Abstract:
Fastening mechanisms for releasably locking a tether are provided. The mechanisms find application with orthopedic internal-fixation implants and make the implants more reliable and their implantation less invasive. A method for releasably locking a tether comprises advancing the tether through a tether aperture in a clamp body. The tether enters the tether aperture in a first plane and exits in a second plane generally transverse to the first plane. Positioning a fastener element in a fastener aperture in the clamp body captures the tether between the clamp body and the fastener element thereby releasably locking the tether in position relative to the clamp body.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of International Patent Application No. PCT/US2009/046492 (Attorney Docket No. 026398-000810PC), filed Jun. 5, 2009, which claims the benefit of provisional U.S. application No. 61/059,543 (Attorney Docket No. 026398-000800US), filed Jun. 6, 2008, the full disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to medical methods and apparatus. More particularly, the present invention relates to orthopedic internal fixation such as methods and devices for restricting spinal flexion in patients having back pain, other spinal conditions, providing fracture fixation in long bone and trochanteric fractures or other orthopedic applications where a tether may be employed. 
         [0004]    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 discs located at the L 4 -L 5  or L 5 -S 1  junctions of the spine. Pain tends to be exacerbated when patients put their lumbar spines into flexion (i.e. by sitting or bending forward) and relieved when they put their lumbar spines into extension (i.e. by standing or arching backwards). Flexion and extension are known to change the mechanical loading pattern of a lumbar segment. When the segment is in extension, the axial loads borne by the segment are shared by the disc and facet joints (approximately 30% of the load is borne by the facet joints). In flexion, the segmental load is borne almost entirely by the disc. Furthermore, the nucleus shifts posteriorly, changing the loads on the posterior portion of the annulus (which is innervated), likely causing its fibers to be subject to tension and shear forces. Segmental flexion, then, increases both the loads borne by the disc and causes them to be borne in a more painful way. Discogenic pain can be quite disabling, and for some patients, can dramatically affect their ability to work and otherwise enjoy their lives. 
         [0005]    Pain experienced by patients with discogenic low back pain can be thought of as flexion instability, and is related to flexion instability manifested in other conditions. The most prevalent of these is spondylolisthesis, a spinal condition in which abnormal segmental translation is exacerbated by segmental flexion. The methods and devices described should as such also be useful for these other spinal disorders or treatments associated with segmental flexion, for which the prevention or control of spinal segmental flexion is desired. Another application for which the methods and devices described herein may be used is in conjunction with a spinal fusion, in order to restrict motion, promote healing, and relieve pain post-operatively. Alternatively, the methods and devices described should also be useful in conjunction with other treatments of the anterior column of the spine, including kyphoplasty, total disc replacement, nucleus augmentation and annular repair. General orthopedic or surgical applications are envisioned where a tether, cable or tape may be employed. An example is tronchanteric fracture fixation in which a cerclage device is wrapped around the bone and is attached and tightened to facilitate fracture healing. Similarly, the device may also be used in conjunction with a cerclage device for the fixation of long bone fractures. 
         [0006]    Patients with discogenic pain accommodate their syndrome by avoiding positions such as sitting, which cause their painful segment to go into flexion, and preferring positions such as standing, which maintain their painful segment in extension. One approach to reducing discogenic pain involves the use of a lumbar support pillow often seen in office chairs. Biomechanically, the attempted effect of the ubiquitous lumbar support pillow is also to maintain the painful lumbar segment in the less painful extension position. 
         [0007]    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 vertebra. Fusion may or may not also include instrumentation of the affected spinal segment including, for example, pedicle screws and stabilization rods. Fusion is not usually recommended for discogenic pain because it is irreversible, costly, associated with high morbidity, and has questionable effectiveness. Despite its drawbacks, however, spinal fusion for discogenic pain remains common due to the lack of viable alternatives. 
         [0008]    An alternative method, that is not commonly used in practice, but has been approved for use by the United States Food and Drug Administration (FDA), is the application of bone cerclage devices which 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 devices 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 dynamic elastic resistance to flexion across a range of motion. The purpose of bone cerclage devices and 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, which can lead eventually to adjacent segment morbidity. 
