A comparison of pressure generated by cordless gingival displacement techniques.

STATEMENT OF PROBLEM: Handling properties of cordless gingival displacement materials is not well understood, resulting in incorrect use. Insufficient displacement of the gingival margin may result in a poor impression. PURPOSE: This study investigated the pressure generated by a cordless displacement paste with respect to different techniques. MATERIAL AND METHODS: Two chambers with dimensions of 5 × 5 × 2 mm were made from Type IV stone and silicone material to simulate a rigid and elastic environment. A pressure gage was embedded into the wall of the chamber, and a paste material (Expasyl) was injected into the different chambers. The final pressures generated by the Expasyl were recorded by Chart 5 software and Power Lab system. This was repeated by using a displacement cord (KnitTrax) as a control for the study. The different loading methods for the Expasyl material were compared with 1-way ANOVA (α=.05). RESULTS: The mean pressure generated during placement of the Expasyl paste material in the silicone chamber was 143 kPa, which is significantly lower (P=.001) than the pressure generated by the KnitTrax cord (5396 kPa). Manipulating Expasyl after placement resulted in a pressure reduction of 73% in the stone chamber and 29% in the silicone chamber. CONCLUSIONS: Pressure generated by Expasyl is minimal compared to the cord system. Pressure is generated during the injection of the Expasyl, and subsequent manipulation reduced the final pressure. Handheld and motorized delivery guns produce similar pressure, but the motorized gun was found to have a more constant pressure delivery.

Development of a dental model hardener.

The paper presents a novel method of strengthening dental gypsum materials by infiltrating with a low-viscosity autopolymerising monomer. Gypsum specimens were infiltrated with diluted butyl-cyanoacrylate monomer at different concentrations. Nanoindentation was employed to measure their hardness and elastic modulus. A standard single-edge-notched beam (SENB) method was used to measure fracture toughness. Biaxial strength was measured using a universal test machine with a piston-on-ring jig. Scanning Electron Microscopy (SEM) and affiliated energy dispersive X-ray spectroscopy (EDX) were used to observe the fracture topography and the depth of infiltration. Infiltration and autopolymerisation of the cyanoacrylate improved the biaxial strength and fracture toughness of the gypsum by around 39% and around 30%, respectively. The hardness and elastic modulus for the treated specimens were also higher, albeit slightly. The proposed method provides a convenient way to strengthen and repair dental gypsum models in situ.

A novel polymer infiltrated ceramic for dental simulation.

Simulation of tooth preparation using rotary cutting instruments is viewed as beneficial and essential in dental training. Various types of materials have been used for simulation systems in dental preclinical training. However, the phantom tooth materials used for simulation have not changed significantly for decades and they are acknowledged to be different from natural teeth. This study investigated the mechanical properties and microstructure of a widely used phantom tooth material and compared them with a novel, polymer infiltrated, ceramic. It was concluded that the polymer infiltrated ceramic has mechanical properties more similar to natural teeth than current phantom tooth materials, suggesting that it might be a good candidate material for phantom teeth for trainees to acquire initial tactile sense for tooth preparation.

Micromechanical evaluation of mineralized multilayers.

The biomechanical stability of osseointegrated implants is of particular importance, especially the stability which is achieved from structural manipulation at the interface between the implant surface and the bone tissues. Nanoscale beta-tricalcium phosphate-immobilized titanium was prepared by discharge into a physiological buffered saline solution. Compared with hydroxyapatite, it has been shown to be effective in generating a bone-like chemical structure on the surface by cooperative interaction between osteoblastic cells and the beta-tricalcium phosphate. The present study, after cell cultivation, investigates the nanostructures and biomechanical property differences of a mineralized layer formed on two samples of nano-calcium phosphate-immobilized titanium. A scanning probe microscope study revealed that the mineralized tissue formed on the beta-tricalcium phosphate samples after 1 week of cell culture showed significantly higher roughness, compared with hydroxyapatite samples. Nanoindentation micromechanical evaluation of the in vitro generated multilayered structures exhibited thicker bone-like mineralized layers on the beta-tricalcium phosphate samples. A successful modification of titanium implants through the cooperative interaction between osteoblastic cells and nano beta-tricalcium phosphate is anticipated.

