Modular Pedicle Screw System

The present invention relates to a method and apparatus which is used to retain spinal elements in a spatial relationship. More particularly, a modular pedicle screw assembly of the invention consists of a base fastener and connecting headpieces which are varied in length and angularity in order to provide a clinician a greater degree of flexibility in stabilizing the spine when performing spinal fusions in the cervical, thoracic, and lumbar regions.

Important considerations for spinal instrumentation used to promote spinal fusion are safe insertion, rigid fixation, and ease and adaptability of implantation. Historically, Harrington rods were an early type of spinal instrumentation used to stabilize the thoracic and the upper lumbar regions to obtain correction and stability for fusion. This instrumentation involved long rods attached to hooks, which were inserted over the bony arches (lamina) of the spinal canal. The instrumentation was subject to failure when the hooks either broke the lamina or became displaced and thus, the intended stabilization and correction was lost.

Subsequent spinal instrumentation included the use of a longitudinal member, such as a plate, to interconnect a series of pedicle screws such as that disclosed in U.S. Pat. No. 5,129,899 issued to Laura C. Small ("Small, et al"). In this system, the clinician must strive to place the series of pedicle screws into the bone in longitudinal alignment with as little lateral displacement and angular deflection as possible so that the plate may be attached to the series of pedicle screws in order to achieve a solid biomechanical relationship. However, due to variances in the spinal column or imperfect placement by the clinician, the screws are often inserted at an angle or must be placed laterally outside the longitudinal axis defined by the plate and other screws. While slight angular deviations of screw placement of up to 20.degree. may be overcome by the use of contoured washers, larger angular deflections and the problems associated with lateral displacement of the pedicle screws cannot be overcome by the instrumentation disclosed in the Small, et al. patent.

The development of the Kambin Offset Bolt for the use with the device disclosed in the Small, et al. patent allows greater flexibility when the pedicle screws cannot be placed in the desired longitudinal relationship. The Kambin system, which is offered by Smith & Nephew and known to those of skill in the art, uses bolts having lateral extensions that allow attached threaded shanks to reach the plates in a more desirable biomechanical relationship. The offset bolt, however, has several drawbacks. First, insertion of the offset bolt requires a larger area of clearance to obtain insertion. This often leads to insertion difficulty when the bolt interferes with the spinal retractors, other elements of the spine, or other screws which have already been implanted. This interference can cause other implanted screws to loosen when impinged. In addition, once in place, the offset bolt can only overcome translational malalignment, but not angular malalignment of the pedicle screws.

Another system is disclosed in U.S. Pat. No. 5,129,900 ("Asher, et al."). There, pedicle screws are connected to a longitudinal member, such as a rod. Lateral deviations may be cured by the use of connector members which include oblong openings. (Col. 4 lines 54-67). Like the offset screw taught by the Small et al. patent, however, the device disclosed in the Asher, et al. patent cannot overcome the difficulties associated with angular malalignment.

The present invention preserves the advantages of using a pedicle screw and longitudinal member while providing new features and advantages not found in the previous instrumentation. Specifically, the invention overcomes both translational malalignment and angular malalignment by providing a modular pedicle screw system having a fastener adapted to receive a plurality of headpieces with various angular and lateral length configurations which are, in turn, connected to a longitudinal member or members to obtain rigid stabilization of the spine.

In one embodiment of the present invention, a pedicle screw is provided having an exterior thread along its lower portion and an upper portion adapted to securely receive a headpiece. The headpiece includes a neck which has a bore that defines an inner wall having a mating surface which is adapted to slidingly engage or mate with a mating surface defined by the outer wall of the upper portion of the headpiece. The headpiece further includes a projection which terminates in a connector which is adapted to engage a longitudinal member such as a plate or rod.

In another embodiment of the present invention, the mating surface of the outer wall of the upper portion of the pedicle screw is frusto-conical in shape with a fluted outer or "male" surface; the headpiece including a corresponding inner or "female" portion. Mating the two components in this manner provides greater lateral and rotational stability as well as greater ease of insertion.

In yet another embodiment of the present invention, an angulated headpiece is provided. The headpiece of this embodiment includes a neck portion having a projection which extends out of the neck at various angulations and terminates with a connector for engaging a longitudinal member. Angulating the projection allows for the correction of angular malalignment of the fastener.

In still another embodiment of the present invention, a translational headpiece is provided. The headpiece of this embodiment includes lateral sections which extend outwardly from the neck and may range in length from about 5 millimeters to 3 centimeters. The lateral sections also terminate in a connector for engaging a longitudinal member and allow for the correction of translational malalignment of the fastener.

