AU764997B2 - Resonant frequency stimulator and recorder - Google Patents

Description

WO 00/27335 PCT/AU99/00974 "Resonant frequency stimulator and recorder" Field of the Invention The present invention relates to a device and method for the noninvasive stimulation and monitoring of bone formation. In particular, the present invention relates to a device and method of generating a mechanical strain in bone tissue to promote healing and growth of bone tissue.

Background Art Typically, when a bone is fractured or broken and re-set, a physician will assess the healing process based on clinical findings including swelling, tenderness, abnormal movement and radiographic imaging of the fracture site. Based on these findings, the physician will determine the amount of load the bone is able to bear and when joint movement should commence.

If limb movement is not implemented at the correct time, joint contractures and muscle atrophy can occur. If mobilisation is introduced too early, however, there is a risk of re-fracture and malunion of the fracture ends. Similar examination may also be used in the assessment of osteoporosis and other bone disorders.

Clearly, there is a requirement for a rehabilitation programme based on accurate and quantifiable data reflecting the various stages of bone tissue growth.

It is known that as a fracture heals, a callus is formed between the fractured ends. The formation of the callus, and the healing process in general, involves the strengthening and stiffening of the bone at this site.

Such a change in the mechanical properties of bone has a direct relationship with the resonant frequency of bone. For example, as a fractured bone heals, and its bending or torsional stiffness increases, so does the resonant frequency of the bone; and similarly, as the amount of mineral per unit volume (and hence density) of an osteoporotic bone reduces, so does the resonant frequency of that bone. Accordingly, the analysis of the resonant frequency of a bone following fracture, or of an osteoporotic bone, or of a bone with some other form of pathology, can provide a physician with yet another tool for studying the process of bone formation.

Various methods of promoting bone tissue growth have been proposed.

Such methods include electrical (voltage and current) signal stimulation, magnetic field stimulation, and ultrasound stimulation.

WO 00/27335 PCT/AU99/00974 2 Mechanical loading methods have also been proposed. Such methods normally are performed at a low frequency or repetition. In US 5376065 (McLeod et al), a method is described wherein bone tissue is stimulated with a longitudinal strain at a physiologically relatively high frequency of In this case, a patient is placed on a spring mounted assembly in such a way as to mechanically load the bone. Once the total weight is loaded, the bone is caused to resonate at the natural frequency for the spring.

Mechanical methods of bone growth stimulation have also been described. What is clear from these methods is that there appears to be a window of strain magnitude, which stimulates the maintenance and/or healing of bone tissue. This strain magnitude varies according to the maturity of the bone tissue, the site of the bone and its usual loading conditions in vivo. The nature of the strain may be compression, tension or torsion. It may also be bending, since bending is a combination of compression on one side and tension on the other. In a given bone, the window would be expected to be different for each mode of strain. There are, therefore, strains that are too low in magnitude to produce a beneficial effect, and conversely strains which are too high, and thus cause damage or destruction of bone tissue.

Attempts to modify these strains to a beneficial effect have been made by altering the design and load characteristics of external and internal fixation devices by encouraging patient physical activity. Difficulties with these methods include that the strain cannot be controlled and that they rely on the patient being able to weight bear sufficiently to impart the load.

Disclosure of the Present Invention According to a first aspect, the present invention comprises a device for promoting or maintaining bone mass including: a vibration stimulator that provides a vibratory stimulus to a bone; (ii) a vibration detector that detects the vibratory response of a bone to the vibratory stimulus and produces a signal representative of the vibratory response; (iii) a control means that processes the signal representative of the vibratory response to identify at least one resonant frequency of the bone and then causes the vibration stimulator to provide a vibratory stimulus to the bone at, or approximate, the bone's at least one resonant frequency.

WO 00/27335 PCT/AU99/00974 3 In preferred embodiment.;, the vibration stimulator provides a vibratory stimulus over a range of frequencies.

