[Effect of occlusal thickness design on the fracture resistance of endocrowns restored with lithium disilicate ceramic and zirconia].

OBJECTIVE: This study aimed to investigate the effect of occlusal thickness design on fracture resistance of endocrowns restored with lithium disilicate ceramic and zirconia. METHODS: A total of 24 artificial first mandibular molars were randomly divided into four groups with six teeth in each group as follows: group lithium disilicate ceramic-2 mm (lithium disilicate ceramic, with an occlusal thickness of 2 mm and a retainer length of 4 mm); group lithium disilicate ceramic-4 mm (lithium disilicate ceramic, with an occlusal thickness of 4 mm and a retainer length of 2 mm); group zirconia-2 mm (zirconia, with an occlusal thickness of 2 mm and a retainer length of 4 mm); and group zirconia-4 mm (zirconia, with an occlusal thickness of 4 mm and a retainer length of 2 mm). After adhesive cementation (RelyX Ultimate Clicker), all specimens were subjected to thermocycling (10 000 cycles). The specimens were subjected to fracture resistance testing at a 135° angle to the teeth at a crosshead speed of 0.5 mm·min⁻¹ in a universal testing machine. Data were analyzed with ANOVA and Tukey's HSD test by SPSS 15.0. The failure modes were classified. RESULTS: The fracture resistances of groups lithium disilicate ceramic-2 mm, lithium disilicate ceramic-4 mm, zirconia-2 mm, and zirconia-4 mm were (890.54±83.41), (2 320.87±728.57), (2 258.05±557.66), and (3 847.70±495.99) N respectively. Group zirconia-4 mm had the highest fracture resistance, whereas group lithium disilicate ceramic-2 mm had the lowest. CONCLUSIONS: The fracture resistance of molar endocrown with zirconia is higher than that with lithium disilicate ceramic. Increasing the occlusal thickness can improve the fracture resistance but increase the risk of fracture of abutment.

Factors affecting the length of fiber post and core structure: a literature review / 口腔疾病防治

@#An id eal balance between the post length within root and the coronal extension should exist. Some theories advocated the use of the longest post as possible as the apical seal is not disturbed. Others advocated that the post should be longer than the crown or that the post should be a certain fraction of the length of the root. The conventional post length has been equal to 2/3-3/4 of the root length from half a century ago. Most of these theories have emphasized the post length in order to achieve sufficient retention and rigidity for coronal restoration. However, dental materials are evolving.With the use of fiber post and resin cement, stress distribution and post retention are improved over conventional metal post and zinc phosphate or glass-ionomer cement. Therefore, with improved bonding strength of fiber post, the post length within root should be minimized to maintain sufficient coronal rigidity. Factors affecting fracture resistance balance of a post-and-core system include the amount of Ferrule height (FH), Post length in root (PLIR), Post length in bone (PLIB), Abutment height (AH), Core length (CL) and Post length out of bone (PLOB). In view of the factors mentioned above, this paper investigated the mechanical factorsin a post-and-core system with literature review.