1. Field of the Invention
This application relates to a method and an interbody disc with compliant natural and/or artificial filler material for restoring spinal motion between vertebral bodies between which a natural spinal disc has been removed in whole or in part.
2. Description of Related Art
In the field of spinal surgery, many treatment options exist to treat spinal pain, nerve impingement and spinal instability where a natural disc has failed in whole or in part. One such treatment is the removal of a damaged disc and its replacement with an intervertebral spacer which promotes fusion of bone between the separated vertebral bodies. This type of procedure when successfully completed, will result in a large bone mass between the vertebral bodies which will stabilize the column to a fixed position. See
Ball and socket type disc arthroplasty devices have been tried for over 30 years. See U.S. Pat. Nos. 5,676,701 and 6,113,637. Their design rational is to allow motion in the hopes of reducing higher loads to adjacent structures. These have shown some success but also failures. A ball and socket type device requires no energy to rotate. Thus, the work absorbed by the device during rotation is zero. The rotation centers may be favorable at one specific instantaneous center of rotation present in a natural healthy disc, but is never correct nor favorable for all movements. This forces abnormal loads on adjacent structures. Materials needed for a stable ball and socket device are often very stiff or incompressible, thus any axial loads and especially shock loads through the device are almost completely transferred to the adjacent structures. A patient expecting a favorable outcome with a ball and socket lumbar disc arthroplasty device may find unfavorable results if repeated axial loads I shocks (along the spine axis) are a common occurrence.
U.S. Pat. No. 4,309,777 to Patil, discloses an artificial disc with internal springs intended to flex. The device relies solely on the internal springs to provide the mechanical flexing motion. U.S. Pat. No. 5,320,644 to Baumgartner, discloses a different type of a mechanical flexing device. This device uses overlapping parallel slits forming leaf springs, which may contact in abrupt load paths, yielding impact stress. U.S. Pat. Nos. 6,296,664, 6,315,797 and 6,656,224 to Middleton, attempt to solve the disadvantage of abrupt load paths with a device containing a pattern of slits to allow for a more continuous load path. Middleton's device further includes a large internal cavity defined by the exterior wall. The internal cavity may be packed with bone to rigidly fuse adjacent vertebral bodies or capped with opposing plugs which limit the device's motion. Middleton's devices are intended to have a continuous load path with no abrupt load stops. These devices must be sufficiently stiff to support the anatomical average and extreme loads, thus too stiff to provide soft fusion as defined hereinafter.
U.S. Pat. No. 6,736,850, to Davis, discloses a pseudoarthrosis device containing small (0.25 to 2 mm inner diameter), flexible, permeable material tubes as to allow fibrous ingrowth. This device is very soft and may collapse under normal loads and may likely not form bone within the small inner diameters.
See published application nos. US20060217809A1; US20060200243A1; US20060200242A1; US20060200241A1; US20060200240A1; and US20060200239A1. It is the apparent attempt of the intervertebral prosthetic discs disclosed in these latter publications, to restore full intervertebral motion. However, these devices, as a result of their design, may be soft and very flexible resulting in artificial discs capable of absorbing little energy when subjected to shock loads. Computer simulations and mechanical validations of discs obviously patterned after some of these designs showed that it takes minimal loads (e.g., less than about 5 lbs for the cervical and less than 20 lbs for the lumbar) to compress the devices. While the weight required to be supported by an individual's spinal column will, to a great extent, depend on the individual's size, the weight to be supported in the cervical, thoracic and lumbar regions, will range from about 5 to 30 lbs, 30 to 60 lbs and 60 to 150 lbs or more in the cervical, thoracic and lumbar regions, respectively. Computer simulations also demonstrated that the use of a spiral slot or slit extending from the outer to the inner wall and encircling the disc two or more times as is illustrated in some of the publications is probably the reason for this lack of stiffness. A device which is too soft, will fully collapse when the patient is vertical, allowing for no additional movement to absorb impact energy. These types of soft spring devices, believed to have a stiffness of about 2.0 newtons(N)/mm, for use in the cervical region, and about 22.0N/mm in the lumbar region. Some of the patents/publications do show a vertical hole in the device, but apparently it came about for manufacturing purposes not for functionality. These patents do not describe or imply an intended fusion.
