilizing at least a portion of the spinal column including at least two
 vertebrae. The method includes: providing an elongate member having a
 length extending between the two vertebrae and being at least partially
 formed of a pseudoelastic shape-memory material displaying reversible
 stress-induced martensitic behavior at about body temperature; securing
 the elongate member to the two vertebrae; transforming at least a portion
 of the shape-memory material into a martensitic state as a result of the
 imposition of the stress onto the elongate member during relative movement
 between the two vertebrae; and applying a substantially constant
 restorative force to the two vertebrae when the shape-memory material is
 in the martensitic state to provide stabilization to the at least a
 portion of the spinal column.
 It is one object of the present invention to provide a device and method
 for stabilizing at least a portion of the spine, and more particularly the
 cervical region of the spine.
 Further objects, features, advantages, benefits, and aspects of the present
 invention will become apparent from the drawings and description contained
 herein.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
 For the purposes of promoting an understanding of the principles of the
 invention, reference will now be made to the embodiments illustrated in
 the drawings and specific language will be used to describe the same. It
 will nevertheless be understood that no limitation of the scope of the
 invention is hereby intended, such alterations and further modifications
 in the illustrated devices, and such further applications of the
 principles of the invention as illustrated herein being contemplated as
 would normally occur to one skilled in the art to which the invention
 relates.
 FIGS. 1-2 depict a spinal stabilization system 20 according to one
 embodiment of the present invention for stabilizing at least a portion of
 the vertebral column. Stabilization system 20 is shown attached to the
 cervical region of the vertebral column, extending across a plurality of
 spinal motion segments, such as cervical vertebrae V. However, it should
 be understood that system 20 may also be utilized in other areas of the
 spine, such as the thoracic, lumbar, lumbo sacral and sacral regions of
 the spine. It should also be understood that system 20 can extend across
 any number of vertebrae V, including two adjacent vertebrae V.
 Additionally, although system 20 is shown as having application in an
 anterior approach, system 20 may alternatively be applied in other
 surgical approaches, such as, for example, a posterior approach.
 In a typical grafting procedure, one or more adjacent pairs of vertebra V
 may be fused together by way of a graft or implant (not shown) positioned
 in the disc space between the adjacent vertebrae V. The implant may be a
 bone graft, an artificial fusion device, or any other type of interbody
 device that is insertable into the disc space to promote fusion between
 the adjacent vertebrae V. One purpose of the stabilization system 20 is to
 prevent excessive loads from being placed on the graft structures in
 response to even small degrees of spinal motion. However, it should be
 understood that stabilization system 20 can be used in conjunction with
 fusion or non-fusion treatment of the spine.
 In accordance with the present invention, stabilization system 20 includes
 an elongate member 22 positioned along a portion of the vertebral column.
 In the illustrated embodiment, the longitudinal member is an elongated
 stabilization plate sized to span a distance between at least two
 vertebrae V. Although elongate member 22 has been illustrated and
 described as a spinal plate, it should be understood that elongate member
 22 can also be configured as a spinal rod or any other type of
 longitudinal element for use in conjunction with a spinal fixation system.
 It should also be understood that any number of plates 22, including a
 pair of plates 22 positioned on opposite sides of the spine, could be used
 to provide stabilization to the vertebral column. Stabilization plate 22
 is secured to the upper and lower vertebrae V.sub.U, V.sub.L (FIG. 1) by a
 plurality of bone anchors, shown in the form of bone screws 24. However,
 other types of bone anchors are also contemplated, such as, for example,
 spinal hooks. A locking device 26 engages the adjacent bone screws 24 to
 prevent bone screws 24 from loosening and backing out. In the illustrated
 embodiment, the locking device 26 is a screw extending through each end
 portion of the plate 22 and into engagement with the heads of adjacent
 bone screws 24. However, other types of locking devices are also
 contemplated, such as, for example, a pop rivet, a retainer fabricated
 from a shape-memory alloy configured to change shape in response to a
 change in temperature or the release of stress, a locking washer rotatably
 displaceable between an unlocked position and a locked position, or any
 other type of locking mechanisms known to those of skill in the art. An
 example of a locking washer for use with the present invention is
 disclosed in U.S. patent application Ser. No. 09/399,525 entitled
 "Anterior Cervical Plating System" filed on Sep. 20, 1999, the contents of
 which are hereby incorporated by reference. Further details regarding
 spinal stabilization system 20 are described more fully below.
