Different Zirconia Surface Treatments: Strategies to Enhance Adhesion in Zirconia-Based Dental Restorations

Abstract

All-ceramics have been widely used in dentistry. The favorable mechanical properties and high biocompatibility of zirconia ceramics lead to using it as an alternative to the conventional porcelain-fused-to-metal (PFM) restorations. However, its major limitation is the difficulty of achieving proper bonding due to the chemical inertness of its surface. The current work aims to show the application of the zirconia in dental fields and to discuss the various surface treatment options available. Hydrofluoric acid treatment of the zirconia surface did not result in durable bonding. A combination of mechanical and chemical preparation is essential to reach an activated surface. Different mechanical preparation methods were used as airborne-particle abrasion, tribo-chemical silica coating, low-fusion porcelain layers application, selective infiltration etching, laser irradiation, plasma spraying and zirconia ceramic powder coating. Chemical preparation involves the use of silane and primers. However, the proper universal bonding technique of zirconia has yet to be established, owing to insufficient supporting evidence in literature. This review highlights the use of zirconia ceramics in dental field and its proper bonding protocol achieved via different surface treatments.

INTRODUCTION

A wide variety of dental materials are currently available in the dental field. There is a continuous desire to achieve terraforming and more aesthetic materials than the available materials are increasing. The ceramic-metal system, also known as porcelain-fused-to-metal (PFM), was the gold standard during the 1960s due to the limited mechanical properties of pure ceramics. It has high longevity and strength; moreover the underlying metal framework withstands heavy mastication forces, but metallic ions cause gingival discolorations, and it lacks the aesthetic properties due to its limited light transmission crucial for natural appearance.[1] Eventually, metal-free restorations were developed, shifting the use of ceramo-metallic system to fully ceramic system. this transition mainly aimed to improve the aesthetic properties of the restoration by mimicking the natural appearance of the teeth due to its high optical properties. Ceramics have highly attractive clinical properties such as high strength, good biocompatibility, and chemical stability. They combine both mechanical and optical properties needed in the dental restorations.[2]

The use of dental ceramics has increased significantly with the advancement of dental materials and growing aesthetic demand. Ceramics have become a mainstream material in restorative dentistry, as they offer numerous advantages, including excellent mechanical properties, high aesthetic properties, volumetric and color stability, biocompatibility and decreased thermal conductivity. Over time, dental restorations have shifted from metal alloys to ceramic materials in performing various dental prosthetics like on lays, inlays, veneers, full coverage crowns and bridges, implant-supported prostheses and other restorations.[3] Ceramic materials have been continuously improved, and different types have been developed, as silicate ceramics, polycrystalline ceramics, and resin-based ceramics.[4]

METHODS OF SEARCH

Zirconia surface treatments that improve their bonding strength were the subject of a thorough literature search utilizing PubMed, Scopus, ScienceDirect, and Google Scholar. Zirconia ceramic, surface treatment, and resin cement bonding were among the keywords. Studies published in English between 2000 and 2024 were included. Articles without data on enhancing adhesion or unrelated to zirconia were not included.

TYPES OF CERAMICS

Scientific articles have presented a wide variety of ceramic classification systems, but only one is discussed in this article. Dental ceramic are generally divided, according to their microstructure and esthetic properties into two mains categories: silicate ceramics and oxide ceramics. Silicate ceramics include feldspathic porcelain and glass ceramics. Glass ceramics are divided into leucite-reinforced ceramics, lithium disilicate ceramics, and glass-filled ceramics. Oxide ceramics are composed of zirconium oxide and aluminum oxide.[1,5]

ZIRCONIA IN DENTISTRY

Zirconia was first used as coping and framework material for veneering with layered porcelain, and later for fabrication of fully contoured monolithic restorations. Superior biological and mechanical characteristics, high clinical success rate, accurate milling process, low wear effect to the opposing dentition and decreased material and fabrication have contributed to zirconia widespread in daily dental practice nowadays.[6]