         [0009]    Another solution involves the use of an elastic structure, such as tethers, coupled to the spinal segment. The elastic structure can relieve pain by increasing passive resistance to flexion while often allowing substantially unrestricted spinal extension. This mimics the mechanical effect of postural accommodations that patients already use to provide relief 
         [0010]    Spinal implants using tether structures are currently commercially available. One such implant couples adjacent vertebrae via their pedicles. This implant includes spacers, tethers and pedicle screws. To install the implant, selected portions of the disc and vertebrae bone are removed. Implants are then placed to couple two adjacent pedicles on each side of the spine. The pedicle screws secure the implants in place. The tether is clamped to the pedicle screws with set-screws, and limits the extension/flexion movements of the vertebrae of interest. Because significant tissue is removed and because of screw placement into the pedicles, the implant and accompanying surgical methods are highly invasive and the implant is often irreversibly implanted. There is also an accompanying high chance of nerve root damage. Where the tip of the set-screw clamps the tethers, the tethers are abraded and particulate wear debris is generated. 
         [0011]    Other implants employing tether structures couple adjacent vertebrae via their processes instead. These implants include a tether and a spacer. To install the implant, the supraspinous ligament is temporarily lifted and displaced. The interspinous ligament between the two adjacent vertebrae of interest is then permanently removed and the spacer is inserted in the interspinous interspace. The tether is then wrapped around the processes of the two adjacent vertebrae, through adjacent interspinous ligaments, and then mechanically secured in place by the spacer or also by a separate component fastened to the spacer. The supraspinous ligament is then restored back to its original position. Such implants and accompanying surgical methods are not without disadvantages. These implants may subject the spinous processes to frequent, high loads during everyday activities, sometimes causing the spinous processes to break or erode. Furthermore, the spacer may put a patient into segmental kyphosis, potentially leading to long-term clinical problems associated with lack of sagittal balance. The process of securing the tethers is often a very complicated maneuver for a surgeon to perform, making the surgery much more invasive. And, as previously mentioned, the removal of the interspinous ligament is permanent. As such, the application of the device is not reversible. 
         [0012]    More recently, less invasive spinal implants have been introduced. Like the aforementioned implant, these spinal implants are placed over one or more pairs of spinous processes and provide an elastic restraint to the spreading apart of the spinous processes occurring during flexion. However, spacers are not used and interspinous ligaments are not permanently removed. As such, these implants are less invasive and may be reversibly implanted. The implants typically include a tether and a securing mechanism for the tether. The tether may be made from a flexible polymeric textile such as woven polyester (PET) or polyethylene; multi-strand cable, or other flexible structure. The tether is wrapped around the processes of adjacent vertebrae and then secured by the securing mechanism. The securing mechanism may involve the indexing of the tether and the strap, e.g., the tether and the securing mechanism include discrete interfaces such as teeth, hooks, loops, etc. which interlock the two. Highly forceful clamping may also be used to press and interlock the tether with the securing mechanism. Many known implementations clamp a tether with the tip of a set-screw, or the threaded portion of a fastener. However, the mechanical forces placed on the spinal implant are unevenly distributed towards the specific portions of the tether and the securing mechanism which interface with each other. These portions are therefore typically more susceptible to abrasion, wear, or other damage, thus reducing the reliability of these spinal implants as a whole. Other known methods use a screw or bolt to draw other components together to generate a clamping force. While these methods may avoid the potentially damaging loads, the mechanical complexity of the assembly is increased by introducing more subcomponents. 
         [0013]    For the aforementioned reasons, it would be desirable to provide improved methods and apparatuses to secure the tethers of such spinal implants together. In particular, such methods and apparatuses should be less invasive and should enable the tether to be more easily, reversibly, repeatably and reliably secured to an implant by a surgeon, in a surgery setting. 
         [0014]    2. Description of the Background Art 
         [0015]    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; 2005/0192581; 2005/0216017; 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; WO2004/052246 A1; WO 2004/073532 A1; and Published Foreign Application Nos. EP0322334 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; A1 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. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    The present invention provides fastening mechanisms and methods for releasably locking a tether for restricting flexion of at least one spinal segment. More particularly, the provided fastening mechanisms and methods relate to improvements to the methods and devices of deploying and implanting spinal implants for the treatment of discogenic pain and other conditions, such as degenerative spondylolisthesis. Specifically, such deployment and implantation methods are made less invasive and more reliable and reversible by the provided fastening mechanisms and methods. 
