Shaped memory artificial disc and methods of engrafting the same

[0001] This application claims the benefit of U.S. Provisional Patent Application 60/404,481, filed Aug. 19, 2002, titled SHAPE MEMORY ARTIFICIAL DISC.

[0002] The present invention relates to artificial intervertebral discs and, more particularly, artificial intervertebral discs constructed from shaped memory alloys.

[0003] The vertebrae of the human spine are arranged in a column with one vertebra on top of the next. Between adjacent vertebrae exists an intervertebral disc that transmits force between adjacent vertebrae and provides a cushion between the adjacent vertebrae.

[0004] Degeneration or other deformities in the intervertebral disc (the “diseased disc”) can cause back pain. When a diseased disc repeatedly impinges upon neurological structures or is determined to be a pain generator, surgeons conventionally treat the diseased disc by surgically removing the diseased disc and inserting a bone graft or other device in the space vacated by the diseased disc. The adjacent vertebrae are then immobilized relative to one another with stabilization hardware. Eventually, the vertebrae grow into one solid piece of bone.

[0005] While fusing the vertebrae into one solid piece of bone is the conventional practice, fusing adjacent vertebrae into a single bone mass is a less than ideal solution. In particular, fusing two or more vertebrae into a single bone mass causes additional stress on the remaining vertebrae and discs accelerating any potential degeneration. Moreover, the fused bone mass may lead to decreased motion and flexibility in the spine. The decreased motion and/or flexibility is exacerbated when three or more vertebrae are fused.

[0006] In order to avoid fusing two or more vertebrae into a single bone mass, prosthetic devices have been developed that attempt to mimic the intervertebral disc, both size and function. The prosthetic device is implanted into the intervertebral space to replace the diseased disc. U.S. Pat. No. 5,458,642, titled SYNTHETIC INTERVERTEBRAL DISC, issued Oct. 17, 1995, to Beer et al. discloses one such prosthetic device. The Beer et al. device includes a plurality of coiled springs interspersed between two endplates. The springs of the Beer et al. device attempt to approximate the function of the replaced intervertebral disc. The Beer et al. device is less than satisfactory because the coiled springs can be damaged and lose their elasticity over time. Further, the coiled springs provide limited shock absorption requiring the use of a compressible pouch of biocompatible material to provide additional shock absorption. Moreover, adjacent vertebrae need significant separation to allow for insertion of the prosthetic device potentially causing trauma to the surrounding structures.

[0007] U.S. Pat. No. 5,676,702, titled ELASTIC DISCPROSTHESIS, issued Oct. 14, 1997, to Ratron, provides another device that attempts to mimic the replaced intervertebral disc. The Ratron device includes the same endplates separated by an elastic post and elastically deformable partitions. The Ratron device is relatively impractical, however, because the placement of the elastic post and the elastically deformable partitions is difficult and varies on a case-by-case basis. Thus, manufacturing the device prior to surgical implantation is difficult. Further, bone or other tissue growth into the intervertebral space can foul the device making it inoperable. Moreover, adjacent vertebrae need significant separation to allow for insertion of the prosthetic device potentially causing trauma to the surrounding structures. Finally, similar to springs, the elastic material may experience plastic deformation causing failure of the prosthesis. Additionally, the elastic material contained in the Ratron device may degrade over time.

[0008] While many artificial intervertebral discs exist, all of them use either coiled springs or plastics to approximate the function of the removed disc. As shown above, these artificial discs suffer many drawbacks. Thus, it would be desirous to develop an improved artificial intervertebral disc.

[0009] To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an artificial intervertebral disc is provided. The artificial intervertebral disc comprises an upper and a lower endplate. The upper and lower endplate attach to lower surface of the upper vertebral body and the upper surface of the lower vertebral body respectively. A core resides between the upper and lower endplate comprising a shaped memory alloy.

