US20050049708A1 - Devices and methods for the treatment of spinal disorders - Google Patents

Abstract

Devices and methods for treating a damaged intervertebral disc to reduce or eliminate associated back pain. Dynamic bias devices and reinforcement devices are disclosed, which may be used individually or in combination, to eliminate nerve impingement associated with the damaged disc, and/or to reinforce the damaged disc, while permitting relative movement of the vertebrae adjacent the damaged disc.

Description

FIELD OF THE INVENTION
• The present invention generally relates to spinal implants. Specifically, the present invention relates to implantable devices and methods for the treatment of spinal disorders associated with the intervertebral disc.
BACKGROUND OF THE INVENTION
• Back pain is one of the most common and often debilitating conditions affecting millions of people in all walks of life. Today, it is estimated that over ten million people in the United States alone suffer from persistent back pain. Approximately half of those suffering from persistent back pain are afflicted with chronic disabling pain, which seriously compromises a person's quality of life and is the second most common cause of worker absenteeism. Further, the cost of treating chronic back pain is very high, even though the majority of sufferers do not receive treatment due to health risks, limited treatment options and inadequate therapeutic results. Thus, chronic back pain has a significantly adverse effect on a person's quality of life, on industrial productivity, and on heath care expenditures.
• Some forms of back pain are not chronic and may be simply treated by rest, posture adjustments and painkillers. For example, some forms of lower back pain (LBP) are very common and may be caused by unusual exertion or injury. Unusual exertion such has heavy lifting or strenuous exercise may result in back strain such as a pulled muscle, sprained muscle, sprained ligament, muscle spasm, or a combination thereof. An injury caused by falling down or a blow to the back may cause bruising. These forms of back pain are typically non-chronic and may be self-treated and cured in a few days or weeks.
• Other types of non-chronic back pain may be treated by improvements in physical condition, posture and/or work conditions. For example, being pregnant, obese or otherwise significantly overweight may cause LBP. A mattress that does not provide adequate support may cause back pain in the morning. Working in an environment lacking good ergonomic design may also cause back pain. In these instances, the back pain may be cured by eliminating the culprit cause. Whether it is excess body weight, a bad mattress, or a bad office chair, these forms of back pain are readily treated.
• However, some forms of back pain are chronic and are the result of spinal disorders which are not readily treated. Such spinal disorders may cause severe back pain, the origin of which may or may not be certain. A prevalent clinical theory is that pain arises from physical impingement of the nerve roots or the spinal cord. Such nerve impingement may have of a number of different causes, but generally results from either a disc protrusion or from narrowing of the intervertebral foramina which surround the nerve roots. Another clinical theory is that damage to the disc, either from injury, degradation or otherwise, causes physical impingement of the disc nerves, which are primarily disposed about the periphery of the annulus, but may grow into fissures of a damaged disc.
• Disc protrusions may be caused by a physical injury to the disc or by natural degradation of the disc such as by degenerative disc disease (DDD). Physical injury may cause damage to the annulus fibrosus which allows a portion of the disc, such as the nucleus pulposus, to protrude from the normal disc space. DDD may cause the entire disc to degenerate to such a degree that the annulus fibrosus bulges outward, delaminates or otherwise separates such that a portion of the disc protrudes from the normal disc space. In either case, the disc protrusion may impinge on a spinal nerve root causing severe pain. Impingement on the nerve root may also be caused by conditions unrelated to the disc such as by a spinal tumor or spinal stenosis (abnormal bone growth), but disc protrusions are the most common cause. Depending on the cause and nature of the disc protrusion, the condition may be referred to as a disc stenosis, a disc bulge, a herniated disc, a slipped disc, a prolapsed disc or, if the protrusion separates from the disc, a sequestered disc.
• Nerve root impingement most often occurs in the lumbar region of the spinal column since the lumbar discs bear significant vertical loads relative to discs in other regions of the spine. In addition, disc protrusions in the lumbar region typically occur posteriorly because the annulus fibrosus is thinner on the posterior side than on the anterior side and because normal posture places more compression on the posterior side. Posterior protrusions are particularly problematic since the nerve roots are posteriorly positioned relative to the intervertebral discs. When a posterior disc protrusion presses against a nerve root, the pain is often severe and radiating, and may be aggravated by such subtle movements as coughing, bending over, or remaining in a sitting position for an extended period of time.
• A common treatment for disc protrusion is discectomy, which is a procedure wherein the protruding portion of the disc is surgically removed. However, discectomy procedures have an inherent risk since the portion of the disc to be removed is immediately adjacent the nerve root and any damage to the nerve root is clearly undesirable. Furthermore, discectomy procedures are not always successful long term because scar tissue may form and/or additional disc material may subsequently protrude from the disc space as the disc deteriorates further. The recurrence of a disc protrusion may necessitate a repeat discectomy procedure, along with its inherent clinical risks and less than perfect long term success rate. Thus, a discectomy procedure, at least as a stand-alone procedure, is clearly not an optimal solution.
• Discectomy is also not a viable solution for DDD when no disc protrusion is involved. As mentioned above, DDD causes the entire disc to degenerate, narrowing of the intervertebral space, and shifting of the load to the facet joints. If the facet joints carry a substantial load, the joints may degrade over time and be a different cause of back pain. Furthermore, the narrowed disc space can result in the intervertebral foramina surrounding the nerve roots to directly impinge on one or more nerve roots. Such nerve impingement is very painful and cannot be corrected by a discectomy procedure.
• As a result, spinal fusion, particularly with the assistance of interbody fusion cages, has become a preferred secondary procedure, and in some instances, a preferred primary procedure. Spinal fusion involves permanently fusing or fixing adjacent vertebrae. Hardware in the form of bars, plates, screws and cages may be utilized in combination with bone graft material to fuse adjacent vertebrae. Spinal fusion may be performed as a stand-alone procedure or may be performed in combination with a discectomy procedure. By placing the adjacent vertebrae in their nominal position and fixing them in place, relative movement therebetween may be significantly reduced and the disc space may be restored to its normal condition. Thus, theoretically, aggravation caused by relative movement between adjacent vertebrae (and thus impingement on the nerve root by a disc protrusion and/or impingement from bone may be reduced if not eliminated.
• However, the success rate of spinal fusion procedures is certainly less than perfect for a number of different reasons, none of which are well understood. In addition, even if spinal fusion procedures are initially successful, they may cause accelerated degeneration of adjacent discs since the adjacent discs must accommodate a greater degree of motion. The degeneration of adjacent discs simply leads to the same problem at a different anatomical location, which is clearly not an optimal solution. Furthermore, spinal fusion procedures are invasive to the disc, risk nerve damage and, depending on the procedural approach, either technically complicated (endoscopic anterior approach), invasive to the bowel (surgical anterior approach), or invasive to the musculature of the back (surgical posterior approach).
• Another procedure that has been less than clinically successful is total disc replacement with a prosthetic disc. This procedure is also very invasive to the disc and, depending on the procedural approach, either invasive to the bowel (surgical anterior approach) or invasive to the musculature of the back (surgical posterior approach). In addition, the procedure may actually complicate matters by creating instability in the spine, and the long term mechanical reliability of prosthetic discs has yet to be demonstrated.
• Many other medical procedures have been proposed to solve the problems associated with disc protrusions. However, many of the proposed procedures have not been clinically proven and some of the allegedly beneficial procedures have controversial clinical data. From the foregoing, it should be apparent that there is a substantial need for improvements in the treatment of spinal disorders, particularly in the treatment of nerve impingement as the result of damage to the disc, whether by injury, degradation, or the like.
SUMMARY OF THE INVENTION
• The present invention addresses this need by providing improved devices and methods for the treatment of spinal disorders. As used herein, the term spinal disorder generally refers to a degradation in spinal condition as the result of injury, aging or the like, as opposed to a spinal deformity resulting from growth defects. The improved devices and methods of the present invention specifically address nerve impingement as the result of damage to the disc, particularly in the lumbar region, but may have other significant applications not specifically mentioned herein. For purposes of illustration only, and without limitation, the present invention is discussed in detail with reference to the treatment of damaged discs in the lumbar region of the adult human spinal column.
• As will become apparent from the following description, the improved devices and methods of the present invention reduce if not eliminate back pain while maintaining near normal anatomical motion. Specifically, the present invention provides dynamic bias devices and reinforcement devices, which may be used individually or in combination, to eliminate nerve impingement associated with a damaged disc, and/or to reinforce a damaged disc, while permitting relative movement of the vertebrae adjacent the damaged disc. The devices of the present invention are particularly well suited for minimally invasive methods of implantation.
• The dynamic bias devices of the present invention basically apply a bias force to adjacent vertebrae on either side of a damaged disc, while permitting relative movement of the vertebrae. By applying a bias force, disc height may be restored, thereby reducing nerve impingement. Specifically, by restoring disc height, the dynamic bias devices of the present invention: retract disc protrusions into the normal disc space thereby reducing nerve impingement by the protrusions; reduce the load carried by the facet joints thereby eliminating nerve impingement originating at the joint; restore intervertebral spacing thereby eliminating nerve impingement by the intervertebral foramina; and reduce pressure on portions of the annulus thereby alleviating nerve impingement in disc fissures.
