Clinical trials in zirconia: a systematic review

Abstract

Zirconia is unique in its polymorphic crystalline makeup, reported to be sensitive to manufacturing and handling processes, and there is debate about which processing method is least harmful to the final product. Currently, zirconia restorations are manufactured by either soft or hard-milling processes, with the manufacturer of each claiming advantages over the other. Chipping of the veneering porcelain is reported as a common problem and has been labelled as its main clinical setback. The objective of this systematic review is to report on the clinical success of zirconia-based restorations fabricated by both milling processes, in regard to framework fractures and veneering porcelain chipping. A comprehensive review of the literature was completed for in vivo trials on zirconia restorations in MEDLINE and PubMed between 1950 and 2009. A manual hand search of relevant dental journals was also completed. Seventeen clinical trials involving zirconia-based restorations were found, 13 were conducted on fixed partial dentures, two on single crowns and two on zirconia implant abutments, of which 11 were based on soft-milled zirconia and six on hard-milled zirconia. Chipping of the veneering porcelain was a common occurrence, and framework fracture was only observed in soft-milled zirconia. Based on the limited number of short-term in vivo studies, zirconia appears to be suitable for the fabrication of single crowns, and fixed partial dentures and implant abutments providing strict protocols during the manufacturing and delivery process are adhered to. Further long-term prospective studies are necessary to establish the best manufacturing process for zirconia-based restorations.

Three- to nine-year survival estimates and fracture mechanisms of zirconia- and alumina-based restorations using standardized criteria to distinguish the severity of ceramic fractures

“…One explanation is the inclusion of patients with signs of bruxism who are often excluded from clinical studies [29,30]. Typically, these patients severely abraded their occlusal surfaces and provoked critical ceramic fractures with exposures of the zirconia cores.…”

The influence of zirconia coping designs on the fracture load of all-ceramic molar crowns

“…Recently, review report by Al-Amleh et al 19) showed that chipping of the veneering porcelain is confirmed to be an ongoing problem with zirconia all-ceramic-based restorations. At the beginning of introduction of zirconia restorations, uniform thickness coping was used.…”

The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio

“…Y 2 O 3 (approximately 3 mol%) is the most widely used stabilizer for dental zirconia (Y-TZP) [2]. In response to mechanical stimuli such as tensile stress at the crack tips, the stabilized tetragonal zirconia transforms to the more stable monoclinic phase with a local increase in volume of approximately 4-5 vol% [10]. This increase in volume closes the crack tips, effectively blunting crack propagation [10].…”

“…In response to mechanical stimuli such as tensile stress at the crack tips, the stabilized tetragonal zirconia transforms to the more stable monoclinic phase with a local increase in volume of approximately 4-5 vol% [10]. This increase in volume closes the crack tips, effectively blunting crack propagation [10]. It is this transformation-toughening process, which gives zirconia its strength and toughness, exceeding all currently available dental ceramics [10].…”

“…This increase in volume closes the crack tips, effectively blunting crack propagation [10]. It is this transformation-toughening process, which gives zirconia its strength and toughness, exceeding all currently available dental ceramics [10]. From an optical point of view, Y-TZP is white and can be easily coloured by addition of traces of rare earth elements [2].…”