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. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

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.

Introduction. According to American Joint Replacement Registry, particle mediated osteolysis represents 13 % of the knee revision surgeries performed in the United States. The comprehension of mechanical and wear properties of materials envisioned for TJR is a key step in product development. Furthermore, the maintenance of UHMWPE mechanical properties after material modification is an important aspect of material success. Initial studies conducted by our research group demonstrated that the incorporation of ibuprofen in UHMWPE had a minor impact on UHMWPE physicochemical and mechanical properties. Drug release was also evaluated and resulted in an interesting profile as a material to be used as an anti-inflammatory system. Therefore, the present study investigated the effect of drug release on the mechanical and biological properties of ibuprofen-loaded UHMWPE. Experimental. UHMWPE resin GUR 1020 from Ticona was for sample preparation. Samples with drug concentrations of 3% and 5% wt were consolidated as well as samples without anti-inflammatory addition through compression molding at 150 °C and 5 MPa for 15 minutes. Mechanical properties were evaluated via the tensile strength experiment (ASTM D638) and dynamic mechanic tests. Wear resistance was measured using the pin on disc (POD) apparatus. Finally, cytotoxicity analysis was conducted based on ISO 10993–5. Results. Dynamic-mechanic analysis demonstrated no difference in flexion modulus and stress for all materials (Table 1). No difference was also verified during cyclical loading experiments (Table 1), which indicates that the drug concentration added to material composition did not affect these properties. POD experiments were proposed to evaluate wear resistance of ibuprofen-loaded UHMWPE samples considering the combination of materials similar to those employed in TJR. Results from POD tests are presented in Table 1. Volumetric wear was close to zero for all samples after 200 thousand cycles. Comprehension of the effect of drug release on mechanical properties is essential to estimate how the material will behave after implantation. Therefore, mechanical properties were assessed after 30 days of ibuprofen release and the results were compared with those obtained in samples as prepared (Table 2). Initial results demonstrated a decrease in elastic modulus in samples prepared with ibuprofen. However, no difference was verified between UHMWPE, UHMWPE 3% IBU and UHMWPE 5% IBU after ibuprofen release. Finally, cell viability of UHMWPE 3% IBU and UHMWPE 5% was found to be superior to 100% (Figure 1). Therefore, both materials can be considered nontoxic. Conclusions. Ibuprofen-loaded UHMWPE did not demonstrate a significant influence on the mechanical and biological behavior of UHMWPE. Dynamic-mechanical tests demonstrated constancy for all samples under analysis. Wear testing resulted in gravimetric wear close to zero, for all tested materials. Mechanical properties conducted after 30 days of ibuprofen release also had a positive outcome. Although presenting a difference in modulus prior and after release tests, modulus and tensile yield stress remained inside acceptable range indicated to UHMWPE used in orthopedic implants. Furthermore, after drug elution UHMWPE 3% IBU and UHMWPE 5% IBU recovered original UHMWPE properties. Cytotoxicity assessment was performed and both ibuprofen-based formulations were considered nontoxic according to ISO 10993–5. For any figures or tables, please contact the authors directly

Poly (vinyl alcohol) (PVA) hydrogel with high water content is one of the potential materials for artificial cartilage. In the previous study, the wear behavior of PVA hydrogel prepared by freeze-thawing (FT) method (PVA-FT gel) showed the excellent friction and wear property in simulated biological environment. However, the improvement of mechanical strength and wear resistance would be also needed for clinical application of PVA hydrogel as artificial cartilage. The different kind of physically-crosslinked PVA hydrogels prepared by cast-drying (CD) method (PVA-CD gel) and hybrid method of FT and CD (PVA-CD on FT hybrid gel) were also developed, and these two hydrogels have different mechanical properties and showed low friction compared with PVA-FT gel in saline. In this study, PVA hydrogel prepared by CD and hybrid methods were newly developed and friction and wear behavior of PVA-CD gel and PVA-CD on FT hybrid gel were evaluated in simulated biological environment. A sliding pair of an ellipsoidal reciprocating upper specimen of hydrogel and a flat stationary lower specimen of hydrogel was tested in reciprocating friction test. The thicknesses of PVA-CD gel and PVA-CD on FT hybrid gel were 2.0mm and 1.7mm, respectively. The applied load was 2.94 N. The sliding velocity was 20 mm/s and the total sliding distance was 1.5 km. In this study, solutions that contain hyaluronic acid, phospholipid and proteins were prepared as simulated synovial fluid and used as a lubricant for friction test. Molecular weight of sodium hyaluronate was 9.2×10. 5. L-alpha dipalmitoylphosphatidylcholine (DPPC) was selected as phospholipid constituent and was dispersed in saline as liposome. This liposomal solution was used as a base lubricant. Albumin and gamma-globulin, which are main protein constituents in natural synovial fluid, were used as additives as protein constituents. As shown in Fig.1, PVA-CD gel showed low friction such as below 0.02 at initial state of friction test. However, friction coefficient of PVA-CD gel rapidly increased and reached to about 0.5. In contrast, PVA-CD on FT hybrid gel kept low friction within the friction test. After friction test, many deep scratches were observed on the worn surface of PVA-CD gel (Figs. 2(a)-(c)). In contrast, the original surface structure of PVA-CD on FT hybrid gel almost remained while some scratches were observed (Figs. 2(d)-(f)). These results indicated that PVA-CD gel could show low friction but low wear resistance. The hybridization of FT and CD improved the wear resistance of PVA-CD gel. Therefore, the hybridization of FT and CD method is one of the prospective preparation methods of artificial cartilage with low friction and low wear. It is important to elucidate the mechanism of excellent lubricating property of PVA-CD on FT hybrid gel and develop the highly-functioned artificial hydrogel cartilage with low friction and high wear resistance