Several of the above references disclose the use of mechanical springs or bellows as the means to separate adjacent vertebrae while providing movement therebetween during flexure and extension. Such spring arrangements, beside their other problems, such as fracture at attachment points to end plates, provide little shock and energy absorption capability because they either fully compress at normal loads, or fracture at high loads.
There is a need for an intervertebral disc replacement or spacer for simulating the motion and energy shock absorption characteristics of a natural disc. To this end our novel intervertebral disc and method relies on a combination of mechanical flexure elements and bone and/or soft tissue infiltration within the disc to accommodate such motion and compliant filler materials such as a mixture of natural bone and/or artificial material for infiltration within the disc to accommodate such motion and energy absorption.
Overview
A desirable condition, which we term soft fusion, can be created between a patient's adjacent vertebral bodies in which the natural disc has failed in whole or in part by a) removing the failed disc or failed portion thereof; b) installing an artificial intervertebral disc between the two vertebral bodies; c) the disc providing one or more selected continuous or discontinuous channels of limited size for bone to form and fuse into one or more continuous or discontinuous struts between the vertebral bodies; d) the device being stiff enough to support the bodies in their natural spaced relationship while allowing limited motion and flexible enough to transfer sufficient energy to the bone struts to create one or more conditions of nonunion joints or pseudoarthrosis resulting in living nonrigid bone growth; and e) the disc being further arranged to limit its movement to an amount which is sustainable by the disc without resulting in fatigue failure during an anticipated lifetime.
The cortical/cancellous bone of a vertebrae, particularly in the lumbar region, is very stiff. For example, a vertebral body 30 mm in diameter with cortical bone around the outer 5 mm and cancellous bone (softer bone) on the inner area, which is 25 mm in height, will have an axial stiffness of approximately 235,000 N/mm or 235 KN/mm. The stiffness (axial) of a disc enabling soft fusion in accordance with the present invention should be between about 50 to 4000 N/mm, preferably within the range of about 200 to 1500 N/mm and most preferably between about 400-800N/mm. The size of the bone accommodating channel(s) should occupy about 10-35% (or less) and preferably about 12% to 25% of the total area of the disc facing the vertebral body to be supported.
A condition of soft fusion is illustrated in
The bone strut 6, extending between the vertebral bodies, has formed regions of pseudoarthosis or nonunion locations 6a. The nonrigid bone struts along with the mechanical properties of the artificial disc accommodate additional energy absorption with increased movement per given load simulating, to a significant extent, the performance of a natural disc.
Preferably in addition to the inclusion of an open continuous or discontinuous core(s) 17 to accommodate the bone strut(s) the spacer will include generally horizontally oriented tissue accommodating channels (“tissue channels”) 22, 24 to promote vascularization and fibrous tissue ingrowth.
The added advantage of tissue channels in conjunction with the bone strut forming channel(s) is that upon each loading and unloading cycle of the spine, nutrients and cellular waste will be pumped through tissue channels forming fibrous tissue within the tissue channels (vascularization). The nutrients and cellular waste are also pumped in and out to the bone strut(s). The disc may be “tuned” to match the deflection per load ratios to that of a natural healthy disc. The additional benefit of the soft tissue vascular areas (or bone void areas) is that soft tissue provides little initial resistance to compression but provides increasing resistance to an increasing compressive load. The natural disc is also softer at lower compressions than at higher compressions (axial or bending). A soft fusion device, infiltrated with adequate soft tissue in the tissue channels or voids will produce a device which is nearly as soft as the implanted device or natural disc, when subjected to light loads, and then become stiffer with increased compression or bending, just as a natural disc will. This unique ability of a soft fusion device with applied vascular cellular inputs promotes a device which will closely mimic a natural healthy disc embracing the ability for the soft tissue to heal due to vasculization or to fuse upon a lack of device motion due to non use or device collapse or flexural element(s) failure.
