Effect of type of polymerization on different properties of dental composites.

The aim of this study is to assess the influence of plasma lamps on the properties of the composites compared to the influence of conventional polymerization. Vickers hardness tests, three-point bending tests, and measurement of the shrinkage marginal gap by scanning electron microscopy were carried out on three resin composites (Tetric Ceram, Z-100 and Inten-S) irradiated with to lamps (Flipo) plasma and Astralis 7 halogen lamps). With a 3-second exposure, the results of Vickers hardness and resistance to flexion (excepting values for Z-100) were lower for the composites cured by the Flipo plasma lamp, than after 40-second curing by the conventional halogen lamp (Astralis 7), notably at a depth of 3 mm. With a 5-second exposure the results of Vickers hardness and resistance to flexion obtained using the plasma lamp approached those obtained by using the halogen lamp. Whatever the polymerization protocol used, the measurements of the gap between the tooth and the filling are very similar except for Z-100/Astralis 7, for which shrinkage results are more important. For any one resin composite and lamp used, the shrinkage values obtained at a depth of 4 mm are twice higher than those obtained at the surface. In conclusion, for a 3-second exposure the level of polymerization obtained by plasma curing is lower than the one obtained by halogen curing, particularly in depth. On the other hand, 5-second plasma curing results recommends the use of this kind of lamp.

Mechanical properties of two orthodontic adhesive materials polymerized with halogen and plasma curing.

OBJECTIVES: The aim of this study is to compare the efficiency of two polymerization techniques (halogen curing--Astralis 7 and plasma curing--Flipo), with two orthodontic adhesive materials (Enlight, a composite resin, and Fuji Ortho LC, a glass ionomer cement). METHODS: The efficiency of the polymerization techniques was shown by two mechanical tests. The hardness test was carried out on the exposed and non-exposed surfaces using 10 x 4 x 3-mm samples, polymerized either by halogen curing (40 seconds) or by plasma curing (5 seconds). The three-point bending tests were carried out on 2 x 2 x 25-mm samples polymerized as above. The samples were kept 1 hr at room temperature, then for 24 hrs in distilled water at 37 degrees C. RESULTS: Whatever the polymerization technique used, the results are similar for hardness and flexion, with the exception of the hardness tests carried out after polymerization with the Flipo light on the surface not directly exposed. CONCLUSION: In orthodontic practice, both polymerization techniques can be used. But a multi-bracket session can be long, and the reduction of time spent in the chair obtained by using plasma lamps seems to make this technique preferable.

Influence of autoclave sterilization on the surface parameters and mechanical properties of six orthodontic wires.

Orthodontic wires are frequently packaged in individual sealed bags in order to avoid cross-contamination. The instructions on the wrapper generally advise autoclave sterilization of the package and its contents if additional protection is desired. However, sterilization can modify the surface parameters and the mechanical properties of many types of material. The aim of this research was to determine the influence of one of the most widely used sterilization processes, autoclaving (18 minutes at 134 degrees C, as recommended by the French Ministry of Health), on the surface parameters and mechanical properties of six wires currently used in orthodontics (one stainless steel alloy: Tru-Chrome RMO; two nickel-titanium shape memory alloys: Neo Sentalloy and Neo Sentalloy with Ionguard GAC; and three titanium-molybdenum alloys: TMA(R) and Low Friction TMA Ormco and Resolve GAC). The alloys were analysed on receipt and after sterilization, using surface structure observation techniques, including optical, scanning electron and atomic force microscopy and profilometry. The mechanical properties were assessed by three-point bending tests. The results showed that autoclave sterilization had no adverse effects on the surface parameters or on the selected mechanical properties. This supports the possibility for practitioners to systematically sterilize wires before placing them in the oral environment.

Trabecular alveolar bone microarchitecture in the human mandible using high resolution magnetic resonance imaging.

