Summary. Prosthetic UHMWPE added with vitamin E and crosslinked UHMWPE are able to decrease significantly the adhesion of various bacterial and fungal strains limiting biomaterial associated infection and consequent implant failure. Introduction. Polyethylene abrasive and oxidative wear induces overtime in vivo a foreign-body response and consequently osteolysis, pain and need of implant revision. To solve these problems the orthopaedic research has been addressed to develop new biomaterials such as a crosslinked polyethylene with a higher molecular mass than standard Ultra High Molecular Weight Polyethylene (UHMWPE), and consequently a higher abrasive wear resistance and an antioxidant (vitamin E)-added UHMWPE to avoid oxidative wear. Nevertheless a feared complication of implant surgery is bacterial or fungal infection, initiated by microbial adhesion and biofilm formation, and related to the biomaterial surface characteristics. Staphylococci are the most common microorganisms causing biomaterial associated infection (BAI), followed by streptococci, Gram-negative bacilli and yeasts. With the aim to prevent BAI, the purpose of this study was to evaluate the adhesion of various microbial strains on different prosthetic materials with specific surface chemical characteristics, used in orthopaedic surgery. Methods. We compared the effects of vitamin E-added UHMWPE and crosslinked UHMWPE with that of standard GUR 1020 UHMWPE, upon the adhesion of ATCC biofilm-producing strains of Staphylococcus epidermidis, S. aureus, Escherichia coli and Candida albicans. After different incubation times the samples were sonicated to release the attached microorganisms and spread onto agar to quantify colony forming units (UFC)/ml. The biomaterials were physico-chemically characterised by means of scanning electron microscopy (SEM), water contact angle (CA) measurements and attenuated total reflectance (ATR)-fourier transform infrared (FTIR) spectroscopy, before and after adhesion assays. The experiments were assayed in triplicate and repeated a minimum of three times. A statistical analysis on results was conducted. Results. No significant difference of the surface roughness, CA and ATR-FTIR spectroscopy was found among the different biomaterials. After 3 and 7 h of incubation microbial adhesion rates were similar with no statistically relevant differences among the samples assayed. On the contrary, after 24 and 48 h of incubation a significantly (p<0.05 and p<0.01) different adhesion trend was achieved on the three biomaterials, highlighting a microbial adhesion significantly lower on vitamin E-added UHMWPE and crosslinked UHMWPE compared with that on standard UHMWPE. Discussion/Conclusion. Standard UHMWPE, vitamin E-added UHMWPE and crosslinked UHMWPE were chosen because of their diffusion in the clinical use. Previously we showed that vitamin E addition to the UHMWPE reduces the adhesive ability of various staphylococcal strains, compared with standard UHMWPE, and we correlated this results with its antioxidant properties. The results of this study indicate a quite similar significant reduction of bacterial and fungal adhesion on either vitamin E-added UHMWPE and crosslinked UHMWPE, if compared to standard UHMWPE at 48h. Further analysis on the chemical- physical characteristics of the UHMWPE surfaces and on their morphology are needed to explain the different adhesions

Summary Statement. Vitamin E-UHMWPE particles have a reduced osteolysis potential in vivo when compared to virgin, highly cross-linked UHMWPE in a murine calvarial bone model. Introduction. Ultra high-molecular weight polyethylene (UHMWPE) particle-induced osteolysis is one of the major causes of arthroplasty revisions. The lack of particle clearance from the joint inevitably leads to the upregulation of the inflammatory cascade, resulting in bone resorption and implant loosening. Recent in vitro findings (Bladed CL et al. ORS 2011 and J Biomed Mater Res B Appl Biomater, 2012) have suggested that UHMWPE wear particles containing vitamin-E (VE) may have reduced functional biologic activity and decreased potential to cause osteolysis. This is of significant importance since VE-stabilised cross-linked UHMWPEs were recently introduced for clinical use, and there is no in vivo data determining the effects of wear debris from this new generation of implants. In this study we hypothesised that particles from VE-stabilised, radiation cross-linked UHMWPE (VE-UHMWPE) would cause reduced levels of osteolysis in a murine calvarial bone model when compared to virgin gamma irradiated