Fracture toughness and fractal analysis of ceramic benchmark materials.

Previous studies have reported various methods of measuring the fracture toughness of brittle ceramics. The purpose of the present research was to use a new method of fractal dimension measurement on benchmark materials (silica glass, Viosil SX, Shin-Etsu, n = 13, and silicon nitride standard reference material, SRM2100, NIST, n = 10), to compare the fracture toughness calculated using different methods, and to study the effect of noise filtering on the fractal dimension and fracture surface roughness. Fracture toughness was determined using surface crack in flexure method according to ASTM C1421 and fractal analysis method. Fractal dimension was determined using the Minkowski Cover algorithm on atomic force microscope scans of epoxy replicas of fracture surfaces. The mean ± standard deviation of fracture toughness using surface crack in flexure method and fractals method were 0.97 ± 0.18 MPa·m1/2 and 1.03 ± 0.07 MPa·m1/2 for silica glass and 4.62 ± 0.14 MPa·m1/2 and 2.54 ± 0.07 MPa·m1/2 for silicon nitride, respectively. The mean ± standard deviation of fractal dimension was 2.17 ± 0.03 for silica glass and 2.13 ± 0.01 for silicon nitride. The mean ten-point roughness (Rz) before and after noise filtering was 89 ± 102 nm and 87 ± 101 nm for silica glass and 355 ± 132 nm and 357 ± 134 nm for silicon nitride, respectively. Noise filtering had no significance on the fracture surface roughness of the two materials. The newly developed fractal analysis method can be used to predict the baseline fracture toughness of specimens with unknown failure stress.

Reliability and failure behavior of CAD-on fixed partial dentures.

OBJECTIVES: To estimate the reliability and failure behavior of fixed partial dentures (FPDs) fabricated using the CAD-on technique. METHODS: FPDs (n=25) were fabricated using a CAD/CAM system: IPS e.max ZirCAD - Crystall./Connect and IPS e.max CAD (Ivoclar). The restoration type ("three-unit bridge") and design method ("multilayer") based on Biogenerics were used. Framework and porcelain structures were united using a fusion ceramic (Crystall./Connect, Ivoclar). Mechanical fatigue was tested in a servohydraulic load frame machine at a cyclic loading (frequency: 2Hz; load ratio: 0.1). Based on previous data from specimens tested in fast fracture, three different stress profiles were used. The lifetime data were analyzed using an inverse power law-Weibull cumulative damage model (ALTA PRO, Reliasoft). All failed specimens were examined under a field emission scanning electron microscope. RESULTS: Porcelain chipping was the predominant (60%) mode of failure for FPDs tested in fast fracture and connector failure was predominant (67%) under fatigue. For fast fracture data, the Weibull modulus (ß) of FPDs was 7.8 combining the two failure modes. When chipping and connector fracture data were analyzed separately, ß values were 7.9 and 2.9. For the step stress fatigue test, ß values were lower than estimated using fast fracture, being 1.6 for connector fracture and 1.3 for porcelain chipping. SIGNIFICANCE: The test method (fast fracture or fatigue) significantly influenced the reliability of FPDs fabricated using the CAD-on technique, but it did not influence their failure behavior.

The Tennessee study: factors affecting treatment outcome and healing time following nonsurgical root canal treatment.

AIM: To determine factors that may influence treatment outcome and healing time following root canal treatment. METHODOLOGY: Root filled and restored teeth by pre-doctoral students were included in this study. Teeth/roots were followed-up regularly, and treatment outcome was evaluated at every follow-up appointment (healed, healing, uncertain or unsatisfactory). Host (age, immune condition, pulp/periapical diagnosis, tooth/root type, location and anatomy) and treatment factors (master apical file size, apical extension, voids and density of root filling) were recorded from patient dental records. Univariate, bivariate and multivariate analyses were performed to determine the impact of the factors on treatment outcomes and healing times. RESULTS: A total of 422 roots from 291 teeth met the inclusion criteria with a mean follow-up period of 2 years. The preoperative pulp condition, procedural errors during treatment, apical extension and density of root fillings significantly affected the treatment outcome. The average time required for a periapical lesion to heal was 11.78 months. The healing time increased in patients with compromised healing, patients older than 40 years, roots with Weine type II root canal systems, root canal systems prepared to a master apical file size <35, and roots with overextended fillings (P < 0.1). CONCLUSION: Multiple host and treatment factors affected the healing time and outcome of root canal treatment. Follow-up protocols should consider these factors before concluding the treatment outcome: patient's age, immune condition, as well as roots with overextended fillings, root canal systems with smaller apical preparations (size <35) or roots with complex canal systems. Intervention may be recommended if the treatment quality was inadequate or if patients became symptomatic.

