Papers by Author: Shi Bao Li

Abstract: Objective: To evaluate the fluoride release rates and fluoride recharge capability of a experimental resin composite, a compomer, a giomer and a resin-modified glass ionomer cement. Methods: Ten disc specimens of each material were prepare and polished with abrasive paper, followed by clearing with oil-free compressed air jet. The specimens were placed in a plastic vessel containing 5ml deionized water immediately after fabrication. Each specimen was removed and put to a new a vessel containing deionized water each day, and the sample solution was test for fluoride concentration each day using a compound fluoride ion selective electrode and a conductivity meter. All specimens were removed from container after 28 days and were recharged by immersing in a fluoride foam solution for four minutes. Then fluoride release from these recharged samples was measured daily for 7 days. Recharge was repeated three times. Result: Fuji II LC demonstrated the highest fluoride release ( p <0.01), followed by experimental resin and Compoglass. Both Beautufil and Charisma presented the lowest fluoride release ( p <0.01) during the first two weeks. All materials except Charisma demonstrated a sharp decline of fluoride release, followed by a comparatively stable stage of fluoride release for about twenty days. All materials presented fluoride release increase substantially one day after recharge but declines rapidly almost to the baseline level after 2 days except for Fuji II LC that showed a continual fluoride release for 5 days. Conclusion: The experimental composite resin has a capability of fluoride release and recharge that are comparable to compomers.

Abstract: A novel PMMA-ZrO₂ composite (PZC) was prepared by resin infiltrated to ceramic method. The composite mechanical properties were evaluated and correlated to its microstructure. Partially sintered zirconia ceramics (PSZC) were made by isostatic pressing and partially sintering. Subsequently, the PZC was prepared by vacuum infiltrating prepolymerized MMA into PSZC, followed by in-situ polymerization. When PSZC-70% was used as the matrix, the bending strength, elastic modulus, and fracture toughness of the prepared composite i.e PZC-70% were 202.56±12.09 MPa, 58.71±3.98 GPa, and 4.60±0.26 MPa·m^1/2, corresponding to 25.69%, 23.31%, and 169.01% improvement, respectively, in comparison with the control matrix. Among them, the fracture toughness improvement was the most prominent. According to SEM images of the fracture surfaces, each pore of zirconia skeleton was filled by organic resin contributing to the bending strength improvement. These weak interfaces between zirconia skeleton and organic resin absorbed energy and terminated the growth of microcracks which might be responsible for significant improvement in fracture toughness. This PZC material is anticipated to be a new member of the dental CAD/CAM family.

Abstract: Objectives: To compare the permeability of human dentin and a porous ceramic with two different compositions of perfusion fluid, in vitro. Methods: Dentin discs were cut from extracted third molar at a level close to the pulp chamber and ceramic discs were cut from a porous ceramic block. The convective permeability of the discs was measured in a fluid transport model using de-ionised water or 1:3 diluted bovine serum and the hydraulic conductance was determined. Results: In the negative controls, no flow was detected. The hydraulic conductance values for the dentin discs and the ceramic discs when water was used as the perfusion fluid were 0.069±0.023and 0.047±0.019ml min^-1 cm^-2 cm H₂O^-1, respectively. When the discs were perfused with the diluted bovine serum the mean (standard error of the mean) hydraulic conductance values for the dentin and the ceramic discs were 0.036±0.016 and 0.012±0.0055 ml min^-1 cm^-2 cm H₂O^-1, respectively. Conclusion: Basing on the modern technique it is possible to fabricate discs of hydroxylapatite ceramic to act as substitutes for dentin barriers in cytotoxicity test.

Abstract: To evaluate the effects of specimen grips on the measurement of the micro-tensile bond strength (mTBS) to dentin. Methods: Twelve extracted human molars were sectioned to expose mid-coronal dentin. Each surface was ground with 600-grit SiC paper. Four adhesives: Prime & Bond NT (Dentsply, USA), Contex (DMG, German), Adper Prompt (3M/ESPE, USA) and Clearfil S³ Bond (Kuraray, Japan) were applied to the polished surfaces followed by creation of composite buildups. After 24 hr storage in 37°C water, the teeth were sectioned perpendicular to the adhesive interface to produce multiple beams of composite-bonded dentin, approximately 0.8 mm² in cross-sectioned area. Half of the specimens were attached to testing grips A which did not contain positioning pins and another half were attached to the testing grips B which contained positioning pins. All specimens were tested using a universal testing machine at a crosshead speed of 1.0 mm/min. Results: Specimens tested using the grips A presented lower mTBS than using the grips B (P<0.01). Many specimens tested using the grips A showed mix failure or cohesive failure within composite, and most of the failures were adhesive for specimens tested using the grips A. Conclusion: Specimen grips without positioning pins cannot accurately present mTBS and the grips with positioning pins can more accurately present mTBS.

Abstract: The objective of this study is to compare the degradation of three strontium-containing calcium phosphate cement (Sr-CPC) and one calcium phosphate cement without strontium (CPC) in vivo. Three Sr-CPCs, containing 1%, 5%, 10% strontium respectively, and a CPC without strontium were tested in this study. The specimens in rod-shape (2 mm  6 mm) were prepared, and were implanted in the erector spine muscle of 15 New Zealand rabbits. After 4, 8 and 12 weeks, 5 rabbits were sacrificed respectively and the specimens were taken out, cleaned, dryed and weighed. The weight losses of the specimens were calculated and the data were analyzed by ANOVA. The results showed that the CPC containing 5%, 10% strontium showed obviously higher degradation rates at the three observation periods than that containing 1% strontium and that without strontium (P<0.05). Addition of 1% strontium into CPC did not increase degradation rate (P>0.05), and the CPC containing 5% and 10% strontium showed no difference in degradation rate at the three observation periods (P>0.05).

Abstract: The aim of this investigation is to study the effect of SnO coatings on the bond strength between titanium and porcelain. The coatings were produced by sol-gel method and heat-treatment at 300°C. Once the coatings have been formed on the titanium substrates, self-made porcelain was fused. The specimens with pre-oxidation before porcelain fused were set as control group. It was shown SnO gel formed completely at 300°C and was composed by SnO and SnO2 in the form of a uniform film. The bond strength between titanium and porcelain of the specimens with SnO coatings was greater than those in control group statistically, which might prove that the coatings reduced the oxidation of titanium surface in the porcelain fusing progress. EDS result showed almost all fractures between titanium and porcelain occurred at the oxide layer. Si and Sn might attend the reaction at interface. It was concluded that the SnO coatings produced in this study can improved the titanium-porcelain bond strength.

Abstract: Partially sintered zirconia ceramics (PSZCs) for dental uses were prepared from zirconia nanopowder via isostatic pressing and partially sintering. The open porosities, pore diameters, grain sizes and mechanical properties of the ceramics with different densities were studied. The results show that the pores formed in the PSZCs are all open pores, with a diameter distribution of 60nm~130nm and a grain size distribution of 120~170nm. The machinability becomes worse when the density of PSZC is higher than 75% of the theoretical density, so a ceramic named PSZC-70% with density of 70%TD was selected as the target material. Its bending strength is 168 MPa and fracture toughness is 1.8 MPa·m1/2. A dental restoration framework can be obtained via machining the PSZC-70% on a dental CAD/CAM system.