Cementation of Zirconia-Based Toothborne Restorations: A Clinical Review

The clinical success of a dental restoration is determined by its resistance to fracture, marginal fit, esthetic outcome, and survival. Although the marginal fit of zirconia restorations has been shown to be equal to that of metal ceramics,^7,8 reliable marginal seal is also important for long-lasting restorations. The most common factors that influence the marginal fit are finish line configuration, cement space dimensions, fabrication process, and cementation. Cementation of zirconia restorations can be a simpler process compared to the delivery/cementation of other all-ceramic systems. Zinc-phosphate cement,^9-14 glass-ionomer cement (GIC),^15-18 resin-modified GIC,^5,19 and composite-resin cement have all been used for cementation of zirconia restorations.^13,20-27

Loss of abutment tooth vitality, abutment tooth fracture, and secondary caries are predominant biologic complications for crowns.¹ Loss of retention has been reported for both metal-ceramic and all-ceramic restorations, and it may occur more frequently in restorations cemented with zinc-phosphate or GIC compared to restorations cemented with composite-resin cements.¹³ Cement choice may impact the long-term survival rates of restorations if the retention form of the tooth preparation is less than ideal. High rates of loss of retention of single crowns may be related to the total angle of convergence (ie, taper) of the tooth preparation and abutment height rather than cement type.^28-30

The most common problems associated with zinc-phosphate cements are ditching at the restoration margin, leakage, and secondary caries.^20,31 Although no direct correlation was observed between secondary caries and cement type for zirconia-based crowns that were luted with either composite-resin cement, GIC, zinc-phosphate cement, or temporary cement, the rate of crown loosening after 3 years was 12.5% for zinc-phosphate cement (Phosacem, Ivoclar Vivadent, ivoclarvivadent.com) and 6.6% for self-adhesive modified composite-resin cement (RelyX^™ Unicem, 3M Oral Care, 3m.com).²⁶

In a split-mouth clinical study after 8-year follow-up, estimated survival, which was defined as no loss of retention or no loss of pulp vitality, was 96% (95% with zinc-phosphate cement and 97% with resin-modified GIC) for metal-ceramic and zirconia crowns. The difference between resin-modified GIC and zinc-phosphate cement was shown not to be statistically significant.³² Despite the encouraging retentive capacity of zinc-phosphate-cemented zirconia restorations, after 5 years' follow-up of three-unit fixed partial dentures that were cemented with either composite-resin cement (Panavia^™ F, Kuraray, kuraraydental.com) or zinc-phosphate cement (De Trey^® Zinc, Dentsply Sirona, dentsplysirona.com), visible evidence of ditching along the margins was seen in the zinc-phosphate-cemented group.⁴ Three-unit zirconia bridges (Cad.esthetics system, Denzir AB, denzir.com) were rated satisfactory and only one restoration was recemented after a 5-year observation period. Pitted surfaces and ditching were reported in 30% of bridge retainers that were cemented with phosphate cement.¹⁰

GICs are used mainly to cement cast-metal and metal-ceramic restorations with adequate retention and resistance form. Although their ability to bond chemically on tooth surface makes GICs attractive for clinical application, traditional GICs may not be considered as well-suited for high-strength ceramic restorations such as zirconia because of their low strength and solubility in a moist environment³³ compared to resin-modified GICs.³⁴ Zirconia-based bridges that were fabricated with zirconia margins and conventionally luted with resin-modified GIC (RelyX^™ Luting, 3M Oral Care) were successful at 5 years of clinical service in terms of marginal adaptation, and no loss of retention or secondary caries was reported.⁵ Although microleakage scores did not show statistical difference between adhesive-resin cement and GIC for zirconia crowns,³⁵ adhesive methods may provide better retention, improve marginal adaptation, and prevent microleakage.^36,37

