Patent Application
20140012382
Spinal disc herniation, a common ailment, and degenerative disc disease often induce pain, as well as neurologically and physiologically debilitating consequences, for which relief becomes paramount. If conservative treatments fail, the more drastic measures of discectomies and spinal fusion may be indicated. The latter treatment, while providing short term relief, limits spinal mobility and often leads to excessive forces on facet joints adjacent to the fusion and may create further problems over time. Drastic treatments are usually unable to restore normal disc function. The loss of disc function has led to a number of disc prostheses that attempt to provide natural motion.
The literature documents that the Instantaneous Axis of Rotation (IAR) during sagittal rotation of the superior vertebra with respect to the inferior vertebra of a Functional Spinal Unit (FSU) in the cervical spine moves significant distances during flexion and extension of the spine (Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474). This motion can vary widely between functional spinal units on an individual spine and between individuals and can depend on age, time of day, and the general health and condition of the intervertebral discs, facet joints, and other components of the FSU and spine. A moving IAR means that the superior vertebra can both rotate and translate while moving with respect to the inferior vertebra of an FSU.
Researchers have attempted to design a successful intervertebral disc for years. See, for example, the devices described by Salib et al., U.S. Pat. No. 5,258,031; Marnay, U.S. Pat. No. 5,314,477; Boyd et al., U.S. Pat. No. 5,425,773; Yuan et al., U.S. Pat. No. 5,676,701; and Larsen et al., U.S. Pat. No. 5,782,832, which describe designs that limit motion to rotation about a ball and socket when two plates are in contact. As the literature points out (Bogduk N. and Mercer S., “Biomechanics of the cervical spine. I: Normal kinematics”, Clinical Biomechanics, Elsevier, 15(2000) 633-648; and Mameren H. van, Sanches H., Beursgens J., Drukker, J., “Cervical Spine Motion in the Sagittal Plane II: Position of Segmental Averaged Instantaneous Centers of Rotation-A Cineradiographic Study”, Spine 1992, Vol. 17, No. 5, pp. 467-474), devices that restrict motion in such a manner do not allow the natural motion of the vertebrae, either for sagittal plane motion or for combined sagittal, lateral and axial motion. Further, when the two plates, as described in the cited patents, are not in contact, the devices are unable to provide stability to the intervertebral interface, which can allow free motion and lead to disc related spondylolisthesis, FSU instability and excessive facet loading.
Advances in disc arthroplasty design have resulted from intense patent activity in this area. Nonetheless, motion coupling between flexion-extension, axial rotation, and lateral bending and other functional spinal units involved in the overall spinal motion, increases the complexity and difficulty in developing a prosthetic disc replacement that realizes natural spinal motion. Producing such complex disc motions with strictly articulating joints is at best problematic.
Additional motion complexity required of a spinal disc derives from motion constraints dictated by facet joints. Complex facet joint surfaces in an FSU can significantly influence and constrain sagittal, lateral and axial motions. The orientation of these facet surfaces varies with FSU location in the spine and induces wide variations in motion parameters and constraints. The complex motion of a superior vertebra with respect to the associated inferior vertebra of an intact spinal joint segment, certainly in the cervical spine, cannot be realized by a simple rotation or simple translation, or even a combination of rotation and translation along a fixed axis, and still maintain the integrity and stability of the spinal segment, in particular the facet joints.
Natural spinal motions, therefore, can place severe requirements on the design of a prosthetic disc, requiring a minimum of six independent degrees of freedom to fully enable complete spinal motion. The number of independent mechanical degrees of freedom of a disc modeled on rigid body assumptions, however, can be reduced by one or more degrees of freedom at a joint stop or singularities of configuration, limiting further motion along the reduced degrees of freedom. Using only mechanically articulating joints, therefore, can be problematic in resolving complex disc motion. Adding flexure motion capability between relatively rigid mechanical joints within a disc prosthesis can enhance the prosthesis responsiveness when in configurations that restrict certain motions of mechanical articulating joints. On the other hand, mechanical articulating joints can provide structurally stronger and more robust motion generation with less stress. A combination of articulating joints and flexure motion structures, therefore, can enhance performance by utilizing the features of both. Such a combination of mechanical and flexural joints would represent a hybrid disc prosthetic system, hybrid because it employs both (approximately) rigid-body articulating joints and flexure links between those joints. A device utilizing a hybrid system, being one that employs rigid-body-modeled, articulating, mechanical joints linked by a nucleus, a portion of which can be compliant, can enable a total disc replacement to perform the complex, coupled motions required of a natural disc.
