NIOM scientists were invited to the national dental conventions in Sweden and Norway this autumn. Head of Laboratory Hilde M. Kopperud, researcher Simen E. Kopperud and guest-researcher Frode Staxrud from NIOM interacted in the two-hour lecture which presented data from both clinical and laboratory studies, and was entitled:
Tooth-coloured restorations – choice of materials, procedures and longevity
Longevity of dental restorations is affected by factors related to the material, the procedure, the patient and the dentist. A questionnaire study has revealed that while Norwegian dentists chose actively among the different categories of direct restorative materials in 1995, composite resin is by far the most used restorative material for posterior Class II direct restorations today (95%). Composites are basically a mixture of resin and filler particles. The resin is an organic matrix of different monomers. Large, stiff and hydrophobic monomers (e.g. BIS-GMA) give the material strength and reduce polymerization shrinkage and water uptake. Smaller, less viscous monomers (e.g. TEGDMA) are necessary to improve the consistency of the material, and will also increase the degree of conversion. More hydrophilic monomers (e.g. HEMA) are used where a good contact with the tooth structure is necessary. A large variation in size distribution and chemical composition of the filler particles is now used to optimize the material properties.
Clinical studies performed in the 1990’s showed significantly better performance of amalgam restorations compared to composites. However, studies published later show comparable longevity of both materials, with annual failure rates (AFR) ranging from 1% to 7%. Reasons for failure of composite resins are mainly secondary caries and fractures. Saucer-shaped preparations in a Norwegian study have shown significantly higher failure rates than traditional Class II restorations. This indicates that some mechanical retention in small Class II preparations could improve the longevity of approximal restorations. Unpublished data from the same study also show that tooth surfaces in contact with newly placed composite restorations have significantly higher risk of developing caries.
Composites are bonded to the tooth. A new golden standard for bonding of restorations to tooth substance is recently introduced in dentistry. While a three-step etch-and-rinse procedure was recommended previously, is it now claimed that it is better for the dentine if a mild self-etching primer is used, followed by bonding material. Enamel should be separately etched prior to applying primer. The bond strength is affected by water, since some chemical bonds will be broken by hydrolysis. Therefore hydrophilic residues from the primer should be evaporated before applying the bonding material. Some manufacturers add MDP or silane to achieve better bonding to old restorations.
Metal primers are recommended when repairing metal-containing restorations. Studies performed at NIOM show that – when repairing old composite resin restorations – bonding is significantly improved if a silane is applied to the old composite prior to bonding and placing the new restoration.
As materials with larger layer thicknesses are introduced, as well as high-intensity curing devices advocating short curing times, an awareness of proper curing is important. A longer curing time will produce a larger curing depth. We recommend increasing the curing time, rather than using high-intensity curing lamps, which may cause thermal damage to soft tissues and may present blue-light hazards to the eye.
Longevity of restorations is strongly related to patient and dentist related factors. Restorations placed in caries active patients have a much higher failure rate compared with low-risk patients. Likewise, restorations in molars have 2–3 times as high risk of failure compared with restorations in premolars. Although little evidence is available, the dentist is considered one of the most important factors for longevity of restorations in clinical studies, since the dentist both decides which lesions need to be filled, chooses the restorative material, prepares the cavity, places the restoration, follows up the patient postoperatively and finally decides when the restoration should be replaced.
NIOM Newsletter November 2014