|Year : 2017 | Volume
| Issue : 1 | Page : 22-27
Predicting the dental implant stability based on the antiresonance phase of a piezo-based impedance sensor
Paramita Banerjee1, Abhijit Chakraborty2, Ratna Ghosh3, Bhaswati Goswami3
1 Department of Applied Electronics and Instrumentation Engineering, Guru Nanak Institute of Technology, Kolkata, West Bengal, India
2 Department of Periodontology, Guru Nanak Institute of Dental Sciences and Research, Kolkata, West Bengal, India
3 Department of Instrumentation and Electronics Engineering, Jadavpur University, Kolkata, West Bengal, India
|Date of Web Publication||30-Jun-2017|
157/F, Nilgunj Road, Panihati, Kolkata - 700 114, West Bengal
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: The stability of dental implants (DIs) in in vivo tests can be determined using noninvasive resonance frequency analysis technique. A low-cost piezo-based sensor has been developed for this purpose which uses a readily available two-terminal piezo element, to which a metal substrate is adhesively glued for attaching the implant. Aim: The attainment of implant stability in dynamic tests using this sensor must be standardized in terms of the major antiresonance (AR) in the impedance phase responses using sensor-DI assembly. This will be used to predetermine the dimensions of the glued metal substrate in the sensor design. Materials and Methods: Multiple sensors with varying sensor dimensions were developed. Static and dynamic impedance studies were performed on these and corresponding sensor-implant assemblies. Static tests as well as in vitro tests with the sensor-implant assembly dipped in a standardized dental plaster mixture were performed in controlled laboratory conditions. Results: The probability of acceptance of the hypothesis has been checked using binomial distribution with a significance level of 5%. Statistically observed that for 95% of the cases where the DI becomes stable in dental plaster, both AR phase and AR frequency (ARF) return to their corresponding static values. Furthermore, for a piezo element, whose ARF is within 6–6.6 kHz, the sensor yields maximal phase when the length of the metallic strip is 2 cm. Conclusions: Experimental validation supports both claims. Hence, this work can be extended to in vivo DI stability determination and design aspects of the corresponding sensor.
Keywords: Antiresonance frequency, dental implant stability, impedance response
|How to cite this article:|
Banerjee P, Chakraborty A, Ghosh R, Goswami B. Predicting the dental implant stability based on the antiresonance phase of a piezo-based impedance sensor. J Int Clin Dent Res Organ 2017;9:22-7
|How to cite this URL:|
Banerjee P, Chakraborty A, Ghosh R, Goswami B. Predicting the dental implant stability based on the antiresonance phase of a piezo-based impedance sensor. J Int Clin Dent Res Organ [serial online] 2017 [cited 2018 Oct 16];9:22-7. Available from: http://www.jicdro.org/text.asp?2017/9/1/22/201734
| Introduction|| |
Measurement of the stability of some specific types of dental implant (DI), when placed inside the jaw, can be done reliably using resonance frequency analysis (RFA) technique based on voltage magnitude responses of a sensor. In this paper, the primary focus has been shifted to the measurement of the antiresonance frequency (ARF) obtained from the instantaneous sensor impedance phase responses. An in vitro study with a two-terminal piezo-based impedance sensor indicated that the value of the frequency corresponding to maximum phase, denoted as ARF in this study, finally returns to its value at the first instant when the DI was just dipped into the mixture. This needs to be validated for the present modified design of the aforementioned sensors, which can then be used with any standard two-stage implant system.
| Materials and Methods|| |
The basic element of the sensor is a low-cost radial thin disc two-terminal piezo element of diameter 2 cm and thickness 0.25 mm, which is available in the commercial market as a piezo buzzer. Two connecting wires are soldered on the electrodes of the piezo buzzer at the specified positions [Figure 1]a. To form the sensor, a circular strip of the same diameter as the buzzer, which is cut from an aluminum sheet of thickness 0.29 mm, is attached on the reverse side of the piezo using commercially available Araldite standard epoxy adhesive. This circular sheet is extended into a perpendicularly folded rectangular arm of width 7 mm, hereafter referred to as the L arm [Figure 1]b. This L arm has a groove at the free end to hold the DI in position [Figure 1]c.
|Figure 1: piezo-based sensor (a) with soldered wires on front side, (b) aluminum substrate with L arm glued on the reverse side, and (c) attached with dental implants|
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The first study involves the comparison of impedance magnitude and phase responses. Both impedance magnitude and impedance phase responses have been recorded and compared for static as well as dynamic in vitro tests to establish the viability of using the impedance phase response for implant stability determination. For the in vitro tests, the DI is dipped into a dental plaster mixture and readings are recorded till the mixture sets completely. It is to be mentioned that dental plaster, which is used in this present study, is not a material substitute for bone. However, it is known that changes in stiffness may occur at the implant-tissue interface during bone formation, and healing can be mimicked using the change in state from liquid to solid that occurs during the setting of dental plaster mixture. This simple design thus allows a change in the stiffness to be monitored over a convenient period. All responses have been acquired with a 4294A Agilent Impedance Analyzer (Keysight Technologies, USA) in Z-θ mode in the range of 1–20 kHz with a frequency resolution of 192 Hz.
