Effect of feldspathic porcelain layering on the marginal fit of zirconia and titanium complete-arch fixed implant-supported frameworks

Statement of problem. Veneering with porcelain may adversely affect the marginal fit of long-span computer-aided design and computer aided manufacturing (CAD -CAM) implant-supported fixed prostheses. Moreover, data regarding the precision of fit of CAD-CAM fabricated implant-supported complete zirconia fixed dental prostheses (FDPs) before and after porcelain layering are limited. Purpose. The purpose of this in vitro study was to evaluate the effect of porcelain layering on the marginal fit of CAD-CAM fabricated complete-arch implant-supported, screw-retained FDPs with presintered zirconia frameworks compared with titanium. Material and methods. An autopolymerizing acrylic resin fixed complete denture framework prototype was fabricated on an edentulous typodont master model (all-on-4 concept; Nobel Biocare) with 2 straight in the anterior and 2 distally tilted internal-hexagon dental implants in the posterior with multiunit abutments bilaterally in canine and first molar locations. A 3-dimensional (3D) laser scanner (5600 ARTI; Zirkonzahn) was used to digitize the prototype and the master model by using scan bodies to generate a virtual 3D CAD framework. Five presintered zirconia (ICE Zirkon Translucent - 95H16; Zirkonzahn) and 5 titanium (Titan 5 - 95H14; Zirkonzahn) frameworks were fabricated using the CAM milling unit (M1 Wet Heavy Metal Milling Unit; Zirkonzahn).The 1-screw test was applied by fixing the frameworks at the location of the maxillary left first molar abutment, and an industrial computed tomography (CT) scanner (XT H 225 - Basic Configuration; Nikon) was used to scan the framework-model complex to evaluate the passive fit of the frameworks on the master model. The scanned data were transported in standard tessellation language (STL) from Volume Graphics analysis software to PolyWorks analysis software by using the maximum-fit algorithm to fit scanned planes in order to mimic the mating surfaces in the best way. 3D virtual assessment of the marginal fit was performed at the abutment-framework interface at the maxillary right canine (gap 3) and right first molar (gap 4) abutments without prosthetic screws. The facial or buccal aspects of the teeth on frameworks were layered with corresponding porcelain (Initial Dental Ceramic System; GC) and CT-scanned again using the same protocol. Marginal fit measurements were made for 4 groups: titanium (Ti) (control), porcelain-layered titanium (Ti-P) (control), zirconia (Zir), and porcelain-layered zirconia (Zir-P). 3D discrepancy mean values were computed and calculated, and the results were analyzed with a repeated measures 3-way ANOVA using the maximum likelihood estimation method and Bonferroni adjustments for selected pairwise comparison t-tests (alpha=.05). Results. The 3D fit was measured at gap 3 and gap 4. Statistically significant differences in mean 3D discrepancies were observed between Zir-P (175 gm) and Zir (89 mu m) and between Zir-P and Ti-P (71 mu m) (P<.001). Conclusions. Porcelain layering had a significant effect on the marginal fit of CAD-CAM fabricated complete-arch implant-supported, screw retained FDPs with partially sintered zirconia frameworks. 3D marginal discrepancy mean values for all groups were within clinically acceptable limits (<120 mu m), except for the layered zirconia framework.