WO2004011092A1 - Method and device to treat back pain - Google Patents

Abstract

Back pain is attributable, at least in part, to the development of fissures (15) in the annulus fibrosus (8) of the intervertebral disc. The disc is treated by positioning an inductively heatable implant (17) within the nucleus pulposus (7) of the disc (1), preferably close to the inner wall of the annulus fibrosus. The implant (17) is exposed to rf energy and heats up. Heat is passed from the heated implant to the interior of the disc, thus providing relief to the patient.

Description

METHOD AND DEVICE TO TREAT BACK PAIN

RELATED APPLICATIONS This application claims priority from U.S. Provisional Patent Application Serial No. 60/398497, filed on July 25, 2003, and incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method and devices to modify human or animal intervertebral disc tissue, especially annular fissures which are the cause of back pain. The invention uses an implant and an inductive heating process of such.

BACKGROUND OF THE INVENTION

The intervertebral disc 1, see FIGs. 1-3, primarily serves as a mechanical cushion between the vertebral bones 2a and 2b, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulposus 7 ("nucleus"), the annulus fibrosus 8 ("annulus") and two vertebral end plates . The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.

The annulus 8 of the disc is a tough, outer fibrous ring which binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fiber layers 12 of the annulus 8 consist of fifteen to twenty overlapping multiple plies 12, and are inserted into the superior and inferior vertebral bodies at roughly a forty degree angle 16 in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.

Immersed within the annulus 8, positioned much like the liquid core of a golf ball, is the nucleus 7. The healthy nucleus 7 is largely a gel-like substance having a high water content, and like air in a tire, serves to keep the annulus 8 tight yet flexible. The nucleus- gel moves slightly within the annulus 8 when force is exerted on the adjacent vertebrae with bending, lifting, etc.

The nucleus and the inner portion of the annulus 8 have no direct blood supply. In fact, the principal nutritional source for the central disc arises from circulation within the vertebral body. Microscopic, villous-like fingerlings of the nucleus and annulus 8 tissue penetrate the vertebral end plates and allow fluids to pass from the blood across the cell membrane of the fingerlings and then inward to the nuclear tissue. These fluids are primarily body water and the smallest molecular weight nutrients and electrolytes. The natural physiology of the nucleus 7 promotes these fluids being brought into and released from the nucleus by cyclic loading. When fluid is forced out of the nucleus 7, it passes again through the end plates and then back into the richly vascular vertebral bodies. This cyclic loading amounts to daily variations in applied pressure on the vertebral column (body weight and muscle pull) causing the nucleus 7 to expel fluids, followed by periods of relaxation and rest, resulting in fluid absorption or swelling by the nucleus 7. Thus, the nucleus 7 changes volume under loaded and non-loaded conditions. Further, the tightening and loosening effect stimulates normal annulus 8 collagen fibers to remain healthy or to regenerate when torn, a process found in all normal ligaments related to body joints. Notably, the ability of the nucleus 7 to release and imbibe fluids allows the spine to alter its height and flexibility through periods of loading or relaxation.

Chronic lumbar pain is a common cause of disability, with an estimated 5% of the American population suffering from chronic, disabling low back pain. Acute low back pain often responds to physical therapy and activity modification.

According to Kevin J. et al. Intradiscal electrothermal therapy for the treatment of discogenic back pain, Applied Radiology, July 2001 , approximately 90% of acute low back pain sufferers will have a resolution of their pain within 6 tol2 weeks, but epidemiological studies have shown that more than 60% of patients will suffer from recurrent symptoms. It has been reported that the prevalence of discogenic pain among patients with chronic low back pain approaches 40%.

According to Saal J.A. et al., Intradiscal Electrothermal Therapy for the treatment of chronic discogenic low backpain, Operative Techniques in Orthopedic Vol. 10 No 4; S271 - 281, 2000, patients with chronic lumbar pain fall into two clinical categories: chronic recurrent and chronic persistent. Chronic recurrent patients have multiple pain flares with varying durations of two weeks to three months. Many patients within this group will begin to have more frequent recurrences with fewer pain-free intervals as time passes. Chronic persistent patients have persistent symptoms that do not abate and last longer than three months. The disc has been shown to be the pain source in the vast majority of patients with chronic symptoms. Carey T. et al., Beyond good prognosis: Examination of an inception cohort of patients with chronic low backpain, Spine 25:115- 120, 2000, reported that patients who do not experience a resolution of their back pain within three months of onset had a poor prognosis for further recovery. When assessed at 22 months, patients continued to complain of low back pain and were dissatisfied with their outcomes. Von Korff, M., Studying the natural history of back pain, Spine 19:2041S-2046s, 1994, reported that although 80% of patients had resolution of their acute low back pain in 12 weeks, 60% of the patients experienced recurrent symptoms. The natural history of the degenerating disc includes the loss of nuclear hydrostatic pressure, which leads to buckling of the annular lamellae. This phenomenon leads to increased focal segment mobility and increased shear stress to the annular wall. Moore R. J. et al., The origin and fate ofherniated lumbar intervertebral disc tissue, Spine 21; 2149-2155, 1996, showed that this process continues to delaminating (separation into layers) and fissuring of the annular wall. Annular delaminating has been shown to occur as a separate and distinct event from annular fissures. Fissures can be radial or concentric. In addition, electron microscopy has shown microscopic fractures of collagen fibrils with disc degeneration. The progressive degeneration of the disc, manifested by any of these morphologic changes, has been shown to alter disc mechanics, see Schmidt T. A. et al, The stiffness of lumbar spinal motion segments with a high intensity zone in the annulus fibrosus, Spine 23:2167-2173, 1998.