         [0017]    A first aspect of the present invention provides a device for restricting flexion of a spinal segment. The device includes a constraint device having a tether structure and a compliance member. The constraint device is adapted to be coupled with adjacent spinous processes or a spinous process and a sacrum. The constraint device is also adapted to provide a force resistant to flexion of the spinal segment. A locking mechanism is coupled with the constraint device and includes a clamp body and a fastener element. The clamp body has a fastener aperture and a tether aperture. The fastener aperture is sized to receive the fastener element, and the tether aperture is sized to receive the tether structure. A portion of the tether structure is disposed in the tether aperture and the fastener aperture, and the fastener element is disposed in the fastener aperture such that the tether structure is captured between the fastener element and the clamp body. 
         [0018]    The constraint may have a dimension that is adjustable to allow tightening over the spinous processes or spinous process and sacrum when the spinal segment is in a neutral position. The constraint device may provide an elastic resistance to flexion of the spinal segment beyond the neutral position in the range from 7.5 N/mm to 20 N/mm. The clamp body is coupled with the constraint. The fastener element may be disposed at least partially in the fastener aperture. The tether is captured between the fastener element and the clamp body. 
         [0019]    In many embodiments, the constraint may provide an elastic resistance to extension beyond the neutral position below 3 N/mm. In some embodiments, the elastic resistance may be below 0.5 N/mm. In some embodiments, the constraint is pre-tensioned to provide an initial resistive force to flexion which must be overcome prior to initiating deformation. The initial resistive force may be in the range from 5N to 25N. 
         [0020]    In many embodiments, the constraint may comprise a superior tether structure adapted to couple to the superior spinous process, an inferior tether structure adapted to couple to the inferior spinous process or sacrum, and at least one compliance member. The tether structures are substantially non-distensible and the compliance member provides the resistance to flexion. In some embodiments, the device may comprise at least two compliance members arranged to lie on either side of a midline when the tether structures are placed over the spinous processes or spinous process and sacrum. 
         [0021]    In various embodiments, the tether or tether aperture may include various features. The tether aperture may comprise a rectangular shaped slot. The tether may enter the tether aperture without deformation thereof. The tether may be deformed in order to be inserted into the tether aperture. In some embodiments, the tether may have a width and the tether aperture may have a width smaller than the tether width. The tether enters the tether aperture in a first plane and the tether exits the tether aperture in a second plane generally transverse to the first plane. 
         [0022]    In some embodiments, the fastener element comprises a screw threadably engaged with the fastener aperture. The screw may comprise a head having a diameter larger than the aperture diameter. The tether may be captured between the screw head and the clamp body. A surface of the clamp body may support the screw head and prevent bending of the screw. The screw may comprise a head having surface features adapted to press into the tether when the screw is tightened. The screw may comprise a driver feature adapted to receive a tool to permit rotation of the screw. The driver feature may be, for example, a Phillips head, a slotted flat head, a Torx head, or a hex head. The fastener element may rotationally lock the tether in position relative to the clamp body. 
         [0023]    In some embodiments, the fastener element comprises a position indicator adapted to provide visual, tactile, or audible feedback to an operator on the relative position of the fastener element with respect to the clamp body. The position indicator may comprise detents or calibration marks on either the fastener element or the clamp body and the indicator may be radiopaque to permit visualization under x-ray, fluoroscopy or other radiographic techniques. 
         [0024]    In another aspect, the invention provides a method for releasably locking a tether. The tether is advanced through a tether aperture in a clamp body. The fastener element is positioned in a fastener aperture in the clamp body. The tether enters the tether aperture in a first plane and then exits the clamp body in a second plane generally transverse to the first plane. The tether is captured between the clamp body and a surface of the fastener. The fastener element thereby releasably locks the tether in position relative to the clamp body. The clamp body may have additional surfaces that support the screw in reaction to bending or other loads that may be induced in the screw by the clamping action. 
