Masayuki Kyomoto

Wear resistance of artificial hip joints with poly(2-methacryloyloxyethyl phosphorylcholine) grafted polyethylene: comparisons with the effect of polyethylene cross-linking and ceramic femoral heads.

Aseptic loosening of artificial hip joints induced by wear particles from the polyethylene (PE) liner remains the ruinous problem limiting their longevity. We reported here that grafting with a polymer, poly(2-methacryloyloxyethyl phosphorylcholine (MPC)) (PMPC), on the PE liner surface dramatically decreased the wear production under a hip joint simulator condition. We examined that the effect of properties of both PE by cross-linking and femoral head by changing the materials on wearing properties of PE. The PMPC grafting on the liners increased hydrophilicity and decreased friction torque, regardless of the cross-linking of the PE liner or the difference in the femoral head materials. During the hip joint simulator experiments (5 x 10(6) cycles of loading), cross-linking caused a decrease of wear amount and a reduction of the particle size, while the femoral head materials did not affect it. The PMPC grafting abrogated the wear production, confirmed by almost no wear of the liner surface, independently of the liner cross-linking or the femoral head material. We concluded that the PMPC grafting on the PE liner surpasses the liner cross-linking or the change of femoral head materials for extending longevity of artificial hip joints.

Self-initiated surface graft polymerization of 2-methacryloyloxyethyl phosphorylcholine on poly(ether ether ketone) by photoirradiation.

In the present paper, we reported the fabrication of a highly hydrophilic nanometer-scale modified surface on a poly(ether ether ketone) (PEEK) substrate by photoinduced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) in the absence of photoinitiators. Photoirradiation results in the generation of semibenzopinacol-containing radicals of benzophenone units in the PEEK molecular structure, which acts as a photoinitiator during graft polymerization. The poly(MPC)-grafted PEEK surface fabricated by a novel and simple polymerization system exhibited unique characteristics such as high wettability and high antiprotein adsorption, which makes it highly suitable for medical applications.

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.

Effects of mobility/immobility of surface modification by 2-methacryloyloxyethyl phosphorylcholine polymer on the durability of polyethylene for artificial joints

Surface modification is important for the improvement in medical device materials. 2-Methacryloyloxyethyl phosphorylcholine (MPC) polymers have attracted considerable attention as surface modifiable polymers for several medical devices. In this study, we hypothesize that the structure of the surface modification layers might affect the long-term stability, hydration kinetics, wear resistance, and so forth, of medical devices such as artificial joints, and the poly(MPC) (PMPC) grafted surface might assure the long-term performance of such devices. Therefore, we investigate the surface properties of various surface modifications by using dip coatings of MPC-co-n-butyl methacrylate (PMB30) and MPC-co-3-methacryloxypropyl trimethoxysilane (PMSi90) polymers, or photoinduced radical grafting of PMPC and also the effects of the surface properties on the durability of cross-linked polyethylene (CLPE) for artificial joints. The PMPC-grafted CLPE has an extremely low and stable coefficient of dynamic friction and volumetric wear as compared to the untreated CLPE, PMB30-coated CLPE, and PMSi90-coated CLPE. It is concluded that the photoinduced radical graft polymerization of MPC is the best method to retain the benefits of the MPC polymer used in artificial joints under variable and multidirectional loads for long periods with strong bonding between the MPC polymer and the CLPE surface, and also to retain the high mobility of the MPC polymer.

Ceramic Versus Cobalt-Chrome Femoral Components; Wear of Polyethylene Insert in Total Knee Prosthesis

The present study aimed to determine the effect of femoral component materials and sterilization methods on wear properties of total knee prostheses by using a knee simulator test and retrieval analysis. The simulator test revealed that ultrahigh molecular weight polyethylene (UHMWPE) inserts had remarkably lower wear against the ceramic femoral component than against the Co-Cr femoral component. However, the retrieval study revealed no significant difference in the linear wear between the former and the latter. The alumina ceramic/UHMWPE insert combination showed a mild wear. However, whether cross-linking by gamma-ray sterilization reduces wear remained unconfirmed. In contrast, oxidative degradation and/or delamination was confirmed. Thus, we conclude that alumina ceramic/ethylene oxide gas-sterilized UHMWPE insert in a total knee prosthesis might exhibit a good wear resistance.

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.

Superlubricious surface mimicking articular cartilage by grafting poly(2-methacryloyloxyethyl phosphorylcholine) on orthopaedic metal bearings.