• The reinforcement devices of the present invention basically reinforce a damaged disc, restore disc height and/or bear some or all of the load normally carried by a healthy disc, thereby reducing nerve impingement. Some embodiments of the reinforcement members of the present invention have a relatively small profile when implanted, but are very rigid, and thus serve to reinforce the disc, particularly the annulus. By reinforcing the disc, and particularly the annulus, disc protrusions may reduced or prevented, thereby eliminating nerve impingement by the protrusions. Other embodiments have a relatively large profile when implanted, and thus serve to increase disc height and/or to bear load. By increasing disc height, the advantages discussed previously may be obtained. By bearing some of the load normally carried by a healthy disc, the load may be redistributed as needed, such as when a dynamic bias device is used.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIGS. 1A and 1B illustrate left lateral and posterior views, respectively, of a portion of the adult human vertebral (spinal) column;
• FIG. 2A illustrates a left lateral view of an intervertebral disc disposed between adjacent vertebrae, wherein the disc is partially protruding from the normal disc space and the disc height is reduced;
• FIG. 2B illustrates a left lateral view of an intervertebral disc disposed between adjacent vertebrae as inFIG. 2A, wherein dynamic bias devices and reinforcement devices of the present invention, which are illustrated schematically, restore normal disc height and eliminate the disc protrusion;
• FIGS. 3A-3C schematically illustrate adynamic bias device100 in accordance with the present invention;
• FIGS. 4A-4B schematically illustrate left lateral and posterior views, respectively, of dynamic bias devices of the present invention mounted to adjacent vertebrae equidistant from the median plane;
• FIGS. 5A-5B schematically illustrate left lateral and posterior views, respectively, of a dynamic bias device of the present invention mounted to adjacent vertebrae in the median plane;
• FIGS. 6A-6B illustrate end and exploded views, respectively, of a bushing in accordance with a first embodiment of the present invention;
• FIG. 6C illustrates a posterior view of the bushing shown inFIGS. 6A-6B mounted to a spinous process;
• FIG. 6D illustrates a posterior view of the spinous process shown inFIG. 6C, detailing the counter-bore;
• FIGS. 7A-7B illustrate end and exploded views, respectively, of a bushing in accordance with a second embodiment of the present invention;
• FIGS. 8A-8B illustrate end and exploded views, respectively, of a bushing in accordance with a third embodiment of the present invention;
• FIGS. 9A-9B illustrate end and exploded views, respectively, of a bushing in accordance with a fourth embodiment of the present invention;
• FIG. 10A illustrates a side view of a dynamic bias device in accordance with a first embodiment of the present invention;
• FIG. 10B illustrates a side view of the dynamic bias device shown inFIG. 10A subjected to a compression load;
• FIG. 10C illustrates a cross-sectional view of the dynamic bias device shown inFIG. 10A;
• FIG. 11A illustrates a cross-sectional view of a dynamic bias device in accordance with a second embodiment of the present invention;
• FIG. 11B illustrates a cross-sectional view of a dynamic bias device in accordance with a third embodiment of the present invention;
• FIGS. 12A-12B illustrate rear and side views, respectively, of a dynamic bias device in accordance with a fourth embodiment of the present invention;
• FIG. 12C illustrates the dynamic bias device shown inFIGS. 12A-12B subjected to a compression load;
• FIG. 13A illustrates a side view of a dynamic bias device in accordance with a fifth embodiment of the present invention;
• FIG. 13B illustrates a side or rear view of a dynamic bias device in accordance with a sixth embodiment of the present invention;
• FIG. 13C illustrates a rear view of a dynamic bias device in accordance with a seventh embodiment of the present invention;
• FIGS. 14A-14D illustrate tools of the present invention for implanting the reinforcement members;
• FIGS. 15A-15J illustrate steps for implanting a self-expanding reinforcement member;
• FIGS. 15K-15L illustrate steps for implanting an inflatable reinforcement member;
• FIGS.15M-1SR illustrate steps for implanting reinforcement bars; AND
• FIG. 16 illustrates a bias force v. displacement curve for the dynamic bias device.
DETAILED DESCRIPTION
• The following detailed description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention.
• With reference toFIGS. 1A and 1B, the lower portion of an adulthuman vertebral column10 is illustrated in left lateral and posterior views, respectively. The upper portion of thevertebral column10 includes the thoracic region and the cervical region, which are not shown for purposes of simplified illustration only. The lower portion of thevertebral column10 includes thelumbar region12, thesacrum14 and thecoccyx16. Thesacrum14 and thecoccyx16 are sometimes collectively referred to as the pelvic curvature.
• Thevertebral column10 includes an axis ofcurvature60 which generally forms a double-S shape when viewed laterally. Thevertebral column10 also includes amedian plane70 which is a sagittal plane bisecting thevertebral column10 into symmetrical left lateral and right lateral portions. In posterior views, themedian plane70 appears as a line.
• Thelumbar region12 of thevertebral column10 includes five (5) vertebrae20 (labeled L1, L2, L3, L4 and L5) separated byintervertebral discs50. Thesacrum14, which includes five (5) fused vertebrae30 (superior vertebra30 labeled S1), is separated by asingle disc50 from thecoccyx16, which includes four (4) fusedvertebrae40. Although not labeled, theintervertebral discs50 may be referenced by their respective adjacent vertebrae. For example, thedisc50 between the L4 and L5lumbar vertebrae20 may be referred to as the L4L5 disc. Similarly, thedisc50 between the L5lumbar vertebra20 and the S1sacral vertebra30 may be referred to as the L5S1 disc.
• Although eachvertebra20/30/40 is a unique and irregular bone structure, thevertebrae20 of the lumbar region12 (in addition to the thoracic and cervical regions) have common structures. Eachvertebra20 of thelumbar region12 generally includes abody portion21 and a vertebralarch portion22/23 which encloses the vertebral foramen (not visible) in which the spinal cord is disposed. Thevertebral arch22/23 includes twopedicles22 and twolaminae23. Aspinous process24 extends posteriorly from the juncture of the twolaminae23, and twotransverse processes25 extend laterally from eachlamina23. Fourarticular processes26/27 extend inferiorly26 and superiorly27 from thelaminae23. The inferiorarticular process26 rests in the superiorarticular process27 of the adjacent vertebra to form a facet joint28.
• The five (5)vertebrae30 of thesacrum14 are fused together to form a single rigid structure. Thesacrum14 includes a mediansacral crest31 which roughly corresponds to the spinous processes of thevertebrae30, and two intermediate sacral crests32 which roughly correspond to the articular processes of thevertebrae30. The sacral laminae33 are disposed between the median31 and intermediate32 sacral crests. Two lateral sacral crests34 are disposed on either side of thesacral foraminae35. Thesacrum14 also includes a pair ofsacral wings36 which defineauricular surfaces39. The superior (S1)sacral vertebra30 includes two superiorarticular processes37 which engage the inferior articular processes26 of theL5 lumber vertebra20 to form a facet joint, and thebase38 of the superior sacral vertebra S1 is joined to theL5S1 disc50.
• Eachintervertebral disc50 includes anannulus fibrosus52 surrounding anucleus pulposus54, which are more clearly visible inFIG. 15A. Theposterior annulus52 is generally thinner than theanterior annulus52, which may account for the higher incidence of posterior disc protrusions. As used herein, a disc protrusion generically refers to any portion of the disc that protrudes from the normal disc space. Common clinical conditions that may be characterized as a disc protrusion include a disc stenosis, a disc bulge, a herniated or sequestered disc, a slipped disc, and a prolapsed disc. Generally, a disc protrusion results in a decrease in disc height proportional to the volume of the protrusion. A degenerative disc may sometimes only involve the loss of disc height, and may or may not involve any significant protrusion. However, both degenerative discs and a disc protrusions usually involve some loss in disc height.
• A common theory is that eachintervertebral disc50 forms one support point and the facet joints form two support points of what may be characterized as a three point support structure between adjacent vertebrae. However, in thelumbar region12, the facet joints28 are substantially vertical, leaving thedisc50 to carry the vast majority of the load. As between theannulus fibrosus52 and the nucleus pulposus54 of thedisc50, it is commonly believed that thenucleus54 bears the majority of the load. This belief is based on the theory that thedisc50 behaves much like a balloon or tire, wherein theannulus22 merely serves to contain thepressurized nucleus54, and thenucleus54 bears all the load.
• However, this theory is questionable since theannulus fibrosus52 comprises 60% of thetotal disc50 cross-section, and thenucleus pulposus54 only comprises 40% of thetotal disc50 cross-section. In addition, theannulus fibrosus52 is made of 40-60% organized collagen in the form of a laminated structure, whereas thenucleus pulposus54 is made of 18-30% collagen in the form of a relatively homogenous gel. It seems a more plausible theory is that theannulus fibrosus52 is the primary load bearing portion of thedisc50.
• With reference toFIG. 2A, a left lateral view of anintervertebral disc50 disposed between adjacent vertebrae20_S(superior) and20_I(inferior) is illustrated, wherein thedisc50 is partially protruding56 from the normal disc space and the disc height is reduced. Although thedisc50 is shown to include aprotrusion56, the reduction in disc height may or may not be accompanied with aprotrusion56 as discussed previously. For example, if thedisc50 is degenerated, the disc height may be reduced with or without a correspondingprotrusion56.
• It should be understood that the vertebrae shown inFIGS. 2A and 2B generically refer to any two adjacent vertebrae or any series of adjacent vertebrae, and thatlumbar vertebrae20_Sand20_Iare specifically shown for purposes of illustration only. This generic method of illustrating vertebrae also applies to the remainder of the Figures.
• With reference toFIG. 2B, a left lateral view of theintervertebral disc50 disposed betweenadjacent vertebrae20_Sand20_Iis illustrated as inFIG. 2A. However, in this Figure,devices100 and200 of the present invention, which are illustrated schematically, eliminate thedisc protrusion56 and restore normal disc height. Specifically, one or moredynamic bias devices100 and one ormore reinforcement members200 are utilized, either in combination or individually.
• Thedynamic bias device100 restores disc height and, by conservation of disc volume, retracts the protrusion into the normal disc space thereby reducing nerve impingement by the protrusion. Restoring disc height also reduces the load carried by the facet joints thereby eliminating nerve impingement originating at the joint, restores intervertebral spacing thereby eliminating nerve impingement by the intervertebral foramina, and reduces pressure on portions of the annulus thereby alleviating nerve impingement in disc fissures.
