Post-Operatively Adjustable Spinal Fixation Devices

Abstract

A system for spinal fixation with a non-rigid portion at least one of the caudal or cephalad terminus. Various devices and techniques are described for transition from a rigid fixation construct to a less rigid support structure applied to a “soft zone” that will help share the stress created on the spinal levels caused by the fixed levels below. In specific embodiments the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.

Description

BACKGROUND

Field

The present disclosure relates generally to medical devices, and specifically to surgical instruments and methods for performing spinal procedures.

Background

The spine is critical in human physiology for mobility, support, and balance. The spine protects the nerves of the spinal cord, which convey commands from the brain to the rest of the body, and convey sensory information from the nerves below the neck to the brain. Even minor spinal injuries can be debilitating to the patient, and major spinal injuries can be catastrophic. The loss of the ability to bear weight or permit flexibility can immobilize the patient. Even in less severe cases, small irregularities in the spine can put pressure on the nerves connected to the spinal cord, causing devastating pain and loss of coordination.

The spinal column is a bio-mechanical structure composed primarily of ligaments, muscles, bones, and connective tissue that forms a series of vertebral bodies stacked one atop the other and intervertebral discs between each vertebral body. The spinal column provides support to the body and provides for the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs; complex physiological motion between these parts; and protection of the spinal cord and the nerve roots.

The stabilization of the vertebra and the treatment for spinal conditions is often aided by a surgically implanted fixation device which holds the vertebral bodies in proper alignment and reduces the patient's pain and prevents neurologic loss of function. Spinal fixation is a well-known and frequently used medical procedure. Spinal fixation systems are often surgically implanted into a patient to aid in the stabilization of a damaged spine or to aid in the correction of other spinal deformities. Existing systems often use a combination of rods, plates, pedicle screws, bone hooks, locking screw assemblies, and connectors, for fixing the system to the affected vertebrae. The system components may be rigidly locked together to fix the connected vertebrae relative to each other, stabilizing the spine until the bones can fuse together.

Whatever the treatment, the goal remains to improve the quality of life for the patient. In the vast majority of cases this goal is achieved, however in some instances patients who receive implants to treat the primary pathology develop a secondary condition called junctional disease. Most commonly this occurs at the proximal or cephalad area of spinal instrumentation and is then termed “adjacent segment pathology.” Clinical Adjacent Segment Pathology (CASP) refers to clinical symptoms and signs related to adjacent segment pathology. Radiographic Adjacent Segment Pathology (RASP) refers to radiographic changes that occur at the adjacent segment. A subcategory of CASP and RASP that occurs at the proximal end of the instrumentation is termed proximal junctional kyphosis (PJK). PJK may be defined in several manners and commonly is specified as kyphosis measured from one segment cephalad to the upper end instrumented vertebra to the proximal instrumented vertebra with abnormal value defined as 10° or greater. In practice this often means that the patient's head and/or shoulders tend to fall forward to a greater degree than should normally occur. Sometimes the degree is significant.

Adjacent segment pathology can occur as either a degenerative, traumatic or catastrophic condition and sometimes as a result from a combination of factors. Degenerative conditions are ones that occur over a period of time, normally 5 or 6 years but can occur at an accelerated rate particularly with altered mechanics related to spinal fusion. As a result the patient's head and/or shoulder region(s) fall forward gradually over time. Traumatic and catastrophic conditions occur as a generally sudden shifting of the vertebral body immediately cephalad to the upper end instrumented vertebra and can lead to sudden changes in spinal alignment with marked symptoms noted by the patient.

Whether the condition is degenerative, traumatic, or catastrophic, the exact cause of adjacent segment pathology is uncertain. Without wishing to be bound by any hypothetical model, it is generally believed that adjacent segment pathology and more specifically PJK is a result of excess strain and stress on the proximal instrumented spinal segment which is then at least partially transferred to the bone structures, disc, ligaments and other soft tissues, causing a loss of normal structural integrity and mechanical properties. The resultant effect can be a forward (i.e. kyphotic) shift of the adjacent non-instrumented vertebral body. One such theory is that this strain and stress is caused by suboptimal alignment and/or balance of the screw and rod construct. Another theory is that the rigidity of the screw and rod construct causes the problem in that the transition from a motion-restrained segment to a motion-unrestrained segment is too much for the non-instrumented (unrestrained) segment to handle over time. Yet another theory speculates that the specific level at which the proximal instrumented vertebra is located is of vital importance in that some levels may be better suited to handle a proximal termination of a fixation construct than others.

