A novel model to evaluate the fatigue resistance of NiTi instruments: Rotational and axial movement at body temperature.

To develop a model to test cyclic fatigue resistance of TruNatomy instruments undergoing rotational and axial movement at body temperature. A total of 288 Prime and Medium instruments were subjected to cyclic fatigue testing in simulated canals (at 37°C) using a model with either rotational movement only or rotational and axial movement simultaneously. Two different sized canals and three different types of curvatures were tested for each instrument (30/0.04 and 30/0.06 for Prime; 38/0.04 and 40/0.06 for Medium). The number of cycles to failure (fatigue resistance) was recorded. Rotational and axial movement of instruments led to greater fatigue resistance compared with rotational movement alone. Apical curvatures led to greater fatigue resistance than curvatures in the coronal and middle third. The developed dynamic model at body temperature to evaluate fatigue resistance of instrument closer simulates clinical scenarios.

Fracture toughness of conventional, milled and 3D printed denture bases.

OBJECTIVES: The aim of this study was to determine and compare fracture toughness (KIC) and work of fracture (WOF) of a conventional (C) denture base, using the notchless triangular prism (NTP) specimen KIC test, with CAD/CAM (milled, M) and 3D-printed (P) materials at 7 d and 90 d. METHODS: Lucitone 199 (C), Lucitone 199 CAD (M) and Lucitone Digital Print (P) (Dentsply, USA) were used to fabricate NTP specimens. Samples were stored in 37 °C water for 7 d (20/group) and 90 d (20/group) and conditioned in 23 °C water for 1 h prior to testing. For testing, samples were secured in custom-made jigs and loaded in tension until crack arrest/failure. The maximum-recorded load was used to calculate KIC. The results were analyzed by two-way ANOVA, Scheffé multiple mean comparisons (α = 0.05), independent t-tests and Weibull. WOF (in KJ/m2) was calculated by dividing the area under the load-displacement graphs by twice the corresponding crack-arrested cross sectional area of the fractured surfaces. RESULTS: The results have shown that: 1) the tested materials had significantly different KIC (P > C > M; p < 0.005) and WOF at both 7d and 90d; 2) ageing in 37 ºC water for 90 d resulted in a significant decrease of KIC in the C and M groups (p < 0.001) and of WOF in all groups. SIGNIFICANCE: The tested P denture base exhibited significantly higher KIC and WOF, suggesting that it could be more resistant to crack propagation than the C and M materials tested. Water storage for 90 d significantly decreased KIC of C and M materials and WOF of all.

Characterisation of deformed or separated nickel-titanium retreatment instruments after clinical use - A multicentre experience: Defect profiles of clinically-used retreatment instruments.

OBJECTIVES: The present study examined the defect characteristics of clinically-used, discarded nickel titanium rotary retreatment instruments and analysed the impact of clinical use on their metallurgical properties. METHODS: 92 XP-endo Shaper (XPS; FKG Dentaire) and 20 XP-endo Finisher (XPFR; FKG Dentaire) instruments with structural deformation or separation were collected after retreatment from four endodontic clinics over a 20-month period. The types of defects and their relative locations were recorded. The lateral and fractured surfaces of the separated instruments were examined with scanning electron microscopy. Differential scanning calorimetry was used to investigate the thermal behaviour of new, deformed and fractured instruments. RESULTS: 77 (84%) XPS and 4 (20%) XPFR had an area with structural change while 15 (16%) XPS and 16 (80%) XPFR were fractured. All unfractured, deformed XPFR showed unwinding close to the coronal end of the flute. Fractures in XPS and XPFR were often close to the coronal end of the flutes or the expanding segments of the insruments. Most of the XPS fractures were torsional failure (67%) while XPFR failed predominantly by cyclic fatigue (81%). The austenite-finishing temperature of XPFR (40 °C) was higher than that of XPS (35 °C). Both XPS and XPFR exhibited 2-stage phase transformations. CONCLUSIONS: Torsional failure was more prevalent in XPS instruments and fatigue failure was more prevalent in XPFR instruments. amongst the investigated instruments, XPFR were more likely to separate without warning whereas XPS frequently exhibited plastic deformation. The latter may be used as a pre-separation forewarning sign during clinical retreatment. CLINICAL SIGNIFICANCE: The failure mode of XP-endo Shaper and XP-endo Finisher used clinically for retreatment appeared to be different. Plastic deformation, the forewarning sign of instrument seaparation, occurs when XP-endo instruments are used for retreatment.

