WO2006021891A1

WO2006021891A1 - Apical constriction locator

Info

Publication number: WO2006021891A1
Application number: PCT/IB2005/003414
Authority: WO
Inventors: Jonas Martynaitis; Rimantas Povilas Masiulis
Original assignee: Jonas Martynaitis; Rimantas Povilas Masiulis
Priority date: 2004-07-21
Filing date: 2005-07-13
Publication date: 2006-03-02
Also published as: GB2431358A; SE0401910D0; GB2431358A8; GB0701779D0; GB2431358B; GB2431358B8

Abstract

The present invention relates to instrumentation and method for determining the distance between a dental probe located in a root canal of a tooth and the apical constriction. Two excitation signals are applied between the dental probe and an electrode located in electrical contact with oral mucosa and using these two signals comprising alternating voltages of different frequencies a signal is measured and a lower frequency signal is used to cancel errors due to media dependencies by applying an error signal obtained from a low frequency signal to the high frequency component using an analogue negative feedback system. The high frequency signal then contains all distance information and is error eliminated. Thus by sampling only the high frequency signal an accurate and stable measure of the distance is obtained.

Description

Apical Constriction Locator

Field of the invention

The present invention relates to a device and method for locating the apical constriction in a tooth and specifically to a device and method for locating the apical constriction (apex physiologicum) with high accuracy using a signal transformation technique of a dual frequency approach.

Background of the invention

During root filling of a damaged tooth it is necessary to clean the root canal properly and to an exact location close to the apical constriction of the tooth. If an improper cleaning of the root canal has been done there is a high risk that the filling material, e.g. flexible filler, will not heal and stabilize properly and cause further problems for the patient. If the cleaning process is done to far down the root canal and into the apical constriction and eventually penetrating the jaw tissue the filling material may penetrate into the jaw tissue and damage blood tissue and nerve tissue irreversible and cause swelling and pain for the patient.

Current standards within the field include methods such as using a reamer, file, or similar dental probe for cleaning the root canal. During the process an x-ray image is taken with the dental probe inserted into the root canal and the remaining distance to the apical constriction is measured on the x-ray image and it is thus possible to control the location of the apical constriction with respect to the dental probe. However, this is slow process and exposes the patient to unnecessary x-ray radiation; therefore other techniques have been developed for monitoring the location of the apical constriction continuously during the cleaning process.

Such systems include measuring the impedance between the dental probe (using a conductive probe) and some point in the mouth close to the tooth under treatment, often the oral mucosa. It has been shown in the literature that it is possible to monitor the impedance changes during the cleaning process however; the existing techniques have drawbacks in the form of inaccuracy or are unable to take mixed material compositions in the root canal into account. Often a mixture of blood, tooth tissue, water, or other substances is apparent inside the root canal during the cleaning process, disturbing the impedance measurement and increasing the risk of cleaning too far down into the apical constriction or to short of it.

Some of the first attempts to solve the problem of having a continuous measuring principle may be found in US patents 5,049,069 and 5,063,937 wherein systems for using a single resistive or a combination of resistive and capacitive components using a single AC signal has been demonstrated but they both lack the accuracy needed for safe operation and is unable to take the conditions inside the root canal into consideration (such as wet or dry conditions).

The second solution group involves using two alternating frequencies in the measurement in order to be able to measure both the resistive and capacitive components of the root canal impedance. These techniques, demonstrated by for instance US patents 5,017,134, 5,080,586, and 5,211,556, use a principle based on the measurement of a voltage difference between the two different frequencies, however, still using this kind of solution it is difficult to handle different material compositions present in the root canal and they need calibration during each measurement, thus lowering the user friendliness of the instrument.