         [0025]    The provided method may further comprise various steps and/or features. The fastener element may comprise a screw and capturing the tether may comprise threadably engaging the screw with the fastener aperture. The fastener element may comprise a screw and rotating the screw in a first direction locks the tether in position while rotating the screw in a second direction opposite the first may unlock the tether. The indicator indicates the position of the fastener element relative to the clamp body. Advancing the tether through the tether aperture may comprise advancing the tether therethrough without deformation of the tether or the tether may be deformed in order to fit in the tether aperture. A position indicator indicating the position of the fastener element relative to the clamp body may be monitored. 
         [0026]    In yet another aspect of the present invention a method for releasably locking an orthopedic, surgical tether comprises advancing the tether through a tether aperture in a clamp body and positioning a fastener element having a head, in the clamp body. Thus, the tether is captured between the clamp body and the head of the fastener element thereby releaseably locking the tether in position relative to the clamp body. 
         [0027]    These and other embodiments are described in further detail in the following description related to the appended drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic diagram illustrating the lumbar region of the spine. 
           [0029]      FIG. 1A  a schematic illustration showing a portion of the lumbar region of the spine taken along a sagittal plane. 
           [0030]      FIG. 2  illustrates a spinal implant of the type described in US 2005/0216017A1. 
           [0031]      FIGS. 3A-3B  illustrate additional tissue surrounding the spinous processes. 
           [0032]      FIGS. 4A-4M  show an exemplary method of surgically implanting a spinal device. 
           [0033]      FIGS. 5A-5K  show an exemplary fastening mechanism using a screw clamp. 
           [0034]      FIGS. 6A-6B  illustrate the use of a tether and fastening mechanism for trochanteric fixation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 1  is a schematic diagram illustrating the lumbar region of the spine including the spinous processes (SP), facet joints (FJ), lamina (L), transverse processes (TP), and sacrum (S).  FIG. 1A  is a schematic illustration showing a portion of the lumbar region of the spine taken along a sagittal plane and is useful for defining the terms “neutral position,” “flexion,” and “extension” that are often used in this disclosure. 
         [0036]    As used herein, “neutral position” refers to the position in which the patient&#39;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 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 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. 
         [0037]    Furthermore, as used herein, “flexion” refers to the motion between adjacent vertebrae in a spinal segment as the patient bends forward. Referring to  FIG. 1A , as a patient bends forward from the neutral position of the spine, i.e. to the right relative to a curved axis A, the distance between individual vertebrae L on the anterior side decreases so that the anterior portion of the intervertebral disks D are compressed. In contrast, the individual spinous processes SP on the posterior side move apart in the direction indicated by arrow B. Flexion thus refers to the relative movement between adjacent vertebrae as the patient bends forward from the neutral position illustrated in  FIG. 1A . 
         [0038]    Additionally, as used herein, “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  FIG. 1A . As the patient bends backward, the anterior ends of the individual vertebrae will move apart. The individual spinous processes SP on adjacent vertebrae will move closer together in a direction opposite to that indicated by arrow B. 
         [0039]      FIG. 2  shows a spinal implant of the type described in related U.S. Patent Publication No. 2005/02161017 A1, the contents of which are herein incorporated by reference. As illustrated in  FIG. 2 , an implant  10  typically comprises an upper strap component  12  and a lower strap component  14  joined by a pair of compliance members  16 . The upper strap  12  is shown disposed over the top of the spinous process SP 4  of L 4  while the lower strap  14  is shown extending over the bottom of the spinous process SP 5  of L 5 . The compliance member  16  will typically include an internal element, such as a spring or rubber block, which is attached to the straps  12  and  14  in such a way that the straps may be “elastically” or “compliantly” pulled apart as the spinous processes SP 4  and SP 5  move apart during flexion. In this way, the implant provides an elastic tension on the spinous processes which provides a force that resists flexion. The force increases as the processes move further apart. Usually, the straps themselves will be essentially non-compliant so that the degree of elasticity or compliance may be controlled and provided solely by the compliance members  16 . 
         [0040]      FIG. 3A  is a side view of the lumbar region of the spine having discs D separating the vertebral bodies V. The supraspinous ligament SSL runs along the posterior portion of the spinous processes SP and the interspinous ligament ISL and multifidus tendon and muscle M run alongside of and attach to the spinous processes SP.  FIG. 3B  is a posterior view of  FIG. 3A . 