A soft fusion device may take on many different forms and structures which will be as individualized as the anatomical location, desired outputs, and designer preferences but encompass the spirit of the invention. Obviously, the device must have a stiffness less than that of bone, but sufficient to maintain the supported vertebrae in a desired spaced relationship when the spine is subjected to light loads and flexible enough to transfer sufficient energy to bone strut(s) to create nonunion joints 6a when the spine is subjected to additional loading. Bone growth between the vertebral bodies outside of the selected bone accommodating channels is to be inhibited by limiting the available void volume, orientating the voids in a direction generally tangent to load paths, adding cellular inputs to specific void areas, filling the voids with a fluid or softer material and/or other means. The bone channels shown in this application are generally vertical and generally continuous. This is not a requirement for a soft fusion device. The device may have multiple channels in varying directions which do not need to be continuous. A discontinuous bone channel or an interrupted channel may extend ⅓ the total device height from one vertebral body and ⅓ the total distance from the opposing vertebral body and the device may be interrupted within the middle ⅓ of the device, for example. A channel extending at an angle from the endplate, at 60 degrees from vertical for example, may be useful to in allowing for more axial compression than a vertical channel. All these variations are allowable and in the spirit of a soft fusion device.
It may be possible for an artificial spinal prosthesis or disc to accomplish the same degree of limited motion, load dampening, and energy absorption of a soft fusion device but without the living bone struts (and preferably soft tissues layers) created by soft fusion, it will not have the unique ability to adapt to the patient's loading conditions, repair itself when broken, and have the unique ability to fully support the vertebral column in the unlikely event that the underlying interbody disc fails.
It is to be noted that the creation of a soft fusion state after the installation of a soft fusion hybrid device, in accordance with this invention, is dependent upon a patient's level of activity. For example, if a patient is sedimentary, i.e., moves very little, the bone formed with the channel(s) will become dense and rigid limiting the motion and energy absorption while protecting the spinal column stability. If the patient is more active, i.e., subjecting the struts and the device to additional loads, e.g., walking, lifting, etc., the bone core(s) will be less solid, i.e., fractured, not fully formed and/or infiltrated by soft tissue, allowing for more motion and energy absorption. This type of soft fusion/hybrid device will be able to change throughout the life of the patient, just as the body is able to remodel for given inputs. If the mechanical dampening and flexible members of a soft fusion device fatigue, crack and fail, the device will slightly collapse. The collapse will limit the motion and eliminate the dampening action of the device thus transferring the energy to the supporting bone strut(s), promoting additional bone fusion and support.
Mathematical Rational
The theory behind soft fusion may be best understood by analyzing only the differences between a soft and rigid fusion rather than attempting to analyze actually true loads, deflections, and energy absorption capabilities. This is done by starting with basic equations.
Axial Deflection (δ) in the cephalic/caudal direction is equal to,
where P is the applied force, L is the length of the strut (disc height), A is the cross-sectional area and E is the modulus of elasticity.
Bending curvature
either in flexion/extension or lateral bending is equal to:
where M is the applied bending moment and I is the moment of inertia.
Soft fusion works by displacing under applied forces more than possible with a ridged fusion. Strain energy (U) is defined as the energy uptake or energy absorbed by the deformation of the material by the applied load or:
U=∫
0
x1
P*dx (Eqn. 3)
where P in an applied force and the integral of x from 0 to x1 is the deformation. Deformation noted in equations 1 and 2 may be inserted into equation 3 to determine the actually strain energy.
Many assumptions must be made to analyze the forces and deflections through the vertebral column and associated structures in order to accurately determine strain energy or energy absorption. However, the validity of soft fusion may be proven by simply comparing the variables unique between soft and rigid fusions. For the abovementioned device these are 1) the cross-sectional area of the bone strut verse the cross-sectional area of a rigid fusion 2) the presence or absence of the device in conjunction with the bone strut and for sake of comparison to arthroplasty ball and socket devices, 3) the modulus of elasticity.
To first look at the axial energy absorbed with the first set of variables we only need to define the cross-sectional area of a soft fusion as approximately 0.785 cm̂2 and the cross-sectional area of a rigid fusion as 15.4 cm̂2. These are typical cross-sectional areas seen within the lumbar region. By then setting the strain energy of a soft fusion to US and that of a rigid fusion to UR the relation between the two becomes:
With equal assumptions to both soft and rigid fusions and with all variables except the cross-sectional areas equal, equation 4 becomes:
In other words, a fully formed soft fusion bone channel will absorb 19.6 times more axial energy than a rigid fusion based solely on the area of available bone. The soft fusion device will reduce this number to some degree, depending on the stiffness of the actual device. Such constricted bone growth should not fully form in active patients or become fractured with high patient generated forces. When this occurs the presence of nonunions and fibrous tissue within the defined strut location(s) will only aid the soft fusion energy absorption capabilities by softening the hybrid bone, tissue, and implanted device creating a condition of a controlled pseudoarthrosis.