 Referring to FIGS. 3-7, shown therein are various details regarding the
 stabilization plate 22. Plate 22 has a longitudinal axis L extending along
 its length and includes an elongated central portion 30 and a pair of
 connection portions 32 disposed at opposite ends of central portion 30. In
 the illustrated embodiment, central portion 30 and connection portions 32
 are formed integral to plate 22, thus forming a unitary structure or
 construct. However, it should be understood that connection portions 32
 can be formed separate from central portion 30 and attached thereto by any
 method known to one of ordinary skill in the art, such as, for example, by
 fastening or welding. Plate 22 is at least partially formed of a
 shape-memory material that exhibits pseudoelastic characteristics or
 behavior at about human body temperature, the details of which will be
 discussed below. It should be understood that the terms "pseudoelastic"
 and "superelastic" have identical meanings and are used interchangeably
 throughout this document. In one embodiment of the present invention, the
 entire plate 22 is formed of the shape-memory material. However, it should
 be understood that only central portion 30 need be at least partially
 formed of the shape-memory material, with the connection portion 32 being
 formed of any suitable biocompatible material, such as, for example,
 stainless steel or titanium.
 SMAs exhibit a "shape-memory" characteristic or behavior in which a
 particular component formed of a shape-memory alloy ("SMA") is capable of
 being deformed from an initial "memorized" shape or configuration to a
 different shape or configuration, and then reformed back toward its
 initial shape or configuration. The ability to possess shape-memory is a
 result of the fact that the SMA undergoes a reversible transformation from
 an austenitic state to a martensitic state. If this transformation occurs
 due to a change in temperature, the shape-memory phenomena is commonly
 referred to as thermoelastic martensitic transformation. However, if the
 martensitic transformation occurs due to the imposition of stress, the
 shape-memory phenomena is commonly referred to as stress-induced
 martensitic transformation. The present invention is primarily concerned
 with stress-induced martensitic transformation.
 SMAs are known to display a superelastic phenomena or rubber-like behavior
 in which a strain attained beyond the elastic limit of the SMA material
 during loading is recovered during unloading. This superelastic phenomena
 occurs when stress is applied to an SMA article at a temperature slightly
 higher than the temperature at which the SMA begins to transform into
 austenite (sometimes referred to as the transformation temperature or
 A.sub.s). When stressed, the article first deforms elastically up to the
 yield point of the SMA material (sometimes referred to as the critical
 stress). However, upon the further imposition of stress, the SMA material
 begins to transform into stress-induced martensite or "SIM". This
 transformation takes place at essentially constant stress, up to the point
 where the SMA material is completely transformed into martensite. When the
 stress is removed, the SMA material will revert back into austenite and
 the article will return to its original, pre-programmed programmed or
 memorized shape. This phenomena is sometimes referred to as
 superelasticity or pseudoelasticity. It should be understood that this
 phenomena can occur without a corresponding change in temperature of the
 SMA material. Further details regarding the superelastic phenomena and
 additional characteristics of SIM are more fully described by Yuichi
 Suzuki in an article entitled Shape Memory Effect and Super-Elasticity in
 Ni-Ti Alloys, Titanium and Zirconium, Vol. 30, No. 4, October 1982, the
 contents of which are hereby incorporated by reference.
 There is a wide variety of shape-memory materials suitable for use with the
 present invention, including shape-memory metal alloys (e.g., alloys of
 known metals, such as, for example, copper and zinc, nickel and titanium,
 and silver and cadmium) and shape-memory polymers. While there are many
 alloys which exhibit shape-memory characteristics, one of the more common
 SMAs is an alloy of nickel and titanium. One such alloy is nitinol, a
 bio-compatible SMA formed of nickel and titanium. Nitinol is well suited
 for the particular application of the present invention because it can be
 programmed to undergo a stress-induced martensitic transformation at about
 normal human body temperature (i.e., at about 35-40 degrees Celsius).