cross-linked UHMWPE. Methods. Study groups were the following: 1) Radiation cross-linked VE-UHMWPE, approximately 0.8% by weight, diffused after 100 kGy; 2). Radiation cross-linked virgin UHMWPE (virgin UHMWPE); 3). Shams. Particle generation and implantation: UHMWPE was sent to Bioengineering Solutions (Oak Park, IL) for particle generation. After IACUC approval, C57BL/6 mice (n=12 for each group) received equal amount of particulate debris (3mg) overlying the calvarium and were euthanised after 10 days. Micro-CT scans: High resolution micro-CT scans were performed using an X-Tek HMX ST 225 with a set voltage of 70 kV and current of 70 µA. Topographical Grading Scale: Each calvarial bone (interparietal, right and left parietal, right and left frontal) was blindly scored using the following scale: 0=No osteolysis, defined as intact bone; 1=Minimal osteolysis, affecting 1/3 or less of the bone area; 2=Moderate osteolysis, affecting at least 2/3 of the bone area; 3=Severe osteolysis, defined as completely osteolytic bone. Histological Analysis: H&E and TRAP staining was performed on tissue to confirm the micro-CT findings and to quantify osteoclasts. Statistical Analysis: Inter-rater analysis was performed using Cohen's kappa analysis. An inter-rater coefficient >0.65 was considered as high inter-rater agreement. Comparison between groups was made using one-way ANOVA with post hoc Bonferroni correction for multiple comparisons. Correlations are reported as Spearman's rho. A p-value<0.05 was considered statistically significant. Results. More than 83% of the VE-UHMWPE and more than 85% of the virgin UHMWPE particles measured less than 1 µm in mean particle size. The mean particle size for VE-UHMWPE was 1.12 µm (range 0.28 to 79.08 µm), while virgin UHMWPE particles measured 1.22 µm (range 0.28 to 82.04 µm). There was a statistically significant greater level of osteolysis visualized on the topographical grading scale in calvaria implanted with virgin UHMWPE wear particles. The micro-CT findings were confirmed histologically. A greater amount of inflammatory tissue overlaying the calvaria was observed in the virgin UHMWPE group when compared to both shams and VE-UHMWPE groups. Post hoc analysis revealed significant difference between VE-UHMWPE and virgin UHMWPE for the topographical osteolysis grading score (p = 0.002) but no difference in osteoclast count (p = 0.293). Discussion/Conclusion. This is the first in vivo study reporting the effects of clinically-relevant UHMWPE particles generated from a VE-UHMWPE implant that is in current clinical use. These results suggest that VE-UHMWPE particles have reduced osteolysis potential in vivo when compared to virgin, highly cross-linked UHMWPE in a murine calvarial bone model. Arthroplasty procedures using VE-UHMWPE might be less susceptible to peri-prosthetic loosening caused by wear debris

Currently, between 17% of patients undergoing surgery for adult spinal deformity experience severe instrumentation related problems such as screw pullout or proximal junctional failure necessitating revision surgery. Cables may be used to reinforce pedicle screw fixation as an additive measure or may provide less rigid fixation at the construct end levels in order to prevent junctional level problems. The purpose of this study is to provide insight into the maximum expected load during flexion in UHMWPE cable in constructs intended for correction of adult spine deformity (degenerative scoliosis) in the PoSTuRe first-in-man clinical trial. Following the concept of toppinoff, a new construct is proposed with screw/cable fixation of rods at the lower levels and standalone UHMWPE cables at the upper level (T11). A parametric FE model of the instrumented thoracolumbar spine, which has been previously validated, was used to represent the construct. Pedicle screws are modeled by assigning a rigid tie constraint between the rod and the lamina of the corresponding spinal level. Cables are modeled using linear elastic line elements, fixing the rod to the lamina medially at the cranial laminar end and laterally at the caudal laminar end. A Youngs modulus was assigned such that the stiffness of the line element was the same as that of the cable. An 8 Nm flexion moment was applied to the cranial endplate. The maximum value of the force in the wire (80 N) is found at the T11 (upper) level. At the other levels, forces in the cable are very small because most of the force is carried by the screw (T12) or because the wires are force shielded by the contralateral and adjacent level pedicle screws (L2, L3). The model provides first estimates of the forces that can be expected in the UHMWPE cables in constructs for kyphosis correction during movement. It is expected that this approach can help in defining the number of wires for optimal treatment

