Intraoral repair of fractured ceramics: a literature review [version 1; peer review: awaiting peer review]

Background: Fracture or chipping of veneering ceramic resulting in aesthetic and functional issues is a frequent technical complication encountered with fixed dental prostheses. Treatment options include extraoral or intraoral repair of the ceramic restoration; the latter being minimally invasive and cost-effective. We reviewed the various intraoral repair techniques for ceramic fractures and their efficiency in the last decade. Methods: A literature search was carried out between 2017 and 2022 using the PubMed database with keywords: intraoral, repair, ceramics, porcelain, and ‘porcelain fused to metal’. Screening of abstracts and full texts was carried out to determine the final list of eligible studies. Results: Twenty-one eligible studies were included in the review which consisted of 17 in vitro studies, three case reports, and one prospective clinical study. Researchers and dentists preferred repairing cracked veneered zirconia-based restorations intraorally if the restoration is in acceptable condition. However, successful intraoral repair of veneering fractures relies on the bond between the previous restoration and the new repair material, the right adhesive resin, and adequate surface conditioning. The review indicated that the best technique to fix chipping in a complex made of zirconia core and veneering ceramic is to treat the veneering ceramic with hydrofluoric acid before covering the core ceramic with silica. Conclusion: Intraoral repair is an efficient and economic procedure without the need for repeated sessions. However, the success and longevity of the restoration rely on the technical skills of the clinicians and adherence to prescribed protocols.


Introduction
Modern cosmetic dentistry places a strong emphasis on aesthetics, and thus finds a wide usage of ceramic and porcelain in teeth restoration to achieve the most natural-looking aesthetics. However, the accelerated usage of dental porcelains is accompanied by the risk of fractures (Hammond et al. 2009). Owing to its brittle nature, porcelain veneering, when exposed to masticatory forces, clenching, and moisture causes cracks or chipping (Rosentritt, Steiger et al. 2009). Studies have shown that the most frequent technical issue associated with dental restorations were ceramic fractures, which were followed by porcelain chipping and loss of retention (Vagropoulou, Klifopoulou et al. 2018). According to a systemic literature review, chipping of the veneering ceramic is one of the most frequent technical complications occurring at a rate of 12.7% following a three-year observation period (Pjetursson, Sailer et al. 2015). Consequently, patients with ceramic fractures might have aesthetic and functional issues that need an immediate fix. Direct repairs have been suggested in such conditions where the broken prosthesis displays good adaptation and acceptable aesthetics (Mesquita, Al-Haj Husain et al. 2021).
In bilayered dental prostheses, such as metal-ceramic fixed dental prostheses (FDP), fractures in veneering ceramic may be treated either by replacing the existing restoration or by repairing it intraorally. Removal of FDP can be challenging as it can lead to deformation of the metal and iatrogenic fracture of the sound tooth tissues that can seriously compromise the longevity of the tooth. Additionally, replacing FDPs is typically a difficult and expensive process that adds significantly to chairside time, which is not always well received by the patient (Özcan and Volpato 2017). Furthermore, it is associated with the risk of harming the tooth that serves as the abutment (Kocaa gao glu, Manav et al. 2017).
On the other hand, intra-oral ceramic restoration repair is minimally invasive and most cost-effective making it an ideal choice (Aslam, Hassan et al. 2018). Furthermore, intraoral repair has recently been suggested as a potential therapy alternative if the indications and treatment techniques are suitable (Agingu, Zhang et al. 2018). Repairing the ceramic restoration intraorally is a temporary but effective solution. The easiest way is to polish the surface and prevent future failures. If the repair cannot be done by polishing, the missing part is replaced with composite resin, or the chipped part is cemented using resin cement. A new veneer layer is prepared and bonded to the existing restoration (Kimmich and Stappert 2013).
Intraoral repair is a simpler technique to treat veneer ceramic fractures than total replacement or extraoral repair methods (Polat, Tokar et al. 2021). Veneering porcelain flaws that expose the restoration's core material create a special challenge for intraoral repair. Particularly with metal frameworks, it can be challenging for the dentist to match the colour of the framework with veneer without compromising the aesthetic outcome. To achieve functional success, the dentist must effectively bond the veneering porcelain to the core material (Kimmich and Stappert 2013). However, selecting the best system that would deliver a trustworthy result is often difficult for clinicians. Hence, this article reviews studies that have investigated intraoral repair techniques for ceramic fractures and their efficiency in the last five years.

