Artificial knee joints are continuously loaded by higher contact stress than artificial hip joints due to a less conformity and much smaller contact area between the femoral and tibial surfaces. The higher contact stress causes severe surface damage such as pitting or delamination of polyethylene (PE) tibial inserts. To decrease the risks of these surface damages, the oxidation degradation of cross-linked polyethylene (PE) induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization should be prevented. Vitamin E (VE), as an antioxidant, blended PE (PE(VE)) has been used to solve the problems. In addition, osteolysis induced by PE wear particles, bone cement and metallic debris is recognized as one of the important problems for total knee arthroplasty (TKA). To decrease the generation of PE wear particles, we have developed the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface having high wear resistance. In this study, we have evaluated the surface, mechanical under severe oxidative condition, and wear properties of PMPC-grafted cross-linked PE(VE) (PMPC-CLPE(VE)) material for artificial knee joints.

Untreated and PMPC-grafted 0.1 mass% VE-blended PE (GUR1020E resin) with a gamma-ray irradiation of 100 kGy for cross-linking and 25 kGy for sterilization were prepared (CLPE(VE) and PMPC-CLPE(VE), respectively). Surface properties were evaluated by Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscope (TEM) observations. Surface wettability and frictional property were measured by static water contact angle measurement and ball-on-plate friction test. To evaluate the oxidation degradation resistance, mechanical and physical properties such tensile test, izod impact test, small punch test and cross-link density measurement before and after accelerated aging were measured. Wear properties of the tibial inserts were examined by using knee simulator in the combination of Co-Cr-Mo femoral components according to ISO14243-3. Gravimetric wear, volumetric penetration and the number of generated wear particles were measured.

By the FT-IR measurements and TEM observation, P–O peaks attributed to MPC unit and uniform PMPC layer with 100–200 nm thick was observed only on PMPC-CLPE(VE) surface. Static water contact angle of CLPE(VE) was almost 100 degree, while that of PMPC-CLPE(VE) decreased significantly to almost 35 degree. There was no significant difference in the mechanical and physical properties between CLPE(VE) and PMPC-CLPE(VE). Moreover, both the CLPE(VE) and PMPC-CLPE(VE) maintained these properties even after the accelerated aging of 12 weeks [Fig. 1]. Blended VE in CLPE would act as radical scavengers to prevent oxidation degradation. In the knee simulator wear test, the PMPC-CLPE(VE) tibial inserts showed about a half gravimetric wear compared to the CLPE(VE) tibial inserts [Fig. 2]. This would be due to the significant differences observed in wettability of the surface. Water thin film formed on the hydrated PMPC graft layer, would act as significantly efficient lubricant.

From these results, the PMPC-CLPE(VE) is expected to be one of the great bearing materials not only preventing surface damages due to higher contact stress and oxidation degradation but also improving wear resistance, and to provide much more lifelong artificial knee joints.

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To prevent aseptic loosening resulting from osteolysis induced by polyethylene (PE) wear particles in THA, it is necessary to develop a high wear-resistance bearing material. We have investigated the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface. High wear-resistance of PMPC-grafted surface has been revealed in the hip simulator wear test of 20 million cycles. Additionaly, in THA, oxidation degradation induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization is also regarded as serious issue. Recently, gas plasma (GP) sterilization has been used as a less residual radical sterilization method. In this study, we ask a question: the GP sterilization would affect to PMPC surface and/or PE substrate? Hence, we investigated surface chemical, wear, mechanical, physical and oxidation properties of GP sterilized PMPC-grafted highly cross-linked PE (CLPE).

GP-sterilized CLPE and PMPC-grafted CLPE (CLPE (GP) and PMPC-CLPE (GP), respectively; GUR 1020 resin, 75 kGy irradiation), and 25 kGy-gamma-sterilized PMPC-grafted CLPE (PMPC-CLPE (g); GUR 1020 resin, 50 kGy irradiation) were evaluated. Surface property of PMPC layer was evaluated by X-ray photoelectron spectroscopy (XPS), fourier-transform infrared (FT-IR) spectroscopy, fluorescence microscope and cross-sectional transmission electron microscope (TEM) observations. Wettability and lubrication of the PMPC-CLPE surface were evaluated by static water contact angle measurement and ball-on-plate friction test, respectively. Wear properties of the acetabular cups were examined by using hip simulator in the combination with Co-Cr-Mo femoral heads. To evaluate the GP sterilization effect to the CLPE substrate, tensile test, izod impact test, small punch test, gel content, residual radical concentration and oxidation degradation were conducted. Oxidation degradation was evaluated as oxidation index by using a FT-IR spectroscopy.