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 × 10. 6. 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 × 10. 6. 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 × 10. 6. 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. Radiation cross-linked UHMWPE is preferred in total hip replacements due to its wear resistance [1]. In total knees, where stresses are higher, there is concern of fatigue damage [2]. Antioxidant stabilization of radiation cross-linked UHMWPE by blending vitamin E into the polymer powder was recently introduced [3]. Vitamin E greatly hinders radiation cross-linking in UHMWPE [4]. In contrast peroxide cross-linking of UHMWPE is less sensitive to vitamin E concentration [5]. In addition, exposing UHMWPE to around 300°C, increases its toughness by inducing controlled chain scission and enhanced intergranular diffusion of chains, simultaneously [6]. We present a chemically cross-linked UHMWPE with high vitamin E content and improved toughness by high temperature melting. Methods and Materials. Medical grade GUR1050 UHMWPE was blended with vitamin E and with 2,5-Di(tert-butylperoxy)-2,5-dimethyl-3-hexyne or P130 (0.5% Vitamin-E and 0.9% P130). The mixed powder was consolidated into pucks. The pucks were melted for 5 hours in nitrogen at 300, 310 and 320°C. One set of pucks melted at 310°C was accelerated aged at 70°C at 5 atm. oxygen for 2 weeks. Tensile mechanical properties were determined using ASTM D638. Izod impact toughness was determined using ASTM D256 and F648. Wear rate was determined using a bidirectional pin-on-disc (POD) tester with cylindrical pins of UHMWPE against polished CoCr discs in undiluted, preserved bovine serum. Results. The vinyl index increased as a function of temperature (Fig 1a). Cross-link density steadily decreased and impact strength increased with increasing vinyl index (Fig 1b). The ultimate tensile strength (UTS) was not affected by HTM (Table 2). Impact strength was significantly improved for all treatment temperatures (P<0.05) and wear was significantly increased only for the sample melted at 320°C (Table 2). Discussion. High temperature melting (HTM) was shown to increase toughness of UHMWPEs presumably due to controlled chain scissioning and increased intergranular diffusion of chains [6]. For radiation cross-linked UHMWPE, it was shown that an increase in elongation-at-break and impact strength could be obtained without sacrificing wear resistance up to an elongation of about 500% [7]. This vitamin E-blended, peroxide cross-linked, high temperature melted UHMWPE has very high oxidation resistance due to its high antioxidant content, high wear resistance due to cross-linking and much improved toughness, representing an optimum joint replacement surface. For figures/tables, please contact authors directly.

Introduction. In vivo, UHMWPE bearing surfaces are subject to wear and oxidation that can lead to bearing fatigue or fracture. A prior study in our laboratory of early antioxidant (AO) polyethylene retrievals, compared to gamma-sterilized and highly cross-linked (HXL) retrievals, showed them to be more effective at preventing in vivo oxidation. The current analysis expands that early study, addressing the effect of:. manufacturing-variables on as-manufactured UHMWPE;. in vivo time on these initial properties;. identifying important factors in selecting UHMWPE for the hip or knee. Methods. After our prior report, our IRB-approved retrieval laboratory received an additional 96 consecutive AO-retrievals (19 hips, 77 knees: in vivo time 0–6.7 years) of three currently-marketed AO-polyethylenes. These retrievals represented two different antioxidants (Vitamin E and Covernox) and two different delivery methods: blending-prior-to and diffusing-after irradiation cross-linking. Consecutive HXL acetabular and tibial inserts, received at retrieval, with in vivo time of 0–6.7 years (260 remelted, 170 annealed) were used for comparison with AO-retrievals. All retrievals were analyzed for oxidation and trans-vinylene index (TVI) using a Thermo-Scientific iN10 FTIR microscope. Mechanical properties were evaluated for 35 tibial inserts by uniaxial tensile testing using an INSTRON load frame. Cross-link density (n=289) was measured using a previously published gravimetric gel swell technique. Oxidation was reported as maximum ketone oxidation index (KOI) measured for each bearing. TVI was reported as the average of all scans for each material. Cross-link density and mechanical properties were evaluated as a function of both TVI and oxidation. Results. Minimal increase in oxidation was seen in these AO-retrievals, out to almost 7 years in vivo. In contrast, HXL-retrievals showed increasing KOI with time in vivo (annealed-HXL = 0.127/year, remelted-HXL = 0.036/year, p<0.001). HXL oxidation rate was higher in knees (0.091/year) than in hips (0.048/year), p<0.001. Cross-link density (XLD) correlated positively with TVI for both HXL (Pearson's correlation=0.591, p<0.001) and AO (Pearson's correlation=0.598, p<0.001) retrievals. AO-materials had higher TVI for the same or similar XLD than did HXL polyethylene. XLD correlated negatively with KOI for HXL retrievals (Pearson's correlation=−0.447, p<0.001). Mechanical properties varied by material across all materials evaluated, with tensile toughness correlating negatively with increasing TVI (Pearson Correlation=−0.795, p<0.001). Discussion. Irradiation cross-linking has been used effectively to improve wear resistance. Residual free radicals from irradiation are the target of AO-polyethylene, to prevent loss of UHMWPE XLD, resulting from in vivo oxidation of free radicals as seen in HXL retrievals, and toughness, resulting from oxidation or initial remelting. Despite different manufacturing variables, AO-polyethylene retrievals in this cohort had minimal oxidation and no change in XLD or toughness due to oxidation. However, toughness did vary with irradiation dose as did cross-link density. To achieve the same level of cross-linking as HXL-polyethylene required a higher irradiation dose in blended AO-polyethylene. AO-polyethylenes evaluated in this study had toughness that decreased with irradiation dose, but avoided loss of toughness due to remelting. Because AO-polyethylenes did not oxidize, they did not show the decrease of cross-link density, and potential loss of wear resistance, seen in HXL-polyethylene. For any figures or tables, please contact authors directly