Summary Statement. In the most recent type of highly cross-linked UHMWPE, stabilised by vitamin E, the majority of this anti-oxidant cannot be leached out. Even more, the vitamin E molecules are grafted to the UHMWPE polymer backbone by an ether bond. Introduction. Today, highly cross-linked, vitamin E stabilised UHMWPE is clinically accepted as bearing material in joint replacements. Little is known about the chemistry of this antioxidant in the polymer after irradiation. The present investigation presents a model for the chemical nature of the trapping of vitamin E in PE. Method. UHMWPE type GUR 1020 (Ticona GmbH, Kelsterbach/Germany) was blended with 0.1 % vitamin E (Merck KGaA, Darmstadt/Germany), compression moulded at Mathys Ltd Bettlach in-house and cross-linked with γ-irradiation dose of nominally 100 kGy. To assess the extent of vitamin E leachable out, three 0.3 mm sections were cut from the centre of the samples. By extraction in heptane for 48 h at 98 °C, this amount of vitamin E trapped in the polymer was determined by Fourier transform infrared spectroscopy (FTIR) as relative vitamin E index (RVEI). The nature of the extracted substances was analysed by GC-MS. For solids, many of modern spectroscopic methods are not applicable. Therefore, 0.1 % vitamin E were dissolved in two model hydrocarbons (cyclohexane and n-octane) and irradiated at the same 100 kGy γ-dose. In order to determine the chemical bond vitamin E – hydrocarbon after irradiation, these liquid solution samples were analysed by different spectroscopic methods, such as GC-MS, MALDI-TOF-SIMS, HPLC and NMR. Results. Extraction experiments showed that only 23 % of the vitamin E could be extracted by heptane after irradiation whereas from a non-irradiated control sample, all vitamin E was extracted. GC-MS confirmed that the extracted vitamin E was chemically unchanged. Analysing the model hydrocarbons after irradiation, the GC-MS-chromatogram of the cyclohexane solution showed a single peak of the formal cyclohexene adduct of vitamin E. Illustrates this adduct, cyclohexyl-6-O-α-tocopherolether. Contrariwise, the same analysis of the n-octane solution revealed three formal octane adducts. By preparing references substances these three peaks could be attributed to ethers of vitamin E bonded at three different, but chemically equivalent CH. 2. positions on the eight carbon atom chain of n-octane. The single mass peak of the cyclohexane solution arises from the six chemically equivalent carbon atoms in this cyclic hydrocarbon. The 100 kGy γ-dose transformed 76 % of the vitamin E in the n-octane solution to the corresponding ethers and 68 % of the vitamin E in the cyclohexane to cyclohexyl ether. Therefore we postulate that in highly cross-linked, vitamin E stabilised UHMWPE the vitamin E is grafted to the polymer carbon backbone by an ether bond at the phenolic OH group of the vitamin E molecule. Conclusion. Upon irradiation, vitamin E is grafted to the UHMWPE polymer backbone to a large amount. This portion of antioxidant cannot leach out. Therefore, vitamin E stabilised HXLPE is protected from oxidation and ageing by a chemically grafted, quasi internal antioxidant

Summary. Low energy irradiation of vitamin E blended UHMWPE is feasible to fabricate total joint implants with high wear resistance and impact strength. Introduction. Irradiated ultra-high molecular weight polyethylene (UHMWPE), used in the fabrication of joint implants, has increased wear resistance. But, increased crosslinking decreases the mechanical strength of the polymer, thus limiting the crosslinking to the surface is desirable. Here, we used electron beam irradiation with low energy electrons to limit the penetration of the radiation exposure and achieve surface cross-linking. Methods. Medical grade 0.1wt% 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. (950–980cm. −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 × 6.35mm) in accordance with ASTM F648. Cubes (1 cm) from 0.1wt% 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. 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

