Analysis of Bone Mineral Density and Bone Quality of Cortical Bone in the Human Hyoid Body and Histological Observation of the Entheses.

The hyoid is the only bone in the human body that is completely independent, not forming a joint with any other bone; its position is maintained by the suprahyoid and infrahyoid muscles, as well as several ligaments. The purpose of this study was to ascertain the effect of the functional pressure arising from these muscles and ligaments on the hyoid body structure from its bone mineral density, bone quality, and histological observations. The area between the mesial-most part of each lesser horn and the center of the hyoid body was divided equally into four measurement regions. We conducted histological investigations at each measurement region and observed the entheses. To analyze bone mass and bone quality, we also measured bone mineral density (BMD) and analyzed biological apatite (BAp) crystallite orientation in the same regions. Histological observations identified periosteal insertions and fibrocartilaginous entheses. There was no significant difference in BMD between any of the measurement regions, but the preferential orientation of BAp crystallites was stronger in the infrahyoid muscles and ligaments, where fibrocartilaginous entheses are found, than in other places. This suggests that the functional pressure at these sites might exert a major effect not only on the morphological characteristics of the entheses but also on bone quality.

The Wear Behavior of Glass-Ceramic CAD/CAM Blocks against Bovine Enamel.

The wear of enamel and crown restorative materials often occur by occlusion. The purpose of this study was to evaluate the wear volume between glass-ceramics used for CAD/CAM blocks (lithium disilicate: Initial LiSi block (LIS), IPS e.max CAD (IPS), zirconia-reinforced lithium silicate glass-ceramics: Celtra DUO (DUO), VITA Suprinity (VITS) and feldspar-based glass-ceramics: Vitablocs Mark II (MAK)) and bovine tooth enamel using a two-body wear test, the hardness, three-point bending strength, micro-structure and the element components of glass-ceramics. The data were analyzed using a one-way analysis of variance and Tukey's multiple comparison test (α = 0.05). IPS and DUO with relatively large size crystal gain had significantly larger abrader wear volumes. Zirconia-reinforced lithium silicate glass-ceramics (DUO, VITS) caused significantly greater wear volume in antagonist enamel. MAK with scale-shape crystals grains produced distinct scratches after wear tests, both in the material itself and in the enamel. A strong correlation between the mechanical properties (hardness, three-point bending strength) and wear volume could not be confirmed. The type of glass-ceramic, size, and shape of the crystal grains affected the wear behavior of the glass-ceramics for CAD/CAM blocks. Therefore, dentists should consider that wear behavior varies with crystal structure, size, and shape in glass-ceramics for CAD/CAM blocks.

Effect of Surface Treatments of Polyetherketoneketone as a Post Material on Shear Bond Strength to Root Dentin using Two Types of Resin Cement.

PURPOSE: This study investigated the effects of mechanical and chemical pretreatment of polyetherketoneketone (PEKK) on shear bond strength (SBS) to root dentin using two types of resin cement. MATERIALS AND METHODS: A total of 100 PEKK specimens were prepared and polished. Sixty specimens were mechanically treated by sandblasting (50-µm Al2O3 for 10 s at 0.2-0.25 MPa), and the remaining 40 were untreated. Self-adhesive resin cement and conventional resin cement were used. PEKK specimens bonded to root dentin using self-adhesive resin cement were classified into three groups by pretreatment method: (1) untreated PEKK, (2) mechanical pretreatment (sandblasted PEKK), and (3) both mechanical and chemical pretreatment (PEKK sandblasting as well as application of Scotchbond Universal; 3M Oral Care). Conventional resin cement was pretreated following the same steps (1-3) as those followed for self-adhesive specimens. Each group included 10 specimens. PEKK specimens after surface treatments were examined using SEM. SBS tests were performed using a universal testing machine, and data were statistically analyzed using one-way ANOVA and Tukey's multiple comparison test (p < 0.05). RESULTS: No significant difference was observed between cements with and without sandblasting. However, self-adhesive specimens with both mechanical and chemical pretreatments demonstrated higher SBS than other cements with or without pretreatment. CONCLUSION: Mechanical pretreatment by sandblasting did not improve the PEKK-root dentin SBS. However, combined mechanical (sandblasting) and chemical pretreatment (ScotchBond Universal) significantly improved the SBS between the PEKK and root dentin.

Wear Behavior between Aesthetic Restorative Materials and Bovine Tooth Enamel.

Tooth enamel wear occurs because of daily mastication and occlusion. This study investigated the wear behavior of bovine teeth against aesthetic restorative materials in vitro. Abrader specimens were fabricated using four tooth-colored restorative materials (zirconia, lithium disilicate glass ceramic, dental porcelain, and resin composite), with bovine tooth enamel as a control. Flattened bovine tooth enamel was used as the substrate specimen. These materials were characterized by Vickers hardness tests and surface roughness measurements. Two-body wear tests between the abrader and substrate specimens were performed, and the worn topographies were evaluated using a contour-measuring instrument and 3D laser microscope. The restorative materials and bovine tooth enamel had similar surface roughness but different hardness and wear behaviors. Bovine teeth showed the largest wear in tooth-tooth contact as the abrader and substrate specimens. Compared to bovine teeth, zirconia, lithium disilicate glass ceramic, and dental porcelain showed greater hardness and less wear on their surfaces, and less substrate wear of the opposite tooth enamel. The lowest hardness resin composite showed intermediate wear on its surface, resulting in the lowest substrate wear. Accordingly, dentists should pay attention to the selection of restorative materials to reconstruct their morphologies owing to different wear behaviors.

