Papers by Author: Keiichi Sasaki

Abstract: The dental implants have been increasingly used for replacing missing teeth over the past three decades. However, its mechanical strength remains a major concern to dental clinicians and patients. To investigate failure modes and fracture loads of implant-supported fixed partial bridge, eXtended Finite Element Method (XFEM) was employed in this study. The 3D finite element (FE) models simulating full mandibular teeth and implant-supported three-unit fixed partial denture (FPD) were developed to determine the crack initiation and propagation in the dental prostheses. The failure modes and fracture loads are compared for three typical treatment scenarios: namely three-unit FPD supported by two implants at left second premolar and first molar (named as cantilever Model-IIP), second premolar and second molar (bridge Model-IPI), and first and second molars (cantilever Model-PII). The XFEM analyses show that the bridge Model-IPI exhibited considerably high fracture resistance than the other two configurations. Model-IIP displays the worst fracture strength of these three case scenarios. The results provide a basis for clinical assessment of mechanical strength for implant-supported FPD or other restorative devices.

Abstract: This study aimed to evaluate the biomechanical responses in the peri-implant bony structure installed with the fixed partial dentures (FPDs). Unlike traditional configuration, the FPD considered here comprises a superstructure and is supported by three implants. The computational model of mandibular bone and the implant prosthesis were constructed based on patient-specific computerized tomography (CT) images and Computer Aided Design (CAD) tools. To better reflect the real clinical situation, the 3D real-time loading data of maximum voluntary clenching measured using piezo-electric force transducers in patient were adopted in the 3D finite element (FE) analyses (FEA). The von Mises equivalent stress, maximum shear stress, equivalent strain and strain energy density in the peri-implant bone regions are quantified. The peak stresses and strains in the peri-implant bone were observed around the neck of the implant, indicating risk of micro-motion and bone resorption. In this study, we successfully conducted a computational simulation in silico based on in vivo 3D force measurement of a specific patient. The results provided important biomechanical data for clinical treatment, potentially helping enhancing the longevity and reliability of the implant-supported FPD restoration.

Abstract: This study aims to analyze the functional contact pressure induced by Removable Partial Denture (RPD) by using a 3D finite element (FE) model constructed based on patient specific CT scans. This model was validated against the in vivo test results. The outcomes demonstrate that the finite element simulation has the capability of quantifying localized stress distribution in a complicated denture-mucosa contact problem, with a reasonable matching to clinical measurements of occlusal force and pressure distribution. The methodology is of considerable clinical implication to improve the long term outcomes of the denture treatment.

Abstract: This study aimed to create a thick hydroxyapatite (HAp) film on the surface of a human tooth by using a newly developed powder jet deposition (PJD) device for dental handpieces, and sought to examine the microstructural and mechanical properties of the resulting HAp film. The film was evaluated on three-dimensional view, surface roughness, Vickers hardness, and bonding strength before and after artificial aging through thermal cycling (555°C) for 500 cycles (30 sec for each cycle, 20 sec of dwell time).The HAp particles in the deposited film were densely packed, and the HAp films three-dimensional microstructure and its rough surface were maintained after thermal cycling. There was no significant difference in either the HAp films Vickers hardness or the bonding strength between the film and the enamel substrate before and after thermal cycling. The HAp films created in this study demonstrated excellent microstructural and mechanical properties even after the application of thermal stress. We demonstrated the possibility of using a new type of powder jet deposition (PJD) method we developed to form a new type of interface between the tooth and biomaterials. Consequently, we propose the use of this method in new dental treatments.

Abstract: The surface oxide films were prepared by Electron Cyclotron Resonance (ECR) plasma oxidation on Ti substrates. Octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) peaks were formed after calcification by supersaturated calcium and phosphate solutions. Calcification ability was enhanced with increasing the oxidation time and the total pressure of ECR plasma treatment during oxidation. The results demonstrated that the calcium phosphate nucleation and the deposition can be controlled by various ECR plasma conditions.