Tearing and delaminating of the annulus can cause chronic pain. Mechanoreceptors in the disc wall have been shown to discharge with disc mobilization (Robert S. et al., Mechanoreceptors in intervertebral discs, Spine 20:2645-2651, 1995).

A scenario for chronic discogenic pain is created when any combination of annular fissures, delaminating, or micro fractures of collagen fibrils leads to mechanical distortion of annular lamellae. A combination of mechanical and neural properties creates an interplay that leads to chronic discogenic pain. Disc bulging is always associated with annular degeneration and fissures; however, this phenomenon does not always create clinically significant low back pain.

Lumbar disc herniation is a distinct pathologic entity. Lumbar intervertebral disc herniation typically occurs as a result of annular degeneration leading to weakening of the annulus fibrosis, leaving it susceptible to annular Assuring 15 and tearing. Nuclear migration caused by annular disruption leads to the most common forms of clinically recognized lumbar disc herniation (LDH) (i.e., contained, extruded, and sequestered).

The spinal canal location of disc trespass will determine the type of neural compromise and clinical pain pattern. The degree of neural compromise cannot be judged accurately by the size, type, or location of the disc material (Saal J.A., Natural history of nonoperative treatment of lumbar disc herniation, Spine 21; 2S-9s, 1996).

Classic disc herniation predominantly creates leg pain. Some contained disc herniation, especially central herniation, creates a mixed pattern of both back and leg pain. There are three general types of disc pathologies: a. The leg pain disc, which is the classic disc herniation with nuclear migration; b. The back pain disc, which is the internally disrupted disc (discogenic pain) with annular pathology creating back pain, and variable amounts of leg and buttocks pain; and c. A mixed pattern, such as with small-contained disc herniation and central hemiation.

In addition, the postoperative disc can be a source of pain that mimics internal disc derangement. Certainly not all chronic lumbar pain is discogenic. It is estimated, however, that more than 50% of patients with chronic low back pain have the disc as the primary source of pain. Intervertebral disc abnormalities have a high incidence in the population and may result in pain and discomfort if they impinge on or irritate nerves. Disc abnormalities may be the result of trauma, repetitive use, metabolic disorders and the aging process and include such disorders but are not limited to degenerative discs (i) localized tears or fissures in the annulus fibrosus, (ii) localized disc hemiations with contained or escaped extrusions, and (iii) chronic, circumferential bulging disc. Disc fissures occur rather easily after structural degeneration (a part of the aging process that may be accelerated by trauma) of fibrous components of the annulus fibrosus. Sneezing, bending or just attrition can tear these degenerated annulus fibers, creating a fissure. The fissure may or may not be accompanied by extrusion of nucleus pulposus material into or beyond the annulus fibrosus. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc. Even if there is no visible extrusion, biochemicals within the disc may still irritate surrounding structures. Disc fissures can be debilitatingly painful. Initial treatment is symptomatic, including bed rest, pain killers and muscle relaxants. Another disc problem occurs when the disc bulges outward circumferentially in all directions and not just in one location. Over time, the disc weakens and takes on a "roll" shape or circumferential bulge. Mechanical stiffness of the joint is reduced and the joint may become unstable. One vertebra may settle on top of another. This problem continues as the body ages and accounts for shortened stature in old age. With the increasing life expectancy of the population, such degenerative disc disease and impairment of nerve function are becoming major public health problems. As the disc "roll" extends beyond the normal circumference, the disc height may be compromised, foramina with nerve roots are compressed. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and foramina through which nerves pass. This condition is called lumbar spondylosis.

Treatment options for discogenic pain include conservative measures such as physical therapy and steroid injection; and more aggressive strategies include surgery with discectomy and spinal fusion.

Traditional, conservative methods of treatment include bed rest, pain medication, physical therapy or steroid injection. Upon failure of conservative therapy, spinal pain (assumed to be due to instability) has been treated by spinal fusion, with or without instrumentation, which causes the vertebrae above and below the disc to grow solidly together and form a single, solid piece of bone. The procedure is carried out with or without discectomy. Other treatments include discectomy alone or disc decompression with or without fusion.

Spinal fusion with cages have been performed when conservative treatment did not relieve the pain. The fissure may also be associated with a herniation of that portion of the annulus.

Treatment methods include reduction of pressure on the annulus by removing some of the interior nucleus pulposus material by percutaneous nuclectomy. However, complications include disc space infection, nerve root injury, hematoma formation, instability of the adjacent vertebrae and collapse of the disc from decrease in height.

Nuclectomy can be performed by removing some of the nucleus to reduce pressure on the annulus. However, complications include disc space infection, nerve root injury, hematoma formation, and instability of adjacent vertebrae.

These interventions have been problematic in that alleviation of back pain is unpredictable even if surgery appears successful. In attempts to overcome these difficulties, new fixation devices have been introduced to the market, including but not limited to pedicle screws and interbody fusion cages. Although pedicle screws provide a high fusion success rate, there is still no direct correlation between fusion success and patient improvement in function and pain. Studies on fusion have demonstrated success rates of between 50% and 67% for pain improvement, and a significant number of patients have more pain postoperatively. Therefore, different methods of helping patients with degenerative disc problems need to be explored.

Necrotic lesioning, cutting, ablation, coagulation, and even physical therapy are some of the areas in which heat has been applied in medical care. Recently, the use of heat therapy has expanded to controlled contraction, or shrinkage, of collagenous tissues. A new, minimally invasive, fluoroscopically guided treatment option has been developed called intradiscal electrothermal therapy (IDET).

