Determination of degree of conversion as a function of depth of a photo-initiated dental restoration composite—III application to commercial Prodigy Condensable™

In this third work, we evaluated the degree of conversion (DC) versus depth of dental filling composite named Prodigy Condensable™ using infrared spectroscopy. Confirming previous results, there was a gradual reduction of DC with increasing depth but the composite exhibited extreme values of DC, an upper value (45.9%) on the surface and a lower one (6.1%) at a depth of 5 mm. The composite presented the worst performance among materials studied. The composite formulation was 80% of inorganic fillers and BisfenolA/dimethacrylate (BisGMA) (18%)/triethyleneglycoldimethacrylate (TEGDMA) (2.0%) as monomers. As stated before, type/ratio/viscosity of monomers and type, amount, size and size distribution of fillers all together had an important role in the cure reaction contributing to the final performance of the composite.     

Corrosion-stable nickel- and cobalt-based alloys for dental applications

In a comparative study the in vitro corrosion behavior of a selection of nickel- and cobalt-based alloys for application in dentistry containing no noble metals was studied with slow scan cyclic voltammetry. The obtained breakthrough potentials, the repassivation behavior and further typical features of the cyclic voltamograms are correlated with the chemical composition as measured with electron beam microanalysis. Surface inhomogenities detected with the latter method are discussed with respect to the electrochemical behavior. For all alloys stabilities in terms of breakthrough potential superior to previously reported data for nickel-base alloys are found.     

Use of the molybdenum-thiocyanate-rhodamine 6G ternary complex for spectrophotometric molybdenum determination without extraction

A modified thiocyanate method without extraction by using rhodamine 6G as a secondary ligand was developed. Molybdenum in 1.0×10⁻² M HCl, after the addition of ascorbic acid, was heated for 10 min in a 90 °C water bath for reduction. Suitable amounts of glycerine, Triton X-100, rhodamine 6G solutions and 2+1 (v/v) 9 M H₂SO₄+3 M KHSO₄ were added in this order. This solution was allowed to cool to room temperature and the absorbance at 570 nm was measured against a reagent blank 45 min after the addition of thiocyanate solution and the second aliquot of Triton X-100 solution. The complex was stable for at least 4 h, the order of reagent addition was important, and thiocyanate should be in large excess. Beer’s law was obeyed over the range 0.9×10⁻⁶ to 1.1×10⁻⁵ M Mo with the molar absorptivity being 1.1×10⁵ l mol⁻¹ cm⁻¹. The R.S.D. for the determination of 0.7 mg Mo l⁻¹ was 1.83% (n=8). Possible interferences of various cations and anions on molybdenum determination were studied. The proposed method was applied to the determination of molybdenum in a dental alloy, Wiron 99.     

Electrochemical technique for the determination of fractal dimension of dental surfaces

Fractal dimension of a carious tooth surface was determined using an electrochemical method. The method was based on time-dependent diffusion towards electrode surfaces, which is one of the most useful and reliable methods for the determination of fractal dimension of electrode surfaces. For this purpose, the tooth was covered with a gold layer, which acted as an electrode in electrochemical experiments. It is suggested that the fractal dimension can be used as a quantitative measure of the state of dental surfaces. The method presented demonstrates the power of electrochemical techniques for the determination of fractal dimension of surface of non-conducting objects.     

STUDY OF THE EFFECT OF TWO BPO/AMINE INITIATION SYSTEMS ON THE FREE-RADICAL POLYMERIZATION OF MMA USED IN DENTAL RESINS AND BONE CEMENTS

ABSTRACT

The kinetics of the free radical bulk polymerization of methyl methacrylate (MMA) was studied by DSC, using the benzoyl peroxide (BPO)/amine initiation system. N,N dimethyl-4-aminophenethyl alcohol (DMPOH), which is a newly synthesized and used amine in the preparation of acrylic dental resins and bone cements was examined, and the results compared to the most commonly used in these applications amine, the N,N dimethyl-p-toluidine (DMT). For both amines, the effect of the molar ratio of BPO/amine and of the reaction temperature, on the polymerization kinetics was investigated. The prepared polymers were characterized by determination of the average molecular weights (M¯ _n and M¯ _w ) and molecular weights distribution (M¯ _w /M¯ _n ) using Gel Permeation Chromatography. DMPOH was found to lead in slightly higher polymerization rates, lower gel times and lower molecular weights than DMT. The values of these parameters for both amines were influenced by the molar ratio of BPO to amine, when the product of the concentrations of these was kept constant. The highest polymerization rate occurred in the lowest gel time, resulting in polymers with the lowest molecular weight, and was observed when a molar ratio of about 1.5 BPO/amine was used. However, the final monomer conversion was found to be independent of the molar ratio and amine used. The activation energy of polymerization was found to be 51.8 kJ/mol K for BPO/DMPOH and 47.1 kJ/mol K for BPO/DMT.     