The invention instructed herein meets a variety of criteria for an artificial intervertebral disc suitable for implantation within a Functional Spinal Unit (FSU). When implanted, the invention can provide up to six independent degrees of freedom from a neutral position and enables the device to exercise complex motions dictated by spinal muscles, and motions consisting of simultaneous, compound rotations coupled with simultaneous multi-axis translations. A moving Instantaneous Axis of Rotation (IAR), typically dictated by flexion and extension in the cervical spine, for example, can be successfully tracked by the embodiments of the subject invention utilizing a movable rotation center and appropriate multi-axis translations. Shock absorption and flexure along and about multiple axes are other features that can be utilized with the devices of the subject invention.
In one embodiment, the devices comprise two endplates, a nucleus, and an optional, elastic boot attached to the endplates and enclosing the contents of the device, namely, the nucleus
The endplates can provide outer surface features that allow fusion of the plates to the superior and inferior vertebrae of a FSU and can prevent expulsion of the invention immediately after implanting. This disclosure instructs, but the invention is not limited to, several endplate geometries. The perimeter of a horizontal section of an endplate can be a general closed curve of which a circle, ellipse, and rounded-corners squares, rectangles, and trapezoids describe simple instances.
The nucleus can have compliant and rigid sections or in a particular embodiment comprise only compliant materials and, in another embodiment, only rigid materials, and yet another embodiment, a combination of compliant and rigid materials.
In specific embodiments, a nucleus can be configured as an integrated, graded polymer or as a tripartite construction comprising two or more, usually three, elements that can be overmolded, fitted or bonded together, or it can be a single unified element of the same material. One embodiment comprises a viscoelastic spherical cap, core, and sled. In one example of a graded polymer nucleus, the cap, core, and sled can consist of possibly different thermoplastics with different mechanical properties to be over-molded so as to create a single unit with five regions consisting of three primary regions, each comprising a particular material and two transitions, or interface, regions therebetween. Each transition region can comprise a mixture of the material between two different primary regions to which it interfaces. Mechanical properties of the nucleus can transition from one primary region to another primary region through an interface region. Such an interface region can vary in thickness from between approximately 0.025 mm to approximately 0.1 mm as the material transitions from one concentration of material to another. Such graded interface regions can be advantageous in handling shear stresses compared to abrupt interface transitions that occur when the three elements comprise separate parts to be fitted together.
A continuously graded nucleus can be generated by varying the mechanical moduli both axial and radially using additive manufacturing techniques with viscoelastic materials.
The mechanical properties chosen for each of the three regions can dictate the choice of polymer or other material for each element, which do not have to be different. For one embodiment, all three elements (cap, core, and sled) consist of the same polymer; for another embodiment, the same polymer is used for all three regions (single unit). Variations of both types of embodiments are instructed herein. This disclosure instructs, but the invention is not limited to, several different embodiments of a nucleus, all of which can be constructed from one to three different polymers that can be joined together by one or more combinations of techniques, for example, possible combinations of overmolding, chemical bonding, thermal or sonic welding, or mechanical bonding.
In particular embodiments, the invention employs, but is not limited to, a disc shaped sled and a rounded-corners square sled. Other embodiments of sled perimeter geometries can include any closed curve, such as, for example, an ellipse. From the perimeter geometries of a sled and its corresponding bottom endplate cavity, the associated range of motion of the sled within the bottom endplate cavity can be determined by one skilled in the art. Such variations, which provide the same function, in substantially the same way, with substantially the same result, are within the scope of the subject invention.