The principle of measurement can be explained as follows. It is expected that the recorded antiresonance phase (ARPh) as well as the ARF of the sensor are nonlinear functions of the change in stiffness σ(t) of the implant-dental plaster mixture with time, t and by extension, of the dynamically changing implant-jaw bone and tissue interface. Thus, in this study, the time-changing ARF and the corresponding ARPh of the inserted implant, acquired from the instantaneous impedance phase responses of the attached sensor, have been used to monitor the bonding of the DI with the surrounding dental plaster mixture to ascertain its final stability. It is expected that both these parameters will reach a stable value when the implant is bonded.
A preliminary study conducted on 30 piezo elements showed that for each piezo, the impedance responses were identical within 1.00°. Hence, in studies stated herewith, the average response of a particular sensor configuration has been considered.
In the next study, it is necessary to select a sensor with the maximal ARPh to categorize the dynamic responses and ascertain the implant stability. Hence, the impedance phase responses of the piezo, the corresponding sensor, and the sensor implant assembly have been analyzed with the objective of determining the suitable length of the L arm of the sensor.
For this, 12 piezos that exhibit ARF between 6000 and 7000 Hz have been used to prepare sensors but without any groove at the free end. The L arm of the metallic strip of all sensors was initially taken as 3 cm and thereafter, the length was reduced to 2 cm, then to 1 cm. In an independent study performed on 3 sensors, it was observed that the ARF as well as the ARPh of the three sensors with groove and without groove for all three lengths remain fairly close. Hence, the results for a sensor without groove have been treated equivalently as those for a sensor with groove in this case.
To obtain insight about the prediction of DI stability, four sensors, each with two cm length of the L arm and with a DI of dimension 3.5 mm × 9 mm inserted into the groove, have been used in the in vitro study to determine the criteria for predicting the attainment of implant stability. The experimental methodology used for this study is stated below.
The impedance responses of all four static sensor-implant assemblies are recorded and the values of ARPh and the corresponding ARF are identified.
Ten in vitro tests in total have been conducted using four sensors. In each trial, 4 g of dental plaster powder is mixed with 3 cc of water and thereafter, the spatula used for preparing the mixture is rotated same number of times and in the same direction to maintain an uniform consistency of the mixture.
The sensor was connected to the impedance analyzer while the implant screwed to it was suspended precisely into the center of the inner hollow cavity of a predesigned circular container containing a fixed volume of the dental plaster mixture. This is done to ensure that a fixed height of the DI is dipped into the mixture.
The dental plaster gradually hardens with time. To maintain a uniform rate of setting of the mixture, the environmental temperature is maintained same during the experiment. As stated earlier, this is expected to mimic the changes in stiffness at the implant-tissue interface during bone formation.
The impedance responses were recorded at regular intervals of 61 s each for a total duration of about 2 h to ensure that the hardening process was complete.
For the statistical analysis, nonparametric hypothesis tests were performed to find the statistical significance of the proposed claims. The tests are to determine whether the claims are true against the alternative hypothesis; hence, binomial distribution has been considered. The significance level considered is 5%. There are only two outcomes, true or false. Hence, the probability of claim is the same as that of the alternative, P = 0.5 [Table 1] and [Table 2].
|Table 1: Statistical analysis of criteria proposed for selection of sensor length|
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|Table 2: Statistical analysis of the condition for predicting dental implant stability in dental plaster|
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| Results|| |
The first part of the result provides comparison of impedance magnitude and phase responses. The static impedance magnitude and phase responses of a two-terminal piezo element, the corresponding sensor with L arm of length 2 cm, the sensor implant assembly as well as dynamic responses when the implant is just dipped into dental plaster mixture and when the mixture has set completely [Figure 2]a.
|Figure 2: (a) plots of impedance magnitude response of piezo, sensor, sensor attached to a DI, and first instant just after DI in plaster. (b) Plots of impedance phase response of piezo, sensor, sensor attached to a DI, and first instant just after DI in plaster|
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It is evident that the impedance magnitude achieves local minima at the system resonances, but these are comparable with the values at higher frequencies [Figure 2]a. On the other hand, the achieved local maxima at the ARs are much less than the magnitude at 1 kHz frequency. Thus, identifying the resonance frequency (RF) or ARF unambiguously from the impedance magnitude responses becomes difficult. Conversely, the impedance phase of the sensor is nominally about −90° and it changes significantly at all system ARs and particularly in the maximal case ARPh [Figure 2]b, thus, justifying the identification of ARF from the impedance phase responses.
The second part of the result pertains to the determination of sensor length. These are depicted in a bar graph of ARF and the corresponding ARPh of all 12 piezos [Figure 3]a and [Figure 3]b while the corresponding observations are stated below.
|Figure 3: plots of (a) ARF of piezos and respective sensors of varying lengths. (b) Plots of ARPh of piezos and respective sensors of varying lengths|
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The ARF of a sensor with an L arm is shifted by 5.4 ± 2.0 kHz from that of the underlying piezo element. In nine out of twelve cases, the shift is within 4.4 ± 0.6 kHz.