[Determination and Weibull analysis of the strength of dental veneering porcelains].

OBJECTIVE: To determine the strength of the veneering porcelain stored in artificial saliva. METHODS: Standard beams with a size of 3 mm x 2 mm x 20 mm made from the body, incisal, and translucent porcelains of SHOFU Vintage were stored in the artificial saliva for 30 days. Twenty samples from each material were tested for the strength by the 3-point flexural test at a crosshead speed of 0.5 mm. The data were analyzed by the two-parameter Weibull method. RESULTS: The strength, Weibull modulus (m) and characterization strength (male0) were (75.5 +/- 5.6) MPa, 17.3, and 77.6 MPa for Body porcelain, respectively; (73.6 +/- 5.0) MPa, 16.5, and 76.0 MPa for incisal porcelain, respectively; and (72.9 +/- 4.5) MPa, 16.1, and 75. 5 MPa for translucent porcelain, respectively. CONCLUSION: There are no differences in strength between the three dental porcelains. Weibull analysis describes the strength of the porcelains better than the average and standard deviation.

Enamel - a "metallic-like" deformable biocomposite.

OBJECTIVES: The aim of this study is to compare the mechanical responses of enamel with dental-used metals and to show that enamel has mechanical properties similar to metals rather than ceramics. METHODS: Four dental-used metals: cast alloy, gold alloy, titanium and amalgam, were compared with enamel. Pure hydroxyapatite (HAP) was used as the ceramic analogue of enamel. A Berkovich and a spherical indenter were selected for nanoindentation tests. The stress-strain (H-a/R) curves and indentation creep behaviour were investigated and analyzed. RESULTS: Although the primary composition of enamel is hydroxyapatite, the indentation stress-strain curves and creep behaviour of enamel was totally different to HAP. Enamel had similar stress-strain response to that of cast alloy and gold alloy, all of which showed work-hardening effect. Titanium and amalgam had curves that showed lower stress at comparable strain than enamel and showed no work-hardening. Amalgam exhibited the greatest creep behaviour, followed by Titanium, enamel and gold alloy. Similar to HAP, the cast alloy had very limited creep response. Only enamel showed significant backcreep at minimum load. This may be a consequence of the memory behaviour of the minor protein component within enamel. CONCLUSIONS: The small remnant volume fraction of protein fragments have endowed enamel with metallic-like mechanical properties, which impart it with an ability to sustain repetitive cyclic contact loading over the life of the host. It may be better to choose metallic-like dental restorative materials to sustain the severe cyclic contact behaviour experienced and to protect the opposing teeth from excessive damages.

Characterisation of enamel white spot lesions using X-ray micro-tomography.

OBJECTIVES: The aim of this study was to characterise the mineral density (MD) of natural enamel white spot lesions (WSLs) using X-ray micro-tomography calibrated with different density hydroxyapatite phantoms. METHODS: Seven natural WSLs from four extracted non-carious premolar teeth were scanned at a voxel size of 7.6 microm using a desktop X-ray micro-tomography system. Five hydroxyapatite phantoms (sintered pellets of hydroxyapatite powder) with densities ranging from 1.52 to 3.14 g/cm(3) were used as calibration standards for each scan. Three-dimensional image reconstruction enabled MD gradients throughout the lesion to be quantified using an MD calibration equation derived from hydroxyapatite phantoms. Background noise generated during the measurement of MD was reduced using a Gaussian filter. RESULTS: Gaussian filter reduced the signal-to-noise ratio (standard deviation) significantly while the basic MD information (average value) remained intact. The mineral gradients through the WSLs examined were compared and are discussed in terms of existing literature. The MD of sound enamel, apparent intact surface layer of WSL, and lowest level of WSL was found to be 2.65-2.89 g/cm(3), 2.23-2.58 g/cm(3) and 1.48-2.03 g/cm(3), respectively. Our MD results are comparable with other studies. CONCLUSIONS: X-ray micro-tomography is a sensitive in vitro technique capable of characterising and quantifying MD of small non-cavitated WSLs. This method has a promising potential for future carious and quantitative remineralisation studies.