By the XPS and FT-IR measurements, phosphorus peak and P-O peak attributed to grafted PMPC were observed, respectively. Uniform PMPC layer (100–200 nm thick) was observed on both surfaces of PMPC-CLPE (g) and PMPC-CLPE (GP) [Fig. 1]. Water contact angle of CLPE (GP) was almost 100 degree, while those for PMPC-CLPE (g) and PMPC-CLPE (GP) decreased dramatically to almost 10 degree. Dynamic coefficient of friction of PMPC-CLPE (g) and PMPC-CLPE (GP) was lower than that for CLPE (GP). In the hip simulator wear test, PMPC-CLPE (g) and PMPC-CLPE (GP) cups showed significantly lower amount of wear than that of CLPE (GP) [Fig. 2]. The number of the wear particles was extremely less in PMPC-CLPE (g) and PMPC-CLPE (GP), though the size was not different of all cases. Water thin film might be formed at the grafted PMPC layer, which acted as significantly efficient lubricant. There was no difference in the mechanical and physical properties among three groups. Oxidation index for PMPC-CLPE (GP) after acceleration of aging was lower than that of PMPC-CLPE (g). The GP sterilization might affect only to the PMPC-grafted surface, whereas gamma irradiation affects also to the PE substrate.

From these results, the PMPC-CLPE (GP) is expected to be one of the great bearing materials having not only high-wear resistance but also high-oxidation resistance, which could give further longevity of implantation.

The modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) is known to increase the hydration of the surfaces and decrease the wear of the substrates. PMPC grafting to acetabular liner of total hip arthroplasty showed a drastic reduction of cross-linked polyethylene (CLPE) wear in a long-term hip simulator test and achieved a good short-term clinical result. To apply this technique to other joint prostheses, the wear resistance under various conditions needs to be evaluated because every joint has a different wear mode. ASTM F732 gives a method that disk shaped polymer specimen is loaded with hemispherical pin using pin-on-disk tester, which is suitable for hydrated polymer because the lubricant is supplied every loading cycle on the surface. The purpose of this study is to evaluate the performance of PMPC-grafted hydrated CLPE under multidirectional wear condition in anticipation of applying PMPC to various prostheses.

The CLPE disks of 3 or 6-mm in thickness were machined from a bar stock. The PMPC was grafted onto the CLPE surfaces using a photoinduced polymerization of MPC in aqueous medium. All disks were irradiated with a total amount of 75-kGy gamma-ray. The wear resistance of the CLPE and PMPC-grafted CLPE disks against Co-Cr-Mo alloy pin was evaluated using Ortho-POD pin-on-disk tester. The disks were fixed to the tester with a Ti-6Al-4V alloy plate that has screw hole in the center. The test conditions were a static load of 213 N, sliding shape of 5 mm × 10 mm rectangular, frequency of 1 Hz and maximum cycles of 1.0 × 10⁶ [Fig. 1]. Gravimetric wear was determined by weighing the disks and soak controls were used to compensate for the fluid absorption. After the wear test, volumetric changes of sliding and backside surfaces of disks were evaluated using a noncontact optical three-dimensional profiler.

The PMPC-grafted surface showed decrease in the gravimetric wear drastically [Fig. 2]. The thickness of CLPE had no substantial effect on the wear resistance. Three-dimensional profile measurements of sliding surfaces detected a substantial volumetric penetration; the corner of sliding track were deeper than the straight-line portion. Backside extrusion was observed in all disks. The thickness of CLPE affected both volumetric penetration and backside extrusion for both untreated and PMPC-grafted CLPE. The PMPC grafting had no discernible effect on volumetric changes [Fig. 3].

Results of this study revealed: (1) the PMPC-grafted surface decreases wear of CLPE, however, the thickness of disk has no effect, in contrast, (2) thinner thickness of CLPE increases the volumetric changes including penetration in sliding surface and extrusion in back surface but the PMPC-grafted surface has no effect. Gravimetric wear did not correlate with the volumetric penetration in sliding surface because the volumetric penetration might be caused by not only the wear but also the creep deformation. In conclusion, hydrated bearing surface and thickness of bearing substrate are essential for the wear and fatigue resistance properties for an increasing longevity of artificial joint. In addition, PMPC grafting is a promising technique for increasing the longevity of various joint prostheses.

