Kenichi Saiga

Biomimetic hydration lubrication with various polyelectrolyte layers on cross-linked polyethylene orthopedic bearing materials

Natural joints rely on fluid thin-film lubrication by the hydrated polyelectrolyte layer of cartilage. However, current artificial joints with polyethylene (PE) surfaces have considerably less efficient lubrication and thus much greater wear, leading to osteolysis and aseptic loosening. This is considered a common factor limiting prosthetic longevity in total hip arthroplasty (THA). However, such wear could be mitigated by surface modification to mimic the role of cartilage. Here we report the development of nanometer-scale hydrophilic layers with varying charge (nonionic, cationic, anionic, or zwitterionic) on cross-linked PE (CLPE) surfaces, which could fully mimic the hydrophilicity and lubricity of the natural joint surface. We present evidence to support two lubrication mechanisms: the primary mechanism is due to the high level of hydration in the grafted layer, where water molecules act as very efficient lubricants; and the secondary mechanism is repulsion of protein molecules and positively charged inorganic ions by the grafted polyelectrolyte layer. Thus, such nanometer-scaled hydrophilic polymers or polyelectrolyte layers on the CLPE surface of acetabular cup bearings could confer high durability to THA prosthetics.

Lubricity and stability of poly(2-methacryloyloxyethyl phosphorylcholine) polymer layer on Co―Cr―Mo surface for hemi-arthroplasty to prevent degeneration of articular cartilage

Migration of the artificial femoral head to the inside of the pelvis due to the degeneration of acetabular cartilage has emerged as a serious issue in resurfacing or bipolar hemi-arthroplasty. Surface modification of cobalt-chromium-molybdenum alloy (Co-Cr-Mo) is one of the promising means of improving lubrication for preventing the migration of the artificial femoral head. In this study, we systematically investigated the surface properties, such as lubricity, biocompatibility, and stability of the various modification layers formed on the Co-Cr-Mo with the biocompatible 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer by dip coating or grafting. The cartilage/poly(MPC) (PMPC)-grafted Co-Cr-Mo interface, which mimicked a natural joint, showed an extremely low friction coefficient of <0.01, as low as that of a natural cartilage interface. Moreover, the long-term stability in water was confirmed for the PMPC-grafted layer; no hydrolysis of the siloxane bond was observed throughout soaking in phosphate-buffered saline for 12 weeks. The PMPC-grafted Co-Cr-Mo femoral head for hemi-arthroplasty is a promising option for preserving acetabular cartilage and extending the duration before total hip arthroplasty.

Surface grafting of biocompatible phospholipid polymer MPC provides wear resistance of tibial polyethylene insert in artificial knee joints

OBJECTIVE Aseptic loosening of artificial knee joints induced by wear particles from a tibial polyethylene (PE) insert is a serious problem limiting their longevity. This study investigated the effects of grafting with our original biocompatible phospholipid polymer 2-methacryloyloxyethyl phosphorylcholine (MPC) on the insert surface. METHODS The hydrophilicity of the PE surface was determined by the contact angle of a water droplet, and the friction torque was measured against a cobalt-chromium alloy component. The wear amount was compared among PE inserts with or without cross-linking and MPC grafting during 5x10(6) cycles of loading in a knee joint simulator. The surfaces of the insert and the wear particles in the lubricant were subjected to electron and laser microscopic analyses. The mechanical properties of the inserts were evaluated by the small punch test. RESULTS The MPC grafting increased hydrophilicity and decreased friction torque. In the simulator experiment, the wear of the tibial insert was significantly suppressed in the cross-linked PE (CLPE) insert, and even more dramatically decreased in the MPC-grafted CLPE insert, as compared to that in the non-cross-linked PE insert. Surface analyses confirmed the wear resistance by the cross-linking, and further by the MPC grafting. The particle size distribution was not affected by cross-linking or MPC grafting. The mechanical properties of the insert material remained unchanged during the loading regardless of the cross-linking or grafting. CONCLUSION Surface grafting with MPC polymer furnished the PE insert with wear resistance in an artificial knee joint through increased hydrophilicity and decreased friction torque.

Grafting of poly(2-methacryloyloxyethyl phosphorylcholine) on polyethylene liner in artificial hip joints reduces production of wear particles.