OBJECTIVES: Cancellous bone microarchitecture in the mandible can influence the success of dental implant osseointegration. The aims of this study were to explore the feasibility of two-dimensional (2D) high resolution magnetic resonance imaging (MRI) for the evaluation of trabecular bone architecture and to compare architecture parameters derived from MR images between different areas in the mandible, and between sex and dental status. METHODS: 45 mandibular bone specimens (8 mm thickness) were cut from 15 fresh cadavers. MR images were obtained at 2 T using a multislice 2D spin-echo sequence. After MR image binarization, histomorphometry parameters, i.e. bone area fraction (BAF), specific perimeter (La), trabecular bone width (Tb.Wi), trabecular bone separation (Tb.Sp) and trabecular network anisotropy (R), were computed from the mean intercept length. The angle between trabeculae and tooth axis and the Euler-Poincaré characteristics (EPCs) were also computed. RESULTS: BAF, Tb.Wi and R were significantly higher in male specimens compared with female specimens, whereas Tb.Sp was significantly lower. The apparent Tb.Wi was found to be significantly higher in dentate specimens compared with edentulous ones. The highest anisotropy, corresponding to the smallest R value, was seen in the incisal specimens. The preferential orientation of the trabeculae was close to perpendicular to the tooth axis, especially in the molar specimens. CONCLUSION: BAF, La, Tb.Wi, Tb.Sp and R, the most variable parameters, may potentially have a relationship with the biomechanical competence of trabecular bone and play a role both in primary stabilization of dental implant and the time needed before loading.

Electrochemical studies of the corrosion behaviour of titanium and the Ti-6Al-4V alloy using electrochemical impedance spectroscopy.

OBJECTIVES: The aim of this study was to compare the electrochemical behaviour of two materials used in oral implantology. METHODS: The resistance to corrosion of Ti grade 2 and the alloy Ti-6Al-4V was studied in an artificial saliva solution. It has been observed that the passivation of titanium by an oxidised layer can be shown both by cyclic voltametry and by electrochemical impedance spectroscopy. Moreover, this latter technique, rarely used in odontology, opens up interesting perspectives, enabling a more quantitative approach to the resistance of the passive layer to be adopted. RESULTS: Also, the impedance data recorded for Ti grade 2 and the alloy Ti-6Al-4V, in the artificial saliva solution were shown that Ti grade 2 has a higher resistance to corrosion and a thicker oxide layer than the alloy Ti-6Al-4V. SIGNIFICANCE: The fact that the electrochemical properties of Ti-6Al-4V are lower than that titanium's ones indicate than a release of ions aluminum and or vanadium ions in the body can occur. This is why we recommend to the dental practitioners to preferably use titanium in implantology.

Influence of gamma-irradiation sterilization on the structural defects of sapphire single crystals (alpha-Al(2)O(3)).

The influence of sterilization by gamma rays on the structure and the electrical behaviour of sapphire single crystal (alpha-Al(2)O(3)) was studied successively by thermoluminescence, by cathodoluminescence and by observation of the scanning electron microscope mirror effect. The mirror method allowed us to measure the capacity of an insulating material to trap electrons. The structural analysis of the alpha-Al(2)O(3) showed that there were oxygen vacancies, as well as chromium and titanium impurities. It was possible to demonstrate that these defects, especially the oxygen vacancies, are in a different state after a 30 kilogray irradiation. The valency state changes of these defects and the presence of trapped charges are accompanied by a deformation of the crystalline lattice which results in a modification of its electrical properties. At room temperature, the irradiated alpha-Al(2)O(3), unlike non irradiated alpha-Al(2)O(3), is capable of trapping electrons. It can be concluded that gamma-ray sterilization modifies the cohesive energy of alpha-Al(2)O(3), which could lead to mechanical changes (surface charge, friction, wear, fracture strength, em leader) in this material.

Comparison of corrosion behaviour in presence of oral bacteria.

The aim of this study was to compare the resistance of the corrosion of dental alloys in a solution containing oral bacteria named Actinomyces viscosus (ATCC19246). In this paper, we explain the choice of this precise species of bacteria, then specify its culture in artificial saliva and the experimental precautions needed to avoid the pollution by other bacteria. The electrochemical behaviour of two dental alloys (Ni-Cr alloy and gold-based alloy) was investigated by electrochemical means in sterile Fusayama artificial saliva (AS), AS enriched with sterile yeast extract (YE) and YE modified by introducing bacteria (AV). Open-circuit potentials, potentiodynamic curves, polarization resistance and impedance spectroscopy are the electrochemical procedures selected for this work. It has thus been shown that the open-circuit potential of the non-precious alloy is always lower than that of the gold precious alloy, and the colonization of metal surface by bacteria caused a drop in open circuit potential. The electrochemical impedance spectroscopy results have shown that the electrolyte resistance decreased between the AS, YE and AV milieu, in the presence of bacteria a slight decrease in polarization resistance was observed with the precious alloy and an increase with the non-precious alloy. The drop in the electrolyte resistance cannot explain the change in polarization resistance. The influence of Actinomyces viscosus might be essentially due to the consumption of oxygen at the metal/electrolyte interface of the specimen. For the non-precious alloy, the absence of oxygen (instigator of corrosion) led to an increase in polarization resistance whereas the slight decrease for the precious alloys might be justified by the organic and inorganic metabolites released by bacteria in to the electrolyte. The scanning electron micrography after electrochemical analysis, confirmed the absence of contaminants. These preliminary results demonstrate the unquestionable influence of this bacteria on the corrosion behaviour of the alloys studied, however, further studies are necessary.

