Osteogenesis stimulator with digital signal processing

A stimulator and a method for electrical stimulation of bone to promote osteogenesis is disclosed in which surface electrodes positioned around an incision site transmit an interferential current that has a base medium frequency alternating current between 1K-20 KHz. A digital signal processor generates a sine-wave-like waveform from a pulse generator which after further processing is used to generate two circuits for use in producing the interferential current. The effective area of stimulation is controlled by placement of electrodes and electrode orientation. Amplitude modulation of electrical circuits created at the electrode placements also augments the effective area of stimulation.

FIELD OF THE INVENTION

The present invention is generally related to osteogenesis and, more particularly, is related to an apparatus and method for the electrical stimulation of bone to promote osteogenesis, and aids in the treatment of osteoporosis.

BACKGROUND OF THE INVENTION

Spinal fusion surgery in the clinical setting has a reported failure (non union) rate of anywhere from 5% to 35%. While many variables are thought to affect the success rate of spine fusion procedures, most attempts to improve the clinical outcome have focused on internal or external fixation techniques that augment the biomechanical stability of the fusion mass. More recently, however, pre-clinical investigations have sought to understand the role and importance of biological and physical factors in the healing and subsequent stabilization of the fusion mass. Electrical stimulation in various forms has been demonstrated to improve the overall fusion rate in clinical populations.

However, current technology uses electrical leads that are implanted in the bony fusion mass, which must be left behind during explantation of the battery cell. Also, patient compliance is an issue with the current techniques employed in clinically attachable bone growth surface electrode stimulators.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an apparatus and method for osteogenesis and the treatment of osteoporosis using electrical stimulation of the bone. The present invention utilizes an interferential current that has a base medium frequency alternating current between 1 KHz and 20 KHz. An interferential current is set up between two circuits that are arranged in a cross-pattern on the subject's targeted area of stimulation. Where the circuits superimpose in a cross-pattern, the resultant beat frequency will be the difference between the frequencies of the two circuits and will usually range between 0-250 Hz and can be dynamic, and the amplitude will be additive and greater than either circuit alone. Multiple levels of spinal fusion can be treated depending upon the electrode placement and modulation pattern selected. The amplitudes of the outputs in the respective circuits may be modulated to increase the area of targeted stimulation. Interferential current allows improved directional control and depth of penetration in comparison to other standard osteogenesis stimulators.

Briefly described, in architecture, one embodiment of the invention, among others, can be implemented as follows.

Digital signal processors (DSPs) are used for improving the accuracy and reliability of digital signals that are used extensively in the communications field. Digital signal processing works by standardizing or clarifying the output of a digital signal. In this embodiment, the digital signal processor is used to shape multiple pulsatile waveforms to approximate the output of a sine-wave generator. In another embodiment of the invention, the digital signal processor is replaced with a field-programmable gate array (FPGA). An FPGA is an integrated circuit that can be programmed in the field after it is manufactured and therefore allows users to adjust the circuit output as the needs change. Both the DSP and the FPGA process a digital signal into a pseudo-sine-wave current waveform from the digital pulses generated by a pulse generator. The pseudo-sine-wave current waveform is transmitted through surface electrodes at a targeted area creating an interferential current. That interferential current is used for osteogenesis.

Embodiments of the present invention can also be used to provide methods for electrical stimulation of bone to promote osteogenesis. In that regard, one embodiment of such a method, among others, can be broadly summarized by the following procedure:

A first and second pair of surface electrodes are positioned on a subject's skin surface at a targeted area. A digital signal processor (or a field-programmable gate array) is connected to the surface electrodes. A pulse generator is connected to the digital signal processor and generates digital pulses. The digital signal processor processes the digital pulses to approximate the output of a sine-wave generator (pseudo-sine-wave output). The processed pseudo-sine-wave is transmitted to the surface electrodes wherein an interferential current is produced and generated at the targeted area. The interferential current is used for osteogenesis and the treatment of osteoporosis.

