Patent Application
20210007783
The present invention relates generally to medical methods and apparatus. More particularly, the present invention relates to methods and devices for restricting spinal flexion in patients having back pain or other spinal conditions.
A major source of chronic low back pain is discogenic pain, also known as internal disc disruption. Patients suffering from discogenic pain tend to be young, otherwise healthy individuals who present with pain localized to the back. Discogenic pain usually occurs at the lower lumbar discs of the spine (
Such discogenic low back pain can be thought of as flexion instability and is related to flexion instability that is manifested in other conditions. The most prevalent of these is spondylolisthesis, a spinal condition in which abnormal segmental translation is exacerbated by segmental flexion.
Current treatment alternatives for patients diagnosed with chronic discogenic pain are quite limited. Many patients follow a conservative treatment path, such as physical therapy, massage, anti-inflammatory and analgesic medications, muscle relaxants, and epidural steroid injections, but typically continue to suffer with a significant degree of pain. Other patients elect to undergo spinal fusion surgery, which commonly requires discectomy (removal of the disk) together with fusion of adjacent vertebrae. Fusion is not usually recommended for discogenic pain because it is irreversible, costly, associated with high morbidity, and of questionable effectiveness. Despite its drawbacks, however, spinal fusion for discogenic pain remains common due to the lack of viable alternatives.
An alternative method, that is not commonly used in practice, but has been approved for use by the FDA, is the application of bone cerclage devices that can encircle the spinous processes or other vertebral elements and thereby create a restraint to motion. Physicians typically apply a tension or elongation to the devices that applies a constant and high force on the anatomy, thereby fixing the segment in one position and allowing effectively no motion. The lack of motion allowed after the application of such a device is thought useful to improve the likelihood of fusion performed concomitantly; if the fusion does not take, these devices will fail through breakage of the device or of the spinous process to which the device is attached. These devices are designed for static applications and are not designed to allow for a dynamic elastic resistance to flexion across a range of motion. The purpose of bone cerclage devices and the other techniques described above is to almost completely restrict measurable motion of the vertebral segment of interest. This loss of motion at a given segment gives rise to abnormal loading and motion at adjacent segments, leading eventually to adjacent segment morbidity.
Recently, a less invasive and potentially more effective treatment for discogenic pain has been proposed. A spinal implant has been designed which inhibits spinal flexion while allowing substantially unrestricted spinal extension. The implant is placed over one or more adjacent pairs of spinous processes and provides an elastic restraint to the spreading apart of the spinous processes which occurs during flexion. Such devices and methods for their use are described in U.S. Pat. No. 7,458,981, which has common inventors with the present application.
Implants used for applying elastic constraint to spinal segments as taught in the '981 patent must meet two generally conflicting objectives. First, the implants must be very robust and have a high fatigue strength since they will be subjected to use over millions of cycles after implantation as the patient goes about daily life. For a given desired stiffness, the principle way in which to increase fatigue strength is to increase the size of the implant. Generally, however, smaller implants having a lower profile are easier to implant, are better tolerated by the patient, and lead to fewer complications.
In addition to the limitations on size and strength, as discussed above, implantable elastic constraints as described in the '981 patent can, in some cases, allow excess flexion despite the elastic constraint which is applied as the spinous processes of the spinal segment move apart. Should such excess flexion occur, the patient can experience pain and the implant itself can experience greater stress and fatigue than intended.
For these reasons, it would be desirable to provide improved spinal implants and methods for their use in inhibiting flexion in patients suffering discogenic pain. It would be particularly desirable if such improved devices would be robust in use with very high fatigue strengths while having a minimum size and correspondingly reduced implantation profile. It would be further desirable if, in addition to the strength and size characteristics, the elastic constraints were to inhibit or prevent excess flexion of the treated spinal segment during use. At least some of these objectives will be met by the invention as described herein below.
U.S. Pat. No. 7,458,981 has been described above. US 2005/0192581 describes an orthopedic tether which can have a stiffness from at least 1 N/mm to at least 200 N/mm and which can be used for many purposes, including wrapping spinous processes. U.S. 2008/0312693 describes a spine stabilization unit comprising a spring with an internal motion limit. Other patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO 2004/052246 A1; WO 2004/073532 A1; and Published Foreign Application Nos. EP 0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. S186-S191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.
