Mikio Iwamoto

Ceramic Versus Cobalt-Chrome Femoral Components; Wear of Polyethylene Insert in Total Knee Prosthesis

The present study aimed to determine the effect of femoral component materials and sterilization methods on wear properties of total knee prostheses by using a knee simulator test and retrieval analysis. The simulator test revealed that ultrahigh molecular weight polyethylene (UHMWPE) inserts had remarkably lower wear against the ceramic femoral component than against the Co-Cr femoral component. However, the retrieval study revealed no significant difference in the linear wear between the former and the latter. The alumina ceramic/UHMWPE insert combination showed a mild wear. However, whether cross-linking by gamma-ray sterilization reduces wear remained unconfirmed. In contrast, oxidative degradation and/or delamination was confirmed. Thus, we conclude that alumina ceramic/ethylene oxide gas-sterilized UHMWPE insert in a total knee prosthesis might exhibit a good wear resistance.

The Characteristics of the New Ceramic Material for Artificial Joints

A new zirconia-toughened alumina ceramic (JMM-ZTA) for a ceramic-on-ceramic hip replacement has been developed. The JMM-ZTA has a fracture toughness superior to zirconia, and the JMM-ZTA/JMM-ZTA combination has superior wear-resistant performance to alumina/alumina. Moreover, the JMM-ZTA shows an excellent crystalline phase stability. Therefore, the JMM-ZTA is believed to be useful for artificial hip prostheses.

Radiographic and retrieval wear analyses of the first generation highly cross-linked polyethylene cup against a ceramic femoral head

In this study, the in vivo wear of highly cross-linked polyethylene (CLPE) cups against alumina ceramic femoral heads was evaluated by radiographic and retrieval analysis. The radiographic wear of six ethylene oxide gas-sterilized (i.e., non-cross-linked) conventional polyethylene (PE) cups with the mean follow-up of 20.9 years and 60 CLPE cups with the mean follow-up of 7.4 years was measured. The retrieved 16 PE cups with clinical use for mean 21.5 years and 10 CLPE cups with clinical use for mean 2.9 years was evaluated as a retrieval analysis. In the radiographic analysis, the linear wear of CLPE cups was significantly lower (99% reduction) compared to conventional polyethylene cups. The results of retrieval analyses for both cups were similar to those of radiographic analyses. Even when third-body wear occurred during clinical use, no surface damage was observed on the surface of ceramic femoral heads. The surface is not sensitive to third-body wear, and hence, the ceramic femoral head has a great advantage in terms of the wear of CLPE under third-body wear conditions. In conclusion, CLPE cups used with alumina ceramic femoral heads in total hip arthroplasty should have favorable wear resistance in several in vivo situations.

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.

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.

Ceramic Total Knee Prosthesis with Smooth Surface Keeps High Wear Resistance in Long Clinical Use

In the late 1970s, based on good clinical results in total hip prostheses, the use of a combination of alumina ceramics and ultra-high molecular weight polyethylene (UHMWPE) was begun for total knee prostheses (TKPs) in order to reduce UHMWPE wear. In this study, to examine the in vivo efficacy of the alumina ceramic bearing surfaces of TKPs, we compared retrieved alumina ceramic TKPs with cobalt-chrome (Co-Cr) alloy TKPs by surface observations and linear wear measurements. In scanning electron microscopic observations, many scratches due to clinical use were observed only on the retrieved Co-Cr alloy femoral components. The damage in the form of scratches on the articulating surface was linear and was produced by rubbing against the Co-Cr alloy surface. The linear wear rate of the retrieved Co-Cr alloy TKPs was 0.027–0.358 mm/year. In contrast, the wear of the retrieved alumina ceramic TKPs was stably low and linear; the linear wear rate was estimated to be 0.026 mm/year. The lower wear rate and milder nature of wear observed in the TKPs with the combination of UHMWPE inserts and alumina ceramic femoral components suggest the possibility of the retention of high performance even during prolonged clinical use.

Wear and Corrosion Resistance of Low Temperature Degradation Free ZTA for Artificial Joint

Low-temperature-degradation-free (LTD-free) zirconia-toughened alumina (ZTA) has been developed as an alternative to alumina and zirconia. It has been reported that ZTA has more beneficial mechanical characteristics than alumina, because of the stress-induced transformation and the spontaneous transformation of the zirconia phase that occurs for some ZTA in a hydrothermal environment. To achieve high-reliability artificial-joint prostheses, it is necessary to improve both the mechanical characteristics and phase stability of the joint material simultaneously. Thus, in this study, we evaluate the wear characteristics and corrosion resistance of LTD-free ZTA using a hip wear simulator, so as to confirm the long-term stability and reliability of this material in clinical use. The ZTA with advantageous mechanical characteristics used in this study has extremely high wear and corrosion resistance. Therefore, this ZTA could be quite useful as a bearing material in artificial joints for longer-term clinical use. It is also confirmed that the newly developed LTD-free ZTA is extremely stable under sliding conditions. Thus, it can be expected that the initial superior surface characteristics will be maintained through long-term clinical use.

29 to 24 Year-Clinical Results of Total Hip Arthroplasty Cemented with HA by Interface Bioactive Bone Cement (IBBC)

To improve the fixation of bone and bone cement in THA we used Interface Bioactive Bone Cement technique (IBBC) since 1982. For prostheses 28 mm-alumina head with polyethylene socket (Kyocera Co.) were used. In IBBC not-resorbable pure crystalline HA granules (0.3– 0.5 mm) were smeared on the bone surface in one to three layers just before packing bone cement. Group 1: IBBC was used in six joints in 1982. Group 2: In 25 patients, conventional bone cement technique (non-IBBC) in the one hip (in 1985 to 1986) and IBBC technique in the other hip (in 1986) were performed in the same patients. Group 3: In 65 patients (71 joints) IBBC were performed. Neither radiolucent line nor osteolysis appeared in all case in group 1. In group 2 in Non-IBBC the appearance rate of radiolucent line and osteolysis were rather high, however in IBBC they were extremely low. In Non-IBBC loosening were seen in 5 joints in the acetabulum and in 2 joints in the femur. However, in IBBC there was no loosening. In group 3 the radiolucent line and the osteolysis appeared extremely low. There was no loosening. In Non-IBBC the appearance rate of radiolucent line and the osteolysis was very high and the rate of loosening was 20%. However, in IBBC, radiolucent line and osteolysis will be prevented eternally.

Clinical Experiences of Ceramic Total Knee Arthroplasty More than 26 Years

In the late 1970s, we started the clinical use of total knee prostheses (TKPs) composed of alumina ceramic. In this study, we investigated the long-term clinical performance of ceramic TKPs. First-generation ceramic TKPs were used between 1981 and 1985; second-generation TKPs, between 1990 and 1996 and third-generation TKPs, between 1993 and 1998. We examined the findings of clinical radiographic observation. A total of 137 first-generation ceramic TKPs were followed up for 20–23 years after implantation. All the rates of loosening, sinking and revision were higher with cementless fixation than with cemented fixation. In the second- and third-generation TKPs, all the components were implanted using bone cement. In 249 joints that were followed up for 6–14 years, neither loosening nor sinking was observed. No osteolysis was observed in any case. We compared the wear of metal TKPs and ceramic TKPs that were retrieved after long-term use. Metal TKPs exhibited a higher wear rate than ceramic TKPs, and they also exhibited scratched surface damages. The lower wear rate and considerably less surface damage observed in our previous study suggest the long-term durability and performance of ceramic TKPs.