Fracture Characteristics of Dental Ceramic Crown according to Zirconia Coping Design

Abstract

Fracture characteristics of the ceramic crown according to the zirconia coping design have been researched using various experimental methods. The 3D contour dimension of a mandibular first molar acrylic model tooth was scanned with Optical scanner (S600, Zirkonzahn, Italy), and then a classic crown preparation was created in CAD software (Zirkonzahn.Modellier, Zirkonzahn, Italy). The CAD file of the prepared tooth was imported into a milling machine, and acrylic resin (PMMA) dies were generated. Four different coping designs based on the thickness of zirconia marginal collar were used: 0.0mm zirconia collar (Group1), 0.5mm (Group2), 1.0mm (Group3), and 2.0mm (Group4). Copings were fabricated with CAD/CAM System (CAD CAM M5, Zirkonzahn, Italy) and veneering porcelain (IPS e.max Ceram) was build-up with layering technique. All the samples were cemented (RelyX Unicem, 3M/ESPE) onto corresponding dies. All specimens were tested in 4 groups (n=10 for each group) based on the thickness of zirconia collar. Fracture load test was performed on eight crowns from each group, and the remaining crowns (n=2) were subjected to fatigue test. Fracture strength was tested with a universal testing machine at a crosshead speed of 1.0mm/min to the vertical axis of the tooth 0.5mm lingually down the mesio-distal cusp. For the testing of fatigue failure, specimens were subjected to 50,000 cycles of cyclic loading 200N, at a rate of 15Hz and then loaded dynamically at a crosshead speed of 1.0mm/min, until failure. Fracture location and fracture surface of failed crowns were observed with a field emission scanning electron microscope (FE-SEM S-4800, Hitachi, Japan). Mean of fracture strength values of veneering porcelain were: 1411N for Group1, 1428N for Group2, 1596N for Group3 and 1621N for Group4. And according to increasing of thickness on zirconia collar, zirconia coping showed more fracture strength value with increasing of fracture strength of porcelain veneered it up. And examined using FE-SEM, two interesting features were shown from the fracture surface, sudden fracture and gradual fracture. And failure of specimens was mainly occurred by crack growth initiated from porosities within the veneering porcelain. In conclusion, according to the fracture test, the more thickness of zirconia collar, the higher fracture strength of veneering porcelain. Thus, zirconia coping design with 2.0mm marginal collar width is recommended for both functional longevity and esthetics of zirconia-ceramic restoration.