Fracture Toughness, Flexural Strength, and Flexural Modulus of New CAD/CAM Resin Composite Blocks.

PURPOSE: To determine and compare the fracture toughness, flexural strength and flexural modulus of four new, commercially available CAD/CAM resin composite blocks and one new CAD/CAM lithium disilicate glass-ceramic block, tested under dry and aged conditions. MATERIALS AND METHODS: Three dispersed-fillers resin composite blocks, CERASMART, KZR-CAD-HR2, and CAMouflage NOW, one polymer-infiltrated ceramic network resin composite block, Enamic, along with Obsidian, a lithium disilicate glass-ceramic block, were characterized. Fracture toughness was determined through the notchless triangular prism specimen test, while flexural strength and flexural modulus were determined by three-point bend testing. Blocks were cut and ground to obtain (6 × 6 × 6 × 12) mm prisms and 10:1 span-to-thickness ratio bars (n = 25/group); half of the resin composite specimens were aged in 37°C distilled water for 30 days before testing. Fractured surfaces were characterized using a scanning electron microscope. Results were analyzed using Weibull statistics and two-way ANOVA, followed by Scheffé multiple means comparisons (α = 0.05). RESULTS: With regards to fracture toughness, KZR stood out among resin composites with a dry value of 1.37 MPa·m1/2 ; this was significantly affected by ageing, while the fracture toughness of the other dispersed-fillers resin composite blocks was not. Obsidian had the highest fracture toughness at 1.47 MPa·m1/2 . With regards to flexural strength, Obsidian > CERASMART = KZR > CAMouflage > Enamic. The flexural strength of the resin composites was lowered by ageing. Enamic was found to have the highest flexural modulus among the resin composites (33.02 GPa), but its value was significantly lower than that of Obsidian (76.46 GPa); flexural modulus was not affected by ageing. CONCLUSION: There was a significant difference in flexural strength between the materials, but not unanimously in flexural modulus and fracture toughness. The tested resin composite block materials had inferior flexural strength, flexural modulus and fracture toughness compared with the tested lithium disilicate glass-ceramic block (Obsidian). Enamic, the polymer infiltrated ceramic network material, had a significantly higher flexural modulus than the dispersed-fillers materials. Ageing had a deleterious impact on the flexural strength of all RCB, while its effect on the flexural modulus was insignificant. The selection of any restorative material requires a thorough analysis of its advantages and limitations to inform the clinical decision in a case-by-case approach.

Shear bond strength vs interfacial fracture toughness - Adherence to CAD/CAM blocks.

OBJECTIVE: To compare shear bond strength (SBS) and interfacial fracture toughness (IKIC) results when assessing the effect of surface roughness and thermocycling on the adherence of a resin composite luting agent (RCLA) to a CAD/CAM resin composite block (RCB). METHODS: Tetric CAD HT along with the recommended bonding system, Adhese Universal and Variolink Esthetic LC, were used. Surface roughness was achieved with 600/320/60 grit SiC papers. Samples were stored 24h in 37°C water or thermocycled 10000× (5°C-55°C) prior to testing. Results were analyzed by univariate ANOVA and Scheffé modified t-tests (α=0.05). Fractured specimens were viewed with a stereo microscope and selected specimens with a scanning electron microscope. RESULTS: SBS results showed a significant difference between the 60 grit group and the other groups, both after 24h and thermocycling. A large number of SBS samples showed cohesive fracture or subsurface damage in RCB. Thermocycling led to a significant decrease in SBS in all groups. IKIC results showed no significant differences due to surface preparation after 24h storage in 37°C. After thermocycling, there was a significant difference between the 60 and the 600 grit groups. All KIC samples fractured adhesively at the RCB surface. KIC of the RCLA was significantly higher than IKIC of all groups. SIGNIFICANCE: The results endorse the use of fracture mechanics methodology for the assessment and characterization of adherence, while identifying difficulties in its implementation. The results suggest also that adherence to CAD/CAM RCB may be limited by the strength of the resin composite block - adhesive interface.

"CAD-on" Interfaces - Fracture Mechanics Characterization.

