Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/70—Spinal positioners or stabilisers ; Bone stabilisers comprising fluid filler in an implant
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/70—Spinal positioners or stabilisers ; Bone stabilisers comprising fluid filler in an implant
  • A61B17/7001—Screws or hooks combined with longitudinal elements which do not contact vertebrae
  • A61B17/7002—Longitudinal elements, e.g. rods
  • A61B17/7019—Longitudinal elements having flexible parts, or parts connected together, such that after implantation the elements can move relative to each other
  • A61B17/7023—Longitudinal elements having flexible parts, or parts connected together, such that after implantation the elements can move relative to each other with a pivot joint
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/03—Automatic limiting or abutting means, e.g. for safety

Definitions

• Degenerative disc disease is an important public health problem with multiple dimensions: personal, social, and professional. It is also well recognized that facet arthritis is associated with disc degeneration, and this is typically attributed to loss of disc height and consequently increased posterior column loads. However, in addition to disc height loss, intervertebral kinematics becomes progressively erratic with increasing disc degeneration, being characterized by significant variability in the instantaneous axis of rotation (IAR) position (centrode). Since spinal movement is constrained by both the disc and facet joints, disc material property deterioration with degeneration also influences facet forces. Unfortunately, the influence of IAR position fluctuations on facet loads, and consequently arthritis risk, has not been previously investigated or reported.
• IAR instantaneous axis of rotation
• Dynamic stabilization can take on many forms, from those providing assistance using mechanical devices (e.g. partial disc replacement, posterior dynamic stabilization), to those relying on biologic processes (tissue regeneration/repair).
• mechanical devices e.g. partial disc replacement, posterior dynamic stabilization
• biologic processes tissue regeneration/repair.
• IAR instant axis of rotation
• Posterior dynamic stabilization has the advantage of leaving the disc space intact and being facilitated by a less invasive surgical procedure.
• Current posterior dynamic stabilization technologies are incremental improvements of traditional rod and screw fusion systems that incorporate either flexible rods or articulating rod/screw attachments. These systems however, do not support the natural IAR.
• anterior intervertebral shear is significant at this level. Consequently, the facet joints are critical for preventing spondylolisthesis and constraining inter-segmental motion.
• An aspect of the invention is a method of stabilizing adjacent vertebrae.
• the method includes the steps of installing a first anchor in a first vertebra and a second anchor in a second vertebra adjacent to said first vertebra, and coupling an articulating linkage to said first and second anchors.
• the articulating linkage constrains one or more components of motion between the first and second vertebrae while allowing the first vertebra to move along the path of the IAR of the first vertebra.
• the linkage is configured to constrain non-physiologic motion between the first and second vertebrae.
• the IAR of the first vertebra comprises an axis that the first vertebra rotates about and travels along as it moves from one position to another.
• Coupling an articulating linkage may be achieved by attaching a first member to the first anchor and a second member to the second anchor, and establishing one or more hinges about one or more respective pivot points, wherein the one or more hinges link the first member to the second member, and wherein the one or more pivot points correlate to the IAR of the first vertebra.
• first member is coupled to the second member via a first articulating link having a first pivot point on the first member and a second pivot point on the second member, and a second articulating link having a third pivot pint on the first member and a fourth pivot point on the second member.
• first member and first anchor are rigidly fixed to each other such that they move in unison along with the first vertebra.
• the second member and second anchor are rigidly fixed to each other such that they move in unison along with the second vertebra.
• the first or second anchor may comprise any one of known fastening means available in the art, such as a pedicle screw installed in a pedicle of the vertebra.
• the linkage is installed in a posterior region of the vertebrae.
• the first vertebra comprises the L5 vertebra
• the second vertebra comprises the S1 vertebra.
• the articulating linkage allows the L5 vertebra to rotate and translate with respect to the S1 vertebra.
• the rotation and translation of the L5 vertebra follows that path of the IAR of the L5 vertebra. More particularly, the L5 IAR intersection with the midsagittal plane moves cephalid relative to the S1 endplate during flexion, and posterior during extension.
