BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic showing the leadless bone stimulation system in application at a tibial fracture site.
FIGS. 2
a and 2b are block diagrams showing the components of the acoustic controller-transmitter and acoustic receiver-stimulators of the present invention.
FIG. 3 illustrates representative acoustic and electrical signals useful in the systems and methods of the present invention.
FIG. 4 is a schematic showing the leadless bone stimulation system in application at the spine.
FIGS. 5
a, 5b, and 5c are schematic illustrations showing components of the present invention adapted for use with devices commonly used for connecting bone fractures.
FIGS. 6
a through 6c depict various embodiments for an implantable receiver-stimulator utilizing a planar transducer.
DETAILED DESCRIPTION OF THE INVENTION
The systems and devices described here comprise a controller-transmitter device that will deliver vibrational energy and information to one or more implanted receiver-stimulator device(s) that will convert the vibrational energy to electrical energy of a form that can be used to electrically stimulate bone. The vibrational energy can be applied with ultrasound as a single burst or as multiple bursts or as a continuous wave with appropriate selection of the following parameters:
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Parameter
Value Range
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Ultrasound frequency
20 kHz-10 MHz
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Burst Length (#cycles)
3-continuous
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Stimulation Pulse Duration
0.1 μsec-continuous
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Duty Cycle
0-100%
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Mechanical Index
<1.9
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The controller-transmitter device contains one or more ultrasound transducers of appropriate size(s) and aperture(s) to generate sufficient acoustic power to achieve the desired stimulation at the location of an implanted receiver-stimulator device. Additionally, multiple implanted receiver-stimulator devices may be placed within the region insonified by the controller-transmitter device. Multiple receiver-stimulator implants can function simultaneously. It is also possible for multiple devices to function independently, either by responding only to a specific transmitted frequency, or through the use of a selective modulation technique such as pulse width modulation, or through encoding techniques such as time-division multiplexing. Further, the characteristics of the acoustic energy generated by the controller-transmitter, including the frequency, burst length, duty cycle, and mechanical index are selected such that stimulation of bone growth occurs due to the insonification of the bone tissue, as is previously known in the art. Such ultrasonic stimulation would be simultaneously present with the applied electrical stimulation of this invention to provide a combined therapy more beneficial than either ultrasonic or electrical stimulation alone. In this adaptation, the receiver-stimulator device would be constructed to operate at the same acoustic parameters required for ultrasonic stimulation and would intercept at least a portion of the applied acoustic energy for conversion into electrical stimulation energy.
In the implanted version, the controller-transmitter device containing the transmitting transducer is implanted typically just beneath the skin in the subcutaneous space. In the non-implanted version, the transducer portion is placed over the skin near the targeted bone and acoustic gel or other means is placed between the transducer face and the skin surface to ensure adequate acoustic coupling.
An example of such an electro-acoustic stimulation system as a simple bone stimulation system is illustrated in FIGS. 1, 2, and 3.
In FIG. 1, a controller-transmitter device 1 containing circuitry to provide stimulation control and ultrasound transmission, plus means to communicate with an outside programmer 3 is implanted just beneath the skin, and generally oriented such that the transmission is over the targeted bone fracture site. An ultrasound signal is transmitted by this device 1 through intervening tissue to the receiver-stimulator device 2 containing means to receive this acoustic energy and convert it into an electrical current which may then be applied to the attached electrodes. Alternatively, the ultrasound transmission is configured such that the targeted bone fracture site receives sufficient ultrasonic energy to promote bone healing, in addition to providing the receiver-stimulator device with sufficient energy to provide electrical stimulation. In FIG. 1, this receiver-stimulator device 2 is shown attached to a section of bone in a tibial fracture. However, it should be noted that the receiver-stimulator 2 could also be attached to any bone or site near any bone that is the target of treatment. The receiver-stimulator device 2 is shown here as a small button-shaped device that would be affixed to the bone. Other appropriate shapes could be cylindrical, hexagonal, oblong, etc. Alternatively, the functional components of the receiver-stimulator may also be separated. In one embodiment (not shown) the electrodes are applied directly to the bone or to tissue near the bone and connected by small wires to the receiver. This embodiment would adapt the electrode to be shapeable, malleable configurations that conform to the bone as flexible wraps, cages, bindings, etc. or that could be placed near the bone. Electrodes may be adapted that are round, long, segmented, etc. to increase surface area or to control current density at the electrode. Electrodes may be placed on opposing sides of the bone in linear alignment with the bone or in any arrangement suitable for the size and location of the bone and the targeted bone healing site.