Data sources and search strategy
The literature search was conducted using the PubMed database for studies that investigated intraoral repair techniques for ceramic fractures. All studies published in English from January 2017 through August 2022 were considered for review. The search strategy was derived from appropriate keywords representing the concepts: "intraoral repair", "ceramics", "porcelain", "porcelain fused to metal", and "efficacy". The primary outcome of the review included the efficacy of the intraoral repair technique used. Articles were excluded if published in a language other than English, not published in the last five years, or if full text is unavailable.
After applying the criteria, study selection was undertaken by first screening the abstract and title followed by the full-text versions of relevant studies to determine the final list of eligible studies. Data were extracted from the studies following the completion of full-text screening. All data were summarized. No statistical analysis was performed.

Results
The literature search from PubMed retrieved 21 eligible studies that were included in this review to understand the intraoral repair techniques used for treating ceramic fractures. The findings of the included studies have been summarized in Table 1.
The current literature review included 17 in vitro studies, three case reports, and one prospective clinical study. Various ceramic types were included in these studies such as glass-based ceramics with three subclasses: predominantly glass (feldspathic glass), moderately filled glass (leucite), and highly filled glass (leucite disilicate based); glass-infiltrated ceramics (In-Ceram groups); and non-glass-based ceramics: polycrystalline ceramics (alumina, zirconia). Likewise, Tokar et al. found that erbium, chromium: yttrium scandium gallium garnet (Er, Cr: YSGG) laser surface treatments with different pulses can be safely recommended in the treatment of fractured zirconia ceramics and can strengthen the SBS between zirconia and composite resin. Shorter pulse laser irradiation performed better compared to longer pulse rates in terms of mean SBS (Tokar, Polat et al. 2019).
Sattabanasuk et al. assessed the efficacy of resin adhesion to glass-ceramic surfaces that had previously undergone various treatments and found that silane solution could be used to achieve mechanical and/or chemical retention. The study found that regardless of the tested adhesive, the resin-ceramic bond depended upon both mechanical and chemical retention. The authors suggested that adding a silane monomer to the adhesive formulation may not be the best way to achieve silanization and hence recommend regularly applying a silane solution to a glass-ceramic surface to ensure resin adherence (Sattabanasuk, Charnchairerk et al. 2017).
Polat et al. compared the effectiveness of Er, Cr: YSGG laser with short and long pulse durations to different surface roughening techniques for repairing zirconia ceramics with various surface configurations and found there were no discernible variations among sandblasting and Er, Cr: YSGG laser treatments (Polat, Tokar et al. 2021). Tatar and Ural investigated surface treatments for chipping to reduce the problems related to bonding between hybrid materials and composites and found that the most effective method was air abrasion with silica-coated aluminum oxide (Tatar and Ural 2018). A similar study by Libecki et al. where the aim was to check the efficacy of different surface treatment options (air abraded, polished surface, or ground using a special silicon carbide bur), reported that the TBS is high when the ceramic is conditioned with air abrasion or roughened with silicon carbide bur (Libecki, Elsayed et al. 2017). Ozdemir and Yanikoglu assessed the SBS of nano-hybrid and nano-ceramic composite resins to feldspathic porcelain (Vita and Ivoclar) and found the latter to possess lower SBS compared with the former (Ozdemir and Yanikoglu 2017). A study by Chen et al. evaluated if silane contamination of dentin impacts the binding strength of the tissue and found that there were no adverse effects on SBS due to contamination of dentin with silane before etching (Chen, Hammond et al. 2017).

