Practical aspects in restoring the morphology and functionality of the dento-maxillary system using full-coverage prosthetic restorations (Part I)

Introduction

Dental prosthetics is a branch of dentistry that deals with the design, manufacture and maintenance of prosthetic parts intended to replace and prevent deterioration of oral structures. It uses techniques, materials and methods to improve the health of the structures of the dento-maxillary system. The role of dental prosthetics is represented by the replacement of teeth and tissues in their vicinity, but also by the treatment of occlusal and/or temporomandibular joint disorders, the restoration of masticatory efficiency and, most importantly nowadays, the improvement of the orofacial esthetic appearance^(1-4).

General data

When the first edentulous span occurs, it is advisable to treat it as quickly as possible, so as not to create complications at its level, which can subsequently make the future therapeutic approach difficult. Establishing prosthetic treatment immediately after advanced coronal destruction of some masticatory units, or immediately after the appearance of edentulous spans, prevents the onset of masticatory, occlusal, functional, esthetic, phonetic disorders, pathological migrations and periodontal diseases. And, in these cases, both the restoration of masticatory function and the restoration of esthetics and phonation must be taken into account^(1-4).

Oral and dental esthetics have become the most important objective of any restorative treatment today. Thus, new techniques and technologies, modern materials and methods have been developed which allow the manufacturing of prosthetic rehabilitations that are as natural as possible. Regardless of the technique used, the main goal of prosthetic rehabilitation is to improve the esthetic appearance of the smile. Next, several interesting technological aspects will be presented which intervene in the technological flow of manufacturing full-coverage single tooth and/or multiple teeth prosthetic restorations^(1-4).

Case presentations

Case 1

In this case, a metal-ceramic bridge was chosen in the lateral area, for which teeth 2.4, 2.5 and 2.7 were used as retainers, with 2.6 as pontic. The metallic substructure was obtained by the conventional lost wax technique, using of casting the Co-Cr-based metallic alloy and an automatic casting machine.

In a first stage, the dentist performed both the appropriate endodontic treatments and the related grinding of the abutment teeth of the future dental bridge, thus obtaining the necessary preparations for the impression. The functional impression was made in a standard impression tray using dual-consistency silicone condensation materials (putty and light body), and the mandibular antagonist impression was also made in a standard impression tray using silicone condensation material of putty consistency.

Next, the models were cast (the maxillary functional model was cast from type IV dental stone, and later it was sectioned in order to obtain a sectional model, while the mandibular antagonist arch model was cast from type III dental stone), as well as their mounting in a simulator, based on the occlusal record made by the dentist in the specialized office. Subsequently, the metal substructure was fabricated:

• first the wax pattern was made, and then it was prepared for investing;
• the invested wax pattern was placed inside of a heating furnace to remove de wax and to obtain the mold; the heating temperature of the mold was about 960°C, after which the Co-Cr-based metal alloy was cast to obtain the metal substructure using an automatic casting machine;
• after the completion of the casting and the slow cooling, the metal substructure was de-vested, processed and checked for fitting on the functional model, it was later sent to the dental office for try-in;
• finally, the sandblasting process of the metal component was also carried out, in order to subsequently veneer it with ceramic masses for esthetic purposes.

The ceramic masses were applied to the metal component to obtain the esthetic component. Initially, the metal component was introduced into the ceramic firing furnace for oxidation, thus obtaining an indispensable layer in the fusion process of ceramic masses to dental alloys. The first layer was represented by wash-opaque, used to facilitate the adhesion of subsequent layers. It was followed by the opaque layer with role of blocking the metal color from showing through the thickness of the ceramic mass. Over the first layers, the dentin and enamel layers were applied to obtain the appropriate shape and color for the restored teeth. Each applied layer was followed by a firing stage according to parameters well defined by the manufacturer. The final appearance of the fully esthetic metal-ceramic prosthetic restoration was achieved by applying and subsequently firing the glaze layer. The prosthetic restoration was sent to the dental office for cementation.