• the articulating linkage is configured to allow the L5 vertebra to rotate substantially forward during flexion, and substantially backward during extension.
• the apparatus includes a first anchor configured to be installed in a first vertebra, a second anchor configured to be installed in a second vertebra adjacent to said first vertebra, and an articulating linkage coupling said first and second anchors.
• the articulating linkage is configured to constrain one or more components of motion between the first and second vertebrae while allowing the first vertebra to move along the path of the IAR of the first vertebra.
• the articulating linkage comprises a first and second members configured to be attached to the first and second anchors respectively, and one or more hinges centered about one or more respective pivot points, wherein the one or more hinges link the first member to the second member, and the one or more pivot points correlate to the IAR of the first vertebra.
• Another aspect is an apparatus for dynamically stabilizing adjacent vertebrae.
• the apparatus comprises a superior anchor configured to be installed in a superior vertebra; an inferior anchor configured to be installed in an inferior vertebra adjacent to the superior vertebra, and means for rotatably linking the superior anchor with the inferior anchor such that one or more components of motion between the superior and inferior vertebrae are constrained while allowing the superior vertebra to move along the path of the IAR of the superior vertebra.
• the linking means is configured to constrain non- physiologic motion between the first and second vertebrae.
• the linking means articulates about one or more pivot points that correlate to the IAR of the first vertebra.
• the superior anchor comprises a superior pedicle screw configured to be installed in a pedicle of the superior vertebra.
• the inferior anchor comprises a inferior pedicle screw configured to be installed in a pedicle of the inferior vertebra.
• the superior vertebra comprises the L5 vertebra
• the inferior vertebra comprises the S1 vertebra.
• the linking means is configured to allow the L5 vertebra to rotate and translate with respect to the S1 vertebra. Ideally, the rotation and translation of the L5 vertebra follows that path of the IAR of the
• the linking means may be configured to allow the L5 vertebra to rotate substantially forward during flexion, and substantially backward during extension.
• Another aspect is an apparatus for stabilizing first and second adjacent vertebrae, comprising a dynamic stabilization assembly configured to be implanted in relation to the first and second vertebrae, wherein the first and adjacent vertebra comprise a vertebral joint having at least one IAR associated with the first and second vertebrae.
• the dynamic stabilization assembly is configured to allow at least a portion of the vertebral joint to rotate substantially about a first IAR corresponding to a first range of motion associated with the vertebral joint.
• the dynamic stabilization assembly is further configured to allow at least a portion of the vertebral joint to rotate substantially about a second IAR corresponding to a second range of motion associated with the vertebral joint.
• the first IAR and second IAR have different locations with respect to a disc plane associated with the vertebral joint.
• the position of the first IAR with respect to the second IAR shifts substantially laterally across the disc plane during the first range of motion.
• the position of the first IAR with respect to the second IAR shifts substantially vertically along a spinal axis of the first and second vertebrae during the second range of motion.
• the dynamic stabilization assembly comprises a plurality of members coupled to the first and second vertebrae, wherein the plurality of members are configured to constrain motion of the vertebral joint while allowing at least a portion of the vertebral joint to move in accordance with the IAR.
• the plurality of members may comprise a four-bar linkage, or other type of dynamic stabilization constraint that allows motion in accordance with the IAR .
• the linkage may comprise a plurality of pivot points associated with the IAR.
• the method further includes restraining motion of the vertebral joint while allowing at least a portion of the vertebral joint to rotate substantially about a first IAR corresponding to a first range of motion associated with the vertebral joint.
• FIGS. 1 A and 1B show a graphical demonstration of the orientation of the instant axis of rotation for five sectors of flexion/ extension (FIG 1A) and lateral bending (FIG. 1B).
• FIGS. 2A and 2B show the intersection of the axes illustrated in FIGS
• FIG. 1A and 1 B with the sagittal plane (FIG 2A) and frontal plane (FIG. 2B) are superimposed on the L5/S1 spinal segment.
