How useful is SBF in predicting in vivo bone bioactivity

Ceramics used for the repair and reconstruction of diseased or damaged parts of the musculo-skeletal system, termed bioceramics, may be bioinert (alumina, zirconia), resorbable (tricalcium phosphate), bioactive (hydroxyapatite, bioactive glasses, and glass-ceramics), or porous for tissue ingrowth (hydroxyapatite-coated metals, alumina). Applications include replacements for hips, knees, teeth, tendons, and ligaments and repair for periodontal disease, maxillofacial reconstruction, augmentation and stabilization of the jaw bone, spinal fusion, and bone fillers after tumor surgery. Carbon coatings are thromboresistant and are used for prosthetic heart valves. The mechanisms of tissue bonding to bioactive ceramics are beginning to be understood, which can result in the molecular design of bioceramics for interfacial bonding with hard and soft tissues. Composites are being developed with high toughness and elastic modulus match with bone. Therapeutic treatment of cancer has been achieved by localized delivery of radioactive isotopes via glass beads. Development of standard test methods for prediction of long-term (20-year) mechanical reliability under load is still needed.

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Bioceramics: From Concept to Clinic

Bioceramics : from concept to clinic

High-strength bioactive glass-ceramic A-W was soaked in various acellular aqueous solutions different in ion concentrations and pH. After soaking for 7 and 30 days, surface structural changes of the glass-ceramic were investigated by means of Fourier transform infrared reflection spectroscopy, thin-film x-ray diffraction, and scanning electronmicroscopic observations, in comparison with in vivo surface structural changes. So-called Tris buffer solution, pure water buffered with trishydroxymethyl-aminomethane, which had been used by various workers as a "simulated body fluid," did not reproduce the in vivo surface structural changes, i.e., apatite formation on the surface. A solution, ion concentrations and pH of which are almost equal to those of the human blood plasma--i.e., Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+ 2.5, Cl- 148.8, HCO3- 4.2 and PO4(2-) 1.0 mM and buffered at pH 7.25 with the trishydroxymethyl-aminomethane--most precisely reproduced in vivo surface structure change. This shows that careful selection of simulated body fluid is required for in vitro experiments. The results also support the concept that the apatite phase on the surface of glass-ceramic A-W is formed by a chemical reaction of the glass-ceramic with the Ca2+, HPO4(2-), and OH- ions in the body fluid.

Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W.

Titanium and titanium alloys are widely used in biomedical devices and components, especially as hard tissue replacements as well as in cardiac and cardiovascular applications, because of their desirable properties, such as relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance, and biocompatibility. However, titanium and its alloys cannot meet all of the clinical requirements. Therefore, in order to improve the biological, chemical, and mechanical properties, surface modification is often performed. This article reviews the various surface modification technologies pertaining to titanium and titanium alloys including mechanical treatment, thermal spraying, sol–gel, chemical and electrochemical treatment, and ion implantation from the perspective of biomedical engineering. Recent work has shown that the wear resistance, corrosion resistance, and biological properties of titanium and titanium alloys can be improved selectively using the appropriate surface treatment techniques while the desirable bulk attributes of the materials are retained. The proper surface treatment expands the use of titanium and titanium alloys in the biomedical fields. Some of the recent applications are also discussed in this paper.

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Surface modification of titanium, titanium alloys, and related materials for biomedical applications

The average electronic oxide polarizability α02− of numerous single component oxides has been calculated on the basis of two different properties: linear refractive index n0 and energy gap Eg, which have demonstrated remarkable correlation. The optical basicity Λ of the oxides has been estimated on the basis of average electronic oxide polarizability calculated from the refractive index Λ(n0) and the energy gap Λ(Eg). A good agreement has been observed between the optical basicity data obtained using independent initial quantities. The simple oxides have been separated into three groups according to the values of their oxide polarizability. The α02− values (above 3 A) obtained for PbO, Sb2O3, and Bi2O3 have been attributed to the high cation polarizability and the presence of a lone pair in the valence shell of the cation.

Electronic oxide polarizability and optical basicity of simple oxides. I

Glass-ceramic A-W, containing crystalline apatite and wollastonite in a MgO-CaO-SiO2 glassy matrix shows high bioactivity as well as high mechanical strength, but other ceramics containing the same kinds of crystalline phases in different glassy matrices do not show the same bioactivity. In order to investigate the bone-bonding mechanism of this type of glass-ceramic, surface structural changes of the glass-ceramics after exposure to simulated body fluid were analyzed with various techniques. A solution with ion concentrations which are almost equal to those of the human blood plasma was used as the simulated body fluid, instead of Tris-buffer solution hitherto used. For analyzing the surface structural changes, thin-film x-ray diffraction was used in addition to conventional techniques. It was found that a bioactive glass-ceramic forms a Ca, P-rich layer on its surface in the fluid but nonbioactive ones do not, and that the Ca, P-rich layer consists of carbonate-containing hydroxyapatite of small crystallites and/or defective structure. These findings were common to those of Bioglass-type glasses. So, we conclude that the essential condition for glass and glass-ceramic to bond to bone is the formation of the surface apatite layer in the body environment but it is not essential to contain apatite within the material. Bioactivity of glass and glass-ceramic can be evaluated in vitro by examining the formation of the surface apatite layer in the simulated body fluid described above.

Ca, P‐rich layer formed on high‐strength bioactive glass‐ceramic A‐W

For the purpose of analyzing the crystallization process of glass by DTA, the modified Kissinger-type equation was derived on the basis of the nucleation and growth equations, and the validity of the equation was ascertained by applying to the crystallization of Li 2 O·2SiO 2 glass whose kinetic data regarding crystallization are already well-known. The modified Kissinger-type equation is identical with the so-called Kissinger equation only when the crystallization starts at the surface and grows towards the inside of the glass one-dimensionally. The conditions required for applying the Kissinger plot or modified Kissinger-type plot to the crystallization of glass were discussed, and it was concluded that the crystallization mechanism should be known in order to obtain the meaningful activation energy.

Kinetic study of crystallization of glass by differential thermal analysis—criterion on application of Kissinger plot

A suitable relationship between the linear refractive index n0, the energy gap Eg, and the nonlinear refractive index n2 has been looked for on the basis of experimental and theoretical data reported in the literature for numerous simple oxides. It has been established that the nonlinear refractive index increases with increasing linear refractive index and decreasing energy gap of the oxides. This is related to the increasing metallicity of the oxides. Oxides with a high nonlinear refractive index posses a metallization criterion of approximately 0.30–0.45.

Sumio Sakka

Papers

Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W.

Electronic oxide polarizability and optical basicity of simple oxides. I

Ca, P‐rich layer formed on high‐strength bioactive glass‐ceramic A‐W

Kinetic study of crystallization of glass by differential thermal analysis—criterion on application of Kissinger plot

Linear and nonlinear optical properties of simple oxides. II