In vitro assessment of polishing efficiency for additive-manufactured Co-Cr alloy clasps.

PURPOSE: The processes and methods of treating and polishing additive-manufactured (AMed) cobalt-chromium (Co-Cr) alloy clasps were assessed in vitro to determine their suitability for constant clinical use and the reduction of dental technician work. METHODS: AMed Aker clasps were fabricated by selective laser sintering of approximately 50 µm Co-Cr alloy powders. After the nodules and fins on the inner surface of the AMed clasps were removed and morphological correction was performed, a dental technician manually polished the clasps as a control. Four surface treatments, barrel finishing, shot peening, and wet and dry electropolishing, were performed to obtain smooth surfaces. In addition, hybrid manufacturing, which integrates repeated laser sintering and high-speed milling for one-process molding, was added to this study. After observing the treated surfaces using SEM and Hybrid Laser Microscope (HLM) the surface roughness, fitness accuracy, and retentive forces of the treated AMed clasps were measured, and their polishing efficiencies were compared. RESULTS: Similar to manual polishing, dry electropolishing yielded the smoothest surfaces in all treatments. The fitness accuracy of all clasp regions and treatment methods ranged from 80 to 140 µm, without significant differences among the treatment methods. All treated clasps showed acceptable retentive forces for clinical use, and hybrid manufacturing and wet electropolishing showed significantly higher forces. CONCLUSIONS: AMed Co-Cr clasps with all surface treatments could be clinically used if additional slight manual polishing was performed; however, each processing condition should be carefully selected.

Fitness accuracy and retentive forces of milled titanium clasp.

Since cast titanium prostheses have many drawbacks, multi-directionally forged titanium grade 2 (MDF) was developed, and the application of the milling process was proposed for improving the titanium clasp. This in vitro study evaluated milled titanium clasps, including MDF titanium. Milling clasps were manufactured with commercially pure (CP) titanium grade 2 (CP 2), grade 4 (CP 4), Ti-6-Al-4V, and MDF. As a control, a CP 2 cast titanium clasp was fabricated in the conventional manner. No porosities and catastrophic failures were observed in the four milled titanium clasps. Fitness accuracy and retentive forces of milled CP 2 and CP 4 tended to be worse, and the milled MDF showed the higher retentive forces (12.45 N) than did cast and milled CP 2 clasps (9.32 N and 4.42 N). Milled titanium clasps can be recommended for longer-term clinical use as compared to cast clasps.

Clinical application of an intraoral scanner and CAD/CAM system for a Kennedy class I partially edentulous patient.

Traditionally, removable partial dentures (RPDs) have been made by using an elastic impression material and casting from a framework wax pattern on a refractory cast. In this short communication, the procedures for digitally fabricating removable partial dentures using an intraoral scanner (IOS) and additive manufacturing system (AM) are described. The adaptation accuracy of the RPD using IOS and AM techniques was evaluated subjectively as good or satisfactory.

Trueness and fitting accuracy of maxillary 3D printed complete dentures.

Purpose The trueness and fitting accuracy of complete dentures (CDs) manufactured digitally from wax dentures have not been investigated yet. This study evaluated the trueness and fitting accuracy of maxillary CDs manufactured using computer-aided design technology.Methods CD bases were manufactured from fully edentulous maxillary casts using a milling machine and three three-dimensional (3D) printers (two stereolithography apparatus (SLA) and one digital light processing (DLP)). 3D printing was performed using an SLA printer with eight build support angles (0° to 315°). As a control, a CD base was conventionally fabricated using a heat-polymerized PMMA resin. After the tissue surfaces of the casts and the cameo surfaces of all the CD bases were scanned, their STL data were superimposed with a best-fit alignment. The deviations of all the CD bases were evaluated using data-matching software.Results The milled CD bases showed higher trueness and fitting accuracy compared with the 3D-printed and conventional bases. SLA showed a higher fitting accuracy than DLP. The best support angles for the fitting accuracy were 45° and 225°. The fitting accuracy of the SLA 3D-printed CD bases with an angle of 45° was comparable to or slightly higher than that of conventionally fabricated bases.Conclusions The milled CD bases showed an excellent fitting accuracy. The SLA-printed CDs demonstrated a clinically acceptable fitting accuracy.

Laboratory efficiency of additive manufacturing for removable denture frameworks: A literature-based review.

