Veneering with ceramic masses of single tooth prosthetic restorations in lateral areas (Part I)

Introduction

Single tooth fixed prosthetic restorations are small-sized prosthetic works with high accuracy, made with the help of the dental technique laboratory, in order to restore the morphology, physiognomy and functionality of a tooth or part of a tooth, as well as to restore its position on dental arch. These prosthetic parts can be made of dental alloys, polymers or ceramic masses^(1-3).

General data

Metal-ceramic crowns are completely esthetic single tooth prosthetic restorations that are very often used as a treatment option in prosthetic rehabilitation. Their preferential use is due both to the esthetic component that offers a natural aspect and to the metal component responsible for resistance and hardness, serving as a support for the ceramic masses. The two main components make metal-ceramic restorations quite widely used and popular both among dental practitioners (dentists and dental technicians) and among patients^(1-3).

In the following, a series of extremely interesting technological aspects will be presented, regarding the manufacture of metal-ceramic fixed single tooth prosthetic restorations in the lateral areas (maxillary and mandibular), by veneering them with ceramic materials.
 

Case presentation

A 35-year-old female patient presented to the dental office due to severe pain in quadrant 1, in the lateral area. After the examination, the dentist found that the upper first molar, 1.6, was the one causing the pain, due to an advanced caries that was not treated in time, which was very close to the pulp chamber. Thus, the dentist, in agreement with the patient, chose as a treatment solution the prosthetic rehabilitation using a metal-ceramic full coverage crown, this being a resistant, durable and esthetic structure. After the dentist performed the appropriate endodontic treatment at the level of tooth 1.6, the tooth was prepared as abutment using diamond burs, in order to manufacture the metal-ceramic full coverage crown.

In a first stage, both the maxillary functional impression and antagonistic mandibular arch impression were made using standard impression trays and condensation silicone material in double consistency, putty and light body. The occlusal registration was performed using a specially designed addition silicone material.

The two impressions obtained were washed under tap water for 15 seconds, after which they were decontaminated by immersion in chemical substances with antimicrobial potential, following the instructions recommended by the manufacturers.

After washing, disinfecting and drying the impressions, the functional sectional model was obtained using Pinax Pro system. For both functional and antagonistic models, type IV dental stone was used.
 

The die spacer was applied on the surface of the removable abutment, with the purpose of keeping space for the dental luting cement which will bond the crown on the abutment, as well as for leveling the dental abutment surface of any small irregularities. Afterwards, the abutment was insulated with gypsum separating fluid and inserted into the wax bath, in order to make the wax coping of the future metal infrastructure of the metal-ceramic crown. For a more precise cervical fitting, the wax pattern was reduced at the cervical level by 1.5-2 mm, so that the remaining space was occupied by the cervical wax. In order to provide support for the ceramic mass, the wax pattern was modeled respecting the anatomic shape of the tooth but undersized, ensuring the resistance of the ceramic mass at the level of the entire future metal support.
 

After the wax pattern has been made, it is left to cool, and later, a surface tension reducer solution is applied for investing. The wax sprue rod is applied, then the rod is positioned on the crucible former cone to be invested. The investing material is gradually poured inside the wax pattern, to avoid the formation of air voids on the surface of wax. The investing material is left to set (about 35 minutes), and the mold is heated up to 950^oC to burn out the wax. The dental alloy is melted and casted in to the mold and left for cooling. The casting is devested and sandblasted to remove any remaining investing material form the surface. The sprue rod is removed, and the metal coping is processed. To avoid cracks in the veneer material, the metal coping will be polished. After the fitting of the metal coping is checked on the functional model and on the patient tooth, the coping is sandblasted and then heated in a ceramic furnace to obtain a layer of oxides on surface for bonding of ceramic material.
 

Once the metal structure has been oxidized, the opaque paste of color A3 is prepared and applied on the surface of the coping by brushing. The application of the first opaque layer is done with the help of a tool with a rounded tip, covering the entire external surface of the coping in a thin and uniform layer, after which it is inserted into the special furnace for firing the ceramic mass. In order for the opaque material to be uniform and to completely cover the surface of the metal framework, it is necessary to apply two layers, each application being followed by a firing. The role of the opaque is to prevent the color of the metal framework from showing through the ceramic mass and to form the bond between the me­tal and the ceramic mass through the previously formed oxide layer. A ceramic shoulder can be produced using Margin material for a natural aspect of the restoration.
 

The physiognomic component is applied by layering ceramic masses: deep dentin, dentin, dental enamel. To avoid confusing different types of effects during the layering process, dyes specific to ceramic masses were used. So, for each type of ceramic mass and effect, a color was assigned:

• pink – deep dentine;
• red – dentine A3.5;
• blue – TI 2 enamel;
• green – opal 3;
• purple – violet opal.