The geometry of the sled within a bottom endplate cavity, together, determines the freedoms and range of motion. For example, a disc shaped sled with radius r within a circular cavity with radius r+δ can allow the disc to move from the center of the cavity δ mm in any direction, realizing two degrees of freedom in polar coordinates. A square sled s mm on a side within a bottom endplate cavity s+δ can allow the sled centroid to move from the cavity centroid δ mm in either the y or z direction independently (refer to
While the nucleus of the invention can be embodied using several different means of manufacture, the abstract functional, or operational characteristics, can be described for all versions without differentiation. The nucleus maintains the intervertebral space, provides two articulating mechanical joint interfaces, one with each endplate; can provide shock absorption or not, depending upon an embodiment selected; and can allow translation of the Center-of-Rotation (COR) of the spherical cap. Movement of the COR of the spherical cap, can help accommodate variations in placement of the device at implant and variations in the location of the Center-of-Rotation (COR) of the top endplate with respect to the bottom endplate from level-to-level in the cervical spine.
When two surfaces can move relative to each other to form a lower pair joint, but no externally applied forces are necessary to maintain contact between the surfaces, then the joint is said to be profile-closed. A series of mechanically linked, profile-closed joints comprises a kinematic chain.
In one embodiment, a spherical cap region at the top of the nucleus interfaces with a conforming spherical cavity in a top endplate to form a three rotational degrees of freedom “ball-and-socket” type joint. These interfacing spherical surfaces define a lower pair spherical joint. This spherical joint is not shown profile-closed in the figures, but it can be made so as discussed below.
Moving caudally, the spherical cap material can transition to a region, called the core, consisting of the same, or different, material as the spherical cap. The core can transition to a region, called a sled, consisting of the same or different, material as the core or cap. When joined together the spherical cap, core, and sled comprise the nucleus of the device. The sled can transport the rest of the nucleus and top endplate and establishes a planar joint with the bottom endplate in which the sled can move. Advantageously, this arrangement can provide up to two independent translational degrees of freedom and one independent axial rotation, which can be a redundant rotation since the spherical cap can also provide axial rotation. Together the spherical ball and socket joint and the planar joint provide 6-degrees-of-freedom (6-DOF) of which 5-DOF are independent.
In an embodiment where a portion of the nucleus is compliant, the nucleus allows a continuum number of possible motion modalities, an important one of which is compression along the y-axis (refer to
For a graded polymer nucleus embodiment, the transition from cap to core to sled can be gradual, as in, for example, an overmolded case. For tripartite construction, the transition can be more abrupt. A single polymer nucleus can have no material transition regions. A nucleus with compliant portion(s) can provide shock absorption for axial compression up to approximately 1 mm, which can provide a third, independent translation degree of freedom, as mentioned previously. In a particular embodiment, the translation due to the compression of compliant portions of the nucleus and the motion of the sled can provide three, mutually orthogonal, i.e. independent, translational degrees of freedom. These degrees of freedom, coupled with the three rotational degrees of freedom of a spherical cap can complete the realization of six independent degrees of freedom of motion of the top endplate with respect to the bottom endplate.
In addition to providing 6-DOF, 5 independent DOF and one redundant DOF by means of two articulating mechanical joints, the compliant portions of the nucleus can provide a continuum of redundant freedom by means of flexion, extension and torsion about any axis. This redundancy allows accommodating complex, coupled joint motions of normal spinal motion dictated by spinal muscles and tendons, even when joint limits of the articulating mechanical joints have been reached.
