Method and device for detection of a tooth root apex

[0002] The present invention relates to dental equipment, and more particularly, to measuring equipment for dental endodontics.

[0003] In root canal therapy, a dental probe, such as a reamer or file, is inserted into the canal and manipulated to remove undesired material A flexible filler substance is then placed in the root canal that is sealed with a rigid material. If the canal is not completely cleaned before filling and sealing, debris left inside the canal can prevent proper healing. The probe must therefore be inserted all the way to the apex of the root canal during cleaning to remove all debris. On the other hand, if the probe is inserted too deeply, the tool penetrates the jaw tissue, causing swelling and unnecessary trauma for the patient. It is therefore essential to precisely determine when the probe tip is located at the root canal apex so that the canal can be cleaned fully without excessive trauma to the patient.

[0004] Locating the apex is difficult because the narrow canal does not provide a clear viewing path and fluids can partially fill the canal. In one method, the probe is inserted into the canal and the tooth is x-rayed. In the x-ray image, the metal probe contrasts with the surrounding tooth and body tissue so that the positions of the probe tip and the apex can be compared. If the probe tip is not at the apex, it is inserted deeper into the canal and a new x-ray image is obtained. This method is unreliable, time-consuming, costly, and entails excessive exposure of the patient to X-ray radiation.

[0005] It is also known to locate a root apex by inserting a conductive probe into the root canal and placing an electrode in contact with the patient's body, usually in or near the mouth. As the probe is moved through the canal towards the apex, electrical measurements across the probe and electrode are made. For example, U.S. Pat. No. 5,063,937 discloses an apex detector utilizing an AC signal at a single frequency. The probe tip is determined to be at the apex when the result of a calculation based upon these measurements is within a predetermined range. However, methods that utilize a single frequency AC signal for apex positioning are not accurate in the vicinity of the apex.

[0006] U.S. Pat. No. 5,096,419 to Kobayashi et al., discloses locating the apex by making impedance measurements at two frequencies as the probe is moved towards the apex. A detector determines that the probe tip as at the apex when the ratio of the two impedance measurements is within a predetermined range. U.S. Pat. No. 5,759,159 to Masreliez discloses locating a root apex based upon the amplitude or phase of voltage measurements across the probe and the electrode as the probe is moved towards the apex. However, amplitude and phase measurements are affected by inevitable signal distortion Moreover, the multiple frequency methods are accurate only near the apex and cannot indicate the probe position before the probe tip approaches the apex region.

[0007] With prior art apex locators, whether single or multiple frequency, the frequency or frequencies of the test signals, as well as the internal impedance, of the devices are fixed by the manufacturer, and cannot be changed during operation. However, the electrical parameters of the tissues of the root canal and jaw may vary significantly from patient to patient, from one tooth to another of the same patient, and even from one canal to another in the same tooth. Fixed test frequencies and internal impedance of the device obviously cannot be optimal for all cases. Hence, the precision afforded by these devices is limited.

[0008] In the following description and set of claims, two explicitly described, calculable or measurable variables are considered equivalent to each other when the two variables are substantially proportional to each other.

[0009] The present invention provides a method and device for locating the apex of a root canal. The device comprises two electrodes. A conductive probe inserted into a root canal forms the first electrode. A second electrode contacts the patient's body, typically in the mouth area. An AC voltage test signal V_gis applied to a driver having an internal output impedance Z_o. The internal output impedance of the device and the impedance of the tissues between the two electrodes form a parametric voltage divider. The test signal V_gmay be a single frequency or a multi-frequency signal. The internal output impedance Z_omay be kept constant during the measurements, or be made to vary. As the probe tip is moved through the canal towards the apex, voltage measurements V_lacross the probe and electrode are continuously made and a microprocessor calculates one or more test scores based upon the voltage measurements. The current position of the probe in the canal is then determined on the basis of the test scores.

[0010] In the following description and set of claims, the portion of the root canal near the apex is referred to as “region B” of the canal. The rest of the canal is referred to herein as “region A” of the canal. Determining the position of the probe tip may simply involve determining whether the tip is in region A, in region B, at the apex, or in the jaw tissue In this case, as the probe tip is moved towards the apex, the test score is compared to a first numerical value. When the test score is less than the first numerical value, the user is informed that the tip is in region A. When the test score first equals the first numerical value, the user is informed that the tip is in region B. When the test score first reaches a second numerical value, the user is informed that the probe tip is at the apex. When the test score exceeds the second numerical value, the user is informed that the tip has passed the apex and entered the jaw tissue. The position of the probe tip in the canal may be indicated to the user on a graphical or numerical display, or by means of a sensible signal such as a bell.