Aseptic loosening of the artificial hip joint with osteolysis due to the wear particles from polyethylene cup has remained as a serious issue. To reduce this wear and develop a novel artificial hip joint system, we produced a superlubricious metal-bearing material: for this, we grafted a 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer onto the surface of a cobalt-chromium-molybdenum (Co-Cr-Mo) alloy. For ensuring long-term benefit retention of poly(MPC) on the Co-Cr-Mo alloy for application as a novel artificial hip joint system, several issues must be considered: strong bonding between poly(MPC) and Co-Cr-Mo surface, high mobility of free end groups of the poly(MPC) layer, and high density of the introduced poly(MPC). Considering these issues, we introduced a 3-methacryloxypropyl trimethoxysilane (MPSi) intermediate layer and a photoinduced graft polymerization technique to create a strong covalent bond between the Co-Cr-Mo substrate and the poly(MPC) chain via the MPSi layer. The thickness and density of the poly(MPC) layer on the surface increased with the MPC concentration and photoirradiation time. The grafted poly(MPC) layer successfully provided super-lubricity to the Co-Cr-Mo surface. The poly(MPC)-grafted crosslinked polyethylene/poly(MPC)-grafted Co-Cr-Mo or cartilage/poly(MPC)-grafted Co-Cr-Mo bearing interface mimicking natural joints showed an extremely low friction coefficient of 0.01, which is as low as that of natural cartilage interface. A superlubricious metal-bearing surface would enable the development of a novel biocompatible artificial hip joint system-artificial femoral head for partial hemiarthroplasty and metal-on-polymer/metal type for total hip arthroplasty.

Cartilage-mimicking, high-density brush structure improves wear resistance of crosslinked polyethylene: a pilot study.

BackgroundIn natural synovial joints under physiologic conditions, fluid thin-film lubrication by a hydrated layer of the cartilage is essential for the smooth motion of the joints. The considerably less efficient lubrication of artificial joints of polyethylene is prone to wear, leading to osteolysis and aseptic loosening and limiting the longevity of THA. A nanometer-scale layer of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) with cartilage-mimicking brushlike structures on a crosslinked polyethylene (CLPE) surface may provide hydrophilicity and lubricity resembling the physiologic joint surface.Questions/purposesWe asked whether the photoirradiation time during graft polymerization would affect the density and stability of the PMPC layer and the PMPC-grafted surface would enhance the durability of artificial joints. We investigated the effect of photoirradiation time and the resultant characteristics of the PMPC layer on the durability of the CLPE.MethodsFor each of the PMPC-grafted CLPE surfaces with various photoirradiation times (six groups: 0 [untreated CLPE], 11, 23, 45, 90, and 180 minutes), 18 sample pieces (total of 108 samples) were evaluated in surface analyses, and four cups (total of 24 samples) were evaluated in a hip simulator test.ResultsThe density of the PMPC layer increased with an increase in the photoirradiation time. The hip simulator test confirmed the PMPC-grafted CLPE with a high density of the PMPC layer exhibited minimal wear as compared with the untreated CLPE. High-density PMPC grafting appears essential for maintaining the high wear resistance of the PMPC-grafted CLPE. To obtain a high-density PMPC layer, the photoirradiation time must be greater than 45 minutes.ConclusionsThe cartilage-mimicking, density brushlike structure of the PMPC-grafted CLPE could extend high durability to acetabular cups in THA.Clinical RelevanceOur in vitro findings suggest the wear performance of CLPE acetabular cups in THA can be improved by this approach.

The prevention of peritendinous adhesions by a phospholipid polymer hydrogel formed in situ by spontaneous intermolecular interactions

Preventing peritendinous adhesions after surgical repair of tendon is difficult. In order to establish an ideal anti-adhesion material, we prepared a spontaneously forming hydrogel by mixing the aqueous solutions of two polymers, poly(MPC-co-methacrylic acid) (PMA) and amphiphilic poly(MPC-co-n-butyl methacrylate) (PMB), in the presence of Fe(3+). This PMA/PMB/Fe(3+) hydrogel (MPC polymer hydrogel) had a honeycomb microstructure with nanometer-scale pores, which resist cell invasion but allow the passage of cytokines and growth factors for tendon healing. The dissociation rate of the hydrogel could be controlled by changing Fe(3+) concentration, and by examining the viscoelasticity of the hydrogel, we determined the optimal Fe(3+) concentration to be 0.05 M. We then examined the effects of the in situ application of this MPC polymer hydrogel containing 0.05 M Fe(3+) by using two animal models: the rat Achilles tendon model and the chicken flexor digitorum profundus tendon model. In both models, macroscopic and histological observation revealed that peritendinous adhesions were significantly decreased by the hydrogel application. Mechanical analyses revealed that the hydrogel prevented peritendinous adhesions but did not affect the tendon healing. Because of its characteristic microstructure and excellent biocompatibility, we believe that the MPC polymer hydrogel will be ideal for preventing peritendinous adhesions.