Preliminary studies have shown that the DSP pseudo-sine-wave interferential current produces a significantly higher level of bone growth and in particular, increased secondary or lamellar bone growth. The DSP generates individual pulses of differing widths. When those differing width pulses are driven into a transformer, they produce an output that simulates a sine wave (hence the term pseudo-sine-wave, or sine-wave-like waveform). That pseudo-sine-wave produces a “ringing” effect and harmonics are formed due to the interaction of the pulses. The waveform that is generated looks “fuzzy” in comparison to a waveform that is generated by a standard true sine-wave generator and it is believed that that harmonic ‘ringing’ has a superior effect on the osteoblastic activity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention and modifications thereof will now be described with reference to the drawings.

FIG. 1shows a stimulator100for the electrical stimulation of bone to promote osteogenesis utilizing an interferential current110that has a base medium frequency alternating current between 1K-20 KHz. The interferential current110is set up between two circuits118,120that are arranged in a cross-pattern. A first pair of surface electrodes108,208are positioned on a subject's skin surface112at one set of diagonal corners of a targeted area214(see FIG.2). A second pair of surface electrodes108,208is then positioned at the other set of diagonal corners of the targeted area214. A digital signal processor102is connected to the first and second pairs of surface electrodes108. When a signal generating source104is connected to the digital signal processor102, a sine-wave-like waveform signal output106is created. The results indicate that currents produced by surface interferential stimulation has positive effects on the tensile strength and the formation of mineralized surface area at fusion sites. The fibrous connective tissue formed between autograft fragments promotes the biomechanical properties of the fusion mass. The digital signal processor102improves the accuracy and reliability of digital signals. The digital signal processor102processes the multiple pulses116from the signal generating source104to approximate a sine-wave (pseudo-sine-wave or sine-wave-like). The digital signal processor102generates individual pulses106of differing widths and resultant amplitudes. When those differing pulses106are driven into a transformer (not shown), the pseudo-sine-wave is produced. A pulse generator104is connected to the digital signal processor102and supplies a pulsed digital signal output116to the digital signal processor102. The digital signal106processed by the digital signal processor102creates a first circuit118and a second circuit120at the first and second pairs of surface electrodes108,208, respectively. Where the first and second circuits118,120superimpose, the resultant beat frequency (which is preferably between 1 and 250 beats/second) will be the difference between the frequencies of the two circuits, and the amplitude will be additive and-greater than either circuit alone (FIG.2).

Multiple levels of spinal fusion can be treated depending on the placement of the first and second pairs of electrodes308and by modulating the amplitudes of the outputs of the first and second circuits318,320(see FIG.3). Modulating the outputs of the first and second circuits318,320increases the area of the targeted stimulation. The depth of modulation can vary from 0 to 100% and depends on the direction of the currents established by the first and second circuits318,320. When the first and second circuits318,320intersect at 90°, the maximum resultant amplitude and the deepest level of modulation is half-way between the two circuits (45° diagonally). (See FIG.2). Thus, also, the number of vertebral segments stimulated can be controlled by the placement of the first and second pairs of electrodes308and their spatial orientation as described above. The area of stimulation can be augmented by modulation of the amplitudes of the outputs of the two circuits.

FIG. 4illustrates the interferential current306with the sine-wave-like waveform that is generated by the digital signal processor102. An example of a suitable commercially available digital signal processor102is the RS 4i manufactured by RS Medical of Vancouver, Washington. A field-programmable gate array (not shown) can also be used to shape multiple pulsatile waveforms to approximate the output of a sine-wave generator instead of the digital signal processor102described above. The FPGA is an integrated circuit that can be programmed in the field after it is manufactured and allows its user to adjust the circuit output as desired.

In an alternative embodiment, as described above, the digital signal processor may be replaced with the FPGA. Whereas DSP processors typically have only eight dedicated multipliers at their disposal, a higher end FPGA device can offer up to 224 dedicated multipliers plus additional logic element-based multipliers as needed. That allows for complex digital signal processing applications such as finite impulse response filters, forward error correction, modulation-demodulation, encryption and applications such as utilized in the present invention.

Preliminary studies have been conducted by the inventors using the present invention and the results have been summarized by bone growth quantification. The results support the claim that the use of electrical stimulation singly, or in combination with other techniques, augments and enhances healing and effectivenss of biologics for osteogenesis (including but not limited to growth factors, Bone Morphogenetic proteins, Hydroxyapatite, Autogenous Bone Grafts, Human Bone Allografts, Demineralized Bone Matrix (DBM) and systemic use or local application of anti-resorptive drugs (i.e., Foxomax, Raloxifene and others)).