By neglecting the minimal effects of the Soft Fusion device and only comparing the bone strut to a cobalt chromium ball and socket device we see that the strain energy relationship in axial compression is approximately equal to:
As seen in equation 6, a cobalt chromium articulating device is extremely poor at absorbing axial impacts.
Similar bending calculations are currently omitted because of their redundancies to this application but would show similar results.
Suitable Intervertebral Disc Structure for Enabling Soft Fusion
A preferred intervertebral motion restoring disc for supporting adjacent vertebral bodies in their natural spaced relationship after a natural disc has been partially or wholly removed in accordance with the present invention has upper and lower surfaces for engaging the faces of the vertebral bodies to be supported and a support structure between the surfaces having a stiffness within the range previously discussed. The disc defines one or more generally vertically oriented continuous or discontinuous bone growth channels of limited cross-sectional area enabling bone struts to form therein extending at least partially and preferably completely between the bodies. The disc (with its stiffness characteristics) and the resulting bone strut or struts are arranged so that predetermined axial and/or bending loads thereon, e.g., normal loads, loads associated with standing or walking, will not fully compress the disc allowing a narrowing of the distance between the supported bodies during normal motion and create one or more pseudoarthrosis or fibrous nonunion locations along the length of the strut(s) to provide soft fusion thereby limiting a complete rigid strut formation. The disc further fully compresses at predetermined excessive forces in order to protect the flexural members of the device from overloading and failure. The unique combination of one or more pseudoarthrosis bone struts and the mechanical disc supporting structure results in the condition of soft fusion as previously discussed. Such controlled and limited fusion, i.e., soft fusion, provides limited motion, both translational and rotational and energy and shock absorption characteristics surpassing that of a rigid fusion while preserving vertical column stability.
First, vertebral column stability is particularly important in that it prevents disc induced or allowed kyphosis and scoliotic curvatures as seen with ball and socket type devices. Some prior art articulating devices will often settle into a fully rotated position when the soft tissue is unable to stabilize the spinal column. A soft fusion disc provides a force towards the central position assisting to stabilize the spinal column. Second, disc stability is important in that the continuous or discontinuous bone channels will likely form some degree of bone with soft tissue infiltration. This will greatly aid in preventing device expulsion, a failure mode seem with other non-fusion devices.
One such intervertebral disc acceptable for providing soft fusion and particularly designed for anterior insertion in the lumbar/thoracic region includes a pair of end plates (or layers) with each end plate having an outer intervertebral engaging surface for buttressing against a respective vertebral body and an inner surface. A plurality of interleaved first and second axial dampening plates (or layers) are sandwiched and secured between the inner surfaces of the end plates.
Each of the individual dampening plates define a peripheral outer wall and an inner generally cylindrical open bone accommodating core aligned along a longitudinal axis which will be generally aligned with the patient's spinal column when installed. Every other pair of axial dampening plates may be bonded, e.g., welded, together adjacent the inner core (or machined) leaving a generally planar space therebetween extending outwardly from the bonded area beyond the outer walls. The remaining pairs of axial dampening plates may be bonded, e.g., by welding, together along their peripheral walls (or machined) leaving a generally planar space therebetween extending from the bonded area to the open core. This arrangement provides alternating spaces extending from the core outwardly and from the peripheral walls inwardly which allows the end plates and the vertebral bodies to which they are secured to have limited translational motion parallel to the longitudinal or spinal axis and limited pivotal motion about the axis while dampening both motions. The channels formed between the plates and particularly the channels extending inwardly from the peripheral wall will accommodate tissue infusion and function as tissue channels.