The IDET process takes about an hour to complete and is performed as follows:

1. The procedure is performed with a local anaesthetic and mild intravenous sedation.

2. A hollow introducer needle is inserted into the painful lumbar disc space using a portable x-ray machine for proper placement. 3. An electrothermal catheter (heating wire) is then passed through the needle and positioned along the back inner wall of the disc (the annulus), the site believed to be responsible for the chronic pain.

4. The catheter tip is then slowly heated up to 90 °C for 15-17 minutes. 5. The heat contracts and thickens the collagen fibers making up the disc wall, thereby promoting closure of the tears and cracks. Tiny nerve endings within these tears are cauterized (burned), making them less sensitive. 6. The catheter is removed along with the needle and, after a short period of observation, the patient goes home. 7. A lumbar support is worn for 6 to 8 weeks, followed by an appropriate course of physical therapy.

The IDET procedure is then repeated whenever needed.

The disc itself is a virtually avascular structure, and heat does not travel as easily in it as in other soft tissues. This environment allows heat to be held in the tissue with relatively little fluctuation during treatment. Adjacent structures are protected from thermal injury by the vascular circulation outside the disc, which quickly dissipates any heat escaping from the disc.

A catheter based heating system, as described in U.S. Patent Nos. 6,290,715 and 6,261 ,311 , has the ability to deliver energy at the point of contact. Heat is transferred by conduction from the catheter to the adjacent tissue. Temperature sensors deliver feedback to the generator, which adjusts power levels as necessary to reach and maintain set target temperatures. Identifying the precise temperatures needed to affect the disc tissue and having the power to produce the required temperature in the disc offers a high degree of accuracy and consistency to this treatment. Letcher F. et al., The effect of radiofrequency current and heat on peripheral nerve action potential in the cat, J. Neurosurg. 29_:42-47, 1968, established that irreversible nerve blocks occur at 45°C in the brain, and Cosman E. R. et al., Theoretical aspects of radiofrequency lesions in the dorsal root entry zone. Neurosurgery, 15:945- 950, 1984, used 45°C isotherms for neural tissue lesioning. The intradiscal temperatures generated by the SpineCATH device, taught in U.S. Patent No. 6,290,715, which are in the range of 50 °C to 75 °C, are in a range suitable for creating thermo-coagulation of neural tissue.

Collagen contraction, or shrinkage, has been documented in the use of nonablative laser energy on joint capsular tissue and, application in the glenohumeral joint capsule (Fanton G. S., et al., Electro thermally assisted capsule shift (ETAC) procedure for shoulder instability, Am. J. Sports Med., (in press), and Hect P. et al., The thermal effect of radio frequency on joint capsular properties: An in vivo histological study using a sheep model. Am. J. Sports Med. 26; 808-814, 1998). Research has shown that there is a direct correlation between the amount and duration of heat applied to tissue and the resulting collagen contraction (see, for example, Hayashi K„ et al., The effect of nonablative laser energy on joint capsular properties: An in vitro mechanical study using a rabbit model, Am. J. Sports Med.. 23:482-487, 1995; Hayashi K. et al., The effect of nonablative laser energy on the ultra structure of joint capsular collagen, Arthroscopy 12:474-481, 1996; Hayashi K. et al., The effect of nonablative laser energy on joint capsular properties: An in vitro histological and biochemical study using a rabbit model, Am. J. Sports Med.

24:640-646, 1996; Lopez M. et al., The effect of radio frequency energy on the ultra structure of joint capsular collagen, Arthroscopy 14:495-501, 1998; Obrzut L. et al., The effect of radio frequency energy on the length and temperature properties of the glenohumeral joint capsule, Arthroscopy 14:395-340, 1998). The breaking apart of the heat sensitive bonds of the collagen fibrils causes tissue shrinkage. The optimal temperature for collagen contraction is reported to be 65 °C; the lowest practical is 60 °C. It is unclear whether there is an additional shrinkage effect over 75 °C. The typical treatment regimens with the SpineCATH system is of the range of 60 °C to 75 °C for collagen contraction. Thermal shrinkage of collagen is also dependent on the duration of the application of heat. Lower temperatures over a longer period of time result in shrinkage comparable with that achieved with a higher temperature over a shorter period of time. Under these parameters, IDET is capable of causing shrinkage of the collagenous fibers of the disc annulus. Typically, the IDET therapy has to be repeated several times to be successful. The repeated placement of the catheter is both costly and painful for the patient. SUMMARY OF THE INVENTION

Intradiscal applications of other available thermal energy sources have limitations when compared with temperature-controlled thermal resistive heating. Radio frequency delivered by an intradiscal needle electrode cannot transfer the necessary range of thermal energy to achieve the desired effects, and a needle delivery system cannot access the broad expanse of the posterior annular wall. Intradiscal laser energy is potentially hazardous to bone and nerve tissue because of the lack of temperature control and extent of tissues heated.

It has been shown that intradiscal electrothermal therapy IDET can be very effective in back pain therapy. Since back pain is a constant problem to a patient the procedure has to be repeated over, during a long period. It is therefore of advantage to have a procedure in which the interventional process is replaced by a non-interventional process. This invention proposes an implant which is deployed in the spinal disk and which can be repetitively heated by an inductive heating process. The replacement of a heating catheter with a permanent, or semi-permanent, implant eliminates the need for repeatedly invasive processes. The invention further suggests materials and shapes of the implant and an inductive heating device.