• Thedynamic bias device100 basically applies a bias force to theadjacent vertebrae20_Sand20_Ito which it is connected, but allows relative movement of thevertebrae20_Sand20_I. Thedynamic bias device100 is conceptually similar to a spring attached to theadjacent vertebrae20_Sand20_I. Thedynamic bias device100 applies a bias force (usually repulsive) between thevertebrae20_Sand20_Iwhen the disc height is normal or less than normal. The bias force is preferably set such that the disc height is normal with normal posture and loading, and increases with posterior flexure and/or added vertical load. The details of the design and use of thedynamic bias device100 will be discussed in greater detail hereinafter, particularly with reference toFIGS. 3A-3C,4A-4B,5A-5B,10A-10C,11A-11B,12A-12C, and13A-13C.
• Becausemost protrusions56 are posterior, thedynamic bias device100 is preferably mounted posterior to the axis ofcurvature60. Locating thedynamic bias device100 posterior to the axis ofcurvature60 shifts the load carried by thedisc50 from the posterior portion of the disc to the anterior portion of thedisc50. Locating thedynamic bias device100 posterior to the axis ofcurvature60 also reduces the load carried by the facet joints. Preferably, thedynamic bias device100 applies a substantially vertical bias force, with the direction independent of displacement.
• Because more load will be shifted to the anterior portion of thedisc50 with a posterior mounteddynamic bias device100,reinforcement members200 may be placed in theanterior annulus52, to effectively bolster the anterior portion of the disc. Thereinforcement members200 may be used to reinforce the disc, restore disc height and/or bear the load normally carried by annulus. Thereinforcement members200 are relatively rigid and thus serve to reinforce thedisc50 where inserted. In addition, thereinforcement members200 may have a relatively large profile when implanted and thus increase disc height. Thereinforcement members200 are particularly beneficial if thedisc50 is degenerated, or if thedisc50 will likely become degenerated with the change in load distribution. The details of the design and use of thereinforcement members200 will be discussed in greater detail hereinafter, particularly with reference toFIGS. 14A-14D and15A-15R.
• As mentioned previously, one or moredynamic bias devices100 and one ormore reinforcement members200 may be utilized, either alone or in combination. Specifically: one or moredynamic bias devices100 may be used alone; one ormore spacer devices200 may be used alone; and one or moredynamic bias devices100 and one ormore reinforcement members200 may be used in combination. If a combination ofdevices100/200 is used, it is believed that the use of one or more posteriordynamic bias devices100 in combination with one or moreanterior reinforcement members200 is most effective in treatingposterior protrusions56, facet joint degradation, and nerve impingement by intervertebral foraminae, which are believed to be the most common culprits of chronic LBP.
• As an alternative to the arrangement shown inFIG. 2B, two or moredynamic bias devices100 may be attached to the vertebrae on opposite sides ofvertebrae20_Sand20_I. Specifically, one or moredynamic bias devices100 is connected tovertebra20_Sand the vertebra immediately superior tovertebra20_S, and one or moredynamic bias devices100 is connected tovertebra20_Iand the vertebra immediately inferior to vertebra201. With this arrangement, thedynamic bias devices100 are primarily applying a traction force to effectively pullvertebrae20_Sand20_Iapart, thereby eliminating thedisc protrusion56 and restoring normal disc height.
• With reference toFIGS. 3A-3C, thedynamic bias device100 is schematically illustrated under conditions of no-load, compression load (L_C), and traction load (L_T), respectively. Thedynamic bias device100 includes a pair ofattachment members102, abias member104, and ahousing106.Attachment members102 facilitate attachment of thedynamic bias device100 tovertebrae20_Sand20_I, as shown inFIG. 2B.Bias member104 functions to apply a bias force between theattachment members102.Housing106 functions to separate the moving portions ofdynamic bias device100 from the surrounding muscle, ligaments and other tissue when thedynamic bias device100 is implanted.
• Attachment members102 may comprise a wide variety of mechanical connection designs, and may incorporate into their design, or be used in combination with, other machine elements not specifically mentioned herein. For purposes of illustration only, the eachattachment member102 is shown as loop which may be connected to the vertebrae by fasteners and bushings, specific examples of which are described in detail with reference toFIGS. 6A-6D,7A-7B,8A-8B and9A-9B. These specific examples are provided by way of example, not limitation. Those skilled in the art will recognize that theattachment members102 may comprise or include screws, rivets, spikes, keys, pins, cotters, splines, couplings, bushings, washers, and the like, without departing from the scope or spirit of the present invention.
• The primary function of theattachment members102 is to fixedly secure the ends of thebias member104 to thevertebrae20_Sand20_I. Preferably, theattachment members102 are secured to thevertebrae20_Sand20_Isuch that translational movement is minimized or eliminated, and such that rotational movement is permitted between eachattachment member102 and eachvertebrae20_Sand20_I. Providingattachment members102 with these functional attributes permits thedynamic bias device100 to effectively transmit a bias force to eachvertebrae20_Sand20_I, allow relative movement therebetween, and minimize stress on thevertebrae20_Sand20_Iat the attachment points.
• Bias member104 functions to apply a bias force, either attraction or repulsion, between theattachment members102. The bias force generally increases or decreases with displacement of the ends of thebias member104, as with a conventional spring. In addition, the bias force may increase or decrease with the time derivative of displacement (i.e., velocity) of the ends of thebias member104, as with a conventional damper or shock absorber. As shown inFIG. 3B, thebias member104 compresses in response to a compression load (L_C), thereby increasing or decreasing the bias force. Similarly, as shown inFIG. 3C, thebias member104 extends in response to a traction load (L_T), thereby increasing or decreasing the bias force.
• If thedynamic bias devices100 are attached tovertebrae20_Sand20_I(compression embodiment), as shown inFIG. 2B, the bias force of thebias member104 increases in response to a compression load (L_C), and decreases in response to a traction load (L_T). In addition, thebias member104 normally operates in compression. Preferably, the bias force of thebias member104 is adjusted such that the disc is restored to a more normal height when thedynamic bias device100 is implanted. Because the disc height is usually initially less than normal, thedynamic bias device100 is attached to the vertebrae with thebias member104 preloaded in compression or with thevertebrae20_Sand20_Iin traction or otherwise spread apart. In this manner, for a given posture, the disc height will be larger following implantation of thedynamic bias device100 than prior to implantation.
• If thedynamic bias devices100 are attached to the vertebrae on opposite sides ofvertebrae20_Sand20_I(traction embodiment), as discussed as an alternative to the arrangement shown inFIG. 2B, the bias force of thebias member104 decreases in response to a compression load (L_C), and increases in response to a traction load (L_T). With this latter arrangement, thebias member104 normally operates in tension. Because thebias member104 normally operates in tension with this arrangement, thebias member104 may simply comprise a member that is rigid or semi-rigid in tension, such as a cable. Also with this arrangement, the bias force of thebias member104 is adjusted such that the disc is restored to a more normal height when thedynamic bias device100 is implanted. Further with this arrangement, because the disc height is usually initially less than normal, thedynamic bias device100 is attached to the vertebrae with thebias member104 preloaded in tension or with thevertebrae20_Sand20_Iin traction or otherwise spread apart.
• With either arrangement, thedynamic bias device100 preferably operates with substantially linear displacement substantially parallel to the axis ofcurvature60. However, the amount of displacement will be evenly shared between thedynamic bias devices100 in the traction embodiment, whereas the compression embodiment requires the full displacement to be assumed by eachdynamic bias device100. The following ranges of displacement are given with reference to the compression embodiment. When mounted near the posterior portion of adjacent spinous processes, thedynamic bias device100 may have a total (i.e., maximum) displacement preferably in the range of 1.0 to 3.0 cm to accommodate full posterior to anterior flexure in the L5-S1 region, 0.5 to 1.5 cm to accommodate full posterior to anterior flexure in the L4-L5 region, and 0.25 to 1.0 cm to accommodate full posterior to anterior flexure in the L1-L4 region.
• Within these ranges of displacement, it is preferable thatbias member104 operate within its elastic limit, as dictated by the chosen material and geometry of thebias member104. In addition, because the bias member preferably should be able to withstand 1.0 to 10 million fatigue cycles, it is preferable thatbias member104 operate within its fatigue limit, as dictated by the chosen material and geometry, for the full range of displacement.
• As mentioned previously, the bias force may generally increase or decrease with displacement of the ends of thebias member104, as with a conventional spring. In this situation, the bias force (F_B) is generally governed by Hooke's Law where F_B=KΔX, wherein F_Bis linearly proportional to the displacement (ΔX) as dictated by the spring constant (K) of thebias member104. Also as mentioned previously, the bias force may increase or decrease with the time derivative of displacement (i.e., velocity) of the ends of thebias member104, as with a conventional damper or shock absorber. In this situation, the bias force (F_B) is generally linearly proportional to the derivative of displacement (ΔX/ΔT) as dictated by the damper constant (P) of thebias member104. Preferably, the bias force F_BOf thebias member104 is adjusted such that the disc is restored to a more normal height when thedynamic bias device100 is implanted. The bias force F_Bmay be adjusted by selecting the spring constant (K) and/or damper constant (P) of thebias member104 and by pre-loading (compressing) thebias member104 an initial displacement ΔX_i.
• The necessary bias force F_Bmay be roughly calculated as a function of body weight (BW), the distance of the mounteddynamic bias device100 from the axis ofcurvature60, and the mechanical properties of the surrounding tissues (muscle tissue, connective tissue, joints). The normal net load carried by thelumbar region12 is roughly 30% BW when laying down, 140% BW when standing, 185% BW when sitting, 215% BW when bending forward, and 250% BW when slouching.
