Developments in the field of plastics have made laboratory-made composite restorations an adequate alternative to ceramic restorations. This article answers some questions about the composition of dental composites and presents an examination of three different veneering composites. In a statement published at the same time, ZTM Annette v. Hajmasy shares her perspective on composite materials in everyday laboratory life. to the statement
What determines the properties of composite?
In addition to mono-functional methacrylates, a combination of multi-functional monomers is predominantly used as an organic matrix. The polymerization of these bi-functional monomers results in three-dimensionally networked structures. The result is a single, networked macromolecule. Since the 60s, bisphenol glycidyl methacrylate (BisGMA) has been a commonly used monomer and main component of the matrix. In order to reduce the high viscosity of BisGMA, shorter monomers such as UDMA, TEGDMA etc. are added to the matrix.
The viscosity of the monomer depends on its molecular weight. The higher the molecular weight, the more viscous the monomer. The higher the viscosity, the lower the shrinkage after polymerization (curing) as well as the mechanical properties.
Basically, all monomers have a similar structure. At their ends there are C=C double bonds, which enable polymerization to other monomers. In the middle is the “residue” that determines the properties of the monomer. Since the C=C double bonds only make up a small portion of the molecule, the polymerization shrinkage is reduced compared to conventional PMMA. The rest of the monomer molecule remains unchanged during the reaction. In addition to the choice of monomers, the type and amount of fillers influence the mechanical properties (hardness, strength, abrasion resistance, polishability) of the veneering composites.
A high proportion of filler improves the mechanical properties. The viscosity can therefore be controlled by the choice of monomers and the filler.
Why is silanization of the fillers necessary?
Veneering composites consist of an organic monomer matrix, inorganic fillers, initiators and stabilizers. In order to be able to permanently incorporate the fillers into the monomer matrix, they must be silanized. In most cases, a bipolar, organic molecule (e.g. methacryloxypropyltrimethoxysilane) is used as the composite silane. Its task is to connect the fillers with the inorganic monomer matrix. The filler is coated with the silane. Glass, glass ceramics, silicates, fumed silicon oxide or prepolymers are used as fillers.
For permanent silanization, a high proportion of silicon dioxide in the glass matrix is necessary.
What is special about a veneering composite?
Although the composition of veneering composites is similar to direct dental composites, they have better mechanical properties. One reason for this is the tempered polymerization in the polymerization oven. Veneering composites achieve a higher conversion rate and thus a lower residual monomer content.
A high conversion rate also increases the resistance of the veneering composite to chemical attacks.
Which polymerization devices are recommended?
Veneering composites are light-curing one-component systems that are cured with light of specific wavelengths. Since polymerization devices not only work with light, but also with temperature or vacuum, depending on the manufacturer, not every polymerization device is equally suitable for all veneering plastics.
When choosing and using the polymerization device, the information provided by the veneering plastic manufacturer must always be taken into account.
Examination of three different veneering composites
Problem presentation
Flexural strength, hardness, surface roughness, discoloration and abrasion resistance are among the important properties of veneering composites. Due to their composition, modern veneering plastics are said to have improved mechanical properties. However, their long-term durability has not yet been sufficiently investigated.
Aim of the investigation
The aim of the presented study was to test the aging of three different veneering plastics. The materials examined differ greatly in the choice of monomers, fillers, filler content and thus in their processing.
Material and method
Three indirect veneering composites (GC Gradia, VITA VM LC, Sinfony) were tested
- Flexural strength (N = 165 per veneering resin, n = 15 per group)
- Martens hardness (n = 10)
- Surface roughness (n = 10)
- Discoloration rate (n = 30)
- Abrasion resistance (n = 6).
After the initial flexural strength measurement, the remaining test specimens were stored in water or aged for 1, 7, 28, 90 or 180 days in thermal cycling. Hardness, surface roughness and flexural strength were then measured. The discoloration test specimens were divided into three groups: coffee, black tea and red wine and the discoloration was measured. The abrasion resistance was determined after 120.000, 240.000, 640.000 and 1.200.000 mechanical and thermocyclic loads. Properties such as hardness, surface roughness, discoloration rates and abrasion resistance were determined longitudinally. This means that these properties were always determined on the same test specimens during the aging process. The bending strength is determined in a destructive test. That's why new test specimens were repeatedly made and aged for different lengths of time. All data were then statistically evaluated.
Results
All three veneering resins showed promising results. However, when looking at the five properties tested, Sinfony (3M Espe) showed the best results, followed by GC Gradia (GC Europe) and VITA VM LC (Vita Zahnfabrik).
Author: A. Kieschnick, Berlin (www.annettkieschnick.de)
ADDITIONAL INFORMATION
- Full article published: Stawarczyk B, Egli R, Roos M, Özcan M, Hämmerle CH. The impact of in vitro aging on the mechanical and optical properties of indirect veneering composite resins. J Prosthet Dent 2011 Dec;106(6):386-98.
- Go to the Statement from everyday laboratory life by ZTM Annette v. Hajmasy!