Flexural strength and reliability of monolithic and trilayer ceramic structures obtained by the CAD-on technique.

OBJECTIVE: To evaluate the flexural strength, Weibull modulus, fracture toughness, and failure behavior of ceramic structures obtained by the CAD-on technique, testing the null hypothesis that trilayer structures show similar properties to monolithic structures. METHODS: Bar-shaped (1.8mm×4mm×16mm) monolithic specimens of zirconia (IPS e.max ZirCAD - Ivoclar Vivadent) and trilayer specimens of zirconia/fusion ceramic/lithium dissilicate (IPS e.max ZirCAD/IPS e.max CAD Crystall./Connect/IPS e.max CAD, Ivoclar Vivadent) were fabricated (n=30). Specimens were tested in flexure in 37°C deionized water using a universal testing machine at a crosshead speed of 0.5mm/min. Failure loads were recorded, and the flexural strength values were calculated. Fractography principles were used to examine the fracture surfaces under optical and scanning electron microscopy. Data were statistically analyzed using Student's t-test and Weibull statistics (α=0.05). RESULTS: Monolithic and trilayer specimens showed similar mean flexural strengths, characteristic strengths, and Weibull moduli. Trilayer structures showed greater mean critical flaw and fracture toughness values than monolithic specimens (p<0.001). Most critical flaws in the trilayer groups were located on the Y-TZP surface subjected to tension and propagated catastrophically. Trilayer structures showed no flaw deflection at the interface. SIGNIFICANCE: Considering the CAD-on technique, the trilayer structures showed greater fracture toughness than the monolithic zirconia specimens.

Lifetime comparison of Y-TZP/porcelain crowns under different loading conditions.

OBJECTIVES: To compare the lifetime of Y-TZP/porcelain crowns under three different load conditions using step-stress accelerated lifetime testing. METHODS: The Y-TZP frameworks were milled using CAD/CAM, veneered with a porcelain and cemented onto dentine analogue dies. Specimens were divided according to the occlusal load condition (n=20): central fossa load (CFL), cusp tip load (CTL) and sliding contact (SC). For CFL and CTL, the cyclic load was applied parallel to the long axis of the preparation using a ceramic piston. For SC, the axial load was associated to 1mm lateral displacement at the disto-lingual cusp. Different stress profiles were used. Failures were detected with an acoustic system. A Weibull distribution (95% confidence boundary) was used to analyse the data, and fractographic principles were used to evaluate fractured specimens. RESULTS: The acoustic monitor was able to detect the initial crack. The probability of failure (at 300 N load and 200,000 cycles) was statistically greater for CTL (0.63; 0.44-0.81) compared to CFL (0.23; 0.12-0.43). The Weibull modulus of CFL (2.1; 1.5-3.6) was greater than for SC (0.7; 0.5-1.2), with no difference in the lifetime. All specimens failed by chipping, which originated predominantly at the contact (66.7%) on CTL, and in the bulk of the porcelain on CFL (100%) and SC (80%). CONCLUSIONS: Contact at the cusp tip is more harmful than at the central fossa. Data from sliding contact are less consistent than from axial contacts, but more clinically relevant. CLINICAL SIGNIFICANCE: The loading condition of Y-TZP/porcelain crowns can influence on the probability of failure and failure mode. The contact at the cusp tip is more harmful than at the central fossa, where the stress is better distributed. Sliding contact is a better simulator of the chewing cycle compared to axial contacts.

Calving fluxes and basal melt rates of Antarctic ice shelves.

Iceberg calving has been assumed to be the dominant cause of mass loss for the Antarctic ice sheet, with previous estimates of the calving flux exceeding 2,000 gigatonnes per year. More recently, the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front. So far, however, no study has reliably quantified the calving flux and the basal mass balance (the balance between accretion and ablation at the ice-shelf base) for the whole of Antarctica. The distribution of fresh water in the Southern Ocean and its partitioning between the liquid and solid phases is therefore poorly constrained. Here we estimate the mass balance components for all ice shelves in Antarctica, using satellite measurements of calving flux and grounding-line flux, modelled ice-shelf snow accumulation rates and a regional scaling that accounts for unsurveyed areas. We obtain a total calving flux of 1,321 ± 144 gigatonnes per year and a total basal mass balance of -1,454 ± 174 gigatonnes per year. This means that about half of the ice-sheet surface mass gain is lost through oceanic erosion before reaching the ice front, and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking. In addition, the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves. We find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering and enhanced discharge. We suggest that basal mass loss is a valuable metric for predicting future ice-shelf vulnerability to oceanic forcing.