A third solution group is represented by US patents 5,096,419, 5,295,833, and international application WO 01/47414, wherein attempts to eliminate the root canal media conditions is done. These solutions measure the ratio of impedances or voltages of two different frequencies when the dental probe is moved in the root canal. These techniques are based on some simplifications of the algorithm governing the measurement analysis and it is also necessary to measure two signals at the same time instant, these two problems with the above mentioned solutions lead to an unstable and inaccurate measurement. US patent 6,221,031 discloses a system for using a square signal as excitation signal and relies on the measurement of two or three parameters in order to be able to analyze the result and deduce the position of the dental probe with respect to the apical constriction leading to a complex technique.

Summary of the invention

It is the object of the present invention to remedy some of the above mentioned problems and provide a device for measuring the location of the apical constriction independent on the media composition in the root cana! and with high accuracy in order to have a user friendly instrument being able to measure continuously during the cleaning process.

A preferred embodiment of the present invention, a device for measuring a location of an apical constriction of a tooth is provided, the device comprise: generation means for generating two excitation signals; separation means for separating the signals; feedback means for feeding back an error signal to the signals in order to reduce errors, such as dependence on signal path media type; and acquisition means for acquiring a separated high frequency component to a processing unit; wherein the excitation signal is applied between a first electrode on a dental probe located in a root canal of a tooth and a second electrode, and a resulting signal depending on an impedance between the dental probe tip and the second electrode is separated into a high and a low frequency component in the separation means, the low frequency component is compared to a reference signal resulting in an error signal fed back to the resulting signal producing an error compensated signal wherein the high frequency component is error compensated and comprise accurate information about the distance, the high frequency signal is separated from the resulting signal and acquired in the acquisition means.

The device may further comprise summation means for combining the two signals into one combined excitation signal or first switching means for applying the generated two excitation signals in an alternated manner. Optionally the device may further comprise second switching means for steering the resulting error compensated signal to appropriate separation means, the second switching means operate in synchrony with the first switching means.

Yet another preferred embodiment of the present invention, a method for increasing the measurement accuracy in a continuous monitoring system for location of an apical constriction in a tooth is provided, comprising the steps of: generating two alternating excitation signals at different frequencies and applying the signals between a first electrode on a dental probe located inside a root canal of a tooth and a second electrode; separating a low frequency signal from a resulting signal depending on impedance between the first electrode on the dental probe and the second electrode, and comparing the low frequency signal with a reference value, resulting in an error signal; applying the error signal to the resulting signal through a negative feedback system, creating an error compensated signal; separating a high frequency component comprising information about a distance between the dental probe tip and apical constriction from the combined error compensated signal; and acquiring the separated high frequency signal, comparing the separated high frequency signal to calibration data in order to transform measurement data to an accurate distance between the dental probe tip and apical constriction.

In the method, the step of applying the two generated alternating signals may further comprise a step of combining the two signals into one excitation signal using a summation means or applying the signals in a alternating manner using a first switching means for applying the generated two excitation signals in an alternated manner. The method may further comprise a step of using second switching means for steering the resulting error compensated signal to appropriate separation means, the second switching means operating in synchrony with the first switching means.

Still another preferred embodiment of the present invention, a system for measuring a location of an apical constriction of a tooth is provided, the system comprise: a first electrode on a dental probe for location in a root canal in a tooth; a second electrode; signal generation means for producing two signals with alternating voltages at different frequencies as excitation signal or signals; a measurement device for error compensating, measuring, and producing signals indicative of distance between the dental probe tip and an apical constriction in the tooth; and a display unit for displaying the signals indicative of the distance; wherein the excitation signal is applied between the first electrode on the dental probe located in a root canal of a tooth and the second electrode, and a resulting signal depending on an impedance between the first electrode on the dental probe and the second electrode is separated into a high and a low frequency component in the separation means, the low frequency component is compared to a reference signal resulting in an error signal fed back to the resulting signal producing an error compensated signal wherein the high frequency component is error compensated and comprise accurate information about the distance, the high frequency signal is separated from the combined resulting signal and sampled in the sampling means.