Resonant frequency analysis can be used as a non-invasive technique of structural assessment. The analysis relies upon the physical principle that structures vibrate maximally, or are most easily vibrated, at one or more frequencies determined by the structure's physical dimensions and material properties. In the case of a bone, the boundary conditions or environment, such as the overlying soft tissue and joint position, also impacts on the bone's resonant frequency.

In one embodiment, the vibration stimulator can provide a vibratory stimulus in the form of a broadband or random frequency stimulus, or a sweeping signal. The stimulator can be a mechanical device such as an electromagnetic shaker, a rotating mass or an impact mallet. Other types of pulsed or frequency energy stimuli can be utilised, such as ultrasonic vibration.

In one embodiment of the invention, the device is adapted to maintain stimulation of the bone by the vibration stimulator at, or approximate, the at least one resonant frequency of the bone such that the bone is stimulated at the correct frequency to optimise treatment.

During bone healing, the dimensions, internal structure and stiffness of the callus changes as it remodels. The boundary conditions are also changing. By bending, a strain is generated in the bone. Stimulating a bone to vibrate in a bending mode will cyclically load the bone. Determination of the bone resonant frequency allows determination at any given time during healing, the frequency of vibration to be caused to the bone at which it is most easily vibrated in a given bending mode or shape. The determined bone resonant frequency may be in either acceleration, velocity, or displacement dimensions.

In another embodiment of the invention the vibration stimulator maintains stimulation of the bone for between 10 and 60 minutes.

In a still further embodiment of the invention, the vibration stimulator provides a vibratory stimulus that is of a relatively high frequency and preferably in the order of between 50Hz and 1000Hz.

In another embodiment the vibration stimulator and the vibration detector are applied directly to the surface of the skin of a patient and the vibratory stimulus imparted to the bone transcutaneously. This may include WO 00/27335 PCT/AU99/00974 4 bones with external frames or casts in situ, or bones with internal implants in situ including: plates, screws, intramedullaiy devices, or ingrowth or ongrowth implants.

In a further embodiment, the vibration stimulator and vibration detector are attached to an external frame or pin, and the vibratory stimulus imparted and recorded via these external structures.

In a still further embodiment, the vibration detector is an accelerometer or a plurality of accelerometers.

In another embodiment, the control means includes signal processing means that processes the signal representative of the vibratory response In a further embodiment, the signal processing means can include an amplifier that amplifies the signal representative of the vibratory response.

In yet a further embodiment, the signal processing means can include an analogue to digital converter that converts the signal representative of the vibratory response so that it can be processed by the control means.

In a further embodiment, the control means is a computer which processes the signal transmitted from the vibration detector and displays the signal representative of the vibratory response of the bone both numerically and graphically.

In a still further embodiment, the device is used to promote bone mass at a fracture site of a bone.

In another embodiment, the device is used to promote bone mass in a bone affected with osteoporosis.

In a further embodiment, the device is used to maintain the mass of a bone and thereby prevent the onset of osteoporosis.

In yet a further embodiment, the device is used to promote bone growth into ingrowth implants, and/or onto outgrowth implants.

In another embodiment, the device is used in the lengthening of limbs wherein the bone of a limb is fractured and bone growth at the fracture site promoted.

In yet another embodiment still, the device is used in the promotion of healing of non-union and/or of incorporation of a bone graft.

In a second aspect, the present invention provides a method of treating a patient suffering from a bone disorder including the steps of: providing a vibratory stimulus to a bone; detecting the vibratory response of the bone to the vibratory stimulus; WO 00/27335 PCT/AU99/00974 generating a signal representative of the vibratory response; processing the signal to identify at least one resonant frequency of the bone and providing a signal to adjust the vibratory stimulus to the bone such that it is at, or approximate, the bone's at least one resonant frequency.

In a still further embodiment, the device of the invention is portable and the method of treatment may be administered at the patient's home. The patient need simply apply the vibration stimulator and the vibration detector to the affected limb, or other body part, and activate the device for a predetermined period of time: preferably between 10 and 60 minutes. The treatment is preferably carried out on a daily basis for a period of time until the desired increase in bone strength has been achieved, and preferably for at least three months.