 Moreover, nitinol has a very low corrosion rate and excellent wear
 resistance, thereby providing an advantage when used as a support
 structure within the human body. Additionally, implant studies in animals
 have shown minimal elevations of nickel in the tissues in contact with the
 nitinol material. It should be understood, however, that other SMA
 materials that exhibit superelastic characteristics are contemplated as
 being within the scope of the invention.
 The central portion 30 of plate 22 is at least partially formed of an SMA
 material and has an initial or "memorized" shape or configuration (see
 FIG. 4a), and a different shape or configuration (FIG. 4b) when deformed
 through the imposition of stress onto plate 22. If the central portion 30
 is reshaped or deformed while at a temperature above the transformation
 temperature A.sub.s, the central portion 30 will automatically recover
 toward its initial shape or configuration when the stress is removed from
 plate 22. In one embodiment of the present invention, the plate 22 is
 secured to the upper and lower vertebrae V.sub.u, V.sub.l while in a
 substantially unstressed initial configuration where virtually all of the
 SMA material is in an austenitic state. Upon the imposition of stress onto
 plate 22, caused by relative movement between the upper and lower
 vertebrae V.sub.u, V.sub.l, at least a portion of the SMA material is
 transformed into reversible stress-induced martensite. Upon the reduction
 or removal of stress, at least a portion of the SMA material is
 transformed back into austenite. It should be understood that the plate 22
 may be pre-stressed prior to being secured to the upper and lower
 vertebrae V.sub.u, V.sub.l, thus initially transforming a portion of the
 SMA material from austentite into SIM. In this case, the SMA material will
 never attain an entirely austenitic state when the stress imposed onto
 plate 22 by the upper and lower vertebrae V.sub.u, V.sub.l is removed.
 Referring specifically to FIG. 4a, central portion 30 is shown in an
 initial, unstressed configuration. Central portion 30 has an
 accordion-like shape, defining a series of alternating ridges 34 and
 grooves 36 extending along longitudinal axis L and facing laterally
 outward relative to longitudinal axis L. When in its initial
 configuration, central portion 30 has an initial, unstressed length
 l.sub.1. Preferably, each of the alternating ridges 34 and grooves 36 has
 a substantially triangular shape, with the outermost tip 35 of ridges 34
 being rounded to avoid trauma to adjacent tissue, and the innermost
 portion of grooves 36 defining a partially cylindrical surface 37.
 However, it should be understood that ridges 34 and grooves 36 can take on
 other shapes as well, such as, for example, an arcuate shape, an
 undulating curve shape, or a square or rectangular shape. When central
 portion 30 is in its initial configuration, each of the ridges 34 and
 grooves 36 have an initial amplitude a.sub.l, as measured from base line B
 to the outermost tip 35 and the innermost point of cylindrical surface 37.
 Preferably, the partially cylindrical surface 37 has a diameter somewhat
 larger than the minimum distance between adjacent ridges 34.
 In the illustrated embodiment, a number of the alternating ridges 34 and
 grooves 36 are defined along each of the laterally facing sides 38a, 38b
 of central portion 30, with the ridges and grooves defined along side 38a
 being disposed laterally opposite respective ones of the ridges and
 grooves defined along side 38b, thereby defining laterally opposing pairs
 of ridges 34p and laterally opposing pairs of grooves 36p. A number of
 openings or slots 40 extend through central portion 30 intermediate the
 laterally opposing pairs of ridges 34p. Preferably, slots 40 have a
 substantially oval shape, with each of the slots 40 having laterally
 extending side walls defining opposing concave surface 42 and an initial
 slot width w.sub.l when central portion 30 is in its initial, unstressed
 configuration. However, it should be understood that slots 40 can take on
 other shapes as well, such as, for example, circular, elliptical, diamond
 or other geometric shapes as would occur to one of ordinary skill in the
 art. Slots 40 span virtually the entire distance between the opposing
 pairs of ridges 34p, having opposing ends 44 positioned proximately
 adjacent the outermost tips 35 of opposing pairs of ridges 34p. In a
 preferred embodiment, the opposing ends 44 of slots 40 each define a
 partially cylindrical surface 45. Preferably, the partially cylindrical
 surface 45 has a diameter somewhat larger than the minimum distance
 between the opposing concave surfaces 42. The configuration of central
 portion 30 can alternatively be described as having a pair of laterally
 opposing thin strips of material 46 extending along longitudinal axis L,
 each having a zig-zag or corrugated shape and being linked together by a
 number of laterally extending linking portions 48.