• With reference toFIG. 16, a bias force versus attachment point displacement curve for thedynamic bias device100 is shown. The bias force is intended to be sufficiently high to spread the attachment points (e.g., processes of adjacent vertebrae) and restore normal disc height in all postures. For example, in normal standing posture, the bias force is sufficiently high to spread the attachment points as shown inFIG. 16, such that more normal disc size and shape is obtained. As the spine is placed in flexion and extension, the amount of force carried by thedynamic bias device100 will change as a function of the spring properties, including the spring constant (K) and the compression length of the spring.
• In a preferred embodiment, the bias force is sufficient to shift the pre-implant (normal posture) distance to the post-implant (normal posture) distance. To prevent excessive compression of the disc, particularly the posterior disc, it is also preferred that the bias force increase significantly as the attachment points come closer, as by extension, lifting and/or poor posture. This is facilitated by the natural increase in bias force of the spring as the distance decreases, and is aided by the damper pad and the compression limit (bottomed out) of the spring. In addition, because the dynamic bias device is intended to limit excessive compression of the posterior disc, and not necessarily intended to limit flexion of the spine, it is also preferable that the bias force approach zero (spring fully extended) at a distance which is less than the extension limit of the dynamic bias device.
• Thus, by way of example, not limitation, the bias force may be in the range of 1% to 30% BW when laying down. With other postures after implantation, the bias force may be estimated by subtracting the contribution of body weight from the load carried bylumbar region12, which is approximately 50% BW (head=5% BW; arms=9% BW; trunk=36% BW). As such, the bias force may be in the range of 10% to 90% BW when standing.
• With reference toFIGS. 4A-4B and5A-5B, left lateral and posterior views ofdynamic bias devices100 are schematically illustrated as being mounted to adjacentspinous processes24_Sand24_Iofadjacent vertebrae20_Sand20_I. When two or moredynamic bias devices100 are utilized per pair of vertebrae as shown inFIGS. 4A and 4B, thedynamic bias devices100 are preferably mounted substantially equidistant from themedian plane70, or otherwise symmetric about themedian plane70, in order to avoid causing lateral bias or curvature of thespine10. Note that thedynamic bias devices100 may be mounted substantially vertical as shown or at an angle to themedian plane70 and satisfy these criteria. When only onedynamic bias device100 is utilized per pair of vertebrae as shown inFIGS. 5A and 5B, thedynamic bias device100 is preferably mounted in or near themedian plane70 for the same reason.
• Although it is preferable to have the dynamic bias device(s)100 near themedian plane70, substantially equidistant from themedian plane70, or otherwise symmetric about themedian plane70, it is possible to have multipledynamic bias devices100 mounted asymmetrically while maintaining balanced bias forces about themedian plane70. The objective is to avoid causing lateral bias or curvature of thespine10, which is a function of balancing bias forces and moments about themedian plane70.
• The bias forces are vectors which have a magnitude governed by the properties of thebias member104, and a direction dictated by the mounting position of thedynamic bias device100. Eachdynamic bias device100 has two bias force vectors, one for eachattachment member102 at each attachment point. Each bias force vector has a moment arm equal to the distance from the attachment point to themedian plane70. For each attachment point, the product of the moment arm and the vertical component of the bias force vector is the moment or torque applied to thespine10, and the horizontal component of the bias force vector is the shear applied to thespine10. Thus, in order to minimize curvature of thespine10, all of the moments are balanced about themedian plane70 In order to minimize lateral bias on thespine10, all of the horizontal components of the bias force vectors are balanced about themedian plane70. The easiest way to accomplish this result, of course, is to mount thedynamic bias devices100 symmetrically about themedian plane70. However, those skilled in the art will recognize that asymmetric mounting arrangements that substantially meet these criteria are also possible.
• Further, becausemost protrusions56 are posterior, the dynamic bias device(s)100 is/are preferably mounted posterior to the axis ofcurvature60. This is advantageous because loss of disc height is most common in theposterior disc50, the largest amount of mechanical advantage about the anterior disc is obtained posterior to the axis ofcurvature60, and the posterior portions of the vertebrae are easiest to access less invasively. However, the dynamic bias device(s)100 may be mounted at any position relative to the axis ofcurvature60 depending on the location of theprotrusion56, as long as the dynamic bias device(s)100 is/are near themedian plane70, substantially equidistant from themedian plane70, or otherwise symmetric about themedian plane70 as discussed above.
• Given these criteria, there are many suitable mounting locations or attachment points for thedynamic bias device100. Some of the possible attachment points are labeled A-N inFIGS. 1A and 1B. Attachment points A-G refer to the lumbar vertebrae20 (L1-L5), and attachment points H-N refer to the sacral vertebrae30 (particularly S1). The attachment points A-G of thelumbar region12 are equally applicable to the thoracic and cervical regions of thespine10, which are not illustrated for purposes of simplicity only.
• In thelumbar region12, attachment points A and B refer to the left lateral and right lateral surfaces of thespinous process24; attachment point C refers to the posterior surface of thespinous process24; attachment points D and E refer to the posterior surfaces of the left andright laminae23; and attachment points F and G refer to the distal ends of the left and right transverse processes25.
• In thesacrum14, attachment points H and I refer to the left lateral and right lateral surfaces of the superior mediansacral crest31; attachment points K and L refer to the posterior surfaces of the sacral laminae33 between the mediansacral crest31 and the intermediate sacral crests32; and attachment points M and N refer to posterior surface between the intermediate sacral crests32 and the lateral sacral crests34.
• A wide variety of sets of attachment points are possible, a non-exhaustive list of which is set forth herein. For singledynamic bias device100 mounting, the nomenclature is (X₁Y₁) where X₁is the attachment point on vertebra X, and Y₁is the attachment point on vertebra Y. For doubledynamic bias device100 mounting, the nomenclature is (X₁Y₁, X₂Y₂) where X₁is the attachment point of the firstdynamic bias device100 on vertebra X, Y₁is the attachment point of the firstdynamic bias device100 on vertebra Y, X₂is the attachment point of the seconddynamic bias device100 on vertebra X, and Y₂is the attachment point of the seconddynamic bias device100 on vertebra Y. Vertebrae X and Y refer to any two different vertebrae, which are usually, but not necessarily, adjacent. In addition, vertebrae X and Y may be superior and inferior, respectively, or vice-versa.
• To illustrate the attachment point nomenclature, reference may be made toFIGS. 4B and 5B. InFIG. 4B, a firstdynamic bias device100 is attached to the left lateral surface of the two spinous processes, and a seconddynamic bias device100 is attached to the right lateral surface of the two spinous processes. Thus, the set of attachment points for the arrangement ofFIG. 4B is (AA, BB). InFIG. 5B, only onedynamic bias device100 is attached to the posterior surface of the two spinous processes. Thus, the set of attachment points for the arrangement ofFIG. 5B is (CC).
• By way of example, not limitation, the following sets of attachment points may be used to satisfy the above-referenced criteria with regard to balancing moments and forces about themedian plane70. For singledynamic bias device100 mounting: (CC); and (CJ) are preferred. For doubledynamic bias device100 mounting: (AA, BB); (DD, EE); (FF, GG); (AH, BI); (DK, EL); (FK, GL); (DM, EN); and (FM, GN) are preferred. Also for doubledynamic bias device100 mounting: (AD, BE); (AF, BG); (AK, BL); (AM, BN); (DH, El); (DF, EG); (FH, GI); (CA, CB); (CH, CI); (CD, CE); (CF, CG); (CK, CL); (CM, CN); (JA, JB); (JH, JI); (JD, JE); and (JF, JG) are possible. For more than double mounting, any combination of these sets may be used. Generally, the more posterior the attachment points, the less invasive the procedure will be. Attachment points A, B, C, H and I are preferred for this reason. In addition, to avoid interfering with the motion of the vertebrae, thedynamic bias device140 is preferably disposed laterally or posteriorly of the spinous processes24, as opposed to under and between the spinous processes24.
• Thedynamic bias device100 may be attached to these points by conventional surgical techniques, except as described herein. The posterior musculature and connective tissues may be dissected to expose the desired attachment points. If desired, anydisc protrusions56 may be removed, in whole or in part, using a conventional discectomy procedure. Also if desired, any other abnormal spinal growths or protrusions may be removed. However, formany disc protrusions56, it is anticipated that conventional traction or separation techniques may be employed to temporarily retract theprotrusion56 into the normal disc space until the dynamic bias devices are implanted.
• In order to establish separation of the vertebrae, the spine may be placed in traction or conventional intervertebral separation tools may be used. Alternatively, thedynamic bias device100 may be preloaded such that when the device is released after attachment, the bias force establishes the desired amount of separation.
• Pilot holes are drilled as needed, such as for the use ofbushings330,340 and/or350 (described with reference toFIGS. 7A-7B,8A-8B and9A-9B hereinafter). If attachment points A, B, H and I are to be used, such as with the use of bushing320 (described with reference toFIGS. 6A-6D hereinafter), ahole90 and counter-bore92 may be drilled into thespinous process24. The device(s)100 are then attached to the desired attachment points in accordance with the hardware being used, and the site is subsequently surgically closed.
• With reference toFIGS. 6A-6D,7A-7B,8A-8B and9A-9B, various embodiments ofbushings320,330,340, and350, respectively, are illustrated. As mentioned previously, theattachment members102 may comprise a wide variety of mechanical connection designs, and may incorporate into their design, or be used in combination with, other machine elements such asbushings320,330,340, and350.Bushings320,330,340, and350 are adapted to mount one or twodynamic bias devices100. As illustrated,bushings320,330,340, and350 are adapted to receiveattachment members102 in the form of loops or the like, but may be modified to receive other structures. A primary function ofbushings320,330,340, and350 is to isolate movement of theattachment members102 from the vertebrae to which they are attached. Thus, the bushing to bone (vertebrae) interface is static, while the bushing to attachment member interface is dynamic. This reduces if not eliminates the abrasive degradation of the vertebrae due to the attachment of thedynamic bias device100. The orientation of the vertebral surface at the attachment points will determine the best bushing scheme.
