DEVICE AND METHOD FOR CONTROLLING THE STABILITY OF A DENTAL IMPLANT

Description

TECHNICAL FIELD

[0001] This disclosure relates to a device and method for controlling the stability of a dental implant inserted at least partially into a bone. BACKGROUND

[0002] A dental implant typically takes the form of an artificial tooth root, usually made of titanium alloy, placed inside the upper or lower jawbone. A dental prosthesis element is then screwed into the implant. Often, after the dental implant is placed, a healing period is necessary for bone cells to colonize the buried surface of the implant and for it to be "osseointegrated," that is, integrated into the bone, without the interposition of fibrous tissue at the bone-implant interface, or border, between the bone and the implant.

[0003] Since the dental implant acts as an intermediary between the prosthesis element and the jawbone and transmits forces to the bone support, it must be well integrated into the latter. During the aforementioned healing period, the implant is left to rest in order to achieve its integration into the bone, so that it can then be able to withstand the loads that will be exerted on it. After the healing period, the surgeon places the prosthesis element in the dental implant. Measuring the proper integration of the implant into the bone, or osseointegration, is therefore essential for the success of the treatment, in particular to determine precisely when to end the healing period and load the implant with the prosthesis element.

[0004] Patent document EP 2503954 describes a device for monitoring the stability of a dental implant in a bone and an associated device. This method consists of calculating, from an echogram, an indicator whose value makes it possible to evaluate the osseointegration of the implant. Although this indicator is satisfactory under particular conditions, there remains a need for an indicator making it possible to finely evaluate the osseointegration of

[0005] the implant under more general conditions.

[0006] The article by Romain Vayron et al., "Assessment of In Vitro Dental Implant Primary Stability Using an Ultrasonic Method", Ultrasound in Medicine & Biology, vol. 40., no. 12, pages 2885-2894, discloses the features of the preamble of claim 1. GENERAL PRESENTATION

[0007] The present disclosure relates to a device for controlling the stability of a dental implant inserted at least partially into a bone, the implant having a free end emerging at the surface of the bone and an end buried in the bone, located opposite the free end. This device comprises an ultrasonic transducer adapted for: be coupled directly or indirectly to the implant, emit an ultrasonic wave propagating inside the implant toward the buried end, collect the reflected ultrasonic wave at a contact interface between the implant and the bone, and provide a measurement signal representing the reflected ultrasonic wave.

[0008] The device also includes a processing unit adapted to calculate, from the measurement signal, an indicator whose value makes it possible to evaluate the integration of the implant in the bone.

[0009] The proposed indicator corresponds to the average energy of the measurement signal, over a time interval t1-t2 starting at a first instant t1 and ending at a second instant t2, the first instant t1 being between 20 and 80 µs after the start of the emission of the ultrasonic wave by the transducer. Such an indicator proves to be particularly sensitive to the osseointegration of the implant and therefore allows for a fine evaluation of this.

[0010] The present disclosure also relates to a method for controlling the stability of a dental implant inserted at least partially into a bone, the implant having a free end emerging at the surface of the bone and an end buried in the bone, located opposite the free end. The method comprises the following steps: emitting an ultrasonic wave propagating inside the implant towards the buried end, collecting the reflected ultrasonic wave at a contact interface between the implant and the bone, and providing a measurement signal representing the reflected ultrasonic wave, calculating, from the measurement signal, said indicator.

[0011] Of course, the above-mentioned device can be used to implement the method.

[0012] In addition to the characteristics mentioned above, the proposed device and method may have one or more of the following characteristics, considered in isolation or in technically possible combinations. The first instant t1 is between 30 and 60 µs. This further improves the sensitivity of the indicator. The second instant t2 is, at a minimum, separated from the first instant t1 by 5 µs (i.e. t2 ≥ t1 + 5 µs) and, at a maximum, equal to 200 µs. The ultrasonic transducer is adapted to emit an ultrasonic wave whose central frequency is at least equal to 5 MHz. Below this value, the collected measurement signal is less representative of the bone-implant interface and, therefore, the calculated indicator characterizes this interface less well. In particular, the central frequency can be at least equal to 8 MHz and, for example, approximately equal to 10 MHz. Ultrasonic transducers having a central frequency of 10 MHz are easily found commercially, at low cost, and this frequency proves to be well suited to the intended application.The ultrasonic transducer has an active surface adapted to be in contact with the free end of the implant, directly or via an intermediate part, parallel to a bearing surface defined by the free end of the implant, so as to emit an ultrasonic wave propagating inside the implant in a propagation direction perpendicular to the active surface of the transducer. The processing unit comprises amplification means for amplifying the measurement signal, before calculating the indicator, the measurement signal being amplified with a gain adjusted so as to obtain saturation of the signal up to the first instant but not beyond. Furthermore, the gain can be variable as a function of time and increase, for example continuously or in successive stages, between the first and second instants. This makes it possible to increase the size of the time interval t1-t2 without obtaining saturation of the signal, and therefore to gain in sensitivity.