Nanopore formation on the surface oxide of commercially pure titanium grade 4 using a pulsed anodization method in sulfuric acid.

Titanium and its alloys form a thin amorphous protective surface oxide when exposed to an oxygen environment. The properties of this oxide layer are thought to be responsible for titanium and its alloys biocompatibility, chemical inertness, and corrosion resistance. Surface oxide crystallinity and pore size are regarded to be two of the more important properties in establishing successful osseointegration. Anodization is an electrochemical method of surface modification used for colorization marking and improved bioactivity on orthopedic and dental titanium implants. Research on titanium anodization using sulphuric acid has been reported in the literature as being primarily conducted in molarity levels 3 M and less using either galvanostatic or potentiostatic methods. A wide range of pore diameters ranging from a few nanometers up to 10 µm have been shown to form in sulfuric acid electrolytes using the potentiostatic and galvanostatic methods. Nano sized pores have been shown to be beneficial for bone cell attachment and proliferation. The purpose of the present research was to investigate oxide crystallinity and pore formation during titanium anodization using a pulsed DC waveform in a series of sulfuric acid electrolytes ranging from 0.5 to 12 M. Anodizing titanium in increasing sulfuric acid molarities showed a trend of increasing transformations of the amorphous natural forming oxide to the crystalline phases of anatase and rutile. The pulsed DC waveform was shown to produce pores with a size range from ≤0.01 to 1 µm(2). The pore size distributions produced may be beneficial for bone cell attachment and proliferation.

Effect of cryogenic treatment on nickel-titanium endodontic instruments.

AIM: To investigate the effects of cryogenic treatment on nickel-titanium endodontic instruments. The null hypothesis was that cryogenic treatment would result in no changes in composition, microhardness or cutting efficiency of nickel-titanium instruments. METHODOLOGY: Microhardness was measured on 30 nickel-titanium K-files (ISO size 25) using a Vicker's indenter. Elemental composition was measured on two instruments using X-ray spectroscopy. A nickel-titanium bulk specimen was analysed for crystalline phase composition using X-ray diffraction. Half of the specimens to be used for each analysis were subjected to a cryogenic treatment in liquid nitrogen (-196 degrees C) for either 3 s (microhardness specimens) or 10 min (other specimens). Cutting efficiency was assessed by recording operator choice using 80 nickel-titanium rotary instruments (ProFile 20, .06) half of which had been cryogenically treated and had been distributed amongst 14 clinicians. After conditioning by preparing four corresponding canals, each pair of instruments were evaluated for cutting efficiency by a clinician during preparation of one canal system in vitro. A Student's t-test was used to analyse the microhardness data, and a binomial test was used to analyse the observer choice data. Composition data were analysed qualitatively. RESULTS: Cryogenically treated specimens had a significantly higher microhardness than the controls (P < 0.001; beta > 0.999). Observers showed a preference for cryogenically treated instruments (61%), but this was not significant (P = 0.21). Both treated and control specimens were composed of 56% Ni, 44% Ti, 0% N (by weight) with a majority in the austenite phase. CONCLUSIONS: Cryogenic treatment resulted in increased microhardness, but this increase was not detected clinically. There was no measurable change in elemental or crystalline phase composition.

SEM/EDS evaluation of porcelain adherence to gold-coated cast titanium.

The adhesion between titanium and dental porcelain is related to the diffusion of oxygen to the reaction layer formed on cast-titanium surfaces during porcelain firing. The diffusion of oxygen could be suppressed by coating the titanium surface with a thin gold layer. This study characterized the effects of gold coating on titanium-ceramic adhesion. ASTM grade II CP titanium was cast into a MgO-based investment (Selevest CB, Selec). The specimen surfaces were air abraded with 110-microm Al(2)O(3) particles. Gold coating was applied on titanium surfaces by three methods: gold-paste (Deck Gold NF, Degussa-Ney) coating and firing at 800 degrees C for three times, single gold-paste coating and firing followed by sputter coating (40 mA, 500 s), and sputter coating (40 mA, 1000 s). Surfaces only air abraded with Al(2)O(3) particles were used as controls. An ultra-low-fusing dental porcelain (Vita Titankeramik, Vident) was fused on titanium surfaces. Specimen surfaces were characterized by SEM/EDS and XRD. The titanium-ceramic adhesion was evaluated by a biaxial flexure test (N = 8), and area fraction of adherent porcelain (AFAP) was determined by EDS. Numerical results were statistically analyzed by one-way ANOVA and the Student-Newman-Keuls test at alpha = 0.05. SEM fractography showed a substantial amount of porcelains remaining on the gold-sputter-coated titanium surfaces. A new Au(2)Ti phase was found on gold-coated titanium surface after the firing. Significantly higher (p <.05) AFAP values were determined for the gold-sputter-coated specimens compared to the others. No significant differences were found among the other groups and the control. Results suggested that gold coatings used in this study are not effective barriers to completely protect titanium from oxidation during the porcelain firing, and porcelain adherence to cast titanium can be improved by gold-sputter coating used in the present study.