One of the highest-strength ceramics in dentistry is zirconia (zirconium-dioxide, ZrO₂). It is one of the most stable materials and is compatible with CAD/CAM technology. It has high flexural strength (>1 GPa) and increased fracture toughness (K_IC = 9–10 MN/m³/²) compared to the metal-based fixed prostheses. However, it partially transmits light, so it is considered semi-opaque material.[1] Crowns, bridges, and implant superstructures are made using zirconia because of its properties.[7]

ZIRCONIA CRYSTALLOGRAPHY

Zirconia has three major crystal phases, which vary depending on temperature. At room temperature and up to 1170°C, zirconia is in its monoclinic phase. If the temperature rises above approximately 1170°C and up to 2370°C, it transforms into a tetragonal structure. When the temperature increases beyond 2370°C, zirconia transforms into a cubic structure. In clinical applications, the tetragonal phase zirconia is particularly preferred for its toughness and aesthetic appeal.[5]

ZIRCONIA SURFACE TREATMENT

Since oxide ceramics are free of glass phase and have high crystalline content, zirconia is not etchable that represents a major limitation. The conventional protocols for bonding such as hydrofluoric acid etching followed by silane coupling agents-used with glass ceramics do not allow true chemical and micromechanical adhesion to zirconia.[1] As a result, for luting agents to perform durable bond with zirconia surfaces, the surface area must be increased, and an active surface must be created.[8] Therefore, proper surface treatment, type of luting agent and cementation protocol play the significant role in the success of clinical indirect ceramic restorations and increases their bonding and flexural strength.[9]

Untreated zirconia surfaces exhibit low bond strength and more adhesive failures compared to pre-treated zirconia surfaces. There is no universal zirconia surface pre-treatment protocol. Former pre-treatments were suggested as hydrofluoric acid chemical etching and grinding surface using diamond burs to create roughness, have been proposed, but none of them resulted in the needed bonding strength.[10] Various mechanical, chemical, and combined chemo–mechanical zirconia surface pre-treatment methods have been employed to enhance the zirconia-resin bonding. These include airborne-particle abrasion (APA), tribo-chemical silica coating (TSC), laser irradiation, plasma spraying, selective infiltration etching (SIE) and low-fusion porcelain layers application.[6,8]

Succeeding the surface treatment, the application of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) bifunctional monomer to the surface is recommended due to its two functional groups: the phosphate ester group, which bonds chemically to metal oxides, and the methacrylate group, which bonds to the resin matrix of the resin cement. Also, proper resin cement is used to provide the needed long-term durable zirconia-resin bonding. This technique is established in literature due to its positive impact on the bonding performance.[11]

SALINIZATION OF ZIRCONIA

Application of silane coupling agents is crucial for achieving durable bonding between zirconia base structure and resin cement because it acts as link between inorganic zirconia surface and organic resin cement. Salinization enables a chemical bond with the silica coating deposited on the zirconia surface. The silane coupling agent consists of two functional groups at each end of its molecules, that connect the unpolymerized resin matrix with the inorganic surface.[12]

Acid-catalyzed hydrolysis of the alkoxy groups (RO3Si-) of the silane molecule through its reaction with water leads to formation of silanol groups (SiOH).[8] The formed silanol groups (Si–OH) react with hydroxyl groups (OH) on the silica particles coating the zirconia, resulting in the formation of siloxane covalent bonds (Si–O–Si).[12] Moreover, the end of the silane molecules bonds with the methacrylate groups of the resin adhesive agent. This forms a strong network of siloxane covalent bonds between the ceramic surface and methacrylate resin. However, this processes not applicable to untreated zirconia-based ceramic surfaces due to the absence of a silica phase.[8]

Nevertheless, siloxane bonds at the bonding interface deteriorate over time in the oral environment. Research suggested that using newly developed silane chemicals promotes the stability of zirconia/resin interface to aging environment under hydrolytic conditions through the adoption of MDP-containing primers or compatible resin cements.[13]

AIR-BORNE PARTICLE ABRASION (APA)