Particular embodiments of the viscoelastic core can also support a plurality of hydraulic circuits that assist in shock absorption and can provide synovial joint type operation by forcing fluids between the sliding and rotating surfaces. In a particular embodiment, a hydraulic fluid can be a sterile, normal saline solution encapsulated by a boot, which can isolate the core and other working components from environment or biological fluids. In an alternative embodiment, there is no boot and the hydraulic fluids circulated by the device are environmental/biological fluids. Contraction and expansion of the central hydraulic cavity and transport channels of the viscoelastic core, as it compresses and then relaxes, can supply the pumping action of the fluid. In one embodiment, duck bill valves, or other valving techniques, located at the openings of hydraulic channels within a central hydraulic chamber, can allow fluid to flow into the chamber, but not out of the chamber. Duck bill valves, or other valving techniques, can also establish one-way flow out of the central chamber through its top and bottom portals. Such a configuration, during operation, can supply the “synovial” fluid-like action for the spherical and planar joints, respectively, reducing wear and friction.
In one embodiment, progressing still further caudally, the viscoelastic core transitions to a region designated as a sled. The most caudal end or underneath surface of the sled can mate with a planar surface within a cavity in the bottom endplate to form a planar joint. For a disc shaped sled, the sled can slide up to approximately ±1 mm in any direction from the geometric centroid, or from the vertical axis, of the cavity plane. A rounded-corners square sled 2-2-3, can slide up to approximately ±1 mm in the z direction and ±1 mm in the x direction from the centroid, or from the vertical axis, of the cavity plane, where z and x define the plane of motion (refer to
In one embodiment of a disc sled, the orifice in the bottom endplate cavity provides a tight fit for insertion. Once inserted, a cavity lip can prevent the sled and nucleus from being expelled from between the plates during nominal operation, thus, creating a profile-closed planar joint. In an alternative embodiment of a rounded-corners square sled, a separate sled retaining ring, in lieu of a cavity lip, can be fixedly or removeably attached to the bottom endplate, after the sled has been inserted in the bottom endplate cavity. In a further embodiment, the bottom end plate cavity can be made larger than the sled dimensions to easily accommodate such a placement. After fixing the sled retaining ring to the bottom endplate, there will be a retaining lip around the cavity, to retain the sled, hence the entire nucleus, between the plates, even during motion of the sled within the cavity, thus, creating a profile-closed planar joint.
In one embodiment, a viscoelastic boot in slight tension, when fixed to the endplates, separates the invention's interior space from its external environment and can provide resistance to extension forces and general torsion. Such a boot also can protect the mechanism from osteoblasts and other cell matter from fouling the mechanism during the healing process.
In order that a more precise understanding of the above recited invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. It should also be understood that the drawings presented herein may not be drawn to scale and that any reference to, or implication of, dimensions in the drawings or the following description are specific to the embodiments disclosed. Any variations of these dimensions that will allow the subject invention to function for its intended purpose are considered to be within the scope of the subject invention. Thus, understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered as limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which
The subject invention pertains to embodiments of a device capable of providing motion with up to six independent degrees-of-freedom and redundant flexure degrees of freedom. Specifically, articulating mechanical joints of the invention can provide five independent degrees of freedom and one redundant axial rotation degree-of freedom. Flexure between the two mechanical joints provides an additional axial degree of freedom and allows a continuum of motions within the rated modulus of the material. In particular, flexure in the invention accommodates compression-extension motions along the axial axis of the FSU independent of the articulation joint degrees of freedom. Embodiments of the device can further simultaneously provide reaction to compressive, tension and torsion loads. More specifically, the subject invention pertains to embodiments of a device capable of approximating the potential motion between two vertebrae in a spine
The following description will disclose embodiments of the subject invention that can be useful in the medical fields encompassing spinal surgery and, in particular, to devices and methods for correcting, replacing, or approximating natural movement between two adjacent vertebrae within a Functional Spinal Unit (FSU). More specifically, the embodiments disclosed herein can be useful for the treatment and/or removal of spinal disc herniation and degenerative disc disease. However, a person with skill in the art will be able to recognize numerous other uses that would be applicable to the devices and methods of the subject invention. While the subject application describes, and many of the terms herein relate to, a use for implanting within a spine, particularly for the treatment of spinal disc herniation and degenerative disc disease, other uses and modifications thereof that will be apparent to a person with skill in the art and having benefit of the subject disclosure are contemplated to be within the scope of the present invention.
