Patent Application
20090276047
The present invention relates generally to an implantable prosthesis for repairing damaged intervertebral discs. More particularly, the present invention relates to a rail-based modular disc prosthesis of predetermined size and shape.
The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, including the two vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints. The disc consists of the end plates at the top and bottom of the vertebral bones, the soft inner core, called the nucleus and the annulus fibrosis running circumferentially around the nucleus. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. A normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosis. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus is not very effective in withstanding compression and shear forces.
As people age the intervertebral discs often degenerate. This degeneration of the intervertebral discs may lead to degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones.
As the annulus loses stability it tends to bow out and may develop a tear allowing nuclear material to extrude. Breakdown products of the disc and facet joint, including macroscopic chunks, microscopic particles, and noxious biochemical substances build up. These breakdown products stimulate sensitive nerve endings in and around the disc, producing low back pain and sometimes, sciatica. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted.
Degenerative disc disease is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain.
Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: heating the annular region to destroy nerve endings and strengthen the annulus; applying rigid or semi-rigid support members on the sides of the motion segment or within the disc space; removing and replacing the entire disc with a non-flexible, articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebra adjacent the affected disc.
Until recently, spinal fusion has generally been regarded as the most widely used treatment to alleviate back pain due to degenerative disc disease. While this treatment is effective at relieving back pain, all discal motion is lost in the fused spinal motion segment. The loss of motion in the affected spinal segment necessarily limits the overall spinal mobility of the patient. Ultimately, the spinal fusion places greater stress on the discs adjacent the fused segment as these segments attempt to compensate for lack of motion in the fused segment, often leading to early degeneration of these adjacent spinal segments.
Current developments are focusing on treatments that can preserve some or all of the motion of the affected spinal segment. One of these methods to stabilize the spinal motion segment without the disadvantages of spinal fusion is total disc replacement. Total disc replacement is a highly invasive and technically demanding procedure which includes removing the cartilaginous end plates between the vertebral bone and the disc, large portions of the outer annulus and the complete inner nucleus. If the entire disc is removed, typically an artificial prosthesis is placed in the disc space. Many of the artificial disc prosthesis currently available consist of a soft polymer to act as the nucleus. The soft polymer is interposed between two metal plates that are anchored or attached to the vertebral endplates. A summary of the history of early development and designs of artificial discs is set forth in Ray, “The Artificial Disc: Introduction, History and Socioeconomics,” Chpt. 21, Clinical Efficacy and Outcome in the Diagnosis of Low Back Pain, pgs. 205-225, Raven Press (1992). Examples of these layered total disc replacement devices are shown, for example, in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and 6,533,818.
These types of artificial total discs have several disadvantages. First, because the artificial disc prosthetics are relatively large, they require relatively large surgical exposures to accommodate their insertion. The larger the surgical exposure, the higher the chance of infection, hemorrhage or even morbidity. Also, in order to implant the prosthetic, a large portion of the annulus must be removed. Removing a large portion of the annulus reduces the stability of the motion segment, at least until healing occurs around the artificial disc. Further, because the devices are constructed from rigid materials, they can cause serious damage if they were to displace from the disc space and contact local nervous or vascular tissues. Another disadvantage is that rigid artificial disc replacements do not reproduce natural disc mechanics. Finally, relative movement between the hard surfaces of the metal plates of many artificial discs and the vertebral bone will tend to cause erosion of the vertebral endplates. Such endplate erosion can lead to instability, subsidence, and/or neurological or vascular damage.
An alternative to total disc replacement is nuclear replacement. Like the artificial disc prosthetics, these nuclear replacements are also inert, somewhat flexible, non-biological prosthetics. The procedure for implanting a nuclear replacement is less invasive than the procedure for a total disc replacement and generally includes the removal of only the nucleus and replacement of the nucleus with a prosthetic that may be malleable and provide cushioning that mimics a natural disc nucleus. Examples of the prosthetics used for nuclear replacement include: the Ray implant (U.S. Pat. Nos. 4,772,287 and 4,904,260), the Bao implant (U.S. Pat. No. 5,192,326), the Sulzer spiral implant (U.S. Pat. No. 5,919,235), and the Replication Medical implant (U.S. Pat. No. 6,726,721).