[0010] The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

[0019] Some embodiments of the present invention are described with reference to FIGS. 1 to 5. FIGS. 1–5 generally show the present invention on an eye level, off the shelf view with fixation spikes in phantom. One of skill in the art, on reading the below disclosure, will recognize that the exact configuration of the present invention will depend, in part, on the anatomy of the patient. In particular, FIG. 1 shows a cross section of an anterior view of a portion of a spinal column 100 with an artificial disc 102. FIG. 2 shows a lateral, elevation view of spinal column 100 with artificial disc 102. Disc 102 is implanted in an intervertebral space 104 situated between an upper vertebral body 106 and a lower vertebral body 108. Disc 102 includes an upper endplate 110, a lower endplate 112, and a core 114. Upper endplate 110 and lower endplate 112 should be formed of a biocompatible metal including shaped memory alloys, other metallic alloys, or plastic. Conventionally, endplates are made from titanium, but any biocompatible materials are satisfactory. If, for example, upper endplate 110 and lower endplate 112 are made from shaped memory alloys, the plates could be placed in a compact state (deformed, rolled, coiled, or the like) and activated once placed. Upper endplate 110 is coupled to upper vertebral body 106 using, for example, fixation spikes 116 (shown in phantom on FIGS. 1, 2, and 3). Fixation spikes could be replaced with screws or other conventional coupling devices. FIG. 4 shows an alternative attachment means where, for example, endplates 110 and 112 have fixation tabs 118 through which screws 120 are inserted to couple endplates 110 and 112 to upper vertebral body 106 and lower vertebral body 108.

[0020] Upper endplate 110 and lower endplate 112 are shown with a lattice and/or biochemical coating to enhance bone ingrowth and encourage longterm fixation of the plates to the vertebral bodies. Alternative to coating the plates with bone growth material 122, the plates could have other devices to encourage bone growth, such as, ridges, ribs, scars, striations, or the like. Further, a layer of adhesive or tape could be applied to assist in fixation of the plates.

[0021] Core 114 comprises a shaped memory alloy (SMA). SMAs are a group of materials that demonstrate an ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at a predefined temperature, and upon exposure to thermal manipulation, will return to the pre-deformation shape. Some SMA material is considered to be two-way shaped memory alloys because they will return to the deformed shape upon proper thermal activation. SMAs include Ag—Cd alloys, Cu—Al—Ni alloys, Cu—Sn alloys, Cu—Zn alloys, Cu—Zn—Si alloys, Cu—Zn—Sn alloys, Cu—Zn—Al alloys, In—Ti alloys, Ni—Al alloys, Ni—Ti alloys, Fe—Pt alloys, Mn—Cu alloys, Fe—Mn—Si alloys, and the like. Currently, Ni—Ti alloys (a.k.a. Nitinol) are considered a good SMA for medical applications. Making core 114 out of SMAs provides the ability to implant a compact artificial disc during the procedure requiring less distraction of upper vertebral body 106 and lower vertebral body 108. An activation signal would be provided to cause core 114 to expand to the surgically desired shape. The reduction in distraction reduces the surgical trauma associated with the implant. Many SMAs are thermally activated, however, activation signals may be any number of signals, such as, for example, thermal, electrical, magnetic, radiation, or the like.

[0022] While core 114 could be a solid piece of SMA material. It is believed core 114 would function better if core 114 comprises a series of SMA members 124 extending between upper endplate 110 and lower endplate 112. As shown, each SMA member 124 is a loop 602 (shown in FIG. 6) of SMA material to provide some vertical elastic deformation ability. As shown in FIG. 6, SMA members 124, however, could have a number of constructions, including, for example, a series of columns 604, an arc 606 or curved shape 608, a flanged surface 610 coils 612, a zigzag pattern 614, or the like. Further, each member 124 could have different or the same construction and made out of different or the same SMAs. The variation of the shapes and materials would provide surgical control of the forces associated with core 114. In particular, the SMAs could be chosen, shaped, and designed to specifically replicate and resist axial, rotational, sagital, and coronal forces using conventional design methods.

