Hirokazu Amino

Needs of Bioceramics to Longevity of Total Joint Arthroplasty

Wear on alumina / UHMWPE-THP decreased by 25-30% of that on met al / UHMWPE in hip simulator test and clinical results. Wear on THP of alum ina / alumina was near zero in hip simulator test. In knee simulator test, UHMWPE wear against lumina decreased to 1/10 of that against metal. Clinically we have no revision case due to PE we ar problems for 23 years. In retrieved cases, UHMWPE surface against alumina was very s mooth. On UHMWPE surface against metal, many fibrils and scratches were found. In IBBC loosening in acetabulum occurred in 2.5% in 268 joints in only early cases at 16 to 14 years after THA. Only one joint was revised. At revision THA with massive bone defect, HA granules were filled. Socket migrations in two joints and partial spaces in two joints occurred in total 40 joints at 17 to 5 years. Bioceramics was found to be indispensable in enduring total joint arthroplasty . Introduction In order to keep the longevity of total joint arthroplasty, extrem ely low wear bearing materials and maintaining adequate fixability to the bone forever are desired. In our experimental and long term clinical experiences, it has been found that bioceramics, including bioinnert and bioactive ceramics, have been playing a maj or p rt in enduring total joint arthroplasty. The production of particulate wear debris from implant materials a nd subsequent osteolysis has been recognized as the major cause of long term failure in tot al hip replacement. The basic strategy to address the problem of osteolysis should be to reduce the number of polyethylene particles generated by improving the materials at the articulating count erfaces. The use of a ceramic femoral head has been advocated especially in young active patients because it produces less polyethylene wear compared with a conventional metal femoral head. However, an attempt to eliminate the use of polyethylene has been made through the use of metal-on-metal and ceramic-on-ceramic articulations. In 1970, to increase the wear resistance of polyethylene, wear te sts w re performed on RCH 1000 [ultrahigh molecular weight polyethylene (UHMWPE), molecular we ight, 106] irradiated at several levels of high-dose gamma radiation emitted by 60Co. The wear ra te was smallest at 100 Mrad. Sockets cross-linked by gamma radiation at 100 Mrad were used clinically from 1971 to 1978. We also experimentally confirmed that UHMWPE (molecular weig ht, 6 x 106) showed less wear in an alumina-on-UHMWPE combination than in the metal-on-UHMWPE combina tion. In 1977, we began to use 28 mm alumina balls. In our clinical experience, it was found that the thicker the poly ethylene socket, the lower the wear rate. To use a thicker UHMWPE sockets, the femoral head size was decreased with time: 26 mm alumina femoral heads were used from 1989 to 1994, 22 mm alumina femoral h eads were used from 1994 to 1995, and 22 mm zirconia femoral heads were used from 1995 to 1996. S ince 1996, Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 735-754 doi:10.4028/www.scientific.net/KEM.240-242.735

Wear, Friction, and Mechanical Investigation and Development of Alumina-to-Alumina Combination Total Hip Joint

Since 1962, when Charnley proposed and used ultrahigh-molecular-weight polyethylene as the material of choice for the low-friction hip joint, many clinical successes have been achieved. However, there are also many cases in which the excessive wear of particles of polyethylene can result in late aseptic loosening. To solve the critical issues resulting from polyethylene wear, a new all-alumina hip joint was developed by considering the design from the viewpoints of friction, wear, and mechanical strength. The all-alumina hip joint, with 28 mm head diameter, 5–35 itm radial clearance, <0.02 μm Ra surface roughness, <± 1 μm sphericity, and semispherical with polyethylene armored inlay design, has good friction and wear properties, as well as strength. By using the inlay with polyethylene liner for assembly in the metal shell and dispersing the load to the alumina inlay and metal shell, a reduction in stress is expected. Providing as much shock-absorbing capability as possible to an all-alumina hip joint appears to be required for safety.