Papers by Author: M.P. Hofmann

Abstract: Mineral trioxide aggregate (MTA) is a Portland cement (PC) based material used for sealing root canals however it has a long setting time which is undesirable for dental applications. This study investigated the effect of three different calcium sulphate additions for accelerating the initial setting of a PC based dental material, whilst attempting to maintain its high compressive strength and low relative porosity. Anhydrous calcium sulphate (CaS), Plaster of Paris, calcium sulphate hemihydrate (PoP) and Gypsum, calcium sulphate dihydrate (Gyp) were each added to PC at 5wt%, 10wt% and 20wt%. Initial setting times, compressive strengths and relative porosity were measured using the Gilmore Needles Test, a universal testing machine and a helium pycnometer respectively. Scanning electron microscopy (SEM) was used to observe any microstructural changes in cements. PoP and CaS had the most profound influence on the setting of PC. 20wt% CaS had the greatest effect on the setting time of PC (10min) although decreased the compressive strength by up to 40%, which may have arisen from the formation of microcracks, observed by SEM analysis. Additions of 10wt% PoP and CaS may have the potential to reduce the long setting time of PC based dental materials.

Abstract: The present study evaluated the influence of different radiopacifiers on an established mineral trioxide aggregate (MTA)-like Portland cement system. Commercial MTA contains bismuth oxide (Bi2O3) as the radiopacifier and inert alternatives tested were barium sulphate (BaSO4), lanthanum oxide (La2O3) and tantalum pentoxide (Ta2O5). The radiopacity of the different formulations was measured with densitometry, whilst relative porosity was determined using helium pycnometry and compressive strength was measured as an indicator for the potential longevity of the alternative cement formulations. At 20wt% radiopacifier content the order of radiopacity was, Bi2O3 (3.71mm Aluminium equivalent), Ta2O5 (2.76mm Al), La2O3 (1.85mm Al) and BaSO4 (1.48mm Al). The commercial MTA control had a radiopacity of 3.65mm Al. The addition of all radiopacifiers caused a deterioration in strength and increased porosity; the incorporation of 20wt% Bi2O3 reduced strength from 68MPa by 36%. Ta2O5-containing cement had a strength comparable with that containing Bi2O3 (43MPa), whilst La2O3 and BaSO4 addition generated even weaker cements. Commercial MTA had a similar strut density to the Bi2O3- containing model system although had a lower strength (34MPa) due to its higher porosity. Bi2O3 appeared to be the best of the investigated radiopacifiers for this endodontic filling material as it provided the highest mechanical stability, lowest porosity and was the most radiopaque.

Abstract: Mineral trioxide aggregate (MTA) is a slow setting Portland cement (PC) based dental material for endodontic applications. The present study investigated the effect of adding either CaCl2 or Plaster of Paris (PoP) as setting accelerators on the development of the material properties and microstructure with reaction time for a PC based model system. Mechanical strength, density and relative porosity were measured after 1, 10 and 30days and the microstructure was assessed using scanning electron microscopy (SEM). The strength of all cements increased with time whereas material density and relative porosity decreased due to the progress of the hydration reaction. Cements with 5-10% CaCl2 in the liquid phase had a higher final strength and lower porosity than cements modified with 20wt% PoP in the cement powder, whilst PoP modified cement had a shorter setting time of 15min compared with 60min for 10% CaCl2 addition. The microstructure of the two modifications were noticeably different, with the CaCl2 modified cement having more interconnected needle-like crystals than seen in PoP modified cements, which may explain the higher strength of this cement.

Abstract: The effect of mechanical mixing on compressive strength, relative porosity and reliability of strength data of a brushite forming cement at different powder to liquid ratios (PLRs) was investigated. Mean compressive strengths were measured, associated reliability (Weibull moduli) and survival probability distributions of the data sets were analysed. Relative porosities were determined using helium pycnometry. For low PLR (2.2g/ml), no significant differences in compressive strength were observed for either mechanical or hand mixed samples, although reliability of the former was significantly increased. At high PLR (3.4g/ml), mechanically mixed cements exhibited approximately twice the mean compressive strength compared with hand mixing, although Weibull moduli remained statistically similar. At medium PLR (2.8g/ml) strength and reliability of cements were similar and independent of mixing regime. For all PLRs, a significant decrease in porosity of mechanical- compared with hand-mixed cements was observed. Mechanical mixing of a brushite cement can provide lower porosity, increased reliability and higher strength.

Abstract: This study investigated the influence of the addition of various proteins to the liquid phase (albumin, fibrinogen and foetal bovine serum (FBS)) on the mechanical strength and setting time of a brushite forming calcium phosphate cement. Additions of 1wt% protein to the liquid phase led to a deterioration in compressive strength of the set cement by up to 50%. The setting time was not affected by adding albumin and FBS but was increased by 50% with admixtures containing fibrinogen. The conversion of the reactants, β-tricalcium phosphate and monocalcium phosphate, to brushite was found to be unaffected by addition of up to 10wt% proteins.

Abstract: Time resolved infrared spectroscopy (FTIR) and isothermal differential scanning calorimetry (DSC) were used for the first time to monitor the chemical reaction in a fast setting brushite forming calcium phosphate cement. It was found that the reaction percentage at a given time was dependent on temperature and not powder to liquid (P/L) ratio. Both methods showed that there was, within the temperature range investigated, a single autocatalytic like setting reaction within the cement paste. Final conversion of the reactants was found to be unaffected by temperature and P/L ratio.

Abstract: This study sought to examine the efficiency of coating cement powder reactants in order to reduce the solubility rate of reactants and thereby increase setting times of cement systems. In this investigation magnesium and sodium stearate salts were used to coat the highly soluble monocalcium phosphate monohydrate (MCPM) powder component of a hydraulic brushite forming calcium phosphate cement system with b-tricalcium phosphate (b-TCP) as other component. The results showed that stearate coating of the MCPM reactant could lead to a 100% increase in setting and working times without affecting compressive strength of the set cement when applied with the appropriate P/L-ratio.