Dental material science is separated into a distinct field of knowledge, and all dental materials are divided into three main classes depending on the chemical structure — ceramics, metals, and polymers [13,14,15,16,17].
In modern dentistry, high functional and aesthetic requirements are imposed on materials intended for the manufacture of temporary protective prostheses [8,9,10,11,12]. And in dentistry, a combination of materials of different chemical nature is used, since none of the materials can be considered ideal [16,18].
The “ideal” material for dentistry must fully meet the following requirements: be biocompatible, resist the effects of the oral environment, provide a strong and permanent connection with the hard tissues of the teeth, fully reproduce their appearance, have a complex of physical and mechanical properties corresponding to the properties of the replaced tissues and, if possible, contribute to the recovery and regeneration of biological tissues [16,19,20].
Requirements for temporary protective materials.
Literature analysis has shown that temporary protective prostheses provide a comfortable state of the patient if they fully meet the requirements for temporary protective prostheses:
— provide thermal insulation of the tooth tissues from temperature influences and precise fit, eliminating edge see page;
— to avoid correction or alteration of permanent restorations ensure the stability of the tooth position, excluding its displacement;
— provide a full function of chewing and thus prevent the development of neuromuscular imbalance in the temporomandibular joint;
— provide a uniform redistribution of chewing load on the periodontium of remaining teeth;
— provide an accurate edge fit. Inflammation caused by the overhanging edges of dental crowns can cause proliferation, recession, and bleeding when removing an impression or cementing a permanent restoration;
— has the same color and anatomical shape of natural teeth, thereby providing a good aesthetic and phonetic result;
— do not interfere with the hygienic treatment of the oral cavity. If the gum tissue remains healthy during the use of dental prostheses, this reduces the likelihood of problems during cementation and restoration;
— provide modeling of rational parameters of the future design of a permanent dental prosthesis;
— contribute to the rapid adaptation to the permanent prosthesis.
Properties of protective dental materials.
The analysis of material properties has not only theoretical but also practical significance associated with the regulation and prediction of properties by changing the chemical and compositional structure, technological modes of production for various areas of dentistry. The entire set of properties of dental materials can be classified into physical, mechanical, chemical, aesthetic, biological, and technological, which determine the possibility of manufacturing an orthopedic structure from a particular material [16,22]. The basic properties of dental materials include:
— physical: thermal conductivity, changes in linear dimensions and volume depending on temperature humidity and other factors, sorption of oral fluid, the possibility of galvanic currents (for metals), optical characteristics that determine the aesthetic quality of dental replacement, etc.;
— chemical: properties that provide molecular changes in the material as a result of chemical interactions; this class is associated with such important processes for use in dentistry as curing and adhesive interaction of the replacement material with the surrounding tissues;
— mechanical: a separate group of physical properties aimed at overcoming functional loads that affect the recovery materials and impose certain requirements on them.
The following requirements are applied to biocompatible dental materials: do not damage the pulp and soft tissues of the oral cavity; do not contain diffusion-capable substances of damaging action, sensitizing substances; do not have carcinogenicity; form reliable adhesive compounds with hard tissues of the tooth; have a regenerating and healing effect [16,22].
The biocompatibility of the material is assessed by the types of their effects on the body — it is general (resorptive) — allergic, toxic and local — mechanical, locally toxic, locally allergic, changes in temperature perception [16].
All materials for temporary structures are classified into categories, depending on the type of body tissue that the material should contact and the time of contact [21,22].
The aesthetic properties of materials and biocompatibility indicators are “tied” to their physical and chemical characteristics [16,22].
The mechanical properties of dental materials are very significant because chewing and other functional loads are forces that act on dental materials when replacing lost tooth tissues [16].
The most important characteristic of any material is its strength. It represents the ability of a dental prosthesis to resist the loads applied to it, without collapsing and without showing excessive and, especially, irreversible deformation [21,22,23].
