Romain Vayron

Biomechanical determinants of the stability of dental implants: Influence of the bone–implant interface properties

Dental implants are now widely used for the replacement of missing teeth in fully or partially edentulous patients and for cranial reconstructions. However, risks of failure, which may have dramatic consequences, are still experienced and remain difficult to anticipate. The stability of biomaterials inserted in bone tissue depends on multiscale phenomena of biomechanical (bone-implant interlocking) and of biological (mechanotransduction) natures. The objective of this review is to provide an overview of the biomechanical behavior of the bone-dental implant interface as a function of its environment by considering in silico, ex vivo and in vivo studies including animal models as well as clinical studies. The biomechanical determinants of osseointegration phenomena are related to bone remodeling in the vicinity of the implants (adaptation of the bone structure to accommodate the presence of a biomaterial). Aspects related to the description of the interface and to its space-time multiscale nature will first be reviewed. Then, the various approaches used in the literature to measure implant stability and the bone-implant interface properties in vitro and in vivo will be described. Quantitative ultrasound methods are promising because they are cheap, non invasive and because of their lower spatial resolution around the implant compared to other biomechanical approaches.

Micro-Brillouin Scattering Measurements in Mature and Newly Formed Bone Tissue Surrounding an Implant

The evolution of implant stability in bone tissue remains difficult to assess because remodeling phenomena at the bone-implant interface are still poorly understood. The characterization of the biomechanical properties of newly formed bone tissue in the vicinity of implants at the microscopic scale is of importance in order to better understand the osseointegration process. The objective of this study is to investigate the potentiality of micro-Brillouin scattering techniques to differentiate mature and newly formed bone elastic properties following a multimodality approach using histological analysis. Coin-shaped Ti-6Al-4V implants were placed in vivo at a distance of 200 μm from rabbit tibia leveled cortical bone surface, leading to an initially empty cavity of 200 μm×4.4 mm. After 7 weeks of implantation, the bone samples were removed, fixed, dehydrated, embedded in methyl methacrylate, and sliced into 190 μm thick sections. Ultrasonic velocity measurements were performed using a micro-Brillouin scattering device within regions of interest (ROIs) of 10 μm diameter. The ROIs were located in newly formed bone tissue (within the 200 μm gap) and in mature bone tissue (in the cortical layer of the bone sample). The same section was then stained for histological analysis of the mineral content of the bone sample. The mean values of the ultrasonic velocities were equal to 4.97×10(-3) m/s in newly formed bone tissue and 5.31×10(-3) m/s in mature bone. Analysis of variance (p=2.42×10(-4)) tests revealed significant differences between the two groups of measurements. The standard deviation of the velocities was significantly higher in newly formed bone than in mature bone. Histological observations allow to confirm the accurate locations of the velocity measurements and showed a lower degree of mineralization in newly formed bone than in the mature cortical bone. The higher ultrasonic velocity measured in newly formed bone tissue compared with mature bone might be explained by the higher mineral content in mature bone, which was confirmed by histology. The heterogeneity of biomechanical properties of newly formed bone at the micrometer scale may explain the higher standard deviation of velocity measurements in newly formed bone compared with mature bone. The results demonstrate the feasibility of micro-Brillouin scattering technique to investigate the elastic properties of newly formed bone tissue.

Variation of the ultrasonic response of a dental implant embedded in tricalcium silicate-based cement under cyclic loading.

The use of tricalcium silicate-based cement (TSBC) as bone substitute material for implant stabilization is promising. However, its mechanical behavior under fatigue loading in presence of a dental implant was not reported so far because of the difficulty of measuring TSBC properties around a dental implant in a nondestructive manner. The aim of this study is to investigate the evolution of the 10 MHz ultrasonic response of a dental implant embedded in TSBC versus fatigue time. Seven implants were embedded in TSBC following the same experimental protocol used in clinical situations. One implant was left without any mechanical solicitation after its insertion in TSBC. The ultrasonic response of all implants was measured during 24 h using a dedicated device deriving from previous studies. An indicator I based on the temporal variation of the signal amplitude was derived and its variation as a function of fatigue time was determined. The results show no significant variation of I as a function of time without mechanical solicitation, while the indicator significantly increases (p<10(-5), F=199.1) at an average rate of 2.2 h(-1) as a function of fatigue time. The increase of the indicator may be due to the degradation of the Biodentine-implant interface, which induces an increase of the impedance gap at the implant surface. The results are promising because they show the potentiality of ultrasonic methods to (i) investigate the material properties around a dental implant and (ii) optimize the conception of bone substitute materials in the context of dental implant surgery.