PURPOSE: To apply fracture mechanics methodology to determine the interfacial fracture toughness of the interfaces present in "CAD-on" crowns consisting of CAD/CAM milled lithium disilicate veneers glass-fused to CAD/CAM milled yttrium oxide stabilized tetragonal zirconia polycrystal framework. MATERIALS AND METHODS: The notchless triangular prism specimen fracture toughness test was used to determine interfacial fracture toughness. Four groups, each consisting of (6 × 6 × 6 × 12) mm prisms (n = 22), were produced. Half-size [(6 × 6 × 6 × 6) mm] specimens of IPS e.max CAD and IPS e.max ZirCAD were approximated under vibration with Crystal Connect fusing glass and sintered according to manufacturer's guidelines to obtain the following three interfaces: (1) e.max CAD/Crystal Connect/e.max CAD (Group I); (2) Zir CAD/Crystal Connect/Zir CAD (Group II); and (3) Zir CAD/Crystal Connect/e.max CAD (Group III). For Group IV (control, based on the "press-on" veneering technique), half-size [(6 × 6 × 6 × 6) mm] IPS e.max ZirCAD prisms were coated with ZirLiner and pressed with IPS e.max ZirPress ingots to obtain (6 × 6 × 6 × 12) mm prisms. All specimens were tested using a computer controlled material testing machine. Results were analyzed with one-way ANOVA, Scheffé multiple means comparisons (α = 0.05) and Weibull statistics. All fractured surfaces were characterized with a light microscope. Selected fractured surfaces were characterized under a scanning electron microscope. RESULTS: All experimental groups demonstrated a cohesive mode of failure in the fusing glass layer. The number and size of defects appeared to correlate with the variability of fracture toughness values. There were no significant differences between the fracture toughness of the "CAD-on" interfaces (p = 0.052). The results suggested that the fracture toughness of Crystal Connect limited the interfacial fracture toughness values. The "CAD-on" fracture toughness value (Group III) was significantly greater than that of the ZirPress "press-on" control (Group IV) (p < 0.001). CONCLUSION: The "CAD-on" process results in stronger bonding between veneer and framework, compared to conventional veneering. The clinical use of "CAD-on" crowns could therefore be advocated. The selection of any restorative material requires a thorough analysis of advantages, limitations and results from clinical studies to inform the clinical decision in a case-by-case approach.

Interfacial Fracture Toughness of Adhesive Resin Cement-Lithium-Disilicate/Resin-Composite Blocks.

PURPOSE: Resin composite blocks (RCB) are advocated as alternative to ceramic blocks (CB). Prior to use, adherence to these materials should characterized. This study aimed to test the null hypothesis (H0 ) that material and surface treatment combinations do not influence interfacial fracture toughness (KIC ) of a self-cured adhesive resin cement [RelyX Ultimate (RXU)] to RCB or CB, under nonaged and aged conditions. MATERIALS AND METHODS: Two RCB, Lava Ultimate (LU) and Enamic (EN), and one CB, IPS e.max Press (EMP) were used. Half-size [(6 × 6 × 6 × 6 mm)] specimens were prepared for EMP (n = 30), EN (n = 30), and LU (n = 60). RCB specimens were prepared by wet cutting/grinding, while CB specimens were pressed. Surfaces of EMP and EN were preconditioned with hydrofluoric acid (5%); surfaces of LU were sandblasted with either 27 µm alumina (LUS) or 30 µm silica-modified alumina Rocatec soft (LUR). All specimens were bonded with Scotchbond Universal adhesive and RXU. Additionally, twenty (4 × 4 × 4 × 8 mm) RXU specimens were prepared. All specimens were stored in water at 37°C and tested after 1 and 60 days. Interfacial KIC was determined with the notchless triangular prism specimen KIC test. Results were analyzed with two-way ANOVA and Scheffé multiple means comparisons (α = 0.05). Preconditioned and selected fractured surfaces were characterized with scanning electron microscopy. RESULTS: At 24 hours, LUS-RXU and LUR-RXU had significantly higher interfacial KIC than EN-RXU and EMP-RXU and were not different from KIC of RXU. Aging lead to a significant decrease in KIC of RXU and interfacial KIC of LUS-RXU, LUR-RXU, and EMP-RXU; interfacial KIC of EN-RXU was not affected. Based on the results, H0 was rejected. CONCLUSION: Under the conditions of this study, at 24 hours, interfacial KIC of LUS-RXU and LUR-RXU was superior to EMP-RXU and EN-RXU. Aging in water at 37°C did not affect interfacial KIC of EN-RXU but adversely affected KIC of RXU and the other interfacial KIC . CLINICAL IMPLICATIONS: The results suggest that RXU and its adherence to LU and EMP deteriorates upon exposure to water at 37°C. In making clinical decisions related to material selection, practitioners should consider in vitro results.