Preferably the dampening plates are provided with one or more flexion slots between the outer peripheral walls and the inner cores to provide increased flexing action. The periphery of plates preferably follow the contour of the disc which they are to replace, e.g., an outer, generally convex, peripheral wall merging with a generally concave inner wall. As an option, a rotational dampening subassembly, to provide limited rotational motion between the end plates, can be inserted into the sandwiched axial dampening plates. Such assembly comprises an inner generally circular planar torsional dampening spring member with a helical slot, mounted between upper and lower torsional plates so that one of the torsional plates can rotate through a limited angle relative to the other. Alternatively, the spacer may be formed with about a 1½ turn or helical slot extending from the exterior wall to the central core(s) eliminating the interleaved plate construction as will become apparent in reference to the appended drawings.
The plates may be made of a suitable biocompatible material such as a titanium, cobalt or stainless steel alloy and or super elastic metals, e.g., nitinol, which in the sandwiched assembly, has sufficient strength and flexibility (stiffness) to withstand the anticipated stresses while providing the desired motion requirements to allow nonrigid bone struts to form within the open core.
In one method of construction the assembly is built plate by plate (or layer by layer) with the individual plates joined by diffusion, laser or electron beam welding or perhaps with a mechanical interference fit only.
The assembly may be constructed in various configurations adapted to site specific in vivo locations such as anterior, anterior lateral, lateral, lateral posterior or posterior spinal interbodies, interspinous dampening spacers, interconnecting pedicle screw dampening members or other posterior element stabilization devices.
An intervertebral disc particularly designed for the cervical region of the spine is formed with upper and lower surfaces for engaging the respective vertebrae faces to be supported and a generally elliptical partially obstructed open core for accommodating the formation of one or more bone struts. The spacer includes generally planar semicircular soft tissue integration channels extending inwardly from a peripheral wall to a location short of the open core. The tissue channels are interleaved with planar channels extending outwardly from the core to a location short of the peripheral wall.
While providing various examples of intervertebral prosthetic discs and a method for accommodating the creation of soft fusion within the discs advances the state of this art, we now propose improvements to provide a superior prosthesis and method by filling the channels defined in the intervertebral prosthetic discs with a material which is less stiff than typical cortical bone including some cancellous bone used in the prior art devices. For example, a filler material having a flexural stiffness less than 10-12 GPa will improve the load compliance and flexibility of the intervertebral prosthesis.
Where a cortical/cancellous bone blend is to be used as the filler material, cancellous bone, which has a GPa of the order of 4 GPa, should comprise at least the predominate, if not, the sole constituent of the blend. This flexural stiffness is reported to be the average for cancellous bone. The use of such softer filler materials in the discs will allow for a more compliant and energy absorbing device even in the absence of a nonunion joint or pseudoarthosis. A softer filler material will in effect alleviate the need for a nonunion or pseudoarthosis by the formation of a more compliant yet stable fusion.
An interbody disc, in accordance with the present invention, has (a) upper and lower surfaces for engaging the faces of the adjacent vertebral bodies between which a failed natural disc has been partially or wholly removed, (b) an exterior wall and one or more generally vertically oriented continuous or discontinuous channels (c) a sufficient stiffness to support the separated vertebrae in substantially their naturally spaced relationship while allowing limited motion and flexibility when subjected to a predetermined load to alter the distance between the vertebrae and thereby transfer load and energy to the any material filling the channels or voids and (d) a bio-compatible filler material disposed within the channels, the filler material being compliant and softer than cortical bone, e.g., having a flexural stiffness of less than about 10-12 GPa. The filler material combined with the device characteristics—will dampen the loads and energy transfer prior to the device contacting on the internal stops which will then in turn prevent fatigue failure.
Where human bone is selected as the filler material, cancellous bone is the first choice. As a second choice cancellous and cortical bone can be blended with cancellous bone being the predominate portion of the blend such as a ratio of cancellous to cortical bone within the range of about 80% to 20% and preferably about 60%+ to 40%.
Other naturally harvested materials (either from the patient, a donor or an animal) suitable for use in the blend can include any substances softer than bone, such as portions of the removed disc. Morselized bone or bone weakened by gamma sterilization is more compliant then cortical bone and may also be useful as a filler material.