         [0041]      FIGS. 4A-4M  illustrate an exemplary surgical method of implanting a spinous process constraint such as the embodiment of  FIG. 2 . One of the first steps to surgically implant a spinal implant is to make an incision to access the spinal area of interest.  FIG. 4A  shows the lumbar region of back K after an incision I has been made through the patient&#39;s skin.  FIG. 4B  illustrates the lumbar region of the spine after the incision I has been made through the patient&#39;s skin. Multifidus muscle and tendon M have been refracted with refraction tools TR to expose the spinous processes. 
         [0042]    After the incision has been made, a piercing tool T having a sharp distal end may be used to access and pierce the interspinous ligament ISL while avoiding the supra spinous ligament SSL, creating an interspinous ligament perforation P 1  superior of the first spinous process SSP of interest. This surgical approach is desirable since it keeps the supra spinous ligament intact and minimizes damage to the multifidus muscle and tendons and other collateral ligaments. As shown in  FIG. 4C , from the right side of the spine, tool T accesses and pierces the interspinous ligament ISL adjacent of the first spinous process SSP of interest. The distal end of tool T is shown in dotted line. Alternatively, tool T may access and pierce the interspinous ligament ISL from the left side instead. The distal end of tool T is coupled with tether  102 , parts of which are also shown in dotted line. In addition to accessing and piercing the interspinous ligament ISL, piercing tool T also advances or threads tether  102  through perforation P 1 . As shown in  FIG. 4D , tool T is then removed, leaving tether  102  positioned through perforation P 1 . 
         [0043]    Multifidus tendon and muscle M is not shown in  FIGS. 4C and 4D  so that other elements are shown more clearly. 
         [0044]      FIG. 4E  is a posterior view of a section of the spine after the above steps have been performed. Often times, the distal tip TI of tool T is detachable. As shown in  FIG. 4E , after tool T accesses and pierces the interspinous ligament ISL with distal tip TI, distal tip TI is detached from tool T and is left in place in perforation P 1  (shown in dotted line) above the first spinous process SSP of interest. Tether  102  lags behind tip TI. In some cases, distal tip TI may fully pierce through interspinous ligament ISL. In these cases, distal tip TI has passed through the interspinous ligament ISL while a portion of tether  102  is left in place in perforation Pl. 
         [0045]    After tip TI or a portion of tether TH is left in place in perforation Pl, another tool may couple with tip TI and pull tip TI such that it drags tether  102   a  and compliance element  104   a  to its appropriate position relative to the spine, as shown in  FIG. 4F . Compliance element  104   a  is coupled to tether  102   a  and is used to provide a force resistive to flexion of spinous processes SP. Compliance element  104   a  includes a fastening mechanism or fastening element  106   a  and may further comprise a spring, a tensioning member, a compression member, or the like. Related compliance members are described in commonly owned U.S. patent application Ser. No. 12/106,103 (Attorney Docket No. 026398-000410US), the entire contents of which are incorporated herein by reference. 
         [0046]    The steps of accessing the ISL, piercing the ISL, and threading tether  102  through a perforation are then repeated for the opposite, lateral side of the spine for an adjacent spinous process ISP, inferior of the first superior spinal process SSP of interest. As shown in  FIGS. 4G and 4H , tool T accesses the interspinous ligament from the left side of the spinal midline and pierces the interspinous ligament ISL, creating a second perforation P 2  located inferior of a second spinous process of interest, labeled as inferior spinous process ISP. As shown in  FIG. 4G , the inferior spinous process ISP of interest is directly adjacent to and inferior of the first superior spinous process SSP of interest. However, it is entirely possible to perform the described procedure starting with the inferior spinous process ISP first instead of the superior spinous process SSP, for example, perforation P 2  may be created before perforation P 1 . It is also possible that there may be a gap of one or more spinous processes SP between the spinous processes of interest. Multifidus tendon and muscle M is not shown in  FIGS. 4G and 4H  for clarity of the other shown elements. 