Assessment of In Vitro Dental Implant Primary Stability Using an Ultrasonic Method

Dental implants are used for oral rehabilitation. However, there remain risks of failure that depend on the implant stability. The objective of this study is to investigate whether quantitative ultrasound technique can be used to assess the amount of bone in contact with dental implants. Ten implants are first inserted in the bone samples. The 10 MHz ultrasonic response of each implant is measured using a dedicated device and an indicator I is derived based on the amplitude of the signal. Then, the implant is unscrewed by 2 π radians and the measurement is realized again. A statistical analysis of variance was carried out and revealed a significant effect of the amount of bone in contact with the implant on the values of I (p value < 10⁻⁵). The results indicates the feasibility of quantitative ultrasound techniques to assess implant primary stability in vitro.

Ultrasonic evaluation of dental implant osseointegration

Dental implants are widely used for oral rehabilitation. However, there remain risks of failure which are difficult to anticipate and depend on the implant osseointegration. The objective of this in vivo study is to determine the variation of the echographic ultrasonic response of a dental implant to bone healing around the implant interface. Twenty one dental implants were inserted in the femur of seven New Zealand white rabbits. Two animals were sacrificed after a healing duration of two weeks, three animals after six weeks and six animals after eleven weeks. The 10 MHz ultrasonic response of the implant was measured just after the implantation using a dedicated device positioned at the emerging surface of each dental implant. The measurements were realized again before the sacrifice with the same device. An indicator I˜ was derived based on the amplitude of the rf signal obtained for each configuration. The bone-Implant Contact (BIC) ratio was determined by histological analyses. The average value of the relative variation of the indicator I˜ obtained after initial surgery and after the corresponding healing period varies between 7% and 40%. A Kruskal-Wallis test (p<0.01) revealed a significant decrease of the value of the indicator I˜ as function of healing time. The indicator I˜ was significantly correlated (R(2)=0.45) with the BIC ratio. The results show that the ultrasonic response of a dental implant varies significantly as a function of healing time, which paves the way for the development of a new quantitative ultrasound (QUS) method in oral implantology.

Influence of healing time on the ultrasonic response of the bone-implant interface

The aim of the present study is to investigate the effect of bone healing on the ultrasonic response of coin-shaped titanium implants inserted in rabbit tibiae. The ultrasound response of the interface was measured in vitro at 15 MHz after 7 and 13 weeks of healing time. The average value of the ratio r between the amplitudes of the echo of the bone-implant interface and of the water-implant interface was determined. The bone-implant contact (BIC) was measured by histomorphometry and the degree of mineralisation of bone was estimated qualitatively by histologic staining. The significant decrease of the ultrasonic quantitative indicator r (p = 2.10⁻⁴) vs. healing time (from r = 0.53 to r = 0.49) is explained by (1) the increase of the BIC (from 27% to 69%) and (2) the increase of mineralization of newly formed bone tissue, both phenomena inducing a decrease of the gap of acoustical impedance.