• With reference toFIGS. 6A-6B, end and exploded views, respectively, of abushing320 are illustrated.Bushing320 is particularly suitable for attachment to thespinous process24 as shown inFIG. 6C, or attachment points A, B, H and I as illustrated inFIG. 1A. Bushing320 may be attached to thespinous process24 utilizing aconventional fastener300, which includesbolt302,nut304 andwashers306 and308. Preferably, thefastener300 is a lock fastener such that it will not have a tendency to unscrew with relative motion of theattachment members102. However, thenut304 is not tightened so much as to inhibit rotational movement of theattachment members102.Fastener300 may alternatively comprise a key and pin (e.g., cotter pin). When fully assembled, theattachment members102 are disposed around the shaft of thebolt302 on either side of thebushing320 and between thewashers306 and308.
• Bushing320 includes amale fitting321 which fits into afemale fitting324. Themale fitting321 includes ashank portion322 and ahead portion323. Similarly, thefemale fitting324 includes ashank portion325 and ahead portion326. Thefemale fitting324 has an inside diameter sized to accommodate theshank322 of themale fitting321, and themale fitting321 has an inside diameter sized to accommodate thebolt302 of thefastener300. The outside surface of theshank322 of themale fitting321 and the inside surface of theshank325 of thefemale fitting324 may include mating threads.
• The size of thehead323/326 to bone interface is preferably maximized to minimize stress concentration and to distribute torsional loads over a large surface area. The size of thefemale shank25 and the corresponding size of thehole90 drilled through thespinous process24 are chosen to minimize stress concentration and minimize the loss of bone integrity. A counter-bore92 may be used to flatten and thereby maximize the contact surface area of thehead323/326 to bone interface, as illustrated inFIG. 6D.
• The materials of thefastener300 andbushing320 may comprise any suitable implantable material capable of withstanding high fatigue. For example, all components could be comprised of300 or400 series stainless steel, titanium alloy6-4, or MP35N alloy. Preferably, all components would be made of the same or similar material to reduce galvanic corrosion. The surfaces of thefastener300 andbushing320 that engage theattachment members102 of thedynamic bias device100 are preferably smooth to reduce friction and wear. The surfaces of thebushing320 that engage the vertebrae may have a roughened surface (e.g., knurled) to reduce the likelihood of relative movement therebetween. In addition, the surfaces of thebushing320 that engage the vertebrae may have a porous sintered surface to facilitate solid bone growth, thereby further securing thebushing320. Coatings and surface treatments may be utilized to reduce or increase friction where desired, and biological response where tissue interface is likely.
• With reference toFIGS. 7A-7B, end and exploded views, respectively, of abushing330 are illustrated. Except as described herein,bushing330 is substantially the same in design, function and use asbushing320.Bushing330 is adapted to mount one or (preferably) twodynamic bias devices100.Bushing330 is particularly suitable for attachment points C and J as illustrated inFIG. 1B. Bushing330 may be attached to thevertebrae20/30 utilizing aconventional bone screw310, which may be modified in diameter, length and thread type for the particular attachment site and condition.
• Bushing330 includes twomale fittings331 which fit into afemale fitting334. Themale fittings331 each include ashank portion332 and ahead portion333. Similarly, thefemale fitting334 includes twoshank portions335 and twohead portions336. Thefemale fitting334 has an inside diameter sized to accommodate theshanks332 of themale fittings331, and themale fittings331 have an inside diameter sized to accommodate thebolt302 of thefastener300. The outside surfaces of theshanks332 of themale fittings331 and the inside surfaces of theshanks335 of thefemale fitting334 may include mating threads. When fully assembled, theattachment members102 are disposed around theshanks335 on either side ofheads336 of thefemale fitting334 and between theheads333 of themale fittings331.
• With reference toFIGS. 8A-8B, end and exploded views, respectively, of abushing340 are illustrated. Except as described herein,bushing340 is substantially the same in design, function and use asbushing330.Bushing340 is adapted to mount onedynamic bias device100.Bushing340 is particularly suitable for attachment points C, D, E, F, G, J, K, L, M and N, but may also be used for attachment points A, B, H and I as illustrated inFIGS. 1A and 1B. Bushing340 may be attached to thevertebrae20/30 utilizing aconventional bone screw310, which may be modified in diameter, length and thread type for the particular attachment site and condition.
• Bushing340 includes amale fitting341 which fits into afemale fitting344. Themale fitting341 includes ashank portion342 and ahead portion343. Similarly, thefemale fitting344 includes ashank portion345 and ahead portion346. Thefemale fitting344 also includes aflange347 connecting thebone screw310 to thebushing340. Thefemale fitting344 has an inside diameter sized to accommodate theshank342 of themale fitting341, and themale fitting341 has an inside diameter sized to accommodate thebolt302 of thefastener300. The outside surface of theshank342 of themale fitting341 and the inside surface of theshank345 of thefemale fitting344 may include mating threads. When fully assembled, theattachment member102 is disposed around theshank345 on thefemale fitting344 and between theheads343/346 of thefittings341/344. When mounted, the axis of theshank345 ofbushing340 is oriented parallel to the mounting surface.
• With reference toFIGS. 9A-9B, end and exploded views, respectively, of abushing350 are illustrated. Except as described herein,bushing350 is substantially the same in design, function and use asbushing340.Bushing350 is adapted to mount onedynamic bias device100.Bushing350 is particularly suitable for attachment points C, D, E, F, G, J, K, L, M and N, but may also be used for attachment points A, B, H and I as illustrated inFIGS. 1A and 1B. Bushing350 may be attached to thevertebrae20/30 utilizing aconventional bone screw310, which may be modified in diameter, length and thread type for the particular attachment site and condition. In this particular embodiment, thefastener300 is formed integrally with thebone screw310.
• Bushing350 includes amale fitting351 which fits into afemale fitting354. Themale fitting351 includes ashank portion352 and ahead portion353. Similarly, thefemale fitting354 includes ashank portion355 and ahead portion356. Thefemale fitting354 has an inside diameter sized to accommodate theshank352 of themale fitting351, and themale fitting351 has an inside diameter sized to accommodate thebolt302, which is integral with thebone screw310. The outside surface of theshank352 of themale fitting351 and the inside surface of theshank355 of thefemale fitting354 may include mating threads. When fully assembled, theattachment member102 is disposed around theshank355 on thefemale fitting354 and between theheads353/356 of thefittings351/354. When mounted, the axis of theshank355 ofbushing350 is oriented perpendicular to the mounting surface.
• With reference toFIGS. 10A-10C, side views of adynamic bias device110 are illustrated in a no-load condition, in a compression load condition, and in cross-section, respectively. Except as described herein,dynamic bias device110 is substantially the same in design, function and use as thegeneric device100 described previously.Dynamic bias device110 includes abarrel111 in whichpiston112 is slidably disposed. A bias member in the form of aspring113 is disposed in thebarrel111. Longitudinal displacement of thebarrel111 relative to thepiston112 causes compression (or extension) of thespring113. Thespring113 provides a bias force which increases (or decreases) linearly with displacement as discussed previously. A flexible housing (not shown) may be placed about thedynamic bias device110 to isolate the movingparts111/112 from the surrounding tissue when implanted.
• Anadjustable arm114 may be connected to thepiston112. Thearm114 and thebarrel111 includeholes115 or other suitable attachment members, which may be used incombination bushings320,330,340 and350, to attach thedynamic bias device110 to the vertebrae. Theadjustable arm114 and thepiston112 may include mating threads such that rotation of thearm114 causes thearm114 to effectively lengthen or shorten thepiston112. This allows the distance between theholes115 to be varied to accommodate different attachment locations and different anatomies. This also allows the dynamic bias device to be preloaded by extending the effective length of thepiston112 beyond the distance between attachment points.
• Acollar116 is provided to limit the extended length of thedynamic bias device110. Thecollar116 may include threads that mate with threads inside thebarrel111 such that thecollar116 is adjustable, and thus the extended length is adjustable. Thecollar116 may also include an elastomeric bumper pad to dampen impact between thepiston112 and the collar when thedevice110 is filly extended. Similarly, aelastomeric bumper pad117 may be provided in thebarrel111 to dampen impact between thepiston112 and thebarrel111 when thedevice110 is filly collapsed.
• With reference toFIG. 11A, a cross-sectional view of adynamic bias device120 is illustrated. Except as described herein,dynamic bias device120 is substantially the same in design, function and use asdynamic bias device110 discussed with reference toFIGS. 10A-10C.Dynamic bias device120 includes abarrel121 in whichpiston122 is slidably disposed. Abias member123 in the form of a compressed or evacuated fluid (liquid or gas or a combination of both) is disposed in thebarrel121 and sealed relative to thepiston122 bypiston ring128. Thebarrel121 andpiston122 may define a closed volume or anexhaust reservoir129 may be used as shown. Thebias fluid123 is in fluid communication with theexhaust reservoir129 by way of an exhaust port through the wall of thebarrel121. Theexhaust reservoir129 may comprise an expandable annular bag as shown, or other suitable structure. If a closed volume is used, longitudinal displacement of thebarrel121 relative to thepiston122 simply causes a change in pressure of thefluid123. If anexhaust reservoir129 is used as shown, longitudinal displacement of thebarrel121 relative to thepiston122 causes a change in pressure of the fluid123 and flow offluid123 into theexhaust reservoir129 via the exhaust port. The pressure of the fluid123 and the size of the exhaust hole dictates the bias force which increases (or decreases) with the time derivative of displacement as discussed previously.
• A flexible housing (not shown) may be placed about thedynamic bias device130 to isolate the movingparts121/122 from the surrounding tissue when implanted. The housing may be used to define theexhaust reservoir129. Anadjustable arm124 may be connected to thepiston122. Thearm124 and thebarrel121 includeholes125 or other suitable attachment members to attach thedynamic bias device120 to the vertebrae. Theadjustable arm124 and thepiston122 may include mating threads to effectively lengthen or shorten thepiston122. Anadjustable collar126 may be provided including mating threads such that thecollar126 is adjustable, and thus the extended length of thedynamic bias device120 is adjustable. Thecollar126 may include an elastomeric bumper pad (not shown) and anelastomeric bumper pad127 may be provided in thebarrel121 to dampen impact between thepiston122 and thebarrel121.