An image analysis method for the study of cell adhesion to biomaterials.

This fluorescence image analysis method for the quantitative determination of cell adhesion on biomaterials allows bone cells labelled with propidium iodide to be counted automatically, directly on their support. The reliability of the estimation by fluorescence image analysis was validated by comparison with visual counting and with results obtained by an electronic particle counter. In this way it was possible to demonstrate that the adhesive properties of bone cells are dependent on the type of substrate--enstatite (MgO, SiO2, CaO-P2O5-Al2O3), Thermanox (modified polyethyleneterephthalate), or glass. In contrast, the spread of the cell cytoplasm, labelled with fluorescein isothiocyanate and measured by image analysis, does not vary significantly according to the substrate. The characterisation by SKIZ tessellation of the spatial cell arrangement shows that the bone cells have a random organisation on Thermanox and glass, whereas they form aggregates on enstatite.

Magnetic resonance imaging of rodent teeth.

In this paper, the anatomical structures of rat jaws are studied using magnetic resonance imaging (MRI) with high spatial resolution. In vivo microimages of teeth from 3- and 12-week-old (young and adult) rats have been performed. A 2 T nuclear magnetic resonance microscope was used to collect, with multiple orientations, non-invasive 2D data images using the spin-echo technique. MRI appears well suited to give images of the oral area and may represent a useful tool for diagnosis of dental diseases and more particularly of pulp pathologies.

Measurement and evaluation of galvanic corrosion between titanium/Ti6A14V implants and dental alloys by electrochemical techniques and auger spectrometry.

The purpose of this study was to investigate, in different experimental conditions, the galvanic corrosion phenomena which can exist between a dental suprastructure and a dental implant. The electrochemical behavior of 7 alloy superstructures with titanium and titanium alloy (Ti6A14V) implants was investigated by electrochemical means in Fusayama-Meyer de-aerated saliva and Carter-Brugirard (AFNOR) non de-aerated saliva. Different techniques were used to obtain the value of the galvanic coupling current and potential for each couple. All showed very low corrosion rates, ranging from 10(-6) to 10(-8) A. Surface analysis confirmed these results.

Effects of new machinable ceramic on behavior of rat bone cells cultured in vitro.

The behavior of cultured rat bone cells growing on modified polyethylene terephthalate (mPET), glass, and machinable ceramic substrates containing enstatite (MgO, SiO2) and glass (CaO-P2O5-Al2O3) was studied. Cell attachment was measured directly on the substrates using an image analysis system. Electron microscopy observations and the MTT test revealed that cells are able to spread and proliferate on the material surface, keeping a healthy ultrastructure on all materials tested in the present study. After having colonized the surface of the materials, as shown by immunocytochemistry, the cells synthesize an osteoid-like matrix composed of osteocalcin, type I collagen, and fibronectin fibrils. The titration of alkaline phosphatase activity showed that the cells grown on the ceramic exhibit a greater osteogenic activity than those grown on controls (glass and mPET). This osteogenic activity results in a mineralization of the extracellular matrix in cultures on ceramic or plastic whereas only few calcium phosphate crystallite traces were revealed by Von Kossa staining on glass. Enstatite constitutes, therefore, an environment compatible with in vitro bone cell life.

Cytokeratin expression in human oral gingival epithelial cells: in vitro regulation by titanium-based implant materials.