The present invention provides methods and apparatus for relieving symptoms of lumbar pain associated with flexion of a spinal segment of a patient. The lumbar pain may arise from a variety of particular conditions such as those described previously herein. The devices and methods will dynamically limit flexion of at least one spine segment by increasing the bending stiffness of the spinal segment by a preselected amount, typically in the range from 0.1 Nm/deg to 2 Nm/deg, preferably from 0.4 Nm/deg to 1 Nm/deg. Usually, the bending stiffness is increased by coupling an elastic constraint between a superior spinous process and an inferior spinous process or between an L5 spinous process and a sacrum of the patient. The elastic constraint may have an effective elastic tensile stiffness in the range from 7.5 N/mm to 40 N/mm, where the constraint may be positioned at a distance in the range from 25 mm to 75 mm in a posterior direction from a center of rotation of the spinal segment. The “effective elastic tensile stiffness” is defined as the elastic tensile stiffness present between the inferior and superior attachment location resulting from the stiffness contributions of all elements or components of the elastic constraint. The bending stiffness will be increased during flexion (but not extension) of the spinal segment, usually being increased over the full range of flexion. The full flexion-extension range of motion of the spinal segment will typically be from 3° to 20°, usually from 5° to 15°. The flexion portion of the total range of motion of the spinal segment is expressed as an angle measured relative to the neutral position (defined below) and will typically be from 2° to 15°, usually from 4° to 10°. The bending stiffness will be increased over at least 75% of the full range flexion, usually over the full range of flexion as well as 25% of the extension range of motion.
In addition to dynamically limiting flexion of the at least one spine segment by increasing the segment's bending stiffness, the devices and methods of the present invention further provide for a “hard” limit or stop on flexion to both reduce the risk of the patient suffering from over flexion of the spinal segment and to reduce the mechanical load on the elastic constraint devices. Usually, the flexion limit is provided by a separate elongation limit or limiting element which is flexible but substantially non-distensible and which may be attached between vertically adjacent spinous processes (or an L5 process and a sacrum) in parallel with the elastic constraint. “Substantially non-distensible” as used in this application is defined as having a higher tensile stiffness than the compliance element; usually having a tensile stiffness which is at least twice that of the compliance element, and preferably having a tensile stiffness which is at least ten times that of the compliance element. The elongation limiting element will be attached so that there is some slack or excess length present when the spinal segment is in its neutral position. Thus, as the spinal segment initially undergoes flexion, the elastic constraint will provide the desired increase in bending stiffness while the elongation limit applies little or no force between the spinous processes. Once the spinal segment reaches the desired maximum flexion, however, the elongation limit will reach its maximum extension and prevent further separation of the spinous processes, thus protecting both the patient from over flexion and the elastic constraint from excess stress.
The elongation limit may have any one of a variety of configurations, but will usually comprise a tether, cord, or cable made from a very flexible but substantially non-distensible (i.e. very high tensile stiffness) material (as defined above) which prevents further separation of the spinous processes as soon as it goes taut. In an exemplary embodiment, the tether may consist of a single cord, e.g., formed from ultra high molecular weight polyethelene fibers, braids, cords, tubes, or the like. In alternative embodiments, the elongation limit may be incorporated into the elastic constraint so that the elongation limit is in its slack state when the elastic constraint is in its neutral position between the spinous processes of the spinal segment.
The preferred methods and systems of the present invention will provide minimum and preferably no elastic resistance to extension of the spinal segments. The preferred elastic constraint systems of the present invention will be coupled to the spinous processes via flexible straps which, by virtue of their placement around the spinous processes and their flexible nature, will impart no force to the spinous processes as they move together during extension. Furthermore, the implants of the present invention will usually be free from structure located between adjacent spinous processes, although in some cases structure may be provided where the structure does not substantially interfere with or impede the convergence of the spinous processes as the spine undergoes extension. While some small amount of elastic resistance to extension might be found, it will preferably be below 3 N/mm, more preferably below 1 N/mm, and usually below 0.5 N/mm.