Case 2

A 37-year-old female patient presented to the dental office complaining of sensitivity at the level of teeth 3.6 and 3.7. Following the clinical and radiological examination, massive coronal destruction was revealed in both teeth, due to severe carious processes.

In order to restore the coronal integrity, but especially the masticatory efficiency, the dentist decided to prepare the two dental units (3.6, 3.7), after previously performing the endodontic treatment, in order to create two dental crowns veneered with ceramic masses on a zirconia support, due to the increased resistance of this material to high masticatory forces. In a first stage, an intraoral scan of the prosthetic field was performed. Briefly, the manufacturing stages of these prosthetic restorations can be described as follows:

1. Importing STL files into the digital software. Esta­blishing the limit of preparations at the abutment teeth level.

2. Designing the future resistance substructure.

3. Milling the zirconium oxide structures.

4. Sintering process.

5. Layering the ceramic masses.

Case 3

A 38-year-old patient suffered an accident that resulted in some of the front dental units on the maxillary left quadrant no longer remaining intact. The patient presented to the dental office, wanting to restore the esthetic appearance. Thus, the dentist, with the patient’s consent, decided to create a three-unit bridge, with two retainers, on teeth 2.1 and 2.3, and a pontic for tooth 2.2, on a zirconium oxide structure veneered with ceramic masses, in order to obtain a more natural appearance. After preparing the abutment teeth, an intraoral scan of the prosthetic field was performed. The work steps were the same as in the previous case:

1. Importing STL files into the digital software. Esta­blishing the limit of preparations at the abutment teeth level.

2. Designing the future resistance substructure.

3. Milling the zirconium oxide structures.

4. Sintering process.

5. Layering the ceramic masses.

Discussion

Achieving esthetic results using full-coverage crowns has taken on a new meaning with the introduction of digital techniques for manufacturing prosthetic restorations. While initially the emphasis was on metal-ceramic restorations, in which the metal component was obtained through the lost wax technique, today the pro­sthe­tic possibilities are much more varied⁽⁴⁾.

The advantages of new technologies can be partially offset by the experience of the dental technician regarding the shape, size and placement in the dental arches. In contrast, new materials bring added value through their color^(5,6). The color of dental alloys is very different from that of hard dental structures, requiring the application of opaque materials to mask it^(7,8). In the case of substructures made of milled ceramic or zirconia, the color is similar to the hard dental structures, these materials being available in different colors^(9,10). For this reason, the esthetics of such restorations have a much more natural appearance compared to the widely used metal-ceramic restorations. The use of a zirconia substrate increases the resistance of the restorations, making them recommended in the lateral area, not only in the anterior area^(11,12).

Conclusions

After reviewing this material, several extremely interesting aspects can be concluded, as follows:

• Fixed dental prosthodontics has evolved so much in recent years that patients have begun to desire more esthetic prosthetic restorations, not just functional ones.
• With the help of the CAD/CAM technology, prosthetic restorations are obtained at a faster pace, compared to the usual classical methods, while also increasing the quality and proportion of successful treatments.
• The use of full-coverage prosthetic restorations, both metal-ceramic and zirconia supported, is increasingly popular among both practitioners and patients.
• The esthetic and biological properties of ceramic masses indicate them for a very wide use in dental practice.
• Ceramic masses are contraindicated in patients with bruxism. In these cases, semi-physiognomic prosthetic restorations are recommended, with occlusal surfaces made of metal alloys.

Acknowledgement. Viorel Ștefan Perieanu and Ion Ale­xan­dru Popovici are corresponding authors (Viorel Ștefan Perieanu: viorel.perieanu@umfcd.ro; Ion Alexandru Popovici: aipopovici@gmail.com).

Autor corespondent: Viorel Ștefan Perieanu E-mail: viorel.perieanu@umfcd.ro; Ion Alexandru Popovici E-mail: aipopovici@gmail.com

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

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