• FIG. 3 shows an example of link points for a four-bar linkage that generates normal flexion/extension motion for the L5/S1 interspace superimposed over an L5/S1 spinal joint.
• FIG. 4 shows a schematic representation of posterior dynamic stabilization linkage instrumentation in lateral view superimposed over FIG. 3.
• FIGS. 5A-C show three positions of the schematic representation of the posterior dynamic stabilization assembly shown in FlG. 4, demonstrating its ability to guide L5 through a physiologic flexion/extension movement.
• FIG. 6 illustrates the l_5/ S1 joint and respective coordinate system.
• FIG. 7 shows a schematic diagram of L5/ S1 , and shows 40° sacral slope and 850 N load in standing position
• FIG. 8 shows testing device with wedge to simulate constrained L5 posture in flexion, extension, and bending for investigating L5/S1 kinematics.
• FIGS. 5A-C show three positions of the schematic representation of the posterior dynamic stabilization assembly shown in FlG. 4, demonstrating its ability to guide L5 through a physiologic flexion/extension movement.
• FIG. 6 illustrates the l_5/ S1 joint and respective coordinate system.
• FIG. 7 shows a schematic diagram of L5/ S1 , and shows 40° sacral slope and 850 N load in standing position
• FIG. 8 shows testing device with
• FIG. 9A and 9B show schematic lateral view of L5/S1 facets, with facets open into flexion when the IAR is above the facet level (FIG. 9A), and facets close into flexion when the IAR is below the facet level (FIG. 9B).
• FIG. 10 shows a graph of IAR distance to S1 endplate (ZJ: mm) plotted against facet force variation (N) for each 3° rotation into flexion.
• the present embodiments of the invention relate to providing improved dynamic stabilization in compromised spinal disc joints.
• the present embodiments provide informed solutions as to new experimental methods and observations that have shed new light on the desired performance that artificial dynamic stabilization demonstrate to more closely approximate normal spinal motion.
• the present embodiments more precisely approximate the IAR path (centrode) for normal spinal motion, which has been newly observed in experiments described herein to migrate in three dimensions. Further aspects of these particularly enlightened parameters for dynamic spinal motion are described in additional detail as follows.
• a dynamic stabilization system for use in providing dynamic stability to motion in a compromised spinal joint.
• This system is adapted to provide a centrode of the instant axis of rotation (IAR) that substantially approximates the normal centrode for the respective spinal joint.
• the system is adapted to provide a centrode for the IAR for the respective joint that remains within a range of error of about 25 percent versus the normal centrode. In another mode, the range of error is within about 10 percent of the normal centrode.
• the system is adapted to provide a change in IAR that, during one range of motion of the spinal joint is principally lateral across the disc plane, and in another range of motion of the spinal joint is principally vertical along the spinal axis between vertebral bodies adjoining the disc of the joint.
• FIG. 1 shows the natural centrode location of IAR for five postures (lateral view shown on the left, and AP view shown on the right) according to certain experimental test parameters described in further detail below.
• FIGS. 1A and 1 B illustratel shows a 3-dimensional view of the orientation of the instant axis of rotation (IAR) for five sectors of flexion/ extension (FIG. 1A) and lateral bending (FIG.
• Axes 10 is the IAR for 3° to 6° flexion
• axes 12 is the IAR for 3° to 6° extension
• axes 14 is the IAR for 3° to neutral extension
• axes 16 is the IAR 3° to neutral flexion. The intersection of these axes with the sagittal plane
• FIG. 2A shows an example of a calculated plot for a kinematically- defined linkage for flexion/extension by reference to the natural disc centroid gathered from experimental observation of the L5/S1 interspace, as elsewhere herein described in further detail.
• points T and Q are defined as part of the L5 vertebra
• complementary points R and O are determined using the centrode locations 10, 12, 14, and 16 and points T and Q.
• Points R and O are fixed relative to S1.