In the description that follows, a number of terms are used in relation to the spine, spinal surgery, and medical devices related thereto. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided:
In this disclosure, a Functional Spinal Unit (FSU) or spinal joint is defined herein as two successive vertebrae in a spine, a superior vertebra and an inferior vertebra, including their mutual facet joints, an intervertebral disc attached to the inferior surface of the superior vertebra and the superior surface of the inferior vertebra, and connecting ligaments. Spinal muscles attached to a spinal joint or FSU can induce motion of not only that spinal joint but also influence the motion of other spinal joints due to complex mechanical couplings between spinal joints.
All possible natural motions of a superior vertebra with respect to an inferior vertebra in a healthy spinal joint define the natural workspace of a superior vertebra with respect to an inferior vertebra of a Functional Spinal Unit (FSU). This FSU workspace model treats the vertebrae as rigid bodies, in contrast to a flexible disc, and typically varies from FSU to FSU of an individual spine, and from one individual to another, creating considerable spinal disc prosthesis design problems.
The term “patient” as used herein, describes an animal, including mammals, to which the systems and methods of the present invention can be applied. Mammalian species that can benefit from the disclosed systems and methods include, but are not limited to, apes, chimpanzees, orangutans, humans, monkeys; domesticated animals (e.g., pets) such as dogs, cats, guinea pigs, hamsters; veterinary uses for large animals such as cattle, horses, goats, sheep; and any wild animal for veterinary or tracking purposes.
From bottom endplate to top endplate, for a particular embodiment, the mechanical linkage comprises a planar pair joint linked to a spherical pair joint. A flexible, or rigid, nucleus can establish one surface of a planar pair joint at the caudal end and a spherical surface of a spherical pair joint at the cranial end, and link the two joints together. The connecting link can be compliant in part or rigid, depending upon the embodiment. The planar pair joint can be profile-closed by a snap-fit or by a retaining ring. The spherical pair joint can be profile-closed by using rotational joint stops or sliders. Rotational joint stops and sliders are taught by U.S. Pat. No. 8,277,505 (Doty), which is hereby incorporated by reference for such teachings, including any figures, tables, or drawings pertaining to such teachings.
When the planar and spherical joints are both profile-closed and the nucleus is flexible, there results a hybrid kinematic chain. A hybrid kinematic chain, as defined here, comprises a kinematic chain with one or more, profile-closed, lower-order pair joints, modeled as rigid-bodies, and one or more flexible links connecting such joints.
In this disclosure, the lower pairs can be augmented by the incorporation of ball bearings, roller bearings, sliding bearings, line bearings, fixed bearings, and so forth to create higher pairs. Such modifications are taught in the inventor's U.S. Pat. Nos. 7,361,192; 7,799,080; 7,927,375; 8,226,724; and 8,277,505 provide numerous examples of higher order pair joints that can be substituted or utilized with the embodiments of the subject invention. These references are hereby incorporated, for such teachings, by reference herein, including any figures, tables, or drawings pertaining to such teachings.
The present invention is more particularly described in the following embodiments and examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular for “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise.
Finally, reference is made throughout the application to the “cranial end” and “caudal end.” As used herein, the cranial end 100 is that end that would typically be nearest to the head of a patient. Conversely, the caudal end 200 of the device is that end that would typically be nearest the tail end of a patient (nearest the foot end in a human patient). Similarly, caudally indicates a direction toward the “tail” and cranially a direction toward the head from a current point of reference. These references are for literary convenience and it should be understood that the embodiment of the invention can be used in any orientation within a patient.
