Reported clinical success rates of all-ceramic fixed partial dentures (FPDs) made of high-strength oxide ceramics range between 82.5% and 100%. The main cause of all-ceramic FPD failure is fracture in the connector area. There is, however, no consensus on what connector dimensions are adequate. The aim of this in-vitro study was, therefore, to compare the fracture strength of four-unit Y-TZP FPD cores designed with different connector diameters. A total of 40 four-unit FPD cores supported by end abutments and having two pontics were manufactured in Procera Zirconia. Five groups of FPD cores with connector dimensions of 2.0, 2.5, 3.0, 3.5 and 4.0 mm were produced. All FPD cores underwent a firing programme according to the manufacturer's recommendations for the veneering porcelain, a cyclic preload, thermocycling and finally, load until fracture. Fracture strength was significantly higher for each increase in connector diameter except for the 2.0-mm and 2.5-mm diameters where all fractures occurred during preload. All FPD cores fractured in the connector area. Within the limitations of this in-vitro study, a minimum diameter of 4.0 mm is recommended for all-ceramic zirconia-based FPDs with long spans or replacing molars. Clinical studies are, however, needed to determine adequate connector dimensions.

This is an author produced version of a paper published in Journal of Prosthetic Dentistry. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the published paper:Lövgren, Niklas; Roxner, Rikard; Klemendz, Susanne; Larsson, Christel.(2017). Effect of production method on surface roughness, marginal and internal fit, and retention of cobalt-chromium single crowns. Purpose. The purpose of this in vitro study was to examine 3 different production methods with regard to surface roughness, marginal and internal fit, and the retention of cobalt-chromium alloy single crown copings. Material and methods.A master abutment of a premolar mandibular tooth preparation with 4-mm height and a 0.6-mm deep 120-degree chamfer finish line with a 12-degree angle of convergence was replicated in die stone and scanned. Thirty-six cobalt-chromium alloy copings were produced using 3 different production techniques. Twelve copings were produced by lasersintering (LS), 12 by milling (M), and 12 by milled wax/lost wax (MLW). The surface microstructure of 2 copings in each group was analyzed using interferometry. The remaining 10 copings in each group were used to evaluate marginal and internal fit by using an impression material replica method, and retention was evaluated by using a uniaxial tensile force pull-off test. The copings from each test group (MLW, M, and LS) were cemented with zinc phosphate cement onto resin abutments. Statistical analyses of differences in marginal and internal fit were performed using one-way ANOVA and the Mann-Whitney test. Differences in surface topography were analyzed with the Kruskall-Wallis and Mann-Whitney tests for nonparametric Conclusions. Laser-sintered Co-Cr crown copings showed increased surface roughness and better internal and marginal fit than copings produced by milling or the milled wax/lost wax technique. However, the crown pull-off tests did not reveal any significant differences. CLINICAL IMPLICATIONSCobalt-chromium alloy copings produced by laser-sintering may reduce the risk of complications due to increased surface roughness and improved fit.