Apparatus and method for posterior vertebral stabilization

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The following disclosure is incorporated herein by reference: U.S. application No. 10/860,778 filed Jun. 2, 2004 which carries Applicants' docket no. FSI-2 NPROV and is entitled SPINAL FACET IMPLANT WITH SPHERICAL IMPLANT APPOSITION SURFACE AND BONE BED AND METHODS OF USE.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to orthopedic medicine, and more precisely, to systems and methods for restricting relative motion between vertebrae.

2. The Relevant Technology

Many people experience back pain. Back pain is not only uncomfortable, but can be particularly debilitating. Many people who wish to participate in sports, manual labor, or even sedentary employment are unable to do so because of pains that arise from motion of or pressure on the spinal column. Such pains are often caused by traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine.

In order to alleviate such injuries and pains, spinal fusion techniques have been used for many years to essentially lock two vertebrae together. More recently, artificial discs have been used to replace natural intervertebral discs to correct disc pathologies, while still permitting the adjacent vertebrae to move with respect to each other. Various implants have also been proposed for the partial or complete replacement of vertebral facet joints to alleviate discomfort associated with diseased or atrophied articular processes, while still permitting intervertebral motion.

It has been discovered that excessive anterior/posterior motion between adjacent vertebrae can damage the associated intervertebral disc (i.e., “slip” the disc). Diseased or damaged spinal segments may be especially vulnerable to such damage to the intervertebral disc. Unfortunately, known spinal implants that permit some form of relative motion between the vertebrae generally do not sufficiently restrict the action of shear forces on the vertebrae.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.

FIG. 1 is a perspective view of a portion of a spine including two vertebrae, on which an apparatus according to one embodiment of the invention is bilaterally installed.

FIG. 2 is a caudal, section view of the vertebrae and the apparatus of FIG. 1.

DETAILED DESCRIPTION

The present invention advances the state of the art by providing systems and methods that can be used to restrict relative anterior/posterior motion between adjacent vertebrae. The present invention can be used independently of other corrective procedures, but may advantageously be combined with replacement of one or more vertebral facets. The configuration and operation of selected embodiments of the invention will be shown and described in greater detail with reference to FIGS. 1 and 2, as follows.

Referring to FIG. 1, a perspective view illustrates a portion of a spine 10. FIG. 1 illustrates only the bony structures; accordingly, ligaments, cartilage, and other soft tissues are omitted for clarity. The spine 10 has a cephalad direction 12, a caudal direction 14, an anterior direction 16, a posterior direction 18, and a medial/lateral axis 20, all of which are oriented as shown by the arrows bearing the same reference numerals. In this application, “left” and “right” are used with reference to a posterior view, i.e., a view from behind the spine 10. “Medial” refers to a position or orientation toward a sagittal plane of the spine 10, and “lateral” refers to a position or orientation relatively further from the sagittal plane.

As shown, the portion of the spine 10 illustrated in FIG. 1 includes a first vertebra 24, which may be the L5 (Fifth Lumbar) vertebra of a patient, and a second vertebra 26, which may be the L4 (Fourth Lumbar) vertebra of the patient. The systems and methods may be applicable to any vertebra or vertebrae of the spine 10 and/or the sacrum (not shown). In this application, the term “vertebra” may be broadly interpreted to include the sacrum.

As shown, the first vertebra 24 has a body 28 with a generally disc-like shape and two pedicles 30 that extend posteriorly from the body 28. A posterior arch, or lamina 32, extends between the posterior ends of the pedicles 30 to couple the pedicles 30 together. The first vertebra 24 also has a pair of transverse processes 34 that extend laterally from the pedicles 30 generally along the medial/lateral axis 20, and a spinous process 36 that extends from the lamina 32 along the posterior direction 18.

The first vertebra 24 also has a pair of superior facets 38, which are positioned toward the top of the first vertebra 24 and face generally medially. The natural inferior facets (not shown) of the first vertebra 24 have been resected away, and a pair of inferior facet joint implants 40, or inferior implants 40, has been attached to the first vertebra 24 to replace the natural inferior articular surfaces. Each of the inferior implants 40 is attached to a saddle point 42 of the first vertebra 24. Each saddle point 42 is positioned generally at the center of the juncture of each superior facet 38 with the adjacent transverse process 34.

Similarly, the second vertebra 26 has a body 48 from which two pedicles 50 extend posteriorly. A posterior arch, or lamina 52, extends between the posterior ends of the pedicles 50 to couple the pedicles 50 together. The second vertebra 26 also has a pair of transverse processes 54 that extend from the pedicles 50 generally along the medial/lateral axis 20, and a spinous process 56 that extends from the lamina 52 along the posterior direction 18.

The natural superior facets (not shown) of the second vertebra 26 have been resected away, and a pair of superior facet replacement implants 58, or superior implants 58, has been attached to the second vertebra 26 to replace the natural superior articular surfaces. Additionally, the second vertebra 26 has inferior facets 60, which are positioned toward the bottom of the second vertebra 26 and face generally outward. Each of the superior implants 58 is attached to a saddle point 62 of the corresponding pedicle 50 of the second vertebra 26. Each saddle point 62 is positioned generally at the center of the juncture of the corresponding natural superior facet (not shown) with the adjacent transverse process 54.