Thus there remains a need for continued improvements and new systems for spinal fixation with a specific goal of preventing the occurrence of or reducing the degree of adjacent segment pathology and failures occurring at either the distal junction (DJK) or proximal junction (PJK). The implants and techniques described herein are directed towards overcoming these challenges and others associated with posterior spinal fixation.

SUMMARY

The problems noted above, as well as potentially others, are addressed in this disclosure by a system for spinal fixation with a non-rigid portion at least one of the caudal or cephalad terminus. Various devices and techniques are described for transition from a rigid fixation construct to a less rigid support structure applied to a “soft zone” that will help share the stress created on the spinal levels caused by the fixed levels below. In specific embodiments the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.

In a first aspect, a system for spinal fixation is provided comprising: a first bone anchor, anchored to a first vertebra in a subject, the first bone anchor comprising a first bone fastener attached to a first rod housing; a rigid spinal rod seated in the first rod housing to restrict translation of the rigid spinal rod relative to the first bone anchor; a second bone anchor, anchored to a second vertebra in the subject, the second bone anchor comprising a second bone fastener attached to a second rod housing, wherein the rigid spinal rod is seated in the second rod housing to restrict translation of the rigid spinal rod relative to the second bone anchor; and a compressible spinal connector, connected to the second bone anchor, and anchored to a third vertebra in the subject, the compressible spinal connector comprising a modulation mechanism for modulating at least one of the tension on the compressible spinal connector or the resistance to compression of the compressible spinal connector, wherein said modulation occurs in response to a remote signal.

In a second aspect, a spinal tether assembly for providing non-rigid intervertebral support is provided, comprising: a flexible tether; and an adjustable tensioner connected to exert tension on the flexible tether, the adjustable tensioner comprising a first magnet mounted to rotate in response to a spinning magnetic field; and a tensioning mechanism configured to convert rotation of the magnet to a decrease or increase of tension on the flexible tether, depending on the direction of the first magnet's rotation.

In a third aspect, a dampening spinal rod to adjust friction against tension and compression is provided, comprising: an elongate rigid portion for insertion into a bone anchor; a flared portion for receiving a terminal end of a second spinal rod, the flared portion comprising a rod cavity of sufficient diameter to accept the second spinal rod, and a friction control mechanism configured to modulate friction between the second spinal rod and said dampening spinal rod in response to a remote signal.

In a fourth aspect, a method of fixing the relative positions of a first vertebra and a second vertebra in a subject is provided, the method comprising: anchoring a first bone anchor to the first vertebra, the first bone anchor comprising a first bone fastener attached to a first rod housing; seating a rigid spinal rod in the first rod housing to restrict translation of the rigid spinal rod relative to the first bone anchor; anchoring a second bone anchor to the second vertebra, the second bone anchor comprising a second bone fastener attached to a second rod housing, seating the rigid spinal rod in the second rod housing to restrict translation of the rigid spinal rod relative to the second bone anchor; connecting a compressible spinal connector to the second bone anchor, the compressible spinal connector comprising a modulation mechanism for modulating at least one of the tension on the compressible spinal connector or the resistance to compression of the compressible spinal connector, wherein said modulation occurs in response to a remote signal; anchoring the compressible spinal connector to a third vertebra in the subject; and transmitting the remote signal to the modulation mechanism post-operatively, to cause said modulation to occur.

The above presents a simplified summary in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview. It is not intended to identify key or critical elements or to delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. An example of a spinal fixation system.

FIG. 2. A side view of an embodiment of the dampening spinal rod as part of a larger fixation system, showing only the skeletal system of the subject.

FIG. 3. A dorsal (anterior) view of an embodiment of the spinal fixation system's terminal region having multiple soft connections.

FIG. 4. A perspective view of an embodiment of a tensioner that modules the tension on a flexible tether.

FIG. 5. An embodiment of a locking mechanism for locking a tensioner.

FIG. 6. A cross-sectional view of an embodiment of a telescoping spinal rod.

FIG. 7. An embodiment of a turnbuckle tensioner for modulating the tension on a flexible tether.

FIG. 8. A dorsal (anterior) view of an embodiment of the spinal fixation system's terminal region having multiple soft connections.

FIG. 9. A top view of an embodiment of the external control device.