• With reference toFIG. 11B, a cross-sectional view of adynamic bias device130 is illustrated. Except as described herein,dynamic bias device130 is substantially the same in design, function and use as the combination ofdynamic bias device110 discussed with reference toFIGS. 10A-10C anddynamic bias device120 described with reference toFIG. 11A.
• Dynamic bias device130 includes abarrel131 in whichpiston132 is slidably disposed. A bias member is the form of aspring133A is disposed in thebarrel131. Longitudinal displacement of thebarrel131 relative to thepiston132 causes compression (or extension) of the spring133. The spring133 provides a bias force which increases (or decreases) linearly with displacement as discussed previously. In addition, abias member133B in the form of a compressed or evacuated fluid (liquid or gas) is disposed in thebarrel131 and sealed relative to thepiston132 bypiston ring138.
• Thebarrel131 andpiston132 may define a closed volume or anexhaust reservoir129 may be used as shown. The bias fluid133 is in fluid communication with theexhaust reservoir139 by way of an exhaust port through the wall of thebarrel131. Theexhaust reservoir139 may comprise an expandable annular bag as shown, or other suitable structure. If a closed volume is used, longitudinal displacement of thebarrel131 relative to thepiston132 simply causes a change in pressure of the fluid133. If anexhaust reservoir139 is used as shown, longitudinal displacement of thebarrel131 relative to thepiston132 causes a change in pressure of the fluid133 and flow of fluid133 into theexhaust reservoir139 via the exhaust port. The pressure of the fluid133 and the size of the exhaust hole dictates the bias force which increases (or decreases) with the time derivative of displacement as discussed previously. Thus, thebias members133A/133B effectively act as a combined spring and damper.
• A flexible housing (not shown) may be placed about thedynamic bias device130 to isolate the movingparts131/132 from the surrounding tissue when implanted. The housing may be used to define theexhaust reservoir139. Anadjustable arm134 may be connected to thepiston132. Thearm134 and thebarrel131 includeholes135 or other suitable attachment members to attach thedynamic bias device130 to the vertebrae. Theadjustable arm134 and thepiston132 may include mating threads to effectively lengthen or shorten thepiston132. Anadjustable collar136 may be provided including mating threads such that thecollar136 is adjustable, and thus the extended length of thedynamic bias device130 is adjustable. Thecollar136 may include an elastomeric bumper pad (not shown) and anelastomeric bumper pad137 may be provided in thebarrel131 to dampen impact between thepiston132 and thebarrel131.
• With reference toFIGS. 12A-12B, rear and side views ofdynamic bias device140 are illustrated in no-load condition.FIG. 12C illustrates thedynamic bias device140 subjected to a compression load. Except as described herein,dynamic bias device140 is substantially the same in design, function and use as thegeneric device100 described previously. Although movement of thedynamic bias device140 in compression and extension is substantially linear and parallel to the axis ofcurvature60, as withdynamic bias device100, some lateral or posterior-anterior motion is present, but preferably minimized.Dynamic bias device140 includesbias member142 andloops144 or other suitable attachment members, which may be used incombination bushings320,330,340 and350, to attach thedynamic bias device140 to the vertebrae. Thebias member142 is may be a semi-circular or semi-elliptical leaf spring, which may be a single plate as shown or a series of laminated plates. Relative longitudinal displacement of theattachment members144 causes compression (or extension) of theleaf spring142. Theleaf spring142 provides a bias force which increases (or decreases) with displacement as discussed previously.
• The radius or axis of curvature of theleaf spring142 is preferably maximized such that displacement of theattachment members144 is substantially linear, but should not be so high as to result in buckling or inversion in compression. By way of example, not limitation, the radius or axis of curvature may range from half the distance between the attachment points to approximately 10 cm. Of course, half the distance between attachment points will vary depending on the location of each attachment point, but will likely be in the range of 1.0 to 3.0 cm for attachment points between adjacent processes.
• The displacement of the apex143 is preferably of theleaf spring142 minimized to minimize disturbance of and interference from surrounding tissue (bone, muscle, connective tissue, nerves, etc.). The apex143 may face anteriorly, but preferably faces posteriorly or laterally to reduce interference with tissue close to the spinal column. Thedynamic bias device140, and particularly theleaf spring142, is preferably disposed laterally or posteriorly of thespinous processes24 to avoid interference with movement of the vertebrae.
• With reference toFIGS. 13A-13C, various alternativedynamic bias devices150,160 and170 are illustrated in side and posterior views. Except as described herein,dynamic bias devices150,160 and170 are substantially the same in design, function and use as thedynamic bias device140 discussed with reference toFIGS. 12A-12C.
• Dynamic bias device150 as seen inFIG. 13A includesbias member152 in the form of an articulated leaf spring, andattachment members154. Articulatedleaf spring152 reduces the horizontal range of movement by utilizing a plurality ofarticulations153 having a smaller radius or axis of curvature. The reduced horizontal range of movement of thebias member152 reduces the amount of disturbance and interference from surrounding tissue (bone, muscle, connective tissue, nerves, etc.).
• Dynamic bias device160 as seen inFIG. 13B includes a plurality ofbias members162 in the form of leaf springs (shown) or articulated leaf springs (not shown), andattachment members164. Utilizing a plurality ofleaf springs162 increases stability of thedynamic bias device160 and allows for greater net bias forces to be delivered to theattachment members164 and the vertebrae attached thereto.
• Dynamic bias device170 as seen inFIG. 13C includes a plurality ofbias members172 in the form of leaf springs (shown) or articulated leaf springs (not shown). Thedynamic bias device170 also includesattachment members174 in the form of inverted semi-circular loops. The invertedsemi-circular loops174 permit thedevice170 to be attached to the inferior and superior sides spinous processes of adjacent vertebrae, such that theattachment members174 are disposed between adjacent spinous processes but thebias members172 are disposed laterally of the spinous processes to avoid interference with movement of the vertebrae.
• Dynamic bias devices110,120,130,140,150,160 and170 may be used (i.e., implanted) substantially as described with reference to genericdynamic bias device100. As mentioned previously, one ormore reinforcement members200 may be used in combination with one or moredynamic bias devices100. Thereinforcement members200 may be used to reinforce the disc, restore disc height and/or bear some or all of the load normally carried by the annulus. Thereinforcement members200 are relatively rigid and thus serve to reinforce thedisc50, and particularly theannulus52, where inserted. In addition, thereinforcement members200 may have a relatively large profile when implanted and thus increase disc height.
• The reinforcingmembers200 may be used singularly or in groups, depending on the increase indisc50 height desired and/or the amount of reinforcement of theannulus52 desired. For example, the reinforcingmembers200 may be stacked as illustrated inFIG. 2B or inserted side-by-side as illustrated inFIG. 15R. In addition, the reinforcingmembers200 may be located in virtually any portion of theannulus52. Preferably, the reinforcingmembers200 are substantially symmetrically disposed about themedian plane70 to avoid causing curvature of thespine10. Although the reinforcingmembers200 may be inserted, in part or in whole, into thenucleus54, it is preferable to insert them into theannulus52 for purposes of stability and load carrying. Specifically, to provide stability, it is desirable to symmetrically locate the reinforcingmembers200 as far as reasonably possible from themedian plane70, or to span as great a distance as possible across themedian plane70. In addition, because theannulus52 of thedisc50 is believed to carry the majority of the load, particularly in thelumbar region12, the reinforcingmembers200 are preferably placed in theannulus52 to assume the load normally carried thereby, and reinforce the load bearing capacity of theannulus52, without hindering the normal mobility function of thedisc50.
• The reinforcingmembers200 may comprise expandable members such as self-expandingmembers210 orinflatable members220. Alternatively, the reinforcingmembers200 may comprise unexpandable members such as reinforcement bars230. When implanting each type ofreinforcement member210/220/230, it is preferable to maintain the integrity of theannulus52. Accordingly, space in theannulus52 for the reinforcingmembers200 is preferably established by dilation or the like, although some amount of tissue removal may be used.
• Theexpandable reinforcement members210/220 are useful because they may be delivered in a low profile, unexpanded condition making it easier to traverse the very tough and fibrous collagen tissue of theannulus52. For similar reasons, the reinforcement bars230 are useful because they may have a small diameter and a sharpened tip. Although it is possible to insert theexpandable reinforcing members210/220 into theannulus52 in their final expanded state, it is desirable to deliver theexpandable reinforcing members210/220 into theannulus52 in an unexpanded state and subsequently expand them in order to minimize invasiveness and resistance to insertion.
• The self-expanding reinforcingmember210 may comprise a solid or semi-solid member that self-expands (e.g., by hydration) after insertion into the annulus. Examples of suitable materials for such solid or semi-solid members include solid fibrous collagen or other suitable hard hydrophilic biocompatible material. If the selected material is degradable, the material may induce the formation of fibrous scar tissue which is favorable. If non-degradable material is selected, the material must be rigid and bio-inert. The self-expanding reinforcingmember210 preferably has an initial diameter that is minimized, but may be in the range of 25% to 75% of the final expanded diameter, which may be in the range of 0.3 to 0.75 cm, or 10% to 75% of the nominal disc height. The length of the self-expandingmember210 may be in the range of 1.0 to 6.0 cm, and preferably in the range of 2.0 to 4.0 cm.
• The inflatable reinforcingmember220 may comprise an expandable hollow membrane capable of inflation after insertion into the annulus. An example of a suitable inflatable structure is detachable balloon membrane filled with a curable material. The membrane may consist of a biocompatible and bio-inert polymer material, such as polyurethane, silicone, or polycarbonate-polyurethane (e.g., Corethane). The curable filler material may consist of a curable silicone or polyurethane. The filler material may be curable by chemical reaction (e.g., moisture), photo-activation (e.g., UV light) or the like. The cure time is preferably sufficiently long to enable activation just prior to insertion (i.e., outside the body) and permit sufficient time for navigation and positioning of themember220 in the disc. However, activation may also take place inside the body after implantation. The inflatable reinforcingmember220 preferably has an initial deflated diameter that is minimized, but may be in the range of 25% to 75% of the final inflated diameter, which may be in the range of 0.3 to 0.75 cm, or 10% to 75% of the nominal disc height. The length of theinflatable member220 may be in the range of 1.0 to 6.0 cm, and preferably in the range of 2.0 to 4.0 cm.
