Reinforcing Dentistry Strength Properties, pt. 1
ANALYSIS OF STRENGTH PROPERTIES of mesh metal-composite materials used in reinforcing dentistry by M. Melikian (pt. 1)
We are introducing to your attention the results of the analysis of mechanical properties of reinforced metal mesh composites. These properties particularly depend on location of a strengthened metal mesh to the load direction.
The metal mesh composite is used for reinforced dentistry in treatment,restoration/reconstruction and prosthesis. The study of mechanical properties by а three-point static bend fixture demonstrated that the ultimate tensile strength of the samples depends on the placement position of the metal mesh in the composites with the relation to the direction of the applied load. The installation of the metal mesh at the distance of 1 mm from the bottom surface of а sample in а tension zone practically doesn't change ultimate tensile strength in comparison with control samples. At the metal mesh installation at the distance of 1 mm from the top surface of а sample in а compression zone the maximum load is increased up to 75 %, in comparison with not reinforced samples prepared by using micro-hybrid composite materials.
The development of the crack growth and their expansion, which leads to destruction of samples integrity essentially, depends on the placement position of the metal mesh in the composites. It is proved that the metal mesh prevented development of the crack growth. Process of destruction of а sample begins with the development of the crack growth in non-reinforced zone and finished at the end of the reinforced zone. Due to stretching metal mesh the samples don't split to two parts, and remain ability to resist to а load. The effect of increasing of ultimate tensile strength of the samples occurs by formation of а reinforcing high-strength metal mesh with composite layer in а critical zone of а sample under an indenter. The high-strength reinforcing layer distributes strains more homogeneous. In this case the reinforced composite uniformly distributed а load thereby unloading the corresponding layers of а micro-hybrid composite material. The installation of metal mesh with composites under an indenter (in а contact zone) essentially reduces probability of а crack forming. This can be explained by the decreasing of the maximum values of contact strains due to reinforced mesh installation.
The undoubted advantages of composite materials in comparison with traditional ones made it possible to widely change them in clinical practice. Among benefits of modern composite materials, high physical and mechanical properties, biological inertness, excellent chemical resistance, low shrinkage coefficient, stronger bond and better marginal adhesion to hard tooth tissues should be underlined. With the introduction of composite materials into clinical practice, dentists have the opportunity to conduct sparing odontopreparation with maximum preservation of healthy hard tissues. During the restoration process, the functions and aesthetics of decayed teeth are restored. Yet, despite the obvious advantages, composite materials have a number of disadvantages typical of any artificial material used in dental practice.
Clinical studies have shown that when using reinforced and unreinforced composite materials, various complications arise, which are eliminated by polishing, grinding, partial or complete restoration. It was found that the main causes of complications after restoration are inherent in the physicochemical properties of composite materials. Scientific studies have shown that one of the main drawbacks of composite materials is polymerization shrinkage as a result of the mutual approximation of monomer particles as a result of the action of van der Waals forces. As is well known, in light-cured composites, the organic phase of the uncured material contains free, non-conjugated methacrylate monomers. After the start of light polymerization, these monomers combine in a free radical reaction and form first oligomers and then long chain crosslinked polymerized polymers (a kind of networks). Since the distance between separate components of the formed polymer is less than between individual monomers before the reaction, polymerization leads to a decrease in the volume of the network. This effect is called polymerization shrinkage. As long as monomers can move freely, because they are not yet part of the network, polymerization stress does not develop or develops insignificantly. However, as more and more monomers react, the resulting polymer network becomes rigid, in part due to the increased formation of covalent bonds (crosslinking) between adjacent polymer chains. Together with a decrease in the mobility of monomers, any subsequent shrinkage of the system leads to an increase in polymerization stress (see continuation in the next article).