DETAILED DESCRIPTION

Illustrative embodiments of a system for spinal fixation, parts, and methods for use thereof, are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The system for spinal fixation, parts, and methods for use thereof disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination.

This disclosure describes a variety of transitional or terminal components that may be implanted as part of a spinal fixation construct 5 to decrease the potential for subsequent development of junctional disease or failure. In the examples shown only the cephalad most level (for terminal hardware) or levels (for multilevel transitional hardware) of the fixation construct 5 (e.g. those utilizing the exemplary components described herein) are illustrated. It should be appreciated, however, that the entire fixation construct 5 may extend any number of levels from a single level construct to a long construct spanning multiple spinal levels and multiple spinal regions from the lumbosacral to cervical regions (such as the example construct illustrated in FIG. 1), and with any variety of combinations of known anchors, rods, and connectors. It should also be appreciated that the exemplary terminal and/or transitional components may additionally or alternatively be utilized at the caudal end of the fixation construct. Moreover, although the vertebral fixation systems 5 described herein may be used along any aspect of the spine (e.g. anterior, posterior, antero-lateral, postero-lateral) they are particularly suited for implantation along a posterior aspect of the spine.

A general embodiment of the system comprises a first bone anchor 10, anchored to a first vertebra in a subject, the first bone anchor 10 comprising a first bone fastener 15 attached to a first rod housing 20. A rigid spinal rod 25 is seated in the first rod housing 20 to restrict translation of the rigid spinal rod 25 relative to the first bone anchor 10. The rigid spinal rod 25 is seated in the rod housing 35 of a second bone anchor 30, anchored to a second vertebra in the subject, so as to restrict translation of the rigid spinal rod 25 relative to the second bone anchor 30. A compressible spinal connector 40 is connected to the second bone anchor 30 and anchored to a third vertebra in the subject. The compressible spinal connector 40 has a modulation mechanism 45 for modulating either the tension on the compressible spinal connector 40 or its resistance to compression (or both). The modulation occurs in response to a remote signal. Consequently modulation of the tension and/or resistance to compression does not require access to the device 5 through the patient's tissues, and may be performed post-operatively. The remote signal may be, for example, an electromagnetic signal. A specific example of the remote signal is a spinning magnetic field.

FIG. 1 illustrates an example of a vertebral fixation system 5 of the type that is used with the devices and methods described in this disclosure. By way of example, the illustrated vertebral fixation system 5 is a screw-and-rod construct adapted for implantation along the posterior aspect of the human spinal column. The vertebral fixation system 5 includes a pair of elongate rods (50a, 50b) dimensioned to span multiple vertebral levels, a plurality of threaded bone anchors 55, a plurality of hook-type bone anchors 60, and a plurality of transverse connectors 65 dimensioned to rigidly engage each of the elongate rods (50a, 50b) so as to hold each rod (50a, 50b) in place relative to the other. The transverse connectors 65 may be provided as fixed connectors or adjustable connectors, in any quantity that is required by the surgeon performing the implantation surgery. Proximal bone anchors 70 are provided at the proximal (cephalad) terminus of the assembly 5. Distal bone anchors 75 are provided at the distal (caudal) terminus of the assembly 5. It is contemplated that any of the examples of bone anchors and other transition assemblies described herein may be substituted for the cephalad bone anchors 70 and/or caudal bone anchors 75 which are traditionally rigid and identical to the other bone anchors used throughout the construct 5. It is also contemplated that the examples of flexible or compressible transition segments 80 described herein may replace existing hardware at the cephalad and/or caudal terminus of the vertebral fixation system 5 such that there is no additional surgical footprint realized. It is further contemplated that the examples of flexible or compressible transition segments 80 described herein may augment existing hardware at the cephalad and/or caudal terminus of the vertebral fixation system 5 such that there is additional added surgical footprint realized. This may be more applicable with the various embodiments that can be installed with minimal disruption of additional muscle tissue and/or ligament structure. Finally, as previously noted junctional disease or failure can be a problem at either the cephalad or caudal terminus (or both) of vertebral fixation systems 5. Therefore, although the various examples disclosed herein may be described in terms of cephalad terminus and proximal joint disease (for ease of disclosure) it is to be understood that any of the example embodiments are also applicable and may be used at the caudal terminus of the vertebral fixation system 5 without deviating from the scope of this disclosure. According to one example, a spinal fixation construct 5, like that shown in FIG. 1, is applied to the spinal levels to be fixed. Above the fixed levels a soft-zone is created by applying non-rigid support elements 85 such as tethers or adjustable rods that limit some motion and reduce stress, to the levels of the soft-zone and above, while not inhibiting all motion. The tension applied to the support elements 85 in the soft zone can be adjusted post-operatively and non-invasively to account for changing dynamics in the body, or for any other reason deemed desirable.

