Today's aging society is seeing an increase of patients with rheumatoid arthritis and osteoarthritis, as well as an increase in joint replacement surgery. The artificial joints used in this surgery frequently uses ultra-high molecular weight polyethylene (UHMWPE) as a bearing material. However, UHMWPE wear particles are considered to be a major factor in long-term osteolysis, and implant loosening. Many researchers have reported that the volume and size of particles are critical factors in macrophage activation, with particles in the size range of 0.1 – 1.0 μm being the most biologically active. The micro slurry-jet erosion (MSE) apparatus was introduced to minimize the amount of wear, and increase the size of UHMWPE wear particles by texturing the surfaces of Co-Cr-Mo alloy implants. The MSE apparatus uses a slurry of alumina particles (WA#8000: average diameter 1.2 μm) mixed with water. The slurry and compressed air are mixed within an injection nozzle, which is then applied to the Co-Cr-Mo alloy at high speed to achieve a desired nano-textured surface. In this study, four Co-Cr-Mo alloy surface profiles were prepared. The MSE injection nozzle was fed 40.0 mm in alternating directions across each surface with an orthogonal step of 0.5 mm. The surface M-1 was processed with an injection nozzle feed rate of 1.0 mm/s, and obtained a surface roughness of 5.7 nm. M-2 was processed with a feed rate of 2.0 mm/s, and had a surface roughness of 2.3 nm. The M-4 surface used a 40.0 mm alternating directions surface feed, but with a 1.0 mm orthogonal step, and an injection nozzle feed rate of 0.5 mm/s. It obtained a surface roughness of 4.0 nm. The G-1 surface, with a roughness of 10.0 nm, was processed with the typical lapping method, which is used in conventional artificial joints [Fig. 1]. A pin-on-disk wear tester, capable of multidirectional motion, was used to assess which surface was the most appropriate for artificial joints. The UHMWPE pins were flat ended cylinders, 12.0 mm in diameter, and were placed on the disk with a contact pressure of 6.0 MPa. Tests were carried out in 25% (v/v) fetal calf serum with sodium azide to retard bacterial growth. A sliding speed of 12.1 mm/s, and a total sliding distance of 15.0 km were applied. The wear weight of the MSE textured surface M-1 was significantly lower than the wear weight of the conventional surface. Moreover, the percentages of various wear particle sizes obtained from MSE surface texturing was significantly different from those obtained from the traditional surface.

Cases of intertrochanteric hip fractures as a result of osteoporosis have been increasing in recent years. Treatment of these types of fractures is often performed with intramedullary (IM) nails or compression hip screws (CHS) ^[1]. IM nails are composed of a stem, which is inserted into the medullary canal of the femur, and a lag screw that is placed inside the head of the femur. One problem with this type of device is that both the left and right femurs are fixed with IM nails that have right-hand threaded lag screws. Therefore, on left femurs, the right-handed threads may not provide satisfactory fixation in the bone. This insufficient fixation could cause rotary motion and slippage in the femoral head, which would inhibit fracture healing. This study used three-dimensional finite element analysis (FEA) to examine the fixation and rotational characteristics in reference to the thread direction of the lag screw and the relative angle between the stem and lag screw.

In this study, a 3D CAD model of a left femur and four proximal femoral IM nail designs were analyzed in FEA for stress and displacement. An intertrochanteric femoral fracture was created so that the femoral head and diaphysis were separated. The four IM nails were designed to with either left or right-handed lag screw threads (figure .1) and with relative stem-lag screw angles of 125 or 135 degrees. (Traditional IM nails use a right-handed screw and a relative angle of 125 degrees.)

The results showed the femoral head displacement was smaller when using the left-handed lag screw. It is thought that this difference between the left and right-handed screws is caused by the direction of rotation, which would cause the left-handed screw to tighten and the right-handed screw to loosen within the femoral head. The femoral head displacement also decreased with a screw-stem angle of 135 degrees in comparison to the standard 125 degree angle. The standard right-handed screw with 125 degree relative angle was shown to have the largest displacement of all four types of tested IM nails, whereas the left-handed, 135 relative degree design produced the smallest displacement of all four implants.

These results show how using a left-handed lag screw with proper relative angles in the left femur, effectively reduces femoral head displacement when compared to traditional right-handed lag screw IM implants. This is important for the promotion of intertrochanteric fracture healing.

This study presents the use of precision surface machining on artificial joint bearing surfaces in order to inhibit macrophage activation. Ultra-high molecular weight polyethylene (UHMWPE) is widely used as a bearing material in polymer-on-hard joint prostheses. However, UHMWPE wear particles are considered to be a major factor in long-term osteolysis and implant loosening. Several studies report that wear particle size is a critical factor in macrophage activation, with particles in the size range of 0.1 – 1.0 μm being the most biological active. The surface for a conventional Co-Cr-Mo alloy joint implant generally has a 10.0 – 20.0 nm roughness. After precision machining, the Co-Cr-Mo alloy surface had a 1.0 – 2.0 nm roughness with scattered concave shapes up to 50 nm in depth. This precision surface machining method used a typical lapping method, but the relationship between the slurry and the machining surface was strictly controlled in order to emphasize the micro-erosion mechanism. A pin-on-disc wear tester capable of multidirectional motion was used to verify that the new surface was the most appropriate for joints. Tests were carried out in 25% (v/v) fetal calf serum with sodium azide to retard bacterial growth. UHMWPE pins, 12.0 mm in diameter with a mean molecular weight of 6.0 million, were placed on the Co-Cr-Mo alloy disc at a contact pressure of 6.0 MPa. A sliding speed of 12.1 mm/s, and a total sliding distance of 15.0 km were applied. The new surface reduced the amount of UHMWPE wear, which would ensure the long-term durability of joints. The new surface also enlarged the size of UHMWPE particles, but did not change their morphological aspect. Primary human peripheral blood mononuclear phagocytes were cultured with the particles. The wear particles generated on the new surface inhibited the production of IL-6, which indicates a reduction of induced tissue reaction and joint loosening.

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 textured on Co-Cr-Mo alloy as a counterface material was invented (Figure 1). Although the generally-used surface for a conventional artificial joint has 10 nm roughness (G-1), the nano-level surface has a superfine surface of 1 nm with groove and dimples against the bearing area. The existence probability of groove or dimples, and their surface waviness were adjusted (P-1, 2, 3, 4 and W-1, 2).

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 nano-textured surfaces reduced the amount of UHMWPE wear, this would ensure the long-term durability of artificial joint (Figure 2). 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 3). 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. 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.

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.