The inferior implants 40 on the first vertebra 24 articulate (i.e., slide and/or press) with the superior implants 58 of the second vertebra 26 to limit relative motion between the first and second vertebrae 24, 26 in a manner similar to that of the resected natural articular surfaces. The combination of each inferior implant 40 with the adjacent superior implant 58 provides an apparatus 64 that operates as a prosthetic facet joint. The superior facets 38 of the first vertebra 24 and the inferior facets 60 of the second vertebra 26 are part of natural facet joints that control motion between the first and second vertebrae 24, 26 and adjacent vertebrae (not shown).

As shown, each of the implants 40, 58 is attached to the corresponding saddle point 42, 62 via a fixation member 70 and a castle nut 72. Each of the fixation members 70 may take the form of a pedicle screw, with a distal end having threads implanted in the corresponding pedicle 30 or 50 and a proximal end protruding therefrom, with threads 74 to receive the castle nuts 72 in threaded engagement. Each fixation member 70 has a torquing interface 76, such as the hexagonal recess illustrated in FIG. 1, which enables the fixation member 70 to be rotated into the implanted state through the use of a tool (not shown).

Each of the castle nuts 72 also has a torquing interface 78 that enables the castle nut 72 to be threaded snugly onto the threads 74 of the corresponding fixation member 70. The torquing interface 78 may take the form of crenelations encircling a bore through which the fixation member 70 may protrude. A tool (not shown) may engage the torquing interface 78 to help rotate the castle nut 72 into engagement with the threads 74 and tighten the castle nut 72 to grip the corresponding implant 40, 58 against the corresponding saddle point 42, 62.

The inferior implant 40 has a mounting portion 80, an articulation portion 82, and a stem 84. The mounting portion 80 is attached to the saddle point 42 of the first vertebra 24 via the corresponding fixation member 70 and castle nut 72. In the embodiment of FIG. 1, the mounting portion 80 has a generally semispherical shape that enables adjustment of the orientation of the inferior implant 40 against the saddle point 42 prior to tightening of the castle nut 72. The stem 84 extends from the mounting portion 80 to the articulation portion 82, which is positioned proximate the original location of the resected natural inferior facet.

The articulation portion 82 has an articular surface 86, an abutment surface 90, and a crosslinking extension 92. The articular surface 86 may be oriented generally laterally and anteriorly, like the natural inferior articular surface (not shown). The abutment surface 90 is oriented generally laterally and posteriorly. The crosslinking extension 92 extends almost directly posteriorly to receive a crosslink (not shown) to attach the articulation portions 82 together, thereby stabilizing the inferior implants 40 and ensuring that they do not slip against the first vertebra 24.

As also shown in FIG. 1, each of the superior implants 58 has a mounting portion 100 and an articulation portion 102. The mounting portion 100 may be attached to the corresponding saddle point 62 of the second vertebra 26. The articulation portion 102 is positioned proximate the original location of the resected natural articular surface (not shown). Since this is close to the saddle point 62, no stem is required to connect the mounting portion 100 to the articulation portion 102.

The articulation portion 102 has an anterior flange 104 with an articular surface 106 and a posterior flange 108 with a posterior stabilization surface 110. From the mounting portion 100, the anterior flange 104 protrudes generally anteriorly and medially, while the posterior flange 108 protrudes generally posteriorly and medially. The flanges 104, 108 of each superior implant 58 cooperate to generally encircle the lateral half of the articulation portion 82 of the corresponding inferior implant 40. The articular surface 106 faces and articulates with the articular surface 86 of the inferior implant 40. The posterior stabilization surface 110 faces and articulates with the abutment surface 90 of the inferior implant 40. The geometry of the articulation portions 82, 102 and the manner in which they articulate will be set forth in greater detail in connection with the discussion of FIG. 2.

Referring to FIG. 2, a cephalad, section view illustrates the portion of the spine 10 shown in FIG. 1 along with the bilateral apparatus 64 of FIG. 1. The section is taken just superior to the fixation members 70 and castle nuts 72 that attach the superior implant 58 to the second vertebra 26. Accordingly, only the inferior portion of the first vertebra 24 is visible, along with the articulation portions 82 of the inferior implants 40. The second vertebra 26 and the superior implants 58 are visible almost in their entirety.

As shown, the articular surface 86 and the abutment surface 90 of each inferior implant 40 cooperate to provide a substantially continuous, generally semicircular convex cross sectional shape, with the articular surface 86 facing generally laterally and anteriorly, while the abutment surface 90 faces generally laterally and posteriorly. The articular surface 106 and the posterior stabilization surface 110 of each superior implant 58 similarly cooperate to provide a continuous, generally semicircular concave cross sectional shape. The articular surface 106 faces generally medially and posteriorly, while the posterior stabilization surface 110 faces generally medially and anteriorly.