Similarly, the preferred methods and systems of the present invention will provide a minimum and preferably no elastic resistance to lateral bending or rotation of the spinal segments. The preferred methods and systems of the present invention will usually be coupled to the spinous processes via flexible straps which, by virtue of their placement around the spinous processes and their flexible nature, make it very difficult for the preferred methods and systems of the present invention to provide any resistance to lateral bending or rotation. This is particularly true in the lumbar spine where the range of motion in rotation is usually limited to +−0.3°. While some small amount of elastic resistance to lateral bending or rotation might be found, it will preferably be small.
As used herein, the phrase “spinal segment” refers to the smallest physiological motion unit of the spine which exhibits mechanical characteristics similar to those of the entire spine. The spinal segment, also referred to as a “functional spinal unit” (FSU), consists of two adjacent vertebrae, the intervertebral disk, and all adjoining ligaments and tissues between them. For a more complete description of the spinal segment or FSU, see White and Panjabi, Clinical Biomechanics of the Spine, J.B. Lippincott, Philadelphia, 1990.
As used herein, “neutral position” refers to the position in which the patient's spine rests in a relaxed standing position. The “neutral position” will vary from patient to patient. Usually, such a neutral position will be characterized by a slight curvature or lordosis of the lumbar spine where the spine has a slight anterior convexity and slight posterior concavity. In some cases, the presence of the constraint of the present invention may modify the neutral position, e.g. the device may apply an initial force which defines a new neutral position having some small extension of the untreated spine. As such, the use of the term “neutral position” is to be taken in context of the presence or absence of the device. As used herein, “neutral position of the spinal segment” refers to the position of a spinal segment when the spine is in the neutral position.
As used herein, “segmental flexion” refers to the motion between adjacent vertebrae in a spinal segment as the patient bends forward. Referring to
As used herein, “segmental extension” refers to the motion of the individual vertebrae L as the patient bends backward and the spine extends from the neutral position illustrated in
As used herein, the phrases “elastic resistance” and “elastic stiffness” refer to an application of constraining force to resist motion between successive, usually adjacent, spinous processes such that increased motion of the spinous processes results in a greater constraining force. The elastic resistance or stiffness will, in the inventions described herein, inhibit motion of individual spinal segments by, upon deformation, generating a constraining force transmitted directly to the spinous processes or to one or more spinous process and the sacrum. The elastic resistance or stiffness can be described in units of stiffness, usually in units of force per deflection such as Newtons per millimeter (N/mm). The stiffness may be defined for a single component or compliance member or for the entire structure which may comprise more than one compliance member. In some cases, the elastic resistance will generally be constant (within .+−0.5%) over the expected range of motion of the spinous processes or spinous process and sacrum. In other cases, typically with elastomeric components as discussed below, the elastic resistance may be non-linear, potentially varying from 33% to 100% of the initial resistance over the physiologic range of motion. Usually, in the inventions described herein, the pre-operative range of motion of the spinous process spreading from the neutral or upright position to a maximum flexion-bending position will be in the range from 2 mm to 20 mm, typically from 4 mm to 12 mm. With the device implanted, the post-operative range of motion of the spinous process spreading from the neutral or upright position to a maximum flexion-bending position will be reduced and will usually be in the range from 1 mm to 10 mm, typically from 2 mm to 5 mm. Such spinous process spreading causes the device to undergo deformations of similar magnitude.
As used herein, the phrase “bending stiffness” is defined as the resistance of the spinal segment to bending. The incremental bending stiffness which is provided by the constraints of the present invention may be calculated based on the total elastic tensile stiffness (or elastic resistance) of the constraint circumscribing the spinous processes (or coupling the L5 spinous process to sacrum) and the distance or moment arm between a center of rotation (COR) of the spinal segment and the location at which the elastic constraint is located on the spinous processes. As used herein, the moment arm distance will be expressed in meters (m) and the elastic stiffness ES will be expressed in Newtons per millimeter (N/mm). In the preferred embodiments where two compliance members are positioned “in parallel,” the total elastic stiffness of the constraint structure will be twice the elastic stiffness of a single compliance member. The units of bending stiffness, as used herein, will be Newton-meters per degree (Nm/deg.). The increase in bending stiffness IBS provided by the constraint of the present invention can be calculated by the formula:
IBS=1000 ES×D2×)(7r/180° where the elastic stiffness ES of the device can be measured by testing the device on an Instron® or other tensile strength tester, and the moment arm length D can be measured from radiographs.