Marginal Fit of Lithium Disilicate Crowns Fabricated Using Conventional and Digital Methodology: A Three-Dimensional Analysis.

PURPOSE: To compare the marginal fit of lithium disilicate (LD) crowns fabricated with digital impression and manufacturing (DD), digital impression and traditional pressed manufacturing (DP), and traditional impression and manufacturing (TP). MATERIALS AND METHODS: Tooth #15 was prepared for all-ceramic crowns on an ivorine typodont. There were 45 LD crowns fabricated using three techniques: DD, DP, and TP. Microcomputed tomography (micro-CT) was used to assess the 2D and 3D marginal fit of crowns in all three groups. The 2D vertical marginal gap (MG) measurements were done at 20 systematically selected points/crown, while the 3D measurements represented the 3D volume of the gap measured circumferentially at the crown margin. Frequencies of different marginal discrepancies were also recorded, including overextension (OE), underextension (UE), and marginal chipping. Crowns with vertical MG > 120 µm at more than five points were considered unacceptable and were rejected. The results were analyzed by one-way ANOVA with Scheffe post hoc test (α = 0.05). RESULTS: DD crowns demonstrated significantly smaller mean vertical MG (33.3 ± 19.99 µm) compared to DP (54.08 ± 32.34 µm) and TP (51.88 ± 35.34 µm) crowns. Similarly, MG volume was significantly lower in the DD group (3.32 ± 0.58 mm3 ) compared to TP group (4.16 ± 0.59 mm3 ). The mean MG volume for the DP group (3.55 ± 0.78 mm3 ) was not significantly different from the other groups. The occurrence of underextension error was higher in DP (6.25%) and TP (5.4%) than in DD (0.33%) group, while overextension was more frequent in DD (37.67%) than in TP (28.85%) and DP (18.75%) groups. Overall, 4 out of 45 crowns fabricated were deemed unacceptable based on the vertical MG measurements (three in TP group and one in DP group; all crowns in DD group were deemed acceptable). CONCLUSION: The results suggested that digital impression and CAD/CAM technology is a suitable, better alternative to traditional impression and manufacturing.

Fracture toughness of two lithium disilicate dental glass ceramics.

STATEMENT OF PROBLEM: IPS e.max CAD and IPS e.max Press (Ivoclar Vivadent AG) are lithium disilicate glass ceramics marketed as interchangeable materials indicated for the same clinical uses. However, different crystal sizes of lithium disilicate are formed during the processing of each of these materials, a factor that could lead to significantly different mechanical properties. As mechanical failure is always associated with a crack-initiation/crack-propagation process, fracture toughness (KIC) values could be useful in comparing different ceramics and possibly predicting clinical performance. PURPOSE: The purpose of this in vitro study was to determine and compare the KIC of IPS e.max CAD and IPS e.max Press. MATERIAL AND METHODS: The notchless triangular prism (NTP) specimen KIC test was used to determine and compare the KIC of IPS e.max Press and IPS e.max CAD. Twenty 6×6×6×12-mm NTP specimens of each material were prepared. IPS e.max CAD blocks were cut, ground, and then crystallized, while IPS e.max Press specimens were prepared by pressing IPS e.max Press ingots into molds obtained from 6×6×6×12-mm wax prisms, using the lost wax technique. Each specimen was mounted into a specimen holder, and custom grips were used to attach the specimen holder assembly to a computerized universal testing machine (model 4301; Instron Canada, Inc). The assembly was loaded in tension at a crosshead speed of 0.1 mm/min, and the KIC value was calculated based on the recorded maximum load at fracture. Fractured surfaces were characterized using scanning electron microscopy (SEM). Results were statistically analyzed using Weibull statistics and the Student t test (α=.05). RESULTS: Significantly (P<.05) higher KIC value was determined for IPS e.max Press than for IPS e.max CAD and, based on the Weibull modulus (m), IPS e.max Press was also more reliable. Fractured surfaces, characterized by SEM, showed a marked difference between the 2 materials, suggesting a more complete crystallization in IPS e.max Press, which was most likely responsible for the higher KIC determined. CONCLUSION: Within the limitations of this in vitro study, the results suggest that IPS e.max Press is superior to IPS e.max CAD with regard to the KIC and characteristic Weibull parameters.