In a further embodiment of the present invention, an angulated translational headpiece is provided. The headpiece of this embodiment includes angulated lateral sections which extend outwardly from the headpiece's neck at various angles and lengths. These sections also terminate in a connector for engaging a longitudinal member and allow for the correction of both angular and translational malalignment of the fastener.

Not only does the present invention provide the clinician with the flexibility and adaptability needed to overcome difficult malalignment problems, it offers the patient a number of safety benefits as well. First, the present invention reduces the stress placed on both the placed fastener and patient's bone by reducing the need to manipulate malaligned fasteners. Second, the integrity of the patient's bone is maintained by eliminating the number of placements into the bone since malaligned fasteners do not need to be removed and/or relocated when the present invention is used by the clinician.

Accordingly, an object of the present invention is to provide a modular pedicle screw system and method of implanting the same which is more flexible, safer and easier to implant than present spinal instrumentation and methods.

Another object of the present invention is to provide a modular pedicle screw system and method which can overcome translational malalignment of the fastener.

A further object of the present invention is to provide a modular pedicle screw system and method which can overcome angular malalignment of the fastener.

Yet another object of the present invention is to provide a modular pedicle screw system and method which can overcome both translational and angular malalignment of the fastener.

Still another object of the present invention is to provide a modular pedicle screw system and method which uses a fastener and headpiece having corresponding mating shapes in order to improve both the lateral and rotational stability between the two pieces.

Yet a further object of the present invention is to provide a modular pedicle screw system and method that can be used in combination with existing pedicle screw system employing either longitudinal plates or rods .

These and other features, objects and advantages of the present invention will become apparent from the following description and drawings wherein like reference numerals represent like elements in the several views, and in which:

FIG. 1 is a perspective view of an embodiment using an angulated headpiece having a connector adapted to engage a plate-like longitudinal member and with portions removed to reveal aspects of the invention;

FIG. 2 is a perspective view of the embodiment shown in FIG. 1 which is designed to work with a rod-like longitudinal member and has portions removed to reveal aspects of the invention;

FIG. 3 is a perspective view of an embodiment having a translational headpiece consisting of a plurality of lateral length sections;

FIG. 4 is a perspective view of an embodiment having an angulated translational headpiece consisting of a plurality of angular and lateral length configurations;

FIG. 5 is a posterior view of a spinal section in which an apparatus constructed in accordance with the present invention has been implanted;

FIG. 6 is an enlarged view, partly in section taken generally along the line 6--6 of FIG. 5, illustrating an embodiment threaded into a spinal element and overcoming angular malalignment of the fastener;

FIG. 7 is an enlarged view, partly in section taken generally along the line 7--7 of FIG. 5, illustrating an embodiment threaded into a spinal element and overcoming lateral malalignment of the fastener; and

FIG. 8 is an enlarged view, partly in section taken generally along the line 8--8 of FIG. 5, illustrating an embodiment threaded into a spinal element and overcoming both angular and lateral malalignment of the fastener.

In accordance with a preferred embodiment as shown in FIG. 1, the modular pedicle screw system includes a pedicle screw or fastener 10; a headpiece 30 which is adapted to engage fastener 10; and, a locking element 50 which mechanically affixes fastener 10 to headpiece 30.

Fastener 10 also includes a threaded lower portion 12 for engaging a spinal element and an upper portion 14 which defines an outer wall having a mating surface that is adapted to allow for the sliding engagement of fastener 10 to headpiece 30. One suitable configuration for upper portion 14 that provides rotational and lateral stability is a frusto-conical shape 15 having fluted edges 16 on its outer surface as shown in FIGS. 1 and 2. Upper portion 14 may also be configured as a cylinder, square, hex, or in other shapes persons of ordinary skill would recognize.

Fastener 10 further includes a bore 18 which allows fastener 10 to be cannulated during insertion. The diameter of bore 18 is increased near upper portion 14 to form an internal threaded portion 20 for receiving locking element 50. Bore 18 of fastener 14 may also terminate in a hexed-shaped recess 23 which provides a surface that can engage a suitable insertion tool (not shown). A shoulder 24 may also be provided which acts as a stop that limits the depth fastener 10 can be inserted.

As shown in FIGS. 1 and 2, headpiece 30 includes a neck portion 32 having cylindrical aperture 34 with engagement surface 38. Engagement surface 38 is dimensioned so as to allow head 52 of locking element 50 to form a secure fit with upper surface 38 when inserted.