• Points T, Q, R, and O were derived kinematic (graphical and/or computer generated) analysis of experimental test data described in further detail below.
• the posterior linkage of the present invention include links connecting points R-Q and O-T.
• FIG. 4 shows a schematic lateral view of a posterior dynamic spinal stabilization system 50 in accordance with one embodiment of the present invention.
• the system 50 is adapted to closely approximate the centrode of natural kinematic linkage of the spinal joint (as detailed as points 10, 12, 14, and 16 in FIGS. 2A-2B and FIG. 3.
• the system 50 is defined mechanically via a four-bar kinematic linkage that bridges vertebral anchors (e.g. pedicle screw extensions or the like device known in the art), one pedicle screw 52 in each of the pedicles of the L5 vertebra 22, and another screw 54 in each of the pedicles of the sacrum24.
• vertebral anchors e.g. pedicle screw extensions or the like device known in the art
• the geometry of the linkage is defined by vertebral geometry and the natural intervertebral centrode.
• the linkage is designed to balance the centrode defined for flexion/extension, lateral bending and axial rotation.
• two pedicle screws 52, 54 (which may also comprise porous- coated rods or similar anchoring mechanism) are affixed to adjacent vertebrae (superior vertebra 22 and inferior vertebra 24).
• the pedicle screws 52, 54 may be installed with posterior access to the spine via methods commonly used in the art.
• a superior extension member 56 is rigidly attached to the superior pedicle screw 52 and extends downward toward the inferior vertebra 24.
• An inferior extension member 58 is rigidly attached to the inferior pedicle screw 54 and extends upward toward the superior vertebra 22.
• Upper articulating link 62 and lower articulating link 60 rotatably connect the superior extension member 56 and inferior extension member 58 via flexible hinge joints 64, 66,
• FIGS. 5A through 5C show three modes of operation. As indicated across different ranges of motion, the IAR shifts according to use of this assembly in a manner that more closely approximates the natural centrode.
• FIG. 5A shows the linkage assembly 50 accommodating the position of the superior vertebra 22 at 6 degrees of flexion with the linkage 50 guiding rotation about the appropriate IAR 10.
• FIG. 5B shows the linkage assembly 50 accommodating the position of the superior vertebra 22 at a neutral flexion position with the linkage 50 guiding rotation about the appropriate IAR 16.
• FIG. 5C shows the linkage assembly 50 accommodating the position of the superior vertebra 22 at 6 degrees of extension with linkage 50 guiding rotation about the appropriate IAR 12.
• FIGS. 4 and 5A-C may be provided and used as a stand-alone dynamic fusion device.
• the assembly 50 may be used in conjunction with nucleus replacement or disc biologic regeneration strategies.
• 5A-C is directed primarily toward the L5/S1 joint of the spine. However, is is contemplated that the techniques and systems of the present invention may be used to stabilize a number of other areas of the spine, including L1/L2,
• L2/L3, L3/L4, L4/L5, and other vertebrae in the thoracic and cervical spine can be implemented instead of or in combination with the hinge joints at points T, Q, R, and O shown in FIGS. 4 and 5A-C.
• An elastic or deformable material may be used that allows and restrains motion in the same path as provided by the hinged joints.
• portions of the extension members 56/ 58 and articulating links 60/ 62 may be relieved (or coupled with an elastic material) at the specified locations to have a smaller cross-section to allow bending at the specified location (e.g. at points T, Q 1 R and O).
• the deformable material may comprise a memory material, such as nitinol, or a polymer having similar properties.
• a memory material such as nitinol
• a polymer having similar properties may comprise, without limitation, the following.
• the vertebrae are constrained sufficiently to facilitate distraction during surgical placement.
• the linkage 50 geometry guides the vertebra along the natural centrode.
• the device 50 can be placed via a posterior approach using minimally- invasive techniques. This is shown by way of further example in FIGS. 4 and 5A-C.
• Previously disclosed devices intended to provide posterior dynamic stabilization generally either constrain movement about a fixed axis of rotation, or a variable axis that is far from the natural centrode.
