Simulia Abaqus Dental Implant

Title

Evaluation of dental implant stability in bone phantoms: comparison between a quantitative ultrasound

Running Head (Abbreviated Title)

RFA and ultrasound methods to evaluate implant stability

Authors

Romain VAYRON, PhD, Postdoctoral researcher ; CNRS, Laboratoire Modélisation et Simulation MultiEchelle, MSME UMR CNRS 8208, 61, avenue du Général de Gaulle, 94010

Créteil, Cedex, France

Vu Hieu NGUYEN, PhD, Associate Professor ; CNRS, Laboratoire Modélisation et Simulation MultiEchelle, MSME UMR CNRS 8208, 61, avenue du Général de Gaulle, 94010

Créteil, Cedex, France

Benoît LECUELLE, Bac+2, Engineer Assist ; Centre de Recherche BioMédicale, Ecole Nationale Vétérinaire d’Alfort, 7 Avenue du Général de Gaulle, 94700 Maisons-Alfort France Guillaume HAIAT, PhD, Research Director ; CNRS, Laboratoire Modélisation et Simulation MultiEchelle, MSME UMR CNRS 8208, 61, avenue du Général de Gaulle, 94010 Créteil,

Guillaume Haïat

Laboratoire de Modélisation et de Simulation MultiEchelle, UMR CNRS 8208,

Conflict Of Interest Statement

No conflict of interest.

Author Contribution Statement:

Romain VAYRON: Concept/Design, Data analysis/interpretation, Drafting article, Statistics, Data collection. Vu Hieu NGUYEN: Critical revision of article, Approval of article.

Benoît LECUELLE: Critical revision of article, Approval of article, Data collection. Guillaume HAIAT: Concept / Design, Data analysis / interpretation, Drafting article, Critical revision of article, Approval of article.

Keywords :

Quantitative ultrasound ; Resonance frequency analysis ; Dental implant stability ; Bone phantom ; Bone quality.

Background

Resonance frequency analyses and quantitative ultrasound methods have been suggested to assess dental implant primary stability.

Purpose

The purpose of this study was to compare the results obtained using these two techniques applied to the same dental implants inserted in various bone phantoms.

Materials And Methods

Different values of trabecular bone density and cortical thickness were considered to assess the effect of bone quality on the respective indicators (UI and ISQ). The effect of the implant insertion depth and of the final drill diameter was also investigated.

Results

ISQ values increase and UI values decrease as a function of trabecular density, cortical thickness and the screwing of the implant. When the implant diameter varies, the UI values are significantly different for all final drill diameters (except for two), while the ISQ values are similar for all final drill diameters lower than 3.2 mm and higher than 3.3 mm. The error on the estimation of parameters with the QUS device is between 4 and 8 times lower compared to that made with the RFA technique.

Conclusions

The results show that ultrasound technique provides a better estimation of different parameters related to the implant stability compared to the RFA technique.

Introduction

Dental implant stability, which is determinant for the surgical success 1, is determined by the quantity and biomechanical quality of bone tissue around the implant 2. Two kinds of implant stability may be distinguished. The primary stability occurs at the moment of implant surgical insertion within bone tissue. Dental implant primary stability should be sufficiently high in order to avoid micromotion at the bone-implant interface after surgery, but should not be too high to avoid bone necrosis due to overloading of bone tissue 3. Secondary stability is obtained through osseointegration phenomena, a complex phenomenon of a multi-time and multiscale nature 3, which strongly depends on the implant primary stability.

Dental implant stability remains difficult to be assessed clinically because it depends on the implant properties (geometry, surface properties...), on the patient bone quality, as well as on the surgical protocol. In particular, there is a lack of standardization of the surgical procedures used in dental implantology, for example in the choice of the duration between implant insertion and loading, which may vary from 0 up to 6 months 4. A compromise should be found in a patient specific manner between i) an early (or even immediate 5) implant loading in order to stimulate osseointegration phenomena and ii) a late implant loading in order to avoid degradation of the consolidating bone-implant interface in early postsurgical stages 6.