A Comprehensive Study of Osteogenic Calcium Phosphate Silicate Cement: Material Characterization and In Vitro/In Vivo Testing.

Vertebral compression fractures can be successfully restored by injectable bone cements. Here the as-yet unexplored in vitro cytotoxicity, in vivo biodegradation, and osteoconductivity of a new calcium phosphate silicate cements (CPSC) are studied, where monocalcium phosphate (MCP; 5, 10, and 15 wt%) is added to calcium silicate cement (CSC). Setting rate and compressive strength of CPSC decrease with the addition of MCP. The crystallinity, microstructure, and porosity of hardened CPSC are evaluated by X-ray diffractometer, Fourier transform infrared spectroscopy, and microcomputed tomography (CT). It is found that MCP reacts with calcium hydroxide, one of CSC hydration products, to precipitate apatite. While the reaction accelerates the hydration of CSC, the formation of calcium silicate hydrate gel is disturbed and highly porous microstructures form, resulting in weaker compressive strength. In vitro studies demonstrate that CPSC is noncytotoxic to osteoblast cells and promotes their proliferation. In the rabbit tibia implantation model, clinical X-ray and CT scans demonstrate that CPSC biodegrades slower and osseointegrates better than clinically used calcium phosphate cement (CPC). Histological studies demonstrate that CPSC is osteoconductive and induces higher bone formation than CPC, a finding that might warrant future clinical studies.

Cyclic Fatigue of ProFile Vortex and Vortex Blue Nickel-Titanium Files in Single and Double Curvatures.

INTRODUCTION: The aims of this study were to determine the flexibility of ProFile Vortex (VX) and Vortex Blue (VB) files (Dentsply Tulsa Dental Specialties, Tulsa, OK) and then to evaluate and compare their fatigue resistance in artificial single curvature and 2 different artificial double curvature canals. METHODS: Flexibility of the files (size 25/.04) in bending was assessed according to ISO 3630-1. Both files were subjected to fatigue tests inside artificial canals with a single curvature (group 1: 60° curvature, 5-mm radius) and with 2 different double curvatures (group 2: first [coronal] curve of 60° curvature and 5-mm radius and the second one [apical] of 30° curvature and 2-mm radius and group 3: first curve of 60° curvature and 5-mm radius and the second one of 60° curvature and 2-mm radius). The number of cycles to fracture (NCF) was recorded, and the fracture surface of all fragments was examined with a scanning electron microscope. RESULTS: The bending load was significantly lower for VB files than VX files (P < .05), and the 2 types of files followed different trajectories in identical canals. In group 1, the 2 files had significantly higher NCF than in groups 2 and 3 (P < .05). Both files had significantly higher NCF in group 2 than in group 3 (P < .05). In group 1, VB files had fatigue resistance superior to VX files (P < .05), whereas in groups 2 and 3 their fatigue resistance was not statistically different from each other. The crack initiation of a vast majority of files that fractured in double curvature canals (groups 2 and 3) was localized on either 1 of 2 of the 3 cutting edges. CONCLUSIONS: Double curvature canals represent a much more stressful and challenging anatomy than single curvature canals, and, in them, fatigue resistance may be affected by the degrees and the radii of curvatures as well as by the bending properties of the files.

Assessment of the Internal Fit of Lithium Disilicate Crowns Using Micro-CT.

