Ichiro Shimamura

Influence of number and inclination angle of implants on stress distribution in mandibular cortical bone with All-on-4 Concept

PURPOSE The All-on-4 is used in the therapy of edentulous mandibles. However, few studies have investigated the effect of such implants on supporting bone. To clarify differences in stress in peri-implant cortical bone between 2-patterns of 6 implants and 8-patterns of 4 implants with change in inclination angle based on the All-on-4. METHODS Three-dimensional finite element analysis models based on the mean value of the Japanese edentulous mandible were constructed. Implants 13 or 15 mm in length were inserted between the mental foramina. In the 6-implant model, implants were inserted in parallel. In the 4-implant model, the 2 anterior implants were inserted in parallel and the 2 posterior implants in parallel or on a slant. Implants were splinted with a superstructure. A 50 N load was applied to the occlusal surface under condition A, at 2mm distal from the distal implant, or condition B, at the distal end of the superstructure. Maximum von Mises stress on cortical bone was measured. RESULTS Stress was concentrated around the posterior-most implant on the right side. Under condition A, stress increased with 4 implants and increase in angulation. At 45°, stress increased by 23% of that in the 6-implant model. Under condition B, stress increased with 4 implants, although stress decreased with increase in angulation. At 45°, stress decreased by 45% of that in the 6 implants. CONCLUSIONS The use of 4 implants or inclined implants increased stress on peri-implant cortical bone. However, when used in conjunction with a short cantilever, inclined implants decreased stress on peri-implant cortical bone.

Retainer design for unilateral extension base partial removable dental prosthesis by three-dimensional finite element analysis

PURPOSE The purpose of this study was to evaluate the effect of differences in the retainer design on load on abutment teeth and displacement of the denture base in mandibular unilateral extension base partial removable dental prosthesis to determine the optimum design for a mechanically appropriate retainer. METHODS Three models were used. Each incorporating a removable denture with different designs of retainer positioned. In design A, Akers clasps were positioned over the first and second premolars. In design B, embrasure clasp was positioned over the first and second premolars. In design C, rest and back-action clasps were positioned over the first and second premolars, respectively. Loading on the occlusal surface of the dentures was performed in the vertical, oblique lingual and buccal directions. Load on the abutment teeth and displacement of the denture base were determined by the three-dimensional finite element method analysis. RESULTS Vertical and buccolingual load on the abutment teeth by lingual and buccal direction loading on the denture were smaller in designs B and C than in design A. However, the superioinferior and buccolingual displacement of the denture base by buccal direction loading on the denture were largest in design A. Superioinferior and buccolingual displacement of the denture base by the lingual direction loading on the denture were largest in design C. CONCLUSIONS Under the conditions set in this study, the stability of design A was greater than that of design B or C, but load on the abutment teeth was larger.

Effect of buccal part designs of polyamide resin partial removable dental prosthesis on retentive force

PURPOSE To identify buccal design criteria for retainer on polyamide resin removable partial dentures. METHODS A left upper distal extension model was used for this study. Undercut was given on the buccal side of first and second premolars. Four different retainer designs were created. Undercuts of 0.5 and 0.75 mm were used. All 4 retainer designs started from distal surface of second premolar. The retainer extended to Design 1: covering to the mesial surface of second premolar; Design 2: covering to the distal surface of first premolar; Design 3: covering to the mesial surface of first premolar; and Design 4: covering to the distal surface of canine. For the 0.75 mm undercut experiment, only Design 1 and Design 3 were used. Each experimental denture was pulled and the force maximum required to remove the denture measured. RESULTS On 0.5 mm undercut, no significant difference was found between Designs 1 and 2 or between Designs 3 and 4. Significant differences were found between all other combinations, however. On 0.75 mm undercut, a significant difference was found between the two. CONCLUSION The retentive force of a design for the buccal part of retainers in polyamide resin using large undercut was more effective than that of a design covering to the anterior abutment tooth.

Influence of width and depth of palatal vault on rigidity of palatal strap: A finite element study

PURPOSE The purpose of this study was to develop design criteria for palatal straps according to palatal shape. METHODS The palatal shapes of 60 dentate maxillary stone casts were measured using a contact-type three-dimensional measuring instrument. Based on the average of the 60 palatal shapes, 5 three-dimensional finite element models (FE models) were constructed: a basic FE model, a wide palatal FE model, a narrow palatal FE model, a deep palatal FE model and a shallow palatal FE model. The FE models were simulated unilateral free-end saddle dentures with the maxillary left second premolar and first and second molars missing. The FE models included a denture base, two rests and a palatal strap. The mechanical properties of Co-Cr alloy, Ti-6Al-7Nb alloy and type IV gold alloy were given to the FE models. A vertical load of 50 N was loaded onto the occlusal surface of the left first molar. Vertical displacement of the distal edge of the denture base was then compared between models. RESULTS Compared to the basic FE model, displacement in the wide FE palatal model and shallow palatal FE model was larger, and displacement in the narrow palatal FE model was smaller. The difference in displacement between the deep palatal FE model and basic FE model was small. CONCLUSIONS With a wide and shallow palate, it is necessary to design a reinforced palatal strap. With a narrow palate, the anteroposterior width of the palatal strap can be narrower. With a deep palate, it is not necessary to change the palatal strap design according to the palatal shape.