The purpose of this literature review was to verify the laboratory efficiency of additive manufacturing (AM) systems for removable partial denture (RPD) frameworks. All available relevant articles in English published from 1990 to 2020 were found by searching online databases and by hand research. A total of 17 articles dealt with the surface roughness, fitness accuracy, and retentive forces of AM frameworks. The surface roughness of AM was inferior to that of casting and milling. Whether conventional cast or AM RPD frameworks had superior fitness accuracy could not be clarified. As compared with casting and AM, milling enabled the fabrication of RPD clasps with comparable or better fitness accuracy. Over time, AM clasps had retentive force values of superior consistency as compared with those of conventional cast clasps. Clasps fabricated by repeated laser sintering and high-speed milling could obtain smoother surfaces and more suitable retention than those of AM clasps.

Fitness accuracy and retentive forces of additive manufactured titanium clasp.

PURPOSE: Laboratory and clinical assessment of cast titanium dentures were reported, little is known about additive manufacturing (AM) frameworks. This study evaluated in vitro AM titanium alloy clasps for clinical use. METHODS: After scanning the stainless steel dies to simulate the first molar, an Akers clasp was designed by CAD. AM clasp specimens were manufactured using laser sintering and AM machines from CP titanium grade 2 (CPTi), Ti-6Al-4V (Ti64), and Ti-6Al-7Nb (Ti67) powders. As controls, cast titanium clasps were conventionally made with same three titanium alloys. After nondestructive inspection and surface element analysis, surface roughness, fitness accuracy, initial retentive forces, and changes in retentive forces were measured. RESULTS: Although few porosities and little contamination of AM clasps were confirmed, the AM clasp arm and tip indicated slightly worse fitness accuracy as compared to cast clasps. The initial retentive forces of all titanium clasps showed appropriate retentive forces within the acceptable ranges, AM clasps tended to have lower decreases in retentive forces with up to 10,000 insertion/removal cycles as compared to those of the cast clasps. CONCLUSIONS: AM titanium clasps would be acceptable for clinical use similar to that of cast titanium clasps.

Fitness and retentive force of cobalt-chromium alloy clasps fabricated with repeated laser sintering and milling.

PURPOSE: With computer-aided design and computer-aided manufacturing (CAD/CAM), the study was conducted to create a removable partial denture (RPD) framework using repeated laser sintering rather than milling and casting techniques. This study experimentally evaluated the CAM clasp and compared it to a conventional cast clasp. METHODS: After the tooth die was scanned, an Akers clasp was designed using CAD with and without 50µm of digital relief on the occlusal surface of the tooth die. Cobalt-chromium (Co-Cr) alloy clasps were fabricated using repeated laser sintering (RLS) and milling as one process simultaneously (hybrid manufacturing; HM). The surface roughness of the rest region, gap distances between clasp and tooth die, initial retentive forces, and changes of retentive forces up to 10,000 insertion/removal cycles were measured before and after heat treatment. The HM clasp was compared to the cast clasp and the clasp made by repeated laser sintering only without a milling process. RESULTS: The HM clasp surface was smoother than those of cast and RLS clasps. With the digital relief, the fitness accuracy of the HM clasp improved. The retentive forces of the HM clasps with relief and after heat treatment were significantly greater than for the cast clasp. HM clasps demonstrated a constant or slight decrease of retention up to 10,000 cycles. CONCLUSIONS: HM clasp exhibited better fitness accuracy and retentive forces. The possibility of clinically using HM clasps as well as conventional cast clasps can be suggested.

Complete denture fabrication using piezography and CAD-CAM: A clinical report.

To provide maxillary and mandibular complete dentures for a patient with severe ridge resorption, the denture space was recorded using the piezography technique. After the piezographic space was scanned, a virtual tooth arrangement and festooning were performed within the space using computer-aided design software. The denture bases were milled from a polymethylmethacrylate resin block using computer-aided manufacturing, and commercially available denture teeth were bonded with resin adhesive. Using the piezography technique described, physiologically appropriate complete dentures were fabricated based on the neutral zone concept.

Clinical Evaluation of Implant-Supported Removable Partial Dentures With a Stress-Breaking Attachment.