Effects of flaw size and auto-glaze treatment on porcelain strength.

The functional surfaces of porcelain restorations are often ground to adjust occlusion. This removes the surface glaze and introduces flaws. Re-firing the restoration before final placement produces a self-glaze layer on the surface that may reduce the extent of damage caused by grinding. The objectives of this study were to determine whether re-firing after grinding increases the strength of dental porcelain and to determine whether the effectiveness of this treatment was dependent on the initial flaw size. Six groups, each containing 12 disk specimens, were prepared from experimental body porcelain (No. 36, J.F. Jelenko & Co., Armonk, NY). The specimens were fired under vacuum, ground to a thickness of 1 mm with 240-grit SiC abrasive, and polished through 600-grit on the surface to be subjected to tensile stress. Control Group A consisted of 12 non-indented specimens. Flaws were induced in the remaining groups by means of a Vickers indenter under the following loads: 3.9 N (Group B), 7.8 N (Group C), 11.8 N (Group D), 15.7 N (Group E), and 19.6 N (Group F). Following indentation, half of the specimens from each group were re-fired. The flexure strength of each specimen was determined by means of a piston-on-three-ball biaxial fixture. The TTEST procedure of SAS (1985) indicated a significant difference between the mean strength values of glazed and non-glazed specimens in Group E only (p = 0.55, 0.24, 0.13, 0.07, 0.01, and 0.69 for Groups A to F, respectively). ANOVA with the GLM procedure of SAS (1985) revealed that the mean strength values of groups subjected to five indentation loads were not significantly different (p = 0.45 for glazed specimens and p = 1.00 for non-glazed specimens). These findings support those of Fairhurst et al. (1992) for specimens with smaller flaws that were induced by grinding with 1 micron abrasive grit. The results of the current study indicate that re-firing of porcelain with large surface flaws does not significantly increase the flexural strength.

Sensitivity of catalyst/base ratio on curing of resin luting agents: polymerization exotherm analysis.

OBJECTIVES: Currently, the proposed test of the International Standardization Organization (ISO) for measuring working and setting times of resin luting agents is based on measurement of times to reach specified stages on the polymerization exotherm. The objective of this study was to use this test to investigate the influence of variations in the mass ratios of catalyst paste to base paste on the working and setting times of three dual-cured dental resin luting agents. METHODS: The materials used were Dicor Light Activated Cement (Dentsply International Inc.), Palfique Inlay Resin Cement (Tokuyama Soda Co.), and Vivadent Dual Cement (Vivadent). Fifteen specimens of each material were tested for working time by spatulating mass ratios from 0.7 to 1.3 for 30s at 23 degrees C and recording the time from beginning of spatulation to the time at which a temperature increase occurs. Ten specimens of each material were tested for setting time by spatulating in a similar manner at 37 degrees C and recording the time at which the temperature reaches a maximum value. RESULTS: The data were fitted to the relation, In t = In A + Bm, where t is the time in seconds, m is the mass ratio, and A and B are regression coefficients. The results suggest that working and setting times of the specimens were independent of variations in mass ratio. A comparison among the materials was made by using a multiple linear regression with the relation, In t = In C + Dm + E gamma + Fm gamma, where gamma is a dummy variable to help distinguish between materials, and C, D, E, and F are regression coefficients. The results suggest that differences in materials influence the working time but not the setting time. SIGNIFICANCE: These results infer that variations in mass ratio (+/- 20%) often observed in the clinical setting should not have a significant influence on the working and setting times of resin luting agents.

Mayer-Rokitansky-Kuster-Hauser syndrome with splenosis. A case report.

This is the first reported case of simultaneously occurring pelvic splenosis and Mayer-Rokitansky-Kuster-Hauser syndrome. No other congenital or anatomic abnormalities were observed in the patient, and she had no history of abdominal trauma or surgery. Endometriosis was suspected initially during laparoscopy for pelvic pain. After tissue removal at laparotomy, the histologic evaluation established the diagnosis of splenosis. Biopsy at laparoscopy should be considered to confirm the diagnosis in the presence of a congenital anomaly and lesions suspected of being endometriotic.