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 × 10. 6. [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

Introduction. Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE in knee and hip prostheses after total joint replacement is one of the major restriction factors on the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear of UHMWPE. A number of studies have investigated the factors influencing the wear of UHMWPE acetabular cup liner in hip prosthesis. Most of these studies, however, have focused on the main articulating surfaces between the femoral head and the polyethylene liner. Materials and Methods. In a previous study (Cho et al., 2016), the generations of cold flow into the screw holes in the metal acetabular cup were observed on the backside of the retrieved UHMWPE acetabular cup liners as shown in Figure 1. We focused on the screw holes in the metal acetabular cup (Figure 2) as a factor influencing the wear behavior of polyethylene liner in hip prosthesis. In this study, computer simulations of the generation of cold flow into the screw holes were performed using the finite element method (FEM) in order to investigate the influence of the screw holes in the metal acetabular cup on the mechanical state and wear behavior of polyethylene liner in hip prosthesis. Results. An example of the results of the FEM simulations performed in this study is shown in Figure 3. In the region which the cold flow into the screw holes occurred, it was found that locally high contact stresses which exceed the yield stress of UHMWPE and considerable plastic strains were generated throughout the overall thickness between the backside and top surface of the polyethylene liners. On the contrary, in the case of the polyethylene liner combined with the metal acetabular cup without screw hole, although the regions of high contact stress and high plastic strain had a tendency to be limited around contact surface compared with those of the combination with screw holes, the values of contact stress and plastic strain were lower than the combination with screw holes. Discussion and Conclusions. The results of this study suggest that the cold flow generated by the existence of the screw holes in the metal acetabular cup of hip prosthesis reduces the wear resistance of the UHMWPE acetabular cup liner. It would appear that the cold flow into the screw holes contributes to structural weakening of the UHMWPE and reduction of the polyethylene thickness, thus increase of internal stresses and plastic strains in and around the regions of cold flow. Therefore, it is required that improvement of the screw holes in the metal acetabular cup and/or improvement of fixation method of the metal acetabular cup to a pelvis in order to enhance the wear resistance of the polyethylene liner. For any figures or tables, please contact authors directly

Introduction. In order to reduce polyethylene wear and incidence of osteolysis, and improve the long-term durability of total hip arthroplasty (THA), highly cross-linked polyethylene was introduced for clinical use in substitution for conventional polyethylene. We performed 35 cementless THAs between December 2000 and February 2002, and cross-linked polyethylene was used in these surgeries. The purpose of this study is to research linear wear rate of these hips, and to find the risk factor for high wear rate. Patients and Methods. 32 hips (26 patients) among the 35 could be evaluated at more than 10 years postoperatively. One hip required reoperations due to infection at 8 years postoperatively, and two were lost to followup in less than 10 years. There were 2 males and 24 females, and the observation period was 11.4 years in average (range 10?13 years). The age at the time of operation was 49.4 years in average (range 24?67 years), and body mass index was 22.4 in average (15?34). We used AHFIX total hip prostheses (KYOCERA Medical Corporation) for both acetabular and femoral replacement, and 22 mm Zirconia head was used in all cases. The median cup diameter was 46 mm (range 42?50). Osteolysis and loosening of the implant was evaluated on the anteroposterior radiograph of the hip. Using software for wear measurement (Hip Analysis Suite), linear wear rate and cup inclination angle were measured. Correlation between linear wear rate and age, BMI, cup inclination angle, and cup diameter was investigated using correlation coefficient. Results. Osteolysis and loosening were not found in any cases on the final radiograph. Cup inclination angle was 44.7±4.6 degree, and linear wear rate was 0.034±0.019 mm/y in average. No statistical correlation was found between linear wear rate and age, BMI, cup inclination angle, and cup diameter. Discussion. Long-term clinical performance of cross-linked polyethylene is rarely reported, although it is considered to have excellent wear resistance, and reduces the risk of osteolysis and loosening. Linear wear rate in this series was relatively low, and osteolysis and loosening were not observed. It was demonstrated that cross-linked polyethylene had excellent wear resistance when used in combination with 22 mm zirconia head, and observed for more than 10 years

