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.

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.

Recent books on dental materials

This list has been updated in a new post.

Biocompatibility of Dental Materials‎ by Gottfried Schmalz, Dorthe Arenholt-Bindslev, 2009, 379 pages

Clinical aspects of dental materials: theory, practice and cases by Marcia A. Gladwin, Michael D. Bagby, 2009, 481 pages
(Preview not available)

Dental materials guide by Donna J. Phinney, Judy H. Halstead, 2008, 773 pages

Dental Materials by Lyle Zardiackas, Tracey M. Dellinger, Mark Livingston, 2007, 765 pages
(Preview not available)

Craig's restorative dental materials by John M. Powers, Ronald L. Sakaguchi, 2006, 632 pages
(Preview not available)


Materials and procedures for today's dental assistant by Ellen Dietz-Bourguignon, 2005, 269 pages
(Preview not available)


Dental materials: properties and manipulation by Robert George Craig, John M. Powers, John C. Wataha, 2004, 348 pages
(Preview not available)

Phillips' science of dental materials by Kenneth J. Anusavice, Ralph W. Phillips, 2003, 805 pages
(Preview not available)


Dental materials: clinical applications for dental assistants and dental hygienists by Carol Dixon Hatrick, W. Stephan Eakle, William F. Bird, 2003, 373 pages

Introduction to dental materials by Richard van Noort, 2002, 298 pages




The chemistry of medical and dental materials by John W. Nicholson, 2002, 242 pages



Dental materials and their selection by William Joseph O'Brien, 2002, 418 pages
(Preview not available)



Materials in dentistry: principles and applications by Jack L. Ferracane, 2001, 354 pages

Research methodology: The effect of "material A" on treatment outcome

I've recently discussed with a colleague the possibility to prove or disprove the efficacy of a certain clinical procedure on treatment outcome. Since this is the dental materials blog, I'm going to make the parallel between clinical procedures and dental materials and discuss this matter as if it was about dental materials. From the research methodology point of view, it makes no difference whether it is a dental material or a clinical procedure.

"Randomized control clinical trial" would probably be the most appropriate study design to evaluate whether a certain material (material A) has any effect whatsoever on the outcome of a particular treatment. In a recently published book "Introduction to randomized control clinical trials" by JNS Matthews, there is a very nice definition:

"A randomized concurrently controlled clinical trial is simply an experiment performed on human subjects to assess the efficacy of a new treatment for some condition. It has two key features:

1. The new treatment is given to a group of patients (treated group) and another treatment, often the most widely used, is given to another group of patients at the same time (control group). This is what makes the trial concurrently controlled.
2. Patients are allocated to one group or another by randomization. "(1)

Also, it is very important to note that:
"Trials are applied to many different modes of treatment... for example, new surgical procedures, screening programs, diagnostic procedures etc."(1)
How does this apply to our material A? A double-blind trial would be impossible in this case, because a clinician would always know the details of the treatment. On the other hand, a single-blind trial would be possible and recommended since the patient wouldn't know the details of the treatment in order to exclude the possible placebo effect.
Patient inclusion criteria should be taken into consideration at the beginning of the trial. These include, but are not restricted to, patient age, general health, the diagnosis of the current dental condition, the history of this condition etc. It would be wise to "standardise" the cohort so that the number of variables is reduced as much as possible. For example, root canal treatment of a pulpitis may have a different outcome than the treatment of periapical disease, because of the nature of the two dental conditions and variations in patients' immunological response to any of them. Therefore, it would be recommended that one of the inclusion criteria is the uniformity of clinical diagnosis.
Randomization would be easy using the table of random numbers. It excludes any potential bias and is always preferred to other ways of patient selection, as long as the number of cases in both the treated and control group is the same or as similarly-sized as possible. Most statistical tests are most powerful when the groups being compared have equal sizes.
Then, once the treatment is performed, the treated group would receive material A and the control group would receive placebo. The outcome of the treatment would be monitored over at least 3 years, using the standard parameters for monitoring the outcome of this particular treatment. After the monitoring period, (an) appropriate statistical test(s) would be used to assess the difference in treatment outcomes between the two groups of patients.
Only then would it be possible to claim that material A has any effect on the outcome of this particular dental treatment.
(1) Matthews JNS. Introduction to randomized control clinical trials. 2nd edition. Chapman&Hall/CRC; Boca Raton, FL, USA; 2006.

Upcoming event: ESE Edinburgh 2009

European Society of Endodontology Conference, Edinburgh, UK, September 24-26, 2009.

The European Society of Endodontology Conference will be held next week in the beautiful city of Edinburgh. Unfortunately, I'm not going to take part but I'm looking forward to meeting my fellow colleagues from the University of Belgrade School of Dentistry, Professors Vladimir Ivanovic and Branislav Karadzic and Dr Jugoslav Ilic, who will be presenting their research findings.

Prof. Ivanovic is one of keynote speakers and will give a presentation entitled: "Seeking where, when, why and how to locate apical terminus of the root canal preparation". He will also chair a session on the risks and controversies of local anaesthetics.

Scientific programme comprises more than 30 lectures on various topics of interest in contemporary endodontic practice and science. Dental materials will be discussed during several lectures and this year include the following issues:

• Obturation: concepts, truths and misconceptions, by Prof. G. Glickman, USA
• Changing endodontic concepts and outcomes: the multifaceted use of mineral trioxide aggregate, by Dr G. Bogen, USA
• So much for the endodontics, what about the restoration?, by Prof. R. Ibbetson, UK
• New perspectives in adhesive post endodontic restoration, by Prof. A. Cerutti, Italy
• Fibre posts and dentine adhesion: the true story, by Dr F. Mannocci, Italy

Furthemore, dental materials will be addressed in presentations on freely chosen topics including bioceramics of calcium phosphate in endodontic treatment, rheological studies, apical sealing, bond strength, biocompatibility and cytotoxicity of new and current endodontics sealers, fitting and microleakage around fibre posts.

The full scientific programme can be downloaded here.