In particular, one embodiment of the invention is directed to a method of delivering energy adjacent an inner wall of an intervertebral disc. The method comprises positioning an energy delivery device adjacent the inner wall of the intervertebral disc, and inductively heating the energy delivery device from outside the intervertebral disc. Heat is provided from the heated implant to the inner wall of the intervertebral disc.

Another embodiment of the invention is directed to a system for delivering energy adjacent an inner wall of an intervertebral disc treating back pain. The system comprises a source of rf energy and an rf radiator coupled to radiate rf energy form the source of rf energy. The system also comprises an implantable device capable of being implanted within the intervertebral disc. The implantable device is inductively heated by energy received from the rf radiator when the implantable device is implanted within the intervertebral disc. Another embodiment of the invention is directed to a system for delivering an inductively heatable implant adjacent an inner wall of an intervertebral disc treating back pain. The system comprises an introducer needle adapted for penetrating within the annulus fibrosus of the disc. There is an inductively heatable implant comprising a wire. The inductively heatable implant is disposed within the introducer needle. The introducer needle is adapted to expel the inductively heatable implant into the nucleus pulposus of the disc, and to leave the inductively heatable implant within the nucleus pulposus.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE FIGURES

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 shows a three dimensional view of an intervertebral disc located between two lumbar discs. The spinal nerves branch out lateralis.

FIG. 2 shows a sagittal cross sectional view of a part of the lumbar spinal column without the spinal cord shown.

FIG. 3 a transverse cross sectional view of a middle lumbar intervertebral disc.

FIG. 4 shows is a transverse cross sectional view of a middle lumbar intervertebral disc with an embodiment of a general implant according to principles of the present invention.

FIG. 5 schematically shows an embodiment of an inductive heating system according to principles of the present invention.

FIG. 6 shows a schematic electrical circuit for an embodiment of a power generator - oscillator unit, according to principles of the present invention.

FIGs. 7 A and 7B schematically illustrate two different embodiments of inductor coils useful in reducing the electrical component of the rf (radio frequency) field, according to principles of the present invention.

FIG. 8 shows a graph of eddy current vs. frequency with permeability as a parameter, in accordance with principles of the present invention.

FIG. 9 shows a graph of eddy current vs. frequency with permeability as a parameter, in accordance with principles of the present invention. FIG. 10 illustrates a graph of eddy current vs. frequency with coating thickness as a parameter in accordance with principles of the present invention.

FIG. 11 illustrates a graph of eddy current vs. frequency and permeability in accordance with principles of the present invention. FIG. 12 illustrates a graph of eddy current vs. coating thickness with permeability as a parameter in accordance with principles of the present invention.

FIG. 13 illustrates a graph of eddy current vs. permeability with coating thickness as a parameter in accordance with principles of the present invention.

FIG. 14 illustrates a graph of eddy current vs. coating thickness with frequency as a parameter in accordance with principles of the present invention.

FIGs. 15a-15d show a sequence of views in the horizontal plane of the spinal disc with an embodiment of an implant being deployed, according to principles of the present invention.

FIG. 16 schematically illustrates another embodiment of an inductive heating system according to principles of the present invention.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION FIG. 3 shows a cross-sectional-axial view (horizontal plane) of a spinal disc 1 with a fissure 15 on the left dorsal side, causing the liquid-like nucleus pulposus to "leak", causing pressure on a left spinal nerve 4. The same situation is shown in FIG. 2 in a sagittal view (median plane).

A general approach according to the present invention is schematically presented in FIG. 4. An implant 17 is deployed in the nucleus pulposus 7 on the inner wall of the annulus fibrosus 8. The implant 17 itself or just a portion 18 of is made of a material of a high enough magnetic susceptibility to be heated inductively. The heating may take place by inductively heating the implant 17 using an inductive heating system outside the disc. Where an extra-corporal inductive heating system is used, the patient may receive heat treatment non-invasively, where the only invasion is to position the implant. The implant 17 may be a permanent implant or a temporary implant. The implant 17 may be positioned to treat any portion of the inner wall, including a posterior medial region, a posterior lateral region, an anterior medial region and/or an anterior lateral region of the inner wall of the annulus fibrosus.

A basic set up of an embodiment of a system used for the treatment of back pain is schematically presented in FIG. 5. The patient 19 is lying on the patient bed 24, for example in a prone position. The inductor coil 20 hangs on a inductor holder 21 above the patient 19. The rf (radio frequency) signal, having a particular power and the frequency, is generated by the generator-oscillator unit 22, which may be controlled from a display-console panel 23. It should be appreciated that the set-up illustrated in FIG. 5 is presented as an example of the system. In the illustrated embodiment, the inductor coil 20 does not touch the patient 19.

It may of value, however, if the inductor coil 20 is positioned in close proximity to the patient 19, or even touches the patient 19. The inductor coil 20 may be mounted dorsal to the patient, as shown, or anterior to the patient. The inductor coil 20 may surround the whole patient. It may be of benefit if the patient 19 is sitting or standing dorsal, anterior or even lateralis to the inductor coil 20.

It will be appreciated that the inductive heating system may be oriented differently relative to the patient than shown in FIG. 5. For example, the patient may be positioned on a support so that the patient is within an inductance coil, for example as taught in U.S. Patent No. 6,238,421, incorporated herein by reference. The induction heating process may be carried out using a power generator - oscillator unit, an embodiment of which is illustrated in FIG. 6. The desired frequency range is preferably between 50 Hz and 2 MHz, and more preferably between 100 kHz and 600 kHz and may usually be set to 250 kHz. It has been found that at frequencies above 1 MHz, heating is not restricted to the implant, and the surrounding tissue itself also heats up.