[0013] The above-mentioned features and advantages, as well as others, will become apparent from the following detailed description of examples of embodiments of the proposed device. This detailed description refers to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The attached drawings are schematic and not to scale; they are intended primarily to illustrate the principles of the invention.

[0015] There FIG 1 is an overall schematic representation of an example control device.

[0016] There FIG 2 represents the emission spectrum of the signal corresponding to the ultrasonic wave used.

[0017] There FIG 3 is an example of a measurement signal obtained using the device of the FIG 1 .

[0018] There FIG 4 is a photograph of four identical 1 dental implants, inserted into a resin block with varying levels of depression.

[0019] There FIG 5 is a graph representing the variation of the gain applied to the measurement signal, expressed in dB, as a function of t1, expressed in hundredths of µs (i.e. 10 -8< s).

[0020] There FIG 6 is a graph representing the variation of sensitivity S, expressed as a percentage, as a function of t1, expressed in hundredths of µs (i.e. 10 -8< s). DETAILED DESCRIPTION OF EXAMPLE(S)

[0021] Exemplary embodiments are described in detail below, with reference to the accompanying drawings. These examples illustrate the characteristics and advantages of the invention. It is however recalled that the invention is not limited to these examples.

[0022] There FIG 1 represents an example of a device for controlling the stability of a dental implant inserted into a bone 2. The dental implant 1 is schematically represented in the form of a cylinder. In reality, however, such an implant 1 has a tapered shape and an external thread, so that it can be screwed into the bone 2. For example, the implant has an average diameter of 2.5 to 5.5 mm in diameter and a length of 6 to 16 mm. The dental implant is, typically, made of titanium alloy. Once anchored in the bone 2, the implant 1 has a free end 1a emerging at the surface 3 of the bone 2, opposite an end 1b buried in the bone 2, so that the free end 1a of the implant 1 is likely to be accessible in vivo.

[0023] The control device mainly comprises an ultrasonic sensor 10 and a computing unit 20. The ultrasonic sensor 10, of the piezoelectric type, comprises an ultrasonic transducer 12 for emitting an ultrasonic wave in pulsed mode and receiving the echoes resulting from the reflection of the ultrasonic wave propagating in the implant, and a control circuit 14 for controlling the ultrasonic transducer 12. The ultrasonic transducer 12 is intended to be used in ultrasound mode and placed so that its active surface is coupled, directly or indirectly (i.e. by means of an intermediate part, not shown), with the free end 1a of the implant, and so that this active surface is substantially parallel to the circular bearing surface defined by the free end 1a of the implant. The free end of the implant 1 is thus used as a reception site for positioning the transducer 12 or the intermediate part.

[0024] When an intermediate piece (not shown) is used to couple the transducer 12 to the implant 1, it is adapted to transmit the ultrasonic wave between the transducer 12 and the implant 1. To enable this transmission, the intermediate piece can be mechanically fixed, on one side, to the active surface of the transducer 12 and, on the other, to the implant 1. It is fixed to the active surface of the transducer 12, for example, by gluing, screwing or crimping. It is fixed to the implant 1, for example, by screwing inside a cavity provided in the implant 1. Such a cavity (not shown) opens at the free end 1a of the implant and can extend more or less deeply inside the implant 1 in the direction of its buried end 1b.

