Masking ability of different ceramic systems over a darkened substrate

 

Capacidade de mascaramento de diferentes sistemas cerâmicos sobre um substrato escurecido

 

 

Manuel Tomás B. Radaelli ^I; Leonardo Federizzi ^II; Fabíola Jardim Barbon ^III; Aloísio O. Spazzin ^IV; Noeli Boscato ^V

 

^I DDS and PhD student at Graduate Program in Dentistry, Universidade Federal de Pelotas (UFPel), Pelotas, RS, Brazil, and professor at Department of Prosthodontics, Centro de Estudos Odontológicos Meridional (CEOM), Passo Fundo, RS, Brazil
^II DDS, MSc, and professor at the Department of Prosthodontics, CEOM, Passo Fundo, RS, Brazil
^III DDS and MSc student at Graduate Program in Dentistry, UFPel, Pelotas, RS, Brazil
^IV DDS, MSc, PhD, and professor at the Department of Prosthodontics, CEOM, Passo Fundo, RS, Brazil
^V DDS, MSc, PhD, and professor at Graduate Program in Dentistry, UFPel, Pelotas, RS, Brazil

Correspondence

 

The authors have no conflicts of interest to declare concerning the publication of this manuscript.

 

 

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ABSTRACT

This clinical report describes the masking ability of IPS e.max ceramic (lithium disilicate glass-ceramic and polycrystalline zirconia) used over darkened (metallic post) and natural-colored backgrounds. Initially, medium-opacity lithium disilicate glass-ceramic framework (IPS e.max Press MO) was used over both backgrounds. It was observed that at the minimum thickness recommended by the manufacturer, the medium-opacity lithium disilicate glass-ceramic framework did not provide sufficient masking ability over the darkened background (metallic post). Then, because of its inadequate masking ability, the lithium disilicate glass-ceramic framework over the metallic post was replaced with polycrystalline zirconia (IPS e.max ZirCAD). It was concluded that using polycrystalline zirconia resulted in better masking ability and superior final aesthetic appearance over a darkened background.

Keywords: Ceramic; Translucency; Metallic post; Color; Crown.

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RESUMO

Esse relato de caso descreve a capacidade de mascaramento da cerâmica IPS e.max (dissilicato de lítio, vitrocerâmica, e zircônia, policristalina) usadas sobre substrato escurecido (pino metálico) e com coloração normal. Inicialmente, a infraestrutura de dissilicato de lítio com opacidade intermediária (IPS e.max Press MO) foi usada sobre os dois diferentes substratos. Foi observado que, na mínima espessura recomendada pelo fabricante, o dissilicato de lítio com opacidade intermediária não forneceu capacidade de mascaramento suficiente sobre o substrato escurecido (pino metálico). Então, sobre o pino metálico, a infraestrutura confeccionada à base de dissilicato de lítio foi substituída por zircônia (IPS e.max ZirCAD). Foi possível concluir que o uso da infraestrutura de zircônia policristalina mostrou melhor capacidade de mascaramento e aparência estética final sobre fundos escurecidos.

Palavras-chave: Cerâmica; Traslucidez; Pino metálico; Cor; Coroa.

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INTRODUCTION

The ability of ceramics to match the natural dentition and their appropriate physical and optical properties have made this material the first choice for aesthetic restorations^1-3. However, previous procedures such as endodontic treatments^4,5 or insertion of metallic posts (5) may result in darkened substrates, affecting the final color and jeopardizing the aesthetic appearance of all-ceramic restorations.

The translucency of ceramics has been emphasized as a primary factor in achieving adequate aesthetic outcomes¹. Nonetheless, although glass-ceramic offers adequate optical properties, it is sometimes unable to mask darkened substrates because of its translucency^3,6-8. In these situations, all-ceramic systems with greater opacity and crystalline phase content should be used, since they provide greater masking ability^2,7,9-11.

Several different ceramic systems, designed to improve the final appearance of definitive all-ceramic restorations, have been described in the literature. However, it is a challenge for clinicians to select a ceramic system with the ability to mask the underlying substrate and match the appearance of ceramic restorations with patients' natural teeth, irrespective of differences in substrate color¹². This clinical report describes the masking ability of IPS e.max ceramic (lithium disilicate glass-ceramic and polycrystalline zirconia) over darkened (metallic post) and natural-colored backgrounds.

 

CASE REPORT

This clinical report was approved by the Institutional Review Board at the School of Dentistry, Universidade Federal de Pelotas, Brazil (protocol no. 905.099/2015).