The sending antenna 100 itself may include an inductive component 101 and a capacitive component 102. The antenna 100 is supplied with a radio frequency, high voltage signal 118. The antenna 100 is preferably electrically matched to the oscillator 104 by a matching transformer 103 to maintain efficient operation. The whole system may be supplied with electrical power, for example, 210 kVA at 50 Hz or 60 Hz at the low voltage end 112. A computer 115 may be used to control the voltage at a thyristor unit 110. Feedback 108 may be measured before the voltage is transformed in the transformer 107 to high voltage AC 120 and rectified 106 to load the oscillator 104.

During the inductive heating process, a current flows in the induction coil 20 to produce an alternating magnetic field (AMF). The AMF inductively heats the implant 17, for example due to eddy currents that arise from the AMF. The electric energy supplied by the induction coil is first converted to magnetic energy, which is then converted to heat in the implant 17. The current density in implant 17 is determined to some extent by the skin-effect. The highest current density is reached at the surface of the implant 17. The current density drops off inside the implant exponentially. The inductive heating power (P) is:

wherein: k = Constant

I_IND ⁼ Current in the induction coil μ = Relative permeability of material piece and μ₀ p = Specific resistance of material piece in Ωmm²/m

/ = Frequency in Hz

(magnetic susceptibility + 1 = relative permeability) The constant k can be empirically determined, and is related, at least in part, to the coupling factors of the physical arrangement. The inductive heating power P varies quadratically with the current in the transmitting coil I_IND, and according to the square root of the specific resistance p, the permeability μ and the frequency f. The resistance and permeability are set in by the implant material. The primary goal is to increase frequency and induction current. Increasing the frequency simultaneously increases absorption and decreases the skin-effect. The penetration depth of the current in the implant, due to the skin-effect, is:

The equation is valid where the material piece diameter is at least twice as large as penetration depth of the current. If one increases the specific resistance, the penetration depth of the medium also increases. For a given material, the skin-effect requires a certain minimal frequency at which the coupling eddy currents are effective. When the frequency is made lower, the heating effect is reduced. The heating effect is better when the frequency is increased. However, at a certain value of frequency, above about 1 MHz, the tissue is also heated, along with the implant, and so tissue heating sets an upper limit on the operational frequency.

The correlation among the specific resistance, permeability and coupling power is important. If the specific resistance of the material decreases, one can take advantage of the skin-effect even at low frequencies.

The basic principle of excitation of the heating process may be described in terms of an L-C-parallel-resonant-circuit. Provided that the thickness, d, of the implant is very small compared to the diameter D of the sending inductor loop (d/D < 0.001, d is the diameter of a wire), it is possible to use a simple solution for inductance L:

where R is radius of conductor loop, and d diameter of a wire. As a result, the resonance frequency of the resonant circuit is

The blind current through the coil is calculated as follows:

I = U - ω - C This current causes magnetic field with field strength H. If the implant 17 is positioned within the inductor loop the coil must be placed around the body, but the inductivity of the coil increases with the radius. It is advantageous if one places the implant at a distance 5 cm to 15 cm separated from the inductor coil. If one varies the radius R of the transmitting aerial keeping the distance x to transmitting aerial constant and the simplified assumption of a constant coil's current / in the transmitting aerial, a maximum field strength H is obtained for R x.

As long as x < λ/2π, a round coil (conductor loop) is valid for H along the coil's axle:

where:

N: Number of windings

R: Circuit radius x: Distance to coil's middle in x-direction λ: Wave length

This accounts for the coil's middle point, where:

2R It may be necessary to develop a type of implant 17 that retains all the therapeutic and technical features and improves the ratio between the supply of electro-magnetic energy and the transformed heat from the implant. There is still the task of increasing the efficiency rate of an appropriate type of appliance for warming the new implant. An objective of the present invention is to self-regulate temperature by means of material modification and application of the Curie effect.

At rf frequencies above 1 MHz, and more easily seen above 2 MHz, tissues are seen to heat up. During the development of the inductor coil, we have made the unexpected discovery that, below 1 MHz, the electrical and the magnetic components of the rf field contribute differently to the heating of the tissue itself. Some tissue types heat up when using rf frequencies between 500 kHz and 1 MHz. Once the electrical field component of the rf field was eliminated, tissue heating at a frequency of less than 1 MHz was avoided. Thus, it is advantageous to use an inductor coil that provides shielding for the electrical component.

Two examples of coil arrangements useful for reducing, if not eliminating, the electrical component of the rf field are shown in FIGs. 7a and 7b. FIG. 7a shows an arrangement in which the patient may be located within the coil 27. The inductor coil 27 surrounds the patient. FIG. 7b shows an inductor coil 31 , which may be placed dorsal or anterior to the patient. The patient is located proximal to that side of the arrangement in FIG. 7b, where the metal plates 32 bend. The inductor coils 27 and 31 may be made out of a tube, with water, or other coolant flowing therethrough to cool down the inductor coil itself. This tubing may be formed from copper, for good heat conduction. Metal plates 28 or 32 may be formed as stripes and are located in the two arrangements in such a way that they are parallel to the field lines of the magnetic rf component and perpendicular to the field lines of the electrical component. This results in the magnetic field components of the rf field being passed and the electrical field components of the rf field being blocked. The material for these stripes may be copper for good heat conduction. A copper cooling tube 26 or 30, carrying coolant, may be is attached to the stripes 28 or 32. The whole arrangement may be covered with an electrical insulating cover 25 or 29, which might be made out of any plastic, such as PTFE, PEEK, PE, PP or PU.