• the present invention responds to recent experimental data and observations providing more insight as to the natural centrode, and provides the appropriate systems and methods to more closely approximate this natural motion.
• the lower and upper articulating links 60,62 of spinal stabilization system 50 can be locked (or at least provide a lockable option) so that the spinal stabilization system 50 may be employed to create a solid fusion as with standard fusion hardware. This may provide a benefit in certain particular circumstances for certain patients.
• the spinal stabilization system 50 shown in FIGS. 4 and 5A-C is adapted for posterior dynamic stabilization of the L5/S1 spinal joint shown.
• the spinal stabilization system 50 may be employed to provide similar beneficial results to also more closely approximate the centrode of IAR movement in the normal spine versus prior attempts.
• anterior or lateral dynamic stabilization assemblies and related implant methods may be adapted for use in treating patients according to the information provided herein without departing from the intended broad scope of the various aspects of the present invention.
• engineered combinations of individual implants working together for an overall result may be used.
• a posterior dynamic stabilization assembly may be provided in combination with at least one other implant, such as for example a disc implant (either nucleus or whole disc implant), or for example with an anterior or lateral dynamic stabilization implant, such that the overall assembly working together provides the desired range of motion about a more physiologic centrode of disc rotation.
• a disc implant either nucleus or whole disc implant
• an anterior or lateral dynamic stabilization implant such that the overall assembly working together provides the desired range of motion about a more physiologic centrode of disc rotation.
• Such anatomical variances may include, for example, different considerations at different spine levels, or patient-to-patient variances of anatomy at similar levels along the spine. For example, different sizes, angles, and relative placements of the component parts of the assembly shown may be made available to accommodate such variances.
• further experiments may be conducted similar to those described herein, or appropriately modified by one of ordinary skill based upon an informed review of this disclosure and other available information, to suitably characterize the normal spinal motion across such variable parameters.
• Such experimental observations may then be used to form additional assemblies and methods that are adapted to suitably operate in a manner that approximates the normal motion according to the particular anatomical parameters characterized.
• Information regarding other attempts to which one or more aspects of the present invention is intended to improve includes reference to one or more of the following, by reference: Dynesys from Zimmer Spine; and Isobar TTL from Scient'x USA.
• the goal of this cadaveric biomechanical study was to report and correlate a measure of intervertebral kinematics (the centrode, or the path of the instant axis of rotation) and the facet forces at the L5/S1 motion segment while under a physiologic combination of compression and anterior shear loading.
• Twelve fresh-frozen human cadaveric L5/S1 joints (age range 50 to 64 years) were tested biomechanically under semi-constrained conditions by applying compression plus shear forces in several postures: neutral, and 3 and 6 degrees flexion, extension and lateral bending.
• the experimental boundary conditions imposed compression and shear representative of in vivo conditions during upright stance.
• the 3-D instantaneous axis of rotation (IAR) was calculated between two consecutive postures.
• the IAR positions demonstrate that the L5 vertebral body 22 primarily rotates forward during flexion (IAR close to vertebral body center) and rotates/translates backward during extension (IAR at or below the L5/S1 intervertebral disc). In lateral bending, the IAR obliquity demonstrated coupling with axial torsion due to resistance of the ipsilateral facet.
• the present experiment simultaneously measures spinal kinematics and facet forces during motion in a human cadaveric model of the healthy L5/S1 joint under physiologic compression and shear. This provides a new insight into the centroid of spinal motion, to which the present system and method embodiments variously relate with novel solutions.
• the lumbosacral spine was harvested from 12 human donors aged 50 to 64 at the time of death (8 male and 4 female). Only specimens with no radiographic evidence of bone disease or joint degeneration (osteophytes, disc space narrowing, facet hyperthrophy) were used in this study. Specimen preparation consisted in meticulous removal of muscular tissue so as to retain the integrity of the capsular and ligamentous elements. For each specimen, the superior half of the L5 vertebra and inferior half of S1 vertebra were potted in polymethylmetacrylate (PMMA), so that S1 end-plate was parallel to the PMMA surface and clamping faces.