Alternatively, the increase in bending stiffness of a device could be measured directly by placement on a cadaveric spine segment or a suitable vertebral model. The bending stiffness of the spine segment could be measured with and without the elastic constraint and the increase in bending stiffness provided by the constraint would be the difference between the two values. It would also be possible to calculate the increase in bending stiffness by finite element analysis.
The bending stiffness increase can thus be adjusted by changing the tensile stiffness of the elastic constraint and/or the distance of the moment arm. For example, once the treating physician determines the location of the elastic constraint and the distance between that location and the center of rotation (COR), the physician can then choose an elastic constraint having an appropriate elastic tensile stiffness in order to achieve a target therapeutic increase in the bending stiffness. The location of the center of rotation and the distance of the moment arm can be determined from radiographic images of the target spinal segment, typically taken in at least two positions or postures, such as in flexion and in extension. Typically, the center of rotation will be an average or calculated value determined by measuring translational vectors between the two radiographic positions for two points on a vertebra. Such techniques are described in detail, for example, in Musculoskeletal Biomechanics. Paul Brinckmann, Wolfgang Frobin, Gunnar Leivseth (Eds.), Georg Thieme Verlag, Stuttgart, 2002; p. 105. It would also be possible to employ the instantaneous axis of rotation (IAR), which location varies depending on the degree of spinal flexion or extension. Generally, however, using the COR is preferred since it is a fixed and readily determined value, although the device may affect the location of the COR in some cases.
Thus, the bending stiffness applied by a constraining structure according to the present invention is increased when the spinal segment moves beyond the neutral position and will depend on several factors including the elastic characteristics of the constraining structure, the position of the constraining structure on the spinous processes, the dimensions of the constraining structure, and the patient's anatomy and movement. The constraining structure will usually be positioned so that the upper and lower tethers engage the middle anterior region of the spinous process (25 mm to 75 mm posterior of the COR), and the dimensions of the constraining structure will usually be adjusted so that the tethers are taut, i.e. free from slack, but essentially free from tension (axial load) when the spinal segment is in its neutral position, i.e., free from flexion and extension. As the segment flexes beyond the neutral position, the constraining structure will immediately provide an elastic resistance in the ranges set forth above.
In some cases, the dimensions and assembly of the construct will be selected so that the tethers and compliance members are slightly pre-tensioned even before the compliance members are under load. Thus, the constraining structure may apply a predetermined resistive force, typically in the range from 7.5N to 40N, as soon as the spinal segment flexes from the neutral position. In the absence of such pre-tensioning, the compliance members would apply a zero resistive force at the instant they are placed under load. In all cases, as the segment flexes beyond the treated neutral position, the constraining structure will provide increasing bending stiffness in the ranges set forth above.
Usually, the constraining structures will apply minimal or no bending stiffness when the spinal segment is in the neutral position. In some instances, however, it may be desirable to tighten the constraining structure over the spinous processes so that a relatively low finite bending stiffness force (typically in the range from 0.1 Nm/deg to 2 Nm/deg, usually from 0.4 Nm/deg to 1 Nm/deg) is applied even before flexion while the spinal segment remains at a neutral position. In this case, the additional stiffness afforded by the constraining structure will affect the entire flexion range of motion; as well as a portion of the untreated extension range of motion of the spinal segment.
The relative increase in bending stiffness afforded by the constraining structures of the present invention is advantageous because it allows the constraining structure to cause the treated segment to resist flexion sufficiently to relieve the underlying pain or instability with a reduced risk of injury from excessive force. In particular, the preferred bending stiffness ranges set forth above provide sufficient constraint to effect a significant change in flexion in the typical patient while allowing a significant safety margin to avoid the risk of injury. The bending stiffness provided by the constraints of the present invention will limit the separation of the spinous processes on the treated spinal segment which is desirable both to reduce flexion-related pain and spinal instability.
