Essential toolkit for a dental materials scientist: Search engine (Part I)

MEDLINE

For many clinical researchers, MEDLINE is probably the starting point for any article search. MEDLINE comprises over 5000 journals published worldwide and is the largest part of the PubMed database, a service of the U.S. National Library of Medicine. PubMed also contains other life science journals.

As a result of U.S. National Institutes of Health Public Access policy aimed at increasing free access to articles, Pubmed Central (PMC) has been created as a free digital archive of biomedical and life sciences journal literature. It contains journals which submit articles regularly but also articles published by NIH-funded researchers in journals currently not on the PMC list. Full text is available in either HTML or PDF format.

A particularly useful tool available at PubMed is “MyNCBI” which allows searches to be saved and filtering options and automatic searches set up. It is located in the top right corner of the PubMed homepage and requires registration (free). MyNCBI offers various features but among the most useful are automatic searches and collections.

Automatic search: Once you enter keywords and search results are generated, you should save the search by clicking the “Save Search” option next to the search box. The search is saved to MyNCBI. Then, you can enable automated search in MyNCBI and the results will be emailed to you daily or once a week or month, according to the settings. The same keywords from the initial search will be used every time in the automated search.

Collection: Once you enter keywords and search results are generated, you should save the search by clicking the “Send to” option and selecting “Collections” from a drop down menu. A collection can be made public by selecting the appropriate option in MyNCBI, in which case a direct URL or HTML for web pages and blogs are generated.

You can access MyNCBI through PubMed homepage, but if no PubMed search is intended, then you can use a direct link to MyNCBI

News from jobs.ac.uk

Lecturer in Restorative Dentistry
(Click on the link for more information)

University of Leeds - Leeds Dental Institute
Salary on the Clinical Lecturer scale (£30,685 - £57,084 pa)

Application deadline: 24 October 2009

Resin-based materials: Degree of conversion

Resin-based materials, such as resin-based composites, adhesives, pit & fissure sealants and resin cements, undergo monomer to polymer conversion during both light-activated or chemically-activated polymerisation. Whilst conversion is an inherent property of resin-based materials, the degree of conversion (DC) depends on material chemical composition and curing conditions. The DC affects mechanical properties of resin-based materials, such as wear, fracture toughness, hardness, flexural modulus and fatigue. Less than optimal conversion may compromise mechanical properties but also result in leaching of monomers from restorations.

Traditionally, manufacturers' technical and scientific data did not incorporate results of internal or external tests for the DC. Even most recent materials often lack these data but there seems to be a growing understanding of the importance of this property.

The DC of resin-based materials in dental studies has been determined using various methods for more than two decades, but recently, the most widely accepted and used methods are infrared and Raman spectroscopy. These are based on measuring the changes in either the absorbance or scattering effect of those molecular groups which take part in polymerisation of resin-based materials. The DC is determined as the ratio of absorbance or scattering of these groups and a certain internal standard in uncured and cured material. An internal standard is another molecular group which does not take part in polymerisation and, thus, its infrared absorbance or Raman scattering remains constant before and after polymerisation.

It is very important to point out that the DC indicates the number of unreacted methacrylate or other polymerisable groups and not the amount of unreacted monomers in the polymer. The DC of e.g. 70% indicates that there is 30% of uncreacted groups and not 30% of free, unreacted monomers trapped within the polymer network that could theoretically leach out. Cross-linking monomers in resin-based materials most often contain more than one polymerisable group which means that cross-linking may occur via some but not all such groups. Furthermore, unreacted polymerisable groups always exist at the ends of polymer chains. There have been some estimates that in resin-based composites with the DC of around 70%, the amount of unreacted monomers is actually less than 10%. This depends on material chemical composition and may vary significantly in different resin-based materials.

Dental Research in the UK: Funding

In 2006, the British Society for Dental Research (BSDR) commissioned a position paper on oral and dental research within the United Kingdom which would serve as a foundation and a framework for a national plan for oral and dental research. This paper was written by Iain Chapple, Paula Farthing, David Williams and Michael Curtis in 2006 and updated in December 2007.

According to this strategic review, the UK dental school research income in the period 2000-2004 was less than 2% of medical schools research income. Knowing that the NHS spend on dental care was about 5% of the total NHS healthcare spend, this indicates that dental research was under funded compared to medical research.

The majority of dental research funding was provided by UK government, industry and charities over this five-year period. Funding from research councils, EU and other sources constituted up to one third of the overall funding per year. It is interesting that most of the charitable funding came from the Wellcome Trust since there is no national charitable source dedicated to oral research.

The best financed research in UK dental schools has been basic with more than two thirds of awarded grants. Less than one third of grants was awarded for clinical research in dental schools. These data are based on grants awarded between 2000 and 2005 by the Medical Research Council (cca. £6m), Wellcome Trust (£8.68m), Biotechnology and Biological Sciences Research Council (£cca. 1.2 m) and Engineering and Physical Sciences Research Council (£520k). The percentage of dental materials research funding has not been reported in this position paper.

Some of the identified reasons for under funding dental research in UK dental schools include the lack of representation on review panels of research councils and major charities, the absence of a national charitable source for funding oral research and underscoring grant proposals by internal panels in spite of high scores by external expert reviewers.