PURPOSE: The aim of this study was to compare the internal fit of lithium disilicate crowns fabricated using digital technology with those fabricated by conventional means. MATERIALS AND METHODS: Forty-five lithium disilicate crowns were fabricated: 15 using digital impression and computer-aided design/computer-aided machining technique (group 1), 15 from the same digital impressions, but using a conventional die and laboratory fabrication process (group 2), and 15 using a conventional poly (vinyl siloxane) (PVS) impression and laboratory fabrication process (group 3). Tooth #15 was prepared for all-ceramic restoration on an ivorine typodont, which was digitized and a replica milled in zirconia to serve as master model. The master zirconia model was used for the impression procedures. Duplicate dies of the master zirconia die were made in polyurethane, enabling the internal fit of each crown to be evaluated using X-ray microcomputed tomography. The total volume of the internal space between the crown and die, the mean and maximum thickness of this space, and the percentage of the space that was at or below 120 µm thickness was calculated for each group and statistically tested for significant difference using one-way ANOVA, with post hoc Scheffé analysis. RESULTS: Group 1 crowns resulted in a smaller volume of internal space (12.49 ± 1.50 mm(3)) compared to group 2 (15.40 ± 2.59 mm(3) ) and to those of group 3 (18.01 ± 2.44 mm(3)). The mean thickness of the internal space for group 1 (0.16 ± 0.01 mm) and for group 2 (0.17 ± 0.03 mm) was significantly lower than that of group 3 (0.21 ± 0.03 mm). The average percentage of the internal space of a thickness of 120 µm and below was different between the three groups: 46.73 ± 5.66% for group 1, 37.08 ± 17.69% for group 2, and 22.89 ± 9.72% for group 3. Three-dimensional renderings of the internal space were also created. CONCLUSIONS: The results of this study suggested that pressed and milled IPS e. max crowns from LAVA COS digital impressions had a better internal fit to the prepared tooth than pressed IPS e.max crowns from PVS impressions in terms of total volume of internal space, average thickness of internal space, and percentage of internal space at or below 120 µm.

Dynamic mechanical analysis of high pressure polymerized urethane dimethacrylate.

OBJECTIVES: The aim of this study was to compare the viscoelastic properties of high pressure (HP) polymerized urethane dimethacrylate (UDMA) with those of control, ambient pressure thermo-polymerized and photo-polymerized, UDMA and to assess the effect of varying polymerization parameters (protocol, temperature, and initiator) on the viscoelastic properties of HP polymerized UDMA. METHODS: The viscoelastic properties of the two control polymers, polymerized under atmospheric pressure, and four experimental polymers, polymerized under HP, were determined via dynamic mechanical analysis (DMA), in three point bending configuration. Atomic force microscopy (AFM) was used to characterize fractured polymer surface morphologies. RESULTS: The results showed that: HP-polymerization lead to a polymer with significantly higher Tg and E'rub, indicative of a higher crosslink density; modifying the polymerization protocol resulted in a significant increase in tanδ; increasing the polymerization temperature lead to a significant decrease in E'rub and Tg; and that the polymer with no initiator had the lowest E', E″, Tg, and E'rub and the highest tanδ, suggesting that under this conditions a polymer with significantly reduced crosslink density had been obtained. A characteristic nodular appearance was seen for the two control polymers under AFM, while a modified surface morphology was present in the case of HP polymerized materials. SIGNIFICANCE: The DMA results suggest that polymerization under HP resulted in polymers with an increased crosslink density and that the higher polymerization temperature or the lack of initiator was detrimental to the viscoelastic properties determined. Changes in polymer network morphology were identified by AFM characterization.

High-temperature high-pressure polymerized urethane dimethacrylate-mechanical properties and monomer release.

OBJECTIVE: This study was conducted to determine selected mechanical/physical properties of and monomer release from high-temperature high-pressure (HT/HP) polymerized urethane dimethacrylate (UDMA). METHODS: Flexural strength (σf), hardness, fracture toughness (KIC), and density (ρ) were determined for five UDMA resin blocks produced via different polymerization protocols. High performance liquid chromatography (HPLC) was used to determine monomer release from the five polymers. One way ANOVA, Scheffé multiple means comparisons (α=0.05), and Weibull statistics (for σf) were used to analyze the results. RESULTS: The results showed that HT/HP polymerization resulted in a significant (p<0.05) increase in σf and ρ, along with an increase in Weibull modulus. No significant differences were found in hardness and KIC between the two HT/HP polymerized materials. A significantly lower (p<0.05) monomer release was detected for the HT/HP polymerized groups. SIGNIFICANCE: The results of this study suggest that HT/HP polymerization affects the network structure and leads to UDMA polymers with improved mechanical/physical properties and with dramatically reduced monomer release. The low elution of monomers from HT/HP and HP polymerized materials suggests the achievement of a higher degree of conversion and a lesser degree of inhomogeneity with regards to microgel domains. The results, however, cannot fully explain the dramatic increase in mechanical/physical properties reported previously for RCB, improvements that may be due to a better filler-matrix interaction afforded by HT/HP polymerization.

Randomization in clinical trials: stratification or minimization? The HERMES free simulation software.