[0025] Under these conditions, the ultrasonic wave emitted by the transducer 12 can propagate inside the implant 1, perpendicular to the active surface of the transducer 12, from the free end 1a of the implant towards its buried end 1b, while promoting the phenomenon of interaction of the ultrasonic wave, which propagates in the implant, with a bone-implant contact interface 4 located at the boundary between the external surface of the implant and the bone 2 surrounding it. This interaction phenomenon is essential, as will appear subsequently.

[0026] The control circuit 14 comprises an electrical pulse generator used to excite the piezoelectric element of the transducer 12, which transforms the received electrical pulses into a corresponding ultrasonic wave, which then propagates in the implant 1 towards the buried end 1b. The electrical pulse generator produces, for example, signals of the short pulse or square signal type with rapid rising and falling edges. The control circuit 14 also comprises a receiving circuit receiving the electrical signal, or measurement signal, delivered by the ultrasonic transducer 12 and corresponding to the ultrasonic wave reflected at the contact interface 4 and received by the transducer 12.

[0027] The computing unit 20, connected to the sensor 10, for example via a coaxial transmission line, mainly comprises a computer memory 21 for storing the electrical signal representative of the reflected ultrasonic wave, collected by the reception circuit of the control circuit, and a processing unit 22 configured to process the stored signal, as detailed below. This computing unit 20 may be an independent unit of the microcontroller type or an individual computer. Various equipment may be added, such as a display means and a printing means.

[0028] Taking into account the intended application, the central frequency of the emitted ultrasonic wave is preferably chosen to be at least equal to 5 MHz, in particular at least equal to 8 MHz. The bandwidth measured at -6 dB may be of the order of 80% of the central frequency. The lower limit of the bandwidth may be greater than or equal to 30 MHz. The FIG 2 shows an emission spectrum of the signal corresponding to an ultrasonic wave emitted with a central frequency equal to 10 MHz and a bandwidth A between 6 and 14 MHz.

[0029] The pulse width of the electrical signal sent to the transducer 12 by the control circuit 14 is preferably chosen to be less than or equal to half a period corresponding to the resonance frequency of the ultrasonic transducer 12. For example, for a central frequency equal to 10 MHz, the pulse width is 50 ns. An example of a measurement signal, or echogram, obtained during the control of the stability of an implant is shown in the FIG 3 The amplitude of the signal, on the ordinate, is given in arbitrary units. The time, on the abscissa, is given in µs (i.e. 10 -6< s). In this example, the signal was amplified with a gain of 70 dB, so as to obtain a saturation of the signal up to t1 = 40 µs, the signal being no longer saturated thereafter.

[0030] The processing of the measurement signal representing the reflected ultrasonic wave can include several steps, but essentially consists of calculating an IN indicator whose value makes it possible to evaluate the integration of the implant in the bone.

[0031] This indicator IN corresponds to the average energy of the measurement signal, over a time interval t1-t2 starting at a time t1 and ending at a time t2.

[0032] The average energy can be determined by calculating the integral of the absolute value of the signal amplitude squared, over the time interval t1-t2. In this case, we can have: IN = ∫ t 1 t 2 M t 2 dt where M(t) denotes the measurement signal.

[0033] The average energy can, however, be determined differently, without departing from the scope of the invention. For example, the IN indicator can be calculated according to the following formula: IN = ∫ t 1 t 2 M t dt

[0034] Instead of considering the square of the signal amplitude, or the absolute value of the signal amplitude, we can also consider the envelope E(t) of the measurement signal or any other physical quantity related to the energy contained in the signal, in the time window considered.

[0035] In their research leading to the invention, the inventors discovered that there was an optimal value for the choice of time t1 that would optimize the sensitivity of the indicator and thus allow for a more precise assessment of the osseointegration of the implant. Surprisingly, relatively high values ​​should be chosen for t1. This discovery is illustrated by the following series of experiments. (Materials and methods)

[0036] The experiments were carried out using an ultrasonic probe of the type of the FIG 1 but equipped with an intermediate piece between the transducer 12 and the implant 1. Four strictly identical dental implants 1 were inserted into a resin block with different levels of sinking, as shown in the photograph of the FIG 4 .

[0037] The implants are numbered in order of decreasing depth. The more or less the implants are sunk, the more or less their stability is important. Thus, implant number 1, completely sunk into the resin, is very stable. Its stability corresponds to that of a perfectly osseointegrated implant. Conversely, implant number 4, barely sunk into the resin, is very unstable. Its stability corresponds to that of an implant that is not at all or only very slightly osseointegrated.

