Comparison between interocclusal registration using silicone bite registration material and intraoral scanner on clenching strength.

INTRODUCTION: The purpose of this study was to clarify the influence of clenching strength on interocclusal registration using intraoral scanner (IOS). METHODS: Subjects were eight volunteers. Two experimental conditions were light clenching (LC) and 40% maximum voluntary clenching (MVC). The conventional silicone bite registration and IOS were used for comparison. Occlusal contact areas (OCA) for different clenching strengths were compared, along with variation of measured values (VMV) between recording methods. RESULTS: Significant differences were observed between conditions on OCA and between methods on VMV. CONCLUSION: Clenching strength influenced interocclusal registration using IOS. Int J Prosthodont. 10.11607/ijp.8445.

Effects of hollow structures added by selective laser sintering on the mechanical properties of Co-Cr alloy.

PURPOSE: This study investigates the effects of hollow structures, added by selective laser sintering (SLS), on the mechanical properties of a Co-Cr alloy for providing an optimal structural property to the framework components of removable partial dentures (RPDs). METHODS: The specimens produced using the 3D data of the dumbbell-shaped cylinders were divided into four groups based on the manufacturing method: Cast, Mill, SLS-solid, and SLS-hollow. Tensile tests were performed to measure the mechanical properties of the specimens. The mechanical property values among the four groups were statistically compared using the Kruskal-Wallis test followed by the Steel-Dwass test (α = 0.05). RESULTS: The median elastic modulus was the largest in the Cast, followed by SLS-solid, Mill, and SLS-hollow, with no significant differences observed between all conditions. The median ultimate tensile strength was the largest in the order of SLS-solid, Mill, SLS-hollow, and Cast. The median 0.2% proof stress was the largest in SLS-solid, followed by SLS-hollow, Cast, and Mill. The median elongation was the highest in the order of Mill, SLS-solid, SLS-hollow, and Cast. CONCLUSIONS: With the addition of hollow structures, the elastic modulus decreased while the mechanical strength and proof stress remained high in SLS specimens. In addition, the ISO 22674 standard for dental metals was met, suggesting that SLS may be a possible method to design RPD frameworks with high strength and optimal structural properties.

Digital technology for fabrication of removable dental prosthesis with double crowns combining fiber-reinforced composite and zirconia.

PATIENTS: This clinical report describes the process for fabricating a double-crown-retained removable dental prosthesis combining a fiber-reinforced composite and zirconia using digital technology. An 83-year-old woman presented with gingival swelling around the maxillary right premolar. The swollen tooth was the abutment tooth of a cross-arch fixed partial denture. An intraoral scanner (IOS) and computer-aided design/manufacturing as digital technology were used to plan treatment with a double-crown-retained removable dental prosthesis. A metal-free prosthesis using zirconia for the primary crown and fiberglass-reinforced composite resin for the secondary crown was planned, and the patient consented to the treatment plan. After autotransplantation of a tooth as one of the abutments, the IOS was used to obtain digital scans of the prepared surface of the abutment teeth, opposing dentition, and occlusal relationships. First, primary crowns were milled using zirconia. Next, the intraoral scanner obtained a pick-up impression of the primary crowns, and secondary crowns were designed and milled from the fiber-reinforced composite. After delivery, the patient expressed satisfaction with the functionality, esthetics, and fit of the double-crown-retained removable dental prosthesis. DISCUSSION: Digital technology offers many advantages such as efficient fabrication of double crowns, reduced material costs, improved biocompatibility, and good aesthetics of metal-free materials. CONCLUSIONS: This clinical report describes the application of digital technology for the fabrication of a double-crown-retained removable dental prosthesis combining a fiber-reinforced composite and zirconia, resulting in patient satisfaction.

A new proposal for improving the accuracy of intraoral scanning for partially edentulous residual ridge.

Purpose This study investigated the usefulness of a newly proposed intraoral scanning method, using markers that can be used directly in the oral cavity, in order to improve the accuracy of impression taking of the residual ridge for fabrication of removable partial dentures.Methods An intraoral scanner was used to scan a dental model of a partially edentulous mandibular arch (Kennedy Class I). As markers, pieces of dried pasta were used. The scanning operation was performed under three conditions. In Condition 1, scanning was performed on the remaining teeth and the residual ridge without markers. In Condition 2, scanning of the remaining teeth and residual ridge was performed with markers. In Condition 3, the markers were removed from the model used in Condition 2, and the residual ridge was scanned again. The scanning data of each condition was superimposed on the control data, and the shape error was calculated and compared among the conditions.Results There was a significant difference in trueness of the residual ridge before and after marker application. The application of markers improved the trueness, while maintaining precision. Re-scanning after removing the marker did not affect trueness between before and after re-scanning and the re-scanned region showed shape continuity with the surrounding region.Conclusions The present method using markers that can be used in the oral cavity was effective in improving the accuracy of impression taking at the residual ridge.

Effects of repetitive insertion/removal cycles and simulated occlusal loads on retention of denture retainers.