 Referring now to FIG. 4b, central portion 30 is shown reformed from the
 initial shape or configuration illustrated in FIG. 4a to a different,
 stressed shape or configuration, such reformation occurring in response to
 the imposition of stress caused by relative displacement between the upper
 and lower vertebrae V.sub.u, V.sub.l (FIG. 1). This relative displacement
 can arise through translational movement of upper and lower vertebrae
 V.sub.u, V.sub.l, as occurring during either flexion or extension of the
 spinal column, or through torsional movement, as occurring during rotation
 of the spinal column. The imposition of stress onto central portion 30
 causes at least a portion of the shape-memory material to transform into
 reversible stress-induced martensite. When deformed into its different
 configuration, central portion 30 has a different, stressed length
 l.sub.2, ridges 34 and grooves 36 have a different amplitude a.sub.2, and
 slots 40 are reshaped to define a different slot width w.sub.2. In the
 illustrated embodiment, central portion 30 is elongated or lengthened when
 stressed, thus increasing length l.sub.2 and slot width w.sub.2 while
 decreasing the amplitude a.sub.2. However, it should be understood that
 central portion 30 could alternatively be compressed or shortened when
 stressed, thus decreasing length l.sub.2 and slot width w.sub.2 while
 increasing the amplitude a.sub.2.
 Referring collectively to FIGS. 4a and 7, shown therein are various details
 regarding the connection portions 32. Each of the connection portions 32
 has an inner surface 50 and an oppositely facing outer surface 52. When
 plate 22 is secured to the spinal column (FIGS. 1 and 2), the inner
 surface 50 abuts the upper and lower vertebrae V.sub.u, V.sub.l. Inner
 surface 50 defines a concave lateral curvature C (FIG. 6) extending along
 the longitudinal axis L. Lateral curvature C preferably corresponds to the
 anatomical curvature of the anterior, outer surfaces of upper and lower
 vertebrae V.sub.u, V.sub.l. Outer surface 52 preferably defines a convex
 surface extending along longitudinal axis L to reduce the amount of trauma
 to the adjacent soft tissue when plate 22 is secured to the spinal column.
 Preferably, the central portion 30 of plate 22 defines a corresponding
 concave lateral curvature C along inner surface 51 and a corresponding
 convex outer surface 53. However, it should be understood that the central
 portion 30 and the connection portions 32 can be individually configured
 to accommodate the specific spinal anatomy and vertebral pathology
 involved in any particular application of stabilization system 20.
 Each of the connection portions 32 includes a pair of openings 54 extending
 between the inner and outer surfaces 50, 52 along an axis 56 and
 configured to receive a respective one of the bone screws 24 therein. In
 the illustrated embodiment, the axis 56 of openings 54 extends inwardly
 toward transverse axis T at an angle .alpha..sub.1 (FIG. 7) and outwardly
 toward the end of connection portion 32 at an angle .alpha..sub.2 (FIG.