[0038] Implants number 2 and 3 correspond to a relatively stable and relatively unstable implant, respectively. The results obtained with implants number 2 and 3 were compared to assess the sensitivity of the IN indicator.

[0039] The calculation of the IN indicator is carried out in a time window between t1 and t2, it being understood that the second instant t2 is later than the first instant t1, i.e. t2 > t1. Generally speaking, the value of t2 is chosen to obtain an acceptable compromise between a sufficient duration to obtain relevant information and the need for a satisfactory signal / noise ratio. These general conditions being respected, the inventors found that the value of t2 did not significantly influence the sensitivity of the IN indicator from the moment when the duration of the window t1-t2 was at least equal to approximately 5 µs. Also, in the experiments carried out, the value of t2 was chosen equal to 150 µs for all the measurements. The value of t2 can, however, more generally, be chosen between t1+5 µs and 200 µs. For example, for t1 = 40 µs, t2 can be chosen between 45 and 200 µs.

[0040] A variation of the value of t1 between 5 and 100 µs was considered. For each value of t1 considered, the same procedure was used to adjust the gain so as to be able to visualize the signal with a conventional digitizer. This procedure consists of adjusting the gain so as to obtain a saturation of the signal until the beginning of the time window considered (around the instant t1 therefore), the signal being no longer saturated thereafter. This is illustrated on the FIG 3 which represents an example of a measurement signal obtained during the stability check of implant number 3. The value of t1 considered is 40 µs. The signal was amplified and the gain adjusted to 70 dB so as to obtain a saturation of the signal up to 40 µs, the signal being no longer saturated thereafter.

[0041] This procedure was applied considering implant number 3 because it is the implant that allows obtaining a maximum value of the signal. Note that it is not necessary to have a very precise adjustment of the gain but that it must be the same for the two implants (numbers 2 and 3) considered and for the same value of t1. The Figure 5 shows the variation of the gain, expressed in dB, as a function of t1, expressed in hundredths of µs (i.e. 10 -8< s). The gain increases as a function of t1 because the longer the signal is measured, the more it is necessary to amplify the signal, knowing that it decreases as a function of time.

[0042] Once the gain value was determined for each value of t1, three measurements were made with each implant and each value of t1, using the gain determined previously. The results obtained with implants number 2 and 3 were compared and a sensitivity S(t1) of the IN indicator was estimated for each value of t1. When the values ​​of the IN indicator do not show significant differences between the two implants, the sensitivity S was set equal to 0. Otherwise, the sensitivity S is given by the relative variation of the average of the indicators obtained for implants 2 and 3, i.e.: S t 1 = M t 1,3 − M t 1,2 ∗ 200 M t 1,3 + M t 1,2 where M(t1,i) corresponds to the average of the indicators obtained for implant number i and the indicator calculated with t1. The value of S therefore increases with the sensitivity of the indicator IN. (Results and conclusion)

[0043] There Figure 6shows the variation of the sensitivity S, without unit, as a function of t1, expressed in hundredths of µs (i.e. 10 -8< s). Surprisingly, an optimal value is obtained for a value of t1 located around 50 µs. The sensitivity is good, i.e. greater than or close to 4, for values ​​of t1 between 40 and 80 µs.

[0044] At present, it is difficult to explain the existence of an optimal value for t1 and the fact that this value is relatively high. However, the following hypothesis can be put forward.

[0045] Considering a relatively long propagation time, the ultrasonic wave makes a large number of round trips in the implant (which behaves like a waveguide). Therefore, the longer the time, the more sensitive the amplitude of the measurement signal is to variations in the implant boundary conditions because the ultrasonic wave will have made more round trips in the implant and therefore interacts more significantly with the bone-implant interface. Therefore, if the value of t1 is too low, the useful signal for calculating the IN indicator is included in relatively short times due to the exponential decay of the signal, which does not allow good sensitivity to be obtained due to the nature of the interaction between the ultrasonic wave and the implant. On the other hand, if the value of t1 is too high, the signal becomes strongly affected by noise and it is not possible to obtain good sensitivity either.