Abstract: Surface titanium oxide (TiO2) films were fabricated on implant titanium (Ti) at low temperatures by electron-cyclotron-resonance (ECR) plasma oxidation. The relationship among the oxidization conditions, crystal structure, morphology and osteoconductive property were investigated. Although crystallized TiO2 film was not prepared by thermal oxidation at 300°C, crystallized rutile-type TiO2 film was formed by ECR plasma oxidation at 300°C. Rough morphology was observed in the substrate surface oxidized by ECR plasma. Mixtures of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) were observed after calcification. The XRD peak intensities of the OCP and DCPD formed on the ECR plasma oxidized Ti were larger than those of calcified on the thermal oxidized Ti. ECR plasma oxidation at low temperature would induce osteoconductive calcium phosphate on implant Ti.

Abstract: The present study was designed to investigate whether porous titanium (Ti) having Young’s modulus similar to bone has osteoconductive characteristics in rat critical-sized calvarial bone defect. The effect of coating by octacalcium phosphate (OCP) was also examined. OCP is known as a precursor of initial mineral crystals of biological apatite in bones and teeth. Ti powder was prepared by plasma rotating electrode process in an Ar atmosphere. Then, porous Ti disks, 8 mm in diameter with 1 mm thick, were obtained using the particles ranging from 300 to 500 +m, by sintering at 1573 K without applied pressure. The disks had about 35 vol% in porosity and about 10 GPa in Young’s modulus which corresponds to that of human cortical bone. Newly formed bone was observed so as to fill the pore up at 12 weeks, confirming the ability to conduct the ingrowths of the bone tissue. Although in vitro study showed that proliferation of mouse bone marrow stromal ST-2 cells was inhibited on the dishes coated by OCP rather than the control dish, OCP coating on porous Ti seemed to stimulate the bone formation in vivo. Taken together, it seems likely that porous Ti having Young’s modulus similar to bone shows osteoconductive characteristics to conduct bone ingrowths. OCP could be a potential coating agent to assist bone regeneration on porous Ti.

Abstract: Titania (TiO2) thin films were fabricated on titanium (Ti) substrates at low temperatures by electron cyclotron resonance (ECR) plasma oxidation, and the relationship among the oxidization conditions, crystal structure and osteoconductive property was investigated. Amorphous TiO2 films were obtained below 300°C and crystallized rutile-type TiO2 films were obtained above 400°C. The XRD peak intensity of rutile TiO2 increased with increasing oxidation temperature. Mixtures of octacalcium phosphate (OCP) and Dicalcium phosphate dihydrate (DCPD) peaks were observed after calcification. The intensity of the OCP and DCPD peaks after calcification increased with increasing oxidation temperature. The ECR plasma was significantly effective to prepare crystallized TiO2 films at low temperatures.

Abstract: It has been shown that fluoride ions enhance OCP hydrolysis into Ca-deficient apatite and that fluoridation in hydroxyapatite (HA) affects osteoblast activity. The present study was designed to investigate whether fluoridated Ca-deficient apatite (F-HA) formed via OCP enhances bone regeneration. F-HA was obtained through hydrolysis of the OCP in a solution containing 2 ppm fluoride at 37 °C and pH 7.4. A standardized critical-sized defect was made in the rat calvarium, and granules of F-HA were implanted into the defect. Five rats from each group were fixed through four to twelve weeks after implantation. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) confirmed that F-HA corresponded well to apatite structure. In week four, new bone matrix was formed around F-HA. In week twelve of F-HA group, newly formed bone matrix was more abundant, whereas the implanted F-HA was unresorbed and still remained. A statistical analysis in week twelve showed that the newly formed bone in the defect with F-HA was higher than that with untreated group. The fact that new bone was directly formed on F-HA implant suggests F-HA formed via OCP could be used as a bone substitute material.

Abstract: Nitrogen-doped and -undoped Ti powders were obtained by Plasma Rotating Electrode Process (PREP) in an Ar atmosphere using Ar-2 vol.%N2 and pure Ar, respectively, for plasma flame. Auger electron spectroscopy (AES) revealed that nitrogen is enriched at the surface of nitrogen-doped Ti powder and decreases along with the depth direction. Microstructure of a compact prepared by hot pressing the nitrogen-doped Ti powder shows a shell structure. The outer area is considered to form by the diffusion of nitrogen. This microstructure disappears after annealing at 1373 K for 3.6 ks. It is found that the compressive yield strength of porous Ti compacts can be improved by introducing nitrogen in Ti powder and is superior to that of human cortical bone.