OBJECTIVES: Operative clinical trials are often small and open-label. Randomization is therefore very important. Stratification and minimization are two randomization options in such trials. The first aim of this study was to compare stratification and minimization in terms of predictability and balance in order to help investigators choose the most appropriate allocation method. Our second aim was to evaluate the influence of various parameters on the performance of these techniques. MATERIALS AND METHODS: The created software generated patients according to chosen trial parameters (e.g., number of important prognostic factors, number of operators or centers, etc.) and computed predictability and balance indicators for several stratification and minimization methods over a given number of simulations. Block size and proportion of random allocations could be chosen. A reference trial was chosen (50 patients, 1 prognostic factor, and 2 operators) and eight other trials derived from this reference trial were modeled. Predictability and balance indicators were calculated from 10,000 simulations per trial. RESULTS: Minimization performed better with complex trials (e.g., smaller sample size, increasing number of prognostic factors, and operators); stratification imbalance increased when the number of strata increased. An inverse correlation between imbalance and predictability was observed. CONCLUSIONS: A compromise between predictability and imbalance still has to be found by the investigator but our software (HERMES) gives concrete reasons for choosing between stratification and minimization; it can be downloaded free of charge. CLINICAL RELEVANCE: This software will help investigators choose the appropriate randomization method in future two-arm trials.

Properties of experimental urethane dimethacrylate-based dental resin composite blocks obtained via thermo-polymerization under high pressure.

OBJECTIVES: The aim of this study was to use high-pressure high-temperature (HP/HT) polymerization to produce urethane dimethacrylate (UDMA)-triethylene glycol dimethacrylate (TEGDMA) based resin composite blocks (RCB) suitable for dental computer-aided design/manufacture (CAD/CAM) applications and to compare their physical/mechanical properties to those of a commercial dental RCB. The null hypotheses tested were: (1) there are no differences in the physical/mechanical properties between HP/HT polymerized UDMA-TEGDMA RCB and a commercial RCB; (2) volume fraction filler (Vf) does not affect the physical/mechanical properties of HP/HT polymerized RCB. METHODS: Four UDMA-based experimental RCB were manufactured under HP/HT conditions. A RCB manufactured under the same HP/HT conditions from a commercial resin composite (Z100) and its commercial counterpart CAD/CAM RCB (Paradigm MZ100) were used as controls. Flexural strength (σf), fracture toughness (KIC), and hardness were determined. The results were analyzed using one-way ANOVA, Scheffé multiple means comparisons (α=0.05), and Weibull statistics (for σf). Scanning electron microscopy was used to characterize fractured surfaces. RESULTS: All HP/HT polymerized RCB had superior σf, KIC, and Weibull modulus compared to the commercial dental RCB. The experimental RCB had similar or superior properties compared to HP/HT polymerized Z100 RCB. Fewer and smaller porosities (not quantified) were apparent in HP/HT polymerized RCB. The experimental RCB that contained 65% Vf showed higher porosity, suggesting practical difficulties in filler incorporation beyond a certain Vf. CONCLUSIONS: The results of this study suggested that RCB suitable for dental CAD/CAM applications could be obtained by HP/HT polymerization of resin composites based on pure UDMA.

In vitro studies of calcium phosphate silicate bone cements.

A novel calcium phosphate silicate bone cement (CPSC) was synthesized in a process, in which nanocomposite forms in situ between calcium silicate hydrate (C-S-H) gel and hydroxyapatite (HAP). The cement powder consists of tricalcium silicate (C(3)S) and calcium phosphate monobasic (CPM). During cement setting, C(3)S hydrates to produce C-S-H and calcium hydroxide (CH); CPM reacts with the CH to precipitate HAP in situ within C-S-H. This process, largely removing CH from the set cement, enhances its biocompatibility and bioactivity. The testing results of cell culture confirmed that the biocompatibility of CPSC was improved as compared to pure C(3)S. The results of XRD and SEM characterizations showed that CPSC paste induced formation of HAP layer after immersion in simulated body fluid for 7 days, suggesting that CPSC was bioactive in vitro. CPSC cement, which has good biocompatibility and low/no cytotoxicity, could be a promising candidate as biomedical cement.

Physico-chemical approach to pit and fissure sealant infiltration and spreading mechanisms.