The components in the system 5 are constructed from one or more non-absorbable biocompatible materials. Specific examples of such suitable materials include titanium, alloys of titanium, steel, and stainless steel. Parts of the system 5 could conceivably be made from non-metallic biocompatible materials, which include aluminum oxide, calcium oxide, calcium phosphate, hydroxyapatite, zirconium oxide, and polymers such as polypropylene. Interference with the spinning magnetic field can be reduced by constructing one or more portions of the system 5 from a nonmagnetic or weakly magnetic material. Specific examples of such nonmagnetic non-absorbable biocompatible material include titanium, alloys of titanium, aluminum oxide, calcium oxide, calcium phosphate, hydroxyapatite, zirconium oxide, and polymers such as polypropylene. Examples of weakly magnetic materials include paramagnetic materials and diamagnetic materials. In a specific embodiment, the weakly magnetic material is austenitic stainless steel.

The first, second, and third vertebrae may be adjacent or non-adjacent to one another, in any combination. Thus it is contemplated that the first vertebra will be adjacent to the second, which will be adjacent to the third; the first vertebra will be nonadjacent to the second, which will be adjacent to the third; the first vertebra will be nonadjacent to the second, which will be nonadjacent to the third; and that the first vertebra will be adjacent to the second, which will be nonadjacent to the third.

According to one example the non-rigid support structure 5 is created through the application of a compressible spinal connector 40 in the form of one or more tether assemblies 95, such as those shown in the exemplary embodiment in FIG. 3. In the tether assembly 95 the modulation mechanism 45 is an adjustable tensioner 100 configured to vary the tension on a flexible tether 97. This will control the tension between the second bone anchor 30 and the third vertebra.

The tethers 97 may be attached between the fixation hardware 5 and the soft-zone (e.g. one or more non-fixed levels above), and/or directly between the bone elements of one or more fixed levels and the soft-zone, and/or between two or more of the non-fixed levels in the soft-zone. The tether 97 may be formed of any material suitable for medical use. For example, the tether 97 may be made from allograft tendon, autograft tendon, braided, woven, or embroidered polyethylene, braided, woven, or embroidered polyester, polyether ether ketone (PEEK), or polyetherketoneketone (PEKK). In some instances the tether 97 may be formed of elastic material. FIG. 3 depicts multiple tethers 97 applied to the spine in the soft zone and connected to the fixation construct 5 by different connectors (e.g. an adjustable tension tether-rod connector, adjustable tension cross-connector, and turnbuckle (FIG. 7)—these may be collectively referenced herein simply as adjustable tension connectors 215). It will be appreciated that while shown in use together, either of these connectors 215 may also be used on their own. and in any configuration desired. In use, once the tethers 97 are connected to the fixation construct 5 and the tether 97 coupled to the desired bone structure (or other bone connection element) the tension on the tether 97 can be adjusted. Later, the tension on the tether 97 can be adjusted post-operatively as desired using an external device 155.

A specific example of the adjustable tensioner 100 is a turnbuckle 105 comprising a threaded first end coupler 110, a second end coupler 115, and a rotatable magnet 120 that rotates in response to a spinning magnetic field and that is connected to the threaded first end coupler 110 to cause the threaded first end coupler 110 to rotate about its longitudinal axis when the rotatable magnet 120 rotates. An embodiment of such a turnbuckle 105 is shown in FIG. 7. The illustration is a cross-section of the turnbuckle 105, showing a cylindrical magnet 125 oriented to rotate around its longitudinal axis when exposed to a spinning magnetic field in the right orientation. The threaded first end coupler 110 in the illustration is a hook 130 with a threaded shank 135. The threaded shank 135 runs through a threaded channel 140 in the housing 145 of the turnbuckle 105, which translates rotation of the shank 135 into translation of the hook 130. Alternatively, the magnet 125 itself may contain one or more threaded passages 150 that are engaged with the threaded shank(s) 135. As a result the hook 130 can be extended or retracted by rotating the shank 135. In some embodiments of the turnbuckle 105 both end couplers (110, 115) are hooks (130a, 130b) with threaded shanks 135, the threads oriented such that when the rotatable magnet 125 rotates in a given direction the two hooks (130a, 130b) translate in opposite directions (i.e., they either converge or diverge along their shared longitudinal axis). When the hooks (130a, 130b) are caused to converge, it will increase tension on the connected spinal physiology.