PURPOSE: The stress-breaking ball (SBB) attachment can distribute the occlusal force equally between the alveolar ridge and the implants. The purpose of this study was to evaluate the implant-supported distal extension removable partial dentures (RPDs) with SBB attachment in 10 patients who were partially edentulous. METHODS: This randomized crossover study was designed to compare the function of RPDs with and without healing abutments and SBB attachments to support the posterior aspects of the RPDs. Mandibular jaw movements during mastication and the occlusal force and contact area were measured with a commercially available tracking device and pressure-sensitive sheets. Using a visual analog scale, 4 criteria-chewing, retention, stability, and comfort-were evaluated. All of the data obtained were analyzed using a 1-way analysis of variance (α = 0.05). RESULTS: There were no significant differences in either the mean time or the coefficient of variation among the SBB attachments and healing abutments of implant-supported removable partial dentures (ISRPDs) and conventional removable partial dentures (CRPDs). SBB attachments and healing abutments of ISRPDs had greater forces and contact areas than those of CRPDs with significant differences. For all criteria, patients preferred SBB attachments to healing abutments and CRPDs. CONCLUSIONS: The implant-supported distal extension RPDs with SBB attachment improved denture stability and patients' satisfaction.

Clasp fabrication using one-process molding by repeated laser sintering and high-speed milling.

PURPOSE: A single machine platform that integrates repeated laser sintering and high-speed milling for one-process molding has been developed. METHODS: The Akers clasp was designed using the CAD system (DWOS Partial Frameworks, Dental Wings) and fabricated using repeated laser sintering and a high-speed milling machine (LUMEX Advance-25, Matsuura) with 50-µm Co-Cr particles (CAM clasp). As controls, cast clasps of the same forms were also prepared using conventional casting methods with a Co-Cr alloy and CP titanium Grade 3. After the surface roughness was measured, the gap distance between the clasps and the tooth die was assessed using the silicone film method. The initial retentive force and changes in retention up to 10,000 cycles were also measured. The data were analyzed using two-way ANOVA and Tukey's multiple comparison test (α=0.05). RESULTS: CAM clasps exhibited significantly smoother surfaces than those of cast Co-Cr and CP Ti clasps (p<0.05). However, the gap distances of the CAM clasps were significantly greater than those of the cast clasps (p<0.05). The retentive forces of both CAM and cast Co-Cr clasps were significantly higher than those of CP Ti clasps. (p<0.05). The retention of CAM clasps demonstrated a constant or slight decrease from 1000 up to 10,000 cycles. CONCLUSIONS: The CAM clasp made by repeated laser sintering and high-speed milling can be used effectively as an RPD component.

Fixation of magnet assembly to denture base using alternative resins.

The fixation strengths between conventional/modified magnetic assemblies and denture base resins were evaluated using six alternative resins. Magnetic assemblies with three different undercut wings were prepared. Soft lining materials with added PMMA resin polymer, two photopolymerization denture relining resins, an experimental resin, and a temporary filling resin were used to fix the magnetic assemblies to the denture bases. As a control, a commercially available magnetic assembly without undercut wings and a conventional autopolymerized resin were also prepared. After surface treatments, the magnetic assemblies were fixed using fixation resins, and tensile strengths and attractive forces were measured using an autography. The experimental resin and the temporary filling resin showed retentive forces comparable to those of conventional autopolymerized resins. Although the experimental resin demonstrated satisfactory fixation strengths, it should be necessary to improve its mechanical strength. The temporary filling resin could be used as a permanent fixation material.

Occlusal contact of fixed implant prostheses using functional bite impression technique.

OBJECTIVE: Functional bite impression (FBI) has been described as a definitive impression made under occlusal force after functional generated path (FGP) recording. This study compared the accuracy of occlusal contact of implant-fixed prostheses using the FBI technique and the conventional impression technique. METHODS: Twelve subjects, each missing a single premolar or molar, were selected for this study. The conditions of the occlusal contacts were identified by the modified transillumination method. The occlusal contact condition was determined by comparing the rate of change in the occlusal contact area of the implant-fixed prostheses and both adjacent teeth before and after occlusal adjustment. RESULTS: The rate of change in the occlusal contact area using the FBI technique was 96%, and the rate using the conventional technique was 54%. The occlusal contact of implant prostheses using the FBI technique revealed better accuracy than that of the conventional technique. CONCLUSIONS: Regarding the FBI technique, a precise and functional prosthesis could be produced by completing the maxillomandibular registration, impression, and FGP at the same time.

Influence of thickness and undercut of thermoplastic resin clasps on retentive force.

