Introduction

An equilibrium between remaining tooth structure and providing sufficient retention and resistance-form should be established in a crown preparation. Full veneer metal-crown preparations are generally more conservative of tooth structure than those for either metal-ceramic or all-ceramic restorations. However, there is an increasing demand for metal-free restorations due to an increasing interest in esthetics and concerns about toxic and allergic reactions to certain alloys.^1,2 Both patients and clinicians, especially in the anterior locations, have been seeking suitable metal-free tooth-colored restorations during the last decade.^1-5

It is possible to replicate the esthetic characteristics and vitality of natural teeth by using ceramics.⁶ However, although strong in compression, ceramics can be inherently brittle and weak when placed under tensile and torsional stresses.^7,8 Strong alumina cores have been produced to obtain the all-ceramic restorations with more fracture resistance.^9-11 In-Ceram (Vita Zahnfabrik, Bad Sackingen, Germany) is made of a high alumina core which is subsequently glass infiltrated. However, the challenge of the overlying veneer porcelain is to mask the opaque alumina core and provide the desired contours needed.¹¹

Many factors including the shape and thickness of the restoration; microstructural inhomogeneties; size and distribution of surface flaws; residual processing stresses; the magnitude, direction, and frequency of the applied load; the restoration-cement interfacial defects; the elastic modulus of the restoration components; and environmental effects may be related to crack initiation and propagation within a dental ceramic.^12-15

When the horizontal over-lap is limited, the clinicians’ decision about the patient selection for all-ceramic restorations has been difficult since the mechanical properties of the ceramics remain a concern. Therefore, numerous in vitro studies including the investigation of mechanical properties of all-ceramic crowns can be found in the dental literature.^16-20 However, to date there is no study that solely focuses on the influence of the thickness of an In-Ceram core on the failure of all-ceramic crowns in the dental literature.

This in vitro study was designed to compare the fracture strengths of all-ceramic crowns when supported by three different thicknesses of In-Ceram core. Anterior crown-shaped restorations were utilized since they potentially represent the clinical situation more closely than ceramic disks.²¹