The purpose of this study was to investigate the effects of repetitive insertion/removal cycle tests on denture retainers with simulated occlusal loads on the retentive force and deformation of clasp. Abutment teeth in the form of mandibular secondary premolars and clasp in the form of Akers clasps were prepared. The retentive force of the clasp on the abutment teeth were evaluated before and after undergoing repetitive insertion/removal cycle tests with or without cyclic loading. Changes in the clasp shape were monitored using a 3D scanner and scanning electron microscope. The initial retentive force was approximately 10 N and this value later decreased due to deformation of the clasp tips. In contrast to the non-load group, the load group exhibited a reduction in retentive force during earlier stages. Therefore, cyclic loading was related to a decrease in retentive forces, specifically in the early stages of repetitive insertion/removal cycles.

Trueness and precision of digital impressions obtained using an intraoral scanner with different head size in the partially edentulous mandible.

PURPOSE: It remains unclear whether digital impressions obtained using an intraoral scanner are sufficiently accurate for use in fabrication of removable partial dentures. We therefore compared the trueness and precision between conventional and digital impressions in the partially edentulous mandible. METHODS: Mandibular Kennedy Class I and III models with soft silicone simulated-mucosa placed on the residual edentulous ridge were used. The reference models were converted to standard triangulated language (STL) file format using an extraoral scanner. Digital impressions were obtained using an intraoral scanner with a large or small scanning head, and converted to STL files. For conventional impressions, pressure impressions of the reference models were made and working casts fabricated using modified dental stone; these were converted to STL file format using an extraoral scanner. Conversion to STL file format was performed 5 times for each method. Trueness and precision were evaluated by deviation analysis using three-dimensional image processing software. RESULTS: Digital impressions had superior trueness (54-108µm), but inferior precision (100-121µm) compared to conventional impressions (trueness 122-157µm, precision 52-119µm). The larger intraoral scanning head showed better trueness and precision than the smaller head, and on average required fewer scanned images of digital impressions than the smaller head (p<0.05). On the color map, the deviation distribution tended to differ between the conventional and digital impressions. CONCLUSIONS: Digital impressions are partially comparable to conventional impressions in terms of accuracy; the use of a larger scanning head may improve the accuracy for removable partial denture fabrication.

Evaluation of the rigidity of dentures made of injection-molded materials.

Dentures made of 2 different types of injection-molded thermoplastic resins (polyamide resin and polyester resin) and a denture made of conventional heat-polymerized resin were used to create an experimental model of a mandibular molar region with a two-tooth gap. In the experimental model, a force of 100 N was applied onto the mesial fossa of the first molars of the dentures, and comparisons were performed by measuring the pressure applied under the denture base and the subsidence rate of the denture. The polyamide resin denture showed the highest subsidence, exerted the highest pressure on the underlying mucosa, and showed significant differences with the other types of dentures. The findings showed that polyamide resins have the lowest degree of elasticity, and that when resins with such low elasticities are used in the denture base, they should preferably be reinforced with metals.

Reproducibility of probing depth measurement by an experimental periodontal probe incorporating optical fiber sensor.

BACKGROUND: An experimental periodontal sensor probe (SP) equipped with an optical fiber for recording function was developed. The aim was to test the intraexaminer reproducibility of probing using the SP and to assess the consistency with the manual probe (MP). METHODS: The SP was assembled with an external sheath covering the probe tip of an MP. The sheath was slid backward by the free gingival margin while probing and the sliding distance was detected by the sensor. The probing was conducted with the walking stroke at six sites for four first molar teeth in six maintenance patients with the SP and the MP at a 1-hour interval. The deepest reading in the vicinity of each site was recorded. The measurements were rerecorded 1 week later. RESULTS: The mean depth was 3.03 and 3.08 mm recorded by the MP and SP, respectively. Although no significant difference was found between the probes (P >0.05) in all measurement sites, the mean depth at the lingual site of the upper left first molar was noticeably lower with the SP. For sites ≥7 mm, significantly lower depth was recorded by the SP (P <0.05). Zero discrepancy in duplicate measurements was found in 76% of all sites with MP and 92% with SP. CONCLUSIONS: The reproducibility of the SP was comparable to that of the MP. The results indicate that for sites of maintenance patients with probing depth <7 mm there was excellent agreement obtained by a single examiner using the SP compared to the MP.

Influence of removable partial dentures on the formation of dental plaque on abutment teeth.

PURPOSE: This study aimed to clarify the relation between the shape of the retainers and the plaque formation on abutment teeth to improve the denture design. METHODS: This study observed the buccal surface for the clasps and distoproximal surface for a guide plate of the mandibular first premolar which was the abutment tooth with a direct retainer of a distal extension RPD. The buccal surface was observed in 10 subjects and the distoproximal surface in 14 subjects. The latter cohort was divided into two groups (the group with an open type guide plane and a close type guide plane). All subjects provided their informed consent. The state of plaque accumulation was inspected by standardized photography and bacterial quantification and compared among the cast circumferential (CC), Ibar, wire circumferential (WC) and control (no clasps) in individuals regarding the buccal surface and between the groups with the open type and close type guide plane of the distoproximal surface. RESULTS: No significant differences were observed among the CC, Ibar, WC and control (P>0.05) regarding the buccal surface, while in regard to the distoproximal surface, the group with the open type guide plane had significantly more plaque than the group with the close type guide plane (P<0.05). CONCLUSION: The plaque formation on the buccal surface is not dependent on the types of clasps. It is effective to prepare a guide plane as close to the gingival margin as possible to reduce the plaque accumulation on the distoproximal surface.