         [0047]    As shown in  FIGS. 4H ,  4 I and  4 J, like with the steps shown in conjunction with the first piercing, tether  102   b  is pierced through perforation P 2  and left in place along with distal tip TI of tool T (best seen in  FIG. 4I ). Another tool such as a pair of forceps, is then used to grasp distal tip TI to pull tether  102   b  and compliance element  104   b  in place relative to the spine, as shown in  FIG. 4J . Opposing compliance members  104   a  and  104   b  on opposite sides of spinous processes SP are oriented in opposite directions. Each compliance element  104   a,    104   b  is coupled with their respective tether  102   a,    102   b  and has a respective fastening mechanism or fastening element  106   a,    106   b.  Fastening mechanism  106   a,    106   b  are configured to couple with the tether  102   a,    102   b  of the opposing compliance member  104   a,    104   b.  For example as shown in  FIG. 4K , tether  102   a  is advanced through compliance member  104   b  and is coupled with fastening mechanism  106   b  while tether  102   b  is advanced through compliance member  104   a  and is coupled with fastening mechanism  106   a.  Except for their orientation, compliance members  104   a  and  104   b  are identical. One of skill in the art will appreciate that the tether may enter and exit the fastening mechanism in a number of different directions and configurations, and  FIG. 4K  merely is one exemplary embodiment. 
         [0048]    Fastening mechanism  106  may comprise a driver feature  108 . As shown in  FIG. 4L , the driver feature is adapted to receive a rotating driver tool RT. The driver feature may be a Phillips head, a slotted flat head, a Torx head, a hex head, or the like. Rotation of tool RT, which may be either clockwise or counter-clockwise, changes the configuration of fastening mechanism  106  so as to lock and secure tether  102  in place. This forms a continuous, multi-component tether structure or constraint  110  which couples two spinous processes SP together, as shown in  FIG. 4M . Compliance elements  104   a,    104   b  are used to control flexion between spinous processes SP while tethers  102   a,    102   b  and respective fastening mechanisms  106   a,    106   b  contribute to coupling the spinous processes SP together. Depending on the location of the perforations P 1  and P 2  and the lengths of the compliance elements  104   a,    104   b,  constraint  110  may couple more than two spinous processes SP together. In general, compliance elements  104   a,    104   b  comprise spring-like elements which will elastically elongate as tension is applied through tethers  102   a,    102   b  in an axis generally parallel to the spine. As the spinous processes or spinous process and sacrum move apart during flexion of the constrained spinal segment, the superior tether  102   a  and inferior tether  102   b  will also move apart. Compliance elements  104   a,    104   b  each include spring-like elements which will elastically resist the spreading with a force determined by the mechanical properties of the spring-like element. Thus, constraint  110  provides an elastic resistance to flexion of the spinal segment beyond the neutral position. Constraint  110  is often configured to provide a resistance in the range from 7.5 N/mm to 20 N/mm but the resistance may be below 3 N/mm or even below 0.5 N/mm. Constraint  110  may also be adjustable in certain dimensions to allow tightening over the spinous processes or spinous process and sacrum when the spinal segment is in a neutral position. Other, related tether embodiments and joining methods are disclosed in U.S. patent application Ser. No. 12/106,103 (Attorney Docket No. 026398-000410US), U.S. Patent Publication No. 2008/0009866 (Attorney Docket No. 026398-000140US), U.S. Patent Publication No. 2008/0108993 (Attorney Docket No. 026398-000150US), U.S. Provisional Patent Application No. 60/936,897 (Attorney Docket No. 026398-000400US), the entire contents of which are incorporated herein by reference. 