[0011] Alternatively, determining the position of the probe tip may involve continuously determining the distance of the probe tip from the apex. In this case, the location of the probe in the canal may be displayed continuously on a graphical or numerical display starting from the first insertion of the probe into the canal until the probe tip has reached the apex.

[0012] The test score may involve, for example, the root mean square (RMS) of the voltage measurements V_i. Unlike phase and amplitude measurements, RMS measurements are essentially unaffected by signal distortion. In one embodiment, a test signal V_ghaving a single frequency f₁is used while the internal output impedance Z_ois kept constant, for example 3 Kohm. V_g(f₁) may be, for example, 30-50 mV. As the probe is moved in the canal, the novel test score L₁=S₁(RMS(V_i(f₁,P₀))−RMS(V_i(f₁,P_k))) is calculated where S₁is a scaling factor, and P₀and P_kdesignate, respectively, the initial and current probe positions inside the root canal. S1 may be, for example, about 75. This embodiment may make use of the unexpected finding that when the tip is in region A, the test score L₁is essentially a linear function of the probe tip position in the canal.

[0013] In another embodiment, a test signal V_ghaving two frequencies f₁and f₂is used while the internal output impedance Z_ois kept constant. f₁and f₂may be, for example, in the 400-500 Hz and 8-10 kHz ranges, respectively As the probe is moved in the canal, the novel test score D₁=S₂(RMS(V_i(f₁,P_k))−RMS(V_i(f₂,P_k))) is calculated where S₂is another scaling factor. This embodiment may make use of the unexpected finding that in region B, D₁is a linear function of the distance between the probe tip and the apex.

[0014] In a most preferred embodiment, the test scores L₁and D₁are used simultaneously. The test score L₁is used to locate the probe tip when it is in region A. When L₁first satisfies a predetermined condition that may depend upon the simultaneously obtained test score D₁, the probe tip has passed from region A to region B. The condition may be, for example that L₁=D₁(P_k)−D₁(P₀). The test score D₁is then used to determine the probe tip position. When the value of the test score D₁exceeds a first predetermined threshold T1, the probe tip is close to the apex, for instance, 1 mm from the apex. When the value of the score reaches a second predetermined threshold T2, the probe tip is at the apex. When value of the test score exceeds the second predetermined threshold T2, the probe tip has passed the apex. T1 and T2 may be, for example, 0.196 V, and 0.294V, respectively.

[0015] In another embodiment, V_imeasurements are initially obtained using a testing signal V_ghaving two frequencies f₁and f₂and one or more test scores calculated. V_imeasurements are then performed using a testing signal V_ghaving new frequencies f₃and f₄better suited to the specific case that are determined based upon the test scores obtained with the initial pair of frequencies. For example, initial frequencies f₁and f₂of 1 kHz and 8 kHz respectively may be selected. If a test score obtained using the frequencies f₁and f₂is less than a predetermined value, the frequencies are decreased. If the test scores exceeds this value, the frequencies are increased. The process may be repeated dynamically to arrive at an optimal pair of frequencies for the particular canal.

[0016] In yet another embodiment, voltage measurements are made at a fixed frequency but at two different output impedances Z₁and Z₂of the device. One or more test scores are calculated that may involve, for example, the amplitude, phase, or RMS of the voltage measurements. For example, the test scores L₂=S₁(RMS(V₁(Z₁,P₀))−RMS(V₁(Z₁,P_k))) and D₂=S₂(RMS(V_i(Z₁,P_k))−RMS(V_i(Z₂,P_k))) may be used. The test score L₂is used to locate the probe tip when it is n region A. When L₂first satisfies a predetermined condition that may depend upon the simultaneously obtained test score D₂, the probe tip has passed from region A to region B. The test score D₂is then used to determine the probe tip position. When the value of the test score D₂exceeds a first predetermined threshold T1, the probe tip is close to the apex, for instance, 1 mm from the apex. When the value of D₂reaches a second predetermined threshold T2, the probe tip is at the apex. When value of the test score exceeds the second predetermined threshold T2, the probe tip has passed the apex. T1 and T2 may be, for example, 0.196 V, and 0.294V, respectively.