The various embodiments disclosed herein can allow up to six independent degrees-of-freedom at the center of the Functional Spinal Unit (FSU) workspace. Those degrees of freedom continue to operate until one or more joint limits have been reached. The embodiments of the invention pertaining to approximating the potential motion between two vertebrae in an animal spine can maintain the integrity of the variable intervertebral spacing required during motion. For example, under compression, the intervertebral gap can narrow some and under extension it can widen some.
When appropriately scaled, the invention is capable of tracking workspace movements, within prescribed joint limits, for up to three independent translational and three independent rotational motions of the superior vertebra with respect to the inferior vertebra of the FSU. The flexible nucleus (2-n, n=1 through 7) can allow an infinite number of motions in response to compression, extension, shear, and torsion acting upon it. This is a principle feature of the invention, namely, the device can employ both mechanical, articulating joints and material flexure to realize any motion dictated by spinal muscles. This type of mechanical structure can be called a hybrid mechanical linkage as it includes both lower-ordered pair joints and flexure links, and, if all joints are profile-closed, a hybrid kinematic chain, as defined previously. A hybrid mechanical linkage, or hybrid kinematic chain, establishes two very different mechanisms to realize mobility, rigid-body lower-ordered pair joints and link compliance.
In a patient, the invention can serve, through appropriate mechanical programming of joint stops and scaling, as a disc prosthesis for any point along the spine. For example, in a human patient, the embodiments of the device, with appropriate joint stops and scaling can be utilized as a disc prosthesis from the cervical to the lumbar regions.
For particular embodiments of the invention, the outside perimeter defined by a planar horizontal cross section of endplates 1-1, 1-2, 3-1, 3-2, 3-3 and 3-4 can be trapezoidal, rectangular, square, circular, elliptical, or a general curvate closed curve. While these various geometries can affect the shape of the superior and inferior plates, the internal joint mechanism can typically remain the same, as the two embodiments of external geometries instructed herein, for example, 1-1, and 3-1 are circular and 1-2, 3-1, 3-2, 3-3, 3-4 are rectangular.
FSU angular and translational displacement along the various degrees of freedom, as instructed herein, typically relate to cervical spine applications of the invention in a human patient. However, it should be understood that the invention is not restricted to the cervical spine and can be scaled for larger spinal joints, for example, L4-L5 in the lumbar region.
As shown in
Two more right-handed-coordinate frames, shown in
Reference will be made to the attached figures on which the same reference numerals are used throughout to indicate the same or similar components. In the attached figures, which show certain embodiments of the subject invention, it can be seen that the subject invention comprises, generally, a top endplate 1-1, a bottom end plate 3-1, and a central nucleus 2-1. Variations of these general components are disclosed and shown in the attached figures.
With reference to the attached figures, it can be seen that
It can be seen that, in general, the invention comprises two plates, whose outer surface, in one embodiment, can be configured with keels, spikes, chevrons, or similar such structures. Alternatively, trabecular mesh treatment of those same surfaces can be used, all for fixing the endplates to bone and encouraging osteo-integration. The endplates can be separated by any one of a variety of nucleus embodiments that lock into the bottom plate and create a ball-and-socket joint with the top endplate and a planar joint with the bottom endplate. The various nuclei embodiments of the invention can be unified, integrated, graded, or tripartite polymer nuclei. They can also be configured with or without hydraulic portals and channels and with or without tubular extensions into the core body.
Certain embodiments of the invention, as seen in the figures, comprise two endplates: a top endplate 1-1 or 1-2, for example; a bottom endplate 3-1, 3-2, 3-3 or 3-4, for example; and a nucleus 2-n, n=1 to 7, for example. Nucleus sled 2-n-3, where n=1 to 7, can move planarly upon a horizontal floor (i.e., parallel to z-axis and x-axis plane of F0) of a cavity in the superior surface of a bottom endplate to form a planar joint. The cavity can be circular, as shown by 3-1-3 or 3-2-3, for disc sleds, or rectangular, as shown by 3-3-3 or 3-4-3, for rectangular sleds. Lip 3-1-5 or 3-4-5 or retaining ring 3-2-5 or 3-3-5 can keep the sled and nucleus from being pulled out of a bottom endplate cavity. Spherical cap 2-n-1, wherein n=1 to 7, on the superior surface of nucleus 2-n, where n=1 to 7, conforms to spherical cavity 1-1-3 or 1-2-3 in the inferior surface of top endplate 1-1 or 1-2 to form a spherical joint.