 5). In one specific embodiment, angle .alpha..sub.1 is approximately 6
 degrees and angle .alpha..sub.2 is approximately 12 degrees; however,
 other angles .alpha..sub.1,.alpha..sub.2 are also contemplated as being
 within the scope of the present invention. Preferably, openings 54 are
 identical in size and configuration, and are located symmetrically about
 longitudinal axis L. However, it should be understood that other sizes and
 configurations of openings 54 are also contemplated and that a single
 opening 54 could alternatively be defined in each of the connection
 portions 32. Each of the openings 54 includes a cylindrical bore 58,
 extending through connection portion 32 along axis 56 and opening onto the
 inner surface 50. Openings 54 also include a partially spherical recess
 60, extending from cylindrical bore 58 toward outer surface 52 along axis
 56. Openings 54 additionally include a conical portion 62, extending
 between spherical recess 60 and outer surface 52 along axis 56.
 Preferably, conical portion 62 is flared outwardly at approximately 45
 degrees relative to axis 56.
 Each of the connection portions 32 also includes a fastener bore 66
 extending between the inner and outer surfaces 50, 52 along transverse
 axis T and preferably intersecting the longitudinal axis L to thereby
 position fastener bore 66 intermediate and laterally adjacent bone screw
 openings 54. Fastener bore 66 is adapted to receive a respective one of
 the locking fasteners 26 therein. Specifically, fastener bore 66 includes
 a threaded portion 68 opening onto the inner surface 50 and a conical
 portion 70 extending between the threaded portion 68 and the outer surface
 52. However, it should be understood that other configurations of fastener
 bore 66 are also contemplated. For example, fastener bore 66 need not
 necessarily extend entirely through connection portion 32 in that threaded
 portion 68 can stop short of inner surface 50.
 Referring to FIG. 8, shown therein are various details regarding bone screw
 24. Bone screw 24 includes a head portion 80 connected to a threaded shank
 portion 82 by an intermediate portion 84. Threaded shank portion 82
 defines a number of threads 86 configured to engage vertebral bone and
 sized to pass through the cylindrical bore 58 in connection portion 32.
 Threads 86 are preferably cancellous threads, configured for engagement in
 the cervical region of the spinal column. Additionally, threads 86 may be
 configured to be self-tapping. Further, threads 86 preferably define a
 constant outer diameter along the length of threaded portion 82
 approximately equal to the outer diameter of intermediate portion 84, and
 a root diameter that tapers inwardly toward the intermediate portion 84.
 However, it should be understood that other configurations of threaded
 portion 82 are also contemplated as would occur to one of ordinary skill
 in the art.
 The threads 86 gradually transition into intermediate portion 84 by way of
 a thread run out 88. Intermediate portion 84 has an outer diameter sized
 somewhat larger than the diameter of the cylindrical bore 58 in connection
 portion 32. Intermediate portion 84 transitions into head portion 80 by
 way of a chamfer 90. Head portion 80 includes a lower, partially spherical
 surface 92 configured to be substantially complementary to the partially
 spherical recess 60 of opening 54. Head portion 80 also includes an upper
 conical surface 94, connected to spherical surface 92 by a flattened
 shoulder 96. In one embodiment, conical surface 94 is flared inwardly
 relative to shoulder 96 at approximately 45 degrees. Head portion 80
 further includes a truncated or flattened upper surface 98, through which
 extends a tool receiving recess 100 configured to receive a driving tool
 therein (not shown). In one embodiment, the tool recess 100 is a hexagonal
 recess; however, other shapes are also contemplated as would occur to
 those skilled in the art.
 Referring to FIG. 9, shown therein are various details regarding locking
 fastener 26. Locking fastener 26 includes a head portion 110 and a
 threaded shank portion 112 extending therefrom. Threaded shank portion 112
 defines a number of machine threads 114, configured to engage the threaded
 portion 68 of fastener bore 66 in connection portion 32. Threaded shank
 portion 112 terminates in a sharp point 116 to facilitate insertion of
 locking fastener 26 into fastener bore 66 and to permit easier penetration
 into the upper and lower vertebrae V.sub.u, V.sub.l. Threaded shank
 portion 112 transitions into head portion 110 by way of an outward taper
 118. Head portion 110 includes a lower, conical surface 120 configured
 substantially complementary to the upper conical surface 94 of bone screw
 24. In one embodiment, conical surface 120 is flared outwardly at
 approximately 45 degrees. Head portion 110 further includes an upper
 surface 122, through which extends a tool receiving recess 124 configured
 to receive a driving tool therein (not shown). In one embodiment, the tool
 recess 124 is a Phillips-type recess; however, other types are also
 contemplated as would occur to those skilled in the art.