The stripes 28 or 32 may be 1 to 4 mm in width and 0.2 to 0.5 mm in thickness. The water flow through the induction coil 27 or 31 may be between 4 and 20 liter/minute at 6 bar.

Another approach to inductively heating the implant is to place it in the AMF formed in a magnetic circuit, for example between two pole pieces. One particular embodiment of such a system 1600 is schematically illustrated in FIG. 16. In this embodiment, a generator 1601 produces an AMF that may be guided to a specific location within a subject 1605 by a magnetic circuit 1602. In the illustrated embodiment, the magnetic circuit is c-shaped, although it will be appreciated that other shapes may also be used.

The subject 1605 typically lies upon an X-Y horizontal and vertical axis positioning bed 1606, between the pole pieces 1604 of the c-shaped magentic circuit. The positioning bed 1606 can be positioned horizontally and vertically via a bed controller 1608. The AMF generator 1601 produces an AMF in the magnetic circuit 1602 that exits the magnetic circuit 1602 at one pole face 1604, passes through the air gap and the desired treatment area of the subject 1605, and reenters magnetic circuit 1602 through the opposing pole face 1604, thus completing the circuit.

An operator or medical technician may control and monitor the AMF characteristics and bed positioning via a control panel 1620. When the AMF is generated by an RF generator, the frequency of the AMF may be in the range of about 100 kHz to about 600 kHz, although other frequencies may also be used.

A ring, or circuit, 1602 of low reluctance magnetic material may be specifically formulated for magnetic cores operating at a desired frequency, for example around 150 kHz or 250 kHz. One example of low reluctance magnetic material is Fluxtrol material, commercially available from Manufacturing Inc., Auburn Hills, MI, USA.

One particular advantage of the implant 17 is mainly to be found in the use of a material that possesses increased receptivity for the electromagnetic field strength, which depends on a high degree of magnetic permeability or magnetic susceptibility. A further phenomenon is also put to use, in which the warming of the implant occurs by means of the incidental eddy current losses. Thus, the eddy current is increased through the correct choice of the material and the construction of the implant to the degree that considerably more heat is absorbed with very little additional technological effort. Thus, the energy delivered to the disc via the implant 17 may be controlled so that no vaporization of tissue or other material occurs close to the inner wall of the disc. Furthermore, the energy may be controlled so that no material other than water is removed adjacent the inner wall of the disc, and/or that no destructive lesion is formed adjacent the inner wall of the disc.

When one increases the frequency of the induced H-field, a characteristic and material specific value f_w, the eddy currents then dominate the other effects. f_w= 8 - p / μ ' D² where: p: the specific of the resistance material; μ: the product of permeability and of relative permeability; and D: the thickness of the material.

At high permeability, the frequency typically is far below that of commonly used generator frequencies.

This leads to reduced demand for electrical power and thus a reduction in technical complexity for the power supply system. It is especially advantageous if the implant 17 is formed of a material having a permeability of more than 100. The permeability preferably has a value of several thousand. Some preferred metal alloys include a nickel-iron alloy or a palladium-cobalt alloy, but other alloys, such as nickel- copper, nickel-palladium, palladium-cobalt and nickel-silicon, etc., may also be utilized.

A further advantage is that the metal alloy possesses a Curie temperature that may be used to ensure that implant 17 is maintained below a certain at a maximum temperature. The Curie point of the material, with the help of the alloy composition can, for example, be designed to allow temperatures ranging between 60 °C and 75 °C, and preferably about 65 °C.

The implant made of this alloy may be coated with gold or with a different overlay so that the entire arrangement is corrosion resistant and highly conductive. When simulating the implant with a core and heat conductive gold coating, as illustrated in Figures 8 to 14, the following assumptions are made. The gold coating is varied up to a thickness of 5 μm with 0.5 μm increments. The frequency ranges from 100 kHz to 1 MHz. Relative magnetic permeability is 1 to 2,000. The investigated parameter of all simulations is the in-coupled heat generated or lost due to eddy currents. The eddy current losses are shows as a function of excitation frequency, with the coating thickness being 0.5 micrometers.

Two series of curves are shown in FIG. 10 for increasing permeability, with one series corresponding to a coating thickness of 0.5 μm and the other series corresponding to a coating thickness of 2.5 μm. The in-coupled power, relative to the thickness of the coating, the permeability and the frequency, is shown in FIG. 11. It can be seen that the maximum value is achieved with the thinnest coating (0.5 μm) and highest permeability (2,000) as well as the highest frequency (1 MHz). In FIG. 11, the coating thickness is varied with fixed frequency and permeability. FIG. 12 shows the variation in eddy current as a function of the coating thickness. The maximum in-coupled eddy current loss is also a function of the relative permeability. Above a value of 1,000, the coating thickness lies under 0.5 μm. FIG. 13 shows a graph of eddy current loss as a function of relative permeability, for different coating thicknesses.

A very conductive thin coating around a core with high permeability improves the absorption of the heat generated. The heat energy is primarily produced in the coating.

The thickness of the coating, for example a gold coating, although other materials may also be used, depends on the chosen excitation frequency and on the permeability of the core. At a relative permeability of several thousand, a gold coating is preferably less than 0.5 μm thick if the core only has a diameter of 90 μm. High excitation frequencies (>500 kHz) also lead to the use of thin coatings (<0.5 μm).

The Curie effect is indirectly contained in the permeability variation. Thus, permeability decreases at higher temperatures. The behavior of the permeability temperature ratio is, therefore, material dependent. Depending on the magnitude of the permeability value in a normal state and after warming, the performance can drop by factors up to several 100,000.