Total hip and knee joint prostheses composed of ultra-high molecular weight polyethylene (UHMWPE) and metal or ceramics have been widely applied. Efficacious treatments such as crosslinking, addition of vitamin E and phospholipid coating to UHMWPE have reduced wear and extended the life of joint prostheses. However, wear problems have not yet been completely solved for cases involving severe conditions, where direct contact can occur in mixed or boundary lubrication. In contrast, extremely low friction and minimum wear are maintained for a lifetime in healthy natural synovial joints containing articular cartilage with superior lubricity. Accordingly, joint prostheses containing artificial hydrogel cartilage with properties similar to those of articular cartilage are expected to show superior tribological functions. In establishing the function of artificial hydrogel cartilage as a novel material for joint prostheses, the tribological properties of hydrogel materials used and synergistic performance with synovia constituents are both important. In this study, the lubrication ability and wear resistance properties of poly(vinyl alcohol) (PVA) hydrogels were evaluated by differences in friction and wear properties in reciprocating tests lubricated with saline and simulated synovial fluid. Biphasic finite element (FE) analysis was applied to elucidate the role of biphasic lubrication mechanism in hydrogels. As biocompatible artificial hydrogel cartilage materials, three PVA hydrogels were prepared using the repeated freeze-thawing (FT) method, the cast-drying (CD) method and the hybrid method for laminated gel of FT on CD, which are physically crosslinked with hydrogen bonding but differ in terms of structure and mechanical properties. First the frictional behavior of the ellipsoidal PVA hydrogel specimens was examined in reciprocating tests against a glass plate, which corresponds to simplified knee prosthesis model (Fig.1), with a sliding speed of 20 mm/s under constant continuous loading. As shown in Fig.1, the three hydrogels exhibited different frictional behaviors in a saline solution. It is noteworthy that the hybrid gel maintained very low friction until the end of test. The CD gel showed slightly higher friction and a gradual increase. Meanwhile, the FT gel showed initial medium friction and a gradual increase. Time-dependent frictional behavior was clarified with biphasic lubrication mechanism via biphasic FE analysis. Contact surface observation showed minimal wear without scratches for hybrid gel in saline. Next, simulated synovial fluid composed of 0.5 wt% hyaluronic acid (HA, molecular weight: 920,000 Da), 1.4 wt% albumin, 0.7 wt% gamma-globulin and 0.01 wt% L-alpha dipalmitoylphosphatidylcholine (DPPC), was used to evaluate tribological performance of these gels in physiological condition. As shown in Fig.2, PVA hydrogels in simulated synovial fluid exhibited very low friction, with hybrid gel showing an extremely low friction coefficient of 0.003 in the test. These friction differences were sustained by biphasic FE analysis. Hybrid gel further showed very little wear (Fig.3), which is favorable in terms of hydrogel durability. These results indicate the importance of superior lubricity and wear resistance of PVA hybrid gel for the clinical application of artificial hydrogel cartilage in joint prostheses

Introduction. Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the most common bearing surface used in total joint arthroplasty due to its excellent wear resistance. While radiation cross-linking is currently used, cross-linking using a cross-linking agent such as a peroxide can also be effective with improved oxidative stability, which can be achived by an antioxidant such as vitamin E. The peroxide cross-linking behavior of UHMWPE in the presence of vitamin E was unknown. We investigated the cross-linking behavior and the clinically relevant mechanical and wear properties of peroxide cross-linked, vitamin E-blended UHMWPE. Materials and Methods. Medical grade UHMWPE (GUR1050) was blended with vitamin E and the peroxide (2,5-Dimethyl-2,5-di(t-butylperoxy)hexyne-3 or P130) before compression molding. Various vitamin E (0.1, 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0 wt%) and peroxide concentrations (0.5, 1 and 1.5 wt%) were studied. The cross-link density was calculated as previously described (Oral 2010). The wear rate was determined using a custom-designed pin-on-disc wear tester against CoCr polished discs at 2 Hz and a rectangular path of 5 × 10 mm in undiluted bovine serum (Bragdon 2001). Tensile mechanical properties were determined using Type V dogbones according to ASTM D638. Oxidative stability was determined using oxidation induction testing (Braithwaite 2010). Double-notching and IZOD impact testing was performed according to ASTM D256. Samples prepared with vitamin E concentrations of 0.3 wt% and above and P130 concentrations of 0.5 and 1 wt% were also terminally gamma sterilized. Controls were 150-kGy irradiated vitamin E blends of UHMWPE. Results and Discussion. The cross-link density of peroxide cross-linked UHMWPEs were higher than the irradiated controls at a given vitamin E concentration (For example 250, 301 and 355 mol/dm3 for 0.5, 1 and 1.5 wt% peroxide cross-linked UHMWPE compared to 217 mol/dm3 for 150 kGy irradiated UHMWPE; Figure 1). The cross-link density dependence of wear was similar to radiation cross-linked UHMWPE, resulting in clinically relevant wear rates of 0.5 to 1.5 mg/MC. While the cross-link density of radiation cross-linked UHMWPE became saturated at vitamin E concentrations above 0.3 wt% (Oral 2008), this was not observed in peroxide cross-linked UHMWPE (Figure 2), suggesting more efficient cross-linking in the presence of the antioxidant. The impact strength was 30% higher for the peroxide cross-linked UHMWPEs at the comparable wear rate compared to irradiated controls (72 vs. 56 kJ/m2). The oxidation induction time of all peroxide cross-linked UHMWPEs (up to 57 min) was higher than that of the 0.1 wt% vitamin E-blended, 150-kGy irradiated UHMWPE (6 min). Gamma sterilization of peroxide cross-linked vitamin E blends decreased wear (0.5 wt% peroxide in Figure 3). Thus, peroxide concentration for cross-linking can be reduced if terminal sterilization is used. The mechanical properties and the oxidative stability of the material were not significantly affected by gamma sterilization. Significance. Peroxide cross-linking enabled good wear resistance for high vitamin E concentration blends of UHMWPE (>0.3 wt%), previously not possible by irradiation. Peroxide cross-linking of vitamin E-blended UHMWPE can provide a one-step, cost-effective method to manufacture wear resistant total joint implants with improved oxidative stability