Mode III cleavage of a coin-shaped titanium implant in bone: Effect of friction and crack propagation

Endosseous cementless implants are widely used in orthopaedic, maxillofacial and oral surgery. However, failures are still observed and remain difficult to anticipate as remodelling phenomena at the bone-implant interface are poorly understood. The assessment of the biomechanical strength of the bone-implant interface may improve the understanding of the osseointegration process. An experimental approach based on a mode III cleavage mechanical device aims at understanding the behaviour of a planar bone-implant interface submitted to torsional loading. To do so, coin-shaped titanium implants were inserted on the tibiae of a New Zealand white rabbit for seven weeks. After the sacrifice, mode III cleavage experiments were performed on bone samples. An analytical model was developed to understand the debonding process of the bone-implant interface. The model allowed to assess the values of different parameters related to bone tissue at the vicinity of the implant with the additional assumption that bone adhesion occurs over around 70% of the implant surface, which is confirmed by microscopy images. The approach allows to estimate different quantities related to the bone-implant interface such as: torsional stiffness (around 20.5 N m rad(-1)), shear modulus (around 240 MPa), maximal torsional loading (around 0.056 N.m), mode III fracture energy (around 77.5 N m(-1)) and stress intensity factor (0.27 MPa m(1/2)). This study paves the way for the use of mode III cleavage testing for the investigation of torsional loading strength of the bone-implant interface, which might help for the development and optimization of implant biomaterial, surface treatment and medical treatment investigations.

Finite element simulation of ultrasonic wave propagation in a dental implant for biomechanical stability assessment

Dental implant stability, which is an important parameter for the surgical outcome, can now be assessed using quantitative ultrasound. However, the acoustical propagation in dental implants remains poorly understood. The objective of this numerical study was to understand the propagation phenomena of ultrasonic waves in cylindrically shaped prototype dental implants and to investigate the sensitivity of the ultrasonic response to the surrounding bone quantity and quality. The 10-MHz ultrasonic response of the implant was calculated using an axisymetric 3D finite element model, which was validated by comparison with results obtained experimentally and using a 2D finite difference numerical model. The results show that the implant ultrasonic response changes significantly when a liquid layer is located at the implant interface compared to the case of an interface fully bounded with bone tissue. A dedicated model based on experimental measurements was developed in order to account for the evolution of the bone biomechanical properties at the implant interface. The effect of a gradient of material properties on the implant ultrasonic response is determined. Based on the reproducibility of the measurement, the results indicate that the device should be sensitive to the effects of a healing duration of less than one week. In all cases, the amplitude of the implant response is shown to decrease when the dental implant primary and secondary stability increase, which is consistent with the experimental results. This study paves the way for the development of a quantitative ultrasound method to evaluate dental implant stability.

Ex vivo estimation of cementless acetabular cup stability using an impact hammer.

Obtaining primary stability of acetabular cup (AC) implants is one of the main objectives of press-fit procedures used for cementless hip arthroplasty. The aim of this study is to investigate whether the AC implant primary stability can be evaluated using the signals obtained with an impact hammer. A hammer equipped with a force sensor was used to impact the AC implant in 20 bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the inserted AC implant was impacted four times with a maximum force comprised between 2500 and 4500 N. An indicator I was determined based on the partial impulse estimation and the pull-out force was measured. The implant stability and the value of the indicator I reached a maximum value for an interference fit equal to 1 mm for 18 out of 20 samples. When pooling all samples and all configurations, the implant stability and I were significantly correlated (R(2) = 0.83). The AC implant primary stability can be assessed through the analysis of the impact force signals obtained using an impact hammer. Based on these ex vivo results, a medical device could be developed to provide a decision support system to the orthopedic surgeons.

Variation of biomechanical properties of newly formed bone tissue determined by nanoindentation as a function of healing time

nanoindentation as a function of healing time R. Vayron, E. Barthel, V. Mathieu, E. Soffer, F. Anagnostou and G. Haiat* Modélisation et Simulation Multi Echelle, CNRS, Université Paris-Est, UMR CNRS 8208, 61, Avenue du Général de Gaulle, 94010 Créteil Cedex, France; Laboratoire CNRS/Saint Gobain UMR 125, Surface du Verre et Interfaces, 39, Quai Lucien Lefranc, 93303 Aubervilliers Cedex, France; Laboratoire de Recherches Orthopédiques, CNRS, Université Paris 7, UMR CNRS 7052, 10, Avenue de Verdun, 75010 Paris, France