Junji Ikeda

Mechanical and Phase Stability of Zirconia Toughened Alumina

The JMM-ZTA, a kind of zirconia-toughened alumina, is a bearing ceramic newly developed for artificial joints. Although it has already been reported that mechanical strength and fracture toughness are higher in the JMM-ZTA than in alumina, the stability of the JMM-ZTA has not been studied in detail yet. In the present study, the stability of the JMM-ZTA with respect to mechanical strength and crystalline phases was examined under hydrothermal environment, and the results were compared with those of alumina. Both the 4-point bending strength and the fraction of monoclinic zirconia unchanged even after the aging test at 121°C for 300 h. These results indicate that the JMM-ZTA possesses quite excellent stability to be used as bearing material of artificial joints.

Phase Stability of Zirconia Toughened Alumina Composite for Artificial Joints

The phase stability under aqueous condition and changes in the wear region after the Pin-on-Flat wear testing were evaluated using Raman and fluorescence spectroscopy techniques and X-ray diffractometry for a newly developed zirconia toughened alumina (JMM-ZTA). This study suggests that JMM-ZTA is a highly stable material under aqueous environment in the human body and has a high wear resistance in Pin-on-Flat wear tests. This results from a transformation toughening mechanism operative in JMM-ZTA. Therefore, JMM-ZTA appears to have a possibility to improve the performance of monolithic alumina as a material for hip and knee joint prostheses.

Influence of Complex Frictional Heating and Mechanical Stress Effect for Surface Crystalline Structure Change of Zirconia Femoral Head

Several clinical reports pointed out that the monoclinic transformation of crystalline structure of zirconia femoral head with clinical service time can cause the changes of surface roughness and mechanical properties. To elucidate the relationship between these surface roughness change induced by phase transformation and UHMWPE socket wear, hip simulator wear tests were performed on total hip prostheses (THP) up to 6 million cycles running. Four groups of different surface monoclinic phase fraction of Kyocera zirconia heads were prepared by aging treatment prior to the wear tests. The surface roughness/ wear rate of zirconia heads with 0, 10, 50 and 80mol% monoclinic phase fraction were 0.009/ 3.8, 0.014/ 3.9, 0.012/ 3.7 and 0.010µmRa/ 3.1mg/million cycles, respectively. These results exhibit that the surface roughness change induced by phase transformation does not significantly influence extent of the generation of UHMWPE debris in Kyocera zirconia /UHMWPE THP system. In contrast, the surface roughness exhibited complex relation with the monoclinic phase fraction and therefore the morphological analysis was performed on the zirconia articulating surfaces

Characteristics of Low Temperature Degradation Free ZTA for Artificial Joint

Low temperature degradation free Zirconia toughened alumina (ZTA) has been developed. It is reported that ZTA has higher mechanical strength compared to alumina due to the stress induced transformation and spontaneously transformation of zirconia phase on some ZTA have been occurred. For achieving the higher reliability of artificial joint prosthesis alternative to alumina and other ceramic materials, it is necessary to improve and validate the both mechanical characteristics and phase stability at the same time. We evaluated that microstructure, mechanical characteristics and phase stability of newly developed ZTA (BIOCERAM® AZUL). It was confirmed that four-point bending strength and weibull modulus were extreamly high, and ZTA has higher reliability. There were no significant changes and deterioration in four-point bending strength, crystal structure and wear property with and without accelerated aging test. Newly developed ZTA not only with high mechanical characteristics but also with phase stability could be quite useful as bearing materials in artificial joints for longer clinical use.

Phase Stability and Residual Stress Field in Nano-Structured 3Y-TZP

Phase stability of 3Y-TZP was evaluated by acceleration test in water vapor environment and Raman spectroscopy. We found that the phase stability was affected not only by grain size but also by residual stress field, which could be quantitatively measured by Raman piezo-spectroscopy and controlled by sintering temperature. This study shows that Raman spectroscopy is a very useful technique for non-destructive analysis of 3Y-TZP.

Confocal Raman Spectroscopic Analysis of Ceramic Hip Joints

A survey of confocal Raman/fluorescence microprobe spectroscopic techniques is presented with emphasis placed on surface analysis of artificial hip joints. Suitable instrumental configurations are first explained in some details in order to describe the versatility of the spectroscopic microprobes to biomedical materials analyses. Then, these notions, which represent the foundation for structural and mechanical analyses of joint surfaces, are applied to selected cases of paramount importance in hip arthroplasty.