A 35-year-old woman sought dental care complaining that she was unhappy with the aesthetic appearance of her smile. Intraoral clinical examination revealed interim acrylic resin crowns with poor color matching on maxillary right and left central incisors (Figure 1). Radiographic images showed healthy periodontal support and the right central incisor had previously undergone endodontic treatment and post placement with satisfactory results. Removal of the interim crowns revealed a natural-colored background on the left maxillary central incisor and a darkened background (caused by the metallic post) on the right maxillary central incisor. The teeth were re-prepared to receive metal-free crowns using diamond burs and a chamfer finish line was created, observing the minimum thickness recommended by the manufacturer. Cervical preparation was extended to the cementoenamel junction and smooth margins were created to prevent stress concentration zones (Figure 2). New interim restorations were made using prefabricated acrylic resin teeth (Trilux; VIPI Produtos Odontológicos, Pirassununga, São Paulo, SP, Brazil) and self-cured acrylic resin (Dencôr; Clássico, Campo Limpo Paulista, SP, Brazil), providing both function and aesthetics.

 

 

 

 

 

One month later, the patient returned for the impression step. The soft tissue was displaced using the double cord technique (#000 and #0 Ultrapak cord; Ultradent, Idaiatuba, SP, Brazil) (Figures 3A and 3B), and a full-arch impression was performed with polyvinyl siloxane (Express XT Putty and Light Body; 3M Espe, St. Paul, MN, USA) using a double-mix technique. An opposing arch impression was also taken using irreversible hydrocolloid (Cavex ColorChange; Cavex, Haalen, Netherlands). Color selection was conducted using digital images and a shade guide (Vitapan Classic; Vita-Zahnfabrik, Bad Sackingen, Germany). The left central incisor matched shade B2 (cervical) and B1 (incisal edge), and the right central incisor matched shade B4 (Figures 4A and 4B).

 

 

 

 

 

Stone casts and an interocclusal record at maximum intercuspation position were obtained. After the stone casts had been mounted in a semi-adjustable articulator (A7Fix; Bioart, Jardim Tangará, São Carlos, SP, Brazil), they were sent to the laboratory along with photos for manufacture of the ceramic frameworks. A wax pattern was prepared and pressed using medium-opacity lithium disilicate ingots (IPS e.max Press MO1; Ivoclar Vivadent,Tamboré, Barueri, SP, Brazil) for both crowns, to obtain copings with the minimum thickness recommended by the manufacturer. After verifying the fit of the copings, the veneer porcelain (IPS e.max Ceram; Ivoclar Vivadent, Tamboré, Barueri, SP, Brazil) was applied and fired to achieve the definitive dental shape. Aesthetic, phonetic, and functional evaluations were performed clinically.

At this point, it was observed that the medium-opacity lithium disilicate framework did not mask the metallic post inserted in the right central incisor. Frontal view photos (Figures 5A and 5B) with gamma correction, color supersaturation, and grayscale transformation (Figures 6A to 6F) were taken using a digital single lens refl ex camera (Canon EOS Rebel T5i, Tokyo, Japan) to enable visualization of color discrepancies between the ceramic crowns. The decision was then taken to keep the left ceramic crown and make a new one for the right tooth using a more opaque framework, made of polycrystalline zirconia (IPS e.max ZirCAD; Ivoclar Vivadent, Tamboré, Barueri, SP, Brazil). The same stone casts and photographs were once more sent to the laboratory for fabrication of the new crown. After evaluation of aesthetic and functional aspects, the crowns were cemented using light-cured resin cement (RelyXTMVenner, 3M Espe, St Paul, MN, USA) shade A1, following the manufacturer's instructions for each type of ceramic material. Photopolymerization was performed using a light emitting diode device (Radii-cal; SDI Brasil LTDA, Bayswater, Victoria, Australia) for 40 seconds at each face. All excess cement was removed from the marginal area. Figure 7 shows the crowns 1 month after cementing.

 

 

 

 

 

 

 

DISCUSSION

The ideal choice of ceramic opacity is a crucial element in obtaining clinically acceptable color-matching between crowns fitted over different substrates, since the opacity of the ceramic system can infl uence the final color of the restorations^1,3,12. The aesthetic appearance obtained in this clinical report revealed that a medium-opacity lithium disilicate crown did not mask an underlying darkened background caused by a metallic post. When the lithium disilicate framework was changed for polycrystalline zirconia, it was observed that the replacement material exhibited a better capacity for masking the metallic post.