As pictured in FIG. 7, the turnbuckle 105 can be attached to the spinous process or lamina of two adjacent vertebrae (either directly, or via tethers looped around the lamina or spinous process), and the tension between the vertebrae can be adjusted post-operatively and non-invasively using the external adjustment device 155 to rotate the turnbuckle magnet 120. Though shown only across a single level, turnbuckles 105 could be used at multiple levels. According to one example the turnbuckles 105 can be used selectively to set the tension differently at each level. By way of example, the tension can start out higher closest to the fixed spinal levels, and be sequentially decreased over a series of levels through the soft-zone. In some embodiments of the system a pair of turnbuckles (105a, 105b) is used bilaterally and coupled to tethers 97 looped around the lamina and the superior and inferior coupled vertebrae. A distraction device 160 is also positioned between the spinous processes of the same level. It is contemplated that the distraction device 160 could use a magnetically driven expansion device (such as one utilizing a lead screw coupled to a magnet, similar to that described below, to create linear expansion). This way, both flexion and extension could be effectively controlled, and adjusted post-operatively. Taking it a step further, the addition of a rotatable element 220 within the disc to allow the vertebrae to rotate relative to each other, could facilitate scoliosis correction in both the sagittal and coronal planes using the adjustable turnbuckle 105 and distraction devices 160.

Another embodiment of the adjustable tensioner 100 is a spool 165 about which the flexible tether 97 is wound, and wherein rotation of a spool magnet 170 drives rotation of the spool 165. An example of such an embodiment is shown in FIG. 4, and the remaining description in this paragraph refers to this example. The spool 165 is connected to a body 175 having a rod passage 180 that couples to the rod 50. A setscrew 205 or other element may be used to lock the body 175 to the rod 50. The spool 165 is rotatably connected to the body 175. The tether 97 is attached to the spool 165 such that when the spool 165 rotates the tether 97 is wound up on the spool 165 to create tension. A first end 185 of the tether 97 may be attached to the spool 165 with the second end 190 being otherwise attached to the target bone, another adjustable connector, or to itself (e.g. creating a loop that can be attached to the target bone, or both ends of the tether (185, 190) may be coupled to the spool 165 creating a loop to attach to or around the bone such that both ends of the tether (185, 190) are spooled up together). The spool magnet 170 can be driven by application of a magnetic field to rotate the spool 165. The spool magnet 170 may be a single cylindrical magnet poled north and south across its diameter to form two 180 degree sectors, as in FIG. 4. Alternatively, a quadrupole or multipole magnet may be used. The spool 165 may include a locking mechanism 195, such as a spool 165 and ratchet mechanism 200, to maintain the tension applied. The locking mechanism 195 may be externally controlled similar to the spool magnet 170 such that it can be locked and unlocked if adjustment is needed. According to one embodiment, the locking mechanism 195 may be a set screw 205 situated to inhibit rotation of the spool 165 when engaged. The set screw 205 may be a magnetically driven set screw 225, oriented such that the external drive controller 155 can be positioned to drive only one of the set screw 205 and drive magnet 230, and then prepositioned to drive the other. Alternatively, a locking pin or shaft could be advanced with the set screw 205 to inhibit rotation of the spool 165. In an alternative embodiment shown in FIG. 5, the magnet 230 may drive a gear assembly 380 that rotates two opposing ratchet wheels through 385 which the tether 97 is passed.

