Nano-Level Surface Texturing on Co-Cr-Mo Alloy Inhibits Macrophage Activation in Joint Prostheses

Abstract

An ultra-high molecular weight polyethylene (UHMWPE) is widely used as bearing material in artificial joints, however, UHMWPE wear particles are considered to be a major factor in long-term osteolysis and loosening of implants. The wear particles activate macrophages, which release cytokines, stimulating osteoclasts, which results in bone resorption. The biological activity of the wear debris is dependent on the volume and size of the particles produced. Many researchers reported that the volume and size of particles were critical factors in macrophage activation, which particles in the size range of 0.1–1 mm being the most biological active.

To minimize the amount of wear of UHMWPE and to enlarge the size of UHMWPE wear particle, a nano-level surface texturing on Co-Cr-Mo alloy as a counterface material was invented. Although the generally-used surface for a conventional artificial joint has 10 nm roughness (Surface A), the nano-level textured surface invented has a superfine surface of 1 nm with 3% of groove and dimples against the bearing area. The depths of groove and dimples are less than 50 nm (Surface F).

Pin-on-disc wear tester capable of multidirectional motions was used to verify that the nano-textured surface is the most appropriate for artificial joint. UHMWPE pin with an average molecular weight of 6.0 million was placed in contact with the disc and the contact pressure was 6.0 MPa. The disc and pin were lubricated by a water-based liquid containing the principal constituents of natural synovial fluid. Sliding speed of 12.12 mm/s had been applied for total sliding distance of 15 km.

The superfine surface with nano-level grooves and dimples (Surface F) reduced the amount of UHMWPE wear, this would ensure the long-term durability of artificial joint. The wear particles isolated from lubricating liquid were divided broadly into two categories; one is “simple type” and the other is “complicated type”. The lengths in a longitudinal direction (L_l) and its orthogonal direction (L_s) for each particles (>150) were measured, and the each aspect ratio (= L_l/L_s) was calculated. No significant difference was found in the ratio between simple type and complicated type, and in the distributions of aspect ratios. However, the distributions of L_l, which means the size of UHMWPE wear particle, were dramatically changed by using the nano-textured surface (Figure 2). These results suggest that the nano-textured surface does not change the morphological aspect of UHMWPE particle but enlarges the size of UHMWPE particle.

Cells (RAW264.7, blood, Mouse) were cultured with the particles in supplemented Dulbecco's modified Eagle's medium for 24 h in an atmosphere of 5% CO₂ in air at 37 degrees C, and the quantitative PCR was performed for genetic expression of IL-6 (Figure 3). The wear debris generated on the nano-textured surface inhibited the genetic expression of IL-6, which does not induce the tissue reaction and joint loosening.