Comparison of the accuracy of different handheld-type scanners in three-dimensional facial image recognition.

PURPOSE: Handheld-type scanners are widely used in clinical practice. This study examined the accuracy of handheld-type scanners using plaster statues to assess their performance in facial recognition. METHODS: Twelve 4-mm zirconia balls as measuring points were attached to the facial portions of three types of plaster statue. Six digital facial images of each plaster statue were obtained using one of the following five handheld-type scanners: Artec Eva, Artec Spider, Bellus 3D FaceApp, SNAP, and Vectra H1. Four-millimeter spherical objects were manually placed at the measurement points on the scanned data generated using computer-aided design software and coordinate positions were measured using a contact-type high-resolution three-dimensional measurement device. Consequently, the discrepancy between the distance measured using the contact-type device and that measured using the handheld-type scanner was calculated. The scanning time, processing time, and deviation of the distance between the measuring points were analyzed using two-way analysis of variance and t-test with Bonferroni correction. RESULTS: The scanning and processing times ranged from 15.2 to 42.2 s and 20.7 to 234.2 s, respectively. Overall, 97% of all measured distances by Spider were within ±1.00% deviation; 79%, Vectra; 73%, Eva; 70%, Bellus; and 42%, SNAP. CONCLUSIONS: The performance of handheld-type scanners using plaster statues varied among the different scanners. The scanning time of Eva and the processing time of Bellus were significantly shorter than those of other scanners. Furthermore, Spider exhibited the best accuracy, followed by Eva, Vectra, Bellus, and SNAP.

Influence of block-out on retentive force of thermoplastic resin clasps: an in vitro experimental and finite element analysis.

PURPOSE: The purpose of this study was to determine the effect of block-out preparation, used to eliminate the undercut area, on the retentive force and stress distribution of resin clasps. METHODS: A total of 72 polyester and polyamide resin clasps were fabricated on a premolar abutment crown following six block-out preparations. A combination of two types of vertical block-outs and three types of horizontal block-outs (on the missing side) was used on the abutment tooth. Each clasp was subjected to an in vitro removal test using a universal testing machine. The retentive force and traces of the clasp on the abutment tooth were recorded and analyzed with one-way analysis of variance and post hoc comparisons (α=0.05). Non-linear finite element analysis was performed to assess the stress distributions of the resin clasps. RESULTS: Resin clasps with a vertical block-out of 0.75mm undercut showed significantly higher retentive force than those with the 0.5mm undercut. Resin clasps with horizontal block-out showed significantly lower retentive force than those without horizontal block-out. There was no significant difference between the two thermoplastic resins. The maximum first principal stress of the resin clasp was concentrated under the shoulder of the clasp and strongly influenced by the width of horizontal block-out in the finite element analysis. CONCLUSIONS: This in vitro experiment suggested that a horizontal block-out is necessary even for a 0.5-mm undercut. The design of the resin clasp should be considered from two aspects: retentive force and deformation risk.

The ball-on-disk cyclic wear of CAD/CAM machinable dental composite and ceramic materials.

The purpose of this study was to investigate the wear volume and the principal strain of machinable dental composite and ceramics in simulated mastication. A ball-on-disk wear test was performed for 3,000 cycles in water, using nine ball/disk combinations of three commercial CAD/CAM materials: feldspathic, lithium disilicate glass ceramics, and a highly loaded composite material (n = 7 for each combination). The wear volume was optically measured using a digital scanner and analyzed for statistical differences based on the materials (α = 0.05). We used non-linear finite element analysis to calculate the principal strain. The wear volume of the ball was significantly larger than that of the disk when hardness and fracture toughness of the former was lower than that of the latter and vice versa (P < 0.05). The lithium disilicate glass ceramic constantly showed lower wear volume than the opposing antagonist. Except for the same material pairs of feldspathic and composite, the ball or disk specimen that showed a larger wear in the occluding pair coincided with the one with higher maximum strain. It was not possible to predict the magnitude of wear, whereas the result suggested a strong association between the maximum strain and wear volume of the ceramic surface.