Nuclear replacements are intended to more closely mimic natural disc mechanics. To that end, some nuclear replacements utilize hydrogel because of its water imbibing properties. Hydrogel is also used because of its ability to expand in situ to permit a more complete filling of the excavated nuclear cavity. However, there is a trade-off in that the more expansion the hydrogel achieves, the less robust the end product will be. As a result, hydrogel nuclear disc replacements have generally adopted the use of some formed jacket to contain the hydrogel material. For example, the Ray implant as described in U.S. Pat. Nos. 4,772,287 and 4,904,260 consists of a block of hydrogel encased in a plastic fabric casing. The Bao implant as described in U.S. Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabric shell. While the use of a jacket can result in better structural integrity and less potential extravasation of hydrogel material outside of the nucleus, there is a tendency for jacket encased hydrogel materials to produce a lateral expansive force against the annulus. Another problem with this approach is that the hydrogel material can become too hard for the desired stress response curve of a replacement disc.
Another approach to nucleus replacement involves implantation of a balloon or other container into the nucleus that is filled filling it with a biocompatible material that hardens in situ. Examples of this in situ approach to nucleus replacement include U.S. Pat. Nos. 5,549,679 and 5,571,189. One of the problems with this approach is that the chemical hardening process is exothermic and can generate significant amounts of heat that may cause tissue damage. In addition, there is a possibility that the balloon may rupture during expansion, causing leakage of material out of the bone cavity, which may cause undesirable complications.
Another technique for nucleus replacement involves implanting a multiplicity of individual support members one at a time in the disc space until the cavity is full. Examples of this approach include U.S. Pat. Nos. 5,702,454 and 5,755,797. Because each of the individual components is relatively small, there is a possibility that one or more beads or support members will extrude out of the cavity.
From a mechanical perspective, this technique is limited in the ability to produce consistent and reproducible results because the location and interaction of the multiplicity of beads or support members is not controlled and the beads or support members can shift during and after implantation.
Accordingly, there is a need for a nuclear prosthesis that may be inserted using a minimally invasive procedure and that mimics the characteristics of a natural disc.
The present invention provides a method and apparatus for repairing a damaged intervertebral disc nucleus in a minimally invasive manner with a modular disc prosthesis. The modular disc prosthesis preferably comprises at least three modular segments and at least two rails that operably connect adjacent modular segments. In one embodiment, each modular segment includes a harder inner portion and a softer outer portion. Preferably, the rails operate to slidably connect and interlock adjacent modular segments. A stem portion of the rails that extends outside the periphery of the body of the prosthesis is removable after implantation such that the modular segments form an implanted unitary device that closely mimics the geometry of the disc nucleus cavity.
In one embodiment, a modular disc prosthesis that is adapted to be implanted in an evacuated disc nucleus space includes at least three modular segments each having a width. The first modular segment has a first rail extending at least partially along one side of the width and beyond a periphery of the first modular segment. The second modular segment is slidably connected to the first rail on one side of the width and has a second rail extending at least partially along another side of the width and beyond a periphery of the second modular segment. The third modular segment is slidably connected to the second rail on one side of the width. The prosthesis has an expanded position in which the modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation. The prosthesis also has an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a unitary body having a generally continuous periphery that generally corresponds to the evacuated nucleus disc space with at least a portion of the rails extending beyond the periphery of the body.