To evaluate whether cytokeratin expression in human oral epithelial cells could be influenced by implant materials used in dental surgery, passaged human oral gingival epithelial cells were seeded on commercially pure titanium (CP-Ti) or on Ti6Al4V titanium alloy. Confluence was achieved after about 15 days on both substrates. Cells formed at that time, an organized layer of densely packed polygonal cells, and harbored a filamentous cytokeratin network typical of epithelial cells. Immunochemistry and immunoblot analysis were used to detect modifications of the amount of individual CK polypeptides (CK7, 8, 13, 18 and 19) in function of the culture substrate. Results showed that the level of CK8, CK18 and CK19 expression was not altered whatever the culture substrate used. The expression of CK13 was reduced in epithelial cells cultured on the titanium alloy, as compared with commercially pure titanium. Conversely, the level of CK7 was higher on the Ti6Al4V alloy than on commercially pure titanium. This study suggests that titanium-based implant materials could influence differently the phenotype of oral gingival epithelial cells.

A metabolism study of human masseter muscle by 31P magnetic resonance spectroscopy during long periods of exercise and recovery.

The metabolism of the human masseter muscle was investigated using phosphorus (31p) magnetic resonance spectroscopy (MRS) during long periods of exercise and recovery. Eleven subjects aged 19 to 28 yr were examined by 31p MRS during four consecutive periods of 13 min each: rest, exercise, recovery 1 and 2. For each subject, a biting force equal to 20% of maximum voluntary biting force was applied and controlled during the exercise period to produce maximum fatigue. 31p MR spectra were localized from a 24 cm3 volume of interest using an image selected in vivo spectroscopy (ISIS) sequence and a 6 cm diameter surface coil placed on the left masseter. Compared to the resting level, the phosphocreatine (PCr) content decreased by 26% during exercise, while the inorganic phosphate (Pi) concentration increased by 65%. During the two recovery periods, the Pi content remained decreased compared with the resting level by 36% and 30%, respectively. The Pi/PCr ratio was increased from 0.30+/-0.04 at rest to 0.63+/-0.13 during exercise while the adenosine triphosphate (ATP)/Pi ratio was decreased. The pH decreased from 7.02+/-0.03 to 6.93+/-0.04 during exercise and returned to control level (7.09+/-0.08) only during the second recovery period. These results suggest that the masseter muscle is characterized by high ATP turnover and, therefore, high oxidative phosphorylative activity in agreement with its constitution of predominantly fatigue resistant type I fibers.

In vitro magnetic resonance imaging of rodent teeth.

OBJECTIVE: The anatomic structure of rat teeth was studied and observed using magnetic resonance imaging with high spatial resolution. STUDY DESIGN: The right part of the low mandible of two rats of 3 and 12 weeks old were analyzed. Images with different orientations were performed in a 2 Tesla magnetic field using the spin-echo imaging technique. RESULTS: Highly spatially resolved images revealed details of teeth, and anatomic differences between a young and an adult rat were demonstrated. CONCLUSION: Magnetic resonance imaging is well suited to image the buccal area and may be a useful tool for the diagnosis of dental diseases.

[Anatomical study with MRI of a small muscle: the masseter]. / Etude anatomique par IRM d'un muscle de petite dimension: le masséter.

Magnetic resonance imaging (MRI) provides functional information in an anatomic presentation allowing to distinguish soft tissues with high sensitivity. The goal of this study was to investigate the normal anatomy of the major masticatory muscle, the masseter, both at rest or during contraction by using three dimensional (3D) MRI. Eighteen subjects aged from 19 to 28 years, all in good health, were studied. Several experiments were first realized on phantoms to test the 3D-MRI technique. After reconstruction and segmentation processing, 3D acquisition, enabled obtaining data on the masseter anatomy. The normal anatomical position of the masseter was reported to the skin plan as the mean internal distance (7.9 +/- 0.42 mm) and external distance (15.2 +/- 0.41 mm). While there was no difference between internal distance, for sex or side, the external distance was significantly (p = 0.02) shorter in male (7.7 +/- 0.5 mm) than in female (8.8 +/- 0.4 mm) for both sides. The mean volume for all subjects and both sides (20.3 +/- 1.1 cm3) did not change significantly between rest and exercise. The masseter volume was significantly (p < 0.00001) greater in male (24.2 +/- 2.0 cm3) than in female (16.4 +/- 3.6 cm3) groups. These physiological references may be useful for further MRI investigations of masticatory system pathologies.

Comparative evaluation of polishing systems on the surface of three aesthetic materials.