The system may further comprise summation means for combining the two signals into one excitation signal.

The system may further comprise first switching means for applying the generated two excitation signals in an alternated manner and optionally further comprise second switching means for steering the resulting error compensated signal to appropriate separation means, the second switching means operate in synchrony with the first switching means.

Brief description of the drawings

In the following the invention will be described in a non-limiting way and in more detail with reference to exemplary embodiments illustrated in the enclosed drawings, in which:

Fig. 1 is a schematic depiction of a measurement setup according to a preferred embodiment of the present invention.

Fig. 2 is a schematic illustration of a measurement setup according to another preferred embodiment of the present invention.

Fig. 3a illustrates measurement signals with respect to distance to apical constriction using a preferred embodiment of the present invention and Fig. 3b illustrates the corresponding locations of the probe in the tooth.

Fig. 4 is a schematic illustration of a measurement device according to the present invention.

Fig. 5 is a schematic block diagram of a method according to the present invention.

Fig. 6 is a schematically illustration of an equivalent circuit of the present invention. Detailed description of the invention

In a preferred embodiment of the present invention as shown in Fig. 1, a voltage V_f proportional to the impedance between a first electrode on a dental probe 3 and a second electrode 5 located in contact with the patient close to a tooth 10 under treatment is measured. This is done by applying two different excitation voltages 18 and 19 at different frequencies (V_/ and V_Λ) between the first electrode (not shown) on the dental probe 3 and the second electrode 5 located close to the tooth under treatment, these two different signals may be summed in an adder 1 before being applied to the dental probe 3 electrode located in a root canal 4 of a tooth 10 or applied to the second electrode located close to the tooth under treatment, this second electrode may for instance be located on oral mucosa or any other suitable part of the patient body. In a preferred embodiment of the present invention the signal is applied to the dental probe located on the root canal and the second electrode located close to the tooth is regarded as passive, however, the invention is not limited to this, since the signal may be applied to the second electrode with equally good result.

The "passive" electrode 5 may be located e.g. at the oral mucosa 12 or at any suitable position on the patient body giving an electrical connection to the root canal and acts as a ground electrode or return path of the signal. An equivalent circuit of the electrical setup may be seen in Fig. 6, illustrating a signal generator 601, an internal resistance 602 of the generator, a variable capacitance and a variable resistance representing the dependence on media present in the root canal and the distance between the dental probe and the apical constriction. The media factor is represented with m and the position of the dental probe with d. 605 represents the root canal/apical constriction part of the equivalent circuit.

The preferred embodiment of the present invention may be divided into several separate units, wherein a measuring device comprising the core signal transformation parts, such as handling error compensation and acquisition details of the invention. The functionality of the measuring device will now be described in more detail.

The system measures the respective corresponding voltages affected by the impedance created through the conductive circuit comprising the root canal, the apical constriction and oral mucosa tissue surrounding the base of the tooth. These corresponding voltages V_f/ and V_tΛ are measured over a voltage divider circuitry comprising components 6 and 7, and both signals are amplified in a common amplifier 8 with a gain G. The signals are then separated and detected using separation and detection means 9, for instance by using two filters; one high pass filter and one low pass filter. The lower frequency was chosen in such way that the low frequency component V_tf responds more adequately to media present in the root canal 4 and apical constriction. The modified signal V₁' is compared to a constant reference voltage V_rand an amplified error signal is provided from this comparison 14 and this error signal is fed back to the combined resulting signal using a negative feedback system 7 with resistance R,,, thus providing one single error compensated signal that contains the information of the probe position in the root canal or the apical constriction. The feedback system 7 has a dual functionality in the sense that it forms part of both the voltage divider circuitry and the feedback means. V_r is constant during normal operation but may be changed for calibration purposes.

By providing a single error compensated signal containing all information it is possible to measure with higher accuracy and with a faster response time. The location information of the probe tip is available in the high frequency component V I of the combined resulting signal and may be measured using different data sampling and/or visualization systems.