After implanting an embodiment of the invention in an FSU or spinal joint, outer surface features on the endplates can hold the implant in place and any of a variety of surface treatments can assist in fusing those endplate surfaces to vertebral bone. For example, the endplate outer surfaces 1-1-4, 1-2-4, 3-1-4, 3-2-4, 3-3-4 and 3-4-4 can be treated with titanium plasma sprays or sintering, or one can generate a trabecular type titanium mesh employing Electron Beam Manufacturing. This allows the superior surface 1-1-4 or 1-2-4, of a top endplate to fuse to the superior vertebra and the inferior surface 3-1-4, 3-2-4, 3-3-4 and 3-4-4 of a bottom endplate to fuse to the inferior vertebra of a spinal joint. When spinal muscles dictate complex motion of a superior vertebra of a spinal joint, the top endplate of the embodiment can follow the same motion trajectory as the superior vertebra, since they are fused together. An embodiment of the invention can accommodate the resulting motion of the top endplate by means of a spherical joint, a planar joint and a flexible viscoelastic core.
In one embodiment, a spherical joint can provide up to three independent degrees of rotational freedom about the frontal, sagittal and central axes; a planar joint can provide up to two independent translational degrees of freedom along the frontal and sagittal axes; and the viscoelastic core can provide at least one degree of translational freedom along the central axis of the device. Since these axes are typically perpendicular to each other, they can be linearly independent. Vertebral motions along each axis within the limited range dictated by the spinal muscles, therefore, can be realized by an embodiment of the invention with six degrees of freedom and the range of motion prescribed by spinal kinematics.
In a further embodiment, nuclei 2-n, where n=1 to 7, is a flexible material, such as, for example, viscoelastic plastics, which can also add an infinite variety of motions due to the flexure in the material. In this sense, the invention encompasses the advantage of articulating joints to realize large motions and a viscoelastic core to realize finer motions of compression, extension, shear, torsion, or arbitrary combinations thereof. This joint action incorporates advantages of articulating joints with flexure joints and can be called a hybrid joint mechanism.
In an embodiment, top endplate 1-1 (or 1-2), fused with a superior vertebra and moving in concert with that vertebra, can rotate up to approximately ±10° about an axis comprising a linear combination of frontal (z-axis) and sagittal (x-axis) motions, while simultaneously performing unconstrained axial (y-axis) rotation. A spherical joint formed by nucleus spherical cap 2-n-1, where n=1 to 7, and conforming spherical cavity 1-1-3 (or 1-2-3) in the top endplate's inferior surface can produce these complex, coupled, motions. The independence of the joint rotations means the invention can allow, within joint limits incorporated in the design of the device, complex, coupled rotations normally dictated by the spinal muscles. Flexure of the core can also assist in such motions.
In an embodiment, as top endplate 1-1 (or 1-2) attempts to translate in the horizontal plane, forces between the spherical cavity 1-1-3 (or 1-2-3) and spherical cap 2-n-1, where n=1 to 7, cause the nucleus to slide on its sled in a horizontal plane (z-xis and x-axis plane parallel to F0 z-axis and x-axis plane) and possibly compress or extend the nucleus's viscoelastic core along the central axis (y-axis). At the end of the motion, the top endplate, and the superior vertebra fused to it, will have translated and rotated to the required end position and orientation dictated by the spinal muscles.