Meanwhile, shortening the time of implant loading has become a challenge in recent implant developments to i) minimize the time of social disfigurement and ii) avoid gum loss. As a consequence, accurate measurements of implant biomechanical stability are of interest since they could be used to improve the surgical strategy by adapting the choice of the healing period in a patient-specific manner.

Assessing the implant stability is a difficult multiscale problem because of the complex heterogeneous nature of periprosthetic bone tissue and to remodeling phenomena 7, 8. Different

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approaches have been used to assess the implant stability in vivo. So far, most surgeons still rely on their proprioception because it remains difficult to monitor bone healing in vivo 6. Accurate quantitative methods capable of assessing implant stability are required to guide the surgeons and eventually reduce the risk of implant failure.

Magnetic resonance imaging 9 and X-ray based 10 techniques remain of limited interest to measure implant stability because of diffraction phenomena occurring at the bone-implant interface due to the presence of metal. Therefore, biomechanical methods have been developed, their main advantage consisting in the absence of ionizing radiation, inexpensiveness, portability and noninvasiveness. The measurement of the insertion torque to assess dental implant primary stability has been evoked but such approach remains limited 11 because the result is not only related to the properties of the bone-implant interface and because it cannot be used for secondary stability assessment. The Periotest (Bensheim, Germany) is a percussion test methods 12, 13. Its sensitivity to striking height and handpiece angulation complicates the clinical examination 14 and limits the reproducibility of the measurements. The most commonly used biomechanical technique is the resonance frequency analysis (RFA) 15, which consists in measuring 16 the first bending resonance frequency of a small rod attached to the implant. The RFA technique allows to assess the implant anchorage depth into bone 17, marginal bone level 18 and the stiffness of the bone-implant structure 19, 20. However, RFA cannot be used to identify directly the bone-implant interface characteristics 21. No correlation between the implant stability quotient (ISQ) and bone implant contact (BIC) nor between ISQ and cortical thickness has been evidenced so far 22. Finite element numerical simulation tools showed that the orientation and fixation of the transducer have an important effect on ISQ values 20 obtained with the older Osstell version with an L-shaped, wired transducer.

An alternative method has been developed by our group and consists in using a quantitative ultrasound (QUS) method 23 to investigate the properties of the bone-implant interface. The

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principle of the measurement relies on the dependence of ultrasonic propagation within the implant on the boundary conditions given by the biomechanical properties of the bone-implant interface 24. An in vitro preliminary study was carried out using a prototype titanium cylinder shaped implants inserted in bone tissue, showing the sensitivity of the ultrasound response of the implant to the quantity of bone in contact with the implant 24. The principle of the measurement was then validated experimentally by showing the sensitivity of the echographic response of a planar bone-implant interface to healing time using coin-shaped implant models 25. Significant variations of the ultrasonic response of dental implants embedded in a bone substitute biomaterial (a tricalcium silicate-based cement) was shown to occur when the implants are subjected to fatigue loading 26. More recently, another in vitro study proved the potentiality of QUS methods to assess dental implant primary stability 27. A preclinical validation of the device in rabbits was carried out 25, 28 and showed that i) the measurement was sensitive to healing time and ii) a significant correlation of the measurement with the bone- implant contact (BIC) ratio measured with histology. Moreover, finite difference 29 and finite element numerical simulations 30, 31 were carried out to understand the interaction between an ultrasonic wave and the bone-implant system, leading to a better performance of the device.

The comparison of the experimental results obtained in a controlled configuration using the QUS and RFA techniques would be of interest in order to better assess the performance of the different approaches. The aim of the present study is to compare these the RFA and QUS methods which have been evoked to assess dental implant stability. To do so, our strategy consists in using dental implants inserted in bone mimicking phantoms made of polyurethane because it allows to work under standardized and reproducible conditions. Different parameters related to the implant stability (such as the density of the bone phantom, the thickness of cortical bone, the insertion depth and the drill diameter) were investigated and the related variations of

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different quantities such as i) the RFA response and ii) the ultrasonic response were investigated.

