N. Dorin Ruse

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.

Development and Validation of a Three-dimensional Computational Fluid Dynamics Model of Root Canal Irrigation

INTRODUCTION Root canal irrigation plays an important role in the debridement and disinfection of the root canal system and is an integral part of root canal preparation procedures. The aim was to construct a three-dimensional computational fluid dynamics (CFD) model of root canal irrigation, with a suitable turbulence model, and validate it to provide a novel method for studying the root canal irrigation. METHODS A camcorder was used to record the effect of irrigation in the in vitro model. An exact replica of the geometry and the physical parameters of the in vitro irrigation model were used in CFD analysis, considering four turbulent models. The in vitro irrigation model was used as the reference for the evaluation of the CFD models. RESULTS The result showed that CFD analysis based on a shear stress transport (SST) k-omega turbulence model was in close agreement with the in vitro irrigation model. The in vitro and CFD analyses showed that the irrigant in the curved canal flushes only up to a limited distance beyond the tip of the needle. The results of the CFD analysis also showed that laminar flow exists in the needle lumen and transit the transitional and turbulent flow around the side-vent outlet of the needle and needle tip. CONCLUSIONS The results suggested that CFD based on a SST k-omega turbulence model has the potential to serve as a platform for the study of root canal irrigation.

Propagation of erroneous data for the modulus of elasticity of periodontal ligament and gutta percha in FEM/FEA papers: A story of broken links

OBJECTIVE This brief review essay was triggered by the discovery of two errors that have been perpetuated in the dental literature for the last quarter century and is intended to alert the research community. METHODS An extensive search of the published literature, using PubMed and Web of Science search engines, electronic journal resources, and several trips to the library for manual retrievals of articles were used to retrieve hundreds of articles reporting on finite element modeling - finite element analysis (FEM/FEA) involving periodontal ligament (PDL) and gutta percha (GP). RESULTS The literature search revealed that erroneous values for the modulus of elasticity of PDL and GP were introduced in 1980 and in 1983, respectively. The identified errors range between two to three orders of magnitude and have been used in hundreds of FEM/FEA papers. SIGNIFICANCE The finding casts serious doubts regarding the validity of the results published in hundreds of papers and highlights the importance of checking the references cited and citing, or at least confirming, primary sources rather than citing citations.

In vitro changes in clips and bars used to retain implant overdentures

Although implant-retained overdentures are a less expensive alternative to fixed implant-supported prostheses in certain situations, problems with retentive clips fracturing and needing frequent replacement have been reported. This study compared baseline and posttest retention of metal and plastic clip-retained overdenture analogs and monitored surface changes in bars and clips throughout the testing process. A laboratory model was made with two implant analogs processed into an acrylic resin platform to which three bars were fitted. Two overdenture analogs were made and retained on the model with metal or plastic clips. Each bar-clip assembly was subjected to 5500 insertion and removal cycles to simulate 3 years of in vivo insertion and removal. Although the differences in retention between metal and plastic clips and between clips before and after testing were statistically significant, it is questionable whether they are clinically significant. Neither clip fracture nor loss occurred during this study, which suggests that it may be functional or parafunctional loading and not repeated insertion and removal of an implant overdenture that may cause the stated problems.

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.

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.

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.

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.

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

ObjectivesOperative 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 methodsThe 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.ResultsMinimization 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.ConclusionsA 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 relevanceThis software will help investigators choose the appropriate randomization method in future two-arm trials.

HPLC Analysis of Monomer Release from Conventionally and High Temperature High-Pressure Polymerised Urethane Dimethacrylate Intended for Biomedical Applications

Since monomer release poses significant biocompatibility concerns, the aim of this study was to determine, using HPLC, and compare monomer release from conventionally and high-temperature high-pressure (HT/HP) polymerized urethane dimethacrylate (UDMA) for biomedical applications. Three polymers were made: a) a control, obtained by conventional thermo-polymerization of UDMA with 0.5% (w) benzoyl peroxide (BPO) as initiator; b) an experimental, obtained by HT/HP polymerization of UDMA with 0.5% (w) BPO; and c) another experimental, obtained by HT/HP polymerization of UDMA without initiator. Bar-shaped polymer specimens were immersed in HPLC-grade 75% ethanol for 1 d, 7 d, 14 d, and 28 d prior to monomer determination by HPLC with an Agilent 1260 Infinity Quaternary LC. A Poroshell 120 EC-C18 (4.6x50 mm; 2.7μm) column and elution solvent consisting of HPLC-grade 65% acetonitrile in water, with a flow rate of 1 μL/min, were used. A calibration curve was constructed using standard UDMA solutions in the range of 1x10-5 M to 1x10-7 M. The limits of detection (LOD=2.62x10-6 M) and quantification (LOQ=7.65x10-6 M) for UDMA were determined. The accuracy of the method was confirmed by standard additions. Monomer release was statistically higher in the control group at all-time intervals; the lowest release was detected in the BPO-containing HT/HP polymerized group. The results suggested that there was a significant reduction in free monomer content in HT/HP polymerized UDMA and that polymers obtained under HT/ HP conditions could be more biocompatible