Bone graft substitutes, such as demineralized bone matrix (DBM), calcium sulfate dehydrate (CSD) ceramic-based bone graft extenders, are believed to be satisfactory filler materials. These will have a very low flexural modulus to allow device bending characteristics but will resist compressive forces when contained in a generally vertical channel. Recombinant Human Bone Morphogenetic Protein (rhBMP-2) liquid, Epidermal Growth Factor (EGF) liquid, Platelet Derived Growth Factor (PDGF), Fibroblast Growth Factors (FGFs), Parathyroid Hormone Related Peptide (PTHrp), Insulin-like Growth Factors (IGFs), and Transforming Growth Factor-Betas (i.e., TGF-B1), may also accompany suitable filler materials in order to induce a specific biological response such as bone or soft tissue activity. Another filler material candidate is polyetheretherketone (PEEK) with or without porosity. This synthetic material has mechanical properties very similar to those in cortical bone without porosity and very similar to cancellous bone when used with porosity. In addition, it is highly controllable and stable allowing the disc to be preassembled with the filler material at a factory site.
The method of the present invention entails a) providing a prosthetic disc as discussed above, b) filling the channels with the appropriate filler material, either at the surgical site or at a manufacturing site, c) removing the damaged or failed disc in whole or in part, and d) inserting the filled disc between the separated vertebral bodies.
The structure and function of an intervertebral disc for creating soft fusion and method for accomplishing this desired condition are explained in more detail in the accompanying description of the preferred embodiments taken in conjunction with the appended drawings wherein like components (or locations) are given the same reference numerals.
The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings.
a is an enlarged partial view of the disc of
a is a cross-sectional view of the disc of
Reference will now be made in detail to the preferred embodiments of the invention which set forth the best modes contemplated to carry out the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.
Referring now to
The fixation rings are stepped to provide additional purchase against the vertebral end plates and to fill the convex surface of the adjacent vertebral end plate. The core may be packed with bone to accelerate the formation of the strut or other material. The core 17 and the interior plates layers 14 may be, but preferably are not, shielded from surrounding tissues to prevent tissue integration or device particulate wear explosion. Dacron or polytetrafluroethylene are preferred material to provide device shielding if desired.
Referring now to
The intervertebral disc, with its interleaved plates, has motion yet sufficient stiffness or strength to support the vertebral bodies (7, 8) in their natural spaced relationship while allowing limited motion and dampening the load applied to the bodies. When the separated vertebrae are subjected to normal loads, such as would be experienced by a person standing or walking, the plate will not be fully compressed allowing a narrowing of the distance between the vertebral bodies causing the bone strut formed in the core 17 to fracture or form fibrous nonunion joints at one or more locations along its length. Greater loads, such as jogging or lifting heavy objects, will further aid this process of promoting nonunions.
The fatigue life of the device is preserved by the internal spaces 22 and 24 as shown in
The overall height h (
As a further example for a height h of 0.565″ the spaces 22 and 24 (
A slightly modified disc is shown in
The difference between the intervertebral disc of
As an example, for a support structure 13′ having a height of about 5 mm, the gaps 17′b between the extensions 18i and the adjacent plates may be about 0.015″ while the channels 22 and 24 may have a height of about 0.020″. This difference in the dimensions of the gaps versus the height of the channels allows the spacer to be completely compressed (i.e., along the longitudinal axis) without completely closing the channels 22 and 24 by providing stop means, i.e., contact locations along the central section 17a, to accommodate abrupt loads and to alleviate fatigue failure which may otherwise occur as a result of repetitive loads. This also prevents complete soft tissue compression within the voids 22 and 24 and allows for additional disc bending when fully compressed.
The area of the bone accommodating core 17 or cores should not exceed about 35% and preferably less than about 25%, (e.g., about 10-20%) of the total area of the disc facing the separated vertebral bodies, i.e., in a horizontal plane. The size of the disc and bone strut opening(s) therein will depend upon the size of the vertebral bodies to be supported. As an example, the total area of the openings should have a diameter, if circular, or equivalent dimensions if non-circular, within the ranges of 0.1 to 0.6, 0.1 to 0.7, and 0.2 to 0.7 inches in diameter for the cervical, thorax, and lumbar regions, respectively.