Intraoral repair techniques in the included studies
Various intraoral techniques employed in the studies included in the review are listed below. Passia et al. evaluated the long-term outcome of posterior all-ceramic FDPs made from veneered zirconia ceramic with either a fixed-to-fixed (FF) or a cantilever design (CA). The cumulative 13-year survival rate was 43.2% (FF) (CI: 22.8-66.2%) and 52.5% (CA) (CI: 32.5-71.8); and success rate was 29.5% (FF) (CI:12.1-55.9%) and 22.5% (CA) (CI: 7.9-49.3%) respectively. The survival rates were comparable and failure rates were high, irrespective of the design (Passia, Chaar et al. 2019). An in vitro study analyzed the long-term performance of fabrication techniques on anterior teeth and found that the pressed groups (923.7 N) exhibited significantly higher fracture resistance and crown strength than the milled groups (797.5 N), (p = 0.0002) (Gerogianni, Lien et al. 2019). In a study by Sanal and Kilinc, SBS and color stability of four ceramic veneers (VITAVM 9, VITA VM 13, VITA VMK 95, and IPS e.max Ceram) treated with different repair systems (Vertise Flow, Fusio Liquid Dentin, Constic, and BISCO intraoral repair kit + Filtek Supreme) was examined. The findings of the study indicated that Constic self-adhesive composite resin had the lowest color stability value, but that there was no discernible difference between it and the other repair methods (Sanal and Kilinc 2020).