PURPOSE: The purpose of this study was to investigate if hydrophilicity influences the depth of penetration and the microleakage and to determine if the state of enamel, wet or dry, influences the wettability and sealing capacity of sealants. METHODS: HeliosealF, a hydrophobic sealant, and Embrace, a hydrophilic sealant, were tested. Sections were delimited on each of the polished enamel surfaces of 15 human molars to study the spreading of the selected sealants on wet or dry enamel; 40 molars were used to investigate the depth of penetration and microleakage on both dry and wet enamel. The surface-free energies were determined. RESULTS: The spreading of Embrace was significantly better (P<.01) than that of HeliosealF, but there was no difference between the results obtained on wet or dry enamel (P=.90). While there was no difference in penetration between the 2 sealants (P=.95), their penetration was significantly lower on wet enamel (P=.01). CONCLUSIONS: Sealant penetration and sealing characteristics are better when sealants are applied on dry enamel. Hydrophilicity and wettability did not have a direct influence on penetration or microleakage. A set sealant with a low surface-free energy could be an advantage in reducing bacterial adhesion.

Resin composite blocks via high-pressure high-temperature polymerization.

OBJECTIVES: The aim of this study was to thermo-polymerize under high pressure four commercially available dental resin composites to obtain and characterize composite blocks suitable for CAD/CAM procedures. METHODS: Gradia (GC, Japan), Vita VM LC (Vita Zahnfabrik, Germany), Grandio (VOCO, Germany), and EsthetX (Dentsply, Germany), were selected for this study. Paradigm (3 M ESPE, USA), a CAD/CAM composite block, was included for comparison. Composite blocks were obtained through polymerization at high-temperature high-pressure (HT/HP). Samples for mechanical/physical characterizations were cut from Paradigm and HT/HP composite blocks while control samples were obtained by photo-polymerizing (PP) the materials in molds. Flexural strength (σ(f)), fracture toughness (K(IC)), hardness, and density (ρ) were determined and compared by pairwise t-tests (α=0.05). Fractured surfaces were characterized under a scanning electron microscope. RESULTS: The results have shown that HT/HP polymerization resulted in a significant (p<0.05) increase in σ(f), hardness, and ρ for all composites investigated. Even if K(IC) of all materials was increased by HT/HP polymerization, significant increases were detected only for Gradia and EsthetX. The Weibull modulus of HT/HP polymerized composites was higher than that of PP counterparts. HT/HP materials had higher σ(f), Weibull modulus, and K(IC) compared to Paradigm. The most significant SEM observation of fractured K(IC) specimens from all the materials tested was the presence of fewer and smaller voids in HT/HP polymerized composites. SIGNIFICANCE: The results of this study suggest that HT/HP polymerization could be used to obtain dental resin composite blocks with superior mechanical properties, suitable for CAD/CAM processing.

Three-dimensional numeric simulation of root canal irrigant flow with different irrigation needles.

INTRODUCTION: The purpose of this study was to investigate, by using computational fluid dynamics (CFD), the effect of needle tip design on irrigant flow pattern. METHODS: Parameters of an in vitro irrigation model were used to create CFD models. Experimental data obtained by recording the dynamic fluid distribution during irrigation with 27-gauge notched (Appli-Vac) and side-vented open-ended (Vista-Probe) needles, placed at 3 and 5 mm from the apex of a simulated straight root canal prepared in a plastic block, were used to validate the results of CFD analysis. Two "virtual" needle tip designs were also included in CFD analysis, one with a beveled tip (based on Appli-Vac) and one with side-vent based on Vista-Probe needle but with a closed-end tip. Apical pressure, flow velocity at wall, and flow velocity distribution within root canal were determined by CFD. RESULTS: Flow patterns generated by CFD were in close agreement with the in vitro model. When placed 3 mm from the apex, the irrigant reached, or almost reached, the apex with all 4 needle designs. When placed 5 mm from the apex, the irrigant did not reach the apex with the side-vented needles. Irrigant velocities on canal walls were very low (0-0.7 m/s) compared with that within the needle lumen ( approximately 7 m/s) and varied as a function of needle tip design. Apical pressure was highest with the beveled needle and lowest with the side-vented closed-end needle. CONCLUSIONS: Irrigation needle tip design influences flow pattern, flow velocity, and apical wall pressure, all important parameters for the effectiveness and safety of irrigation. Computational fluid dynamics can be a valuable tool in assessing the implications of needle tip design on these parameters.