Introduction. Aseptic loosening is a major cause of revision of total joint arthroplasty (TJA). Although crosslinked Ultra-high molecular weight polyethylene (UHMWPE) have improved wear resistance, residual radicals remaining in the material have a possibility to increase bio-reactivity of particles [2]. In this study, we attempt to evaluate the effects of irradiation and residual radicals on bio-reactivity of the material with a new method called the inverse culture method [1]. Material and methods. UHMWPE particles (10µm diameter in average, Mitsui chemicals Co., LTD) along with irradiated particles (RAD, 300kGy electron irradiation) and particles annealed after the irradiation (RAD+ANN, 100°C 72 hours) are co-incubated with mouse macrophage cell line RAW264 using the inverse culture method. The amount of TNF-α was measured with ELISA. Results and discussion. The amount of TNF-α released by macrophages reacting with virgin UHMWPE, RAD and RAD+ANN is shown in Figure 1. The horizontal axis represents the total surface area of the particles. The coefficient of determination and inclination of the approximate curve are calculated to analyze the result. The coefficient of determination suggested that cytokines released from macrophage is dose-dependent to the surface area of polyethylene particles, which was consistent with the result of our former study[1]. We use the inclination of the approximation curve in Figure 1 as an index to evaluate the bio-reactivity. The values of the index of virgin, RAD and RAD+ANN were 21×10. -4. gLm. -2. , 100×10. -4. gLm. -2. and 59×10. -4. gLm. -2. The inclination of the approximation line of RAD is significantly larger than that of virgin (test for the difference of regression line angle). These suggest that the irradiation to UHMWPE particles increases their bio-reactivity possibly due to radicals. The increased reactivity cannot be eliminated by annealing (100°C 72 hours) completely. Conclusion. Although electron irradiation increases the bio-reactivity of UHMWPE particles, annealing after the irradiation can decrease it, but cannot restore to original reactivity. For any figures or tables, please contact authors directly

Introduction. Radiation cross-linked UHMWPEs were developed to address osteolysis-induced joint arthroplasty failure by improving wear resistance and reducing associated particulate debris. Introduced clinically fifteen years ago, they are the primary bearing surface in use with excellent clinical outcomes and wear resistance. First generation materials sought to maintain oxidative stability by reducing or eliminating free radicals through thermal treatments, while second generation aimed to further balance oxidation resistance and improve mechanical properties through sequential irradiation and annealing or the incorporation of an antioxidant. Recent reports have identified lipid absorption and cyclic loading as potential in vivo oxidation-inducing mechanisms. In this on-going retrieval study, we report on the current status of oxidative stability in these two generations of UHMWPE bearings. Materials & Methods. Six types of highly cross-linked UHMWPE hip and knee bearings (Table 1) were surgically-retrieved and collected under IRB approval. Standard material analysis was performed on cross-sections of loaded and unloaded bearing surfaces of the components. Thin sections (150 µm thickness) were extracted in boiling hexanes under reflux for 16 hours followed by vacuum drying for 24 hours. FTIR was used to evaluate oxidation and calculated from post-hexane absorbance spectra by normalizing the area under 1740 cm. −1. (1680–1780 cm. −1. ) to the area under 1370 cm. −1. (1330–1390 cm. −1. ), per ASTM F2102-13. Gravimetric swelling of regional cross-sectional blocks (1–2 mm. 3. ) for 2 hours in 130°C boiling xylenes was used to assess cross-link density, per ASTM 2214. Results. Irradiated and melted retrievals all showed detectable (OI>0.1) subsurface oxidation in the articular surface of retrievals (Fig 1). Behavior between materials types differed: 47% of Longevity acetabular liners (MOI=0.14±0.19; Table 2) showed detectable oxidation as opposed to 19% in Marathon retrievals (MOI=0.07±0.08), both with comparable sample sizes and in vivo durations. We saw no concomitant change in the cross-link density, except in one case where OI>1.0. Sequentially irradiated and annealed (X3) retrievals showed the highest incidence of detectable oxidation (76%), highest average maximum oxidation (0.35±0.39), signs of oxidative embrittlement and a loss of cross-link density which correlated with decreasing oxidation (R. 2. =0.30; p-value=0.000016). Oxidation was in both loading regions of X3 knees, while Prolong knees were observed to have oxidation solely at the articular surface. Antioxidant-stabilized E1 retrievals showed low detectable oxidation values (MOI=0.11±0.03) in both regions without change in cross-link density. Discussion. Throughout the first decade of service, irradiated and melted UHMWPE retrievals showed subsurface oxidation, but with little to no impact on material properties. Detectable oxidation and embrittlement were identified in sequentially irradiated and annealed retrievals at shorter time points. Residual free radicals and pre-implantation shelf oxidation, as a result of air permeable packaging, are potential factors behind the higher oxidation at earlier time points. Antioxidant-stabilized retrievals showed no change in their oxidative behavior with the lowest oxidation and variability in this very short 0–3 year follow-up. Continued analysis is needed to understand the second decade of behavior along with longer-term follow-up with patients to understand if these changes could affect clinical outcomes through oxidation-induced changes in material or mechanical properties

