Katsunari Nishihara

Pressure Sintering of Apatite-Collagen Composite

Preparation of a composite consisting of apatite and collagen was attempted. Starting from an aqueous solution of collagen, phosphoric acid, and calcium hydroxide suspension, an apatite (90wt%)-collagen (10wt%) composite of 1.75 g/ml in apparent density, with 2 GPa in Youngs modulus, and 6.5 MPa in compression strength was synthesized at 40 degrees C, 200 MPa, successfully. It was found that the presence of liquid water in the system was essential for the sintering of the composite under high pressures. Without liquid water, the specimen that was pressure treated and brought in atmospheric pressure, broke into small pieces due to residual strain. The prepared composite could be cut by a razor blade, and was stable against immersion in water. The mean size of the apatite crystals in the composite was 10 nm in width and 40 nm in length.

Biomechanical studies on shape effect of hydroxyapatite artificial root upon surrounding jawbone

To study the shape effect upon surrounding tissue of artificial dental root made of synthetic hydroxyapatite, this paper deals with the numerical analysis of the artificial root on functioning jawbone with the aid of the finite element method (FEM). The stress distribution around artificial roots in the shape of a cylinder, a cone, and three types of corrugated cone, including the newly tailored type implanted in the mandibular molar region, was analyzed in the plane strain state. The numerical results showed that the stress distribution was sensitive to the artificial root shape, and that the stress state was distributed in mitigatory way around the roots of the newly tailored form. The pattern of osteogenesis in the animal experiment and the finite element analysis (FEA) pattern showed a close correlation. Osteogenesis was assumed to occur in the weak or moderate stress distribution zone. The principal stress trajectory pattern in the lamina dura around the tailored artificial root was indicated as being either parallel or normal to the root surface. From this study, the biomechanical property of the tooth can be identified as a vehicle of mastication forces which disperse stresses moderately and equally upon surrounding tissues. Also, the periodontal ligament can be identified as a converting system of principal stress trajectories.

Biomechanical studies on newly tailored artificial dental root.

Artificial roots must carry multiple forces during mastication. Stress distribution around a root depends upon the shape, material, and function of the root. Therefore, for biomechanical studies on artificial roots, triad research on the material, shape, and functional effect upon surrounding tissue is essential. For dental implants, there are two different functional systems against the masticatory force, i.e., gomphosis and ankylosis on osseointegration. Stress analyses of functioning new type (gomphosis) artificial roots were carried out in mandibular and maxilla models to study the triad effect using finite element analysis. The authors have already reported histological and biomechanical studies on the shape and functional effect. To observe the material effect biomechanically, artificial roots made of sintered hydroxyapatite and zirconium oxide were analyzed in the models. Thereafter, animal experiments using dogs were carried out to observe bone formation around artificial roots made of hydroxyapatite and zirconium oxide in the mandible and maxilla. The following results were obtained: The patterns of stress distribution around artificial roots of two different materials were not too different, and were exclusively dependent upon the root shape and structure of the jawbone. Around the artificial roots, bone formation coincided with a moderate stress distributing zone and principal stress trajectories. Through these experiments, the following conclusions were obtained: (a) Osteogenesis around artificial roots coincides with the stress distribution patterns. (b) Stress distribution patterns are dependent very little upon material properties but upon both the artificial root shape and the structure of the jawbone. (c) Optimization of the artificial root shape can be obtained by FEA in the models.

Biomechanical Research on Junction System of Bone with Biomaterials

Regarding the junction of bioceramics with original bone, which have quite different material constants of Youngs modulus and Poissons ratio from each other, synostosis (ankylosis) cannot be obtained under severe loading conditions. Therefore, it is necessary to introduce a new junction system for the interface between the biomaterial mechanical organ and original bone. The jointing system of dental root to jawbone reflects on the function against mastication. The interface between different mechanical organs with different materials necessitates a specific juncture system under severe loading because of the disparity of material constants. The authors already reported the result of studies on the shape effect of artificial roots in functioning jawbone by means of finite element analysis. Studies on the functional effect of artificial roots in undulated shape were carried out biomechanically by means of finite element analysis using models to investigate an effective juncture system between bone and biomaterials. The results of finite element analysis were compared with the findings obtained from histological specimens. To observe the juncture state of bioceramics with tubular bone cortex, tubular apatite artificial bone was implanted in the femur of a dog. From these studies, the following results and conclusions were obtained: (a) The fibrous juncture system around bioceramics has an important role, after which the principal stress trajectories are converted; and (b) optimal undulated morphology compatible to the artificial bones juncture system by means of fibrous ligament is essential for remodeling of the bone around the artificial skeletal bone.

DEVELOPMENT OF HYBRID TYPE ARTIFICIAL BONE MARROW USING SINTERED HYDROXYAPATITE

In vivo inducement of hybrid-type artificial bone marrow with hemopoietic inductive microenvironment (HIM) in sintered hydroxyapatite (HA) chamber was carried out. This research is important to disclose the mechanisms of hemopoiesis and is useful for clinical application. In the evolution of vertebrates, cartilage of the inner skeleton changed into bone, having biomechanical properties to form bone marrow cavities. The hemopoietic nests immigrated into the cavities from the spleen. We should be able to induce hemopoietic nests in a hydroxyapatite chamber in place of bone, if we can find optimal structural conditions. Therefore, we tried to artificially induce a hematopoietic field in muscles using sintered porous tubular hydroxyapatite and new type hydroxyapatite plate made by high-pressure gas technique. As a result, not only in the pore sites of tubular hydroxyapatite artificial bone, but at the surface of the new type hydroxyapatite plate implanted in the dorsal muscles, marked differentiation of bone marrow cell clusters of the hematopoietic field could be observed.

Application of Bioactive Ceramics for Functional Surgery in Masticatory Organs

Transplanted and replanted autogenous teeth or implanted artificial roots become easily ankylotic. However, if adequate functional loads are applied to them continuously postop with proper fixation, ankylosis can be prevented. The author found this fact through studies on autogenous tooth replantation 20 years ago. Therefore, not only can tooth transplantation, replantation, and artificial root implantation be done successfully with adequate functional loads, but surgical orthodontics as well as plastic operation of jaw deformity can also benefit in the same way. This therapeutic method is called functional surgery. The ankylotic process can be more effectively prevented by applying bioceramic granules. Using these methods, (a) 30 vital teeth in 23 patients were transplanted or replanted. Follow-up duration was 6 to 96 months. (b) Artificial roots made of compact sintered hydroxyapatite (apatite) were implanted under the new concept of functional surgery. In all, 57 artificial roots in 19 patients were implanted and observed for over 5 years. (c) Masticatory skeletal deformities of the viscerocranium were treated with the new concept of functional surgery applying sintered apatite. For this purpose, artificial bone implantation or simple gnathoplasty, tooth transplantation, and artificial root implantation was carried out in 15 patients, after which mastication training was applied with concomitant corrections of function inclination, i.e., oral-perioral habits. In these clinical experiments, excellent favorable results were obtained. The following conclusion was obtained: Introducing the new concept of functional surgery using bioceramics, not only tooth transplantation, as well as artificial root therapeutics, but deformity of the jaw can be steadily treated.