[A novel synthesized Schistosoma japonicum miracidium attractant imprinted polymer].

A novel molecularly imprinted polymer with adequate attractability for Schistosoma japonicum miracidia was prepared. When adulterated with polyvinyl alcohol (PVA), the fabricated film with good swelling property was formed which can suspend on water and slowly release XF (a chemical to be published). This reusable film can well attract Schistosoma japonicum miracidia, and hopefully be used in the prevention of schistosome infection.

Investigation of dentin hardness in roots exhibiting the butterfly effect.

INTRODUCTION: Most vertical root fractures occur in root canal treated teeth, and they usually run in a buccolingual direction. The butterfly effect is an optical phenomenon seen in some sections of tooth roots. The aim was to investigate the microhardness of dentin in mesiodistal and buccolingual cross sections of roots exhibiting the effect. METHODS: Thirty extracted single-rooted teeth were allocated according to patient age: group 1, 15-24 years; group 2, 25-44 years; and group 3, 45 years and older. Roots were embedded in acrylic and cut into ten 1-mm-thick cross sections. Sections were viewed under a light microscope and coded (1 or 2) according to presence or absence of the butterfly effect. A root scored 20 when all levels featured the butterfly appearance. The 2 teeth with the highest score from each group and 2 control teeth with the minimum score (10) were selected. Two adjacent, consecutive cross sections were chosen from the middle of the roots. Vickers microhardness testing was carried out on the dentin walls. RESULTS: Mean hardness scores were highest mesiodistally (83.7 kgf/mm(2)) and lowest buccolingually (56.4 kgf/mm(2)), a significant difference (P = .028). This trend was found across all age groups. CONCLUSIONS: Root sections with the butterfly effect are harder mesiodistally. This might explain the high prevalence of vertical root fractures that run buccolingually.

A natural functionally graded biocomposite coating--human enamel.

Human enamel has been found to be a coating with excellent mechanical performance, and has undergone extensive investigation and discussion. However, most of the reported studies consider the enamel as a homogeneous anisotropic biocomposite. The current study illustrated the graded properties of the biocomposite from its functional load-bearing direction. Within the thickness of the enamel, from the outer surface towards the enamel-dentin junction (EDJ), the elastic modulus (E(x)) and hardness (H(x)) of enamel exist in an exponential relationship with normalized thickness (x) as E(x)=111.64x(0.18) (R(2)=0.94) and H(x)=4.41x(0.16) (R(2)=0.87) GPa, respectively. Moreover, the creep ability of enamel increases towards the EDJ. The graded properties of the biocomposite can be explained by both microstructural and compositional changes along the thickness of the material towards the EDJ. Finite element analysis indicates that the graded properties of enamel have important roles in reducing the enamel-dentin interface stresses and maintaining the integrity of the multilayer tooth structure. The results provide a new angle to understand the excellent mechanical behaviour of the multilayer tooth structure and may inspire the development of new functionally graded materials and coating structures.

An evaluation of dental operative simulation materials.

The study was to evaluate the performance of different materials used in dental operative simulation and compare them with those of natural teeth. Three typical phantom teeth materials were compared with extracted permanent teeth by a nanoindentation system and evaluated by students and registered dentists on the drilling sensation of the materials. Moreover, the tool life (machinability) of new cylindrical diamond burs on cutting the sample materials was tested and the burs were observed. Although student and dentist evaluations were scattered and inconclusive, it was found that elastic modulus (E) and hardness (H) were not the main factors in determining the drilling sensation of the materials. The sensation of drilling is a reflection of cutting force and power consumption.An ideal material for dental simulation should be able to generate similar drilling resistance to that of natural tooth, which is the machinability of the material.

A novel polymer infiltrated ceramic dental material.