Ii)1 Bone Mimicking Phantoms And Dental Implant

The bone mimicking phantoms used in the present study were made of rigid polyurethane foam (Orthobones®, 3B Scientific, Hamburg, Germany) with different values of bone density and of cortical thickness (1 and 2 mm). Cortical bone was modeled by the material type #40 PCF with a mass density equal to 0.55 g/cm. Three types of trabecular mimicking phantoms were considered (#10, #20, #30 PCF) with mass density values equal to 0.16 g/cm, 0.32 g/cm and 0.48 g/cm, respectively, according to the manufacturer.

All dental implants used in this study were commercial implants manufactured by Zimmer Biomet (Winterthur, Switzerland) under the reference TSVT4B10. The geometrical characteristics are a length of 10 mm and an external diameter of 4.1 mm.

A conical cavity was then created in the block bone test, using color-coded, 10-mm-length surgical drills preconized by Zimmer Biomet and used as a reference protocol by dentists. Different values were considered for the final drill diameter, as described in details in subsection II)3.

II)2 Measurement methods of dental implant stability Two different and complementary methods were used in order to measure the implant stability of each implant in each configuration, which are described in subsection II)3.

A) Resonance Frequency Analysis

For each implant and each configuration, the RFA response of the implant was measured in ISQ units (on a scale from 1 to 100) using the Osstell device (Osstell, Göteborg, Sweden). Figure 1a shows the configuration of the measurements which were realized using a smart peg placed in the implant, as recommended by the manufacturer. Each measurement was performed 5 times (in order to assess the reproducibility of the measurements) in two perpendicular directions denoted 0° and 90° relatively to an arbitrary axis chosen for each block. The average and standard deviation values ISQm and ISQstd of the ten different ISQ values were determined for each implant and each configuration.

B) Quantitative Ultrasound Device

Figure 1b shows the dedicated QUS device, which consists in a 5 mm diameter planar ultrasonic contact transducer (Sonaxis, Besançon, France) generating a 10 MHz broadband (the frequency bandwidth is approximately equal to 6–14 MHz) ultrasonic pulse propagating perpendicularly to its active surface (monoelement transducer used in echographic mode). The probe was attached rigidly to a titanium alloy dental healing abutment which can be screwed into the implant so that the measurements are not influenced by positioning problems of the probe relatively to the healing abutment, as it was the case in previous studies 26. The QUS device was screwed into the implant in order to realize each measurement for each implant and each configuration, as shown in Fig. 1b. The ultrasonic probe was linked to a pulser-receiver via a standard coaxial cable. A transient recorder was used to record the radiofrequency (rf) signal with a sampling frequency equal to 100 MHz.

For each measurement, the transducer was screwed in the implant with a torque of 0.035N.m, which is around 10 times lower than torque values recommended by implant manufacturers 32

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and the echographic measurement was made instantaneously. The transducer was then unscrewed and the same measurement was carried ten times in order to assess the reproducibility of the measurements.

The same signal processing technique as the one used in 26 was used to derive a quantitative ultrasonic indicator UI which had been shown to be related with dental implant stability. The indicator UI was devised to quantitatively estimate the average amplitude of the signal between 10 and 120 µs. Therefore, the envelop S(t) of the rf signal s(t) was determined and the indicator

(1)

where T0=0.01 µs corresponds to the sampling period. The time window chosen to determine the indicator UI was chosen as follows. The upper bound of the time window equal to 120 µs was chosen, which corresponds to a compromise between a sufficient duration to obtain pertinent information and the requirement of a sufficient signal to noise ratio for all rf signals.

Moreover, the time window used to compute the indicator UI starts at a time of 10 µs because the amplitude of the envelop of the rf signals before 10 µs is approximately constant due to a saturation of the amplitude.

