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Introduction

For decades, tremendous effort and research have been performed to develop and generate reliable all-ceramic systems . Despite significant improvements in material properties and excellent esthetics, the overall clinical performance of all-ceramic restorations still fails to match that of metal–ceramic restorations . The trend for development of high-strength ceramics and its use in posterior areas has been a field of constant investigation . Yttria-Stabilized Tetragonal Zirconia Polycrystals (Y-TZP) was introduced as a core ceramic in attempt to reduce restoration bulk fracture. Its high mechanical properties have resulted in successful use of Y-TZP as a core ceramic in short- and medium-term clinical studies, where framework fractures were seldom reported. While Y-TZP provides strength, the clinical success of these restorations has been hampered by fractures within the veneering porcelain.

Studies on the clinical performance of Y-TZP-based restorations have been recently summarized in several review articles . The majority of studies focused on the outcome of fixed dental prostheses and only limited clinical data is available concerning the performance of veneered zirconia posterior crowns. The available evidence concerning short-term clinical studies reveals chip-off fractures rates of 5–9% after 24–36 months for teeth-supported zirconia veneered crowns. When implant-supported, zirconia crowns have presented higher veneer failure rates of up to 25% after 2 years .

Fracture of all-ceramic restorations seems to be fostered by the generation of microcracks due to repetitive occlusal contact, wear and fatigue . In addition, material related factors such as the veneer application technique and the core/framework design resulting in inhomogeneous porcelain support may affect the clinical behavior of veneered Y-TZP restorations . Therefore, attempts to improve the performance of the veneering porcelain on Y-TZP restorations considering their multifactorial essence are warranted. To date, no study has investigated the effect of core/framework designs fabricated by different veneering techniques in a standardized and controlled model. While only one in vitro laboratory study has evaluated the effect of core design on all-ceramic systems on reliability , clinical evaluation of all-ceramic crown design alterations have been primarily based on empirical guidelines via computer aided design/computer aided manufacturing (CAD/CAM) driven modifications than clinically relevant scientific data.

When the first CAD/CAM systems were brought to the market, the majority of the software systems did not allow core design modifications. Excessive veneer layer thickness (sometimes >2.5 mm) was created because of the uniform configuration of the zirconia cores and the independent need to generate an anatomically correct crown . As a result, zirconia framework design modifications, for instance with extending a zirconia collar from the palatal to the interproximal area, derived from the metal-ceramic technique were attempted . Although promising in vitro ) clinical results with no cohesive veneer or core fracture have been reported , evidence-based conclusions on the applicability of this modified framework design cannot be drawn from preliminary clinical case reports as only small samples sizes were observed over a limited timeframe and no control group (uniform thickness framework) was used.

Clinical studies involving fixed dental prostheses with frameworks that resemble the final anatomy of the veneered restoration but with a reduced dimension for the veneer gave a global statement for all restorations included in the study but did not characterize or measure mean core veneer ratios for restorations. This may partially explain the conflicting clinical results related to the framework/core design configuration . As in clinical studies involving single crowns, none of the above mentioned studies included a control group. Hence a sound understanding of core design parameters and mechanical performance warrants further research.

Modification of veneering porcelain application techniques have been another attempt to overcome cohesive failure . Glass-ceramic ingots for pressing veneering ceramics onto zirconia were developed aiming to combine the advantages of the established press technology with the high strength of the zirconia substrate. In vitro studies revealed no differences in failure modes and reliability of standardized trilayer configuration but also in load bearing capacity of crown systems with press veneering ceramics compared to hand-layer veneering . The clinical performance of pressable veneering ceramics for zirconia cores has been questioned but not yet fully investigated in controlled clinical trials. Controversial outcomes of the press veneer application have been described in clinical investigations with reports of no veneer failures in one study , but also veneer fracture rates of 27–49% to 13% after 24–48 months in other studies .

Considering the disparity of the results observed between pressed versus hand-layered veneering porcelains in tandem with the anecdotal understanding of core design modifications, the present study sought to evaluate the reliability and failure modes of molar crowns with either even thickness or anatomically design frameworks veneered with hand-layered or pressable veneer porcelains. This investigation hypothesized that pressed veneer porcelain technique results in an improved reliability levels on either conventional or anatomically designed frameworks compared to hand-layering method.