• PMMA polymethylmetacrylate
• FIG. 7 shows a schematic diagram of L5/ S1 testing assembly 100 in this arrangement, per 40° sacral slope and 850 N load in standing position.
• FIG. 8 illustrates a testing assembly 120, which constrains the L5 posture in flexion, extension, and bending for investigating L5/S1 kinematics.
• the applied load N is uniformly distributed and applied in both shear and compression. Axial torsion was unconstrained. The angle ⁇ was chosen to reflect the average 39 degrees sacral slope in standing position.
• the specimens 112 were loaded with an 850 N vertical force applied via near frictionless elements 112 and 1 14 (e.g. polished steel lubricated with machine oil).
• the force N was chosen to match estimates for L5/S1 in the standing position based on disc pressure and myo-electric measurements, and therefore represents both gravity and muscular loading.
• the 850 N vertical force generated 650 N of disc compression 126 and 550 N of horizontal shear 128 consistent with free body analyses of L5/S1 based on specific morphometric studies.
• the semi-constrained feature of the testing apparatus 110 is such that the location of the resultant force at the frictionless surface varies, and thereby minimizes its distance to the IAR. Consequently, confounding moments about the IAR are minimized.
• the motion from one position to another can be described by the sum of a rotation around a single axis and a translation (perpendicular to the plane of rotation) along this axis.
• the axis is called the helicoidal axis.
• movement occurs around an 'instantaneous axis.
• the instantaneous helicoidal axis is called instantaneous axis of rotation (IAR).
• the transformation matrix is a mathematical description of the rigid-body movement from one position to another, and includes a square 3x3 rotation matrix and a 3x1 translation matrix. Consequently, the helicoidal axis is just an alternate representation of the transformation matrix.
• the transformation matrix was calculated using the method of Kinzel, based on 3-D coordinates of four non-coplanar landmarks placed on the moving vertebra (L5). Then the direction and the position and of the axis was determined in the 3-D space according to the method of Spoor and Velpaus. Finally these data were transformed to a local coordinate frame based on the radiographic anatomy.
• the origin of the orthogonal right-handed frame was the center of the endplate of S1 24, the X-axis being sagittal, the Y-axis coronal, and the Z-axis vertical and perpendicular to the endplate (see FIG. 6).
• IAR direction was described using the inclination (angle ⁇ between the axis and the horizontal plane that is equivalent to a latitude from the S1 endplate) and the declination (D; angle between the axis and the sagittal plane that is equivalent to a longitude).
• IAR position was described as the position of the unique point, P (x p , y p , z p ), of the axis so that the distance OP is the shortest distance from the origin (O) to the axis (P). Therefore, OP is perpendicular to the IAR.
• the compression force transmitted through the left and right facet joints was recorded using thin pressure sensors 130 (e.g. Flexiforce A101-500, Tekscan Inc, South Boston, MA).
• the sensors 130 were introduced into the right and left joint space through a vertical cut in the joint capsule.
• the sensors 130 were 10 mm in diameter, 0.2 mm thick, and made of flexible mylar and contain ink whose resistance varies linearly to the applied force.
• Sensor output was recorded at 5 Hz and averaged using data acquisition software (Labview 6.1, National Instruments,
• the sensor 130 was calibrated by applying pre-determined forces via contact surfaces of different areas and demonstrated that the output voltage varied linearly with the force regardless of the pressure area.
• the calibration ratio was 500 NA/ ( ⁇ 5%).
• ⁇ corresponds to the angle between the endplate and the line 138 between the facet joint and the IAR 136.
• FIG. 1A and 1 B 1 intersections of the IAR 136 and the sagittal plane 18 and the coronal plane through the center of the disc are represented on a lateral FIG. 1A and an AP radiograph FIG. 1 B respectively.
• the diameter of the circles corresponds to the average error in position
• the IAR during flexion and extension, is oriented laterally.