[0017] In either embodiment the position of the probe tip in the canal may be displayed graphically and/or numerically to a user as the probe is moved in the canal. For example, the display may include an illustration of a tooth canal and a marker may; be introduced into the illustration indicating the position of the probe tip in the canal. Once the score D₁or D₂respectively exceeds a first threshold T1 indicating that the probe tip is close to the apex, the scale of the graphic display may be enlarged to provide a more accurate indication of position of the probe tip near the apex. A sensible signal such as a light or a bell may also be used to indicate that the probe is near or at the apex. A visual and/or audio signal may indicate that the probe tip has passed the apex. The invention thus provides a device for locating an apex of a root canal of a tooth, the root canal having a region A and a region B, comprising:

[0018] a a first electrode, the first electrode including a conductive probe adapted for insertion into the root canal, the probe having a tip;

[0019] b a second electrode configured to electrically contact a patient's body;

[0020] c an AC voltage generator configured to provide an AC test voltage signal V_ghaving one or more frequencies;

[0021] d an AC voltage driver being an interface between the voltage generator and the electrodes;

[0022] e a voltage detector coupled to the first and second electrodes, the voltage detector being operative to detect an AC voltage V_iacross the first and second electrodes;

[0023] f an electronic controller coupled to the voltage detector, the controller carrying out the steps of:

[0024] fa calculating one or more test scores, the one or more test scores each having a value that is a function of V_i;

[0025] fb determining, on the basis of the one or more test score values, the position of the probe tip in the canal; and

[0026] fc informing a user of the position of the probe tip in the canal.

[0027] In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0028] FIG. 1 shows an apical position detector according to one embodiment of the invention;

[0029] FIG. 2 shows an apical position detector according to another embodiment of the invention;

[0030] FIG. 3 shows a graphic display of an apical position detector according to one embodiment of the invention;

[0031] FIG. 4 is a flow chart of the steps and control paths for determining probe tip position inside a tooth canal and for visual and audio indication of the position according to one embodiment of the invention; and

[0032] FIG. 5 is a flow chart of the steps and control paths for determining probe tip position inside a tooth canal and for visual and audio indication of the position according to another embodiment of the invention.

[0033] Referring first to FIG. 1, an apex detector, generally indicated 1, in accordance with one embodiment of the invention is shown that includes a microcontroller 2, signal generator 3, output driver 4, amplifier 7, RMS to DC converter 8, analog to digital converter 9, display 10, audio transducer 11 and keyboard 12. The apex detector 1 is connected to a conductive canal probe 5 and a lip electrode 6 via flexible cable 16 of sufficient length. The microcontroller 2 controls the operation of the apex detector 1 according to a computer program stored in a nonvolatile program memory 13 located in the microcontroller 2. The memory 13 may also contain a series of threshold values and other parameters that are accessed by the microcontroller 2 during operation. Random access memory (RAM) 14 located in the microcontroller 2 is used for temporary storage of data during the operation of the apex detector 1.

[0034] The apex detector 1 may optionally include a non-volatile EEPROM or FLASH memory 13 for storing user-defined parameters inputted to the microcontroller 2 by means of the keyboard 12. Memories 13 and 14 are not erased in the absence of a supply power.

[0035] Referring still to FIG. 1, the canal probe 5 is inserted into a canal 24 of a tooth 23. The lip electrode 6 is configured to contact the tissues of the mouth. The root canal 24 is divided into region A 31 and region B 32. The microcontroller 2 drives the signal generator 3 and optionally controls a variable output impedance Z_o21 of the driver 4. In response, the signal generator 3 produces an analog testing signal V_g20 that may be, for example, a sine wave with peak amplitude of 40 mV. The test signal 20 is applied to the electrodes 5 and 6 via output driver 4 and cable 16.

[0036] The measured region 26 of the canal 24 extends from the tip 25 of the canal probe 5 to the root apex 27. Jaw tissue 28 surrounds the tooth 23. The length of the measured region 26 depends upon the depth to which the canal probe 5 is inserted in to the canal 24. The impedance Z_c29 of the tissues between the probe tip 25 and the lip electrode 6 depends upon the length of the measured region 26. The voltage drop V_i22 where V_i=V_gZ_c/(Z_o+Z_c), across the electrodes 5 and 6 is amplified by the amplifier 7 and the amplified signal is inputted to the RMS to DC converter 8. The signal is then digitized by the analog to digital converter 9 and inputted to the microcontroller 2.

[0037] The microcontroller 2 calculates one or more test scores based upon one or more V_imeasurements. When a test score involving two or more frequencies is used, for example, the test score D₁, a multiple frequency test voltage V_gmust be used. In this case the response voltage measurements V_iare separated into their spectral components by the microcontroller 2. More preferably, however, a single frequency test signal V_gis used that rapidly alternates among the different frequencies. Two or more V₁measurements at different V_gfrequencies can be made within sufficiently short time period in which the position of the probe tip inside the root canal essentially does not change.

