It have been reported that the wear volume of vitamin E-containing UHMWPE tested with a knee joint simulator was approximately 30% lower than that of virgin UHMWPE at 5 million cycles. However, the wear resistance mechanism of vitamin E-containing UHMWPE has not yet been clarified. The present study examines the effects of the addition of vitamin E on the frictional properties of ultra-high molecular weight polyethylene (UHMWPE) under several different load and serum conditions.

Friction tests were carried out using a computer-controlled pin-on-disk friction test apparatus. The UHMWPE pin was mounted vertically at the tip of the leaf spring and linear reciprocating sliding motion for 2,000 cycles with an amplitude of 1 mm and a frequency of 1 Hz, was applied under 3 MPa or 30 MPa loading against Co-28Cr-6Mo alloy disk. The lubricant bath was filled with 5 ml of ultrapure water, fresh serum, post-friction (PF) serum or diluted-PF (DPF) which were kept at a temperature of 37°C. The friction force between the UHMWPE pin and the Co-28Cr-6Mo alloy disk was calculated from the displacement of the leaf spring during the sliding motion.

Vitamin E-containing UHMWPE showed a significantly higher friction force than that of virgin UHMWPE in fresh serum lubricant at 30 MPa loading, while there were little differences in either ultrapure water or PF serum or DPF serum. And vitamin E-containing UHMWPE tends to exhibit a lower dynamic friction force within the first few hundred cycles in the case of all serum lubricants at 30 MPa loading. These results suggest that some interaction between the UHMWPE surface and the native conformation proteins was specifically affected by the addition of vitamin E and that some weeping of vitamin E might occur at early stage of sliding. Our results also suggest the importance of the conformational changes of serum proteins for the wear testing.

Introduction: Fibroin sponge is purified silk protein from which high-strength gel sponge can be produced. The purified fibroin sponge causes no immune response. This study evaluates unique performances of the fibroin sponge for articular-cartilage regeneration, and mechanical properties of regenerated cartilage were also measured.

Methods: Refined silk yarn was dissolved in 9M lithium bromide aqueous solutions, and was frozen in −20& #8451 freezer for 12 hours. Hydrogel sponge was formed under the room temperature. Articular cartilage slices were taken from the proximal humerus, distal femur and proximal tibia of 4-week-old Japanese white rabbits. The cartilage slices cut into small pieces and were digested with 0.25% trypsin in DMEM containing antibiotics for 30 min at 37& #8451. After rinsing with Tyrode’s balanced solution and centrifuging at 180 G for 5 min, the chondrocytes were isolated with 0.25% collagenase for 8 h at 37& #8451. These cells were harvested and inoculated into the fibroin sponge. The constructs of the chondrocytes and the fibroin sponge were cultured in DMEM containing 10& #65285 FCS and 50mL L-ascorbate for 4weeks. Indentation test and dynamic visco-elastic measurement were carried out for these constructs.

Results and discussion: Cell density of the inoculated chondrocytes was increased to about five times as much as initial volume. This regenerated tissue was intensely stained with safranin-O fast green and showed a meta-chromatic reaction. This also stained positively with immunostain for type & #8545 collagen, but negatively with immunostain for type & #8544 collagen. Mechanical tests showed that time constants of compressive creep and E’ values were increased with cultivation days, and the peak value and frequency of tan& #948 shifted to a lower amount. The change in dynamic visco-elastic properties of the regenerated cartilage is caused by synthesis of extracellular matrix.