Another embodiment of the compressible spinal connector 40 is a dampening rod 235. The dampening rod 235 is a rod that is both expandable and compressible, and the resistance to expansion and compression is controlled by means of the modulation mechanism 45. The modulation mechanism 45 in this embodiment may take the form of a friction brake 240. The dampening rod 235 accommodates dynamic travel or length adjustment of the rod 235 between the fixed connectors 390. The friction brake 240 can include a set screw 205 that is itself magnetic, or connected to a magnet (“brake magnet”) 245 that may be controllable via an external adjustment device 155. The degree of tension and support provided by the dampening rod 235 can be controlled by increasing or decreasing friction with the set screw 205. Some embodiments of the friction brake 240 can also lock down the rod 235 entirely, to prevent any expansion or compression, should it later become necessary to fix one or more levels in the soft-zone. An embodiment of the dampening rod 235 is shown in FIG. 2. In that embodiment, each rod 235 has a wider bell region 395 and a narrower tail region 400. The tail 400 is about the diameter of an ordinary spinal rod. The bell 395 is open on the inside, and is dimensioned to accommodate a spinal rod (or the tail of another dampening rod 235). As shown in FIG. 2, the friction brake 240 may be, for example, a set screw 205 in a threaded channel 405 positioned to exert compressive force on a spring 250, said spring 250 positioned to exert compressive force against both the compression and expansion of the dampening rod 235. The illustration in FIG. 2 shows the spring 250 positioned to exert compressive force on the tail portion 400 of an adjacent dampening rod 235. The spring 250 in FIG. 2 is a wave spring 410, although other kinds of springs (e.g., helical) are contemplated as well. The set screw 205 may be magnetic (225), or coupled to a magnet, such that a rotating magnetic field in the proper orientation will cause the set screw 225 to rotate, either increasing or decreasing the compressive force that exerts the friction.

A telescoping rod 255 may also be employed in the system. The telescoping rod 255 may be implanted at levels above a fixation construct 5 in patients that are at high risk of developing PJK or other adjacent segment diseases. The rods 255 may be implanted as a prophylactic and used if needed to extend the length for pain relief. An example of the telescoping rod 255 is shown in FIG. 6. As shown in that figure, the telescoping spinal rod 255 comprises a rod magnet 260 configured to rotate when exposed to a spinning magnetic field and cause the telescoping spinal rod 255 to either extend or collapse depending on the direction of the spinning magnetic field. The rod magnet 260 may be a cylindrical permanent magnet (such as a ferrimagnet), but may be another type of magnet. A first elongate element 265 contains a cavity 270, into which fits a second elongate element 275. There may be a dynamic seal between the first 265 and second elongated elements 275, to ensure that no bodily fluids enter the construct. A thrust bearing may be included to reduce friction between the spinning magnetic element and the housing. The second elongate element 275 has an internally threaded region 280 that is engaged to a lead screw 285 coupled to rotate when the rod magnet 260 rotates, and comprising an externally threaded region 290, such that rotation of the lead screw 285 causes the second elongate element 275 to translate relative to the first elongate element 265. Thus, when the magnet 260 rotates, the lead screw 285 will also rotate. The threaded interface between the lead screw 285 and the second elongated element 275 will then cause the second elongated element 275 to translate, with the translational direction being dependent upon the rotational direction of the magnet 260. The threads on the internally threaded region 280 of the first elongate element 265 may be integral, or they may be on the inner surface of another structure, such as a threaded nut.

Whenever the adjustment mechanism is actuated by the rotation of a magnet 120, as a safety precaution, a magnetic immobilization plate 295 may be positioned sufficiently close to the rotatable magnet 120 to cause the rotatable magnet 120 to adhere to the immobilization plate 295 in the absence of a strong external magnetic field. The magnetic immobilization plate 295 will hold the rotating magnet 120 in position, preventing it from rotating, until a stronger magnetic field is applied, such as the rotating magnetic field that is used to adjust the modulation mechanism 45. Like the rotating magnet 120, the immobilization plate 295 may be constructed from a suitable magnetic material, such as a ferromagnetic material. The immobilization plate 295 may be used on its own, or in combination with a locking mechanism 195 as described above.

A specific embodiment of the system is shown in FIG. 3. In that embodiment, the system 5 comprises a third bone anchor 300 (comprising a third bone fastener 305 and a third rod housing 310) anchored to the first vertebra, and a fourth bone anchor 320 (comprising a fourth bone fastener 330 and a fourth rod housing 340) anchored to the second vertebra. A second rigid spinal rod is 25b seated in the third rod housing 310 and the fourth rod housing 340, running roughly parallel to the first rod 25a. A first flexible tether 97a is at least partially wrapped around a spinous process of the third vertebra and connected to both of the first 25a and second rigid spinal rods 25b to exert tension between the third vertebra and the first 25a and second rigid spinal rods 25b. The adjustable tether assembly 95 comprises a second flexible tether 97b encircling the spinous process of the third vertebra and a spinous process of a fourth vertebra, and a tensioner 350 connected to the first 25a and second 25b rigid spinal rods, the tensioner 350 comprising a first magnet 355 mounted to rotate in response to a spinning magnetic field, and a tether connection 360 configured to increase or decrease the tension on the second flexible tether 97b depending on the direction of rotation of the first magnet 355. The tensioner 350 extends or retracts the tether connection 360 depending on the direction of the spinning magnetic field, either reducing or increasing the tension respectively.