In a further embodiment, the device can be adapted such that upon activation by a patient on the second or any other subsequent day of treatment, the vibration stimulator provides a vibratory stimulus at, or approximate, the frequency determined previously as the resonant frequency.

Further, the device preferably can then identify the actual resonant frequency of the bone and adjust the vibratory stimulator to provide a vibratory stimulus at, or approximate, the identified resonant frequency.

The device is preferably adapted to keep a record of the resonant frequency throughout the entire period of time of treatment of the bone. As the resonant frequency will normally gradually increase with bone healing, a record of the rate of healing and the degree of healing of the bone can be provided. The determined rate of healing for a particular patient can be compared with previously determined rates of healing for other patients and so provide the physician with an indication as to whether healing is occurring at a satisfactory rate and to a sufficient degree.

Brief Description of the Drawings By way of example, preferred embodiments of the invention are described with reference to the accompanying drawings in which: Fig. 1 is a simplified view of one embodiment of the present invention.

Preferred Mode of Carrying Out the Invention In the first instance, the device 10 is adapted to determine the specific resonant frequency of a bone 11, and to then subject the bone 11 to stimulation at the specific resonant frequency of the bone 11, and maintain stimulation at that frequency for a period of time. This ensures optimal WO 00/27335 PCT/AU99/00974 6 stimulation and thus optimal promotion of bone mass, whilst at the same time, avoids the risk of over-stimulation or overloading, and thus, fracture of the bone.

The device 10 includes a vibration stimulator 12 which, when activated, stimulates the bone 11 over a range of frequencies, causing vibration of the bone 11. The stimulator 12 is driven by a signal generator housed in the computer 14. The signal generator, when initially activated, can cause the stimulator 12 to vibrate over a range of frequencies. The signal generator can, for example, sweep through this range of frequencies.

Examples of suitable stimulators 12 include a rotating eccentric mass, an electromagnetic shaker, and a variable frequency pulsed ultrasonic transducer. The stimulator 12 can incorporate a stimulus sensor, such as a force transducer, to monitor the stimulus provided to the bone by the stimulator 12.

The vibrations are detected by a detector 13. The detector 13 can comprise an accelerometer or a plurality of accelerometers. The detector 13 transmits the signals to a computer 14 wherein the signals are converted from analogue to digital form and then processed to determine the frequency domain characteristics of the vibratory response. The computer 14 can incorporate an automatic analysing means that determines the peak acceleration/velocity/displacement of the bone and so determines the resonant frequency of the bone.

In another preferred embodiment, the device further includes a display means 17 for displaying numerically and/or graphically any variables detected, affected and/or effected by the device 10. The display means 17 may, for example, provide such a display for the measured characteristics of the vibratory response to allow manual determination of the resonant frequency by a user of the device 10, or a treating physician. The device can also include a manual frequency control 15 for stimulation. Preferably, from the signals received, the computer 14 identifies one of the resonant frequencies of the bone 11 and transmits a signal to the stimulator 12 to stimulate the bone 11 at, or approximate, the one resonant frequency.

The device can incorporate a timer that allows the time of operation of the stimulator 12 to be pre-set prior to activation. Such a time might be set by making an appropriate entry into a software programme running on the computer 14. Preferably, adjustment to the amplitude of the vibratory WO 00/27335 PCT/AU99/00974 7 stimulus can be made by suitable entries into the software running on the computer 14.

Whilst Figure 1 depicts a simple representation of the device whereby the vibration stimulator 12 and the detector 13 are applied to the bone via an external frame 16, it is envisaged that they would be applied either to the skin of the affected limb, wherein the stimulation of the bone could be imparted transcutaneously, or via a frame, or other like device surrounding the limb, and in particular, a plaster cast.