Summary Statement. Novel radiopaque UHMWPE sublaminar cables may be a promising alternative to gliding pedicle screws or titanium sublaminar cables within a growth-guidance system for the surgical treatment of early onset scoliosis. Introduction. Growth-guidance or self-lengthening rod systems are an alternative to subcutaneous growing rods and the vertical expandable prosthetic titanium rib for the treatment of early onset scoliosis. Their main perceived advantage over growing rods is the marked decrease in subsequent operative procedures. The Shilla growth-guidance system and a modern Luque trolley are examples of such systems; both depend on gliding pedicle screws and/or sliding titanium sublaminar wires. However, the unknown consequences of metal-on-metal wear debris are reason for concern especially in young patients. In this study, instrumentation stability, residual growth in the operated segment after surgery and biocompatibility of the novel radiopaque UHMWPE cables as an alternative to gliding pedicles screws or titanium sublaminar wires were assessed in an immature sheep model. Materials and methods. Twelve immature sheep were treated with segmental sublaminar spinal instrumentation: dual CoCr rods were held in place by pedicle screws at the most caudal instrumented level (L5) and novel radiopaque UHMWPE (Bi. 2. O. 3. additive) woven cables were placed at 5 thoracolumbar levels. Lateral radiographs were taken at 4-week intervals to evaluate growth of the instrumented segment. Four age-matched, unoperated animals served as radiographic control. After 24 weeks follow-up, the animals were sacrificed and the spines were harvested for histological evaluation and CT analysis. Results. No neurological deficits and no complications occurred during the initial postoperative period. One animal died during follow-up due to unknown cause. At sacrifice, none of the cables had loosened and the instrumentation remained stable. Substantial growth occurred in the instrumented segment (L5-T13) in the intervention group. No significant difference in growth of the operated segment was found between the intervention and control groups. Histological analysis showed fibrous encapsulation of the novel radiopaque UHMWPE sublaminar cable in the epidural space, with no evidence of chronic inflammation. Discussion. Novel radiopaque UHMWPE cables may be a promising alternative to gliding pedicle screws or titanium sublaminar cables within a growth-guidance system. UHMWPE cables may improve growth results due to the smooth surface properties of the UHMWPE cable and address concerns regarding the consequences of metal-on-metal wear debris

Summary. Fifteen irradiated, vitamin E-diffused UHMWPE retrievals with up to three years in vivo service showed no appreciable oxidation, nor change in material properties from a never-implanted liner, and showed a 94% decrease in free radical content. Introduction. Radiation cross-linking, used to improve wear resistance of ultra-high molecular weight polyethylene (UHMWPE) bearings used in total joint arthroplasty, generates residual free radicals which are the precursors to oxidative embrittlement. First generation materials adopted thermal treatments to eliminate or reduce free radical content, but came with compromises in reduced mechanical properties or insufficient stabilization. A second generation alternative method infuses an antioxidant, vitamin E, into irradiated UHMWPE to stabilise free radicals while maintaining fatigue strength. In vitro studies predict excellent oxidation and wear resistance in vitamin E-stabilised bearings, but the long-term in vivo oxidation behavior, influenced by lipid absorption and cyclic loading, remains largely unknown. Our aim was to investigate in vivo changes in UHMWPE surgically-retrieved explants that were radiation cross-linked and stabilised by vitamin E. Patients & Methods. Fifteen surgically-retrieved irradiated, vitamin E-diffused and inert-gamma sterilised bearings (E1™, Biomet, Inc., Warsaw IN) with in vivo durations ranging from 3 days to 36.6 months were analyzed at unloaded rim/eminence and the articular surface along with one never-implanted component. Total lifetime of components was summed as shelf storage prior to implantation, in vivo duration and ex vivo duration in air. Fourier Transform Infrared Spectroscopy (FTIR) was used to measure carbonyl index (CI; per ASTM F2102-01ε1) both before and after 16 hour hexane extraction to. Extracted thin films were also reacted with nitric oxide to quantify hydroperoxides, an intermediate oxidation product associated with oxidation potential. Cross-link density was calculated from gravimetric swelling analysis per ASTM F2214. Crystallinity measurements were performed regionally using differential scanning calorimetry (DSC). Free radical content was measured by electron spin resonance (Memphis, TN). Results. Irradiated and vitamin E-diffused retrievals showed scratching at the articular surface, but retained machining marks up to three years in vivo, indicative of no measurable wear. Retrievals showed no significant oxidation at the time of surgical removal with maximum post-hexane carbonyl indices in the barely detectable range (MCI=0.029–0.154), located at the surface of retrievals. Ex vivo oxidation was not observed after 18 months of aging in air at room temperature. There was no increase in hydroperoxides (never-implanted HI=0.62±0.04; retrieval HI= 0.62±0.04), nor change in cross-link density (never-implanted: 0.275±0.015 mol/dm. 3. ; retrieval: 0.295±0.016 mol/dm. 3. ) or crystallinity (never-implanted: 58.3±1.4%; retrievals: 60.0±3.5%). Lipid penetration increased with time, showing a higher rate of diffusion in loaded regions. Free radical content was observed to decay with increasing in vivo duration (R. 2. =0.44; p<0.05), and by one order of magnitude (94%) by 36.6 months. A stronger negative correlation (R. 2. =0.65) was observed between the total lifetime of the liner and free radical content. Discussion/Conclusion. The free-radical scavenging activity of the vitamin E appears to successfully prevent both in vivo and ex vivo oxidation for short durations. Without an increase in hydroperoxides, the oxidation cascade initiated by radiation-induced and lipid-derived free radicals appears to have been halted. Retrievals also gave no indication of wear in this timeframe, similar to improved wear resistance seen in first generation materials. Continued monitoring will be necessary at longer implant durations

Summary Statement. In the present hip simulator studies, bearings with the newest generation of HXLPE, stabilised with vitamin E, did not show increased wear under severe conditions, such as accelerated ageing, component mal-orientation and third body wear. Introduction. Unfortunately, acetabular hip components cannot always be implanted in optimal condition. Therefore, we performed hip simulator studies with cups made from highly cross-linked, vitamin E stabilised UHMWPE in i) artificially aged condition, ii) with an inclination angle corresponding to 80 ° in vivo and iii) with third bodies coming from the Ti coating of the acetabular cup. Methods. For these hip simulator studies, seleXys cup inlays, size 28/EE, and RM Pressfit samples 50/28 (Mathys Ltd Bettlach, Switzerland) were used. Standard PE parts and vitamys® inlays (highly cross-linked, vitamin E stabilised UHMWPE) were tested in the same series. PE cups were machined out of sintered GUR 1020 slabs, packaged and gamma-sterilised in inert atmosphere at 30 kGy. The vitamys® material was made in-house by adding 0.1 wt.-% of vitamin E (Merck KGaA, Darmstadt/Germany) to GUR 1020 powder from Ticona GmbH, Kelsterbach/Germany. Cross-linking used 100 kGy gamma-irradiation and the final sterilisation was gas plasma. Artificial ageing was done under pressurised oxygen at 70 °C according to ASTM F2003 for 14 days (standard PE) and 60 days (vitamys®), respectively. The hip simulator test protocol of ISO 14242 was kept for the artificially aged cups, but the inclination angle altered to 80 ° for the test with the steep cup position. In the third test, the test fluid (diluted bovine serum stabilised with sodium azide and EDTA) was altered by adding about 10 Ti particles to the bearing for the first million cycles. This test condition imitates third body wear by particles shed from the coating of the RM cups. All testing was conducted at the RMS Foundation (Bettlach / Switzerland) on a servo-hydraulic six-station hip simulator (Endolab, Thansau/Rosenheim, Germany) at a temperature of 37±1°C. At lubricant change interval of 500’000 cycles, the inlays were measured gravimetrically with an accuracy of 0.01 mg. Results. The wear rate of the standard UHMWPE in the condition aged for 14 days reached 45 mg/Mcycle, corresponding to a 57 % increase over non-aged cups. For the vitamys® cups, the wear rate was virtually unchanged even after 60 days ageing (5.8 mg/Mcycle vs. 5.9 mg/Mcycle). For standard UHMWPE tested with an inclination of 80°, wear was 16% lower than those of the inlays with 45° inclination. Whereas for the vitamys® inlays, the wear rate was about the same for both inclination angles (5.4 mg/Mcycle vs. 5.9 mg/Mcycle,). The addition of Ti particles increased the wear rate of standard UHMWPE to 35 mg/Mcycle. However, vitamys® was hardly affected by the third bodies: the wear rate stood at 7.8 mg/Mcycle. Conclusions. Based on the present simulator study, it seems that hip bearings with the newest generation of HXLPE, stabilised with vitamin E, are exempt from increased wear rate when subjected to severe conditions, such as accelerated ageing, component mal-orientation and third body wear