 Referring once again to FIGS. 1 and 2, shown therein is spinal
 stabilization system 20 securely attached to the upper and lower vertebrae
 V.sub.u, V.sub.l. Initially, plate 22 is positioned across at least two
 vertebrae V, with the inner surface 50 of the connection portions 32
 placed in abutment against an outer surface of the upper and lower
 vertebrae V.sub.u, V.sub.1. The connection portions 32 are then secured to
 the upper and lower vertebrae V.sub.u, V.sub.l by passing bone screws 24
 through openings 54 and driving threaded portion 82 into vertebral bone by
 way of a driver (not shown) inserted in tool receiving recess 100. The
 bone screws 24 continue to be driven into vertebral bone until the lower
 spherical surface 92 of the head portion 80 is placed in abutment against
 the upwardly facing spherical recess 60 of opening 54.
 Conical portion 62 of openings 54 serves to facilitate the insertion of
 bone screws 24 into openings 54. Further, the interaction between
 spherical surface 92 and spherical recess 60 allows the bone screw 24 to
 be oriented relative to axis 56 within a range of angles, limited by the
 interference between the intermediate portion 84 of bone screw 24 and the
 cylindrical bore 58 in connection portion 32. Openings 54 act as a
 countersink for the head portion 80 of bone screws 24, allowing a
 significant portion of head portion 80 to be disposed beneath the upper
 surface 52 of connection portion 32 to thereby minimize the overall height
 or profile of plate 22.
 After the bone screws 24 are driven into the upper and lower vertebrae
 V.sub.u, V.sub.l, thereby securely attaching plate 22 thereto, the locking
 fasteners 26 are then installed to prevent the bone screws 24 from
 loosening and backing out. Specifically, the threaded shank portion 112 of
 fastener 26 is engaged within the threaded portion 68 of fastener bore 66
 and threaded therethrough by way of a driver (not shown) inserted in tool
 receiving recess 124. As the locking fastener 26 is driven through
 fastener bore 66, point 116 pierces the vertebrae and the threaded portion
 68 is driven into vertebral bone, thereby further securing plate 22 to
 upper and lower vertebrae V.sub.u, V.sub.1. Additionally, by embedding
 threaded portion 68 in vertebral bone, the locking fastener 26 is less
 likely to loosen and back out of fastener bore 66. The locking fastener 26
 continues to be driven through the fastener bore 66 until the lower
 conical surface 120 of head portion 110 engages the upper conical surfaces
 94 of the bone screws 24. The abutment of locking fastener 26 against bone
 screws 24 serves to retain bone screws 24 within openings 54, thereby
 preventing bone screws 24 from loosening and backing out. In an
 alternative embodiment of the invention, a washer having a lower conical
 surface may be disposed between the head portion 110 of locking fastener
 26 and the head portion 80 of bone screw 24. Tightening the locking
 fastener 26 would cause the lower conical surface of the washer to engage
 the upper conical surface 94 of bone screws 24 to retain the bone screws
 24 within the openings 54. An example of such a washer is disclosed in
 U.S. patent application Ser. No. 09/399,525 entitled "Anterior Cervical
 Plating System" filed on Sep. 20, 1999, the contents of which have been
 incorporated by reference.
 Referring now to FIG. 10, therein is illustrated a stabilization plate 200
 according to another embodiment of the present invention. Stabilization
 plate 200 extends along a longitudinal axis L. Similar to plate 22,
 stabilization plate 200 is attached to upper and lower vertebrae V.sub.U,
 V.sub.L by way of a plurality of bone screws 24, and a locking screw 26
 that engages the heads of adjacent bone screws 24 to prevent bone screws
 24 from loosening and backing out. Further details regarding plate 200 are
 described more fully below. It should be understood that stabilization
 plate 200 may be used in any application in which the stabilization plate
 22 is used, including those specific applications discussed above.
