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Introduction

Esthetic demands from patients and clinicians are leading to increase usage of all-ceramic restorations instead of porcelain-fused-to-metal (PFM) and full metal crown restorations. This has resulted in a large number of all-ceramic systems becoming available on the dental market. Due to this diversity, appropriate material selection in clinical situations for treating unesthetic teeth has become difficult.

The impressive properties of advanced oxide ceramics has enabled their utility as core structures and usage in more demanding applications where higher stresses are anticipated rather than the more traditional porcelain and metal-ceramics. Yttria tetragonal zirconia polycrystalline (Y-TZP) materials, in particular, are becoming more popular core ceramics due to their high strength of 900–950 MPa and high elastic modulus 200 GPa.

The most common technique to manufacture all-ceramic restorations is to brush-applied veneer the porcelain onto sintered ceramic cores. Recently a heat pressing technique has become available to produce a veneering layer for all-ceramic restorations . Anecdotal evidence and marketing information from dental companies have reported that dental technicians prefer the pressing technique because of its speed, accuracy and stability . In addition framework supported wax patterns can be tried in the mouth enabling adjustments before pressing and sintering without influencing their mechanical properties . In comparison the sintering process is more technique sensitive than pressing because of the brush-applied build-up and firing techniques . However, heat pressing materials have relatively low strength and fracture toughness which limits their use to conservative designs in low to moderate stress environments .

Despite the growing use of zirconia some clinical studies are reporting veneering failure; namely chipping of the veneering porcelain as a major issue. According to Sailer et al.’s study, the chipping of veneer is the most frequent reason for failure with a failure rate of 15.2% after an in-service time of 35.1 ± 13.8 months . Possible reasons for chipping are; insufficient bond strength, excessive tensile stress due to a CTE mismatch, excessive load due to premature contacts, insufficient substrate support, tensile stress established during cooling after firing, especially when a considerable thermal gradient develops through the layered system upon rapid cooling .

Chipping and fracture issues have also been reported with pressed veneering ceramics. Christensen et al. compared the failure rate of PFM restorations with Y-TZP based ceramics both pressed and hand-layered. According to their report, pressed ceramics have a lower failure rate than hand layered restorations after 2 years . This supports Taskonak et al.’s observations comparing pressed to layered veneering over zirconia and metal, where pressed ceramics performed significantly better .

Christensen et al. however found that the failure rate of IPS e.max ZirPress, which was as high as other brush-applied all-ceramic systems, was double that of Noritake’s CZR press material. Moreover, both the brush-applied veneered zirconia system and the pressed to zirconia system had much higher failure rates than PFM restorations. In addition, the most common type of fracture for pressed ceramics to zirconia was chipping of the porcelain, similar to that found in the brush-applied veneered porcelains.

Currently the origin of differences in failure rate between products and between pressed and brush-applied veneered systems is unclear; whether it is formulation differences, difference in intrinsic properties, or the influence of cooling rate as proposed for the brush-applied veneered system. Therefore an investigation of such properties of pressed materials may contribute toward a solution of the high failure rate and chipping associated with Y-TZP based all-ceramic restorations.

The present study has two primary objectives, which were to compare the crystalline phases present of four different pressed ceramics for zirconia to evaluate the mechanical properties of four different pressed ceramics for zirconia; porcelain–zirconia adhesion and flexural strength.

In a companion paper the indentation fracture toughness and the influence of glaze firing conditions on the residual stress development in pressed veneering porcelains are investigated (Choi et al.).