The main objective of joint arthroplasty is to improve activities of daily living of the patient. However, normal daily activities may lead to separation of articular surfaces of an artificial joint, possibly as a result of a combined impact and sliding motion. Therefore, the properties of articular surfaces define the durability of implant materials. Modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) increases the hydration of the surfaces and decreases the wear of the substrates. Hence, a PMPC layer can potentially cushion the impact and improve the resistance of cross-linked polyethylene (CLPE). This study aimed to explore the fatigue and wear resistance of PMPC-grafted hydrated CLPE under impact-to-wear conditions using a pin-on-disk tester.

The surfaces of a CLPE disk (3- or 6-mm thick) were modified with PMPC by photoinduced polymerization and were sterilized using gamma rays. The wear resistance of PMPC-grafted CLPE disks against a Co-Cr-Mo alloy pin was evaluated and compared to that of untreated disks. The disks were fixed to the tester with a metal plate (Ti-6Al-4V alloy) that had a central hole. The test was performed for 2 × 10⁶ cycles of repetitive impact and unidirectional sliding with the maximum load of 150 N, sliding distance of 10 mm, and frequency of 1 Hz [Fig. 1]. Gravimetric wear was determined by weighing the disks, and soak controls were used to compensate for fluid absorption. Volumetric changes in the surfaces of the disks were evaluated using a three-dimensional non-contact optical profiler.

The average gravimetric wear (mg) after 2 × 10⁶ cycles was 0.000/0.120 for CLPE (3/6 mm) and −0.073/–0.137 for PMPC-CLPE (3/6 mm). The weight gain of the PMPC-CLPE disks was due to their greater fluid absorption compared to that of the soak controls under the impact-to-wear conditions, as judged from the fact that during the load-soak in the lubricant this gain was observed for all the disks irrespectively of PMPC grafting. PMPC-grafting decreased the gravimetric wear of CLPE (p < 0.01) in the 6-mm group, whereas the thickness of the CLPE disks had no substantial effect on the wear resistance [Fig. 2]. In all cases, three-dimensional measurements detected a remarkable volumetric penetration in the impact-sliding surfaces and an extrusion of CLPE from the backside surfaces into the hole in the metal plate. Both the volumetric penetration and backside extrusion were smaller in the 6-mm group. The PMPC grafting had no discernible effect on these volumetric changes [Fig. 3]. Even after 2 × 10⁶ cycles of impact loads, mechanical fracture or delamination of the impact-sliding or backside surfaces were hardly observed in all the groups.

The results of this study revealed that: (1) PMPC-grafting of CLPE surfaces decreased the gravimetric wear irrespectively of the disk thickness; and (2) thinner CLPE increased the risk of volumetric changes, including penetration in the impact-sliding surface and extrusion of the backside surface. In conclusion, PMPC grafting can potentially improve the wear resistance of the bearing surface of biomaterials even under impact-to-wear conditions, increasing the longevity of artificial joints.

Introduction

Periprosthetic osteolysis is considered the main problem limiting the longevity and clinical success of artificial hip joints. Aiming at the reduction of the wear particles and the elimination of periprosthetic osteolysis, we have recently developed a novel articular cartilage-inspired technology for surface modification (Aquala® technology) with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) grafting (100–150 nm in thickness) for an acetabular liner in an artificial hip joint. Our previous study on the mechanical and biological effects of PMPC revealed that the grafting decreased the production of wear particles and the bone resorptive responses. However, as well as wear-resistance, oxidation is an important indicator of the clinical performance of acetabular liners. The incorporation of the antioxidant vitamin E has been proposed recently as an alternative to post melting treatment after gamma-ray irradiation to avoid oxidation. The purpose of this study is to investigate the effects of substrate materials, vitamin E-blended cross-linked polyethylene (CLPE), on the oxidative stability and wear resistance of the PMPC-grafted CLPE liner for artificial hip joints.

Despite improvements in techniques and materials, aseptic loosening of artificial hip joints remains as the most serious problem. This study investigated mechanical and biological effects of biocompatible 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer on prevention of aseptic loosening.

To examine mechanical effects of MPC grafting, we performed hip simulator tests (3million cycles) using cross-linked polyethylene (CL-PE) liners with or without nano-grafting of MPC onto articulating surface (MPC liner/CL-PE liner) and PE liner against CoCrMo heads. To examine biological responses of macrophages and osteoblasts, we prepared MPC nanoparticles (500nm). Using in vitro/vivo murine particle-induced osteolysis model, we examined biological effects of MPC nanoparticles on osteoclastogenesis.

