[Show abstract][Hide abstract] ABSTRACT: The mismatch of thermal expansion and contraction between restorative materials and tooth may cause stresses at their interface, which may lead to microleakage. The present work compared the coefficient of thermal expansion (CTE) with the thermomechanical behavior of human and bovine teeth and determined if the CTE is a suitable parameter to describe tooth behavior. Fifteen human third molar and 15 bovine incisor tooth slices (6×5×2 mm) were allocated to 3 groups according to the test environment: G1 - room condition, G2 - 100% humidity, G3 - desiccated and tested in dry condition. Each specimen was weighed, heated from 20 to 70ºC at 10ºC min-1 and reweighed. The CTE was measured between 20 and 50ºC. Fresh dentin (human -0.49% ± 0.27, bovine -0.22% ± 0.16) contracted on heating under dry condition. Under wet conditions, only human teeth (-0.05% ± 0.04) showed contraction (bovine 0.00% ± 0.03) accompanied by a significantly lower (p<0.05) weight loss than in dry specimens (human 0.35% ± 0.15, bovine 0.45% ± 0.20). The desiccated dentin expanded on heating without obvious weight changes (0.00% ± 0.00). The CTE found was, respectively, in dry, wet and dissected conditions in ºC(-1): human (-66.03×10(-6), -6.82×10(-6), 5.52×10(-6)) and bovine (-33.71×10(-6), 5.47×10(-6), 4.31×10(-6)). According to its wet condition, the dentin showed different CTEs. The thermal expansion behavior of human and bovine dentin was similar. A simple evaluation of the thermal expansion behavior of tooth structure by its CTE value may not be appropriate as a meaningful consideration of the effects on the tooth-material interface.
Full-text · Article · Jan 2012 · Brazilian dental journal
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to manufacture and test, in vitro, a novel modification to provide fluoride-releasing orthodontic brackets. Thirty-two orthodontic brackets were drilled to produce a recess (approximately 1.3 mm in diameter and 0.7 mm in depth) at the centre of the bracket base. Four materials, with and without the addition of sodium fluoride, a glass ionomer cement (Ketac Cem micro), a resin-modified glass ionomer cement (RMGIC; GC Fuji Ortho LC), a zinc phosphate (Zinc Cement Improved), and a resin (Transbond XT) were used to fill the recess in the bracket base. Fluoride release was measured daily during the first week and then weekly for 10 weeks. An ion chromatograph with suppressed conductivity was used for free fluoride ion determination. Statistical analysis to determine the amount of flouride release was undertaken using analysis of variance and Tukey's test. During the first 2 weeks, the resin group, with the addition of 38 per cent sodium fluoride added, released significantly more free fluoride (P < 0.05), but after 2 weeks the fluoride release markedly decreased. After 5 weeks, the RMGIC group, with 15 per cent added sodium fluoride, had significantly higher (P < 0.05) daily fluoride release than the other groups. The findings demonstrated that an appropriate fluoridated material can be used as a fluoride-releasing reservoir in a modified orthodontic bracket to enable it to release fluoride over the period of fixed appliance treatment.
Preview · Article · Dec 2007 · The European Journal of Orthodontics
[Show abstract][Hide abstract] ABSTRACT: This study was designed to determine the dimensional changes caused by thermal stimuli of glass ionomers with different glass/matrix ratios. Four cylindrical specimens were made for each of four powder/liquid ratios (3:1, 2.5:1, 2:1 and 1.5:1) for a conventional luting glass ionomer, two high viscosity restorative glass ionomers and a restorative resin-modified glass ionomer. The thermal characteristics were determined using a thermal mechanical analyzer (TMA) by heating the samples from 25 degrees C to 70 degrees C at 10 degrees C per minute. All glass ionomers and the resin-modified glass ionomer lost water on heating. The results of the thermal response of these materials were explained in terms of the opposing effects of thermal expansion and desiccation on heating. The contraction on heating of glass ionomer and related materials was found to relate to the glass/matrix ratio but not directly proportional to it. Materials with lower P/L ratios contracted the most when heated to 70 degrees C. The water loss from conventional and resin-modified glass ionomer with different glass/matrix ratios compensated for their thermal expansion and led to a minimal dimensional change when heated up to 50 degrees C. This outcome may be interpreted as an example of smart behaviour of these materials.
No preview · Article · Jul 2007 · Journal of Materials Science Materials in Medicine
[Show abstract][Hide abstract] ABSTRACT: This study was designed to determine the dimensional changes of glass ionomers caused by thermal stimuli under both dry and wet conditions.
Eight cylindrical specimens (6 mm x 4 mm) were made (using a stainless steel mold) of each of the following materials: a conventional luting glass ionomer, two high viscosity restorative glass ionomers, a resin-modified glass ionomer and a resin composite which was used as a control. The thermal expansion characteristics were determined by a thermal mechanical analyzer (TMA) under wet and dry conditions by heating the samples from 25 to 70 degrees C at 10 degrees C min (-1).
All materials showed contraction on heating in dry ambient conditions. In wet conditions, all glass ionomers maintained their original dimensions on heating, but the resin-modified glass ionomer expanded. The resin composite showed expansion in both wet and dry conditions. The results are explained in terms of the opposing effects of thermal expansion and desiccation on heating.
Under wet conditions glass ionomers maintain their original dimensions when heated. This kind of behavior may be considered as 'smart' behavior.