AM Open Cell porous Ti Structures were investigated for compressive strength, morphology (i.e. pore size, struts size and porosity), and wear resistance with the aim to improve design capability at support of implant manufacturing. Specimens were manufactured in Ti6Al4V using a SLM machine. Struts sizes had nominal diameters of 200µm or 100µm, pores had nominal diameters of 700µm, 1000µm or 1500µm. These dimensions were applied to three different open-cell geometrical configurations: one with unit-cells based on a regular cubic arrangement (Regular), one with a deformed cubic arrangement (Irregular), and one based on a fully random arrangement (Fully Random). Morphological analysis was performed by image analysis applied onto optical and SEM acquired pictures. The analyses estimated the maximum and minimum Feret pores diameter, and the latter was used as one of the key parameters to describe the interconnected network of pores intended for bone colonization. Outcome revealed the systematic oversizing of the actual struts diameter Vs designed diameter; by opposite min. Feret diameters of the pores resulted significantly smaller than nominal pore diameters, thus better fitting within the range of pores dimension acknowledged to favor the osseointegration. Consequently, the actual total porosity is also reduced. Many technologic factors are responsible for the morphologic differences design vs actual, among these the influence of melting pool dimension, the struts orientation during building and the layer thickness have a significant impact. Mechanical compression was performed on porous cylinder samples. Test revealed the Yield Strength and Stiffness are highly sensitive to the actual porosity. Deformation behavior follows densification phenomenon at lower porosity, whereas at higher porosity the Gibson-Ashby model fits for most of the structure tested. The relationship among load direction, struts alignment and the collapse behavior of the unit cell geometries are discussed. Stiffness of the porous structure is evaluated in both quasistatic and cyclic compression. Wear was investigated according to Taber test method. The abrasion resistance is measured by scratching a ceramic wheel against the different AM porous structures along a circular path. Metal debris eventually loss were quantified by gravimetric analysis at different number of cycles. Correlation among AM porous structure geometry, porosity and wear loss is discussed. All the tested structures showed a debris loss within the limit suggested by FDA for the porous coating in contact with the bone tissue. The actual AM porous Titanium unit cell geometry and features are a key design input. In combination with all the other design factors of a device they may result helpful in address the stress shielding and prevent metal debris release issues. The study underlines the importance of the research activity in AM to support Design for Additive Manufacturing (DFAM) capability. For any figures or tables, please contact authors directly

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. Materials & Methods. Vitamin E-blended (0.1 mass%) PE sheet stock was irradiated with a high dose of gamma-rays (100–150 kGy) and annealed for cross-linking (HD–CLPE+E). PMPC grafting onto the HD–CLPE+E liners was performed by a photoinduced polymerization technique. Then, the PMPC-grafted HD-CLPE+E was sterilized by gamma-ray with a dose of 25 kGy. A CLPE with 50 kGy gamma-ray irradiation and 25 kGy gamma-ray sterilization was used as control. Surface properties and oxidative properties of the liners were examined. The wear test was performed using a 12-station hip joint simulator according to the ISO 14242-3. A 26-mm Co-Cr-Mo alloy femoral head component was used for the tests. Results. After PMPC grafting, the peaks ascribed to the MPC unit were clearly observed in both Fourier-transform infrared and X-ray photoelectron spectroscopy spectra. Furthermore, PMPC-grafted CLPE and HD-CLPE+E surface became wettable drastically. Oxidation-induction time of PMPC-grafted HD-CLPE+E was significantly longer compared with non-additive CLPE. After 5.0 million cycles of the simulator test, PMPC-grafted HD–CLPE+E were found to show extremely low and stable wear. Substantially fewer wear particles isolated from lubricants were found for both PMPC-grafted liners than for untreated CLPE liners. Discussion. In this study, we confirmed that the PMPC-grafted layer was successfully fabricated on the HD-CLPE+E surface, and the PMPC-grafted HD-CLPE+E brought high oxidation and wear-resistances. When the surface is modified by PMPC grafting, the PMPC-grafted layer leads to a significant reduction in the sliding friction between the surfaces which are grafted because water thin films formed can act as extremely efficient lubricants. Based on clinical trials and other related evidence, the Japanese government (Ministry of Health, Labour and Welfare) approved the clinical use of PMPC-grafted CLPE without vitamin E acetabular liners in April 2011. Furthermore, and in spite of high-dose gamma-ray irradiation for cross-linking, the substrate modified by vitamin E-blending maintains high oxidation-resistance. Indeed vitamin E is an extremely efficient radical scavenger. Conclusion. In conclusion, the PMPC-grafted HD–CLPE+E provides not only high wear resistance but also high oxidation stability, i.e., life-long durability