The system may be bilateral, in which the network of bone anchors and rods is present on either side of the spine. Such a bilateral system may comprise a second rigid spinal rod 25b seated in an additional rod housing 365 of an additional bone anchor 370 that is anchored in at least one of the first and second vertebrae. As shown in FIG. 1, one or more transverse connectors 65 fastened to the first rigid spinal rod 25a and the second rigid spinal rod 25b may be present for additional stability.

Methods of using the system 5 to fix the relative positions of a first vertebra and a second vertebra in a subject are provided. In a general embodiment the method comprises anchoring a first bone anchor 10 to the first vertebra, the first bone anchor 10 comprising a first bone fastener 15 attached to a first rod housing 20; seating a rigid spinal rod 25a in the first rod housing 20 to restrict translation of the rigid spinal rod 25a relative to the first bone anchor 10; anchoring a second bone anchor 30 to the second vertebra, the second bone anchor 30 comprising a second bone fastener 33 attached to a second rod housing 35; seating the second rigid spinal rod 25b in the second rod housing 35 to restrict translation of the rigid spinal rod 25b relative to the second bone anchor 30; connecting a compressible spinal connector 40 to the second bone anchor 30, the compressible spinal connector 40 comprising a modulation mechanism 45 for modulating at least one of the tension on the compressible spinal connector 40 or the resistance to compression of the compressible spinal connector 40, wherein said modulation 45 occurs in response to a remote signal; anchoring the compressible spinal connector 40 to a third vertebra in the subject; and transmitting the remote signal to the modulation mechanism 45 post-operatively, to cause said modulation to occur. The system 5 may have any of the components and arrangements described above. The compressible spinal connector 40 can be any described as suitable for the system above, including any of the described embodiments of the tether assembly 95, dampening rod 235, and telescoping rod 255.

An example of an external adjustment device 155 used to non-invasively drive the adjustment mechanisms on the various implants described herein is represented in FIG. 9. The external adjustment device 155 is configured for placement on or adjacent to the skin of the subject and appropriately aligned with the magnet to be activated, and includes at least one drive magnet 230 configured for rotation. The external adjustment device 155 further comprising a motor 375 configured to rotate the at least one drive magnet 230, whereby rotation of the at least one drive magnet 230 of the external adjustment device 155 effectuates rotational movement of one or more magnets (e.g., rotatable magnet in the turnbuckle, magnetic set screw, etc). As shown in FIG. 9, the external adjustment device 155 may have two magnets (230a, 230b). The two magnets (230a, 230b) may be configured to rotate at the same angular velocity. They may also be configured to each have at least one north pole and at least one south pole (i.e., a dipole magnet), and the external adjustment device 155 is configured to rotate the first drive magnet 230a and the second drive magnet 230b such that the angular location of the at least one north pole of the first drive magnet 230a is substantially equal to the angular location of the at least one south pole of the second drive magnet 230b through a full rotation of the first 230a and second 230b drive magnets. More complex systems involving quadrupole and multipole drive magnets are also contemplated, as is the use of one or more electromagnets. The foregoing description illustrates and describes the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure. Additionally, the disclosure shows and describes only certain embodiments of the processes, machines, manufactures, compositions of matter, and other teachings disclosed, but, as mentioned above, it is to be understood that the teachings of the present disclosure are capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the teachings as expressed herein, commensurate with the skill and/or knowledge of a person having ordinary skill in the relevant art. The embodiments described hereinabove are further intended to explain certain best modes known of practicing the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure and to enable others skilled in the art to utilize the teachings of the present disclosure in such, or other, embodiments and with the various modifications required by the particular applications or uses. Accordingly, the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure are not intended to limit the exact embodiments and examples disclosed herein. Any section headings herein are provided only for consistency with the suggestions of 37 C.F.R. §1.77 and related laws or otherwise to provide organizational queues. These headings shall not limit or characterize the invention(s) set forth herein.