Discussion
The current review aimed to summarize the available literature on the effectiveness of intraoral repair techniques for ceramic fractures. Twenty-one relevant studies published in English between 2016 and 2022 which discussed various surface treatments and intraoral repair systems were included in the review.
It is a common observation in a dental practice that despite having a high success rate, chipping or fracture of the veneering ceramic is the main cause of clinical failure for veneered zirconia-based restorations (Pjetursson, Sailer et al. 2015). Given the complexities of veneered zirconia-based restorations, clinicians and researchers have explored a variety of intraoral repair techniques to treat the chipping and protect the intact structures. Under appropriate conditions, intraoral repair acts as a potential therapy option. Dental professionals now have a great deal of interest in the scientific basis, therapeutic methods, and clinical effectiveness of the intraoral repair of cracked veneered zirconia-based restorations.
Reviewing the intraoral repair of veneered zirconia-based restorations would improve practitioners' understanding of the advised procedures when zirconia repair is required (Agingu, Zhang et al. 2018).
Understanding the complexities of porcelain fractures is the first step. Porcelain fractures can be static, cohesive, or adhesive. The material used for framework fabrication determines the repair procedure (Aslam, Hassan et al. 2018). Regardless of their high 5-year survival rate of 90.4% (95% CI: 84.8-94.0%), porcelain-veneered zirconia FDPs can eventually crack and get chipped off. This is fundamentally due to the loss of adhesion between zirconia and veneering ceramics (Pjetursson, Sailer et al. 2015). A variety of factors can cause the fracture of veneered porcelain on metal-ceramic crowns, which include lack of sufficient porcelain framework support, intra-ceramic flaws, or parafunctional occlusion (Haselton, Diaz-Arnold et al. 2001). Porcelain fracture can manifest clinically with no metal substrate exposure, modest metal substrate exposure, or significant metal exposure. Although fabricating a new crown seems desirable, it is not always possible. In such cases, the option of intraoral restoration is deemed of immense importance (Haselton, Diaz-Arnold et al. 2001).
Before the build-up of the veneer layer, a succession of thermal sintering cycles and accompanying cooling processes are used to prepare crowns and FPDs. The matching of thermal expansion between the porcelain and the underlying framework, whether metal or ceramic, is crucial to avoid breaking following the firing. Delamination of the porcelain can occur when the coefficient of thermal expansion (CTE) of the framework is significantly higher than that of the porcelain (Swain 2009). Other researchers believe that there should be a similarity in the CTE of both veneering porcelain and substructure to eliminate any stresses in the layered porcelain (Aboushelib, Kleverlaan et al. 2006, Swain 2009). Fracture is a potential risk for veneered ceramics with such residual stress. A tensile stress zone can form below and at the edge of the contact surface of the veneer layer resulting from the combination of compressive residual stress and local stress brought on by mastication (Taskonak, Mecholsky et al. 2005). Several elements influence the development of thermally induced residual stress and contribute to porcelain chipping which includes the veneer's overall thickness, cooling rate, and shape. Guazzato et al. (2010) found that rapid cooling and increased porcelain veneer thickness increase the risk of crack occurrence (Guazzato, Walton et al. 2010). Zirconia may also crack due to the volume expansion (3-4%) occurring during the thermal tempering at high temperatures, which is done to facilitate tetragonal to monoclinic transition (Piconi and Maccauro 1999).
Meeting the requirements for repairing a failed restoration is the next essential step. The establishment of a permanent bond between the previous restoration and the new repair material is the key to a successful repair. Therefore, choosing the right adhesive resin and restorative substance, as well as adequate surface conditioning of the substrate, is important.
To increase bond strength, the surface is roughened using a variety of surface preparation processes (Duzyol, Sagsoz et al. 2016). Acid etching using hydrofluoric acid (HF) is an optimal approach for roughening feldspar ceramic surfaces for resin composite bonding. In comparison with HF treatment alone, the introduction of a silane coupling agent after HF results in stronger resistant bonds. Further, newer surface modifications, such as surface abrading airborne particles, have been developed with advancements in adhesive technology. The initial step in this approach is air abrasion using aluminum oxide particles. Increased resin composite-porcelain bond strengths are reportedly achieved by sandblasting the porcelain's surface with aluminum oxide particles (35-250 μm) (Duzyol, Sagsoz et al. 2016). After air abrasion, chemical adhesion can be generated using specialized primers that interact with a material's surface. To chemically bind the inorganic filler particles to the resin matrix, the most commonly used primer is a silane coupling agent, which is also employed in the construction of composites (Loomans and Özcan 2016).
The wettability of the composite to be utilized as a repair material is increased by applying intermediate adhesive resin to the silanized surface. It is often recommended, but not required, to mix identical composite materials when doing composite-composite repairs due to the variable effects of different substrate materials (Baur and Ilie 2013). Usage of adhesive resin, silanization, and HF acid etching, adhesion to glassy matrix ceramics has been thoroughly established. Optimal surface conditioning of indirect composite restorations can be achieved by using airborne particle abrasion followed by an adhesive resin with a silane coupling agent (Loomans and Özcan 2016).
In the current review, HF etching in combination with laser treatment or silane and traditional adhesive was found to strengthen the link between porcelain and composite resin (Hakimaneh, Shayegh et al. 2020, Benli, Kilic et al. 2022, Kiomarsi, Jarrah et al. 2022). However, durable TBS was observed when surface treatment with air abrasion or roughening the zirconia surface with a silicon carbide bur was offered for the repair of zirconia ceramic after chipping of its veneers (Libecki, Elsayed et al. 2017).

Conclusion
With the limitations of this narrative review, we can conclude that intraoral repair of ceramic fractures offers esthetics and comfort to patients when a restoration cannot be removed or changed. The current review indicated that the best technique to fix chipping in a complex made of zirconia core and veneering ceramic is to treat the veneering ceramic with HF before covering the zirconia core ceramic with silica. Both ceramic substrates must then be covered with adhesive resin and silane. The intraoral repair utilizing resin-based composite materials has significant advantages since it retains most of the restoration, prevents needless loss of healthy tooth structure, and is a quick and simple, reasonably priced procedure with no need for repeated sessions. The skills of the clinicians in using these tools and techniques as well as adherence to prescribed procedures and protocols are crucial to the effectiveness of these treatments.

Data availability
No data are associated with this article.
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