Applying the dentine according to the desired morphology is the first step. A slight oversizing of the crown is aimed at, by applying deep dentin and dentin, thus compensating for the contraction of the ceramic mass during sintering. This creates the volume of the crown according to the neighboring teeth, the occlusal relief, the grooves and the cusps. In order for the dentin-ena­mel transition to be smooth, small cracks are made in the dentin mass, for the most effective interpenetration. Later, the morphological details as well as the outline of the cusps are made by applying enamel, thus resulting in the desired esthetic appearance.
 

After applying all the layers of ceramic mass according to the desired morphology, the prosthetic restoration is inserted into the special furnace for ceramic firing, using the specific program for the first firing. Due to the irregular shape of the cusps, a second correction firing will take place, thus adjusting all the irregularities.
 

The next stage consists in adapting the prosthetic restoration both on the functional model and in the patient’s tooth, both proximally with the neighboring teeth and occlusally with the opposing teeth. The restored tooth – in this case, the maxillary first molar, 1.6, a tooth of particular importance both in functional movements and in the stability of the occlusion – must be given increased attention at the level of occlusion.
 

Once the prosthetic restoration has been fitted, it is ready for the application of the final layer: the glaze. At this stage, the aim is to achieve the desired make-up, by accentuating the grooves and dimples with dark colors, by making a smooth transition from the cervical area to the lighter color at the level of the enamel and the cusps. The prosthetic restoration is placed in the furnace, using the specific program for the glaze layer. Following the application of the final layer of glaze, the esthetic appearance desired by both the dentist and the patient resulted.  
 

Discussion

Metal-ceramic restorations manage to combine the resistance conferred by metal with the aesthetic, natural appearance given by ceramic masses⁽⁴⁾. Although for a long time the focus was only on mechanical resistance in the lateral area, the masticatory forces in this area having the highest values, technological developments have allowed the combination of metal and ceramic without compromising strength^(5-7).

The metallic component is responsible, on the one hand, for the mechanical resistance of the restoration and, on the other hand, for achieving marginal closure^(8-10). Research has shown that the minimum thickness of the metal layer for which the stresses accumulated in the porcelain have the failure values is 0.3 mm. Even the reduction to 0.1 mm did not produce significant changes⁽¹¹⁾.

Another problem of prosthetic restorations made of two materials, in the conditions where the ceramic is thermally processed to obtain the final shape and structure, is represented by the changes that may occur during the ceramic firing process, especially in the case of repeated firings. A study carried out by Gemalmaz et al. revealed that the marginal dimensions of the metal coping are not influenced by the thermal cycles required for the application of ceramic masses, but rather by the contamination of the internal surface of the metal coping with ceramic masses. This contamination is responsible for marginal distortion of metal-ceramic restorations⁽¹²⁾.

The esthetic part of the restoration is achieved by applying ceramic masses on the surface of the metal copings (porcelain fused to metal). The main impediment is represented by the need to mask the unesthetic color of the dental alloy⁽¹³⁾. Numerous studies have shown that the final shade of the restoration is influenced by the dental alloy and the type of ceramic masses used^(14-16). However, the use of ceramic masses in the lateral area requires much lower demands than in the frontal area, and only the use of the esthetic component representing a great evolution from the metallic color of all-metal restorations to the esthetic color given by ceramic masses^(13,17,18).

Conclusions

Metal-ceramic prosthetic restorations are frequently used as a method of prosthetic rehabilitation, in the case of teeth with dental defects, fractures, exaggerated abrasion or color defects. The role of these prosthetic restorations is to improve the physiognomic appearance and restore the functional relationships.

The purpose of this article was to expose the fact that, by using ceramic masses, special results are obtained in the manufacturing of the esthetic component of metal-ceramic crowns, the working technique presenting numerous advantages. These ceramic masses are high-quality materials, the expectations of patients, dentists and dental technicians being met from all points of view.  

Acknowledgement: Viorel Ştefan Perieanu and Oana Elena Amza are corresponding authors (Viorel Ştefan Perieanu: viorelperieanu@yahoo.com; Oana Elena Amza: oana.amza@umfcd.ro). Irina Adriana Beuran and Valentin-Dănuţ Albu have equally contribution with the first author..  

Autori pentru corespondenţă:

Viorel Ştefan Perieanu E-mail: viorelperieanu@yahoo.com

Oana Elena Amza E-mail: oana.amza@umfcd.ro

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

This work is permanently accessible online free of charge and published under the CC-BY.