[0046] Finally, it should be noted that the experiments carried out with a dental implant inserted (at least partially) into a bone agree with the aforementioned results obtained for an implant inserted into a resin. In particular, these experiments also show the existence of an optimal value for t1 and the fact that this optimal value is relatively high. However, although being relatively high, the optimal value of t1 is generally lower than the optimal value obtained with the resin.

Claims

1. A device for controlling the stability of a dental implant (1) inserted at least partially into a bone (2), the implant having a free end (1a) emerging at the surface of the bone and an end (1b) buried in the bone (2), located opposite the free end, the device comprising: - an ultrasonic transducer (12) adapted to be coupled directly or indirectly to the implant (1), to emit an ultrasonic wave propagating inside the implant (1) towards the buried end (1b), to collect the reflected ultrasonic wave at a contact interface (4) between the implant (1) and the bone (2), and to provide a measurement signal representing the reflected ultrasonic wave, - a processing unit adapted to calculate, from the measurement signal, an indicator (IN) whose value allows to evaluate the integration of the implant (1) in the bone (2), the indicator (IN) corresponding to the average energy of the measurement signal over a time interval (t1-t2) starting at a first instant (t1) and ending at a second instant (t2), characterised in that the first instant (t1) is between 20 and 80 µs after the start of the emission of the ultrasonic wave by the transducer (12).

2. The device according to claim 1, wherein the first instant (t1) is comprised between 30 and 60 µs.

3. The device according to claim 1 or 2, wherein the second instant (t2) is, at a minimum, separated from the first instant (t1) by 5 µs and, at a maximum, equal to 200 µs.

4. The device according to any one of claims 1 to 3, wherein the ultrasonic transducer is adapted to emit an ultrasonic wave whose central frequency is at least equal to 5 MHz and, in particular, at least equal to 8 MHz.

5. The device according to any one of claims 1 to 4, wherein the ultrasonic transducer (12) has an active surface adapted to be in contact with the free end (1a) of the implant, directly or via an intermediate part, parallel to a support surface defined by the free end of the implant, so as to emit an ultrasonic wave propagating inside the implant (1) in a propagation direction perpendicular to the active surface of the transducer (12).

6. The device according to any one of claims 1 to 5, wherein the processing unit comprises amplification means for amplifying the measurement signal before calculating the indicator (IN), the measurement signal being amplified with a gain adjusted so as to obtain saturation of the signal up to the first instant (t1) but not beyond.

7. The device according to any one of claims 1 to 6, wherein the processing unit comprises amplification means for amplifying the measurement signal before calculating the indicator (IN), the measurement signal being amplified with a gain which is variable as a function of time and increases between the first and second instants (t1, t2).

8. A method for controlling the stability of a dental implant (1) inserted at least partially in a bone (2), the implant having a free end (1a) emerging at the surface of the bone and an end (1b) buried in the bone (2), located opposite the free end, the method comprising the following steps: - emitting an ultrasonic wave propagating inside the implant (1) towards the buried end (1b), collecting the reflected ultrasonic wave at a contact interface (4) between the implant (1) and the bone (2), and providing a measurement signal representing the reflected ultrasonic wave, - calculating by a processing unit, from the measurement signal, an indicator (IN) the value of which allows to evaluate the integration of the implant (1) in the bone (2), the indicator (IN) corresponding to the average energy of the measurement signal over a time interval (t1-t2) starting at a first instant (t1) and ending at a second instant (t2), characterised in that the first instant (t1) is comprised between 20 and 80 µs after the start of the emission of the ultrasonic wave by the transducer (12).

9. The method according to claim 8, wherein the first instant (t1) is comprised between 30 and 60 µs.

10. The method according to claim 8 or 9, wherein the second instant (t2) is, at a minimum, separated from the first instant (t1) by 5 µs and, at a maximum, equal to 200 µs.

11. The method according to any one of claims 8 to 10, wherein the emitted ultrasonic wave has a central frequency at least equal to 5 MHz and, in particular, at least equal to 8 MHz.

12. The method according to any one of claims 8 to 11, wherein, before calculating the indicator (IN), the measurement signal is amplified with a gain adjusted so as to obtain saturation of the signal up to the first instant (t1) but not beyond.

13. The method according to any one of claims 8 to 12, wherein the gain is variable as a function of time and increases between the first and second instants (t1, t2).