Glazing and Finishing Dental Porcelain: A Literature Review
Ahed Al-Wahadni, BDS, MDSc, PhD
The use of ceramic materials in dentistry can be traced to 1728, when Fauchard suggested its use to restore teeth. Since that time, many in-vitro and in-vivo trials have been conducted for the purpose of improving porcelain to satisfy the cosmetic, mechanical and physical requirements of a restorative material. Glazed porcelain is the restorative material that least encourages plaque accumulation and allows plaque to be easily removed. Glazed porcelain can also duplicate natural tooth surface lustre and characterization.
This paper reviews the literature on one aspect of the clinical handling of dental porcelain how to refinish porcelain when the glaze has been removed during adjustment. We present information about the effect on opposing dental structures of unglazed porcelain, glazed porcelain, and porcelain that has been refinished by various techniques.
Ceramists classify the various stages of the firing or sintering, of dental porcelain as:
1.low bisque the porcelain surface is very porous and will easily absorb water soluble dyes;
2.medium bisque the porcelain surface is still porous and shrinkage will have taken place;
3.high bisque the porcelain surface is now sealed and strong enough to be corrected by grinding prior to final glazing.1
The aim of glazing is to seal the open pores in the surface of a fired porcelain. Dental glazes are composed of colorless glass powder, applied to the fired crown surface, so as to produce a glossy surface.1
The adjustment of a porcelain restoration, for occlusal or contour correction, may have an unfavorable secondary impact on the neighboring teeth, depending on the location of the adjustment. The adjusted rough surface may lead to abrasive wear of the opposing dentition or increase the rate of plaque accumulation.2,3 Unglazed or trimmed porcelain may also lead to inflammation of the soft tissues it contacts.4 Trimming of porcelain may cause some reduction in the strength of a ceramic restoration.5,6
Occlusal contacts between unglazed porcelain and opposing unglazed porcelain or enamel are undesirable because of the high rate of wear of enamel and porcelain. 7 Early researchers agreed that re-glazing was necessary after porcelain adjustment in the clinical setting.8 Many dentists therefore, prefer the porcelain surface of a restoration to be glazed (or re-glazed) prior to cementation.9
A number of more recent studies have suggested that a polished surface may be as acceptable as a glazed surface.10 Many ceramists prefer polishing instead of glazing, to control the surface lustre. Rosentiel et al found that the fracture toughness of polished porcelain was greater than that of glazed porcelain and that both types of finish were equally resistant to staining by coffee.11
The choice of finishing and polishing technique to achieve the optimum smoothness of glazed porcelain has been the subject of a number of studies. Barghi et al found that the smoothness of the surface attained after glazing is not affected by surface treatment prior to glazing.8
Zalkind et al found that glazing a porcelain surface which is reduced by an abrasive instrument will not reduce the resulting roughness. They observed that the only way to produce a surface as smooth as it had been before, is to sandblast the abraded surface with aluminum oxide powder before refiring to produce a natural glaze.12
Sulik and Plekavich 13polished fully maturated porcelain by using a hard rubber wheel, wet pumice and wet tin oxide, sequentially. They found no differences clinically or by means of scanning electron microscopy, between the polished and naturally glazed surfaces of vacuum fired porcelain. Some voids were present on the polished surface which were not evident on the glazed surface.
Smith and Wilson used a series of Soflex discs, designed for finishing composite restorations, to achieve a surface finish on trimmed porcelain surfaces. They found that the surface finish attained with the Soflex discs was comparable to that produced by abrasives commonly used for trimming porcelain surfaces.14
Haywood et al used a series of finishing grit diamonds with diminishing particle sizes (Micron Finishing System), followed by a 30 fluted carbide bur and diamond polishing paste to polish porcelain intraorally. They found this method produced surfaces that were as smooth as glazed porcelain.15
Goldstein found that cups and points made by Shofu were the best instruments available for the final finishing of porcelain.16
Patterson et al examined the surface smoothness produced by a commercial porcelain refinishing kit, incorporating diamond paste (Chameleon Diamond Paste), by using scanning electron microscopy and profilometry.17 They found the paste was capable of achieving a good porcelain surface smoothness on surfaces previously adjusted by fine (red band FG) diamond burs. However, this polishing system was incapable of achieving a surface smoothness comparable to that produced by glazing.