Although the ARPh also changes, there is no such definitive change.
In the neighborhood of the maximum phase peak, typically within ±3.5 kHz of ARPh, the number of AR peaks is three for all lengths of the sensor L arm.
For the piezo elements, whose ARF lies between 6000 Hz and 6600 Hz, the probability of the corresponding sensors which shows higher ARPh when the length of the metallic strip is 2 cm as compared to other lengths is 81% [Table 1].
It must be mentioned here that the ARF of the sensor showed a slight shift, within few kHz, when a DI is attached to it as is to be expected [Figure 2]b. However, independent studies by the authors have not yielded a definitive nature of this change.
The third part of the results pertains to the prediction of DI stability. ARF and ARPh of the static sensor-implant assembly, implant just dipped into dental plaster mixture and last five instants are shown in bar graph [Figure 4]a and [Figure 4]b. The static sensor-implant assembly is referred in the figures as with DI readings. The repeatability of the responses has been established from the total ten in vitro trials conducted using four sensors. The observations from the statistical analysis, performed at a significance level of 5%, are stated in tabular form [Table 2].
|Figure 4: (a) plots of change with time of ARF in in vitro trials. (b) Plots of change with time of ARPh in in vitro trials|
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The ARF of the sensor-implant assembly remains almost unchanged when the implant is just dipped into the dental plaster mixture while there is a significant shift in the ARPh.
For all trials, the ARF in the last five instants remains same and is within ±192 Hz to that of first instant while the probability that the ARF is within ±192 Hz of that of the static sensor-implant assembly is 98%. Hence, both the claims may be accepted.
The probability that the ARPh of all the last five instants is within ±1° is 99%. Furthermore, the probability that the final ARPh is within ±7° of that of the static sensor-implant assembly, or that of the first instant when the DI is just dipped into the mixture, is 94.5% and 82.8%, respectively. Hence, these hypotheses may also be accepted.
| Discussion|| |
The major finding of this study is that the stability of a standard two-stage DI can be ascertained reliably from the ARF and ARPh characteristics acquired from the impedance phase response of a two-terminal piezo-based sensor. It has been validated in in vitro studies with dental plaster mixture that the stability of the DI in dental plaster mixture is ensured when both ARPh and its corresponding ARF become stable for at least 5 min. Moreover, in 95% cases, it is almost certain that these values return back to those of the static sensor-implant assembly within a tolerance limit.
Another significant finding is that the AR characteristics of the piezo to be used for the sensor are also useful in determining the suitable length of the L arm of the sensor. It has been validated that for 95% of the wire connected piezos with their ARF between 6000 Hz and 6600 Hz, the ARPh of the corresponding sensor is maximized when the length of the L arm is close to 2 cm as compared to 3 cm or 1 cm. Hence, it can be claimed that for these piezos, the suitable length of the L arm is 2 cm.
This study thus establishes the unambiguity of determining the DI stability from impedance phase responses as compared to magnitude responses. It also establishes the importance of doing so from the ARF and phase in place of the resonance characteristics. However, these results are based on in vitro studies in dental plaster using standard equipment and in a controlled environment with the assumption that the process of in vitro setting of dental plaster around the inserted implant mimics the long-term in vivo bonding of the DI with the surrounding jaw bone and tissue. Hence, these results have to be revalidated for in vivo studies for implant-bone tissue scenarios in realistic conditions.
Traditionally, DI stability has been measured by several researchers using RFA technique., Contact type and noncontact type sensor designs for specific implant systems developed by Integration Diagnostics AB, Sweden, have utilized the implant stability quotient for specifying implant stability., However, it is known these sensors are implant specific, costly and are affected by electromagnetic induction.
A typical contact type DI stability sensor which can be used with multiple implant system has also been developed. This consists of a three-terminal piezo element and a metal strip that is adhesively glued to it to attach the implant to the sensor. For this sensor, the RF is associated with the peak voltage magnitude measured in the frequency response. To simplify the design and also improve the reliability, the present study utilizes the established concept of utilizing two-terminal piezo elements to monitor structural health using impedance responses.,,,
| Conclusion|| |
This study provides a criterion to predetermine the dimension of an ARF-based DI stability sensor based in terms of the underlying piezo characteristics. Furthermore, it allows for a prior prediction of the stability characteristics of the DI based on the static sensor-implant assembly responses. Thus, this study provides a proof of concept of a reliable yet affordable sensor for DI stability, which can be used with a large variety of commercially available DIs. No controversies have been raised in this study in which an alternative scheme has been proposed.
Further research needs to be done to validate these results in actual in vivo scenarios. To do so, a detailed study has to be done to develop templates with different sensor arm lengths that will maximize the sensor response for the various locations of DI placement in the human jaw. Thereafter, the sensor geometry, tolerances as well as procedures for use have to be established since these affect the sensor responses, and hence, the predictability of implant stability.
Financial support and sponsorship
Administrative and academic support for pursuing Ph.D of Mrs. Paramita Banerjee has been provided by Guru Nanak Institute of Technology. 4294A Impedance Analyzer (Keysight Technologies, USA) was purchased from the financial grant provided by DST PURSE, Jadavpur University.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]