The surface of any dental pulp-capping material has important implications for its clinical success because it is in direct contact with dental tissue, which influences its cytotoxicity. The aim was to determine the chemical composition of the first atomic layers of four pulp-protection agents because these atoms can initiate the pulp healing process.
Biodentine (Septodont), ProRoot MTA (Dentsply), Dycal (Caulk) and TheraCal (Bisco) were prepared (n = 5) according to manufacturer recommendations. The chemical surface composition was analyzed using X-ray photoelectron spectroscopy (XPS), and the bulk composition was analyzed by Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX). Both survey and high resolution XPS spectra of the elements detected were obtained, with element-dependent probe depths of 4–5 nm; the binding energy scale was normalized to the C1s adventitious carbon peak at 285 eV.
There was a significant difference between bulk and surface compositions for all the pulp-capping materials. The calcium surface concentrations at 0 nm and 70 nm were Dycal 7.9% and 15.1%; ProRoot MTA 14.1% and 17%; TheraCal 0% and 3.6%; and Biodentine 17.6% and 33.7%, respectively. Trace amounts of the following elements (<1%) were also found: Ti, S and Zr in Biodentine; Bi in ProRoot MTA and TheraCal; Na, P, Zn and N in Dycal.
The XPS results showed that Ca in the surface layer could vary from 0 to 18%, depending on the material. Aliphatic carbons, from the polymerization reactions, especially in Dycal and TheraCal, were found to mask the other components.
This study compares, for the first time, the chemical composition of the first atomic layers of four pulp-capping materials. This information is relevant because the interaction between pulpar cells and the material’s outermost atomic layer is an important factor for leading the pulpal response.
Calcium-based cements are important tools in the restorative field, for both direct and indirect pulp-capping. They are used for a variety of procedures, and are essential in helping to maintain pulpal vitality when the remaining dentin is believed to be less than 2 mm thick . Until recently, direct pulp-capping had been considered controversial, with the historically-accepted treatment of a pulp exposure being endodontic therapy . The main difficulty with direct pulp-capping has been the unpredictability of the pulpal response. Calcium hydroxide-based cement (i.e. Dycal ) was the gold standard for pulp-capping materials, and has been used since its introduction in 1966 . In the past few decades, several advances have been made in direct and indirect pulp-capping, as well as in new materials, such as hydraulic calcium silicate cements (HCSCs), marketed as being able to improve the success of the treatment . Currently, some of the most prevalent HCSCs include a mineral trioxide aggregate (e.g. ProRoot MTA), a resin-modified calcium silicate (e.g. TheraCal LC) and a bioactive tricalcium silicate (e.g. Biodentine). These products are all based on di- or tri-calcium silicates, but their chemical compositions have subtle differences that drastically influence their clinical performances.
ProRoot MTA is prepared by hand-mixing a liquid and a powder; Biodentine is a powder and liquid that are triturated, similar to amalgam; TheraCal is light-cured. These materials have the potential to release calcium into the surroundings, as does Dycal. Calcium released from a pulp-capping agent is thought to promote mineralization, increase the pH to bactericidal levels and be relatively non-cytotoxic in vivo . Given the similar chemical bases of ProRoot MTA, TheraCal and Biodentine, it would be expected that their effects are similar; however, even subtle differences in chemical structure can influence the responses of the pulp and surrounding tissues and their cytotoxicities.
The bulk chemical composition of the pulp-capping materials is well established . These dentin replacement materials are mainly composed of calcium hydroxide (Ca(OH)2) or calcium oxide (CaO), and components to improve their mechanical properties, biocompatibilities and setting controls. The surface composition of a biomaterial is thought to be the same as that of its bulk; however, because of contact with atmospheric contaminants and their effect on surface thermodynamics, the chemical composition of the biomaterial surface may differ from that of its bulk .
Pulp capping materials can be in direct contact with pulpal cells while pulp exposure. In these situations, these biomaterials should present high biocompatibility levels. For the biocompatibility issue, the most important aspect of the biomaterial is the outermost chemical surface because this layer will be in intimate contact with the tissues. These reactions occur at the interface between cells and the biomaterial , and the nanoscale surface composition of the biomaterial is a major variable that determines the host response . In this context, the in vitro and clinical effects of the HCSCs and Dycal have been studied by many authors; however, information on their nanoscale surface chemical compositions is not presently available. The lack of information could be explained due to the difficult to probe surface in the nanolevel. The use of a powerful surface-sensitive technique, such as X-ray photoelectron spectroscopy (XPS), can help in this issue. XPS are able to characterize the first atomic layers of pulp-capping materials and give insight as to why there are differences in cytotoxicity and calcium release among them .
The aim of this study was to qualitatively and quantitatively characterize the surface chemical compositions, over the first 70 nanometers of depth, of four pulp-capping materials (TheraCal, ProRoot MTA, Biodentine and Dycal), and to compare them to their bulk compositions. The null hypothesis is that there is no difference of the chemical composition between the surface (at nanolevel) and the bulk of the studied pulp capping materials.