Posterior three-unit zirconia-based bridges (Cercon^®, DeguDent GmbH, Dentsply Sirona) veneered with pressable glass-ceramic (Cercon Ceram Express) showed loss of retention on one distal abutment of a mandibular bridge replacing the first molar after 3 years of clinical service. These were cemented with Ketac^™ Cem Aplicap^™ (3M Oral Care) after air-particle abrasion (50 μm aluminum oxide, 500 HPa pressure).¹⁶ In general, short-term studies of less than 5 years' follow-up showed good survival rates for either zirconia-based or metal-ceramic bridges cemented with GICs. Bridges manufactured with computer-aided design/computer-aided manufacturing (CAD/CAM) zirconia showed survival rate of 100% for the frameworks 3 years after cementation with GIC (Ketac^™ Cem, 3M Oral Care). After 3 years, the marginal integrity at the abutment teeth was ranked excellent in 91% of the restorations with small numbers of discoloration on the margin without evidence of caries.³⁸

Adhesive cementation has been clinically proven to be a suitable procedure for the permanent seating of indirect restorations. However, adhesive cementation procedure typically is more technique-sensitive than conventional luting procedures and requires a strict clinical protocol. Zirconia cementation with different composite-resin cements is promoted in different studies if the retention and resistance forms are less than ideal. Although a zirconia-based restoration cemented with a dual-cure, self-adhesive resin cement (RelyX Unicem) showed no debonding after 3-year follow-up³⁹ and 5-year follow-up,²⁷ clinical evaluation of single crowns after up to 7 years of service cemented with either zinc polycarboxylate (Poly-F^®, Dentsply Sirona), composite resin (Panavia^™ F 2.0, Kuraray), or resin-modified GIC (Fuji Plus^®, GC America, gcamerica.com) showed that debonding could be related to taper of the preparation or use of short abutments instead of cement type.^28,40 Self-adhesive composite-resin cements are an attractive option for zirconia-based restorations without requiring any pretreatment, such as etching, priming, air-particle abrasion, or use of a bonding agent, prior to cementation. However, discoloration with no evidence of secondary caries and no pitting along the margins was observed in three-unit veneered zirconia bridges cemented with a composite-resin cement (RelyX Unicem) after 3 years.⁴¹ Cold sensitivity or post-cementation sensitivity after cementation can be an issue with these cements because of their acidic self-adhesive properties. There are reports, however, showing no difference between self-adhesive and total-etch composite-resin cements.^42,43

Crowns and bridges manufactured with four zirconia systems (Cercon, Dentsply Sirona; ZirkonZahn^®, zirkonzahn.com; Lava^™, 3M Oral Care; Katana^™, Kuraray) and cemented using dual-cured composite-resin cement (Panavia F 2.0) showed predictable high survival rates on teeth with no debonding or secondary caries after 4 years of clinical service.⁴⁴ Zirconia crowns bonded with dual-polymerizing, self-adhesive composite-resin cement (RelyX Unicem) showed 80% success and 89% survival rates after 2 to 6 years of service. Loss of cementation was found to be only 4%.⁴⁵ Zirconia-based anterior maxillary crowns with customized copings cemented with dual-polymerizing, self-adhesive composite-resin cement (RelyX Unicem) performed well with no loss of cementation and no secondary caries after 5 years of service with a survival rate of 100%.²⁷

To enhance bonding between composite-resin cement and zirconia, air-particle abrasion of the intaglio surface is advocated. Tribochemical silica coating with an air-abrasion device (CoJet^™ System, 3M Oral Care) may be effective treatment for zirconia intaglio surfaces to increase the bond.⁴⁶ Surface pretreatments such as hydrofluoric acid etching or silanization may not improve the bonding strength of zirconia because of the material's quasi-chemical inertness with limited adhesive luting potential.^47-50 Combined surface treatments using airborne particle abrasion and specific adhesives with a hydrophobic phosphate monomer currently are clinically accepted techniques for bonding to zirconia ceramics.^51-57 Moderate-pressure airborne-particle abrasion in combination with phosphate monomer-containing (10-methacryloyloxydecyl dihydrogen phosphate [MDP]) primer or luting resin is recommended to provide a long-lasting durable bond to zirconia to achieve a decreased decementation rate.⁵⁷ Application of a tribochemical silica coating in combination with an MDP-containing bonding/silane coupling agent mixture may increase the shear bond strength between zirconium-oxide ceramic and phosphate-modified resin cement (eg, Panavia F).