• The reinforcement bars230 may comprise a rigid, solid or hollow bar having a sharpened tip. The reinforcement bars230 may comprises stainless steel mandrels, for example, having a diameter in the range of 0.005 to 0.100 inches, preferably in the range of 0.010 to 0.050 inches, and most preferably in the range of 0.020 to 0.040 inches, and a length in the range of 1.0 to 6.0 cm, and preferably in the range of 2.0 to 4.0 cm. The reinforcement bars230 may be straight for linear insertion, or curved to gently wrap with the curvature of the annulus during insertion. In addition, the outer surface of the reinforcement bars230 may have circular ridges or the like that the permit easy insertion into theannulus52 but resist withdrawal and motion in the annulus following implantation. Other suitable materials for reinforcement bars230 include titanium alloy 6-4, MP35N alloy, or super-elastic nickel-titanium alloy.
• Referring now toFIGS. 14A-14D,various tools410,420 and430 are shown individually and assembled. Thetools410,420 and430 may be used to implant thereinforcement devices210/220/230 discussed above. The tools include a rigid, sharpened,hollow needle410, a semi-rigid, sharpened, hollowcurved needle420, and a sharpenedstylet430. As seen inFIG. 14D, the sharpenedstylet430 fits into thesemi-rigid needle420 which fits into therigid needle410.
• With specific reference toFIG. 14A, the rigidhollow needle410 includes ahollow shaft412 and a grip or handle414. Theshaft412 includes a sharpenedtip413 to facilitate insertion into and pass through the surrounding tissue. Theshaft412 is preferably made of a rigid metal such as a stainless steel hypodermic tube. Thegrip414 may comprise a polymer and may be formed by insert injection molding with theshaft412 inserted into the mold.
• With specific reference toFIG. 14B, the semi-rigidcurved needle420 includes a hollow shaft422 ahub424. Theshaft422, which includes a sharpenedtip423, is longer than therigid needle410 and has an outside diameter sufficiently small to fit into therigid needle410. Theshaft422 is preferably made of a semi-rigid polymer or composite. Theshaft422 includes a curveddistal portion426 that may be straightened (shown in phantom) upon insertion of thesemi-rigid needle420 into the lumen of therigid needle410. Thehub424 may include a fitting425 to facilitate connection to a fluid source or a pressure source (e.g., a syringe).
• With specific reference toFIG. 14C, the sharpenedstylet430 includes aflexible shaft432 and a sharpeneddistal end433. Theshaft432 is longer than the both therigid needle410 and thesemi-rigid needle420, and may have a length on the order of 10 to 60 cm. Theshaft432 also has an outside diameter sufficiently small to fit into thesemi-rigid needle420. Theshaft422 preferably has a flexible but pushable construction incorporating a rigid metal such as stainless steel, or super-elastic nickel-titanium alloy. The sharpenedstylet430 is preferably highly elastic, to resist permanent set upon insertion into thecurved portion426 of thesemi-rigid needle420.
• With general reference toFIGS. 15A-15J, the steps for implanting a self-expandingreinforcement member210 are illustrated. It should be understood that the procedure for implanting asingle member210 in theanterior annulus52 is shown for purposes of illustration, not limitation. All of the variables with regard to quantity, location, orientation, etc. discussed previously may be implemented by varying the generic procedure described hereinafter.
• Initially, the sharpenedstylet430,semi-rigid needle420 andrigid needle410 are assembled as shown inFIG. 14D. As shown inFIG. 15A, the distal portion of theassembly410/420/430 is inserted into thedisc50 as in a conventional discogram procedure. Theassembly410/420/430 is advanced until thedistal tip413 of the rigid needle is proximate the anterior curvature of theannulus52, near the anterior side of thenucleus54, as seen inFIG. 15B. The semi-rigid needle420 (alone or with stylet430) is advanced relative to therigid needle410 until thecurved portion426 of the semi-rigid needle exits thedistal tip413 of therigid needle410 and the desired amount of curvature is established, as seen inFIG. 15C. Thecurved portion426 may be advanced until thetip423 is substantially parallel to the tangent of theanterior annulus52 curvature. The sharpenedstylet430 is advanced relative to thesemi-rigid needle420 to the desired position within theanterior annulus52, as shown inFIG. 15D. Thesemi-rigid needle420 and therigid needle410 are completely withdrawn from thestylet430, leaving the stylet in position as shown inFIG. 15E.
• Aflexible dilator440 is advanced over thestylet430 to dilate theannulus52, as seen inFIG. 15F. Theflexible dilator440 is similar tosemi-rigid needle420 except that the dilator includes a blunt distal tip and is relatively more flexible, and has larger inner and outer diameters. Note that one ormore dilators440 may be advanced co-axially about thestylet430 until the annulus is sufficiently dilated to accept the self-expandable member210. Thestylet430 is then withdrawn from theflexible dilator440 and the self-expandable member210 is introduced into the lumen of theflexible dilator440 using apush bar450, as shown inFIG. 15G. Alternatively, thedilator440 may be removed in favor of a flexible hollow catheter with a large inner diameter to facilitate delivery ofmember210. Thepush bar450 is similar tostylet430 except that the distal tip of thepush bar450 is blunt. Alternatively, thepush bar450 may simply comprise thestylet430 turned around, thus using the proximal blunt end of thestylet430 as thepush bar450. Thepush bar450 is advanced until themember210 is in the desired position, as seen inFIG. 15H. To facilitate positioning themember210, radiographic visualization may be used to visualize the distal end of thepush bar450, which is formed of radiopaque material and may include radiopaque markers. In addition, the member may be loaded with a radiopaque material to facilitate radiographic visualization thereof.
• After themember210 is in the desired position, theflexible dilator440 is retracted from thepush bar450 while maintaining position of themember210 with the push bar. Thepush bar450 is then removed leaving themember210 in place. If necessary, the procedure may be repeated foradditional member implants210. Themember210 is then allowed to expand over time, perhaps augmented by placing thespine10 in traction. Alternatively, thespine10 may be placed in traction prior to beginning the procedure as discussed with reference to the procedure for implantingdynamic bias device100.
• With reference toFIGS. 15K-15L, the steps for implanting aninflatable reinforcement member220 are illustrated. In this procedure, the steps outlined with reference toFIGS. 15A- 15F are followed. Specifically, the same steps are followed up to and including the step of advancing theflexible dilator440 over thestylet430 to dilate theannulus52, and thereafter removing thestylet430 from theflexible dilator440. Using acatheter460, theinflatable member220 is introduced into thedilator440 and advanced until themember220 is in the desired position, as shown inFIG. 15K. Theinflatable member220 is connected to the distal end of thecatheter460, which includes a flexible butpushable shaft462 and aninflation port464. Theflexible dilator440 is retracted from thecatheter460 while maintaining position of themember220.
• With themember220 in the desired position, which may be confirmed using radiographic visualization as described above, theproximal inflation port464 is connected to a syringe (not shown) or other suitable inflation apparatus for injection of the curable filler material. The filler material is then activated and the desired volume is injected into thecatheter460 via theinflation port464, as seen ifFIG. 15L. The filler material is allowed to cure and thecatheter460 is gently torqued to break thecatheter460 from thesolid member220. This break-away step may be facilitated by an area of weakness at the juncture between the distal end of thecatheter460 and the proximal end of themember220. Thecatheter460 is then removed leaving themember220 in place. If necessary, the procedure may be repeated foradditional member implants220.
• With reference toFIGS. 15M-15R, the steps for implanting areinforcement bar230 are illustrated. As seen inFIG. 15M, thedisc50 includes a protrusion orbulge56, which is preferably, but not necessarily, reduced or eliminated before insertion of thereinforcement bar230. This may be done by separating theadjacent vertebrae20. In order to establish separation of thevertebrae20, thespine10 may be placed in traction or conventional intervertebral separation tools may be used. After thebulge56 is reduced or eliminated, similar steps are followed as outlined with reference toFIGS. 15A-15C.
• Delivery of asingle reinforcement bar230 into theposterior annulus52 is illustrated. Specifically, the distal portion of theassembly410/420/450 is inserted into thedisc50 as in a conventional discogram procedure. Theassembly410/420/450 is advanced until thedistal tip413 of therigid needle410 just penetrates the posterior side of theannulus52, as seen inFIG. 15N. The semi-rigid needle420 (alone or with bar230) is advanced relative to therigid needle410 until thecurved portion426 of thesemi-rigid needle420 exits thedistal tip413 of therigid needle410 and the desired amount of curvature is established, as shown inFIG. 15N. Thecurved portion426 may be advanced until thetip423 is substantially parallel to theposterior annulus52.
• Using thepush bar450, thereinforcement bar230 with its sharpened tip is pushed into theannulus52 as seen inFIG. 150. Thereinforcement bar230 is advanced into theannulus52 with thepush bar450 until thebar230 is in the desired position, as seen inFIG. 15P, which may be confirmed using radiographic visualization as described above. Thepush bar450 is then retracted, leaving thereinforcement bar230 in place, as shown inFIG. 15P. Thesemi-rigid needle420 and therigid needle410 are then removed, as shown inFIG. 15Q, or, if necessary, the procedure may be repeated for additionalreinforcement bar implants230, as shown inFIG. 15R. Presence of the reinforcement bars230 serves to keep thedisc50, and particularly thebulge56, in a more normal condition, and to protect against continued bulging, thus easing nerve impingement.
• From the foregoing, those skilled in the art will appreciate that the present invention providesdynamic bias devices100,110,120,130,140,150,160, and170, in addition toreinforcement devices210,220, and230, which may be used individually or in combination, to eliminate nerve impingement associated with a damageddisc50, and/or to reinforce a damaged disc, while permitting relative movement of thevertebrae20_Sand20_Iadjacent the damaged disc. The present invention also provides minimally invasive methods of implanting such devices as described above.
• Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail may be made without departing from the scope and spirit of the present invention as described in the appended claims.