         [0049]    Tethers may be locked and secured in place relative to a compliance member using a screw clamp as seen in  FIGS. 5A-5K . The housing or clamp body  84  of compliance member  80  is configured to secure tether  82  in place with threaded screw  81  but is otherwise similar to compliance members  104   a,    104   b  previously described. Like compliance member  104   a,    104   b,  a pair of compliance members  80  and tethers  84  may be coupled together to form a constraint around spinous processes or a spinous process and sacrum. Housing  84  has a fastener aperture  85  and a tether aperture  87  as shown in the bottom view of  FIG. 5E  and the top view of  FIG. 5F . Tether  82  is advanced through tether aperture  87  as shown in  FIG. 5A . The shaft of screw  81  is positioned in fastener aperture  85  as also seen in  FIG. 5A . Screw  81  often has threads which allow it to couple with fastener aperture  85 . Screw  81  often has male threads while fastener aperture  85  has female threads. Male and female threads may be substituted with one another. Screw  81  preferably does not have sharp edges that will cut or otherwise damage the strap, either during fastening or in service. Screw  81  includes driver features such as a Philips head, a slotted flat head, a Torx head, a hex head, or the like adapted to receive a tool to permit rotation. Screw  81  is rotated and advanced through fastener aperture  85 , capturing and clamping tether  82  between the clamping surface of the head of screw  81  and housing  84 , as shown in the side view of  FIG. 5B  and top vie of  FIG. 5C . Screw  81  may also has surface features adapted to press into the tether when the screw is tightened. For example, the clamping surface may have a specific texture like knurling that would increase or decrease retention strength for a given clamping force based on the smoothness or roughness of the texture. The head of screw  81  has a diameter larger than that of fastener aperture  85 . Housing  84  and/or screw  81  may have position indicators such as detents or calibration marks adapted to provide visual, tactile or audible feedback on the relative position of screw  81  relative to housing  84 . Rotation of screw  81  in an opposite direction loosens the fit of the tether between screw  81  and housing  85 .  FIG. 5D  illustrates the fastener screw  81  and  FIGS. 5E-5F  show the apertures that receive the fastener and tether. 
         [0050]    Tether aperture  87  is often a rectangular shaped slot, although one of skill in the art will appreciate that may geometries may be utilized. In  FIG. 5G  tether aperture  87  may be rectangular and have a width smaller than the width of the tether  82 .  FIG. 5H  shows the opposite side of the tether aperture  87 . Thus, as seen in Fig. SI tether  82  will have to be deformed or folded to fit through the aperture. By deforming tether  82 , tether  82  will be provided with additional flexibility in the directions shown by arrows  88   a,    88   b  in  FIG. 5I . This is advantageous since as tether  82  enters aperture  87 , it generally is biased to flex only in the direction shown by arrow  88   a.  As tether enters aperture  87 , it is folded and then is biased to flex in direction  88   b  which is transverse the direction of bias as tether  82  enters the aperture. Furthermore, tether  82  exits aperture  87  in a direction generally parallel to the direction that a screw of other fixing element enters aperture  85 . This helps to ensure that the physician can easily grasp and adjust tether  82  since a pathway already exists for a fixing element to be engaged with aperture  85 . Further details on this feature will be discussed below. 
         [0051]    Tether aperture  87  may also be generally circular as shown in  FIG. 5J . As seen in  FIG. 5K , tether  82  may be deformed by twisting so that it enters the circular aperture in a first plane and then exits the circular aperture in a second plane generally transverse to the first plane. By twisting tether  82  as described, it is provided with additional flexibility in the directions shown in by arrows  88   a,    88   b.  The deformation of tether  82  may also position the tether so that it can more easily be tensioned in a surgical procedure. Additionally, the deformation of tether  82  allows it to better conform to the anatomy. In  FIG. 5I , the rectangular aperture causes tether  82  to fold and fan-out and in  FIG. 5K , the circular aperture causes tether  82  to twist and fan-out. This enables the tail of the strap to be tensioned dorsally for ease of use, while the opposite, working end changes planes to contour medially and conform to the anatomy. Therefore, the tether can be tensioned from the same direction as application of the driver tool to lock the tether, thus permitting a less invasive procedure. 
         [0052]    While the exemplary embodiments described above illustrate a fastening mechanism that is coupled with a spring-like compliance member, one will appreciate that the fastening mechanism may be used independently of a spring or other internal fixator. Other uses may include applications where a tether is secured with a knot, crimped or the like. These may include cerclage applications such as in trochanteric fixation in addition to application of a substantially rigid tether to multiple spinous processes or lamina.  FIGS. 6A-6B  illustrate the use of a tether and fastening mechanism for trochanteric fixation.  FIG. 6A  shows a tether T wrapped around the tronchanter of a femur F. A fastening mechanism FM releasably locks one end of the tether T, thereby forming a closed loop around the trochanter.  FIG. 6B  highlights the tether wrapped around the trochanter. 
         [0053]    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.