Influence of surface preparation on fracture load of resin composite-based repairs

Fracture Load of CAD/CAM Feldspathic Crowns Influenced by Abutment Material

In vitro studies investigating the mechanical properties of dental reconstructions use various materials to replicate prepared teeth. However, no uniform recommendation exists as to which material is most suitable for standardized testing. The purpose of this study was to identify a material that resembles human dentin in fracture load tests. Sixteen human teeth were scanned with an intraoral scanner to obtain copies of the original crown morphology and were then prepared for crowns. Replica dies of the prepared teeth including the root morphology were fabricated with a Computer-aided design and computer-aided manufacturing (CAD/CAM) system and divided into four groups: (A) reinforced composite (RC); (B) human dentin (HD); (C) polymethyl methacrylate (PM); and (D) hybrid ceramic (HC). Sixty-four feldspar ceramic crowns were designed with the biocopy mode, fabricated with a CAD/CAM system, luted on the dies, and then with the roots embedded in polymethyl methacrylate. Care was taken to position all specimens of the same morphology identically. Thermo-mechanical load cycling was performed in a chewing simulator followed by fractural loading of the crowns. A mixed effect linear model was fitted to the data, and pairwise contrasts were estimated on the marginal means and corrected for multiple testing according to Tukey (α = 0.05). The means for fracture load (N) were 2435 N (95% CI (2162, 2709)) for hybrid ceramic, 1838 N (95% CI (1565, 2112)) for reinforced composite, 1670 N (95% CI (1396, 1943)) for human tooth and 1142 N (95% CI (868, 1415)) for polymethyl methacrylate abutment materials. Post-hoc pairwise contrasts revealed a statistically significant (p < 0.05) difference among all groups except for reinforced composite and human dentin (p = 0.76). The results indicate that the mechanical properties of abutment dies play a significant role for a possible substitution of natural teeth in in vitro studies.

Evaluation of zirconia and zirconia-reinforced glass ceramic systems fabricated for minimal invasive preparations using a novel standardization method

OBJECTIVE

Currently, minimal invasive approaches combining less invasive finish line preparations and reduced ceramic thickness are required. The aim of this study was to evaluate the fracture resistance of two ceramic systems fabricated with two preparation designs using CAD/CAM standardization technology.

MATERIALS AND METHODS

Forty intact human maxillary premolars were divided into two main groups according to the preparation technique. Group H (Horizontal): teeth with shoulder finish line and group V (Vertical): teeth with feather edge. Each main group was subdivided randomly into two subgroups according to the material used. Group CD (Celtra Duo) zirconia-reinforced glass ceramics and group K (KATANA) monolithic zirconia. CAD/CAM was used for standardization of natural teeth preparation. After cementation using self-adhesive resin cement, all specimens were subjected to 5000 thermal cycles and then were loaded until fracture. Failure types were evaluated using Stereomicroscopy and Scanning Electron Microscopy (SEM).

RESULTS

Nonsignificant; the higher mean value was recorded with VCD group (482.5 ± 103.8 N) and VK group (1347.6 ± 177.4 N) vs HCD group (471 ± 107.6 N) and HK group (1255.6 ± 121.3 N). SEM findings showed that fractures occurred mainly at the occlusal side of the crowns.

CONCLUSIONS

Vertical preparation showed a promising alternative to horizontal preparation. Moreover, both Celtra Duo and KATANA crowns can be used in premolar area with 0.5 mm margin thickness.

CLINICAL SIGNIFICANCE

Zirconia-reinforced glass ceramic and monolithic zirconia crowns may not necessitate the preparation of invasive finish lines as the type of finish line did not impair the strength after aging conditions.

Methodology in specimen fabrication for in vitro dental studies: Standardization of extracted tooth preparation

Background

Specimen preparation for in vitro study suffers a general lack of methodological homogeneity, as well as a lack of uniformity in the materials selected for fabricating them. This situation prevents comparisons between studies. When a specimen is not of dental origin it is not possible to study adhesion or bonding techniques realistically. The objective is to design and implement a method of specimen preparation that permits universal standardization for use in in vitro studies.

Material and Methods

A metal stump of specified dimensions was designed and fabricated by hand. It was scanned, the data digitalized, perfecting and standardizing the dimensions. Ten human molars were adapted to the size and shape of a standard milling block. A Cerec 3D inLab Cerec milling unit was used to prepare the molars to match the digitalized model.

Results

Ten specimens with identical dimensions were obtained.

Conclusions

CAD-CAM technology allows the preparation of natural extracted teeth to be standardized and could be used to establish a reproducible method that would facilitate comparison between different in vitro studies, and reduce bias arising from variations in sample fabrication.

Key words:Test ceramic, dentin analog, dentin model, dentin samples, methodology.