Introduction:. Irradiated ultra-high molecular weight polyethylene (UHMWPE), used in the fabrication of joint implants, has increased wear resistance [1]. But, increased crosslinking decreases the mechanical strength of the polymer [2], thus limiting the crosslinking to the surface is desirable. Here, we usedelectron beam irradiation with low energy electrons to limit the penetration of the radiation exposure and achieve surface cross-linking. Methods:. Medical grade 0.1 wt% vitamin E blended UHMWPE (GUR1050) was consolidated and irradiated using an electron beam at 0.8 and 3 MeV to 150 kGy. Fourier Transform Infrared Spectroscopy (FTIR) was used from the surface along the depth at an average of 32 scans and a resolution of 4 cm. −1. A transvinylene index (TVI) was calculated by normalizing the absorbance at 965 cm. −1. (950–980 cm. −1. ) against 1895 cm. −1. (1850–1985 cm. −1. ). TVI in irradiated UHMWPE is linearly correlated with the radiation received [3]. Vitamin E indices were calculated as the ratio of the area under 1265 cm. −1. (1245–1275 cm. −1. ) normalized by the same. Pin-on-disc (POD) wear testing was conducted on cylindrical pins (9 mm dia., 13 mm length, n = 3) as previously described at 2 Hz [4] for 1.2 million cycles (MC). Wear rate was measured as the linear regression of gravimetric weight change vs. number of cycles from 0.5 to 1.2 MC. Double notched IZOD impact testing was performed (63.5 × 12.7 × 6.35 mm) in accordance with ASTM F648. Cubes (1 cm) from 0.1 wt% blended and 150 kGy irradiated pucks (0.8 MeV) were soaked in vitamin E at 110°C for 1 hour followed by homogenization at 130°C for 48 hours. Results:. The penetration of the electron beam for cross-linking was limited at low beam energy and cross-linking of the surface 2 mm was achieved (Fig 1). The wear rate of samples irradiated at 0.8 and 3 MeV was 1.12 ± 0.15, and 0.98 ± 0.11, respectively (p > 0.5). In addition, the wear rate of the surface (0.8 MeV) irradiated UHMWPE was 0.33 ± 0.02 mg/MC 1 mm below the surface. The impact strength of UHMWPE irradiated at 0.8 MeV was 73 kJ/m. 2. and 54.2 kJ/m. 2. for that irradiated at 3 MeV (p = 0.001). Doping with vitamin E and homogenization increased the surface vitamin E concentration from undetectable levels to 0.11 ± 0.01. Discussion:. The wear rate of this surface cross-linked UHMWPE was comparable to uniformly cross-linked UHMWPEs irradiated at higher electron beam energies. Even lower wear rate subsurface suggested the feasibility of machining 1 mm from the surface in implant fabrication. Limiting cross-linking to the surface resulted in higher impact strength compared to a uniformly cross-linked UHMWPE. Vitamin E was optionally replenished by additional doping after cross-linking; an advantage of this method may be increased oxidation resistance. Significance: Low energy irradiation of vitamin E blended UHMWPE is feasible to fabricate total joint implants with high wear resistance and impact strength

Introduction. The most common bearing couple used in total knee arthroplasty (TKA) is ultra-high molecular weight polyethylene (UHMWPE) articulating against a CoCrMo alloy femoral component. Although this couple has demonstrated good clinical results, UHMWPE wear has been identified as one of the principal causes for long-term failure of total knee joint replacements. 1. indicating a need for improvements in TKA bearings technology. The wear resistance of UHMWPE can be improved by radiation crosslinking; however, in order to get the full benefit of this improved wear resistance, an abrasion resistant ceramic counterface is necessary. 2. Since the radiation crosslinking degrades mechanical properties, it is also important to have an optimized radiation dose and subsequent processing. The purpose of this study was to evaluate the long-term wear performance of VERILAST Technology comprising two advanced bearing technologies, abrasion resistant OXINIUM femoral components (OxZr). 3-4. and wear/strength optimized 7.5 Mrad crosslinked polyethylene (7.5-XLPE). 5. Materials and Methods. Three component assemblies of LEGION(tm) cruciate retaining (CR) OxZr femoral components, 7.5-XLPE tibial inserts were tested on an AMTI knee simulator under displacement control at 1 Hz frequency as described previously. 2. The tibial inserts were manufactured from compression molded GUR 1020 UHMWPE, radiation crosslinked to 7.5 Mrad dose, remelted to extinguish free radicals, and sterilized by EtO. The wear test was conducted for 45 Mcycle, which was considered to be a conservative estimate for the amount of cycles that would occur during 30 years of typical in-vivo use based on the relationship between patient age and the number of loading cycles as reported in the literature. 6-8. Results. The predominant wear feature on the 7.5-XLPE inserts was burnishing. There were no signs of fatigue wear or delamination. The mean volumetric wear rate (± SD) of the 7.5-XLPE inserts articulating against OxZr femoral components for 45 Mcycle was 0.58±0.17 mm. 3. /Mcycle. In a previous wear test under substantially identical conditions for 5 Mcycle simulating approximately 3 years of use, the mean volumetric wear rate of CoCr and virgin UHMWPE (CPE) couples was 23.4±2.4 mm. 3. /Mcycle. 2. The mean volumetric wear rate of the OxZr/7.5-XLPE couples was approximately 98% lower compared to the CoCr/CPE couples (p<0.01). After simulating 3 years of use, the mean volumetric wear of OxZr/7.5-XLPE couples (2.67 mm. 3. ) was approximately 98% lower than CoCr/CPE couples (120.42 mm. 3. ) (Figure 1). Furthermore, after simulating 30 years of use, the mean volumetric wear of OxZr/7.5-XLPE couples (22.78mm. 3. ) was approximately 81% lower than the CoCr/CPE couples after simulating 3 years of use (120.42 mm. 3. ) (Figure 2). Discussion. This study demonstrates that coupling OxZr femoral components with 7.5-XLPE inserts results in a TKA bearing combination that provides and maintains significantly lower, long-term wear performance

