Eshmawi, Yousef Tariq; (2022) Novel Resin-composites for Minimally Invasive Restoration of Teeth. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The use of resin restorative materials became predominant with the growing demand for tooth conservation procedures. The development of self adhesive resin composite has simplified their application method. The addition of remineralising and antibacterial agents may further improve the longevity of these restorations by sealing gaps and cracks when applied on caries affected dentine layer. Therefore, it is essential to evaluate the effect of adding these agents on the stability of the experimental composites chemical, physical and mechanical properties. AIMS: The aim was to investigate the effect of MCPM and PLS on the polymerisation stability and paste shelf life of experimental composite formulations, and also evaluate the depth of cure of the experimental formulations and two commercially available restorative materials. In addition, the polymerisation shrinkage, subsequent water sorption-induced mass changes, expansion and hydrogen ion release from four experimental resin-composite formulations were evaluated. The simulated wear resistance of five experimental resincomposite formulations and two commercial restorative materials was also investigated. Furthermore, the aim was to assess the effects of the storage container, time of high-temperature paste aging, sample depth and curing on colour of four experimental resin-composite formulations. MATERIALS AND METHODS: Resin-composite formulations containing PLS (0, 3, 4, 6 or 8 wt%) & double these levels of MCPM and three commercial self-adhesive restorative materials were evaluated. The polymerisation kinetics and paste shelf life were evaluated by using FTIR spectroscopy. Delay times, maximum reaction rate, 50% reaction time (t0.5) and final degree of conversion (%DC) were determined. Resin-composite pastes were stored at room temperature (7 months), 60oC (18 days or 9 months) and at 4oC (9 months). The aged formulations were compared to experimental formulations stored at 4oC and room temperature. Percentage mass and volume change upon water storage were determined gravimetrically regularly up to 11 weeks by using Archimedes Principle and the polymerisation shrinkage was measured using ISO 17304:2013. Storage solution acidity was evaluated at each time point and dried solid content was analysed after 3 months and 1 year using SEM, FTIR and EDX. The wear resistance was evaluated after 3 months and 1 year by using a dual axis-chewing simulator. L*a* b* values for pastes and composite discs were assessed using a spectrophotometer. Pastes samples were stored in sealed brown glass containers at 4oC, 60oC, or 80oC and studied before and after cure. Results were compared with previous data (Pitsillou, 2019) for paste stored in compules. RESULTS: Experimental resin-composite formulations were only mildly affected by additive addition or high-temperature aging. Average experimental composite delay times were 4.9, 7.7 and 24.5s, at 1mm, 2mm and 4mm depths, respectively. The maximum rates of reaction were on average 5, 4 and 2%/s, at 1mm, 2mm and 4mm depth, respectively. The 50% reaction times were 12, 16 and 41s, at 1mm, 2mm and 4mm depth, respectively. At 1mm and 2 mm depth, %DC of 74% and 72% could be achieved with 20s light exposure. 40s light was required to gain an average of 64%DC at 4 mm depth. Experimental formulations delay times and rates of reaction showed major differences when compared to Activa (2, 1 and 11s and 3, 2 and 0.67%/s) and Vertise flow (2, 4 and 10s and 5, 4 and 1%/s) at 1mm, 2mm and 4mm depth, respectively. The polymerisation shrinkage ranged between 3.0 and 4.0%. PLS significantly influenced subsequent initial rate of mass change versus square root of time (p<0.05). After 11 weeks, mass changes were -0.9±0.2, 0.1±0.1, 1.1±0.1 and 1.2±0.1 wt% for F1, F2, F4 and F5, respectively. The final values of volume change were 3.7±0.1, 3.7±0.1, 3.9±0.3, and 2.6±0.1 vol.% for F1 (MCPM 16wt.% and 8wt.% PLS), F2 (MCPM 16wt.% and 4wt.% PLS), F4 (MCPM 8wt.% and 8wt.% PLS) and F5 (MCPM 8wt.% and 4wt.% PLS), respectively. Increasing both additives had a significant impact on volume change (p<0.05). H+ ion release was higher for composite formulations containing high levels of MCPM. The highest average H+ release by 11 weeks was 22±2 micromoles/sample and the lowest was 9±2 micromoles/sample for F1 and F5 respectively. SEM images showed signs of extensive crystallisation from the dried extracts. Elemental analysis from EDX of the dried extracts after 3 months showed P/Cl ratio was approximately 2, 4, 1, 2 as expected from the MCPM: PLS ratios in the evaluated formulations. For FTIR, The MCPM: PLS ratios in dried extracts are 3 times the MCPM: PLS wt.% ratio in the material for F1, F2 and F5. For F4, the ratio is 6 times that in the material. For the wear resistance, the highest surface volume loss after 3 months was 3.40±1.47 mm3 and the lowest was 2.21±0.06 mm3 for F2 and F4, respectively. After 1 year, the highest surface volume loss was 2.89±0.45 mm3 and the lowest was 1.72±0.40 mm3 for F7 (MCPM 8% small particle size and 4% PLS) and F4 respectively. Surface volume loss for commercial materials ranged between 1.20±0.46 to 1.72±0.26 mm3 with the highest average for Fuji II and the lowest for Activa. F7 showed higher surface volume loss and differed significantly to Activa (p=0.046), Fuji II and F4 (p<0.05). The b* values showed greater change than L* and a*. The average b* values for compule-stored, 1mm thick composite paste at 4oC, 25oC and 37oC, covered with acetate were 23 ±1 and 29 ±1.3 without acetate. Following 20s cure, they declined to 6 ±2 and 7.5 ±2 on top and bottom surfaces, respectively. For samples with 4 mm depth, the results ranged between 8.2 to 31.4 with an average of 11.7 ±2.4 for top surfaces and 28.3 ±3.8 for bottom surfaces. The bottom surfaces of the 4mm groups showed higher b* values than the top surfaces and the overall 1mm group, particularly regardless of the curing surfaces. The effect of the composition is evident after long time storage at high temperature as formulations with high PLS became yellower than those with lower PLS. CONCLUSION: The experimental formulations were stable upon high temperature aging. For experimental formulations and commercial materials, 20s light exposure is sufficient for good monomer conversion up to 2mm depth. Increased thickness causes an increase in delay time and a decrease in the rate of reaction. Polymerisation shrinkage was not significantly influenced by MCPM and PLS level. The increase of MCPM and PLS led to higher volume change and H+ release, while increasing PLS led to higher mass change. Reducing MCPM particle size led to lower wear resistance than commercial comparisons. Storage of resin-composite paste in compules instead of sealed containers caused greater colour instability. Sample thickness has a greater effect than high temperature storage conditions on colour stability. High PLS affected the colour stability upon long time storage at high temperature.

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