OBJECTIVES: To evaluate the mechanical behavior of a prototype porous ceramic interpenetrating polymer-ceramic material containing 15-20% polymer. METHODS: After sample preparation, elastic modulus, hardness, stress-strain relationship and indentation creep response were measured by a nanoindentation system. Fracture toughness was measured by the single-edge-notched beam (SENB) method. SEM was employed to observe the fractured surface and analyze the fracture mechanisms. RESULTS: The polymer infiltrated ceramic material has elastic modulus, hardness, and fracture toughness values of 30.14GPa, 2.59GPa, and 1.72 MPam(1/2), respectively. The material illustrates a significant indentation size effect for elastic modulus and hardness, and has similar indentation creep behavior to human enamel. Manufacturing procedures such as the density of pre-infiltrated porous ceramic and processing pressure influence the final properties of the material. SIGNIFICANCE: This polymer infiltrated ceramic material is anticipated to become a new member of the dental CAD/CAM family.

In vitro demineralisation of the cervical region of human teeth.

OBJECTIVE: The aim of this study was to investigate a possible role for demineralisation of the cervical region of human teeth in the development of non-carious cervical lesions (NCCLs). MATERIALS AND METHODS: Freshly extracted human premolars were demineralised and prepared for nanoindentation and scanning electron microscope (SEM) observation. After 1 day or 2 days demineralisation in a solution of pH 4.5, specimens were embedded, cut and polished to 1 µm diamond paste. Nanoindentation was done at the cementum-enamel junction (CEJ) region with an interval of 30 µm, to develop mechanical properties maps. After the indentation, SEM with back-scatter detector was employed to observe the degree of demineralisation at the CEJ. RESULTS: After 1 day and 2 days demineralisation, the mechanical properties of enamel and dentine at the CEJ decreased by ∼50% and ∼90%, respectively. SEM images illustrate that artificial demineralisation generated typical demineralised zones in enamel near the CEJ. Moreover, 2 days demineralisation penetrated the sound enamel at the CEJ, and the dentine beneath was undermined. CONCLUSION AND SIGNIFICANCE: One day and 2 days demineralisation reduced the mechanical properties of teeth at the CEJ significantly. Demineralised enamel and dentine with low mechanical properties are prone to wear and abrasion. The findings of the investigation indicate that acid typical of that produced by dental plaque may compromise the mechanical properties of enamel and dentine at the CEJ to the extent that they would be susceptible to tooth brush abrasion, producing NCCLs.

A suitable base material for composite resin restorations: zinc oxide eugenol.

OBJECTIVE: This in vitro study evaluated the effects of a zinc oxide eugenol (ZOE) base on the mechanical properties of a composite resin restoration. METHODS: Class I cavities were prepared on plastic teeth and filled with ZOE plus composite resin, following standard clinical procedures. The samples were sectioned sagittally and the ZOE-resin interface was exposed. After polishing, nanoindentation was performed on the region near the interface, and elastic modulus and hardness were plotted in the form of a color contour map. SEM was employed to observe the interface between composite resin and ZOE base. RESULTS: In the region close to the ZOE base, the elastic modulus and hardness of composite resin reduced to the values of 9.71+/-0.54 and 0.51+/-0.05 GPa, respectively. Eugenol from ZOE had detrimental effects on the composite resin only to a distance of less than 100 microm from the ZOE base. CONCLUSION: Although eugenol suppresses polymerization slightly, by considering the biological advantages of ZOE, together with the results of the current investigation, ZOE may still be considered a suitable base material for composite resin. Bonding is essential for composite resin restorations over ZOE bases to avoid shrinkage detachment.

A micro-mechanical evaluation of the effects of die hardener on die stone.

The purpose of this study was to investigate the properties of a die hardener penetrated layer and evaluate its protective effects on the surface of die stone. A commercial die hardener (PDQ die hardener, Whipmix corp., USA) was tested on a die stone (GC Fujirock EP die stone, GC Europe, Belgium) and a dental plaster (Dental Stone, United States Gypsum Company, USA). Nanoindentation and micro-scratch tests were performed on both coated and uncoated specimens. The scratch damage was observed by SEM and the penetration depth of die hardener was detected by the affiliated EDX. Upon drying, the die hardener penetrated into the die stone to a depth of 3-5 microm, and deposited a thin film on the surface of die stone. Although the die hardener penetrated layer did not show improved mechanical properties, the die hardener film on the surface did protect the specimens from abrasion damage.

Correlation of mineral density and elastic modulus of natural enamel white spot lesions using X-ray microtomography and nanoindentation.

