Shingo Masuda

Gamma-irradiated cross-linked polyethylene in total hip replacements — analysis of retrieved sockets after long-term implantation

The purpose of this study was to evaluate the mechanical and chemical characteristics of gamma-irradiated, cross-linked polyethylene after long-term service in vivo. Two gamma-irradiated ultra high molecular weight polyethylene (RCH 1000, molecular weight: 10(6)) total hip replacement sockets were retrieved at 15 and 16 years after implantation. Mechanical and chemical characteristics of the sockets were evaluated in comparison with nonirradiated sockets. Significant surface oxidation occurred in the nonirradiated sockets; up to 75% of that seen in the irradiated ones. The mechanical properties of the irradiated sockets were not subject to increased deterioration in the presence of high free radical content. The cross-link was stable and was retained for a long period both in vivo and in ambient air. These data indicate that gamma-irradiated polyethylene was not subject to increased oxidative degradation during long-term service in vivo and confirmed the usefulness of this material as an articulating surface in total hip replacement.

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

Repairing of Osteochondral Defects in Joint Using Beta-TCP/ Carboxymethyl Chitin Composite

Beta-tricalcium phosphate/carboxymethyl chitin composites [TCP/CMCh] of various ratios of TCP granules and CMCh were made and their mechanical properties, handling properties and repair performance for bone defects and for osteochondral defects were investigated. Water pooling ratio of CMCh was approximately 40 times the weight itself. TCP/CMCh of a higher TCP ratio had higher stress at 50%-strain. The stress at 50%-strain of TCP/CMCh with 0, 2.5, 5.0, 7.5, 10 TCP ratios was 0.12, 0.51, 1.08, 1.46, 1.67 (MPa, n=5), respectively. The TCP/CMCh with 5.0 TCP ratios had the best total scores in handling tests. The bone repair rate of TCP/CMCh was TCP ratio 2.5< Blank= TCP ratio 7.5< TCP ratio 5.0. In the implantation test for osteochondral defects, TCP/CMCh was completely absorbed at four weeks after surgery. Regeneration of the articular cartilage was seen with TCP/CMCh and HA/CMCh but not with TCP granules, which remained eight weeks after implantation. The regenerated articular cartilage had remained 32 weeks after implantation. In conclusion, it was demonstrated that this TCP/CMCh composite was a promising material for repairing osteochondral defects.

Low-Wear Effect of High-Dose Gamma-Irradiated Crosslinked Polyethylene in Total Hip Prostheses

The wear rate of crosslinked sockets irradiated in air with gamma-irradiation of 100 Mrad, which were used in total hip arthroplasty clinically between 1971 and 1978, was 18% of that of non-crosslinked sockets. As the sockets were irradiated in air, the surface was oxidized. The weight-bearing portion of retrieved crosslinked sockets presented a clearly outlined pattern, irregularly lined with smooth ripples of 0.1 um. Delamination, flaking, burnishing, and scratch were not observed. In our wear tests, using cylinder-on-flat, sphero-flat reciprocating, and hip simulator test, crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) irradiated with 50 to 1000 Mrad showed extremely low wear, even when the UHMWPE was irradiated in the air and the surface was oxidized. In our hip simulator test with 12 channels using 30% bovine serum as lubricant, crosslinked sockets irradiated with 50 to 150 Mrad, in which the oxidized surface was eliminated, showed no wear. After radiation, the mechanical properties of elongation and tensile strength of polyethylene decreased. The hardness and the ultimate tough strength increased. The chemical properties of gel contents, oxidation (C = O), and double bond (C = C) increased.