The friction torque was about 90% lower in MPC liners than control liners. Total amounts of wear produced from MPC liner was about 1/5and 1/30 of those from CL-PE and PE liners, respectively. Three-dimensional analysis and SEM analysis of MPC liners revealed no or little wear. The effect of MPC nanografting was maintained even after the test, because XPS analysis confirmed the remainder of specific spectra of MPC on the liner surface. When nanoparticles were exposed to cultured mouse macrophages, MPC nano particles were hardly phagocytosed by macrophages and did not enhance the concentration of bone resorptive cytokines and PGE2. Furthermore, culture medium of macrophages exposed to MPC nanoparticles did not induce RANKL expression in osteoblasts and osteoclastogenesis from bone marrow cells. In vivo murine osteolysis model, particle-induced bone resorption was hardly observed in mice implanted MPC nanoparticles.

Some medical devices grafted MPC onto itssurface have been already used under authorization of the FDA. This study demonstrated that MPC grafting markedly decreases wear production. In addition, even if wear particles are produced, they are biologically inert in respect to phagocytosis by macrophages and subsequent resorptive actions, suggesting an epochal improvement of artificial hip joints preventing aseptic loosening.

Rotational acetabular osteotomy (RAO) is a circumacetabular osteotomy of the acetabulum designed to correct the dysplastic hip. In this procedure, the femoral head is covered with the articular cartilage of the acetabulum and the forces of weight-bearing are distributed more evenly. The purpose of this study was to determine whether RAO is effective in delaying the onset of arthrosis in patients with painful hip dysplasia.

We determined the outcome of 20 female patients in whom RAO was performed between 1975 and 1984; all were aged 20 to 29 years at the time of surgery. The pre-operative centre-edge angle of Wiberg was 0 or negative with proximal subluxation of the femoral head. Of these, 10 were lost to follow-up before the age of 42. In these patients, however, radiographs showed no signs of arthrosis at the last follow-up. The remaining 10 patients were examined 15 to 25 years after surgery, when they were 42 to 47 years old. Radiographs revealed findings of arthrosis in only two of them who had had the secondary acetabulum before surgery.

To evaluate the efficacy of preventive medicine, it is necessary to compare the results of intervention with the natural course of the disease. Wiberg reported on the natural history of seven female patients with severe hip dysplasia in 1939. When these patients were 13 to 34 years old, radiographs demonstrated no sign of arthrosis and the centre-edge angle was equal to or less than 12 degrees with proximal subluxation of the femoral head. These hips deteriorated to advanced arthrosis by the age of 42 years. Thus the outcome of our patients was significantly better than the natural course.

In conclusion, our study suggests that RAO is effective in delaying the onset of arthrosis in patients with painful hip dysplasia.

Tartrate-resistant acid phosphatase is contained in multinucleated giant cells of giant cell tumour of bone (GCT) and chondroblastoma (CBL) as well as in osteo-clasts. Yet few studies have so far been done regarding serum acid phosphatase (AcP) level in patients of GCT or CBL. The purpose of this study is to elucidate the clinical significance of serum AcP as a tumour marker for GCT and CBL. Serum AcP value was examined in nine GCT patients and three CBL patients before and after surgery. In the GCT cases, serum AcP values before surgery were high in five cases. They were 14.0 IU/L, 68.7 IU/L, 45.9 IU/L, 21.9 and 31.3 IU/L (normal value; 7.1–12.6 IU/L). They decreased after surgery to 7.7 IU/L (55% of the preoperative value), 8.2 IU/L (12%), 7.8 IU/L (17%), 6.1 IU/L (28%) and 10.0 IU/L (32%), respectively. Serum AcP values before surgery were within normal limits in the remaining four GCT cases. Even in these four cases, postoperative serum AcP level was lower than the preoperative level. Postoperative/preoperative AcP ratios in these four cases were 67%, 80%, 69% and 76%. In the CBL cases, serum AcP values were high in all cases. They were 15.1 IU/L, 13.1 IU/L and 13.7 IU/L. They decreased after surgery to 10.3 IU/L (68% of the pre-operative value), 10.2 IU/L (78%) and 9.7 IU/L (71%), respectively, all within normal limits. Therefore, it is concluded that serum AcP is a useful tumour marker for GCT and CBL in diagnosing the tumour as well as in evaluating the efficacy of treatment.