 Stabilization plate 200 includes an elongated central portion 202 and a
 pair of connecting end portions 32 operably attached to opposite ends of
 central portion 202, such as by welding, fastening, or by any other method
 known to one of ordinary skill in the art. However, it should be
 understood that central portion 202 and connection portions 32 can be
 formed integral to plate 200, thus forming a unitary structure or
 construct. Central portion 202 is at least partially formed of a
 shape-memory material that exhibits pseudoelastic characteristics or
 behavior at about human body temperature. In one embodiment of the
 invention, the entire plate 200 is formed of the shape-memory material.
 However, it should be understood that only central portion 202 need be at
 least partially formed of the shape-memory material, with the connection
 portion 32 being formed of any suitable bio-compatible material, such as,
 for example, stainless steel or titanium.
 The central portion 202 is at least partially formed of an SMA, such as the
 SMA described above with regard to plate 22, and has an initial or
 "memorized" shape or configuration (FIG. 11a), and a different shape or
 configuration (FIG. 11b) when deformed through the imposition of stress
 onto plate 200. If the central portion 202 is reshaped or deformed while
 at a temperature above the transformation temperature A.sub.s, the central
 portion 202 will automatically recover toward its initial shape or
 configuration when the stress is removed from plate 200. In one embodiment
 of the present invention, the plate 200 is secured to the upper and lower
 vertebrae V.sub.u, V.sub.l while in a substantially unstressed, initial
 configuration where virtually all of the SMA material is in an austenitic
 state. Upon the imposition of stress onto plate 200, caused by relative
 movement between the upper and lower vertebrae V.sub.u, V.sub.l, at least
 a portion of the SMA material is transformed into reversible
 stress-induced martensite. Upon the reduction or removal of stress, at
 least a portion of the SMA material is transformed back into austenite. It
 should be understood that the plate 200 may be pre-stressed prior to being
 secured to the upper and lower vertebrae V.sub.u, V.sub.1, thus initially
 transforming a portion of the SMA material from austenite into SIM. In
 this case, the SMA material will never attain an entirely austenitic state
 when the stress imposed onto plate 200 by the upper and lower vertebrae
 V.sub.u, V.sub.l is removed.
 Referring specifically to FIG. 11a, central portion 202 is shown in an
 initial, unstressed configuration. Central portion 202 has a wavy,
 corrugated shape, defining a series of alternating ridges 204 and grooves
 206 extending along longitudinal axis L. Preferably, each of the
 alternating ridges 204 and grooves 206 is arcuate-shaped so as to form a
 series of undulating curves extending along longitudinal axis L.
 Preferably, the ridges 204 and grooves 206 form a sinusoidal pattern
 relative to the base line B. However, it should be understood that the
 ridges 204 and grooves 206 can take on other shapes as well, such as, for
 example, a triangular shape, thus forming a zig-zag pattern, or a square
 or rectangular shape. When in its initial configuration, central portion
 202 has an initial, unstressed length l.sub.1, and each of the ridges 204
 and grooves 206 defines an initial amplitude a.sub.l, as measured from
 base line B.
 Referring now to FIG. 11b, central portion 202 is shown reformed from the
 initial shape or configuration illustrated in FIG. 11a to a different,
 stressed shape or configuration, such reformation occurring in response to
 the imposition of stress caused by relative displacement between the upper
 and lower vertebrae V.sub.u, V.sub.l. This relative displacement can arise
 through translational movement of upper and lower vertebrae V.sub.u,
 V.sub.l, as occurring during either flexion or extension of the spinal
 column, or through torsional movement, as occurring during rotation of the
 spinal column. The imposition of stress onto central portion 202 causes at
 least a portion of the shape-memory material to transform into reversible
 stress-induced martensite. When deformed into its different configuration,
 central portion 202 has a different, stressed length l.sub.2, and the
 ridges 204 and grooves 206 have a different amplitude a.sub.2. In the
 illustrated embodiment, central portion 202 is elongated or lengthened
 when stressed, thus increasing length l.sub.2 while decreasing the
 amplitude a.sub.2. However, it should be understood that the central
 portion 202 could alternatively be compressed or shortened when stressed,
 thus decreasing length l.sub.2 while increasing the amplitude a.sub.2.