Preferably, each modular segment comprises an inner portion and an outer portion. The inner portion includes structure that mates with one of the rails. The outer portion substantially surrounding the inner portion except for the side having one of the rails and the side having structure that mates with one of the rails. In one embodiment, the inner portion of each modular segment and the outer portion of each modular segment are made of polymers of different durometers. Preferably, the inner portion of each modular segment has a compressive modulus from about 70-100 Mpa and the outer portion of each modular segment has a compressive modulus from about 6-20 Mpa. The use of a harder inner portion and softer outer portion as part of an integrated unitary implanted device permits the modular prosthesis of the present invention to more closely mimic the stress response of a biological disc nucleus while simultaneously permitting effective operation of the slidable relationship between adjacent ones of the modular segments.
In one embodiment, locking features are provided to ensure that the modular disc prosthesis is a unitary device both before and after insertion. To prevent the device from being separated prior to insertion, locking features may be provided on the rigid rails to prevent modular segments from being slid back off of the rails. This ensures that each modular segment is connected in its proper position and in the proper order. In addition, locking features may be provided on the modular segments to lock them together upon insertion. This prevents individual segments from dislocating from the assembled prosthetic and migrating within the annulus.
Another aspect of the present invention comprises a method for implanting a modular disc prosthesis. Because the modular disc prosthesis may be implanted one segment at a time, a hole made in the annulus for implantation of the prosthesis may be a fraction of the size of the device in its final assembled form. The modular disc prosthesis is introduced into the patient's intervertebral space through an access tube and the first modular segment is inserted into the disc nucleus space through the small hole in the annulus. The second modular segment is then slid up the first rigid rail using a pushing tool and into the disc nucleus space until the second modular segment interlocks with the first modular segment. The tail stem of the first rigid rail is then severed from the device. In one embodiment, the tail stem of the rigid rails may be removed by a cutting mechanism provided preferably as part of the distal end of the pushing tool. Subsequent modular segments are slid up the adjoining rigid rail with the pushing tool into the disc nucleus space and then interlocked with the previously inserted modular segment in a similar manner. Once all of the modular segments have been inserted and all of the tail stems severed, the modular disc prosthesis is fully inserted into the patient's disc nucleus space and assembled and the access tube and pushing tool may be withdrawn and the access hole is closed up.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
[
Referring to
In a preferred embodiment, hard inner portions 104b, 106b and 108b have an I-beam cross-sectional shape that optimizes flexibility and strength of the hard inner portions. Alternatively, hard inner portions 104b, 106b, 108b, can have a uniformly shaped cross-sectional area to reduce any differences in compressibility of the modular disc prosthesis 100 across the surface area in order to minimize the potential for stress risers to be created in the interface between the outer surface of the modular disc prosthesis 100 and the inner surfaces of the disc space cavity. It will be recognized that various cross-sectional shapes of hard inner portions 102b, 104b, 106b, 108b and 109b can be utilized in accordance with the present invention and that the cross-sectional shapes of the hard inner portions does not need to be symmetric.
Hard inner portion 102b of first modular segment 102 includes first segment interlocking portion 116. Hard inner portion 104b of second modular segment 104 includes second segment interlocking portion 118 and a first slot 128. Hard inner portion 106b of third modular segment 106 includes third segment interlocking portion 120 and a second slot 130. Hard inner portion 108b of fourth modular segment 108 includes fourth segment interlocking portion 121 and a third slot 132. Hard inner portion 109b of fifth modular segment 109 includes a fourth slot 133.
In the preferred embodiment, rails 110, 112, 114, 115 have a noncircular cross-sectional shape, although it will be understood that other cross-section shapes could be utilized and that there is no requirement that all of the rails have similar cross-sectional shapes. It has been found that the noncircular cross-sectional shape as shown (corresponding mating C and sideways T cross-sectional shapes) provides for better alignment of the modular segments and supports larger insertion forces along the axis of the rail.