An important indicator that determines the stiffness and ability of the material to withstand the applied loads without significant deformations is the modulus of elasticity. It is determined by knowing the stress and strain data that occur in a material sample under the action of the applied load force [24,25].
The adhesive ability of dental material is an essential characteristic of any material for the implementation of high-quality prosthetics [26,27,28].
Some authors believe that sometimes adhesion may turn out to be stronger than cohesion, in such cases, when a tensile force is applied, a break occurs in the volume of the less durable of the two materials in contact [27,29].
The material that is applied in the process of obtaining an adhesive compound is called an adhesive, and the material that is applied is called a substrate [30].
There are several mechanisms for the formation of an adhesive compound due to different types of adhesive bonds [27,30,31]. Mechanical adhesion consists of jamming the adhesive in the pores or irregularities of the substrate surface; chemical adhesion is based on the chemical interaction of molecules in the surfaces of two materials that make up the adhesive compound [16].
In most cases, when using materials of various chemical nature for tooth restoration, an adhesive interaction takes place with the inclusion of mechanical, diffusion, and chemical components [21,22].
In recent years, aesthetics in dentistry has gained priority. The indicators that characterize the aesthetic properties of reducing materials include color, translucency, surface, gloss and fluorescence [21,22,32]. Important properties of dental restorative material include hygienic properties, which determine the ability of a material to be cleaned with means for hygienic cleaning of teeth and oral cavity and without changing its properties under their influence [21,22].
Polymers for the manufacture of temporary protective structures.
It should be noted that the industry of using crowns for temporary permanent prosthetics (unlike other orthopaedic directions) began directly with the use of polymer materials. The term “polymers” was first introduced by J. J. Berzelius in 1833, in reference to substances of the same composition but different molecular weight. The synthesis of polymers is carried out by polycondensation and polymerization reactions, and the chemical structure of plastics is synthetic polymer materials [33,34,35].
There are thermoplastic and thermostable plastics. Thermoplastic materials are capable of repeatedly passing into a softened plastic state upon heating (these are materials based on polymers with a linear or branched structure). When reheated, thermostable plastics cannot go into a plastic state. They have a mesh or cross-linked structure that forms when the material is first heated.
The low strength of polymers in comparison with ceramics and metals is clear from the features of the molecular structure, according to which there are strong bonds within the polymer chains and weak — between the chains. Weak secondary bonds between polymer chains allow these chains to slide relative to each other at stresses much lower than the stresses required to break the bonds in the chains themselves [29]. The main attractive feature of plastics is their manufacturability and ease of manufacturing dental products of any, the most complex shapes and purposes [16,19,20]. Neither metals nor ceramics have the same high technological properties as polymer materials.
Composites for the manufacture of temporary protective structures.
The hardness of composites is inferior to that of enamel, but is equal to or even higher than that of dentine [39].
Microfilled composites are thermally conductive with a large proportion of the polymer matrix than fine-filled composites or hybrid composites [16].
The shrinkage of micro-filled composites is greater than that of finely filled and hybrid composites [16, 21, 22].
The original color of polymer orthopaedic materials is much better and lasts longer [16].
The radiopacity of the restorative materials should be slightly higher than the radiopacity of the natural tooth enamel. This result can be achieved by introducing elements with a high atomic number into the filler, such as barium, strontium, and zirconium [16,21,22].
For the direct method of manufacturing temporary fixed structures, various cold polymerization plastics are used — mainly vinyl-ethyl-methacrylic (“Luxatemp”, “DMG”;” Protemp3”,“3MESPE”, etc.) [24]. Table 1 describes the advantages and disadvantages of these polymers.
The emergence of bis-acrylic materials made it possible to simplify and speed up the method of manufacturing temporary fixed prostheses using the direct method. Additional advantages of this group of materials provide properties such as low polymerization temperature, no residual monomer, high strength, smooth surface, no smell, and color stability. When studying the color stability of various types of materials for the manufacture of temporary structures, a number of authors in different years confirmed minimal color changes in bis-acrylic materials [24].