 When secured to at least two vertebrae V, stabilization plates 22 and 200
 serve to stabilize at least a portion of the spinal column, while allowing
 at least limited relative displacement or movement between the vertebrae V
 to restore substantially normal biomechanical function thereto. When
 secured to the upper and lower vertebrae V.sub.u, V.sub.l and stressed in
 response to relative movement between the upper and lower vertebrae
 V.sub.U, V.sub.l, the plates 22, 200 will be reformed from their initial
 shape or configuration to a different shape or configuration, and at least
 a portion of the shape-memory material will be transformed from austenite
 to stress-induced martensite. When in a stress-induced martensitic state,
 the plates 22, 200 exert a substantially constant restorative force onto
 the upper and lower vertebrae V.sub.u, V.sub.l, thereby providing flexible
 stabilization to the vertebral column, and in particular the cervical
 region of the spine. Because the plates 22, 200 are at least partially
 formed of a shape-memory material displaying superelastic or pseudoelastic
 characteristics, when the stress exerted on plates 22, 200 is reduced or
 removed, at least a portion of the shape-memory material will transform
 back into austenite, and the plates 22, 200 will recover toward their
 initial, memorized shape or configuration. Plates 22, 200 are therefore
 compliant, capable of being repeatedly transformed between an initial
 configuration and a different configuration through the imposition and
 release of stress.
 Because the central portions 30, 202 of plates 22, 200 are at least
 partially formed of a shape-memory material exhibiting pseudoelastic
 behavior, they are capable of providing a relatively constant restorative
 forces to the spinal column for correction of various spinal deformities.
 This pseudoelastic behavior of the shape-memory material allows for a
 relatively large degree of recoverable deflection or strain of central
 portion 30, 202 than is possible with conventional materials, such as
 stainless steel or titanium. For instance, most conventional materials are
 capable of being elastically deformed over a relatively small range of
 deflection or strain, and when further stressed begin to deform
 plastically. However, shape-memory materials are capable of recovering up
 to about 8% of deflection or strain, well beyond the yield point of
 conventional materials.
 Moreover, because central portions 30, 202 are each configured to define a
 number of alternating ridges and grooves along the longitudinal axis L of
 plates 22, 200, when stress is applied, a greater degree of flexation or
 deflection is possible than with conventional plates having a flat or
 rectilinear configuration. The spring-like configuration of central
 portions 30, 202 allows for this added degree of flexibility or
 compliability. When central portions 30, 202 are in an initial
 configuration, each has an initial length and the alternating ridges and
 grooves have an initial amplitude. However, when stress is applied to
 plates 22, 200 along the longitudinal axis L, central portions 30, 202
 will each be reformed to a different configuration defining a different
 length and amplitude. When the stress is removed, the spring-like action
 of the central portions 30, 202 will cause each of central portions 30,
 202 to recover toward their initial configuration, length and amplitude.
 By combining the pseudoelastic characteristics of the shape-memory
 material with the spring-like configuration of central portions 30, 202,
 greater degrees of flexation or deflection are possible with stabilization
 system 20 than are currently possible through existing systems.
 While the invention has been illustrated and described in detail in the
 drawings and foregoing description, the same is to be considered as
 illustrative and not restrictive in character, it being understood that
 only the preferred embodiments have been shown and described and that all
 changes and modifications that come within the spirit of the invention are
 desired to be protected. For example, although the system 20 has been
 illustrated and described as a spinal stabilization system, it should be
 understood that plates 22, 200 can also be used as a connector for
 connecting a first member to a second member, and need not necessarily be
 used in conjunction with treatment of the spinal column.