This study compared the efficacy of three different polishing methods on a resin composite, a glass ionomer cement (GIC) and a compomer. Thirty samples of each material were prepared. Ten of them were randomly assigned to one of the three polishing methods. The surface roughness (Ra) of the samples were measured before and after the polishing procedure with a profilometer. There was no statistical difference between the composite and the compomer prior to polishing but statistical difference was found between the GIC and the two other materials. After polishing, the smoothest surfaces were obtained using Sof-Lex discs for the three materials, and the roughest surface, with tungsten carbide burs for the composite and the compomer, and with Enhance kit for the GIC. The composite gave the smoothest surface with the three methods.

Magnetic resonance imaging for the determination of magnetic susceptibility of materials.

A magnetic resonance imaging method for volume magnetic susceptibility estimate of materials immersed in a liquid containing resonant nuclei is proposed. The method uses either the standard spin-echo Fourier imaging technique or the projection-reconstruction technique. Image distortions of cylindrical macroscopic susceptibility inhomomogeneities are analyzed and a correlation is established between the susceptibility value and the size of image deformation along the read gradient. For measurements, the susceptibility of the liquid surrounding the sample (here deionized water) serves as a reference, and characteristic distances between particular points, usually highlights in the distorted image, are related to the susceptibility difference between the two media. Different samples, mainly prosthetic materials with a large susceptibility range from 5 to 200 ppm in absolute value, have been investigated, and the accuracy of susceptibility determination is discussed.

[The demonstration of a relation between premolar force and molar force within the limits of a simplified approach to masticatory system functioning]. / Mise en évidence d'une relation entre la force prémolaire et la force molaire dans le cadre d'une approche simplifiée du functionnement de l'appareil manducateur.

The purpose of this study is to show the existence of a correlation between the premolar and molar clenching forces obtained during a voluntary clenching exercise. The study concerned 32 volunteers aged 21 to 28 with no manducatory problems. The forces were obtained using a device with four Kiowa traducers arranged in a complete Wheatstone bridge. The analysis of the results obtained showed that there was a positive correlation between maximum molar and premolar. The analysis also shows that three results are a direct application of the theory of momentum. This enables us to propose a simple biophysical model to explain the functioning of the masticatory system.

[The determination of the intensity of premolar and molar maximal forces during the isometric contraction of the masticatory muscles due to forced mandibular closure]. / Détermination de l'intennsité des forces maximales prémolaires et molaires au cours de la contraction isométrique des muscles masticateurs par fermeture forcée de la mandibule.

The purpose of this study to quantify the activity of the masticatory muscles. For this we recorded the maximum bite force of opposing teeth during voluntary clenching. The study was carried out on 31 students aged 21 to 28 with a satisfactory dental arch. The measurements were obtained on one side only. Nine thickness of the traducers were used for the premolar zone and two for the molar zone. We determined an optimal thickness enabling the highest measurement: 6.5 mm for the premolars and 5.2 for the molars. The highest measurements were averaged and the following results were obtained: premolars-35 kgf for females and 50 kgf for males; molars 100 kgf for females and 130 kgf for males.

Correlation between magnetic resonance imaging disturbances and the magnetic susceptibility of dental materials.

OBJECTIVES: The purpose of this study was to correlate the size and the orientation of image disturbances observed on specific samples of dental materials with their magnetic susceptibility. The measurement of the magnetic susceptibility was performed in the 10(-5) or 10(-6) range using MRI to establish a classification of dental materials. METHODS: Cylindrical dental alloy samples incorporating gold, silver, and palladium were placed in a Pyrex beaker filled with distilled water. Images were performed at 0.13 Tesla using two-dimensional Fourier transformation and projection reconstruction at 360 degrees imaging methods. The magnetic susceptibilities were obtained by measuring distances between spots having the highest intensity on the image. RESULTS: A very discriminating classification may be established on MRI criteria. This method permits one to determine the dia- or para-magnetic character of the dental materials tested. Only palladium-based alloys have been detected to be paramagnetic with kappa > 0. One of the silver-based alloys did not induce detectable distortion because its susceptibility was very close to that of distilled water. Based on this MRI data, the use of this material may be recommended for applications that may be subjected to MRI evaluation. SIGNIFICANCE: With the increasing use of MRI as a diagnostic tool, it is useful to establish a classification of prosthetic biomaterials compatible with MRI.