Pros and Cons of Porcelain Fused to Zirconia (PFZ) Dental Restorations

In the last few decades, there has been an increasingly huge demand for highly esthetic dental restorations among U.S. dental patients. New developments and advances in all-ceramic dental materials have replaced metal-ceramic systems, such as porcelain-fused-to-metal (PFM) prostheses, with all-ceramic systems. Advancements in bonding techniques resulted in the development of porcelain fused to zirconia (PFZ) restorations, finding superior esthetics compared with PFM restorations.

Metal-free all-ceramic restorations offer better esthetics and biocompatibility than porcelain fused to metal (PFM) prostheses [1]. Although PFZ prostheses offer better esthetics than metal ceramic systems, dentists must evaluate the pros and cons of PFZ prostheses in order to make an informed decision on providing the best dental lab products to their patients. This article will briefly analyze the cons and pros of porcelain fused to zirconia restorations.

Microstructure of PFZ Restorations

Dental ceramics can be classified by their microstructure (i.e., amount and type of crystalline phase and glass composition)[1]. Zirconia oxide (ZrO₂) has a polycrystalline solid-based composition, formed by “directly sintering crystals together without any intervening matrix to from a dense, air-free, glass-free, polycrystalline structure”[1]. Solid-sintered zirconia-oxide ceramic is widely used as a framework material for dental implants, dental crowns, fixed-partial dentures, and other dental lab products.

Conventional dental porcelain is a partially glassy material that contains high amounts of leucite crystals added to aluminum oxide. This pressed-glass ceramic undergoes “dispersion strengthening through the guided crystallization of leucite”[1]. Porcelain is widely used for veneer layering onto a zirconia core (PFZ) or a metal core (PFM).   

Cons of PFZ Restorations

To begin with, clinical research and practices have reported high occurrences of veneer chipping and fractures in all major brands of PFZ systems, especially in posterior prostheses [2]. The porcelain overlay can chip during mastication, most notably at the coping level were thickness is a factor that influences the survival and success of the restoration [3]. According to Agustin et al., porcelain veneer chipping most often occurs as an esthetic defect without affecting the survival of the restoration and is easily repaired by polishing or intraoral repair; it often goes unnoticed by the patient. For this reason, Agustin et al. five-year study reported the survival rates of zirconia-based fixed dental prostheses to be 97-99%.

Although the survival rate is favorable, clinical studies have revealed a high rate of fractures for porcelain-veneered zirconia-based restorations that affect the success of the restoration—varying between 6% and 15% over a 3- to 5-year period. The success of the restoration is dependent on whether the restoration’s esthetics is compromised or not. Agustin et al. believes the reason for the high rate of fractures is uncertain but suspects the bond between the porcelain and the zirconia substructure failed. One likely reason could be the porcelain veneer has a weak flexural strength of 90MPa, contrasted to zirconia’s high flexural strength of 900–1200 MPa [2]. Another likely reason was found in a Baldassarri et al. 2012 clinical study, “the presence of a radial tensile stress in the overlay porcelain of zirconia-ceramic prostheses… may lead to the large clinical chips and fractures of these prostheses.”

Another con to porcelain-veneered zirconia-based restorations is a reported high wear loss of antagonist enamel because porcelain consists of leucite crystal grains that act as an abrasive surface during mastication [4]. Porcelain also consists of feldspathic glass that disappears after wear, leaving large leucite grains to be exposed and act as abrasive materials on the opposing enamel [5]. In fact, Porcelain-veneered zirconia shows higher wear loss on opposing enamel than that of polished zirconia. Rashid reviewed an Odatsu et al. study that used carborundum points, silicone points and diamond polishing paste on zirconia and traditional feldpsathic porcelain to examine the materials surface roughness. They found the “feldspathic porcelain showed highest surface roughness values after finishing and polishing procedures” [7]. They concluded, “a rough surface will abrade the opposing dentition or restoration, and it is highly recommended that the adjusted surface is finished and polished appropriately” [7].

Pros of PFZ Restorations

Despite the low flexural strength of the layered porcelain for PFZ restorations, superior fracture resistance of the zirconia ceramic core is a major pro. According to Miyazaki et al., in a study assessing zirconia-based restorations found the zirconia frameworks rarely got damaged and the complications that did occur were at the veneering ceramic level. In a 10-year clinical study, Sax et al. assessed the long-term survival rate of zirconia-based posterior fixed dental prostheses with zirconia frameworks and found the “zirconia frameworks exhibited very good long-term stability” [6].

In 2012, Agustin et al. analyzed porcelain veneer behavior on zirconia ceramic cores using a scanning electron microscopy (SEM) and found 71.66% of the facture type for zirconia-core restorations were cohesive – meaning the facture only occurred at the surface level without affecting the ceramic-core interface. In another study with the same objective, Saito reported 88.8% of fracture types were cohesive. Since most of the fractures are cohesive in nature, the veneered porcelain can be easily repaired without much concern the ceramic core is fractured [7].

Finally, PFZ restorations have the ability to mask discolored underlying tooth structures because zirconia has opaque characteristics that allow the prepared tooth to be concealed. Since the zirconia core is highly opaque, a porcelain overlay is needed to improve the esthetics of the zirconia core to appear more translucent, more similar to natural teeth. The dual layers of porcelain and zirconia from PFZ restorations allow the prosthetic to hide any discoloration and/or defects from the prepared tooth, making it a top option for patients who may have these issues.

Some studies, particularly Agustin et al., have concluded the survival rate is not dependent on esthetic defects but on the functional survival of the restoration. But with an ever-growing society that demands perfection in esthetics for dental restorations, functionality is not the only characteristic desired for dental restorations by the majority. Although the 1,000 MPa zirconia substructure exhibits good long-term stability, the failure of the layering porcelain can be seen as ultimately a failure of the restoration. After much clinical research, it is safe to say the porcelain layer on PFZ restorations is not reliable for the long-term success of esthetics. Consequently, PFZ restorations are at the risk of failing as an acceptable restoration among U.S. dental patients.

Resource Box:

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References:

1. Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent. 2010;13:195–203. [PMC free article] [PubMed]

2. Baldassarri M, Stappert CF, Wolff MS, Thompson VP, Zhang Y. Residual stresses in porcelain-veneered zirconia prostheses. Dent Mater. 2012;28:873–9. [PMC free article] [PubMed]

3. Triwatana P, Nagaviroj N, Tulapornchai C. Clinical performance and failures of  zirconia-based fixed partial dentures: a review literature. J Adv Prosthodont. 2012;4:76–83. [PMC free article] [PubMed]

4. Agustin-Panadero R, Román-Rodríguez JL, Ferreiroa A, Solá-Ruíz MF, Fons-Font A. Zirconia in fixed prosthesis. A literature review. J Clin Exp Dent. 2014;6:e66–73. [PMC free article] [PubMed]

5. Miyazaki, T., Nakamura, T., Matsumura, H., Ban, S., & Kobayashi, T. (n.d.). Current status of zirconia restoration. Journal of Prosthodontic Research, 236-261.

 

6. Sax C, Hammerle CH, Sailer I. 10-year clinical outcomes of fixed dental prostheses with zirconia frameworks. Int J Comput Dent. 2011;14:183–202. [PubMed]

 

7. Saito A, Komine F, Blatz M, Matsumura H. A comparison of bond strength of layered veneering porcelains to zirconia and metal. J Prosthet Dent. 2010;104:247–57. [PubMed]

7. Rashid, H. (n.d.). The effect of surface roughness on ceramics used in dentistry: A review of literature. European Journal of Dentistry Eur J Dent, 571-571.