Retrieved Ceramic Total Knee Prosthesis in Clinical Use for 23 Years

According to the knee simulator test results in 1970s, the total decrease in thickness of UHMWPE tibial tray in combination with ceramic femoral component [F-Comp] was less than one tenth as that of the combination with metal [1]. These advantages led to development of total knee prosthesis [TKP] with alumina ceramics. In this study, we report the wear surface observation, the clinical wear and the oxidation of the retrieved TKP used clinically for 23 years, comparing with a metal TKP. The retrieved TKP was implanted in 1979, and retrieved on January 9th in 2002. This TKP consisted of an alumina ceramic F-Comp and a UHMWPE tray combined with a alumina ceramic tibial component. Observations of the surface of alumina F-Comp and UHMWPE tray were carried out using SEM. Shape of UHMWPE tray was determined three-dimensionally. Comparing the result with original shape based on the product’s plan, liner wear and volumetric wear were calculated. Oxidation index was determined by Fourier transform infrared spectrophotometry. Alumina F-Comp did not have any scratch on the surface by seeing with naked eye. UHMWPE tray had deformation and scratches obviously. The liner wear rate was 37 micrometer/year and volumetric wear rate was 18.8 mm3/year. The oxidation indexes were 0.6 in the unworn area, 1.2 in the worn area and 0.2 in the inner area. SEM observations of the F-Comp demonstrated no scratch or pit. In contrast, many scratches were clearly observed on the UHMWPE tray. However, higher magnification observations did not demonstrate severe wear, which was shown on the wear analysis of a metallic F-Comp. Oxidation degradation is a problem to solve. However, the low wear rate and mild wear pattern demonstrate that ceramic F-Comp reduced UHMWPE wear.

Functionally Graded Optical Polymer Materials Prepared Using UV-Curable Liquid-Crystals with an Electric Field

UV-curable liquid crystals (LCs) which have a nematic phase at room temperature can be photopolymerized by ultraviolet (UV) irradiation white holding the molecular orientation state in a nematic phase Liquid crystal optical devices with a refractive index distribution property are prepared using the novel LC material. After the graded index properties are produced by a nonuniform electric field, the LC is cured with the voltage and the molecular orientation properties in the LC cell are frozen by the UV irradiation; that is, the functionally-graded-polymer devices are formed. Optical polymer devices such as a microlens array and optical waveguide are easily prepared and demonstrated. Each polymer microlens in the lens array has the parabolic graded-refractive-index profile which shows an excellent focusing property. The polymer optical waveguide can also be prepared using a nonuniform electric field distribution produced by a slit patterned electrode structure and loss parameter is estimated for various waveguide structures.

Clinical Results for 19 to 21 Years on Total Hip Prosthesis with Ceramic Head Combined with UHMWPE Socket

In total hip prosthesis (THP), we have been using alumina ceramic femoral heads to reduce polyethylene debris and the modified cementing technique, which was named “interface bioactive bone cement (IBBC)” method to improve implant fixation. In this study, we investigated the long-term clinical performance of THPs with an alumina ceramic head and IBBC method. From 1986 to 1988, 285 joints (215 patients) were operated on by a senior surgeon, and 265 joints (192 patients) could be followed up. The presence of radiolucent lines, loosening, osteolysis and ultrahigh-molecular-weight polyethylene (UHMWPE) socket wear were observed. In IBBC, a radiolucent line appeared as a ’space’, and loosening appeared as a ’separation’ between the HA layer and the cement. A ‘space’ appeared in 3 joints (1.4%) on the acetabulum and in 4 joints (1.8%) on the femur, while a ‘separation’ appeared in 3 joints (1.4%) on the acetabulum. Osteolysis was noted in 1 joint (0.5%) on the acetabulum and in 2 joints (0.9%) on the femur. No revision surgery was required. In our previous study, we reported that the thickness of the socket affected its clinical wear rate and that the wear rate of sockets with an alumina ceramic head was 20% lower than that of sockets with a metal head. By reducing wear debris through the use of ceramic heads, osteolysis could be reduced. Long-term fixation of THPs to the bone has been achieved by using IBBC. Thus, the long-term clinical results of THPs with an alumina head that were fixed using IBBC were excellent.