Table 1 lists the some ferromagnetic materials that are suitable as starting materials for the process described here.

Table 1: Ferromagnetic Materials

The implant 17 may also be composed of one or more alloys, for example, nickel-copper alloys (described in Table 2), nickel-palladium alloys (described in Table 3), palladium- cobalt alloys (described in Table 4), nickel-iron alloys; and nickel-silicon alloys (described in Table 5).

Table 2: Nickel-Copper Alloys

Table 3: Nickel Palladium Alloys

Material Curie temperature in °C Biocompatibility

Ni Pd in various ratios 43-58 no information

Table 4: Palladium Cobalt Alloys

Material Curie temperature in °C Biocompatibility

Pd 6.15% Co 50 probably

The palladium-cobalt alloy is interesting because, besides having ferromagnetic properties, it also behaves like palladium in pure form. Looking at its material properties, it has an extraordinary corrosion resistance in a very broad pH spectrum. Palladium alloys have been used for quite some time in dental medicine for permanent oral implants, and besides palladium's biocompatibility, there is clinical evidence of mechanical durability. Additionally, there is extensive clinical experience since its introduction in 1986 regarding its use in branchy-therapy with radioactive Pd implants for treating prostate carcinoma. In conjunction with the above named Pd-Co alloy, it is possible to reach a Curie temperature of 50°C in vitro and in calorimetric experiments. Nickel-Iron alloys

Biocompatibility is primarily achieved through the use of a coating of gold or other biocompatible material. In a study of simulated tissue by means of cellulose and a controlled flow of water, a stable Curie temperature of 50°C is maintained at different water flow rates.

Table 5: Nickel-Silicon Alloys

Data exist for in vitro as well as in vivo Ni:Si thermoseeds. The pure uncoated Ni:Si alloys are cytotoxic in vitro and in vivo, and so a coating, e.g. in the form of a plastic catheter, may be used. Furthermore, in production, so-called dendrite arms appear, which may be reduced at considerable cost; however, they do negatively impact the ferromagnetic properties. The process to reduce the dendrite arms leads to considerable irregularities in the surface. Additional materials that may be considered for use in the implant are listed in

Table 6.

Table 6 - Additional Materials

The deployment of the implant is now described with reference to FIGs. 15a-15d. The procedure starts with an introducer needle 33 puncturing through the annulus fibrosus 8, and into the nucleus pulposus 7, as shown in FIG. 15a. A flexible wire implant 34 is pushed outwards from the introducer needle 33, as is shown in FIG. 15b. The wire implant 34 slides along the inner wall 13 of the annulus fibrosus 8. The wire implant 34 may be made in such a way that it does not rupture the inner wall 13, for example, it may be coated with a low friction coating, such as Teflon, or may be polished to achieve low friction. At least one portion 36 of the wire implant 34 is made in a way and with a material as described above of this invention, so that it can be heated inductively. The heatable portion 36 is positioned at the fissure 15 of the annulus fibrosus 8. The wire implant 34 is pushed all the way out of the introducer needle 33 and the introducer needle 33 is removed from the spinal disc 1, as shown in FIG. 15c. In this embodiment of implanted wire 34, the implanted wire 34 is made out of nickel-titanium NiTi shape memory alloy. The NiTi shape memory wire 34 is prepared in such a way, that when heated, it stretches out to form a loop with having a relatively large diameter. This causes the wire implant 34 expand within the nucleus pulposus 7, and more closely hug the inner wall 13 of the annulus fibrosus 8, and to push the heating portion 36 of the wire implant 34 to the fissure part 15 of the annulus fibrosus 8, as shown in FIG. 15d.

The implant may stay in the disc and can be heated as needed to serve as a heating source for the procedure. A typical procedure involves the temperature of the wire implant 34 being raised to a value in the range from 60 °C to 95 °C for 10 to 20 minutes. The wire implant 34 typically has a diameter of 0.5 to 2.0 mm and a length of 30 to 100 mm. The wire implant 34 may be looped several times within the annulus fibrosus 8. The heatable portion 36 may extend over the whole wire implant 34, or may form part of the implant 34. The wire implant 34 may have circular cross-section, although elliptical cross-sections, or cross-sections of other shapes may also be used. In one embodiment of the invention the wire or the fissure adjacent parts of the wire implant may be coated with a medical drug supporting the healing of the fissure. In such an embodiment the temperature of the implant may be only raised to the point where the drug elutes out of the coating. When the temperature is lowered again, the drug elute or diffuse out of the coating, or elutes at a lower rate than at the elevated temperature. Hence, the inductively produced heat may serve as a controlling mechanism for the elution process. As noted above, the present invention is applicable to systems and methods useful for relieving back pain. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

REFERENCE NUMBERS

1 intervertebral disc (discus intervertebralis)

2 vertebrae (corpus vertebrae)

3 spinal cord (cauda equina) 4 spinal nerve, spinal ganglion

5 spinous process

6 spinal branches

7 nucleus pulposus

8 annulus fibrosus 9 nucleus pulposus protruding posteriorly

10 nucleus pulposus protruding into bodies

11 nerval branch to annulus fibrosus of intervertebral disc

12 fiber layers of annulus fibrosus

13 inner wall of annulus fibrosus 14 annulus-dural interface

15 fissure in annulus fibrosus

16 40° angle of fibres

17 implant

18 heating portion of the implant 17 19 patient

20 inductor

21 holder for inductor

22 power generator / oscillator

23 display / console 24 patient bed

25 insulating cover

26 cooling tube for metallic plates

27 inductor coil

28 metallic finger like plates 29 insulating cover

30 cooling tube for metallic plates

31 inductor coil metallic finger like plates introducer needle implant wire distal tip of implant wire heating portion of the implant 34 proximal tip of the implant antenna inductive component of antenna, inductor capacitive component of antenna, capacitor(s) matching transformer oscillator