FIGS. 2
a and 2b show more details of the system described above and shown in FIG. 1. In FIG. 2a the controller-transmitter device 1 comprises: a battery 10, one or more sensors 11, signal processing circuitry 12, a communications module 13, a control and timing module 14, an ultrasound amplifier 15, an ultrasound transducer 16. The battery 10 which provides power for the controller-transmitter may be of a type commonly used in implanted medical devices such as a lithium iodine cell or lithium silver vanadium oxide cell made by Greatbatch, Inc. or which is optionally a rechargeable battery. The one or more sensors 11 are used to detect physiological parameters, such as impedance, temperature, motion, strain, pressure, etc. Sensors may be chosen to measure acute response or to measure chronic progression of response. Suitable sensors are known for the detection of impedance, temperature, motion, strain, pressure, and the like. These sensors are connected to signal processing circuitry 12 and used by the circuitry to adjust delivery of stimulation therapy or to communicate diagnostic information from the sensors. The communications module 13 provides a data path to allow the physician to set device parameters and to acquire diagnostic information about the patient and/or the device. The data path may be by an RF communication link, magnetic coupling, ultrasound pulses, or the like, and would communicate to and from an external unit 3. Device parameters would be used by the control and timing module 14. Device parameters would include adjustments to transmissions, such as power amplitude, pulse duration, duty cycle, and the like. The control and timing module 14 uses device parameters in conjunction with the acquired physiological data to generate the required control signals for the ultrasound amplifier 15 which in turn applies electrical energy to the ultrasound transducer 16 which in turn produces the desired acoustic beam. The controller-transmitter device 1 is encased in a hermetically sealed case 17 constructed of a biologically compatible material, typical of currently existing EBGS devices.
Referring to FIG. 2b, the receiver-stimulator device 2, implanted in the path of the acoustic beam at the location where electrical stimulation is desired, contains an ultrasound transducer 20, an electrical circuit 21, and electrodes 22. Ultrasound transducer 20, typically made of a piezoelectric ceramic material, a piezoelectric single crystal, or piezoelectric polymer or copolymer films, intercepts a portion of the transmitted acoustic energy and converts it into an electrical current waveform from the original alternating nature of the applied ultrasound pressure wave. This electrical signal is applied to an electrical circuit 21 which may be one of a type commonly known as an envelope detector, and which may have one of many known circuit configurations, for example a full-wave rectifier, a half-wave rectifier, a voltage doubler or the like. Electrical circuit 21 produces a voltage pulse with amplitude proportional to the amplitude of the transmitted ultrasound burst and with a pulse length generally equal to the length of the transmitted burst. The circuit 21 may also be of different configurations and functions, and provide output signals having characteristics other than a pulse. This signal is applied then to electrodes 22 made typically of platinum, platinum-iridium, gold, or the like which may be incorporated onto the outer surface of the device, and thus in direct contact with the bone or within close proximity of the bone which is to be treated by stimulation. Alternatively, the electrodes 22 are connected via wires to a main body that consists of the transducer 20 and electrical circuit 21 and the electrodes 22 are adapted to be shapeable, malleable configurations that conform to the bone as flexible wraps, cages, bindings, etc. or that could be placed near the bone. Electrodes may be adapted that are round, long, segmented, etc. to increase surface area or to control current density at the electrode. Electrodes may be placed on opposing sides of the bone or in linear alignment with the bone or in any arrangement suitable for the size and location of the bone and the targeted bone healing site. The receiver-stimulator device 2 is also enclosed within a sealed case 23 of biologically compatible material
Referring also to previously described FIGS. 2a and 2b, FIG. 3 provides detail representing example acoustic and electrical signals of the present system. FIG. 3 first depicts a train of electrical stimulation pulses 31 which have a desired width and are repeated at a desired interval. The controller-transmitter device 1 produces acoustic transmissions 32, for the desired stimulation pulse width and repeated at the desired stimulation pulse interval, which are emitted from the ultrasound transducer 16. Below the waveform 32 is shown an enlargement 33 of a single acoustic burst. This burst again has a desired width, a desired oscillation frequency F=1/t, and also a desired acoustic pressure indicated by the peak positive pressure P+ and peak negative pressure P−. The acoustic pressure wave, when striking the receiving transducer 20 of the receiver-stimulator device 2 generates an electrical signal 34 having frequency and burst length matching that of the transmitted waveform 33 and amplitude proportional to the transmitted acoustic pressure (˜+/−P). This electrical waveform is then rectified and filtered by the circuit 21 producing