It will be understood that in a preferred embodiment, the rails 110, 112, 114, 115 of the present invention have a non-uniform cross-sectional aspect ratio in terms of the height and width of the rail. Preferably, the rails have a relative rigidity along a longitudinal axis of the rail in a dimension of the height of the rail that is greater than a width of the rail, whereas in a dimension transverse to the width of the rail the relative rigidity of the rail permits a greater degree of flexibility such that succeeding modular segments can be moved laterally with respect to one another in the expanded position without deforming the rails. [Insert preferred ranges of dimensions of height and width of the rails]. This differential rigidity in the two dimensions transverse to the longitudinal axis of the rail is important in permitting effective and efficient sliding operation of the adjacent modular segments.
Referring to
In alternate embodiments, modular disc prosthesis may comprise greater or fewer numbers of modular segments and rails, so long as there are at least three modular segments and two rails. For example,
Prior to insertion, modular disc prosthesis 100 further includes first 110, second 112, third 114, and fourth 115 rails. First modular segment 102 is rigidly attached to first rail 115 at first segment interlocking portion 116. Second modular segment 104 is slidably attached to first rail 110 at first slot 128 and rigidly attached to second rail 112 at second segment interlocking portion 118. Third modular segment 106 is slidably attached to second rail 112 at second slot 130 and rigidly attached to third rail 114 at third segment interlocking portion 120. Fourth modular segment 108 is slidably attached to third rail 114 at third slot 132 and rigidly attached to fourth rail 115 at fourth segment interlocking portion 121. Fifth modular segment 109 is slidably attached to fourth rail 115 at fourth slot 133.
As shown in
As shown in the alternate embodiment of
The preferred embodiment is a unitary prosthesis that comes packaged, sterile, and ready for implantation at the surgical site. Since the device is fully preformed and delivered as a unitary implant, the device is under direct operator control until the prosthetic disc nucleus is completely formed. This unitary design limits the need for the surgeon to determine how the cavity should be filled and assures that the components order of insertion and connection cannot be mixed up. The ability to predetermine the size of modular disc prosthesis also allows for the nucleus cavity to be more completely filled and provides a greater degree of control over the uniformity of the stress response of the implant as compared to other kinds of minimally invasive implants. In this regard, it will be understood that the modular disc prosthesis 100 of the present invention may be provided in a variety of different final assembled volumes and shapes to correspond to different sizes and shapes of different evacuated disc cavities.
Modular disc prosthesis is introduced through an access tube that is inserted partially into the disc nucleus space. As shown in
Referring again to
Third modular segment 206 is then slid down the second rail 212 and into the disc nucleus space onto second segment interlocking portion 218 until it is flush with second modular segment 204. This configuration is shown in
In an alternate embodiment, a keystone approach can be used to insert modular disc prosthesis such that the last modular segment inserted into the disc nucleus space is not one of the outside segments. Instead, the outside segments can be the first two segments inserted. This creates a bilateral expansion force as the remaining segments are inserted between the two outside segments. This helps make a tighter fit within the disc nucleus space than does the asymmetric lateral force imparted when the segments are implanted sequentially.
The stem portions of rails 110, 112, 114, 115 that extend beyond the periphery of the body of the modular disc prosthesis 100 can be removed by many different techniques. As shown in
In another embodiment, the stem portions of the rails may be provided with a perforation at the junction with each modular segment such that they can be torn, broken, twisted, or more easily cut off. Cutting may also be accomplished with a wire loop provided to the part. Additionally, heat, laser, or any other local energy source can be used to accomplish the separation. One of skill in the art will recognize that numerous alternative means exist whereby stem rails can be severed from modular disc prosthesis.
Alternatively, modular disc prosthesis may be implanted using an anterior lateral approach. An anterior lateral approach allows for a larger insertion opening to be used while still being minimally invasive. [Insert any further description of this alternate anterior lateral surgical technique.]