The resistance to flexion provided by the elastic constraints of the present invention may reduce the angular range-of-motion (ROM) relative to the angular ROM in the absence of constraint. Angular ROM is the change in angle between the inferior end plate of the superior vertebral body of the treated segment and the superior endplate of the inferior vertebral body of the treated segment when the segment undergoes flexion. Thus, the treatments afforded by the elastic constraints of the present invention will provide a relatively low angular ROM for the treated segment, but typically a ROM higher than that of a fused segment.
While the constraint structures of the present invention will limit flexion, it is equally important to note that in contrast to spinal fusion and immobilizing spinal spacers, the methods and devices of the present invention will allow a controlled degree of flexion to take place. Typically, the methods and devices of the present invention will allow a degree of flexion which is equal to at least about 20% of the flexion that would be observed in the absence of constraint, more typically being at least about 33%. By reducing but not eliminating flexion, problems associated with fusion, such as increased pain, vertebral degeneration, instability at adjacent segments, and the like, may be overcome.
The constraint structures of the present invention will act to restore the stiffness of a spinal segment which is “lax” relative to adjacent segments. Often a patient with flexion-related pain or instability suffers from a particular looseness or laxity at the painful segment. When the patient bends forward or sits down, the painful, lax segment will preferentially flex relative to the stiffer adjacent segments. By adjusting the length, position, or other feature of the devices of the present invention so that constraint structure is taut over the spinous processes when the spinal segment is in its neutral position, the stiffness of the treated segment can be “normalized” immediately as the patient begins to impart flexion to the spine. Thus, premature and/or excessive flexion of the target spinal segment can be inhibited or eliminated.
The protocols and apparatus of the present invention allow for individualization of treatment. Compliance members with different stiffnesses, elongations (lengths of travel), placement location in the anterior posterior direction along the spinous processes and other characteristics can be selected for particular patients based on their condition. For example, patients suffering from a severe loss of stiffness in the target spinal segment(s) may be treated with devices that provide more elastic resistance. Conversely, patients with only a minimal loss of natural segmental stiffness can be treated with devices that provide less elastic resistance. Similarly, bigger patients may benefit from compliance members having a greater maximum elongation, while smaller patients may benefit from compliance members having a shorter maximum elongation.
For some patients, particularly those having spinal segments which are very lax, having lost most or all of their natural segmental stiffness, the present invention can provide for “pre-tensioning” of the constraining structure. As described above, one way to accomplish this is by shortening the constraining structure such that a small amount of tension is held by the constraining structure when the spine is in the neutral or slightly extended initial position. Alternatively, pre-tensioned compliance elements can be provided to pre-tension the constraining structure without changing its length. The tension or compression elements utilized in the compliance members of the present invention, such as coil springs, elastomeric bodies, and the like, will typically present little or no elastic resistance when they are first deformed. Thus, there will be some degree of elongation of the compliance members prior to the spinal segment receiving a therapeutic resistance. To provide a more immediate relief, the tension or compression members may be pre-tensioned to have an initial static resistive force which must be overcome to initiate deformation. In this way, a constrained spinal segment will not begin to flex at the instant the patient begins to flex her or his spine which is an advantage when treating lax spinal segments. Certain specific embodiments for achieving such pre-tensioning are described in detail below.