Claims (20)

1. A method of treating pain associated an intervertebral disc disposed between a first vertebra and a second vertebra in an adult human spine having a median plane and an axis of curvature, the method comprising the steps of:

identifying the intervertebral disc causing pain;

providing one or more implantable devices, each having a first vertebral attachment member, a second vertebral attachment member, and a bias force member applying a bias force therebetween while permitting near normal relative motion therebetween;

attaching the first attachment member of each implantable device to the first vertebra; and

attaching the second attachment member of each implantable device to the second vertebra such that the bias force members exert a net bias force balanced about the median plane to increase separation of the first and second vertebrae.

2. A method as inclaim 1, wherein the first and second vertebrae have spinous processes, and wherein the implantable device is disposed posterior of the spinous processes.

3. A method as inclaim 1, wherein the first and second vertebrae have spinous processes, and wherein the implantable devices are disposed lateral of the spinous processes.

4. A method as inclaim 1, wherein the bias force member of each implantable device is pre-loaded with a compression load prior to the step of attaching the second attachment member.

5. A method as inclaim 1, wherein the first and second vertebrae are partially separated prior to the step of attaching the second attachment member.

6. A method as inclaim 1, wherein the intervertebral disc has a protrusion, further comprising the step of cutting away at least a portion of the protrusion.

7. A method as inclaim 1, wherein the intervertebral disc has an annulus, further comprising the steps of:

providing an implantable reinforcement member; and

implanting the reinforcement member into the annulus of the intervertebral disc.

8. An implantable system for treating an intervertebral disc disposed between a first vertebra and a second vertebra in an adult human spine having a median plane and an axis of curvature, the system comprising:

a first vertebral attachment member;

a second vertebral attachment member; and

a bias member applying a net bias force between the first vertebral attachment member and the second vertebra attachment member, wherein the bias member permits near normal relative motion between the first and second vertebrae, and the net bias force is substantially symmetric about the median plane.

9. An implantable system as inclaim 8, wherein the bias member has a substantially linear displacement.

10. An implantable system as inclaim 9, wherein the displacement is substantially parallel to the axis of curvature of the first and second vertebrae.

11. An implantable system as inclaim 8, wherein the bias member has a range of displacement and the bias member applies substantially no force over a portion of the range of displacement.

12. An implantable system as inclaim 8, wherein the bias member has a range of displacement greater than1.0 cm.

13. An implantable system as inclaim 8, wherein the bias member includes a spring.

14. An implantable system as inclaim 8, further including a bushing to isolate movement of the attachment members from the vertebrae.

15. A method of reinforcing an annulus of an intervertebral disc disposed between a first vertebra and a second vertebra in an adult human spine, the method comprising the steps of:

providing an implantable reinforcement member; and

implanting the reinforcement member into the annulus of the intervertebral disc.

16. A method as inclaim 15, further comprising the step of partially separating the adjacent vertebrae prior to the step of implantation.

17. A method as inclaim 15, wherein a plurality of implantable reinforcement members are provided, and wherein the plurality of reinforcement members are implanted substantially parallel in the annulus.

18. A method as inclaim 15, wherein the reinforcement member is expandable, further comprising the step of expanding the reinforcement member.

19. A method as inclaim 15, further comprising the steps of:

providing a tubular member having a curved tip and a lumen extending therethrough;

inserting the tip of the tubular member into the annulus; and

inserting the reinforcement member through the lumen of the tubular member and into the annulus.

20. A method as inclaim 15, further comprising the steps of:

providing one or more implantable bias devices, each having a first vertebral attachment member, a second vertebral attachment member, and a bias force member applying a bias force therebetween while permitting near normal relative motion therebetween;

attaching the first attachment member of each implantable bias device to the first vertebra; and

attaching the second attachment member of each implantable bias device to the second vertebra.

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