The average and standard deviation values UIm and UIstd of the indicator UI obtained for the ten measurements were determined for each implant and each configuration.

Ii)3 Experimental Protocol

The effect of varying the different parameters described below on the variation of the ISQ and the indicator UI was investigated in this study, as summarized in Figure 2. For each configuration described below, the values of the averaged and standard deviation values ISQm

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and ISQstd (respectively UIm and UIstd) of the ISQ (respectively the ultrasonic indicator UI) were measured.

A) Variation Of Trabecular Bone Density

The effect of modifying trabecular bone properties was first investigated by comparing the results obtained with the two methods described in subsection II)2 for implants fully inserted in different bone mimicking phantoms having the same cortical thickness equal to 2 mm, which corresponds approximately to the clinical situation 33. Three different materials were used to mimic trabecular bone (#10, #20 and #30 PCF). Three implants were inserted with a final drill of 3.4 mm of diameter in each type of bone mimicking phantom, resulting in a total of 9 configurations.

B) Variation Of Cortical Bone Thickness

The effect of changing the cortical bone thickness was then investigated because cortical thickness is known to be an important determinant of implant stability 11. To do so, two bone mimicking phantoms with two different cortical thicknesses (1 and 2 mm) and the same material to mimic trabecular bone (#10 PCF) were considered. Again, three implants were fully inserted with a final drill of 3.4 mm of diameter in each type of bone mimicking phantom, resulting in a total of 6 configurations.

C) Variation Of The Diameter Of The Final Drill

The effect of the diameter of the final drill used to machine the cavity in two given bone mimicking phantom was investigated. The two phantoms used had the same cortical thickness (1 mm) and trabecular bone was mimicked by #10 and #30 PCF. Ten values of diameter of the

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final drill comprised between 2.7 and 3.6 mm with a step of 0.1 mm were considered. Three implants were considered for each diameter tested, leading to a total number equal to 30 configurations.

D) Variation Of The Implant Insertion Depth

Eventually, the effect of the implant insertion depth was investigated because it corresponds to a simple way of imposing a variation of dental implant stability, similarly as what has been done in 28. To do so, three cavities were realized in a given bone mimicking phantom (cortical thickness of 1 mm and #30 PCF for trabecular bone) with a final drill diameter of 3.4 mm.

Then, an implant was screwed in each cavity so that half of the implant was inserted into the cavity. The average and standard deviation values of the ISQ and of the ultrasonic indicator UI were measured. The implant was then screwed by π rad in order to increase the implant surface area in contact with bone mimicking phantom. The two measurements described above (ISQ and UI) were realized again. The procedure was repeated until the implant was completely inserted in bone mimicking phantom, this screwing level is denoted “0”. Then, the implant was then unscrewed by π rad in order to reduce the surface area of the implant in contact with bone block. The same two measurements (ISQ and UI) were again carried out. The procedure was reproduced until the implant was completely detached from the test block.

Ii)4 Statistical Analyses

A one-way analysis of variance (ANOVA) and Tukey-Kramer tests were performed to evaluate the significance of variations of the ultrasonic indicator UI and the ISQ as a function of the different configurations. Statistical differences were defined at a 95% confidence level.

Iii)1 Effect Of Bone Density

Figure 3 shows the results obtained for the nine configurations corresponding to three models of trabecular bone quality (#10, #20 and #30 PCF) with the different methods of implant stability assessment (ISQ and UI). As shown in Fig. 3, the values of ISQm increase when the density of trabecular bone increases, while the opposite behaviour is obtained for the ultrasonic indicator UIm. The error bars represent the reproducibility of the ultrasound and RFA measurements for each configuration. The average value of UISD (respectively ISQSD) obtained for the ultrasonic measurement (respectively RFA measurements) is equal to 0.07 unit (respectively 4 ISQ units).

