Development of a Prototype for a Restorative Dental Cement in Kenya

Abstract

Objective/ Motivation: The aim of this study was to develop a prototype for a restorative dental cement from Portland cement (PC) clinker and related aluminosilicate (AS) materials in Kenya. This was motivated by the need for affordable alternatives in middle-income countries such as Kenya, when dental amalgam, the preferred material, is scheduled for a global phase down by 2020 following the Minamata Convention on Mercury. Study design: This was a quantitative, laboratory-based exploratory study. Study setting: The study was conducted at the Schools of Dental Sciences and Physical Sciences, University of Nairobi, Kenya; World Agroforestry Centre (ICRAF), Nairobi, Kenya; Ministry of Mining, Nairobi, Kenya; and, Otto Schott Institute for Materials Research, Friedrich Schiller University, Jena, Germany. Study materials: The materials evaluated were grey PC clinker, Kaolin, Fly ash (FA) and blast furnace (BF) slag. These were obtained from three cement and one ceramic manufacturing companies in Kenya. Methods: The project was conducted in three phases. In the first phase, PC, Kaolin, FA and BF slag, together with alkaline-activated AS or geopolymers derived from them, were characterized by X-ray Diffraction, Graphite Furnace Atomic Absorption Spectroscopy, Energy Dispersive X-ray Fluorescence and Fluoride Ion Selective Electrode for compositional analysis and laser diffraction for particle size distribution analysis, in comparison to mineral trioxide aggregate (MTA). In the second phase, ionomer-type dental cements derived from clinker or geopolymers were formulated by mixing them with freeze-dried poly (acrylic acid), CaF2 and aqueous tartaric acid. The setting reaction of these cements was followed in real time using Fourier Transform Infra-red Spectroscopy 30s to 24m after mixing. In the final phase, promising cement formulations were evaluated for the following properties of a restorative dental cement: 1. Setting time was determined using Gillmore needle apparatus; 2. Compressive strength was tested using Instron Hydraulic Universal Tester; and, 3. Fluoride ion releasing profile was evaluated by FISE. A costing assessment of the cost of production of the cements was also conducted. Data analysis: R-Studio version 3.4.2 (2017) and Microsoft Excel (2013) were used for descriptive data analysis as well as for hypothesis testing. Continuous data was subjected to analysis of variance (ANOVA) followed by Tukey’s post hoc test at  level of 0.05. Results were presented in form of tables and figures. Results: While MTA and PC comprised mainly of dicalcium and tricalcium silicate phases, geopolymers contained AS phases such as quartz and mullite. The major compounds in the three groups of materials, as determined by EDXRF, were CaO (PC=59.50%wt±7.41, MTA=46.07%wt±10.24, AS=18.35%wt±17.85), SiO2 (PC=25.65%wt±5.68, MTA=19.02%wt±0.91, AS=58.10%wt±13.65) and Al2O3 (PC=5.49%wt±0.491, MTA=2.65%wt±1.07, AS=15.31%wt±6.25). Bi was present only in MTA (24.72%wt±11.92). The major element in the three groups of materials, as determined by GFAAS, was Ca (PC=51623.00μg/g±5182.00, MTA=77083.00μg/g±4612.00, AS= 16328.00μg/g±18110.00). Only FA contained fluoride (43.33μg/g±5.77). There was no statistically significant difference in the mean composition of MTA and Kenyan PC except in the Bi (PC=0%wt, MTA=24.72%wt±11.92, F statistic=164.4, 2df, p0.0001) and Pb content (PC=0.01%wt±0.01, MTA=1.75%wt±0.44, F statistic=44.29, 2df, p0.0001). There was no statistically significant difference in the mean particle size distribution of MTA, PC and geopolymers (after sieving through 120μm mesh sieve, D50 for PC=12.46μm±3.18, MTA=7.23μm±3.43, AS=12.74μm±3.79, F statistic=1.87, 2df, p>0.05). The setting reaction of the experimental cements resembled that of glass ionomer cements, characterized by formation of polyacrylates and tartrates, identifiable as characteristic bands on FTIR spectra. The mixing and setting time of the cements ranged between 30–90s and 135–480s, respectively. On exposure to moisture, cements based on FA/ BF slag geopolymer disintegrated, those based on clinker softened and became rubbery while those based on Kaolin geopolymer remained stable. At 28d, the mean highest strength after incubation at 100% humidity was recorded for the cement derived from EAPC clinker (9.96MPa±3.21) while the mean lowest strength was recorded for the cement based on Kaolin geopolymer (2.00MPa±0.23). Mean average compressive strength tested for all cements increased over time (1d=0.41MPa±0.08, 3d=1.69MPa±0.49, 7d=4.31MPa±0.66, 28d=5.90MPa±1.06), with a statistically significant difference at the different time points (F statistic=82.39, 3df, p<0.0001). Although cements based on Kaolin recorded lowest mean strength values, their behavior was most consistent between 1d and 28d, with all samples breaking and a statistically significant difference between means at all the time points within the group (F-statistic=76.64, df=3, unadjusted p<0.0001, adjusted p=0.000). Cements derived from FA and those containing CaF2 demonstrated fluoride ion release capability, dependent on the amount of fluoride content in the formulation. Kaolin/FA cement demonstrated the highest fluoride ion release (1mmol/L at 28d). Excluding initial investment, the cost of local production of restorative dental cements was found to be less than importation. Conclusion: Kenyan PC and related materials were found to be similar to MTA, a commercial dental product, regarding composition and particle size. These locally available AS materials were capable of forming ionomer-type cements thus may be utilized to design low-cost yet stable glasses for atraumatic restorative treatment (ART) restorations. Specifically, the cements based on Kaolin were identified as a prototype. Further studies should utilize tooth-coloured materials for aesthetically pleasing restorations.