Four priority areas have been proposed in this position paper:

• Establishing a dedicated Oral and Dental Research Charity
• Better representation for oral and dental research on review panels
• Developing critical mass through nationally-coordinated research consortia and
• Encouraging inter-school collaboration.

Light curing of resin-based composites and adhesive systems

Light cured resin-based materials are predominantly used in current dental practice. Light curing protocols have changed over time following changes particularly in light-curing units (LCUs) since the photoinitiator system in these materials has remained virtually unchanged. Though there are attempts to modify the photoinitiator system, the most frequently used one is based on camphorquinone and a tertiary amine.

On the other hand, the LCU technology has been developing in several directions. LCUs comprise four different types of light sources: halogen, light-emitting diode (LED), plasma arc and laser. Halogen and LED LCUs are most often used in dental practice and studied in the dental literature. Light intensity and curing time have been identified as important parameters in monomer conversion which affect mechanical characteristics of the resultant polymer and subsequently its clinical performance. As light intensity has increased from about 500 mW/cm2 which is characteristic of the so-called ‘conventional’ LCUs to more than 700 mW/cm2 in the so-called ‘high-power’ LCUs, most manufacturers recommend shorter curing time. Consensus opinion in the current dental literature is that light energy density (light intensity multiplied by curing time) is a more important determinant of the degree of conversion of resin-based composites (RBCs) and adhesives than light intensity. It is currently recommended to cure adhesive systems for 20 s with LCUs operating at intensities of about 500 mW/cm2 and 10 s with LCUs operating at intensities of more than 700 mW/cm2. For RBCs, the recommended curing time is 40 s with the former LCUs and 20 s with the latter ones. The recommended thickness for each layer of RBCs in the incremental technique is still 2 mm.
Though many LCUs possess additional curing modes, such as soft-start or pulse in order to reduce polymerisation shrinkage of RBCs, there is no scientific evidence that these modes affect the long-term clinical performance of resin-based restorations.

It has been shown that maximum absorption range of camphorquinone is about 468 nm and therefore most LCUs, especially LED and plasma arc, have a very narrow emission range. However, the absorption range of co-initiators may be outside the emission range of such LCUs, thus, leading to insufficient conversion. Most recently, the so-called ‘poly-wave’ LCUs have been introduced on the market in an attempt to cover the absorption range of the entire photoinitiator system and produce maximum conversion for a given material. Future studies will show whether this new approach ensures such monomer to polymer conversion which would lead to better mechanical properties of RBCs and adhesives.

Studies have shown that increased curing distances lead to lower degree of conversion and it has recently been suggested that 6 mm may be a cut-off distance. However, it should be noted that various LCUs and materials may exhibit differences in curing efficiency at various distances. Therefore, as a general rule, the LCU tip should be placed as close as possible to the surface of RBCs and adhesives.

The superficial layer of RBCs and adhesives is insufficiently cured due to oxygen inhibition. It is removed by polishing RBCs but in adhesives, this layer serves as an intermediate zone enabling the formation of the RBC-adhesive bond. It is, therefore, important to use RBCs and adhesives with compatible chemical composition in order to achieve optimal RBC-adhesive bond by interaction of compatible monomers from both materials.

News from jobs.ac.uk

Postdoctoral Research Associate

Tissue engineering scaffolds for bone tissue engineering
King's College London - Department of Dental Biomaterials Science
Salary: £30,000 to £49,999

Application deadline: October 26, 2009.

Filtek Silorane by 3M ESPE

In 2007, 3M ESPE launched a new resin-based composite, Filtek Silorane (FS), and its adhesive system. Both the composite and the adhesive system contain a unique resin monomer based on the combination of siloxanes and oxiranes so it is apparent where the term "silorane" comes from. The polymerisation of FS differs from methacrylate-based composites and adhesives and is claimed to result in reduced polymerisation shrinkage. The cationic polymerisation of FS occurs via the ring opening of the C-O-C epoxide group which ends up in less reduction in molecule distances compared to the free radical polymerisation of methacrylate-based composites. In the latter, monomer interaction via methacrylate C=C groups results in the greater reduction of inter-molecular distances and subsequently greater polymerisation shrinkage.

Most recent studies have shown reduced shrinkage and shrinkage stress and strain for FS compared to methacrylate-based composites. Microleakage and nanoleakage were also reported for FS. Ongoing studies will reveal other properties of FS that may affect its clinical performance.

The dedicated adhesive system is designed to bridge the gap between hydrophilic dentine and hydrophobic FS composite. It contains the Primer and the Bond in separate bottles which are cured as separate layers, unlike any other two-step self-etch adhesive system, where primer and bond are mixed before curing. In Filtek Silorane adhesive system, these layers are not visible on SEM but can be detected using micro-Raman spectroscopy (Santini & Miletic, 2008) At the BSDR symposium on Dental materials it was reported, that after 6 months of storage, the type of failure for FS changes from the adhesive to cohesive as the fracture occurs within the adhesive system. The intermediate zone between FS Primer and Bond of about 1 micron may be the weak link in the failure mechanism and certainly needs further investigation.