MORPHOFUNCTIONAL FEATURES OF THE ORAL MUCOSA AND ITS REGENERATIVE POTENTIAL IN THE AGE ASPECT

Background:Because they are considered biocompatible, tough and look great, zirconia crowns are commonly used instead of metal-ceramic restorations. The advantage of monolithic zirconia is that it does not include a veneer which can easily chip in bilayered restorations. Cementation technique can affect the toughness of the material. The purpose of this study was to compare between the fracture strength of monolithic and bilayered zirconia crowns cemented with either resin or glass ionomer cements. Methods and Materials:Sixteen monolithic zirconia crowns and Sixteen bilayered zirconia crowns were fabricated using CAD/CAM technology and formed two groups called Group A and Group B. Each group was split into two new subgroups (eight each) dependent on the type of used cement, naming the subgroups with AR and BR groups for the adhesive resin cement and AG and BG groups for the glass ionomer cement. All crowns were prepared by air-abrasion and cemented onto epoxy resin dies, after which they were thermocycled (5,000 times, from 5°C to 55°C). Strength at fracture was determined on a universal testing machine and the types of fracture failure were explored using a scanning electron microscope. Results:The fracture strength was much higher in monolithic zirconia crowns than in bilayered crowns, no matter the cement used. The average fracture strength was 2541 ± 349 N for AR, 2578 ± 339 N for AG, 1557 ± 418 N for BR and 1691 ± 526 N for BG. There were no major differences between adhesive and conventional cements used in crowns of the same type (p > 0.05). Even so, there were significant differences (p < 0.001) between crowns made from solid material and those made from two layers. Analysis of failures showed that monolithic crowns suffered major fractures, but the layer-separation problem arises with bilayered crowns. Conclusion:The fracture resistance of monolithic zirconia crowns exceeded that of bilayered zirconia crowns, with this advantage noticeable regardless of the cementation method. Both adhesive resin and glass ionomer cements worked well clinically. For posterior restorations that are best suited to strong crowns, monolithic zirconia may be recommended.