In a first specific aspect of the present invention, a compliance member for attaching inelastic tethers circumscribing spinal processes comprises a body and an elongation limit. The body has a superior tether attachment element and an inferior tether attachment element, and the body defines a tension spring capable of elastic elongation when said attachment elements are drawn apart. The elongation limit is coupled between the superior tether attachment and the inferior tether attachment to prevent elongation of the tension spring beyond a maximum elongation length. Typically, the maximum elongation length is in the range from 2 mm to 15 mm, usually from 5 mm to 10 mm. The constraint structure typically has a total elastic stiffness in the range from 7.5 N/mm to 40 N/mm; and thus a single compliance member in the preferred parallel configuration typically has an elastic stiffness in the range from 3.75 N/mm to 20 N/mm. The compliance member may comprise a variety of elements or components which are able to be attached between the tether attachments of the compliance member body, typically being a non-distensible tether, such as a braided cord with a tensile stiffness greater than 20 N/mm, preferably being greater than 100 N/mm. The non-distensible tether may be secured over an exterior of the body of the compliance member, for example in the form of a braided jacket, tube or sheath. More usually, however, the non-distensible tether will be secured within an interior of the body of the compliance member, for example consisting of a single cord extending from the inferior tether attachment to the superior tether attachment or comprising two or more cords secured between the inferior and superior tether attachments. In a specific embodiment, the tether may be part of an assembly including a base, where the base is secured near one of the tether attachment and the cord looped around an anchor secured near the other of the tether attachments.
The compliance members of the present invention will typically be incorporated into a system for elastically constraining flexion of a spinal segment. Such systems will comprise first and second compliance members, a first non-distensible tether adapted to attach to the first tether attachment element of the first compliance member and to the second tether attachment element of the second compliance member, a second non-distensible tether adapted to attach to the first tether attachment element of the second compliance and to the second tether attachment element of the first compliance member.
In a second specific aspect of the present invention, methods for relieving symptoms of lumbar pain associated with spinal segment flexion comprise coupling and elastic constraint between a superior spinous process and an inferior spinous process or sacrum of the spinal segment. The elastic constraint increases the bending stiffness of the spinal segment in flexion sufficiently to reduce lumbar pain. Of particular interest to the present invention, elongation of the elastic restraint is limited to a maximum elongation length to prevent excessive flexion of the spinal segment, both decreasing the risk of patient discomfort resulting from over flexion of the segment and reducing the maximum stress experienced by the elastic constraint.
Usually, the maximum elongation length for the elastic restraint is in the range from 2 mm to 15 mm, more usually being between 5 mm and 10 mm, with respect to the neutral position of the spinal segment. Limiting elongation typically comprising coupling an inelastic constraint between the superior spinous process and the inferior spinous process, where the inelastic constraint when fully extended is longer than the elastic constraint when coupled to the spinous processes of the spinal segment in a neutral position by a length equal to the maximum elongation length.
Elastic constraint typically increases the bending stiffness of the spinal segment by an amount in the range from 0.1 Nm/deg to 2 Nm/deg. In particular, the elastic constraint may have a total elastic stiffness in the range from 7.5 N/mm to 40 N/mm when the constraint is positioned at a distance in the range from 25 millimeters to 75 millimeters in a posterior direction from a center of rotation of the spinal segment. Optionally, the methods may further comprise adjusting the elastic constraint so that it is taut but not stretched over the spinous processes or L5 spinous process and sacrum when the spinal segment is in its neutral position. The methods may further comprise changing the length of the elastic constraint after it has been coupled to the spinous processes or L5 spinous process and sacrum. Optionally, the bending stiffness may be increased over at least a portion of the full flexion range of the motion of the spinal segment, usually being increased over the entire full flexion range of motion of the spinal segment.
Exemplary spinous process constraints according to the present invention are illustrated schematically in
The right and left compliance members 16 and 18 will usually have similar or identical constructions and include an adjustable attachment component 32 and a fixed attachment component 34 for securing connecting segments of the superior and inferior tether structures 12 and 14. Usually, each compliance member 16 and 18 will have one of the tether structures 12 and 14 pre-attached to the fixed attachment component 34. The two subassemblies can then be introduced onto opposite sides of the spinous processes, and the tether structures placed over the spinous processes or otherwise attached to the vertebral bodies, as generally described in co-pending application Ser. No. 11/875,674 (Attorney Docket No. 026398-000150US), the full disclosure of which is incorporated herein by reference.
