Papers by Author: R.M. Shelton

Abstract: According to the gene repertoire, distinct morphology and the organisation of extracellular matrix, osteoblast development was identified as a series of stages, proliferation, differentiation, matrix deposition, matrix maturation and mineralization. Each of these stages required tightly regulated and functionally coupled expression of genes related to the transcription factors and bone matrix. In this paper, we identify the effects of OCP to the differentiation of osteoblasts from the point of view of differentiation sequence development. Osteogenic medium (Ost MEM) mainly regulated the osteocalcin (OC) mRNA expression in the first week of culture. As culture continued to 24 days, OCP crystal assemblies became the main regulator. This shift in the role that OCP and Ost MEM played in regulation may reflect different biological functions of OC in Ost MEM induced regulation and OCP crystals induced regulation. The up-regulated OC mRNA expression by OCP crystal assemblies may function as a signal to coordinate the activities of osteoblasts and osteoclasts instead of inducing mineralisation at the end of the differentiation sequence of osteoblasts. By comparing the modified expression pattern observed on the OCP crystal particles with the patterns of differentiation sequences, it was found that BMSCs colonising OCP crystal assemblies from day 7 to day 24 matched the process of differentiation in the early stages of matrix deposition. The gene expressions of BMSC cultured in the osteogenic medium (positive control) corresponded with the process from matrix deposition to mineralisation. Hence, the differentiation process of BMSCs on OCP crystal assemblies was different from that found on the positive control. BMSCs could differentiate to osteoblasts that would function as a regulator for osteoclast activities.

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: 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.