Raimondo et al compared the surface finish of unglazed porcelain produced by six different polishing techniques, with that produced by glazing.18 The Shofu kit was the only kit, among those tested, that did not come with polishing paste. It produced a surface that was least acceptable visually, however, its results rated better when examined under the scanning electron microscope. The researchers recommended this kit for smoothing porcelain, if used in conjunction with a porcelain polishing paste containing fine diamond particles.
Grieve et al 19 evaluated three methods of polishing porcelain: a diamond paste; a pumice and water slurry, followed by whiting and a proprietary porcelain finishing kit. A rubber wheel impregnated with carborundum was first applied in all cases. The diamond paste produced the smoothest surface, with the proprietary porcelain finishing kit producing the least smooth surface. Both the diamond paste and the pumice/whiting procedure produced surface finishes comparable to the original glazed surface.
Coarser abrasives give rise to rougher porcelain surfaces. Klausner et al 20 showed that diamond produces the roughest surface and that porcelain finishing stone also produces considerable roughness. These researchers found the Shofu finishing kit was capable of producing as smooth a surface as glazed porcelain.
The efficiency of porcelain polishing was found to be improved when diamond instruments were used at moderate speed, with water spray, or when carbide instruments were used at high speed, without water spray.21
Scurria and Powers examined the surface roughness of two types of ceramic (Ceramco II and Dicor MGC) subjected to five different polishing systems.22 The polishing systems comprised various combinations of: diamonds (45, 25 and 10 mm); a 30-fluted carbide; three silicon carbide-impregnated rubber points; diamond paste (4 and 1 mm); an aluminum oxide point; and two aluminum oxide pastes.The controls specimens were autoglazed Ceramco II and Dicor MGC ceramic blocks milled with a Cerec diamond wheel.
They found that feldspathic porcelain could be polished smoother than glazed porcelain. Dicor polished to a smoother surface than Ceramco II ceramic, by using diamond paste. Finishing diamond points produced the smoothest surface. Smoothness was not improved by using a 30-fluted carbide. For Dicor ceramic, aluminum oxide paste following the use of aluminum oxide points produced a result that was equivalent to that produced by finishing diamonds and gels.
In their in-vitro investigation of enamel wear caused by unglazed, glazed and polished porcelain, Jagger and Harrison 23 found that the rate of enamel wear produced by glazed and unglazed Vitadur porcelain was similar. Porcelain polished with a series of sandpaper disks of increasing fineness (Softlex, 3M) and rubber points (Shofu) produced substantially less enamel wear. The investigators highlight the damage that porcelain can potentially inflict upon enamel, and suggest that porcelain should be polished instead of reglazed after chairside adjustment.
The same authors 24,25 investigated the wear caused by selected restorative materials to opposing dentine and enamel. They found that glazed porcelain produced the greatest amount of wear in dentine, compared to the wear produced by amalgam, gold and microfine composite. The enamel wear produced by glazed and unglazed porcelain was equivalent but greater than the wear produced by the other restorative materials investigated. Hudson et al confirmed these findings, with regard to enamel wear, in a recent study.26
Hacker et al recently investigated the enamel wear caused by low-fusing porcelain (Procera All-Ceramic), feldspathic porcelain (Ceramco), and gold alloy (Olympia, J.F. Jelenko).27 To simulate the oral environment, the testing was performed in a well of fresh, natural, human saliva. Significant differences in restorative material wear were found between the gold and the porcelain materials. The feldspathic Ceramco porcelain caused the greatest wear of enamel.
Table I summarizes the various polishing techniques recommended by different authors. The results of the studies described in Table I support the use of polishing as an alternative of glazing.
A number of studies suggest that the Shofu polishing system produces a surface finish comparable to that of glazed porcelain, 7,20 while others have highlighted the importance of using a diamond paste along with the Shofu system, in order to achieve the best possible finish.9,16 This system contains hybrid diamond points for adjustments, Dura-white stones for recontouring and Ceramiste points for smoothing.
We recommend that any adjusted porcelain restoration should be reglazed or subjected to
a finishing sequence which is followed through to a final stage of polishing with diamond
paste. For polishing, we prefer the Shofu porcelain veneer kit.
Care should be taken clinically to avoid over-reduction of an aluminous porcelain surface. Over-reduction will lead to exposure of a less homogeneous layer of alumina, that will give rise to a more abrasive surface in the finished restoration.
Dr. Al-Wahadniis assistant professor, Jordan University of Science and Technology. Dr.
Martinis director of dental studies, Leeds Dental Institute, UK.
Fig 1: Fig. 1:Kit for finishing adjusted porcelain.