• the IAR intersection with the mid-sagittal plane 18 moves cephalad relative to S1 endplate 26 during flexion, and posterior during extension.
• the IAR positions demonstrate that the L5 vertebral body 22 primarily rotates forward during flexion (IAR close to vertebral body center) and rotates/translates backward during extension (IAR at or below the L5/S1 intervertebral disc).
• Table III illustrate the coordinates of the IAR position in lateral bending, and the IAR declination was 1.8° and similar for every motion sector
• the y-coordinate of IAR position (y p ) varied significantly according the sector of motion during lateral bending (p ⁇ 0.001). Post-hoc tests showed that the IAR moved horizontally towards the bending beyond 3° bending in both directions.
• Table IV shows the facet force in lateral bending.
• Post-hoc tests demonstrated that facet force increased significantly in the first 3° lateral bending.
• the IAR during lateral bending, is oblique relative to the main plane of motion and translates parallel to S1 endplate, toward the side of the bending (Table 2).
• the IAR In lateral bending, the IAR obliquity demonstrates coupling with axial torsion due to resistance of the ipsilateral facet.
• the study investigated various relationships between intervertebral kinematics and facet forces during physiologic motion and loading of the L5/S1 joint. The observed IAR was normally located in the posterior part of the intervertebral disc, and moved superiorly during flexion, posteriorly during extension, and ipsilaterally during lateral bending. As expected, coupled axial rotation was associated with lateral bending.
• the facet force did not show a uniform variation in flexion/extension because of interspecimen variability, it was correlated with the horizontal IAR displacement in lateral bending, such that the facet force increased in the ipsilateral facet.
• the observed IAR was perpendicular to the sagittal plane in flexion/extension and located at the posterior part of the intervertebral disc, which is consistent with prior reports based on planar measurements in vitro and in vivo using the graphical method of Reulaux.
• the Reulaux method calculates the instantaneous center of rotation by drawing bisectors between landmarks on successive radiographs or photographs.
• This 2D method is less accurate compared to the 3D approach used in the current study, which may explain why various of the current observations - including for example but without limitation that the IAR moves superiorly, perpendicular to S1 endplate during flexion, and posteriorly, parallel to the endplate during extension - have not been described previously.
• the IAR path relative to S1 endplate 26 demonstrates that from extension to flexion, the L5 vertebra 22 primarily translates anteriorly at first (i.e., the IAR is low during motion between 6 and 0 degrees of extension), and subsequently rotates forward when at the flexion limit (since the IAR approaches the geometric center of L5 during motion between 3 and 6 degrees of flexion).
• This motion in flexion/extension reflects posture-varying roles of the disc and facet joints in constraining movement, and is consistent with reports that the facet contact area moves upwards into flexion.
• Significant interspecimen variability was observed in the facet force trend with posture that is contrary to the classical notion that facet forces systematically increase into extension.
• FIG. 9A shows a schematic lateral view of the facet joint 132 of the L5/S1 vertebrae. Facet force variation ⁇ F in the facet joints in flexion and extension is related to height H of the IAR 136, assuming that the L5/S1 facets are perpendicular to the S1 endplate 26. Facets open into flexion when the IAR 136 is above the facet level, as shown in FIG. 9A. Facets close into flexion when the IAR 136 is below the facet level as shown in FIG. 9B. [00131] If the facet joint spaces 132 are considered vertical (i.e.
• FIG. 10 shows a graph of IAR 136 distance to the S1 endplate 26 (ZJ: mm) plotted against facet force variation ⁇ F (N) for each 3° rotation into flexion.
• the "grey zone” corresponds to the facet height and hence force sensor 130 location.
• the IAR height H is related to the facet force variation in flexion / extension.
• the present experimental data demonstrates that the IAR height H determines whether the facets 132 open or close during sagittal plane movements (as further described for example by reference to FIGS. 9A and 9B).
