GB2356051A

GB2356051A - Measuring the vascularity within bone tissue using electrical contact impedance measurements

Info

Publication number: GB2356051A
Application number: GB9926247A
Authority: GB
Inventors: Neil Meredith
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-11-06
Filing date: 1999-11-06
Publication date: 2001-05-09
Also published as: GB9926247D0

Abstract

Two electrodes are provided between which electrical resistance through bone tissue is measured. The electrodes comprise one which may be attached to the patients hand or lip. The second electrode can be attached to a drill bit, screw tap, metallic implant or probe. The signal from the electrodes is A to D converted for analysis. The rate of change of resistance gives a measure of the rate of change of vascularity. The vascularity is related to bone vitality and so the system can provide an objective measurement of bone vitality, vascularity or the degree of necrosis.

Description

D 1 2356051 AN APPARATUS AND METHOD FOR MEASURING THE VASCULARITY WITHIN

BONE TISSUE This invention relates to an apparatus and method for measuring the vascularity within bone tissue by measuring the electrical resistance, impedance or conductivity of the bone tissue and its rate of change with time. The method and apparatus according to the invention can also be used to determine bone vitality.

The vascularity of bone tissue varies depending on internal parameters. These include the structure of the bone being a dense cortical plate or a cancellous bone comprising an open trabecular network. The vascularity of the bone may vary locally due to the presence of vessels but will generally relate to the overall structure: the more open trabecular network exhibiting a higher vascularity and hence lower electrical resistance. The vascularity of bone tissue can be related to its ability to heal by repair or regeneration and this can be termed vitality.

A measurement of the vascularity level of bone tissue is of value in determining the healing capacity of the bone following surgical intervention including the placement of screws, plates and dental implants.

US patent, 5,931,795 discloses a method for assessing an individuals risk of bone fracture by evaluating the bone strength of the individual. The method proposes X ray techniques to determine bone strength from calculations derived from measurements of bone density. These measurements of strength reveal information related to the strength of a bone and not its vascularity or healing capacity.

0 2 Devices are known that measure the electrical resistance of teeth in the diagnosis of dental caries. Cavities in the tooth tissue fill with saliva, the ions of which set up conductive pathways causing resistance values to fall. Thus, a measure of the conductivity of the tooth is an indication of the presence of caries. Such devices typically comprise an electrode which is coupled to a surface of the tooth that has been dried, and a second electrode which the patient holds in their hand. A summary of some of these known devices can be found in the paper "Re-evaluation of electrical resistance measurements for the diagnosis of occlusal caries" by Ricketts et al. Published in the British Dental Journal of January 1995.

Although such devices can be used for predicting the presence or absence of caries, the design of the electrodes means that it is difficult if not impossible to accurately predict the conductive path taken by the current. Thus, the device is not suitable for measuring the vascularity of bone.

World Patent, W09632883 discloses an apparatus for noninvasively determining hematocrit. The apparatus utilises the frequency dependent electrical impedance characteristics of whole blood by electrically stimulating a patient body portion containing a vascular compartment with a current source over a range of frequencies. This device seeks to measure, hematocrit, a parameter of blood as a whole and not the relative amount or vascularity of blood within a tissue.

In accordance with one aspect of the present invention there is provided an apparatus for measuring the vitality and vascularity within bone tissue, comprising: two electrodes arranged such that in use one is connected to a drill bit, screw tap, probe or implant inserted into bone and the other is connected via a lip, mucosal or handheld probe; an electrical power source connected between the two electrodes; a sensor for sensing the voltage or current produced by said power source across or 0 3 through the test sample; and a Signal processor responsive to said measured voltage and/or current to calculate a value indicative of said vascularity within said bone tissue.

Thus, the present invention alleviates the disadvantages of the prior art by providing an apparatus comprising two electrodes that can be placed in the bone and on the tissue to accurately measure the conductivity or resistance and from this determine the vascularity of the bone located between the two electrodes. A measurement of the rate of change of conductivity with time provides an indication of relative vascularity at a specific point in the bone relative to the depth of insertion of a drill bit, screw, tap, probe or implant.

Preferably, one electrode is arranged as a clip or probe which can be held by the patient. The other electrode can also be a clip or terminal which provides a contact of low electrical resistance with a drill bit, screw, tap, probe or implant.

In one embodiment the electrode can be connected to a drill bit which is electrically conductive along its length. Thus it is advantageous in that the drill tip can measure regions of low electrical resistance coincident with vascularity as it penetrates the bone.

In another embodiment the electrode can be connected to a screw tap or metallic implant which is conductive along its length. Thus as the screw, tap or implant is inserted into bone, regions of low electrical resistance and high vascularity can be measured.