In a preferred embodiment of the present invention an A/D (analog to digital) converter 11 is used to measure the error compensated position containing signal and a microprocessor 16 is used for analyzing the sampled data and used to control a display unit (not shown). Any kind of microprocessor or computational device and display means may be used as appreciated by the person skilled in the art.

In order to understand how information are transferred from the low frequency signal to the high frequency signal the following mathematical expressions may be used as an illustration :

R,.

K=K -G = V - AV

R₁, + R, and γ' = γ — *z — G ^ώ R_in + R_v

Combining these two equations leads to the final expression:

AV where (1 ) is an error eliminating element. V_h' carries the complete information

about the media in the root canal and the position of the dental probe in the canal.

The ratio of the response signals may be written as:

where

m denotes the dependence on media in the root canal and d the position in the root canal. It is thus possible to write a functional equation for determining the position of the apical constriction in the root canal when the capacitance of the apical constriction of the tooth canal is considered as constant value:

where F(C) denotes the dependence of changes of the root canal capacitance C.

Using the same argumentation it is possible to design another preferred embodiment of the present invention using a switching solution in place of the adder 1 (in Fig. 1) with an optional switch prior to the filtering and detection of the signals. This embodiment is illustrated in Fig. 2 where two excitation signals are controlled by a switch 21a and applied to the dental probe 3. The resulting signal is measured over a voltage divider 6 and 7 and amplified by an amplifier 8. An optional switch 21b may control which filter and detection setup is to be used in the subsequential processing. This embodiment operates in a similar fashion as described above in the first preferred embodiment.

In both cases the two frequencies are chosen in an appropriate manner according to application, in order to obtain good signal to noise ratio (S/N) and good sensitivity. However, in a preferred embodiment the two frequencies are chosen with the lower frequency range within a few hundred hertz to a few kilohertz and the higher frequency range within a few kilohertz; approximately 100 Hz ≤ f_/ < 1 kHz and 1 kHz =ζ f_h ≤ 20 kHz. The only restriction is that f_h is larger than f_/.

Both embodiments eliminate the error in the measurement using an analogue technique, which has inherently high accuracy, which in turn leads to an accurate value of the distance from the tip of the dental probe to the apical constriction as a function of one modified signal of a higher frequency signal (d = F ( V_h' )). It is possible to measure the single signal V_h' with high accuracy since this is an analogue signal and the only limitation is the accuracy of any subsequent measuring system, for instance the resolution of a sampling or digitizing equipment such as an A/D converter (Analog to Digital). Also, since one single error compensated signal contains all information about the location of the probe tip with respect to the apical constriction; the system will have a fast response time minimizing errors and the risk of malpractice. With a slow system it is possible for the dentist to operate to fast and thus damaging the tooth before actually detecting such problems causing the patient discomfort and possibly irreversible damage.

Since VV is used in obtaining the error signal, V_r may be used as a voltage reference for an A/D converter as well, providing even higher stability of the system according to the present invention since V_h' is proportional to V_r.

Using a single measurement signal enables the use of an optimal scaling of a sampling unit such as an A/D converter which leads to higher precision and better linearity of the measured/sampled signal.

In clinical tests and calculations it is possible to reference the sampled/measured value towards a scaling of the distance of the dental probe tip to the apical constriction. This scaling may be installed in a microprocessor or similar computational device calculating the distance and controlling a display unit (not shown) displaying the measured and error corrected distance to the apical constriction during normal operation of the instrument.

Fig. 3a and 3b illustrates the relationship between the actual position of the dental probe tip with respect to the apical constriction in the tooth root canal and corresponding values of V_r 302, V_h' 301, and V₁' 303 as functions of distance. Fig. 3a illustrates the measured/sampled signals 301 and 303 and reference signal 302 as functions of distance, while Fig. 3b illustrates a schematical of a tooth and the corresponding position of the dental probe 304 in the root canal. In Fig. 3a it may be noted that the signal ^' 301 behaves almost linearly until it reaches the apical constriction 305 and changes dramatically inside the apical constriction from the Apex physiologicum 305 to the Apex anatomical 306.