Accordingly, the articular surfaces 106 articulate with the articular surfaces 86 to restrict anterior and medial/lateral motion of the first vertebra 24 with respect to the second vertebra 26. The articular surfaces 106, 86 are shaped to cooperate to replicate the articulation of a natural facet joint; accordingly, the articular surfaces 106, 86 may permit sufficient anterior and medial/lateral motion of the first vertebra 24 with respect to the second vertebra 26 to enable relatively natural flexion, extension, rotation, and lateral bending of the spine 10. To this end, the articular surfaces 106, 86 are shaped in such a manner that relative cephalad/caudal motion is generally unrestricted.

The articular surfaces 106, 86 generally do not restrict posterior motion of the first vertebra 24 with respect to the second vertebra 26. Thus, if the posterior flanges 108 of the superior implants 58 were not present, the first vertebra 24 would be able to relatively freely move posteriorly with respect to the second vertebra 26. Excessive relative anterior/posterior motion would place excessive shearing forces on the intervertebral disc between the first and second vertebrae 24, 26, and potentially injure the intervertebral disc.

The abutment surface 90 and the posterior stabilization surface 110 cooperate to substantially prevent this condition. The abutment surface 90 cooperates with the posterior stabilization surface 110 to restrict posterior motion of the first vertebra 24 with respect to the second vertebra 26. The posterior stabilization surface 110 may thus replicate the growth that occurs posteriorly of many natural superior facet joints to prevent such relative motion.

As illustrated in FIG. 2, the posterior stabilization surface 110 is generally trough-shaped. A “trough-shaped” surface is a surface with opposing sides that are upraised to define a central channel. A trough-shaped surface may be a linearly extruded surface, such as a sectional portion of a cylindrical surface, like the posterior stabilization surface 110 of FIG. 2. A linearly extruded surface is a surface that has substantially the same cross section along a linear length, as though the surface has been formed by extrusion through an opening having the cross-sectional shape. Alternatively, a trough-shaped surface may have a cross sectional shape that remains constant over a nonlinear path, or that does not remain constant over any path at all. A trough-shaped surface may also be a surface of revolution, i.e., a shape rotated through at least a portion of a circular path.

In the alternative to a trough-like shape, a posterior stabilization surface may have any of a wide variety of shapes. Such a surface may have a substantially planar shape, a semispherical shape, a parabolic shape, or a shape defined by more complex mathematical constructs, or any combination thereof.

In the embodiment of FIG. 2, the posterior stabilization surface 110 is substantially continuously formed with the articular surface 106 of the anterior flange 104. Surfaces that are “substantially continuous” with each other are generally surfaces that are not separated by a corner, edge, or break. In alternative embodiments (not shown), a posterior stabilization surface or an abutment surface need not be substantially continuous with an articular surface. Rather, a posterior stabilization surface or abutment surface may be discontinuous from, or even entirely detached from, the corresponding articular surface.

In FIG. 2, the articular surfaces 106, 86 are shown with substantially no play between them. Similarly, the abutment surface 90 and the posterior stabilization surface 110 are illustrated with substantially no play. However, when the first and second vertebrae 24, 26 are moved to different relative positions, as when flexion, extension, rotation, or lateral bending has occurred, more play may be present between the articular surfaces 106, 86 and/or between the abutment surface 90 and the posterior stabilization surface 110. Indeed, in alternative embodiments, gaps may exist between the articular surfaces 106, 86 and/or between the abutment surface 90 and the posterior stabilization surface 110 in all relative dispositions of the vertebrae 24, 26, depending on the range of motion to be allowed by the implants 40, 58. Notably, the implants 40, 58 do permit significant extension of the first and second vertebrae 24, 26.

In yet another alternative embodiments, a wide variety of other structures could be used to restrict posterior motion of a superior vertebra relative to an inferior vertebra. For example, a posterior stabilization surface may be positioned on a flange extending from an inferior implant (not shown), rather than on a superior implant. Such an implant may abut an abutment surface positioned on a superior implant (not shown).

According to other alternative embodiments, a posterior stabilization surface may abut a natural vertebral surface rather than an abutment surface of a second implant. Hence, an implant may be used to limit relative posterior motion of a superior vertebra even if only one or more superior or inferior implants are used. Such implants may then articulate with natural bone structures such as natural articular surfaces. In this application, an articular surface on a vertebra includes both prosthetic and natural articular surfaces.

In other alternative embodiments (not shown), a posterior stabilization surface may be incorporated into a kinematic feature, such as a linkage, that restricts posterior motion of the superior vertebra. In yet another alternative embodiments (not shown), a resilient mechanism such as a spring could be provided and situated such that posterior motion of the superior vertebra can be more gradually and gently restricted. Those of skill in the art will recognize that a wide variety of other alternative embodiments may be constructed within the scope of the present invention.

The present invention has particular relevance to orthopedic medicine, and more particularly to facet joint replacement. However, the principles, structures, and methods of the present invention may be utilized independently of facet joint replacement methods and devices.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. As such the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.