Translucent ceramic systems allow light to be reflected and scattered from the underlying background, thereby affecting the final color of restorations¹³. As a result, translucent restorations can very closely mimic natural teeth, but in such situations, the elements to be restored usually present fewer color changes requiring correction. When more significant color changes are required, the opacity and thickness of the ceramic system should be carefully adjusted, because darkened substrates such as metallic posts can lead to undesirable changes to the final color of glass-ceramic restorations^5,6. In these situations, more opaque ceramic systems should be used to achieve adequate masking^9,11,13,14.

Since the opacity of ceramics is affected by the optical properties, microstructure, and thickness of ceramic systems^13-17, several in vitro studies have been conducted to investigate these issues to improve understanding of their capacity to mask unsatisfactory background shades^{3,5,6,11,18-21}.

In these in vitro studies, increased ceramic thickness and opacity have been linked to better masking ability of all-ceramic restorations over darkened substrates such as metallic posts^3,5,6,16,18. In this clinical report, the restorations observed the minimum thicknesses recommended by the manufacturer (0.5 mm for polycrystalline zirconia and 0.8 mm for the lithium disilicate framework). Additionally, the final thicknesses of the two restorations were different, because it is necessary to provide more space for veneering porcelain stratification in the presence of darkened substrates. Therefore, the left and right central incisor restorations have final vestibular surface thicknesses of 1.2 mm and 1.8 mm respectively.

The amount of light absorbed, reflected, and transmitted by dental porcelain depends on the crystal content, chemical nature, and size of the particles compared to the wavelength of the incident light². The crystal content of the lithium disilicate glass-ceramic is greater than 60% by volume, and the index of refraction of the crystal phase is adjusted to that of the glass matrix to control the translucency of the material. As a result, this material can be produced to different levels of translucency, depending on clinical requirements²². However, because it is considered a translucent ceramic,^2,9,10,23, if there is insufficient material thickness, the color of a darkened substrate will negatively infl uence the aesthetic result of the prosthetic restorative treatment^3,6-8. On the other hand, with a crystal content of 99% by volume and a higher refractive index, polycrystalline zirconia frameworks intercept incident light more efficiently than lithium disilicate frameworks and nullify the influence of underlying structures on the light refl ected to the observer, so they are considered opaque ceramics^2,24. Therefore, restorations manufactured with this material exhibit greater masking ability, which can be very useful in aesthetically demanding clinical situations, such as masking discolored substrates or metallic posts^2,7,9-11.

The IPS e.max ceramic system is an all-ceramic system that uses both lithium disilicate glass-ceramic and polycrystalline zirconia as framework. In this clinical report, the lithium disilicate framework was chosen because of the superior aesthetic properties of this ceramic when compared to polycrystalline zirconia ceramics. According to the manufacturer's table for ingot shade selection²⁵, if a B1 final shade is desired then the medium-opaque (MO 1) or high-opaque (HO 1) ingot should be used to mask darkened substrates with a B4 or yellowish metal shade color. In the treatment described here, the MO 1 ingot was used to make both frameworks because the left substrate had a natural color and it was preferable to refl ect this naturalness. However, even observing the manufacturer's instructions and leaving more space to stratify the veneering porcelain over the right coping, this option proved inadequate and a grayish discoloration was noted when both crowns were compared during clinical assessment. The manufacture of one new restoration with polycrystalline zirconia framework at the minimum thickness recommended by the manufacturer was sufficient to solve this problem, providing excellent color matching between both restorations, as can be observed in the images with digital adjustments.

It is probable that a high opacity ingot of lithium disilicate could completely mask the darkened substrate. However, the masking ability of polycrystalline zirconia is greater, due to the refractive index, and the manufacturer's recommendation for minimum framework thickness is lower, because of its higher compressive strength. The quantity of color discrepancies adjusted with the veneering porcelain over the polycrystalline zirconia coping is smaller, and the space available for stratification is greater. Additionally, the shade of the zirconia blocks in this system is based on that of medium opacity lithium disilicate glass-ceramic ingots, making it even easier to achieve the same final color as the adjacent restoration that was manufactured with the lithium disilicate MO framework.

 

FINAL CONSIDERATION

At the minimum thickness recommended by the manufacturer, the medium opacity lithium disilicate glass-ceramic framework did not provide sufficient masking ability over a darkened background (metallic post). The opacity of the polycrystalline zirconia framework affected the final appearance of the restoration over the metallic post.

 

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Correspondence:
Noéli Boscato
Graduate Program in Dentistry
Universidade Federal de Pelotas
Rua Gonçalves Chaves, 457
CEP 96015-560, Pelotas
RS, Brazil
E-mail: noeliboscato@gmail.com