[0023] When core 114 is not a single solid piece of SMA material, a covering 126 should be deployed around core 114 to prevent tissue, scarring, or bone growth from interfering with disc 102. Covering 126 could be formed of a biocompatible metal, an alloy, or plastic. Conventionally, covering 126 would be a GORTEX® material, but any biocompatible material would function. Alternatively to covering 126, the annulus surrounding the disc could be used. In this case, the surgeon would preserve the majority of the annulus during the discectomy.

[0024] FIGS. 1 and 2 show disc 102 with a “balanced” core 114. Basically, balanced means that SMAs 124 are approximately identical. Referring specifically to FIG. 3, disc 302 is shown. Disc 302 has an “unbalanced” core 314 where the individual SMAs 324 contained in core 314 are designed with different sizes, radii, and elastic deformation coefficients. As implanted, unbalanced disc 302 applies different forces than a balanced disc would deploy. Further, because of the unique feature associated with SMA material, compact SMAs 324c could be implanted in an unexpanded state. Over time, to potentially aid in correcting spinal curvature, thermal activation of SMAs 324c could apply expansion force causing SMAs 324c to become less compact and move disc 302 from unbalanced core 314 to a more balanced core formation. The expansion of SMAs 324c would apply a force represented by Arrows A and would be designed to correct the spines curvature or the like. Alternatively to an unbalanced core, as shown in FIG. 3, multiple discs could be used aligned within the intervertebral space. Each of these multiple discs may be have a balanced core, but the core of the first disc may be different than the core of the second disc, etc., which would approximate the effect of an unbalanced core.

[0025] Conventionally, artificial discs are implanted using anterior surgical techniques. FIG. 5 shows that an artificial disc 600. Artificial disc 600 is essentially the same as disc 102 and disc 302. Disc 600 is divided into a first part 602 and a second part 604. Disc 600 has endplates, bone growth material, and a core similar to the above discs, and those pieces will not be re-explained here. An interlocking portion 606 has a first interlocking piece 608 attached to first part 602 and a second interlocking piece 610 attached to second part 604. While shown exploded for convenience, one of ordinary skill in the art would understand first interlocking piece 608 and second interlocking piece 610 would be intertwined to put disc 600 in a locked position 700. FIG. 7 shows disc 600 with first interlocking piece 608 and second interlocking piece 610 in an unlocked position 702 to allow relative movement between the pieces. While interlocking portion 606 is shown as a relatively simple device, one of ordinary skill in the art would recognize other more elaborate interlocking portions could be designed and used and the simple design shown is for convenience and ease of explanation. Interlocking pieces 608 and 610 can be attached to parts 602 and 604 by being a single integrated unit, screwed, glued, taped, adhered, or the like. Interlocking pieces 608 and 610 would be made of SMAs and are shown in the activated position. Activated, interlocking pieces 608 and 610 engage and hold first part 602 and second part 604 together. To install, however, interlocking pieces 608 and 610 would be in a non-activated position allowing first part 602 and second part 604 to move relative to each other. Using two halves, disc 600 can be installed from a posterior procedure. Implanting an artificial disc using a posterior procedure would be a vast improvement over current anterior implanting procedures because of the reduction in surgical trauma. Once installed, activation of interlocking portion 606 would cause the interlocking pieces 608 and 610 to engage.

[0026] While FIG. 5 shows installing two halves of an artificial disc and linking the two halves together with interlocking portion 606, it would be possible to implant several artificial disc modules in a side-by-side relation. The side-by-side modules could be linked (similar to FIG. 5) or function independently. Using several smaller modules to mimic the removed disc instead of one larger artificial disc would facilitate implantation of the artificial disc using minimally invasive techniques.

[0027] While the invention has been particularly shown and described with reference to an embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.