It is further envisaged that this method of treatment be used in the treatment of several bone disorders to promote bone tissue growth and also to maintain bone mass. Examples include the healing of a fracture site wherein the vibration caused to the bone results in micro-movement, bending or torsion at the site of fracture which in turn leads to promotion of bone healing. The stimulation also causes micro-movement, bending or torsion of the intact portions of the bone, which in turn, leads to promotion of bone formation and prevention of osteoporosis in the intact bone. As the stimulation imparted by device 10 is regulated to be at the same, or approximately the same, frequency as the resonant frequency of the bone, the promotion of bone tissue growth is optimised and occurs at a faster rate than if the bone is simply stimulated at a frequency unrelated to the resonant frequency of the bone.

Similarly, it is readily envisaged that the device 10 could be applied transcutaneously to a bone with a fixation means such as an intramedullary nail holding the bone pieces together. In this manner, the formation of bone between the pieces of bone may be increased by the stimulation of the bone and the fixation means at, or approximate, the bone's resonant frequency.

It can also be envisaged that the device 10 can be transported readily and therefore used in a patient's home. In this way, the device 10 would be pre-programmed such that all the patient need do to use the device would be to attach the vibration stimulator 12 and the vibration detector 13 to the affected limb, or other body part, and activate the device It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (19)

1. A device for promoting or maintaining bone mass including: a vibration stimulator that provides a vibratory stimulus to a bone; (ii) a vibration detector that detects the vibratory response of a bone to the vibratory stimulus and produces a signal representative of the vibratory response; (iii) a control means that processes the signal representative of the vibratory response to identify at least one resonant frequency of the bone and then causes the vibration stimulator to provide a vibratory stimulus to the bone at, or approximate, the bone's at least one resonant frequency.

2. The device of claim 1, wherein the vibration stimulator provides a vibratory stimulus over a range of frequencies.

3. The device of claim 1 or 2, wherein the vibration stimulator provides a vibratory stimulus in the form of a broadband or random frequency stimulus.

4. The device of claim 1 or 2, wherein the vibration stimulator provides a vibratory stimulus in the form of a sweeping signal.

The device of any one of the preceding claims, wherein the stimulator is a mechanical device.

6. The device of claim 5, wherein the mechanical device is selected from the group comprising an electromagnetic shaker, a rotating mass or an impact mallet.

7. The device of any one of claims 1 to 4, wherein the stimulator provides an ultrasonic vibration to the bone.

8. The device of any one of the preceding claims, wherein the stimulator maintains stimulation of the bone at or proximate the at least one resonant frequency of the bone.

9. The device of any one of the preceding claims, wherein the vibration stimulator maintains stimulation of the bone for between 10 and 60 minutes.

The device of any one of the preceding claims, wherein the vibration stimulator provides a vibratory stimulus that is in the range 50Hz to 1000Hz.

11. The device of any one of the preceding claims, wherein the vibration stimulator and the vibration detector are applied directly to the surface of the skin of a patient.

12. The device of any one of claims 1 to 10, wherein the vibration stimulator and the vibration detector are applied to external frames or casts in situ, or bones with internal implants in situ. WO 00/27335 PCT/AU99/00974 9

13. The device according to any one of the preceding claims, wherein the vibration detector is an accelerometer or a plurality of accelerometers.

14. The device according to any one of the preceding claims, wherein the control means includes signal processing means that processes the signal representative of the vibratory response.

The device according to claim 14, wherein the signal processing means includes an amplifier that amplifies the signal representative of the vibratory response.

16. The device according to claim 14 or 15, wherein the signal processing means includes an analogue to digital converter that converts the signal representative of the vibratory response so that it can be processed by the control means.

17. The device according to any one of the preceding claims, further including display means for displaying numerically and/or graphically any variables detected, affected and/or effected by the device.

18. A method of treating a patient suffering from a bone disorder including the steps of: providing a vibratory stimulus to a bone; detecting the vibratory response of the bone to the vibratory stimulus; generating a signal representative of the vibratory response; processing the signal to identify at least one resonant frequency of the bone and providing a signal to adjust the vibratory stimulus to the bone such that it is at, or approximate, the bone's at least one resonant frequency.

19. The treatment of claim 18, wherein the treatment is carried out on a daily basis for a period of time. The treatment of claim 19, wherein the time period is at least three months.