[0038] The microcontroller 2 then determines the position of the probe tip 25 in the root canal 24 based upon the test scores. Determining the position of the probe tip may simply involve determining whether the tip is far from the apex, near the apex or at the apex. Alternatively, determining the position of the probe tip may involve continuously determining the advance of the probe tip 25 in the canal or the distance of the probe tip from the apex. In either case, the position of probe is tip 25 is indicated to the user by the display 10 and/or the audio transducer 11. FIG. 2 shows another embodiment of the invention. The embodiment of FIG. 2 has several components in common with the embodiment of FIG. 1, and identical components in FIGS. 1 and 2 are designated by the same numeral. In the embodiment of FIG. 2, the microcontroller 2 controls a variable input impedance Z_i30 of input interface 17.

[0039] When voltage measurements are carried out using a test voltage V_g20 having a single frequency with two or more values of either the output impedance Z_o21 of the driver 4 (FIG. 1), or the input impedance Z_i30 of the input interface 17 (FIG. 2), the impedance 21 or 30 is varied by microcontroller 2. For example, the impedance Z_o21 of the output driver 4 may alternate between two impedance values Z₁and Z₂where Z₁is a resistive impedance of about 3 kOhm and Z₂is a capacitor of about 20 nF connected serially to resistor of about 3 kOhm. V_i22 is measured for each output impedance Z₁and Z₂. The microcontroller 2 then calculates one or more test scores involving the measured V_i(Z₁) and V_i(Z₂). For example, in region A, L₂or the amplitude or phase of V_i(Z₁) may be used. In region B, D₂, or V_i(Z₁)/V_i(Z₂) may be used as a test score. When D₂is used as the test score, the probe tip is close to the apex, for example, about 1 mm from the apex, when D₂exceeds a first predetermined threshold T1 that may be, for example, 0.196 V. When D₂reaches a second threshold T2, for example, 0.294V, the probe tip as at the apex. When D₂has exceeds the threshold T2, the probe tip has passed the apex and entered the jaw tissue.

[0040] FIG. 3 shows a graphical display, generally indicated 35 that may be used in accordance with the invention. The display includes an illustration of a tooth 36 that is used to depict the probe tip 25 position when the probe tip 25 is located in region A 31 (FIGS. 1 and 2). The root canal 24 is indicated by a sequence of LEDs 37. When the probe tip 25 is in region A, one or more of the liquid crystal diode (LCD) segments are turned on in LCD sequence 37 indicating the position of probe tip 25 in the canal 24. The display also includes an illustration of a canal in the apex region 38 that is used to depict the probe tip 25 position when the probe tip 25 is located in region B 32. The root canal 24 in region B 32 is indicated by a sequence of LCDs 39. When the probe tip 25 is in region B 32, one or more of the LCDs are turned on in LED sequence 39 indicating the position of probe tip 25 in the region B 32 at a higher resolution than when the probe tip 25 is in the region A 31.

[0041] The steps for detecting the advance of the probe 5 inside regions A and B of the root canal 24 and in accordance with a preferred embodiment of the invention using two frequencies are shown in the flow chart of FIG. 4. The operation begins in step 100 with generation of an AC voltage having a predefined frequency f₁. The test voltage is then applied to the lip electrode 6 and the probe 5. Next, in step 101, the microcontroller 2 monitors the RMS of V_i22 through the amplifier 7, the RMS-to-DC converter 8 and the analog-to-digital converter (ADC) 9, and stores the data in memory 14. The ADC 9 continuously provides to the microcontroller 2 a digital signal representing the RMS that depends upon the probe voltages V_i. In step 102, under the control of the microcontroller 2, the signal generator 3 generates another AC voltage with predefined frequency f₂. In step 103 the microcontroller 2 monitors the second RMS value similarly to the step 101 and stores the second value in memory 14. In steps 104 and 105 the microcontroller 2 evaluates the test scores L₁and D₁respectively and stores the resulting values in memory 14.