Air-borne particle abrasion treatment uses the kinetic energy of abrasive grains in a stream of compressed gas (usually air). The accelerated abrasive grains (Al₂O₃ grains) strike the surface of the treated surface (zirconium dioxide) leading to micro-cutting and surface preparation through material removal. This process is termed abrasion, and the treatment is referred to as abrasive blasting.[14] However, research failure analysis showed that the Al₂O₃ sandblasted zirconia bonding to cement is less stable and showed high mechanical aging resulting in failure in the cement/zirconia interface.[10]

This surface modification produces a clean zirconia surface free of impurities and contaminants, increased adhesion between the ceramic and adhesive through the creation of surface microroughness by mechanical means and increased surface energy and wettability.[14] It was found that APA in combination with an MDP monomer-based primer or resin cement led to durable resin-zirconia bonding.[15]

Studies have indicated that APA can generate microcracks that can propagate over time. It negatively affects the fatigue resistance of zirconia restorations and can cause degradation of its mechanical properties and radial cracking of the framework during function. Thus, the effectiveness of mechanical surface treatment is affected by factors such as grain size, applied pressure, and distance. These limitations have been addressed by using smaller-sized particles, lower pressures, and shorter application distances. These adjustments help reduce crack initiation on the treated surface, accelerating the long-term mechanical properties of the ceramic material without compromising its bond strength.[12]

TRIBO-CHEMICAL SILICA-COATING (TSC)

Tribo-chemical silica-coating using silica-coated alumina particles is another method of abrasion used to enhance bonding to zirconia. This method creates micromechanical retention on the surface. Moreover, it chemically activates the surface of the zirconia by deposition of silica particles with grain size 30–110 μm, which promotes the affinity of zirconia for chemical bonding with silane coupling agents.[8]

Previously, in vitro studies have shown that the use of TSC combined with silane or a primer/cement containing MDP forms a more stable and durable bond to zirconia in comparison with the conventional alumina blasting. Moreover, other studies have shown that initially TSC shows high bond strength, but it decreases after long-term storage and thermocycling. Studies suggested that the deterioration of bonding may be due to the deposition of thin layer of silica on the zirconia surface. This layer may promote the penetration of water, therefore causing degradation of the zirconia bonding. Unfortunately, clinical trials are lacking to clearly confirm the long-term efficacy of silica-coated zirconia bonding.[12]

LASER IRRADIATION

Other methods for treating zirconia surfaces include the use of laser systems such as carbon dioxide (CO2), neodymium-doped yttrium aluminum garnet (Nd:YAG) or erbium-doped yttrium aluminum garnet (Er:YAG).

The zirconia surface is irradiated with these lasers to roughen the surface and create micromechanical interlocking. Studies revealed that untreated surfaces and those treated with Er: YAG laser produce similar bonding results. A bubbled, blister-like appearance and microcracks were observed on the irradiated surface due to heat damaging caused by the Er:YAG laser application. This heat-damaged layer has weak attachment to the substrate and may cause cracks under minimal forces.[8]

Nd: YAG laser etching causes melting and fusion of the ceramic layer, followed by solidification that forms a smooth, blister-like surface. In contrast, CO2 laser treatment produces a conchoidal, tears-like surface due to heat induction. Studies have shown that Nd: YAG and CO2 laser etched surfaces achieve sufficient bond strength values.[16]

PLASMA TREATMENT

The atmospheric-pressure low-temperature plasma device operates based on the mechanical piezoelectric resonance application, which amplifies electric energy to generate high voltage. This high voltage causes the surrounding atmospheric gas-specifically high-purity helium gas-to become ionized, thus generating plasma. Helium gas is mainly used because it forms uniform, stable, and mild glow discharges at atmospheric pressure, making it superior to gases such as oxygen, nitrogen, argon, or air. The plasma generated is environmentally friendly, highly efficient, and easy to apply during clinical use.[15]

