Predictable Method for Restoring Endodontically Treated Teeth, Comparing Indirect and Direct Methods and Materials in Aesthetic Dentistry.

Modern restorative dentistry is increasingly focused on treatment approaches that combine long-term clinical effectiveness, preservation of healthy tooth structure, cost-efficiency, and patient satisfaction. These factors are particularly important when treating endodontically treated teeth, which often present with substantial loss of tooth structure and an increased risk of fracture. The choice of restorative material and cementation technique therefore plays an important role in determining the long-term prognosis of these teeth.

Traditionally, endodontically treated posterior teeth have frequently been restored using post-and-core foundations followed by full-coverage crowns. Although this approach can provide adequate mechanical protection, it may require extensive removal of remaining healthy tooth tissue and can involve additional clinical appointments, laboratory procedures, and costs. With the development of adhesive dentistry and modern restorative materials, more conservative treatment options, including inlays, onlays, and overlays, have become increasingly popular. These partial-coverage restorations aim to preserve as much of the natural tooth structure as possible while providing sufficient protection and functional rehabilitation.

Onlays are particularly suitable for structurally compromised posterior teeth because they can restore damaged cusps and occlusal surfaces while avoiding unnecessary removal of unaffected tooth structure. Advances in digital dentistry and computer-aided design and manufacturing (CAD/CAM) have further improved the predictability and precision of indirectly fabricated restorations. Digital impressions, computer-aided design, and automated milling allow restorations to be produced with standardized geometry and high accuracy.

Among the materials currently used for indirect restorations, zirconia and glass-ceramic are of particular clinical interest. Zirconia is characterized by high flexural strength, fracture resistance, biocompatibility, and long-term stability in the oral environment. These properties make it particularly attractive for posterior restorations exposed to high occlusal forces. Glass-ceramic materials, in contrast, offer excellent optical properties and well-established adhesive cementation protocols. They are widely considered a reliable option for highly aesthetic indirect restorations.

Despite the favorable properties of both materials, their clinical performance depends not only on the restorative material itself but also on the quality and durability of the cementation interface. Reliable cementation is essential for maintaining retention, marginal integrity, and resistance to functional stresses over time. The cementation protocol may therefore have a significant influence on the long-term prognosis of indirect restorations.

Conventional adhesive resin cements are widely used for the cementation of indirect restorations and are often considered the reference or "gold standard" against which newer cementation approaches are compared. These systems typically involve several clinical steps, including surface conditioning, priming, adhesive application, and cementation. Although such protocols can provide strong and predictable bonding, they are technique-sensitive and require careful control of each clinical step. The complexity of the procedure may increase treatment time and the possibility of operator-related errors.

Self-adhesive resin cements have been developed as a simplified alternative to conventional adhesive cementation. These materials combine the adhesive and luting functions in a simplified procedure, reducing or eliminating the need for separate etching, priming, and bonding steps. This may decrease clinical treatment time, simplify the procedure, and potentially reduce treatment costs. However, simplified application does not necessarily guarantee equivalent long-term performance. The durability of the adhesive interface can be influenced by moisture, thermal changes, mechanical loading, surface characteristics, and degradation of the cement-tooth or cement-restoration interface.

The cementation of zirconia is of particular interest because zirconia does not respond to conventional hydrofluoric acid etching in the same way as glass-ceramic materials. Consequently, reliable bonding to zirconia depends on appropriate surface conditioning and the chemical interaction between the zirconia surface and the cement. Functional monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP) have been incorporated into modern resin cements and can promote chemical interaction with zirconia surfaces. Nevertheless, the long-term stability of this bond under clinical conditions remains an important area of investigation.

Another factor that may influence the durability of cementation is the preparation and pretreatment of the tooth surface. The condition of enamel and dentin immediately before cementation can affect bond strength and the stability of the adhesive interface. Therefore, the first phase of the present research will investigate the influence of tooth surface pretreatment on the bond strength of self-adhesive resin cement. The in vitro component will assess bond strength following artificial aging, providing information about how different surface preparation approaches affect the durability of the cement-tooth interface.

The results of the in vitro phase will be used to identify an appropriate surface pretreatment protocol for the subsequent clinical investigation. This sequential approach allows laboratory findings to inform the clinical part of the study and provides a more comprehensive assessment of the cementation procedure.

The clinical component of the research will use a split-mouth study design. This design allows different restorative or cementation approaches to be compared within the same patient, thereby reducing the influence of individual factors such as oral hygiene, dietary habits, occlusal forces, saliva characteristics, and general patient-related variability. Thirty patients requiring restoration of two endodontically treated teeth will be included in the clinical study, provided that they meet the predefined eligibility criteria, including healthy periodontal tissues and sufficient tooth structure above the gingival margin.