The tribochemical silica coating process was also tested with zirconia silanization, which resulted in an increased bond strength. Silane/phosphate bonding agent for cementing zirconia restorative material does not necessarily require another mechano-chemical treatment before application of the silane/phosphate bonding agent; however, stable bond strength was achieved with tribochemical silica coating of zirconia and with phosphate monomer and silane. This bonding mechanism is similar to bonding with feldspathic porcelain with silane/MDP bonding agent.⁵⁸ In a study with a 5-year follow-up in which pretreatment of the restorations included airborne particle abrasion with aluminum oxide (110 µm, 200 kPa) and luting with dual-polymerizing, self-adhesive composite-resin cement (RelyX Unicem), none of the zirconia bridges required recementation and no secondary caries was found.⁵⁹ A similar clinical study with three-unit zirconia-based bridges in the premolar and molar regions luted with RelyX Unicem without any surface treatment showed no loss of retention after 5 years of clinical service.⁶⁰ In addition, zirconia-based surveyed crowns (ZirkonZahn) for partial removable dental prostheses bonded with dual-polymerizing, self-adhesive composite-resin cement (RelyX Unicem) showed no loss of retention in a 4-year clinical retrospective study.⁵⁹ Finally, zirconia-based bridges cemented with a self-adhesive resin-based luting system (RelyX Unicem) demonstrated a similar survival rate to metal-ceramic bridges after 4 years of clinical service with no retention loss or decementation.^39,61,62

To clean the abutment tooth before adhesive cementation, mechanical cleaning with pumice paste and slurry is advocated instead of using hydrogen peroxide, ethylenediaminetetraacetic acid (EDTA), or sodium carbonate solution. Air-particle abrasion of the abutment tooth is also an accepted procedure. Some clinicians have considered the use of phosphoric acid for cleaning the intaglio surface immediately after try-in before definitive cementation. Because of zirconia's strong affinity toward the phosphate group, phosphoric acid interacts with zirconia surfaces to form zirconium phosphate. However, zirconium phosphate will be inert to the adhesive cement's phosphate group.⁶³ This can cause lower bond strength between the zirconia-based intaglio surface and the composite-resin cement.⁶⁴

Zirconia-based restorations are considered an acceptable option for restoring anterior and posterior dentitions. Indications for conventional cementation should be critically reviewed, because greater loss of retention occurred in restorations cemented with zinc-phosphate cement or GICs compared to those cemented with composite-resin cements. Self-etching, self-adhesive resin cements were shown in numerous clinical studies to be clinically reliable with better mechanical strength compared to zinc-phosphate cements and GICs. Moreover, self-adhesive resin cements are also clinically appealing to the dentist because the need for pretreatment, such as etching, priming, or using a bonding agent, is eliminated, providing a less technique-sensitive, simple, fast, and predictable cementation protocol. Further well-designed studies with large patient groups and long follow-up times are needed to obtain long-term clinical information on cementation of zirconia-based restorations.

Dr. Raigrodski periodically receives speaker honoraria from 3M Oral Care, though he did not receive any specifically for this article.

Sami Dogan, DDS, Dr. med. dent.
Associate Professor, Department of Restorative Dentistry, University of Washington, Seattle, Washington

Ariel J. Raigrodski, DMD, MS
Affiliate Professor, Department of Restorative Dentistry, University of Washington, Seattle, Washington; Diplomate, American Board of Prosthodontics; Private Practice, Lynnwood, Washington