Many studies have been conducted on the effect of atmospheric-pressure low-temperature plasma treatment on the bond strength of zirconia to resin cement. Research confirms that atmospheric-pressure low-temperature plasma treatment is effective in increasing bonding strength and resists aging by not altering the morphology of zirconia surface, increasing its wettability, and decreasing the carbon/oxygen ratio of zirconia surface.[15]

When comparing sandblasting treatment to atmospheric-pressure low-temperature plasma treatment, sandblasting causes surface roughness and microcracks, that degrade the mechanical properties of zirconia causing radial cracking of the framework during function. Plasma treatment, however, does not alter surface roughness and preserves the zirconia surface. Additionally, plasma treatment demonstrates better wettability than sandblasting treatment. It introduces hydrophilic functional groups into both the organic resin materials and the otherwise inactive zirconia surface. It also improves the compatibility of primers with the surface. Research showed that pretreatment with plasma treatments combined with primers containing MDP significantly improves bond strength.[15,17]

Zirconia surfaces often contain carbon content that results from organic contaminants. These contaminants can be dissociated from the surface by atmospheric-pressure low-temperature plasma treatment through the impact of high-energy ions on zirconia surface. Reducing carbon content exposes more of the zirconia surface, that is clinically beneficial for improving adhesion.[17]

SELECTIVE INFILTRATION ETCHING (SIE)

Selective infiltration etching is another promising surface treatment method for zirconia ceramics. It depends on heat-induced maturation (HIM) process through stressing zirconia grain boundaries by thermal cycles. In addition to selective infiltration of a molten glass agent between the zirconia grains, creating an eroded and porous surface structure. The rearrangement in the surfaces result in the formation of a highly retentive and reactive three-dimensional nanoporous network.

SIE has a unique property of selectivity, as it only affects the zirconia grains in contact with the infiltration glass, allowing the operator to precisely control the etched area. Unlike air abrasion, SIE does not damage or alter the zirconia surface properties that may weaken the material. SIE increases retentive surface roughness without degrading zirconia. Research has shown that bonding achieved via SIE is strong enough to withstand the functional load in high-stress regions.[11]

LOW-FUSING PORCELAIN GLAZE

Another surface treatment proposed for zirconia involves the application of a thin low-fusing porcelain glass layer on the zirconia surface. This veneered layer provides a chemically bonded glass phase that can be acid etched and subsequently treated with silane coupling agents.[13]

The layer is composed of vitreous, amorphous, high silica porcelain (SiO₂) and metal oxide pigments. It can be applied using spray or brush (powder/liquid) techniques. Regardless of the application method, the coated zircons are then fired in a ceramic oven. After firing, the surface is selectively conditioned by hydrofluoric acid. Silane is then applied to establish micromechanical retention and a chemical bond, like that the adhesion protocol for glass ceramics.

This surface treatment improves roughness, wettability silica content of the zirconia surface. It also enables better standardization of the glaze layer and prevents tetragonal-to-monoclinic (t-m) phase transformation.[18] However, some studies have indicated that adding another interface between zirconia and resin cement may elevate the risk of adhesion failure. Researchers have reported that delamination at the zirconia/glaze layer interface, likely due to the mismatch in their coefficients of thermal expansion. Therefore, zirconia surface coating should be performed using specially designed porcelain material to ensure better adhesion ability, optimal thickness, and compatibility.[13]

CONCLUSION

Based on the previous findings, we found that the surface of zirconia oxide ceramic needs to be treated to achieve proper bond strength. Different pre-treatments of the zirconia surface were determined to achieve an operative protocol for proper adhesive cementation through the combination of mechanical and chemical treatments. The most evidence-supported approach to enhance the zirconia adhesion is by combining a conservative mechanical surface treatment (e.g., APA or TSC using optimized parameters) with the application of an MDP-based primer and dual-cure resin cement. Plasma treatment and SIE emerge as superior alternatives due to their minimal invasiveness and strong adhesive potential, but more clinical trials and long-term aging studies are required to confirm their efficacy. However, a standardized operative adhesive protocol has not yet been identified due to a lack of evidence. Also, the choice of cement material is less relevant in terms of adhesion.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.