If top endplate 1-1 (or 1-2) undergoes additional physiologic load, embodiments of the viscoelastic nucleus 2-n, where n=1 to 7, can compress along the central axis of the invention and can compress, extend, twist and bend under the influence of forces and moments-of-force. For certain embodiments, this can be normal operation. When physiologic load reduces on the top endplate during extension, a viscoelastic core can expand up to a non-loaded state, keeping the spherical cap and top endplate spherical cavity in continued contact until that point. Ideally, the expansion capabilities of the viscoelastic nucleus are such that the spherical end cap will maintain contact with the top endplate, even at maximum possible vertebral extension. Thus, at no time will the spherical cap be out of contact with the top end plate.
All displacements and rotations of the joints of the subject invention can be mechanically programmed to specific joint limits by appropriately installed joint stops. The joint stops can be rigid, or, to reduce wear, cushioned with materials falling within a wide range of durometer choices, from between approximately 50 to approximately 100. U.S. Pat. No. 8,277,505 teaches spherical joint stops that can be utilized with the spherical end cap and top end plate of the subject invention. U.S. Pat. No. 8,277,505 is hereby incorporated by reference for such teaching. As further taught by U.S. Pat. No. 8,277,505, the use of spherical joint stops or sliders can keep the nucleus and the top endplate kinematically connected even during extension, since the spherical joint stops and/or sliders generate a profile-closed spherical joint of the spherical pair between the top endplate and the nucleus cap. In such an embodiment, the invention establishes a kinematic chain from bottom endplate to the top endplate, and, if a portion of the nucleus is compliant, a hybrid kinematic chain.
In one embodiment, a boot affixes to and extends between a top endplate 1-1 or 1-2 and a bottom endplate 3-1, 3-2, 3-3 or 3-4. In a more particular embodiment, a protective, fluids/gases impervious, tough, flexible, fiber-reinforced boot fixedly attaches to and between the two endplates. The boot can be designed to be under slight tension in the FSU's neutral position and, thus, oppose increasing extension of the nucleus embodiments described herein. Boot tension also can resist torsion loads on the device. In one embodiment, the boot utilizes a diamond weave fiber matrix. In another embodiment, the boot comprises tough diagonal fibers woven within the boot. The inventor's U.S. Pat. Nos. 7,927,375; 7,361,192 and 7,799,080, 8,226,724, all teach various boot embodiments that can be utilized with embodiments of the subject invention. These patents are hereby incorporated by reference herein, for such teachings, including any figures, tables, or drawings pertaining thereto.
Many previous spinal implants have been unable to imitate the full six-degrees of motion provided by a normal FSU within its workspace. Advantageously, the embodiments disclosed herein are able to provide up to six-degrees of rotational and translational movement, and can also provide profile-closure of at least one or more lower pair joints, for example, the planar and/or spherical pairs. Profile-closed joints help prevent improper or excessive motion between components. The embodiments described herein present a simple, but effective, fully motion capable, spinal implant wherein the nucleus, under normal operation, cannot be withdrawn from its bottom endplate cavity, yet all the while allowing the planar joint to function. With rotational sliders/joint-stops, the top endplate will not separate from the spherical cap of the nucleus without excessive external separation forces, yet all the while allowing the spherical joint to function.
Rotational sliders/joint-stops, as instructed in the inventor's U.S. Pat. No. 8,277,505, can be added to kinematically connect top endplate 1-1 or 1-2 to any spherical cap 2-n-1, where n=1-7, to profile-close the spherical joint (not shown here, but illustrated and instructed in the referenced patent) and simultaneously provide joint stops. Rotational sliders/joint-stops keep the top plate and spherical cap spherical surfaces in contact (profile-closed), even without external loads and, can prevent those circumstances wherein the top endplate spherical surface can otherwise ride upon the cap spherical surface, separating the COR of each and producing high wear curvate line contact between the articulating elements.
The present application claims the benefit of U.S. Provisional Application Ser. No. 61/668,536, filed Jul. 6, 2012, which is hereby incorporated by reference herein in its entirety, including any figures, tables, or drawings.
| Number | Date | Country | |
|---|---|---|---|
| 61668536 | Jul 2012 | US |