the desired pulse 35 with length equal to the burst length of the transmitted waveform 33 and amplitude (VPULSE) proportional to the amplitude of the electrical signal 34. Thus, it can be seen that it is possible in this example to vary the stimulation rate by varying the time between ultrasound bursts, to vary the duration of any one stimulation pulse by varying the duration of the ultrasound burst, and to vary the amplitude of the stimulation pulse by varying the amplitude of the transmitted ultrasound waveform. Circuit 21 could be configured to produce a direct current (DC) output or an alternating current (AC) output, or an output with any arbitrary, pre-determined waveform. Varying the use of signal information within the ultrasound transmission for pulse duration, pulse amplitude, and duty cycle would result in any type of burst sequencing or continuous delivery waveform effective for bone growth and healing. Using signal information in the ultrasound transmission the resultant waveshape may be a square wave, triangle wave, biphasic wave, multi-phase wave, or the like.
In practice, the amount of acoustic energy received by the implanted receiver-stimulator device will vary with ultrasound attenuation caused by loss in the intervening tissue, spatial location of the receiver-stimulator device with respect to the transmitted ultrasound beam, as such a beam is typically non-uniform from edge-to-edge, and possibly with orientation (rotation) of the receiver-stimulator device with respect to the first. Such variation would affect the amplitude of the stimulating pulse for a given ultrasound transmit power (acoustic pressure amplitude). This limitation can be overcome by adjusting the ultrasound transmit power until the resultant stimulation waveform is consistent, a technique similar to that used currently to determine stimulation thresholds at the time of cardiac pacemaker implantation. Another approach would be to adjust automatically using sensing and logic within the first device. The first device would periodically sense the electrical output of the receiver-stimulator device and adjust power transmission accordingly to compensate for any change in the system including relative movement between the transmitting and receiving devices. Yet another embodiment for overcoming this limitation is where the transducer incorporated into the receiver-stimulator device is omni-directional in its reception capability. For example, to improve omni-directional sensitivity, the transducer may be spherical in shape or have specific dimensional characteristics relative to the wavelength of the transmitted ultrasound. Alternatively, multiple transducers are disposed at appropriate angles to reduce or eliminate the directional sensitivity of the device.
Another embodiment of the system is illustrated in FIG. 4. In an application of an electro-acoustic stimulation system for the treatment of spinal fusion, a receiver-stimulator device 2 is shown implanted near the spinal column, with electrodes placed so as to provide electrical stimulation to a specific region of the spine. An external acoustic controller-transmitter device 40 is placed over the area of the implant to activate the stimulation. The external transmitter 40 may be handheld, or worn on the body, attached by a belt, harness, or the like. Controls 41 may be provided to allow the user to adjust ultrasound parameters. Such ultrasound parameters, possibly including amplitude, pulse duration, and pulse repetition frequency, are selected to effect fusion of the bone or bone graft. The external controller-transmitter 40 would comprise an adjustable pulse/frequency generator, ultrasound amplifier, ultrasound transmitter, and battery. Optionally, the battery may be a rechargeable type.
In another embodiment of this invention, the controller-transmitter unit shown in FIG. 4 could be implanted to enable long-term continuous treatment to the spine.
In a different embodiment of this invention, the implanted receiver and stimulation components are incorporated into an associated implanted device. For example, the receiver and stimulation components can be part of a pin, a rod, a cage, or plate used to stabilize a fracture. In such a combined device, the receiver-stimulator is adapted into the form of the associated device to provide the electrical stimulation to facilitate fusion. Referring to FIG. 5a, a metal plate 51 is attached with screws in a typical application to stabilize a severely fractured tibia. In this case the receiving transducer and detector electronics 52 and multiple electrodes 53 are incorporated onto the metal plate, with the electrodes in contact with the bone. An external controller-transmitter device 50 similar to that described above is placed over the implanted plate and held in place with a strap or harness and energized as prescribed. Alternatively the controller-transmitter can be of the type that is fully implanted. Additional applications of such a system are, for example, the incorporation of the receiver-stimulator device into the structure of a prosthetic joint or patched in place while applying bone graft materials. Referring to FIGS. 5b and 5c, a cortical screw 54 is adapted to be a receiver-stimulator including the receiving ultrasound transducer 55, circuitry 56 and electrodes 57. Similarly other associated devices for bone fusion may be adapted to contain the receiving, circuitry, and electrode elements and be used as the receiver-stimulator in the system.