[0042] Once the test scores have been calculated, in step 106 the software checks whether a predefined condition is fulfilled and determines whether the probe tip 25 is in the region A 31 or the region B 32. If L₁>D₁(P_k)−D₁(P₀), the probe tip 25 is in the region A 31, and the software determines the location of probe tip 25 in the canal 24 from the value of the test function L₁stored in the memory 14, and updates the display (step 107). Otherwise the probe tip is in the region B 32, and the software determines the location of probe tip 25 in canal 24 from the value of the test function D₁stored in the memory 14, and updates the display (step 108). In step 109 the test score D₁is compared to the first threshold value T1. If the test score exceeds the first threshold value T1, the probe tip 25 is close to the apex 26, and this is indicated to the user by means of the display 10 and/or audio transducer 11. In step 110 an enlarged image of the apex area is displayed on the display 10, and audio transducer is turned on. The controller then compares the test score D₁to the second threshold value T2 (step 111). If the score D₁does not exceed T2, the probe tip 25 has not yet reached the apex. The exact distance from the probe tip to the apex is determined by microcontroller 2 from the value of the test function D₁stored in the memory 14, and the precise probe tip location is displayed on the enlarged scale on display 10. In step 113 the test score D₁is further compared to the second threshold value T2. If D₁is equal to the second threshold value T2, the probe tip is at the apex, and this is indicated to the user (step 114) by means of the display 10 and/or audio indicator 11. As shown in FIG. 3, the graphical display indicates the current location of probe tip 25 inside the canal 24 of a tooth in the region A 31 and in the region B 32. Alternatively, the position of the probe tip 25 may be presented numerically by a digital display or any other suitable indicating device.

[0043] If D₁exceeds T2 (negative results at both steps 111 and 113), the probe tip 25 is beyond the apex and this is indicated to the user by means of visual and audio alarms (step 115).

[0044] In step 117 the input interface (keyboard 12) is scanned by the software. Once the interface routine (step 118) is complete, the software returns to step 100 to continue the measurements and display the results.

[0045] The steps for detecting the advance of the probe 5 inside regions A and B of the root canal 24 and in accordance with another preferred embodiment of the invention using two impedances are shown in the flow chart of FIG. 5. In this embodiment either the output impedance 21 of the driver 4 (FIG. 1) or the input impedance 30 of the input interface 17 (FIG. 2) is made to vary under the control of the microcontroller 2. The operation begins in step 200 with generation of an AC voltage while the variable impedance is set to Z₁. The test voltage is then applied to the lip electrode 6 and the probe 5. Next, in step 201, the microcontroller 2 monitors the RMS of V₁22 through the amplifier 7, the RMS-to-DC converter 8 and the analog-to-digital converter (ADC) 9, and stores the data in memory 14. The ADC 9 continuously provides to the microcontroller 2 a digital signal representing the RMS that depends upon the probe voltages V_i. In step 202, under the control of the microcontroller 2, the signal generator 3 generates the same AC voltage with the variable impedance set to Z₂. In step 203 the microcontroller 2 monitors the second RMS value similarly to the step 201 and stores the second value in memory 14. In steps 204 and 205 the microcontroller 2 evaluates the test scores L₂and D₂respectively and stores the resulting values in memory 14.

[0046] Once the test scores have been calculated, in step 206 the software checks whether a predefined condition is fulfilled and determines whether the probe tip 25 is in the region A 31 or the region B 32. For example, if L₂>D₂(P_k)−D₂(P₀), the probe tip 25 is in the region A 31, and the software determines the location of probe tip 25 in the canal 24 from the value of the test function L₂stored in the memory 14, and updates the display (step 207). Otherwise the probe tip 25 is in the region B 32, the software determines the location of probe tip 25 in canal 24 from the value of the test function D₂stored in the memory 14 and updates the display (step 208). In step 209 the test score D₂is compared to the first threshold value T1. If the test score exceeds the first threshold value T1, the probe tip 25 is close to the apex 26, and this is indicated to the user by means of the display 10 and/or audio transducer 11. In step 210 an enlarged image of the apex area is displayed on the display 10, and audio transducer is turned on. The controller then compares the test score D₂to the second threshold value n (step 211). If the score D₂does not exceed T2, the probe tip 25 has not yet reached the apex. The exact distance from the probe tip to the apex is determined by microcontroller 2 from the value of the test function D₂stored in the memory 14, and the precise probe tip location is displayed on the enlarged scale on display 10. In step 213 the test score D₂is further compared to the second threshold value T2. If D₂is equal to the second threshold value T2, the probe tip is at the apex, and this is indicated to the user (step 214) by means of the display 10 and/or audio indicator 11. As shown in FIG. 3, the graphical display indicates the current location of probe tip 25 inside the canal 24 of a tooth in the region A 31 and in the region B 32. Alternatively, the position of the probe tip may be presented numerically by a digital display or any other suitable indicating device.

[0047] If D₂exceeds T2 (negative results at both steps 211 and 213), the probe tip 25 is beyond the apex 27 and this is indicated to the user by means of visual and audio alarms (step 215).

[0048] In step 217 the keyboard 12 is scanned by the software. Once the interface routine (step 218) is complete, the software returns to step 200 to continue the measurements and display the results.