During insertion, slots 128, 130, 132, 133 slide along the stem portions of rails 110, 112, 114, 115 and onto segment interlocking portions 116, 118, 120, 121. Slots 128, 130, 132, 133 and segment interlocking portions 116, 118, 120, 121 may be provided with locking features to prevent separation of modular segments 102, 104, 106, 108, 109. Locking features, such as a series of barbs or studs, may be provided such that once a slot is slid onto a segment interlocking portion, it cannot be slid back off of it. A ratchet and pawl may also be used to lock modular segments together. A ratchet release tool may also be provided in case separation of modular segments is desired once they are locked together.
One example of these locking features is depicted in
Alternatively, free movement of modular segments 102, 104, 106, 108, 109 along rails 110, 112, 114, 115 may be allowed until insertion in the body. It will be understood that, depending upon the material configuration of the modular prosthetic 100 and the interface fit, segment interlocking portions 116, 118, 120, 121 may swell due to hydration to lock in the final configuration. The feature may be used alone or in combination with a mechanical locking feature. Alternative methods of locking modular segments together will be appreciated by those skilled in the art.
In the preferred embodiment, modular disc prosthesis 100 is molded from elastomeric biomaterials, preferably polyurethane. Stem rails 110, 112, 114, 115 and hard inner portions 102b, 104b, 106b, 108b, 109b are made from a hard durometer polyurethane, such as a polyurethane with shore hardness 55D or above and compressive modulus of 70 to 100 MPa. Soft outer portions 102a, 104a, 106a, 108a, 109a are made from a soft durometer polyurethane, such as a polyurethane with a shore hardness ranging from 55D to 18A and a compression modulus between 6 and 20 MPa.
In the preferred embodiment, the two different durometer polyurethanes are co-polymerized to create a chemical bond between the two portions of each modular segment 102, 104, 106, 108, 109. In alternate embodiments other polymers such as PEEK, polyethylene, silicones, acrylates, nylon, polyacetyls, and other similar engineering polymers may be used for the hard inner portions or the soft outer portions. For a more detailed description of a preferred embodiment of the multi-durometer polymer compositions of the present invention, reference is made to the previously identified co-pending application entitled, “[fill in final title]”.
In an alternate embodiment, the stem of the tails may be molded from a harder durometer material than soft outer portion and hard inner portion of modular segments. Utilizing this approach allows the rails to be extruded, rather than molded as part of the modular segments. A bond joint can then be made with the hard inner portion external to the periphery of the modular segments to form the unitary design. Extruding the stem portions of the tails makes modular disc prosthesis easier and less expensive to manufacture than a completely molded product.
In the preferred embodiment, the soft outer portion of modular disc prosthesis is deformable in response to normal physiological forces of 30 to 300 pounds. Because of this increased deformability, the prosthesis produces little impingement on the end plates of the intervertebral disc. As a result, the end plates do not flatten out over time and conform to the contours of the implant as is the case with many metal/polymer disc replacement implants.
In an alternate embodiment, outer portion of modular disc prosthesis may be modified to provide for elution of medicants. Such medicants can include analgesics, antibiotics, or bioosteologics such as bone growth agents. The solid polymer outer portion of modular disc prosthesis provides for better and more controllable elution rates than hydrogel materials can. [Insert further discussion of elution, including whether there could be another polymer layer on the very outside that would serve as an “elution” layer”]
Various modifications to the disclosed apparatuses and methods may be apparent to one of skill in the art upon reading this disclosure. The above is not contemplated to limit the scope of the present invention, which is limited only by the claims below.
This application is a continuation of U.S. application Ser. No. 11/372,477, filed Mar. 9, 2006, which also claims the benefit of U.S. Provisional Patent Application No. 60/685,332, filed May 24, 2005, U.S. Provisional Patent Application No. 60/700,459, filed Jul. 19, 2005, and U.S. Provisional Patent Application No. 60/660,107, filed Mar. 29, 2005, the disclosures of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60685332 | May 2005 | US | |
| 60700459 | Jul 2005 | US | |
| 60660107 | Mar 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11372477 | Mar 2006 | US |
| Child | 12435087 | US |