Claims

1. A system for spinal fixation, the system comprising: a first bone anchor, anchored to a first vertebra in a subject, the first bone anchor comprising a first bone fastener attached to a first rod housing;a rigid spinal rod seated in the first rod housing to restrict translation of the rigid spinal rod relative to the first bone anchor;a second bone anchor, anchored to a second vertebra in the subject, the second bone anchor comprising a second bone fastener attached to a second rod housing, wherein the rigid spinal rod is seated in the second rod housing to restrict translation of the rigid spinal rod relative to the second bone anchor; anda compressible spinal connector, connected to the second bone anchor, and anchored to a third vertebra in the subject, the compressible spinal connector comprising a modulation mechanism for modulating at least one of: the tension on the compressible spinal connector orthe resistance to compression of the compressible spinal connector,wherein said modulation occurs in response to a remote signal.

2. The system of claim 1, wherein the remote signal is a spinning magnetic field.

3. The system of claim 1, wherein at least one of the following is at least partially composed of a non-absorbable biocompatible material: the first bone anchor, the second bone anchor, the rigid spinal rod, and the compressible spinal connector.

4. The system of claim 1, wherein the remote signal is a spinning magnetic field, and wherein at least one of the following is at least partially composed of a non-absorbable biocompatible material that is either non-magnetic or weakly magnetic: the first bone anchor, the second bone anchor, the rigid spinal rod, and the compressible spinal connector.

5. The system of claim 1, wherein the compressible spinal connector is a tether assembly.

6. The system of claim 5, wherein the modulation mechanism is an adjustable tensioner configured to vary the tension on a flexible tether between the second bone anchor and the third vertebra.

7. The system of claim 6, wherein the adjustable tensioner is a turnbuckle comprising a threaded first end coupler, a second end coupler, and a rotatable magnet that rotates in response to a spinning magnetic field and that is connected to the threaded first end coupler to cause the threaded first end coupler to rotate about its longitudinal axis when the rotatable magnet rotates.

8. The system of claim 6, wherein the adjustable tensioner comprises a spool about which the flexible tether is wound, and wherein rotation of a spool magnet drives rotation of the spool.

9. The system of claim 6, wherein the adjustable tensioner comprises a locking mechanism configured to maintain tension on the flexible tether when engaged.

10. The system of claim 5, wherein the flexible tether is constructed of a non-absorbable biocompatible material.

11. The system of claim 1, wherein the compressible spinal connector is a dampening spinal rod that is compressible and expandable.

12. The system of claim 11, the dampening spinal rod comprising: an elongate rigid portion for insertion into a bone anchor;a flared portion for receiving a terminal end of a second spinal rod, the flared portion comprising a rod cavity of sufficient diameter to accept the second spinal rod.

13. The system of claim 11, wherein the modulation mechanism is a friction brake configured to vary the resistance of the dampening rod to compression and tension.

14. The system of claim 13, wherein the friction brake comprises a set screw in a threaded channel positioned to exert compressive force on a spring, said spring positioned to exert compressive force against both the compression and expansion of the dampening rod.

15. The system of claim 14, wherein the set screw is magnetic and rotates in the threaded, channel in response to a spinning magnetic field.

16. The system of claim 1, wherein the compressible spinal connector is a telescoping spinal rod positioned within the second rod housing.

17. The system of claim 16, wherein the telescoping spinal rod comprises: a rod magnet configured to rotate when exposed to a spinning magnetic field and cause the telescoping spinal rod to either extend or collapse depending on the direction of the spinning magnetic field;a first elongate element containing a cavity; anda second elongate element dimensioned to at least partially fit within the cavity, and having an internally threaded region;wherein the modulation mechanism comprises a lead screw coupled to rotate when the rod magnet rotates, and comprising an externally threaded region engaged to the internally threaded region of the second elongate element, such that rotation of the lead screw causes the second elongate element to translate relative to the first elongate element.

18. The system of claim 1, wherein a second rigid spinal rod is seated in an additional rod housing of an additional bone anchor that is anchored in at least one of the first and second vertebrae.

19. The system of claim 18, wherein a transverse connector is fastened to the first rigid spinal rod and the second rigid spinal rod.

20. The system of claim 1, comprising: a third bone anchor comprising a third bone fastener and a third rod housing, anchored to the first vertebra;a fourth bone anchor, comprising a fourth bone fastener and a fourth rod housing, anchored to the second vertebra;a second rigid spinal rod seated in the third rod housing and the fourth rod housing; anda first flexible tether at least partially wrapped around a structure of the third vertebra and connected to both of the first and second rigid spinal rods to exert tension between the third vertebra and the first and second rigid spinal rods;