An alternative embodiment of an intervertebral or hybrid disc designed primarily for the cervical region, is illustrated in
The beam is held in place by downwardly extending legs 40i which are formed with or otherwise secured to the lower peripheral wall at 40j (
The bone diversion bar 40h creates channels 401 (
Another alternative hybrid intervertebral disc 42144 is illustrated in
The inner surface 42i (
The abutting surfaces 44d and 42l will only transmit axial compressive and bending loads. This connection will only allow distractional, rotational and translational loads to be carried by the inner spring (formed by the inner cylindrical section 42j), softening the device in those motions. Excessive translations will contact surfaces 44c and 42i and then load the outer spring (formed by the outer cylindrical section 42m). The structure forming the inner and—outer springs is discussed in conjunction with
The upper end 44e of the post 44a is arranged to abut the top 42f of the cavity to limit the compression and vertical articulation of the device.
a illustrates a slight variation of the disc of
Another embodiment of a hybrid intervertebral disc 46148 is illustrated in the side elevational and cross-sectional views of
The voids formed by the spinal slot and the space 49 between the outer and inner surfaces of the hub and the annular post, respectively, provide soft tissue ingrowth locations. The open core will allow bone and/or soft tissue ingrowth.
The ring is preferably free floating within the space created by the surfaces 52e and 52d and smaller in diameter than the distance between such surfaces to allow the inner member to provide a limited amount of articulation, i.e., compression before making contact with both surfaces to stop the articulation resulting from an excessive load. The helical slot and the area surrounding the ring 54 are adapted for soft tissue ingrowth while the open core is adapted to accommodate bone and/or soft tissue ingrowth.
An additional two part disc, suitable for creating soft fusion, is illustrated in
The upper component 62, shown in
The aligned openings 60i and 62e form bone accommodating channels to enable pseudoarthosis struts to form therein, which along with the mechanical characteristics of the disc, provide soft fusion as discussed.
The lower component 66, shown in
As the disc flexes the widened front and back sections adjacent wall 68b and 68c overlying the slots, transition from level to level (vertically) compressing the slots these wall areas tend to widen out. This action allows these wider areas to transition the load to the next bend or level without fatiguing the disc material. By the same token, the narrower mid-section 68a allows more bending, but still without causing fatigue failure. The collapse of the slots with or without soft tissue infused therein serves to limit the compression of the disc due to excessive loads inhibiting fatigue failure.
While providing various examples of intervertebral prosthetic discs and a method for accommodating the creation of soft fusion within the discs advances the state of this art, we now propose improvements to provide a superior prosthesis and method by filling the channels defined in the intervertebral prosthetic discs with a material which is less stiff than typical cortical bone including some cancellous bone used in the prior art devices. For example, a filler material having a flexural stiffness less than 10-12 GPa will improve the load compliance and flexibility of the intervertebral prosthesis.
Where a cortical/cancellous bone blend is to be used as the filler material, cancellous bone, which has a GPa of the order of 4 GPa, should comprise at least the predominate, if not, the sole constituent of the blend. This flexural stiffness is reported to be the average for cancellous bone. The use of such softer filler materials in the discs will allow for a more compliant and energy absorbing device even in the absence of a nonunion joint or pseudoarthosis. A softer filler material will in effect alleviate the need for a nonunion or pseudoarthosis by the formation of a more compliant yet stable fusion.
An interbody disc, in accordance with the present invention, has (a) upper and lower surfaces for engaging the faces of the adjacent vertebral bodies between which a failed natural disc has been partially or wholly removed, (b) an exterior wall and one or more generally vertically oriented continuous or discontinuous channels (c) a sufficient stiffness to support the separated vertebrae in substantially their naturally spaced relationship while allowing limited motion and flexibility when subjected to a predetermined load to alter the distance between the vertebrae and thereby transfer load and energy to the any material filling the channels or voids and (d) a bio-compatible filler material disposed within the channels, the filler material being compliant and softer than cortical bone, e.g., having a flexural stiffness of less than about 10-12 GPa. The filler material combined with the device characteristics—will dampen the loads and energy transfer prior to the device contacting on the internal stops which will then in turn prevent fatigue failure.
Where human bone is selected as the filler material, cancellous bone is the first choice. As a second choice cancellous and cortical bone can be blended with cancellous bone being the predominate portion of the blend such as a ratio of cancellous to cortical bone within the range of about 80% to 20% and preferably about 60%+ to 40%.