Effect of bone defect width and a ferrule on the fracture characteristics of vertically fractured teeth reattached with adhesive resin cement: An in vitro study.

This study aimed to clarify the effects of vertical bone defect width and a ferrule on fracture of the fragments of fractured tooth reattached with adhesive resin cement (reattached tooth). The reattached tooth was built up by a fiber post and composite resin core for abutment and formed to the abutment with or without a ferrule. The vertical bone defect was fabricated with a V-shaped defect in different widths. The fracture load was evaluated using a universal testing machine. The vertical bone defect did not affect the fracture load, but a ferrule increased the root fracture load. For the specimens without a ferrule, debonding between the composite resin core and the root at the coronal loading side and fractures at the apical side of the root were found. In conclusion, the ferrule at abutment could affect fracture load and modes, and the bone defect width did not.

Retention force of fiber-reinforced composite resin post on resin composite for core buildup -Effects of fiber orientation, silane treatment and thermal cycling.

The purpose of this study was to investigate the effects of fiber orientation, silane treatment, and thermal cycling on the retention force of fiber-reinforced composite resin (FRC) posts on resin composite. Two types of FRC posts (linear and woven) were prepared with and without silane treatment. Pull-out test specimens were made of FRC posts and resin composite for core buildup. Specimens were either incubated in distilled water for 24 h or subjected to 10,000 thermal cycles. The maximum fracture load obtained by a pull-out test was defined as the retention force. Fracture modes were observed after the test. Fiber orientation and thermal cycling did not affect the retention force on resin composite, and the retention force was improved by silane treatment. Whereas, fiber orientation affected the fracture mode. The result showed woven fiber orientation could contribute to the mechanically interlocking with the resin composite.

Micro- and nano-bone analyses of the human mandible coronoid process and tendon-bone entheses.

The coronoid process provides attachment to temporalis and masseter muscles, and thus plays an important role in mastication. Tendons connect muscles and bones, mediating the transmission of functional loads to bones. Thus, tendon-bone entheses govern mechanical stress in bones. The preferential orientation of biological apatite (BAp) crystallites, the main mineral component in bones, is an important index for bone quality and function, and is largely influenced by locally applied stress. In this study, we analyzed BAp orientation, Young's modulus, and bone mineral density (BMD) at different sites in the human coronoid process. No differences in BMD were found among the analyzed sites, but BAp crystal orientation was observed to differ. BAp crystallites showed a uni-directional orientation in the mesiodistal direction at the coronoid process apex, but were oriented in the direction vertical to the occlusal plane at other sites. Young's modulus tended to vary according to the BAp orientation. At the apex, a tendon form with characteristics different from those at other sites, including the presence of a fibrocartilaginous layer that may act as a stretching brake to control stress concentration, was observed. These findings suggest that the functional pressure of the temporalis muscle affects bone quality and strength.

In vitro wear behavior of restorative resin composites against bovine enamel.

The purpose of this study was to clarify the effects of fabrication method of restorative resin-based composites on its wear using enamel as antagonist teeth. Wear evaluation was performed via two-body wear test using hemispherical samples of restorative resin-based composite (abrader specimen) fabricated through direct restoration method, indirect restoration method, and CAD/CAM, and bovine enamel (substrate specimen). As a result, there was a difference in wear volume between resin-based composite and bovine enamel depending on the fabrication method. Resin composite used for indirect restoration method showed more wear in both the abrader and substrate specimens. Resin composite used for CAD/CAM crowns showed greater wear volume in the abrader specimen. In conclusion, results clarified that fabrication method of restorative resin-based composite has an influence on the wear of the resin composite itself and enamel as antagonist teeth.

Effects of calcium hydroxide reagent on the bond strength of resin cements to root dentin and the retention force of FRC posts.

The aim of this study was to determine the effects of calcium hydroxide (Ca(OH)2) treatment on bond strength of resin cements to root dentin and retention force of fiber-reinforced composite (FRC) posts. Bovine root dentin was endodontically prepared and treated with Ca(OH)2 for 7 days. Root dentin for bond strength test was adhered to resin-composite with resin cements. For pull-out test, posts consisting of FRC posts and resin-composites were fabricated and cemented to root. Shear bond and pull-out tests were performed using a universal testing machine. No significant differences in bond strength and post retention force were found between Ca(OH)2 treated and untreated groups. Significant differences were found among the cements. A positive correlation was indicated between bond strength of cements and retention force of FRC posts. In conclusion, Ca(OH)2 treatment on root dentin did not affect bond strength of resin cements and retention force of FRC posts.