In one embodiment an insulated probe with an electrically conductive tip can act as the electrode and be inserted into a prepared hole in the bone. The conductive part of the probe can identify regions of vascularity as it passes down the hole.

0 4 In one embodiment the power source is an AC source, preferably a sinusoidal current source (although a square wave source could be used). An AC source reduces possible problems of distortion due to polarisation of the test sample. A current source means that only the resultant voltage need be measured.

In preferred embodiments the apparatus is bandwidth limited and the signals are filtered to reject extraneous interference. Thus, the apparatus may be made less sensitive to interference from other sources.

Advantageously the apparatus comprises a full wave rectifier, low pass filter and an analogue to digital converter to convert the analogue signal to a digital signal. The value of the digital signal provides a direct indication of vitality and vascularity.

In one embodiment the analogue to digital converter is a 16 bit or greater analogue to digital converter, preferably with a sampling rate of at least I OOHz. A high resolution analogue to digital converter means that the apparatus is suitable for use across a wide range of resistances, typically from 10OOhms to 100MOhms and hence a wide range of vascularity levels. A high sampling rate means that rapid changes in vascularity levels can be accurately recorded.

Advantageously, the input impedance of the analyser is greater than lGohms. A high input impedance of the analyser enables it to accurately measure vascularity levels even where the resistance of the bone is quite high. Preferably the apparatus comprises data storage means for storing calibration data. Thus, the apparatus can be calibrated for different probes and for different situations. It can for example be calibrated so that the resistance/conductivity measurements made by the apparatus are translated directly to vitality/vascularity level.

0 5 In one embodiment the signal processor of the apparatus is adapted to calculate bone vitality from the variation in the measured values over time. The apparatus can thus be directly used to produce an objective measure of bone vitality.

Preferably the apparatus comprises display means for displaying the calculated vascularity level and or vitality, advantageously said display means is graphical and is adapted to display the vitality level as it varies with time. In preferred embodiments the apparatus comprises an alarm that is set to be activated when the measured values exceed or fall below at least one preset value.

In preferred embodiments the electrical power source is an AC power source and the method further comprises the step of converting the AC signals to DC signals. The use of an AC power source prevents the sample from becoming polarised, however a DC signal is easier to interpret, thus the method provides for the conversion of the signal.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure I illustrates one electrode attached as a clip to a drill bit inserted into bone tissue according to one embodiment of the invention; Figure 2 illustrates one electrode attached as a clip attached to the lip as an embodiment of the invention; Figure 3 illustrates a block diagram of the electronic circuit of an embodiment of the invention; 0 6 Figure 4 illustrates the relationship between vascularity and AC resistance through a section ofbone.

Figure 5 illustrates the relationship between vascularity and the position of a drill inserted into bone of varying vascularity.

With reference to Figure 1, a clip 10 suitable for attachment to the shank of a drill bit 20 suitable for measuring the vascularity of bone tissue 30,40 is shown. The clip is made from metal and provides a path of low resistance. Alternative embodiments of the electrode, not illustrated, can be attached to different surgical components including screw taps, probes and metallic implants dependent on the surgical technique being performed. The components used will all be metallic and exhibit low electrical resistance.

Figure 2 shows the second electrode attached as a clip to the lip of the patient.

With reference to Figure 3, a device according to an embodiment of the invention comprising an electronic instrument designed to measure the AC electrical resistance and its rate of change between a lip or handheld electrode and a drill bit, screw tap, implant or probe inserted into bone. The instrument relates these measured values directly to vascularity level or vitality.

The analyser determines the resistance of the bone tissue by applying a low current, typically <0.5RA sinusoidal drive to the test sample via the two electrodes. The resistance is given by the ratio of the voltage across the test sample divided by the drive current. This value is directly related to vascularity level and can be converted to it by, for example, calibration. The graph of Figure 4 illustrates the correlation of vascularity level 70 and resistance 80 through a section of dense 50 and porous 60 bone.

0 7 The test range of the instrument is wide rangeing from 10OOhms to 100MOhms and the input impedance of the analyser is greater than IGOhms. The input test leads of the probe are of low capacitance. The system bandwidth is limited and the signals are filtered to reject extraneous interference. A precision full wave rectifier and filter converts the AC voltage into a DC signal representing the value of the resistance and hence also indicative of the value of the vascularity level. The test results may be inverted to give a measure of conductance.

The DC signal may be displayed on a digital or bargraph display or LED (i. e. red/green) and high or low limit trips may be set to sound or indicate alarms at preset intervals. The DC signal is then quantised by an analogue-to-digital converter thereby providing a means for logging data and plotting changes in vascularity level with time. The instrument also includes a timing circuit to measure the time taken for a predetermined vascularity level to be reached. The device finther comprises a microprocessor for performing the signal processing and other processing ffictions, such as calculating vascularity level or bone vitality from the measured values of voltage and or current and their rate of change with time. The device comprises a graphical display for displaying the results calculated by the processor. The unit is powered by a mains power source. Alternative embodiments may be powered by disposable or rechargeable batteries.