Other naturally harvested materials (either from the patient, a donor or an animal) suitable for use in the blend can include any substances softer than bone, such as portions of the removed disc. Morselized bone or bone weakened by gamma sterilization is more compliant then cortical bone and may also be useful as a filler material.
Bone graft substitutes, such as demineralized bone matrix (DBM), calcium sulfate dehydrate (CSD) ceramic-based bone graft extenders, are believed to be satisfactory filler materials. These will have a very low flexural modulus to allow device bending characteristics but will resist compressive forces when contained in a generally vertical channel. Recombinant Human Bone Morphogenetic Protein (rhBMP-2) liquid, Epidermal Growth Factor (EGF) liquid, Platelet Derived Growth Factor (PDGF), Fibroblast Growth Factors (FGFs), Parathyroid Hormone Related Peptide (PTHrp), Insulin-like Growth Factors (IGFs), and Transforming Growth Factor-Betas (i.e., TGF-B1), may also accompany suitable filler materials in order to induce a specific biological response such as bone or soft tissue activity. Another filler material candidate is polyetheretherketone (PEEK) with or without porosity. This synthetic material has mechanical properties very similar to those in cortical bone without porosity and very similar to cancellous bone when used with porosity. In addition, it is highly controllable and stable allowing the disc to be preassembled with the filler material at a factory site.
Between the top plate 80e and the bottom plate 80f are a pair of cantilevered spring plates 80h and 80i which are created by the respective overlapping slots 80i, to provide a controlled spring action.
A transverse rectangular opening 80k extends across the vertical opening 80b to enable bone growth not only from the top and bottom into the disc body through the vertical openings 80a, 80b and 80c, but also from either side through the transverse opening 80k thereby providing further securement of the disc 80.
Finally, a pair of keels 80l are respectively positioned on both the top and bottom surface of the disc 80 to provide a frictional contact with the corresponding surfaces of vertebral endplates.
The filler material 86 was a mixture of cancellous and cortical bone and the bone growth 87 is shown with a discontinuity 88 indicative of movement of the spinal column by the sheep.
Referring now to
The filler material 83 may be inserted into the channel 17 at the surgical site by the surgeon or other attending personnel as shown in
The specific resulting structure of our filler material within an artificial intervertebral disc after a period of time (six months or more) implanted within a patient will vary depending on the activity of the patient. For example, an older and/or less active patient may have a relatively sold infusion of bone growth, particularly in a central core opening of the intervertebral disc. A younger and/or more active patient will experience more motion applied to the intervertebral disc with appropriate flexion that will create and/or maintain openings or discontinuities in the bone growth and/or soft tissue infusion.
The lower component 66, shown in
The prosthesis is completed by filling the channel 64j with an appropriate filler material 83 which is shown in
It is to be noted that the use of the term “adjacent” vertebral bodies includes the fifth lumbar vertebrae and the sacrum. It is also to be noted that the cross-sectional area of the channels to the total cross-sectional area of the disc may exceed the 35% preferred amount.
The method of the present invention entails the steps of a) providing a disc of the-type described herein b) filling the channel or channels with one of the filler materials described previously either at the surgical site or elsewhere, and c) inserting the completed disc between selected vertebral bodies.
There has been described a prosthetic intervertebral disc for restoring the motion between the supported vertebral bodies while enabling the formation of pseudo arthrosistic continuous or discontinuous bone struts having nonunion locations within the disc and between the supported bodies thereby providing a state of soft fusion and optionally accommodating the infusion of soft tissue within generally planar spaces within the disc. The disc may take many structural forms as is illustrated by the accompanying drawings. Variations and improvements to the soft fusion/hybrid disc of the present invention will undoubtedly occur to those skilled in the art without involving a departure from the invention as defined in the appended claims.
This application is a continuation-in-part application from PCT/US06/47902 filed on Dec. 14, 2006 and also claims priority from U.S. Provisional Application Ser. No. 61/072,987 filed on Apr. 4, 2008.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US09/39550 | 4/3/2009 | WO | 00 | 10/1/2010 |
Number | Date | Country | |
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61072987 | Apr 2008 | US |