• the lateral bending experimental data revealed IAR 3-D obliquity, which is due to coupling between lateral bending and axial rotation. That is, if lateral bending were not associated with axial rotation, then the IAR direction would have been perpendicular to the plane of bending. Since bending was applied by simulating the 40° sacral obliquity (FIGS. 7 and 8), the expected IAR inclination would have been 40°.
• the circular area of the force sensor 130 that was used was about half the size of the facet joint 132 surface. As a potential result of this mismatch, it is possible that the facet contact area may have moved beyond measurement area during testing. However, the relatively continuous nature of the facet force measurements between postures suggested that this was not the case. In addition, a vertical cut in the facet joint capsule was necessary for inserting the sensors during preparation. This did not appear to adversely affect segmental kinematics as has been reported by others using pressure Fujifilm paper for mapping facet forces.
• the therapeutic system and method of the present invention combines disc replacement and facet joint modification to provide a highly beneficial and improved result versus disc replacement alone.
• the device therapy regimen of the present invention is adapted to reduce facet forces and thereby protect the joints from iatrogenic arthritis.
• the present data supports the conclusion that the system of the present invention provides a highly beneficial and improved interventional system and method by maintaining an IAR path that is cephalad during flexion, posterior or caudal during extension, and lateral in bending. Such a result more closely approximates the experimentally observed kinematics of the intact L5/S1 level.
• Motion is complex due to position-dependent interaction between disc and facets - during axial rotation, lateral bending, or extension the facets become more engaged (have higher forces) than during flexion. Consequently, during axial rotation, lateral bending, and extension, the IAR moves toward the facet joints. Implants that are meant to facilitate intervertebral motion may conflict with normal motion patterns, and when this occurs it will cause higher than normal force generation in either the facets or discs. Defined, three-dimensional patterns of normal intervertebral motion can therefore serve as a basis for design of dynamic stabilization devices so that the device-constrained motion can more closely match normal and thereby keep tissue stresses (and risk for back pain) minimized.
• the normal motion can be parameterized using the Instant-axis-of- rotation which is a line is space that an object rotates about and translates along as it moves from one position to another.
• the IAR is analogous to the path of a thrown football - the ball is rotating about and traveling along the path.
• Dynamic stabilization in the context of spinal implant of the present invention can be defined as constrained intervertebral movement, where the constraint is meant to eliminate unwanted, non-physiologic motions.
• the premise is that non-physiologic motion patterns are painful by creating elevated stresses in the disc and facets.
• Dynamic stabilization devices generally contact and guide adjacent vertebral movement. Two spaces are generally targeted for dynamic stabilization devices to reside (e.g.
• Posterior devices are generally attached to vertebra by pedicle screws. Intervertebral devices typically attach via metal endplates.
• PDS physiologic dynamic stabilization
• a family of devices may be provided that attach via pedicle screws or pedicle devices.
• Instrumentation related to such approach may include linked, hinged, deforming, or sliding members that, working together, facilitate intervertebral motion as herein described.
• Intervertebral A family of intervertebral devices in accordance with the present invention may include linked, hinged, deforming, or sliding members that work together to facilitate intervertebral motion as described herein. Due to space constraints, the device may include metal endplates with contoured articulating surfaces (which may be, for example, similar in certain regards to previously disclosed 'kinematic' knee replacements).
• contoured surfaces have position-dependent contact points with orientations of the mating surfaces such that, as the vertebra rotates, the surface constraint guides the proper kinematics in all three planes of motion.
• Current data are for L5/S1 , and motion patterns will be different for other spinal levels and thus accommodated in further embodiments properly responding from such information.
• Further embodiments are also to be modified and adapted as appropriate to also more closely approximate spinal motion under axial rotation, which particular dynamic is not specifically characterized in the motion according to the current data or resulting devices and methods for providing medical therapy herein described.
• improved generally applicable motion patterns may be further refined as more specimens are tested and in order to provide optimal prosthetic environment for assisting patients in restoring normal spinal motion.

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  • 2006-09-26 CA CA002622913A patent/CA2622913A1/en not_active Abandoned
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