The device further comprises data storage means for storing calibration data, which is located in a memory within the microprocessor. This data is typically obtained from clinical measurement related to known measurements of resistance or conductance and absolute levels of vascularity measured related to radiographic assessment. Further data relating rate of change of vascularity to bone vitality is also stored.

( - 8 j Figure 5 relates the change in resistance obtained with varying vascularity as a drill bit 100 is inserted through a section of bone 90.

It will be apparent, that the present embodiment has been described by way of example only and that modifications may be made within the scope of the appended claims.

9

Claims

1. An apparatus for measuring the vascularity level within bone tissue, Comprising: two electrodes arranged such that in use one contacts the patients hand or lip and the other penetrates the bone tissue; an electrical power source connected between the two electrodes; a sensor for sensing the voltage or current produced by said power source across or through the test sample; and a signal processor responsive to said measured voltage and or current to calculate a value indicative of said vascularity level within said bone tissue.

2. An apparatus according to claim 1, wherein the intrabony electrode is attached to a drill bit.

3. An apparatus according to claim 2, wherein the intrabony electrode is attached to a screw tap.

4. An apparatus according to claim 3, wherein the intrabony electrode is attached to a metallic implant.

5. An apparatus according to claim 4, wherein the intrabony electrode is attached to a diagnostic probe insulated along its length and having a conductive tip.

6. An apparatus according to any of the preceding claims, wherein said power source is an AC source.

7. An apparatus according to claim 6, wherein said AC source is a sinusoidal current source.

8. An apparatus according to any of claims 6 or 7, wherein the apparatus is bandwidth limited and the signals are filtered to reject extraneous interference.

9. An apparatus according to any of claims 6 to 8, fin-ther comprising a full wave rectifier, low pass filter and an analogue to digital converter to convert the analogue signal to a digital signal.

10. An apparatus according to claim 10, wherein the analogue to digital converter is a 16 bit or better analogue to digital converter.

11. An apparatus according to claim 9 or 10, wherein the analogue to digital converter has a sampling rate of at least I OHz.

12. An apparatus according to any of the preceding claims, wherein the input impedance of the apparatus is greater than I Gohm.

13. An apparatus according to any of the preceding claims, further so utilising data storage means for storing calibration data.

14. An apparatus according to claim 14, wherein said data storage means comprises calibration data, including at least two sets of values of resistance or conductance with associated vascularity level. An apparatus according to claim 13, wherein said data storage means comprises calibration data, including at least two sets of values of resistance or conductance with associated bone vitality.

15. An apparatus according to any one of the preceding claims, wherein said signal processor is adapted to calculate bone vitality from the variation in the measured values over time.

16. An apparatus according to any one of the preceding claims, finther comprising display means for displaying the calculated vascularity level or bone vitality.

17. An apparatus according to claim 16, wherein said display means is a graphical display means adapted to display the variation in vitality level with time.

18. An apparatus according to any one of the preceding claims, further comprising an alarm set to be actuated when the measured values exceed or fall below at least one preset value.

19. A method of measuring the vascularity level of bone tissue comprising the steps of. removing excess surface moisture from an area of the bone to be tested; contacting said test area with a drill bit which is rotated to make a hole in the bone and inserted into said hole; connecting an electrical power source between the electrodes; measuring the current or voltage produced by said electrical power source through or across said test area; calculating a value indicative of the vascularity of said test area from said measured values.

20. A method according to claim 19 wherein a screw tap, metallic implant or probe is inserted into a prepared hole in the test area.

21. A method according to claim 19, wherein said electrical power source is an AC power source and the method further comprises the step of converting the AC signal emanating from said test area to a DC signal.

22. A method according to claim 21, further comprising the step of converting the analogue signals to digital signals prior to measuring them

23. A method according to any of claims 19 to

24 further comprising the step of comparing the measured values with calibration values.

12 24. A method according to any of claims 19 to 24 wherein the steps are repeatedly performed over a period of time.

25. A method according to claim 24, further comprising the steps of measuring the rate of change of vascularity level of the test area over time; determining the vitality of the test area from said rate of change or vascularity level.

26. An apparatus for measuring the vascularity level within bone tissue substantially as hereinbefore described with reference to the accompanying drawings.

27. A method of measuring the vascularity level within bone tissue substantially as hereinbefore described with reference to the accompanying drawings.

28. An apparatus for measuring the vascularity level within bone tissue substantially as hereinbefore described with reference to the accompanying drawings.

29. A method of measuring the vascularity level within bone tissue substantially as hereinbefore described with reference to the accompanying drawings.