Neck 32 further includes a bore or recess 40 which defines an inner wall having a mating surface that is adapted to slidingly engage or mate with upper portion 14 of fastener 10. In the embodiment using a frusto-conical shape, the inner wall would have a similarly configured mating surface 42 and would be configured to mate with the frusto-conical shape of outer wall 15. The inner wall may also have other configurations designed to mate with the other shapes described above. By mating upper portion 14 of fastener 10 with recess 40 of headpiece 30 both lateral and rotational stability between the two pieces may be achieved.

There are a number of variations of headpiece 30 which are built upon the basic structure of headpiece 30 as described above. One is an angulated headpiece 60 (also shown as headpiece 30 of FIGS. 1 and 2), which includes a projection 62 that extends out of neck 32 as shown in FIG. 1. Projection 62 terminates in a connector which can be either a threaded shank 64 (as shown in FIG. 1) or a rod connector 66 having an engagement aperture 65, a threaded bore 68 and a locking screw 69 (as shown in FIG. 2), which are, in turn, adapted to secure a longitudinal member such as plate of a design similar to that disclosed in the Small, et al. patent or a rod 65 of a design similar to that disclosed in the Asher, et al. patent, respectively.

Other headpiece variations, which will be described below, may also terminate in these connectors, but persons of ordinary skill in the art would also recognize that the headpieces may be adapted to work with other types of longitudinal support members as well. Since one of the purposes of the invention is to overcome angular malalignment, projection 62 of angulated headpiece 60 is angled with respect to centerline 63 of headpiece 60 as shown in FIG. 1. The degrees of angulation of projection 62 should vary but should include angulations in the range of about 5.degree. to 45.degree..

FIG. 3 illustrates translational headpiece 70. Translational headpiece 70 includes lateral section 72 which extends outwardly from centerline 77 of headpiece 70 and terminates with a connector 73 as described above. To provide the greatest degree of flexibility to a clinician, the length (L,L',L") of section 72 should vary and include typical lengths ranging from about 5 millimeters to 3 centimeters.

As shown in FIG. 4, another headpiece embodiment is an angulated translational headpiece 80, which is a combination of the previously described headpiece embodiments. Headpiece 80 includes lateral section 82 which extends outwardly from centerline 81 of headpiece 80. In addition, section 82 is also angulated with respect to a horizontal axis shown as line 89. The degrees of angulation (.theta.) should vary and may include angulations in the range of about 2.degree. to 45.degree.. The lengths (L,L',L") of section 82 should also vary as described above.

By providing a modular pedicle screw system with a fastener that accommodates a plurality of at least three different headpieces with various lateral section lengths, various angulated projections and a combination of the two provides the clinician with a system which can overcome many of the malalignment problems encountered during the implantation of spinal instruments as shown in FIG. 5.

In situations involving primarily angular malalignment as shown in FIGS. 5 and 6, fastener 10 will first be secured to a spinal element 90 by a suitable insertion tool which may be a hex-shaped wrench or a wrench having a bore designed to mate with upper portion 14. The clinician will then select one of the angulated headpieces 60 which best compensates for the angular malalignment, if any, and returns the threaded shank or rod connector to a position where it is once again is in longitudinal alignment with the longitudinal member 91 and other fasteners. The headpiece and fastener will then be secured together by set screw 50, and ultimately, a longitudinal member will be securingly retained by a connector as previously described.

In situations involving primarily translational malalignment, as shown in FIGS. 5 and 7, fastener 10 will first be secured to a spinal element 90. The clinician will then select one of the translational headpieces 70 which best compensates for the malalignment and returns connector 73 to a position of proper alignment for securing a longitudinal member 91. Moreover, since the headpiece and fastener are slidingly engaged, the device can be implanted in areas in which there is very little clearance in which to manipulate the headpiece. For example, in instances in which a translational headpiece is used, a fastener can first be inserted and then the headpiece is slid onto the upper portion of the fastener before the two piece are affixed together. By eliminating the need to thread the two pieces together, this arrangement can be used in situations where potential interference from spinal retractors, other elements of the spine, or the other screws which have already been implanted may be encountered. Thus, the clinician can use the present invention in areas which may not be accessible to other offset bolts currently available.

In situations presenting both translational and angular malalignment, as shown in FIGS. 5 and 8, an angulated translational headpiece 80 is used. Again, fastener 10 is first inserted into spinal element 90. The clinician then selects the appropriate headpiece with the correct lateral length and angular deflection which will compensate for the malalignment as shown in FIGS. 5 and 8.

While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those of ordinary skill in the art that changes and other modifications can be made without departing from the invention in its broader aspects. Accordingly, persons of ordinary skill in the art would further recognize that the modular pedicle screw system of this invention may also be useful and could be adapted for use with other applications employing different longitudinal members other than rods or plates.