DC feedback rectifier power high voltage transformer

AC current feedback fuses thyristor unit contactor circuit breaker fuse radio frequency feedback controls / computer front panel external panel high voltage radio frequency (HV-RF) high voltage direct current (HV-DC) high voltage alternating current (HV-AC) input 400V, 50/60 Hz

Claims

WHAT IS CLAIMED IS:

1. A method of delivering energy adjacent an inner wall of an intervertebral disc, the method comprising: positioning an energy delivery device adjacent the inner wall of the intervertebral disc; inductively heating the energy delivery device from outside the intervertebral disc; and providing heat from the heated implant to the inner wall of the intervertebral disc.

2. The method of claim 1, wherein positioning the energy delivery device comprises positioning the energy delivery device adjacent a portion of the inner wall which forms a region of the disc selected from the group consisting of: a posterior medial, posterior lateral, anterior medial and anterior lateral region of the inner wall of the annulus fibrosus.

3. The method of claim 1 , wherein positioning the energy delivery device comprises positioning at least a portion of the energy delivery device adjacent a site of an annular fissure adjacent the inner wall of the disc.

4. The method of claim 1 , wherein positioning the energy delivery device comprises positioning at least a heatable portion of the energy delivery device adjacent a site of an annular fissure adjacent the inner wall of the disc.

5. The method of claim 1 , wherein positioning the energy delivery device comprises introducing the energy delivery device into the nucleus pulposus of the disc using an introducer needle, and expelling the energy delivery device from the introducer needle within the nucleus pulposus.

6. The method of claim 1, wherein the energy delivery device is an implantable wire.

7. The method of claim 1, further comprising expanding the energy delivery device within the annulus fibrosus of the disc to fit against the inner wall of the intervertebral disc.

8. The method of claim 7, wherein the energy delivery device is formed of a shape memory material, and expanding the energy delivery device comprises heating the shape memory material.

9. The method of claim 1, wherein inductively heating the energy delivery device includes delivering a controlled amount of energy adjacent the inner wall such that no vaporization occurs adjacent the inner wall of the disc.

10. The method of claim 1, wherein inductively heating the energy delivery device includes delivering a controlled amount of energy adjacent the inner wall such that no material other than water is removed adjacent the inner wall of the disc.

11. The method of claim 1 , wherein inductively heating the energy delivery device includes delivering a controlled amount of energy adjacent the inner wall such that no destructive lesion is formed adjacent the inner wall of the disc.

12. The method of claim 1, wherein inductively heating the energy delivery device includes exposing the energy delivery device to an alternating magnetic field having a frequency between about 50 kHz and about 2 MHz.

13. The method of claim 1 , wherein inductively heating the energy delivery device includes exposing the energy delivery device to an alternating magnetic field having a frequency between about 200 kHz and about 600 kHz.

14. A system for delivering energy adjacent an inner wall of an intervertebral disc treating back pain, comprising: a source of rf energy; an rf radiator coupled to radiate rf energy form the source of rf energy; and an implantable device capable of being implanted within the intervertebral disc, the implantable device being inductively heated by energy received from the rf radiator when the implantable device is implanted within the intervertebral disc.

15. The system of claim 14, wherein the source of rf energy comprises a power generator and an oscillator.

16. The system of claim 14, wherein the rf radiator comprises an induction coil.

17. The system of claim 16, wherein the induction coil comprises an electrically conducting tube, with a cooling liquid passing through the tube.

18. The system of claim 14, wherein the rf radiator comprises a magnetic circuit having a gap between pole pieces of the magnetic circuit.

19. The system of claim 14, wherein the rf radiator comprises a filter to filter an electrical component of the electromagnetic field radiated by the rf radiator.

20. The system of claim 19, wherein the filter comprises one or more electrically conductive strips oriented non-parallel to the electrical component of the electromagnetic field.

21. The system of claim 14, wherein the rf radiator is a coil and further comprising a patient support to support the patient lying within the coil.

22. The system of claim 14, further comprising a patient support to support the patient lying dorsal or anterior to the rf radiator.

23. The system of claim 14, wherein at least a portion of the implantable device comprises a material having a relative magnetic susceptibility higher than 100.

24. The system of claim 23, wherein at least a portion of the implantable device comprises a material having a relative magnetic susceptibility higher than 100,000.

25. The system of claim 14, wherein at least a portion of the implantable device comprises at least one of cobalt, dysprosium, iron, gadolinium, and nickel, nickel-palladium alloy, palladium-cobalt alloy, nickel-iron alloy and nickel-silicon alloy.

26. The system of claim 14, wherein at least a portion of the implantable device is formed from a shape memory material.

27. The system of claim 26, wherein the shape memory material is nickel-titanium shape memory alloy.

28. A system for delivering an inductively heatable implant adjacent an inner wall of an intervertebral disc treating back pain, comprising: an introducer needle adapted for penetrating within the annulus fibrosus of the disc; and an inductively heatable implant comprising a wire, the inductively heatable implant being disposed within the introducer needle; wherein the introducer needle is adapted to expel the inductively heatable implant into the nucleus pulposus of the disc, and to leave the inductively heatable implant within the nucleus pulposus.