Hidemasa Shimpo

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.

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/Tukeys 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.

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 Tukeys 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.

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.

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.

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.

Denture construction using piezography and functionally generated path technique

Patient: The patient (76 years old, male) visited the hospital with the chief complaint of masticatory impediment due to mandibular severe ridge resorption. A cobalt chrome double metal structure denture with physiological form using piezography and functional occlusal surface form using the functionally generated path technique were delivered. Discussion: Higher denture stability could be obtained because the denture teeth arrangement and cameo surface were located within their individual piezographic spaces. It was thought that not only were denture adjustment times decreased, but also that the dentures contributed to the long-term denture stability of the occlusion by functional occlusion. Conclusion: Complete dentures with functional occlusion and physiological denture form were delivered for mandibular severe ridge resorption which was effective for the long-term stability of masticatory function.