Spatially Resolved Raman Spectroscopy for In-Depth Non-Destructive Residual Stress and Phase Transformation Assessments in 3Y-TZP Bioceramics

A non-destructive assessment of phase transformation and residual stress is presented for a 3 mol.% Y2O3 added ZrO2 ceramic using Raman microprobe spectroscopy. Low CIP pressure has been selected in the sample procedure to increase a potential to transform ZrO2. Aging tests were made and the transformation depth and residual stresses caused by transformation were evaluated by Raman spectroscopy A Raman microprobe technique using a visible wavelength laser coupled with a confocal optical device may enable one to retrieve spatially resolved information along the material subsurface. To demonstrate the potentiality of the confocal technique, aging of a ZrO2 sample has been made in autoclave and phase transformation gradually promoted from the surface towards the sub-surface of the sample (up to ～60 µm, in a sample autoclaved 168 h). Then, a quantitative spatially resolved assessment was attempted on these samples from their surface. The confocal information from the subsurface was compared with results of Raman spectroscopy collected from a cross-section. Accordingly, a quantitative equation was proposed, which allows the quantitative assessment of the thickness of the surface layer, which underwent phase transformation in ZrO2 ceramics, according to in-depth non-destructive assessments.

Characterization of Structural and Chemical Properties of Silver Containing Hydroxyapatite Synthesized by Solid Phase Reaction

In this study, we investigated the physical and chemical state changes of silver in silver-containing hydroxyapatite (AgHA) composites sintered at various heat treatment temperatures. AgHA composites were synthesized by a solid phase reaction at various temperatures using composite powder prepared by ball-milling commercial highly crystalline hydroxyapatite powder and commercial silver oxide powder. Structural characterization of AgHA was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray absorption near edge structure (XANES). The XRD measurement showed that peaks of metallic silver or other silver compounds disappeared at 800°C and reappeared at 1000°C. The a-axis of HA obtained by Rietveld analysis expanded drastically at 800°C. Furthermore, the SEM images showed that silver particles disappeared at 800°C and reappeared at 1000°C, as indicated by the XRD patterns. The XANES spectrum of Ag-L3 edge of AgHA sintered at 800°C was different compared to those of other references. These results suggested that calcium ions were substituted by silver ions in AgHA sintered at 800°C, according to the a-axis expansion of HA lattice caused by the substitution of hydroxyl ions by carbonate ions through the heat treatment, based on the FT-IR measurement. In addition, antibacterial tests showed that AgHA containing various chemical states of silver had good antibacterial activities in proportion to released amounts of silver.

Long-Term Reliability Assessment of Ceramic Femoral Head Based on Microfracture Analysis Using Acoustic Emission Technique

Damage accumulation during compression test of alumina femoral head for artificial hip joint was monitored by acoustic emission (AE) technique. Since a number of mechanical noises due to the friction disturbed AE measurement during the test using conventional apparatus, the testing apparatus was modified. Using the modified apparatus, rapid increase in AE energy was observed before the final fracture. This would be correlated to the maincrack formation which leads to the final fracture. Fracture strength and critical stress for maincrack formation were also computed by FEM. Both exhibited smaller values than bending strengths, which might be caused by the size effect of strength. Finally, reliability assessment based on AE technique for ceramic femoral head was demonstrated.

Kinetics of the Phase Transformation of Non-HIPed Zirconia Femoral Heads

The kinetics of tetragonal-to-monoclinic phase transformation (t→m transformation) in the earlier generation zirconia femoral heads was evaluated by X-ray diffractometry, laser microscopy and Raman microprobe spectroscopy. From previous results of hip-simulator study, it was confirmed that phase transformation on the surface of zirconia femoral heads had little influence on wear rate of UHMWPE sockets, and in some zirconia femoral heads, only a slight increase in monoclinic fraction was observed during hip-simulator test. In this study, we suggest that the models of phase transformation progress during tests in hip-simulator and aging tests are different based on both laser microscopic and Raman/fluorescence spectroscopic observation. Besides this finding, this study shows that Raman spectroscopy is a useful technique for the evaluation of the kinetics of phase transformation in femoral heads after both in vitro and in vivo environmental exposure.