The measures/sampled and calculated information is displayed in a display unit (not shown) in order for the dentist to continuously keep track of the dental probe tip position with respect to the apical constriction. This may be displayed in many ways, e.g. on a computer screen or an analog meter unit, according to the state of the art as will be appreciated by the person skilled in the art. The display on a computer screen may be both in a graphical presentation and a more textual based presentation in the form of figures changing or similar display modes. A loudspeaker may be used and controlled from a control unit or the computational device, controlling a sound indicative of the distance, e.g. in the form of a beeping sound increasing or decreasing in output rate, in the form of a change of sound frequency, or in the form of amplitude changes up or down. The above mentioned solutions for outputting measured data is only given as examples and should not be limited to the above mentioned examples, but it should be appreciated by the person skilled in the art that many other types of solutions exists.

The preferred embodiments of the present invention may be utilized in a measurement device 400 as illustrated in Fig. 4. The device may comprise a computational unit, e.g. a processor 401, memory means (both volatile and non¬ volatile), e.g. a hard disk 402 and RAM 403, connection means 405 in order to communicate with external devices, the communication means include, but is not limited to, Ethernet, USB, optical fiber, RS232, RS484, Centronics, GPIB (general purpose interface bus), I2C, or similar connection solutions. The device may also comprise a display unit (not shown) or means 408 for transmitting display data to an external display unit, e.g. a LCD or CRT screen. For further storage purpose connection means 406 for external storage media may be installed in the device 400, these external storage media may be any suitable type as understood by the person skilled in the art.

In one preferred embodiment of the present invention, a method for improving the accuracy of measurement of the distance to the apical constriction is provided. The method may be illustrated by Fig. 5 and the following steps:

1. Generating two excitation signals at different frequencies (501); 2. Applying the two excitation signals between a first electrode located on the dental probe a second electrode located in electrical connection with the tooth, e.g. on oral mucosa, in order to measure signals present in the dental system for the two different frequencies (502);

3. Separating a low frequency signal from the resulting signal and the separated low frequency signal component is compared to a reference voltage (503);

4. The resulting error signal is fed back using a negative feedback system to the resulting impedance depending signals (504);

5. The resulting error compensated high frequency signal component is separated (505); and

6. The separated error compensated high frequency signal is sampled and compared to calibration data in order to display an accurate distance between the dental probe tip and the apical constriction (506).

Some of the advantages of the embodiments of the present invention are listed below:

1. Since the present invention contains an automatic error eliminating system, eliminating the existing inaccuracy of the ratio of the impedances from the two frequencies, a higher accuracy and more linear measurement is possible.

2. By transferring the measurement information to one variable, it is possible to optimize sampling and/or measurement circuitry leading to higher accuracy. 3. The negative feedback used in the automatic error eliminating system decreases the influence of external factors, such as change in temperature, aging of different components in the system, and other external noise sources, leading to an increased accuracy, increased linearity, and a better overall stability of the measurement device and method.

The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.

Claims

1. A device for measuring a location of an apical constriction of a tooth (10), said device comprise: generation means (18, 19) for generating two excitation signals; separation means (9) for separating signals into high and low frequency components; feedback means (7) for feeding back an error signal in order to reduce errors, such as dependence on signal path media type; and acquisition means (11) for acquiring a separated high frequency component to a processing means (16); wherein: said excitation signal is adapted to be applied between a first electrode located on a dental probe (3) located in a root canal (4) in a tooth (10) of a patient and said second electrode (5), and said separation means (9) is arranged to separate a resulting signal depending on an impedance between said first electrode and said second electrode (5) into a high and a low frequency component, and a comparison means (14) is arranged to compare said low frequency component with a reference signal, resulting in an error signal, wherein said feeedback means (7) is arranged to error compensate said high frequency component to comprise accurate information about said distance by feeding back said error signal to said resulting signal, and said separation means (9) is arranged to separate said high frequency signal from said resulting signal, and said acquisition means (11) is arranged to acquire said high frequency component .