For the indirect restorative component, zirconia and glass-ceramic onlays will be fabricated using standardized digital workflows. The teeth will be digitally scanned using an intraoral scanner, and the restorations will be designed using CAD software and fabricated using CAD/CAM technology. Zirconia onlays will be manufactured from Katana Zirconia STML, while IPS e.max CAD glass-ceramic will be used for the glass-ceramic restorations. Standardization of the digital design and manufacturing procedures will help minimize variations in restoration geometry and fit.

Different cementation protocols will subsequently be compared. The indirect restorations will be cemented using either a conventional adhesive resin cement, Panavia V5 (Kuraray Noritake), or a self-adhesive resin cement, Panavia SA (Kuraray Noritake). The comparison will allow assessment of whether the simplified self-adhesive cementation protocol can provide clinical outcomes comparable to those obtained using the conventional adhesive resin cement.

The clinical performance of the restorations will be evaluated at baseline and during follow-up examinations after 1 and 3 years. The assessment will include restoration-related outcomes such as fractures, cracks, loss of retention, marginal defects, and discoloration. The condition of the restored teeth will also be monitored for secondary caries and endodontic complications. Periodontal parameters, including probing depth, bleeding on probing, and gingival recession, will be assessed to determine whether the different restorative approaches have an influence on the surrounding periodontal tissues.

An important component of the study is the assessment of patient-reported outcomes. Clinical success cannot be evaluated solely by technical parameters, as patients' perception of their treatment is also an important determinant of overall treatment success. Patient satisfaction and well-being will therefore be assessed using standardized questionnaires. This will provide information about how different restorative approaches are perceived by patients and whether differences in treatment complexity, aesthetics, function, or complications influence their overall satisfaction.

The research will also investigate direct composite restoration as a more conservative and potentially lower-cost alternative to indirect restorations. Direct composite restorations remain widely used because they can often be completed with fewer clinical and laboratory steps and require less financial investment. However, there is limited evidence comparing long-term patient satisfaction and clinical outcomes of simple direct composite restorations with those of indirect zirconia and glass-ceramic restorations.

For this purpose, the study will include a direct composite restoration protocol based on the injection moulding technique. This approach uses a digitally planned restoration and a transparent matrix or splint through which flowable composite material is injected and subsequently light-cured. The technique has the potential to provide a minimally invasive and relatively simple restorative solution while maintaining a controlled anatomical shape. However, because published clinical evidence regarding the long-term performance of this approach is limited, its comparison with established indirect restorative methods is of particular interest.

The inclusion of three different restorative approaches-zirconia onlays, glass-ceramic onlays, and direct composite restorations-will allow a broader evaluation of the advantages and limitations of contemporary treatment strategies for endodontically treated teeth. The study will not focus solely on the survival of the restoration but will also consider complications, biological outcomes, patient satisfaction, treatment complexity, and economic factors.

Cost-effectiveness is an important aspect of the research because the most clinically durable treatment is not necessarily the most appropriate or accessible option for every patient. Conventional indirect restorations may involve higher material, laboratory, and clinical costs, while simplified cementation procedures or direct restorative techniques may reduce treatment time and financial burden. However, lower initial costs may not necessarily translate into greater cost-effectiveness if a restoration has a higher risk of complications or replacement. Therefore, the study will consider both the clinical outcomes and associated treatment costs when comparing restorative strategies.

By combining laboratory testing, clinical follow-up, patient-reported outcomes, and economic evaluation, this research aims to provide a comprehensive assessment of contemporary restorative options for endodontically treated teeth. The study will investigate not only whether a restoration remains clinically functional but also how reliably it performs, what complications may occur, how patients perceive the treatment, and whether the treatment represents an efficient use of healthcare resources.

The novelty of the research lies in the comprehensive comparison of different restorative materials and cementation strategies, including the use of a self-adhesive resin cement for zirconia restorations in both the in vitro and clinical components of the project. The split-mouth clinical design will allow direct comparison of cementation protocols within the same patient. In addition, the inclusion of direct composite restoration using the injection moulding technique will provide data on a relatively new restorative approach for which long-term clinical evidence remains limited.

The findings of this research may contribute to the development of more predictable, minimally invasive, and cost-effective restorative protocols for endodontically treated teeth. In particular, the study may help determine whether simplified cementation protocols can provide sufficient long-term clinical performance while reducing treatment complexity and cost. The results may also assist clinicians in selecting restorative materials and cementation techniques according to clinical indications, expected longevity, economic considerations, and individual patient preferences.

Ultimately, the goal of the research is to identify restorative approaches that provide an appropriate balance between clinical durability, preservation of tooth structure, cost-effectiveness, technical simplicity, and patient satisfaction. Such evidence may support more individualized treatment planning and contribute to improving the accessibility and quality of restorative dental care.