The present invention is particularly concerned with the nature of the tension elements 30, and a number of specific embodiments will be described hereinbelow. In general, the tension elements 30 will elastically elongate as tension is applied by the superior and inferior tether structures 12 and 14 through the attachments 32 and 34, in the direction shown by arrow 36. As the spinous processes or spinous process and sacrum move apart during flexion of the constrained spinal segment, the superior and inferior tether structures 12 and 14 will also move apart, as shown generally in broken line in
The tension elements of the present invention will be positioned over adjacent spinous processes, or over the L5 spinous process and adjacent sacrum, in order to increase the bending stiffness of the spinal segment. Referring to
Thus, the positioning of any of the elastic constraints as described herein at a position on the spinous processes SPS and SPI generally indicated by line L will define a moment arm distance dm, as illustrated in
As also shown on
A first exemplary tension element 40 constructed in accordance with the principles of the present invention is illustrated in
Referring now to
A free end 53 of the tether structure 52 may be attached to the adjustable tether connector 42, as illustrated in
Another tether structure (not illustrated) will be attached to the fixed connector 44 at the other end of the tension element 40, typically using a pin (not illustrated). The pin may be anchored in a pair of receiving holes 62, and a free end of the tether wrapped over the pin and firmly attached. Usually, the fixed tether structure will be pre-attached at the time of manufacture so that the treating physician can implant each of the pair of tension members, with one tether structure attached to the fixed tether connector. The remaining free ends of each tether structure 52 may then be deployed around the spinous processes (or attached to a sacrum) in a pattern generally as shown in
Referring now to
Referring now to
Referring now to
A first exemplary tether structure in form of a single cord 402 is illustrated in
Referring to
Referring now to
Referring now to
In addition to the internal “cord” tethers of
The length of the elongation limit may be set either during fabrication or immediately prior to use. In a first fabrication protocol, the compliance member will be adjusted in a jig or other apparatus to the desired maximum elongation. The tether or inelastic cable or cord which will be used as the elongation limit may then be introduced into or over the elastic constraint and pulled until it is taut. Once it is taut, it can be attached to the anchors, exterior, or otherwise to the body of the compliance member. By attaching when the compliance member is in its desired elongated configuration, the proper relative adjustment of the elongation limit can be assured.
In other instances, however, it may be desirable to adjust the elongation limit in situ after the compliance member has been initially implanted. In such cases, the spinal segment can be manipulated to the desired maximum flexion and the elongation limit fixed to the compliance member while the spinal segment remains in the desired flexion.
While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.
This application is a continuation of U.S. patent application Ser. No. 15/726,986 (Attorney Docket No. 48626-705.302), filed Oct. 6, 2017, which is a continuation of U.S. patent application Ser. No. 13/073,620 (Attorney Docket No. 48626-705.502), filed Mar. 28, 2011, which is a continuation-in-part of U.S. patent application Ser. No. 12/535,560 (Attorney Docket No. 48626-705.501), filed on Aug. 4, 2009, (now abandoned), which is a continuation-in-part of U.S. patent application Ser. No. 12/106,103 (Attorney Docket No. 48626-705.201), filed on Apr. 18, 2008, (now U.S. Pat. No. 8,403,961), which claims the benefit of Provisional No. 60/936,897, (Attorney Docket No. 48626-705.101), filed on Jun. 22, 2007, the full disclosures of which are incorporated herein by reference. The present invention is related to but does not claim priority from application Ser. No. 11/076,469, filed on Mar. 9, 2005, now U.S. Pat. No. 7,458,981, which claimed the benefit of prior provisional application 60/551,235, filed on Mar. 9, 2004; application Ser. No. 11/777,366 filed on Jul. 13, 2007; application Ser. No. 11/827,980 filed on Jul. 13, 2007; PCT application no. US 2007/081815 filed on Oct. 18, 2007; PCT application no. US 2007/081822 filed on Oct. 18, 2007; and application Ser. No. 11/975,674 filed on Oct. 19, 2007, the full disclosures of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60936897 | Jun 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15726986 | Oct 2017 | US |
| Child | 16789030 | US | |
| Parent | 13073620 | Mar 2011 | US |
| Child | 15726986 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12535560 | Aug 2009 | US |
| Child | 13073620 | US | |
| Parent | 12106103 | Apr 2008 | US |
| Child | 12535560 | US |