ANOVA shows a significant effect of bone density on the values of UI (p-value < 10-8) and of ISQ (p-value = 0.04). Moreover, Tukey-Kramer tests show that i) the results obtained for UI in #10 PCF (respectively #20 PCF) are significantly different than the results obtained in #20 PCF, p-value < 10-4 (respectively #30 PCF, p-value < 10-7). The results obtained with the RFA measurements are significantly different when comparing implants inserted i) in #10 PCF and #20 PCF (p-value < 3.9*10-8) and ii) in #10 PCF and #30 PCF (p-value < 2.6*10-8).

However, the results obtained with RFA measurements in #20 PCF and #30 PCF are not significantly different (p-value = 0.93).

Iii)2 Effect Of Cortical Thickness

Figure 4 shows the variation of the ISQ and of the UI for different implants inserted in test blocks with 1 mm and 2 mm of cortical thickness. Trabecular bone density is equal to #10 PCF.

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ANOVA shows a significant effect of cortical thickness on the values of the UI (p-value < 10- 9) and on the values of ISQ (p-value = 0.04), although the p-value obtained for ISQ is relatively high.

Iii) 3 Effect Of The Final Drill Diameter

Figure 5 shows the variation of the average and standard deviation values of the ISQ and of the UI as a function of the final drill diameter for implants inserted in the same test block with a cortical thickness of 1 mm and a density of trabecular density equal to #10 PCF. The results show that the values of the UI (respectively ISQ) increase (respectively decrease) as a function of the final diameter drill. In order to clarify the figure, we have chosen to present only one curve (instead of three) because the other results are qualitatively similar to those shown in Fig.

5. ANOVA shows a significant effect of final drilling diameter on the values of the UI (p < 10-12) and on the values of ISQ (p < 10-3). The stars in Fig. 5 indicate the configurations for which the results obtained for the UI and for the ISQ are statistically similar, which was obtained using a Tukey-Kramer test. The results obtained with Tukey-Kramer test indicate that the UI obtained for all final drill diameters are significantly different to the other ones, except for the results obtained for the final drill diameters equal to 2.8 and 2.9. However, the results obtained for ISQ with all drill diameters lower than 3.3 (respectively higher that 3.2) are statistically similar.

Iii) 4 Effect Of The Insertion Depth

Figure 6 shows the average and standard deviations of the ISQ and of the UI for the same implant with different values of the penetration depth. The arrows in Fig. 5 indicate the evolution of the results when the implant is screwed (from a bone level of -5π rad to 0 rad) and

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then unscrewed (from a bone level of 0 rad to -4π rad). The UI value is shown to decrease during the insertion of the dental implant in the bone block while the UI value increases when the implant in unscrewed from the test block. The ISQ values increase during the insertion stages and then decrease when the implant is unscrewed. In order to clarify the figure, we have chosen to present only one curve (instead of three) because the results obtained using the other test block are qualitatively similar to those shown in Fig. 6.

Iv) Discussion

The originality of the present study was to compare two different techniques (RFA and ultrasound measurements) in order to investigate the primary stability of dental implants inserted in artificial bone blocks (Orthobones®). Different stability conditions were considered by varying different parameters such as the type of bone block (trabecular bone density, cortical thickness), the final diameter drill and the insertion depth, in order to simulate different situations mimicking variations of dental implant primary stability.

IV) 1 Physical interpretation and comparison with the literature The values of the ISQ are shown to increase when i) trabecular density increases (see Fig. 3), ii) cortical thickness increases (see Fig. 4), and iii) the insertion depth increases (see Fig. 6).

These results are in agreement with previous studies 11, 14, 17, 34 and can be explained by the higher rigidity of the block-implant system induced by an increase of trabecular density, of cortical thickness and of implant insertion depth.

The values of the UI are shown to decrease when i) trabecular density increases (see Fig. 3), ii) cortical thickness increases (see Fig. 4), iii) the final diameter drill decreases (see Fig. 5) and

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