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
Friday 16 October 2009
Monday 12 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.
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.
Thursday 8 October 2009
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.
Monday 5 October 2009
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.
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.
Saturday 3 October 2009
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.
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.
Friday 2 October 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.
Monday 28 September 2009
Recommendations for conducting controlled clinical studies of dental restorative materials
Inspired by the recent debate, I did a literature search on clinical trials in various dental disciplines. As expected, there are loads of such studies on dental materials and clinical procedures, so the argument that something can't be tested is invalid. Everything can and must be tested using scientifically structured protocols before certain claims are made.
Two years ago, a group of scientists associated with the FDI Science Committee published recommendations for conducting clinical trials on dental materials. These recommendations are related to study design and evaluation criteria.
The following is the abstract of this paper and the full text can be obtained from J Adhes Dent or the first author, Dr Reinhardt Hickel of the University of Munich, Germany hickel@dent.med.uni-muenchen.de
Recommendations for conducting controlled clinical studies of dental restorative materials. Science Committee Project 2/98--FDI World Dental Federation study design (Part I) and criteria for evaluation (Part II) of direct and indirect restorations including onlays and partial crowns.
Hickel R, Roulet JF, Bayne S, Heintze SD, Mjör IA, Peters M, Rousson V, Randall R, Schmalz G, Tyas M, Vanherle G.
J Adhes Dent 2007; 9 Suppl 1:121-147. Erratum in J Adhes Dent. 2007 Dec;9(6):546.
About 35 years ago, Ryge provided a practical approach to the evaluation of the clinical performance of restorative materials. This systematic approach was soon universally accepted. While that methodology has served us well, a large number of scientific methodologies and more detailed questions have arisen that require more rigor. Current restorative materials have vastly improved clinical performance, and any changes over time are not easily detected by the limited sensitivity of the Ryge criteria in short-term clinical investigations. However, the clinical evaluation of restorations not only involves the restorative material per se but also different operative techniques. For instance, a composite resin may show good longevity data when applied in conventional cavities but not in modified operative approaches. Insensitivity, combined with the continually evolving and nonstandard investigator modifications of the categories, scales, and reporting methods, has created a body of literature that is extremely difficult to interpret meaningfully. In many cases, the insensitivity of the original Ryge methods leads to misinterpretation as good clinical performance. While there are many good features of the original system, it is now time to move on to a more contemporary one. The current review approaches this challenge in two ways: (1) a proposal for a modern clinical testing protocol for controlled clinical trials, and (2) an in-depth discussion of relevant clinical evaluation parameters, providing 84 references that are primarily related to issues or problems for clinical research trials. Together, these two parts offer a standard for the clinical testing of restorative materials/procedures and provide significant guidance for research teams in the design and conduct of contemporary clinical trials. Part 1 of the review considers the recruitment of subjects, restorations per subject, clinical events, validity versus bias, legal and regulatory aspects, rationales for clinical trial designs, guidelines for design, randomization, number of subjects, characteristics of participants, clinical assessment, standards and calibration, categories for assessment, criteria for evaluation, and supplemental documentation. Part 2 of the review considers categories of assessment for esthetic evaluation, functional assessment, biological responses to restorative materials, and statistical analysis of results. The overall review represents a considerable effort to include a range of clinical research interests over the past years. As part of the recognition of the importance of these suggestions, the review is being published simultaneously in identical form in both the Journal of Adhesive Dentistry and Clinical Oral Investigations. Additionally, an extended abstract will be published in the International Dental Journal, giving a link to the web full version. This should help to introduce these considerations more quickly to the scientific community.
Two years ago, a group of scientists associated with the FDI Science Committee published recommendations for conducting clinical trials on dental materials. These recommendations are related to study design and evaluation criteria.
The following is the abstract of this paper and the full text can be obtained from J Adhes Dent or the first author, Dr Reinhardt Hickel of the University of Munich, Germany hickel@dent.med.uni-muenchen.de
Recommendations for conducting controlled clinical studies of dental restorative materials. Science Committee Project 2/98--FDI World Dental Federation study design (Part I) and criteria for evaluation (Part II) of direct and indirect restorations including onlays and partial crowns.
Hickel R, Roulet JF, Bayne S, Heintze SD, Mjör IA, Peters M, Rousson V, Randall R, Schmalz G, Tyas M, Vanherle G.
J Adhes Dent 2007; 9 Suppl 1:121-147. Erratum in J Adhes Dent. 2007 Dec;9(6):546.
About 35 years ago, Ryge provided a practical approach to the evaluation of the clinical performance of restorative materials. This systematic approach was soon universally accepted. While that methodology has served us well, a large number of scientific methodologies and more detailed questions have arisen that require more rigor. Current restorative materials have vastly improved clinical performance, and any changes over time are not easily detected by the limited sensitivity of the Ryge criteria in short-term clinical investigations. However, the clinical evaluation of restorations not only involves the restorative material per se but also different operative techniques. For instance, a composite resin may show good longevity data when applied in conventional cavities but not in modified operative approaches. Insensitivity, combined with the continually evolving and nonstandard investigator modifications of the categories, scales, and reporting methods, has created a body of literature that is extremely difficult to interpret meaningfully. In many cases, the insensitivity of the original Ryge methods leads to misinterpretation as good clinical performance. While there are many good features of the original system, it is now time to move on to a more contemporary one. The current review approaches this challenge in two ways: (1) a proposal for a modern clinical testing protocol for controlled clinical trials, and (2) an in-depth discussion of relevant clinical evaluation parameters, providing 84 references that are primarily related to issues or problems for clinical research trials. Together, these two parts offer a standard for the clinical testing of restorative materials/procedures and provide significant guidance for research teams in the design and conduct of contemporary clinical trials. Part 1 of the review considers the recruitment of subjects, restorations per subject, clinical events, validity versus bias, legal and regulatory aspects, rationales for clinical trial designs, guidelines for design, randomization, number of subjects, characteristics of participants, clinical assessment, standards and calibration, categories for assessment, criteria for evaluation, and supplemental documentation. Part 2 of the review considers categories of assessment for esthetic evaluation, functional assessment, biological responses to restorative materials, and statistical analysis of results. The overall review represents a considerable effort to include a range of clinical research interests over the past years. As part of the recognition of the importance of these suggestions, the review is being published simultaneously in identical form in both the Journal of Adhesive Dentistry and Clinical Oral Investigations. Additionally, an extended abstract will be published in the International Dental Journal, giving a link to the web full version. This should help to introduce these considerations more quickly to the scientific community.
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