Fracture resistance of implant- supported monolithic crowns cemented to zirconia hybrid-abutments: zirconia-based crowns vs. lithium disilicate crowns

Abstract

Purpose: The aim of this in vitro study was to investigate the fracture resistance under chewing simulation of implant-supported posterior restorations (crowns cemented to hybrid-abutments) made of different all-ceramic materials.

Materials and methods: Monolithic zirconia (MZr) and monolithic lithium disilicate (MLD) crowns for mandibular first molar were fabricated using computer-aided design/computer-aided manufacturing technology and then cemented to zirconia hybrid-abutments (Ti-based). Each group was divided into two subgroups (n=10): (A) control group, crowns were subjected to single load to fracture; (B) test group, crowns underwent chewing simulation using multiple loads for 1.2 million cycles at 1.2 Hz with simultaneous thermocycling between 5℃ and 55℃. Data was statistically analyzed with one-way ANOVA and a Post-Hoc test.

Results: All tested crowns survived chewing simulation resulting in 100% survival rate. However, wear facets were observed on all the crowns at the occlusal contact point. Fracture load of monolithic lithium disilicate crowns was statistically significantly lower than that of monolithic zirconia crowns. Also, fracture load was significantly reduced in both of the all-ceramic materials after exposure to chewing simulation and thermocycling. Crowns of all test groups exhibited cohesive fracture within the monolithic crown structure only, and no abutment fractures or screw loosening were observed.

Conclusion: When supported by implants, monolithic zirconia restorations cemented to hybrid abutments withstand masticatory forces. Also, fatigue loading accompanied by simultaneous thermocycling significantly reduces the strength of both of the all-ceramic materials. Moreover, further research is needed to define potentials, limits, and long-term serviceability of the materials and hybrid abutments.

Keywords: Abutment; Crowns; Hybrid; Thermocycling; Zirconia.

Fig. 1. Sintering of Zr structures in a Programat S1.

Fig. 2. Fabrication of sample holder (A) Implant positioning and the CS sample cup duplication to create a negative replica of the sample cup, (B & C) Creating the positive replica of the sample cup, (D) The positive sample cup replica with implant and Ti-Base abutment inverted, (E) Acrylic resin poured in the mold and checked in the original chewing simulator sample cup for fitting.

Fig. 3. (A) A jig especially designed for SLF testing, (B) Position of indenter during SLF testing.

Fig. 4. Wear facets visible on the disto-buccal cusp of tested crowns after CS (arrows) using an endodontic microscope at 12×; (A) MZr, (B) MLD.

Fig. 5. Descriptive statistics of fracture load in Newtons for MZr and MLD.

Fig. 6. Fracture pattern for the two tested groups after SLF (A) MZr; fracture along the mesiodistal plane and the lingual developmental groove, (B) MLD; fracture along the mesiodistal plane.

Fig. 7. Representative SEM images after fracture resistance testing showing hackles in both (A) MZr and (B) MLD.