Despite improvements in the techniques, materials, and fixation of total hip arthroplasty, periprosthetic osteolysis, a complication that arises from this clinical procedure and causes aseptic loosening, is considered to be a major clinical problem associated with total hip arthroplasty. With the objective of reducing the production of wear particles and eliminating periprosthetic osteolysis, we prepared a novel hip polyethylene (PE) liner whose surface graft was made of a biocompatible phospholipid polymer-poly(2-methacryloyloxyethyl phosphorylcholine (MPC)). This study investigated the wear resistance of the poly(MPC)-grafted cross-linked PE (CLPE; MPC-CLPE) liner during 15×10(6) cycles of loading in a hip joint simulator. The gravimetric analysis showed that the wear of the acetabular liner was dramatically suppressed in the MPC-CLPE liner, as compared to that in the non-treated CLPE liner. Analyses of the MPC-CLPE liner surface revealed that it suffered from no or very little wear even after the simulator test, whereas the CLPE liners suffered from substantial wears. The scanning electron microscope (SEM) analysis of the wear particles isolated from the lubricants showed that poly(MPC) grafting dramatically decreased the total number, area, and volume of the wear particles. However, there was no significant difference in the particle size distributions, and, in particular, from the SEM image, it was observed that particles with diameters less than 0.50μm were present in the range of the highest frequency. In addition, there were no significant differences in the particle size descriptors and particle shape descriptors. The results obtained in this study show that poly(MPC) grafting markedly reduces the production of wear particles from CLPE liners, without affecting the size of the particles. These results suggest that poly(MPC) grafting is a promising technique for increasing the longevity of artificial hip joints.

Effects of Surface Modification and Bulk Geometry on the Biotribological Behavior of Cross-Linked Polyethylene: Wear Testing and Finite Element Analysis.

The wear and creep deformation resistances of polymeric orthopedic bearing materials are both important for extending their longevity. In this study, we evaluated the wear and creep deformation resistances, including backside damage, of different polyethylene (PE) materials, namely, conventional PE, cross-linked PE (CLPE), and poly(2-methacryloyloxyethyl phosphorylcholine)- (PMPC-) grafted CLPE, through wear tests and finite element analysis. The gravimetric and volumetric degrees of wear of disks (3 or 6 mm in thickness) of these materials against a cobalt-chromium-molybdenum alloy pin were examined using a multidirectional pin-on-disk tester. Cross-linking and PMPC grafting decreased the gravimetric wear of the PE disks significantly. The volumetric wear at the bearing surface and the volumetric penetration in the backside of the 3-mm thick PE disk were higher than those of the 6-mm thick PE disk, regardless of the bearing material. The geometrical changes induced in the PE disks consisted of creep, because the calculated internal von Mises stress at the bearing side of all disks and that at the backside of the 3-mm thick disks exceeded their actual yield strengths. A highly hydrated bearing surface layer, formed by PMPC grafting, and a cross-linking-strengthened substrate of adequate thickness are essential for increasing the wear and creep deformation resistances.

Reducing fretting-initiated crevice corrosion in hip simulator tests using a zirconia-toughened alumina femoral head: REDUCED TAPER FRETTING CORROSION USING ZTA

Taper fretting corrosion is considered a potentially limiting factor for total hip arthroplasty longevity. Recently, attention has been focused on new materials for ceramic femoral heads, for example, zirconia-toughened alumina (ZTA), since they have an alternative bearing surface that can improve the wear resistance. Moreover, ceramics have high chemical stability and corrosion resistance. In this study, we evaluated the effects of ZTA and Co-Cr-Mo alloy femoral heads on their taper fretting and/or corrosion characteristics under a controlled hip simulator test. After the test, less fretting and corrosion were observed in the taper surface of the trunnion against the ZTA femoral head than for that against the Co-Cr-Mo alloy femoral head. In addition, corrosion damages were only observed in the lateral-distal taper surface (noncontact area) of the trunnion in the Co-Cr-Mo alloy femoral head group. The ZTA femoral head group also eliminated the potential for Co ion release into the lubricants from taper corrosion, reducing the possibility of adverse local tissue inflammatory responses. In conclusion, ZTA femoral heads showed markedly less fretting corrosion compared to Co-Cr-Mo alloy femoral heads and have a lower potential for metal ion release.

The effects of presence of a backside screw hole on biotribological behavior of phospholipid polymer-grafted crosslinked polyethylene: EFFECTS OF BACKSIDE HOLE ON TRIBO BEHAVIOR OF PMPC-GRAFTED CLPE

One of the important factors in determining the success of joint replacement is the wear performance of polyethylene. Although highly crosslinked polyethylene (CLPE) is presently used, it is still not adequate. We have developed a surface modification technology using poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) in an attempt to improve wear performance. In this study, we evaluated the wear and creep deformation resistances of 3-mm and 6-mm thick PMPC-grafted CLPE disks, set on a metal back-plate, with and without a sham screw hole. The gravimetric wear and volumetric change of the disks were examined using a multidirectional pin-on-disk tester. PMPC grafting decreased the gravimetric wear of CLPE regardless of the presence of a screw hole, and did not affect the volumetric change. The volumetric change in the bearing and backside surfaces of the 3-mm thick disk with a screw hole was much larger than that of those without a screw hole or those of the 6-mm thick disk, which was caused by creep deformation. PMPC grafting on the bearing surface can be a material engineering approach to reduce the wear without changing the creep deformation resistance, and is a promising surface modification technology that can be used to increase the longevity of various artificial joints.