The use of lithium disilicate in fixed dental prosthesis: from the adhesive interface to clinical performance

Chapter 1 is an introduction to lithium disilicate: its glass ceramic nature, its characteristics, its mechanical properties. The possibility to be etched allows lithium disilicate to be adhesively luted on dental tissues, influencing both the final properties of restoration and the clinical protocol of cementation. The aim of this thesis was investigate the adhesive interface between lithium disilicate, resin cement and dental tissues from different points of view: retention, fracture strength and survival rate. In Chapter 2 the retention of lithium disilicate crowns cemented using two different cementation systems was measured. Twenty extracted mandibular premolars were prepared. Anatomic crowns were waxed and hot pressed using lithium disilicate ceramic. Teeth were divided into two groups (n = 10): (1) self- curing luting composite and (2) glass-ionomer cement (GIC). After cementation, the crowns were embedded in acrylic resin block with a screw base. Each specimen was pulled along the path of insertion in Universal Testing Machine. Failure load in Newtons (N) and failure mode were recorded for each specimen. Failure mode was classified as decementation or fracture. Failure load data were analyzed using one-way analysis of variance (ANOVA). Failure modes were compared using Pearson’s Chi-square test. Mean failure load was 306.6(±193.8) N for composite group and 94.7(±48.2) N for GIC group (p = 0.004). Disilicate crown cemented with luting composite most often failed by fracture; otherwise, crown cemented with glass-ionomer cement most often failed by decementation (p = 0.02). Disilicate full crown cemented with luting composite showed higher failure load compared with conventional cementation with glass-ionomer cement. The interface between tooth, luting composite and lithium disilicate surface was also qualitatively evaluated using a scanning electron microscope (SEM). An extracted restoration-free human molar was stored in physiological solution until it was embedded in an autopolimerysing acrylic resin. A standard preparation for onlay was completed and after preparation an anatomic onlay was waxed on the tooth and then hot pressed using lithium disilicate ceramic. After cementation the sample was dissected using an Automatic Micromet (Remet s.a.s) and the section was analyzed using a SEM. SEM evaluation of the tooth showed the three layers seamlessly; by increasing the enlargement the interface did not change. In Chapter 3 the fracture strength of human teeth restored with lithium disilicate onlays, with and without fiber post build-up, was measured. Twenty human mandibular molars were horizontally sectioned and divided into two groups (n = 10). No treatment was applied in group A. Teeth in group B were endodontically treated, built-up using fiber post and composite core and prepared with a circumferential chamfer providing a 1 mm circumferential ferrule. Lithium disilicate onlays were pressed and luted on teeth using dual-curing luting composite. Teeth were tested under static load. Failures were classified as restorable or not restorable. Failure loads were analyzed with ANOVA. Failure modes were compared using Pearson’s Chi-square tests. The mean fracture loads were 1383.5 N for group A and 1286.3 N for group B. No difference was found (p = 0.6). Ninety per cent of fractures were classified as not restorable in both groups, with no difference (p = 0.8). It was concluded that, for teeth restored with adhesive procedures and lithium disilicate onlays, the presence of build-up with fiber post to provide retention and resistance form (as traditionally stated) does not influence the fracture strength. Finally, in Chapter 4, a retrospective study was conducted to assess the clinical performance of lithium disilicate single restorations adhesively cemented on natural teeth. All patients who received lithium disilicate single restorations between 2009 and 2013 at the dental clinic of the University of Ferrara were recalled for clinical evaluation. A total of 43 partial and total restorations in 17 patients were evaluated from a minimum of 36 months follow-up to a maximum of 81 months follow-up, with a mean follow-up of 51 months. The cumulative survival rate was 97.7%, and the cumulative success rate was 94.2%. Lithium disilicate can be successfully used for single-tooth restorations in both anterior and posterior regions, provided that an adhesive luting protocol is applied.