Thermoplastic resin clasps have been used for esthetic denture rehabilitation. However, details of the design of the clasps have never been thoroughly clarified. This study investigated the retentive forces of thermoplastic resin clasps for non-metal clasp dentures. The retentive forces of all thermoplastic resin clasps depended on the elastic modulus of each resin, undercuts, thickness, and widths of the tested. A clasp with more than 0.5 mm undercut and 1.0 mm thickness is needed for Valplast. Similarly, more than 0.25 mm undercut and 1.0 mm thickness and 0.5 mm undercut and 0.5 mm thickness are required for Estheshot and Reigning, respectively; thus, the recommended clasp arm thickness is 1.0 mm to 1.5 mm for Valplast and Estheshot and 0.5 mm to 1.0 mm for Reigning when the width of the retentive arm is 5.0 mm.

Effect of implant support on mandibular distal extension removable partial dentures: relationship between denture supporting area and stress distribution.

PURPOSE: This study explored the relationship between implant support and the denture-supporting area by comparing the stability of an implant-supported distal extension removable partial denture and a conventional distal extension removable partial denture. METHODS: A model simulating a mandibular bilateral distal extension missing (#34-37 and #44-47) was fabricated using silicone impression material as soft tissue (2 mm thick) on an epoxy resin bone model. The denture base was reduced by 5 mm cutting part of both the retromolar pad and the lingual border. Loads of up to 5 kg were applied, and the pressure and displacement of the RPDs were simultaneously measured and analyzed using the Wilcoxon test (α<0.05). RESULTS: The pressure on the bilateral first molar and the middle areas of the implant-supported distal extension removable partial denture (ISRPD) was significantly less than on the conventional RPD (CRPD). As the supporting area of the denture base decreased, the pressure and the denture displacement of the CRPD were greater than for the ISRPD. CONCLUSION: This study indicated that implant placement at the distal edentulous ridge can prevent denture displacement of the distal extension bases, regardless of the supporting area of the denture base.

Fabrication of fixed implant prostheses using function bite impression technique (FBI technique).

PATIENT: The patient was partially edentulous, lacking both the first mandibular molars. The FBI and the conventional impression technique were used for the fabrication of implant-fixed prosthesis replacing the right and left molars, respectively. In the FBI technique, the definitive impression was made under occlusal force and functionally generated path (FGP) recording at the same time. The right and left occlusal contact areas were compared after completing the implant-fixed prosthesis rehabilitation. DISCUSSION: It has been suggested that accuracy of the impression and maxillomandibular registration is necessary to ensure a satisfactory long-term clinical outcome. The transfer of the exact position of the implants to the working cast is even more important because implants lack the mobility of natural teeth. There are displacement differences between implants and natural teeth under occlusal force. The FBI technique may compensate for this difference in accuracy. CONCLUSION: Using the FBI technique, a precise prosthesis could be produced by completing simultaneously the maxillomandibular registration, impression and FGP.

Effect of arm design and chemical polishing on retentive force of cast titanium alloy clasps.

PURPOSE: Titanium dentures have recently increased in popularity. A decrease in the retentive force of the titanium clasps has frequently been observed. This study investigated the effect of retentive arm design and chemical polishing of titanium clasps. MATERIALS AND METHODS: Four Akers clasps with 0.25-mm and 0.5-mm undercuts were designed so that the retentive arms were placed at 1/2 and 2/3 of the undercut area. Wax patterns were fabricated and invested in phosphate-bonded Al2O3/LiAl SiO6. They were cast with CP Ti Grade 2, CP Ti Grade 3, and Ti-6Al-7Nb using an argon gas-pressure casting unit (Autocast HC-III). After sandblasting, the castings were chemically treated with an HNO3/HF solution. The retentive force (N) was measured up to 10,000 insertion/removal times. The results (n = 5) were analyzed by ANOVA/Tukey's test (alpha= 0.05). RESULTS: The retentive forces significantly decreased with increasing immersion time in the HNO3/HF solution (p < 0.05). The retentive force of the 2/3 undercut was significantly greater than that of the 1/2 undercut (p < 0.05). Excluding the initial insertion/removal period of the 2/3 undercut, there were no significant differences among all the titanium metals tested (p > 0.05). As the number of insertion/removal times increased, the retentive force of the 2/3 undercut greatly decreased. The retentive force of the Ti-6Al-7Nb clasps for the 1/2 undercut decreased the least after 10,000 insertion/removal times. CONCLUSIONS: Chemical treatment for titanium clasps should be performed for 1-5 minutes due to the following factors: accuracy, surface roughness, surface structure, initial retention, and stability of retention. To maintain appropriate long-term retentive force, the retentive arms should be placed in the 1/2 undercut area of the abutment tooth.