Though the uses and configurations differ among the above described example bone stimulation devices, all share the same basic components of a transmitting device and one or more implanted receiver-stimulator devices. The transmitting device, whether in implantable or externally-applied embodiments, and the typical functions that may be incorporated into the transmitting device, have been described. The receiver-stimulator device, in particular with respect to the receiving ultrasound transducer, will have characteristics that are optimized for certain applications.
FIGS. 6
a through 6c depict various possibilities for an implantable receiver-stimulator using a planar transducer. Such an embodiment may, for example, be suitable for surgical implantation for stimulation of bone or may be suitable for incorporation into associated devices such as orthopedic plates or prostheses. FIGS. 6a and 6b show in plan and perspective views, respectively, a receiver-stimulator device 2 having a circular planar ultrasound transducer 80, made typically of a piezoelectric ceramic or single crystal material having electrical contacts deposited on the top and bottom planar surfaces. The size of the ultrasound transducer 80 is selected, for example to be less than one-half wavelength, to optimize receiver sensitivity with respect to orientation. On one surface of the transducer is mounted a circuit assembly 82 containing circuit components 81, the transducer being electrically connected to the circuit components by wiring (not shown). To control any acoustic effects due to combining the components in the receiver-stimulator, the design for mounting of the circuit assembly 82 with the transducer is appropriately chosen; for example, use of air gaps or equivalent. The output of the circuit is connected to two or more stimulation electrodes 84 which are mounted on the outside of an acoustically transmissive and biocompatible casing 83 which also hermetically seals the transducer and circuitry. Electrodes 84 may be positioned on any surface or surfaces of case 83. The planar transducer 80 and case 83 may be circular as shown, or any other shape that may be suitable to the application or intended implant location.
In FIG. 6c, a receiver-stimulator 2 similar to that of FIGS. 6a through 6b is shown, though electrodes 84 are now shown disposed remotely from the case 83, located at the ends of flexible cables 85. Such an embodiment would facilitate precise placement of the stimulating electrodes, perhaps in a situation where it would be otherwise inconvenient or impossible to locate a device having integrated electrodes, as on opposing sides of a bone fracture.
An additional potential benefit of the bone-healing stimulator lies in the reported beneficial aspects of ultrasound exposure alone in accelerating the healing of both bone and soft tissue (bone/ligament/tendon) injuries. In all these devices, combined electrical and ultrasound stimulation would be delivered, providing an enhanced treatment compared to either electrical or ultrasound stimulation alone.
In another embodiment of this invention, the implanted bone stimulation electrodes could be used to deliver therapeutic agents. It is well established that an electric field or ultrasonic field could be beneficially used to enhance the transport of molecules through biological tissue (e.g., iontophoresis, electroporation, or sonophoresis). In one embodiment of this invention, the stimulating electrodes could be coated with a sustained release formulation of a beneficial agent. In another embodiment a reservoir containing a beneficial agent could be attached to the stimulating electrodes. Each time the electrodes are activated the beneficial agent could be released. In yet another embodiment the acoustic energy itself acts as a trigger to release beneficial agent that is contained in a reservoir or in a membrane or the like that is a component of the receiver-stimulator. The beneficial agent could be a bone growth factor, bone cement, stem cells that promote bone healing and growth and the like.
While exemplary embodiments have been shown and described in detail for purposes of clarity, it will be clear to those of ordinary skill in the art from a reading of the disclosure that various changes in form or detail, modifications, or other alterations to the invention as described may be made without departing from the true scope of the invention in the appended claims. For example, while specific dimensions and materials for the device have been described, it should be appreciated that changes to the dimensions or the specific materials comprising the device will not detract from the inventive concept. Accordingly, all such changes, modifications, and alterations should be seen as within the scope of the disclosure.
Patent Application
20070293912