Evaluation of hybrid laser-cut and cast miniature fracture specimens

This paper presents a new hybrid laser-cutting method for producing fracture test specimens from thermosetting materials. The hybrid approach combines casting of a sheet of material with subsequent laser-cutting of the test specimens. The new approach was compared to the traditional casting method using a fracture toughness test. For this study, a compact version of the tapered double cantilever beam (cTDCB) was used as a specimen geometry for both manufacturing methods. The cTDCB specimen is crack length independent, and crack length investigations were performed to ensure the crack length independence of the cTDCB specimens. The specimens that were made by the hybrid laser-cut method were found to be comparable to the specimens obtained by the traditional casting method. Moreover, the laser-cut method provides a fast and accurate method to make a significant number of samples in a reasonable time. These tests show that the hybrid laser-cut method could be a good alternative to the traditional casting method.     

Polymerization and photoinitiation behavior in the light-cured dental composite resins

The polymerization behavior and photoinitiation process in the visible light-cured dental composite resins were studied using ESR and IR spectroscopies. The concentration of methacryl propagating radical corresponded to the transmittance of irradiated light. While the radical was stable and long-lived in the cured resin, its concentration quickly decreased in the oral cavity due to the ventilation of atmospheric oxygen accompanied by respiration. Camphorquinone radical in the presence of tertiary amine easily initiated the radical polymerization. A decrease in monomer consumption was explained by the inhibition effect of oxygen in the photoinitiation process. Post-curing by the residual radical proceeded effectively in the deeper part of the resin.     

电感耦合等离子体原子发射光谱法测定非贵金属齿科材料中痕量铍和镉

提出了电感耦合等离子体原子发射光谱法测定非贵金属齿科材料镍铬合金、钴铬合金及钛合金中痕量镉和铍。用盐酸-硝酸-氢氟酸混合酸溶解试样,使用基体匹配法消除基体的干扰。镉和铍的质量浓度在1.0 mg·L-1以内均与其发射强度呈线性关系,检出限(3s)在0.000 6~0.014mg·L-1之间。方法用于非贵金属齿科材料样品的分析,加标回收率在97.0%~106%之间。     

Optimum selection of the dental implant diameter and length in the posterior mandible with poor bone quality – A 3D finite element analysis

This study aimed to evaluate continuous and simultaneous variations of dental implant diameter and length, and to identify their relatively optimal ranges in the posterior mandible under biomechanical consideration. A 3D finite element model of a posterior mandibular segment with dental implant was created. Implant diameter ranged from 3.0 to 5.0 mm, and implant length ranged from 6.0 to 16.0 mm. The results showed that under axial load, the maximum Von Mises stresses in cortical and cancellous bones decreased by 76.53% and 72.93% respectively, with the increasing of implant diameter and length; and under buccolingual load, by 83.97% and 84.93%, respectively. Under both loads, the maximum displacements of implant-abutment complex decreased by 58.09% and 75.53%, respectively. The results indicate that in the posterior mandible, implant diameter plays more significant roles than length in reducing cortical bone stress and enhancing implant stability under both loads. Meanwhile, implant length is more effective than diameter in reducing cancellous bone stress under both loads. Moreover, biomechanically, implant diameter exceeding 4.0 mm and implant length exceeding 12.0 mm is a relatively optimal combination for a screwed implant in the posterior mandible with poor bone quality.     

Spectroscopic alterations on enamel and dentin after nanosecond Nd:YAG laser irradiation

Laser irradiation on hard tissue has produced a resistant surface that is likely to prevent caries. In this study, human enamel and dentine were exposed to nanosecond pulsed Nd:YAG laser with energy densities of 20-40 J/cm(2) and pulse width of 6 ns inducing chemical changes in these tissues. Infrared analysis of human dental enamel and dentine was performed using the KBr method (2mg sample/300 mg KBr). A correlation between non-lased and lased spectra was performed that gives an indication of the changes in organic and inorganic compounds after laser-tissue interaction. Spectra of teeth simultaneously show the inorganic and organic parts of the tissue. The principal bands: amide bands A, I, II, and III from the collagen-matrix, phosphate from the mineral content, and carbonate bands were identified. The normalized area of peak versus peak position was determined. Changes of the bands attributed to the collagen matrix were verified after Nd:YAG irradiation. The present results suggest a chemical modification of organic and mineral compounds by laser. The spectral results indicated an alteration in the absorption bands relative to, essentially, organic compounds.