Fatigue and Fracture Resistance of Implant-supported Zirconia-based Hybridabutment Crowns: In-vitro Study

Abstract

The current thesis is a combination of review of the literature as well as in-vitro experimental tests conducted which aimed to investigate both the fatigue and the fracture resistance of implant-supported hybrid-abutment crowns manufactured using zirconia-based material. The focus was on the following:  Chapter 2 (Study 1): a systematic review of the literature for in-vitro studies which investigated zirconia-based crowns. It aimed to identify in-vitro survival rate and clarify the most clinically relevant study design to utilise in the series of experimental studies to be conducted in this thesis. After applying the inclusion criteria only 25 articles were included. Five-year cumulative survival rate of zirconia-based implant-supported crowns was lower than tooth-supported crowns (84% and 88.8% respectively). Tooth-supported crowns subjected to wet fatigue showed a lower 5-year cumulative survival rate compared to thermal cycling (62.8% and 92.6% respectively). It was concluded that survival of zirconia-based crowns depends on type of support, type of fatigue test conducted, crown structure, and veneering method. In addition, setting of standardised in-vitro fatigue testing protocols to allow for valid comparability of data was suggested.  Chapter 3 (Study 2): in-vitro experiment to study the influence of using different all-ceramic restorative materials on fatigue and fracture resistance of implant-supported hybrid-abutment all-ceramic crowns. All-ceramic crowns underwent chewing simulation and static load to fracture tests and the obtained data was analysed with One-way ANOVA at a significance level of 5%. Data analysis showed that fracture load of monolithic zirconia crowns was statistically significantly higher than that of monolithic lithium disilicate crowns. Also, exposure to chewing simulation and thermocycling has significantly reduced the fracture load in both all-ceramic materials. The study concluded that implant-supported hybrid-abutment all-ceramic monolithic crowns are capable of withstanding masticatory forces in the posterior region. Moreover, aging caused reduction in fracture resistance of the tested monolithic all-ceramic crowns.  Chapter 4 (Study 3): experimental in-vitro study on the influence of using different crowns structure (mono-layer vs. bi-layer) on fatigue and fracture resistance of implant-supported zirconia-based hybrid-abutment crowns. One-way ANOVA and T-test were used to determine significance between all the tested subgroups. Upon completion of fatigue test; mono-layer crowns had 100% survival rate while bi-layer crowns had 50% survival rate. Statistics showed significantly higher fracture load of mono-layer crowns compared to bi-layer crowns. According to the results of this study, it was concluded that the structure of the implant-supported hybrid-abutment zirconia-based crown plays a major part in determining the crown’s ability to resist fracture. Where mono-layer structure exhibits significantly higher fracture resistance compared to bi-layer structure. Also, resistance to fracture of the zirconia-based mono-layer structure was also affected by fatigue application and aging.  Chapter 5 (Study 4): Experimental in-vitro study on the influence of using different veneering materials and different fabrication techniques (milling vs. hand-layering) on the fatigue and fracture resistance of implant-supported zirconia-based hybrid-abutment crowns. T-test and One-way ANOVA were used to compare means and to evaluate statistical significance between all study groups. Milled crowns had 100% survival rate upon completion of the fatigue testing; while hand-layered crowns had 50% survival rate. Also, milled crowns had statistically significantly higher fracture loads compared to hand-layered crowns at baseline and after fatigue (P ≤ 0.05). The outcome of this study showed that the veneering technique and the veneering material of implant-supported hybrid-abutment bi-layer crowns have an impact on their fracture resistance. It was concluded that milled veneers have the ability to withstand chewing forces present in the oral cavity better than conventionally hand-layered veneers. In addition, neither fatigue nor artificial aging caused any significant reduction in fracture resistant of both techniques.