Contemporary crosslinked polymers didn't just happen. The material was, has, and continues to be studied more than any other bearing surface material used in the total hip and total knee replacement construct. Historical failures and successes provided the information needed to make it the success that it is today as we approach the end of the second decade of extensive use. Recognition that wear particles, not cement, was the major cause of osteolysis was important. Next, understanding that oxidation from free radical formation was deleterious to wear resistant polyethylene was understood and finally, that crosslinking was responsible for magnitude increases in wear resistance. Although manufacturers have developed multiple processes to develop their crosslinked polymers (gamma and e beam radiation, melting and annealing, and most recently the addition of antioxidants) there are excellent 10-year results demonstrating head penetration rates (indicative of wear and creep) in the 0.02 to 0.04 mm/year range for many materials with minimal if any detection of osteolysis on radiographs and close to 0% revised for wear at 10+ years. Are there any cautions? Recently, at 10- to 15-year follow up, some clinically insignificant osteolysis has been noted in one study and in that same study, 36 mm heads had twice the volumetric wear as 32 mm heads, but it was still a relatively low volume compared to the previous generation polyethylenes. We need further follow up, but at two decades of use, crosslinked polymers have dramatically reduced the osteolysis problem

Introduction. Large diameter femoral heads offer increased range of motion and reduced risk of dislocation. However, their use in total hip arthroplasty has historically been limited by their correlation with increased polyethylene wear. The improved wear resistance of highly crosslinked UHWMPE has led a number of clinicians to transition from implanting traditionally popular sizes (28mm and 32 mm) to implanting 36 mm heads. Desire to further increase stability and range of motion has spurred interest in even larger sizes (> 36 mm). While the long-term clinical ramifications are unknown, in-vivo measurements of highly crosslinked UHMWPE liners indicate increases in head diameter are associated with increased volumetric wear [1]. The goal of this study was to determine if this increase in wear could be negated by using femoral heads with a ceramic surface, such as oxidized Zr-2.5Nb (OxZr), rather than CoCrMo (CoCr). Specifically, wear of 10 Mrad crosslinked UHMWPE (XLPE) against 36 mm CoCr and 44 mm OxZr heads was compared. Materials and Methods. Ram-extruded GUR 1050 UHMWPE was crosslinked by gamma irradiation to 10 Mrad, remelted, and machined into acetabular liners. Liners were sterilized using vaporized hydrogen peroxide and tested against either 36 mm CoCr or 44 mm OxZr (OXINIUM(tm)) heads (n=3). All implants were manufactured by Smith & Nephew (Memphis, TN). Testing was conducted on a hip simulator (AMTI, Watertown, MA) as previously described [2]. The 4000N peak load (4 time body weight for a 102 kg/225 lb patient) and 1.15 Hz frequency used are based upon data obtained from an instrumented implant during fast walking/jogging and have previously been shown to generate measurable XLPE wear [2,3]. Lubricant was a serum (Alpha Calf Fraction, HyClone Laboratories, Logan, UT) solution that was replaced once per week [2]. Liners were weighed at least once every million cycles (Mcycle) over the duration of testing (∼ 5 Mcycle). Loaded soak controls were used to correct for fluid absorption. Single factor ANOVA was used to compare groups (a = 0.05). Results. The predominant wear feature displayed on the articular surface of liners was burnishing. There were no signs of fatigue wear or of delamination. Mean wear rates (± std dev) of liners articulated against 36mm CoCr and 44 mm OxZr heads were 3.7 ± 0.4 mm. 3. /Mcycle and 2.7 ± 0.4 mm. 3. /Mcycle, respectively (Figure 1). This difference was statistically significant (p = 0.04). Discussion. Although large diameter heads offer biomechanical advantages, their use in total hip arthroplasty has historically been limited due to correlation with increased polyethylene wear. While highly crosslinked liners exhibit significantly improved wear resistance over conventional UHWMPE, their wear has also been shown to increase with head size [1]. Results presented here indicate that this increase in wear can be negated by using OxZr, rather than CoCr. Specifically, wear of XLPE liners was lower against 44 mm OxZr heads than against 36 mm CoCr heads