**5. Conclusions**

New choices of resins, with better properties compared to acrylics, are now suitable for dental applications. Alternative technologies for processing dental resins, like casting, injection, light-curing and milling are meant to improve their clinical performances.

Long-term deterioration of resin-based dentures in oral environment is still an unsolved problem. This can be improved by bringing innovations in the manufacturing technology, thereby reducing polymer defects and distortions in a warm and humid environment, material's fatigue and ageing. Moreover, we have to consider various possible environmental aggressions such as acids, alcohol, tobacco and thermal fluctuations, the continuous contact with water and salivary enzymes, food remnants, fluctuation of the oral balancing system, the contact with live tissues, bacteria and yeasts. It seems that the enzymes from human saliva are able to produce the softening of the resin surface, probably by hydrolyzing the small oligomers into monomer phases [8].

Choosing the right material for manufacturing partial or full dentures is very important because it has direct effect on their quality and lifetime. Because they are brittle, dentures made up of acrylic resins have a limited life in the mouth. Moreover, following some manufacturing procedures, small defects (holes in the polymer structure) may occur. The continuous mastication stress (repeated movements of low amplitude) and the oral environment have an important role in the degradation of the denture in time [9].

The fracture toughness depends on the type and the nature of the polymer and the reinforcement added components. For example, fracture toughness of a monomethacrylate-based material is lower than that of dimethacrylate-based material. An increase in the fracture toughness can be achieved by adding reinforcement fibres which prevent or slow down the crack growth or by adding rubber-like substances, which increase elasticity [9].

Currently, the selection of dental materials is made particularly in terms of manufacturing technology and aesthetic criteria. These criteria are not sufficient to provide functional, durable and comfortable dentures. In the case of acrylic resins used in dental prosthetics, in addition to aesthetic aspect and strength, elasticity and elongation are also important features. Biomechanical studies of dental resins, implying different technologies, are necessary in order to discover potential causes of failure. These are not only the consequence of material defects and solubility, but can also depend on the technological procedures and processing. Most of the resins distort and fracture at low level of tension dependent on the environmental or loading conditions. In our studies, the values of assessed mechanical properties were lower for the wet samples than for the dry samples, indicating a clear influence of saliva in the biodegradation of the material, with direct consequences biomechanical performances and lifetime for dentures [24, 25].

As far as the allergy potential of acrylic resins is concerned, our results show as follows: *in vivo* tests, both patch test and TNF-alpha showed allergic reactions to acrylate. Both *in vitro* determinations for residual monomer showed a very small and in fact practically undetectable amount in our samples. This led to the conclusion that the manufacturing process was carried out with maximum thoroughness.

Resin-based dental materials have a wide range of applications. But, despite the efforts made to continuously improve their physical, mechanical and aesthetic properties thereby cause side effects regarding their biocompatibility. These situations may lead to local lesions which can prove extremely unpleasant for the patient [16].

In our opinion, the best results in denture manufacturing may be achieved by combining scientific principles with creativity, while the optimal choice of the material and technique has a major importance.