2. The device according to claim 1, further comprising summation means (1) arranged to combine said two signals into one combined excitation signal.

3. The device according to claim 1, further comprising first switching means

(21a) arranged to apply said generated two excitation signals in an alternated manner.

4. The device according to claim 3, further comprising second switching means (21b) arranged to steer said resulting error compensated signal to appropriate separation means (9), said second switching means (21b) is arranged to operate in synchrony with said first switching means (21a).

5. A method for increasing the measurement accuracy in a continuous monitoring system for location of an apical constriction in a tooth (10), comprising the steps of:

- generating two alternating excitation signals at different frequencies; - applying said signals between a first electrode on a dental probe (3) located inside a root canal of a tooth and a second electrode (5) located on a patient;

- separating a low frequency signal from a resulting signal depending on impedance between said first electrode on said dental probe (3) and said second electrode (5), and comparing said low frequency signal with a reference value, resulting in an error signal;

- applying said error signal to said resulting signal or signals through a negative feedback system, creating an error compensated signal;

- separating a high frequency component comprising information about a distance between said first electrode on said dental probe (3) and apical constriction from said error compensated resulting signal; and

- acquiring said separated high frequency signal, comparing said separated high frequency signal to calibration data in order to transform measurement data to an accurate distance between said dental probe tip and apical constriction.

6. The method according to claim 5, wherein said step of applying said two generated alternating signals further comprise a step of combining said two signals into one combined excitation signal using a summation means (1).

7. The method according to claim 5, wherein said step of applying said two generated alternating signals further comprise a step of applying said signals in a alternating manner using a first switching means (21a) for applying said generated two excitation signals in an alternated manner.

8. The method according to claim 7, further comprising a step of using second switching means (21b) for steering said resulting error compensated signal to appropriate separation means (9), said second switching means (21b) operating in synchrony with said first switching means (21a).

9. A system for measuring a location of an apical constriction of a tooth (10), said system comprise:

- a dental probe (3) for location in a root canal (4) in said tooth (10) of a patient;

- a first electrode located on said dental;

- a second electrode (5) located on said patient; - signal generation means (18, 19) for producing two signals with alternating voltages at different frequencies as excitation signals;

- a measurement device comprising separation means (9), comparison means (14), feedback means (7), sampling means (11), and processing means (16) for error compensating, measuring, and producing signals indicative of distance between a dental probe tip and an apical constriction in said tooth; and

- a display unit for displaying said signals indicative of said distance; wherein said excitation signal is adapted to be applied between said first electrode on said dental probe (3) located in a root canal of a tooth (10) and said second electrode (5), and a resulting signal depending on an impedance between said first electrode on said dental probe and said second electrode is separated into a high and a low frequency component in said separation means (9), said low frequency component is compared to a reference signal in said comparison means (14) resulting in an error signal fed back to said resulting signal producing an error compensated signal wherein said high frequency component is error compensated and comprise accurate information about said distance, said high frequency signal is separated from said resulting signal, sampled in sampling means (11), and processed in said processing means (16), and signals indicative of said distance between said dental probe tip and said apical constriction is displayed in said display unit.

10. The system according to claim 9, further comprising summation means (1) for combining said two signals into one combined excitation signal.

11. The system according to claim 9, further comprising first switching means

(21a) for applying said generated two excitation signals in an alternated manner.

12. The system according to claim 12, further comprising second switching means (21b) for steering said resulting error compensated signal to appropriate separation means (9), said second switching means (21b) operate in synchrony with said first switching means (21a).