Our objectives were to correlate the mineral density (MD) and elastic modulus (E) of natural white spot lesions (WSLs) and compare them with analytical and numerical models. Five natural WSLs from four extracted sound premolar teeth were scanned at a voxel size of 7.6µm using a desktop X-ray microtomography (XRMT) system. Five hydroxyapatite phantoms with densities ranging from 1.52 to 3.14gcm⁻³ were used as calibration standards for each scan. MD throughout the WSLs was quantified using an MD calibration equation derived from hydroxyapatite phantoms. Subsequently, teeth were cross-sectioned and the E modulus was measured systematically across the WSLs at intervals of 25 and 50µm using nanoindentation. The MD and E modulus of WSLs correlated well. The relationship may be expressed as E=E°exp(-bP) (R²=0.952) with E° the elastic modulus of the fully dense material, P the porosity and b a constant. The results for sound enamel were compared with Spears model. The limitation of Spears model to the WSLs is discussed and an alternative model developed by Rice for porous materials is proposed. Clinical implications of this work for quantifying de-/remineralization of teeth are pointed out. We conclude that XRMT can be utilized to extrapolate the E modulus of WSLs. This provides a basis for non-destructive, longitudinal analysis of WSLs in de-/remineralization studies of enamel.

Enamel--a functionally graded natural coating.

OBJECTIVE: The aim of this study is to illustrate the graded proper properties of enamel from the outer (near occlusal surface) to the inner region (near enamel-dentine junction) in a cross-sectioned surface and discuss how natural design achieve the graded functions. METHODS: Nanoindentation, Raman spectroscopes, and SEM were employed to compare the inner and outer regions of the cross-sectioned enamel from different angles, namely mechanical properties such as elastic modulus and hardness, indentation energy absorption ability, indentation creep ability, indentation residual stress distribution pattern, compositional differences, and microstructural differences. RESULTS: Inner enamel has lower elastic modulus and hardness but higher creep and stress redistribution abilities than outer counterpart, which is related to the gradual compositional change through the enamel. SIGNIFICANCE: Enamel can be regarded as a functionally graded natural biocomposite, which will require special attention using numerical analysis to fully appreciate the consequences of such a structure for the mechanical behaviour of teeth and restorations placed therein. Moreover, the smart design of nature can be a good model for us in functional graded materials/coatings design and development.

Nanoindentation creep behavior of human enamel.

In this study, the indentation creep behavior of human enamel was investigated with a nanoindentation system and a Berkovich indenter at a force of 250 mN with one-step loading and unloading method. A constant hold period of 900 s was incorporated into each test at the maximum load as well at 5 mN minimum load during unloading. The indentation creep at the maximum load and creep recovery at the minimum load was described with a double exponential function and compared with other classic viscoelastic models (Debye/Maxwell and Kohlrausch-Williams-Watts). Indentation creep rate sensitivity, m, of human enamel was measured for the first time with a value of approximately 0.012. Enamel displayed both viscoelastic and viscoplastic behavior similar to that of bone. These results indicate that, associated with entrapment of particulates between teeth under functional loading and sliding wear conditions, the enamel may inelastically deform but recover upon its release. This behavior may be important in explaining the excellent wear resistance, antifatigue, and crack resistant abilities of natural tooth structure.

[The effect of ion-exchange technique on surface strengthening of dental ceramics].

OBJECTIVE: Observe the effect of the ion-exchange technique on the surface strengthening of the dental ceramics. METHODS: Prepare the standard samples of Vita VMK 95, Vita VMK68 and Shofu Vintage ceramics, 10 pieces for each ceramic. The samples were divided into experiment group and control group. The samples in the experiment group were spread with HX-I ion-exchange cataplasm, and strengthened at the same condition with the control group. Flexure strength and EMPA were tested. All the data were calculated with SPSS 10.0 Stat. Software. RESULTS: The flexure strength of the experiment group was higher than the control group(P<0.05). EMPA showed that the depth of Na(+)-->K(+) ion-exchange was more than 100microm. CONCLUSION: HX-I ion-exchange cataplasm can significantly improve the flexure strength of dental ceramics and get an evident Na(+)-->K(+) ion-exchange.