We examined stainless-steel, cobalt-chrome, titanium and alumina and zirconia ceramic femoral heads retrieved at revision surgery. All the heads had articulated against ultra-high-molecular-weight-polyethylene (UHMWPE) acetabular cups. We studied the simulation of third-body damage and the wear of UHMWPE against the various materials used for the heads. The surfaces of the retrieved heads were analysed using a two-dimensional contacting profilometer. Third-body damage was characterised by the mean height of the scratches above the mean line (R. pm. ). The alumina ceramic and zirconia ceramic retrieved heads were found to have significantly less damage. In laboratory studies the ceramics were also more resistant to simulated third-body damage than the metal alloys. We studied the wear of UHMWPE against the damaged counterfaces in simple configuration tests. The damaged ceramics produced less polyethylene wear than the damaged metal counterfaces. The wear factor of UHMWPE against the damaged materials was dependent on the amount of damage to the counterface (R. p. ). Our study has shown the benefit of using the harder and more damage-resistant ceramic materials for femoral heads

Ultra-high-molecular-weight polyethylene (UHMWPE) components for total joint replacement generate wear particles which cause adverse biological tissue reactions leading to osteolysis and loosening. Sterilisation of UHMWPE components by gamma irradiation in air causes chain scissions which initiate a long-term oxidative process that degrades the chemical and mechanical properties of the polyethylene. Using a tri-pin-on-disc tribometer we studied the effect of ageing for ten years after gamma irradiation in air on the volumetric wear, particle size distribution and the number of particles produced by UHMWPE when sliding against a stainless-steel counterface. The aged and irradiated material produced six times more volumetric wear and 34 times more wear particles per unit load per unit sliding distance than non-sterilised UHMWPE. Our findings indicate that oxidative degradation of polyethylene after gamma irradiation in air with ageing produces more wear

Summary. Four highly cross-linked UHWMPEs except vitamin E-stabilised explants. Introduction. The development of both first and second generation highly cross-linked material focused on stabilizing radiation-induced free radicals as the sole precursor to oxidative degradation; however, secondary in vivo oxidation mechanisms have been identified in both conventional and highly cross-linked UHMWPE, induced by absorbed lipids and cyclic mechanical load. Retrieval studies are reporting in vivo oxidation highly cross-linked retrievals with up to ten year in vivo durations. Preclinical aging tests did not predict these in vivo material changes. With only a decade of these materials in clinical use, retrieval studies are limited to mid-term follow-up. In vitro studies face a challenge in effectively replicating the precise in vivo conditions that lead to this loss of oxidation resistance. In this study, we bypass replicating these in vivo variables by examining surgically-retrieved components, thereby testing material that has been affectively “pre-conditioned” by their in vivo service. After a preliminary post-operative analysis, we subjected retrievals to accelerated aging tests in order to predict the extent to which their oxidative stability had been uniquely compromised in vivo. Patients & Methods. Twenty-four highly cross-linked retrievals of four manufacturing methods (n=6 each of Longevity™, Prolong™, X3™ and E1™) and in vivo durations (1–4 years) were analyzed post-operatively and after accelerated aging (70°C, 5atm O. 2. for 2 weeks; ASTM F2003). Never-implanted components (n=1) of each material type were also aged. Infrared microscopy was used to evaluate lipid absorption, oxidation (per ASTM F2102-01ε1) and hydroperoxide levels after 16 hrs of nitric oxide staining for oxidation potential, and gravimetric swelling analysis assessed cross-link density (ASTM F2214). Results. All retrievals contained absorbed lipids penetrating below both loaded (penetration depth=1.3 ± 0.5 mm) and unloaded (0.6 ± 0.2 mm) surfaces. Each material type subset contained retrievals with and without detectable oxidation after in vivo service (Max OI=0.01–0.94). After aging, all post-irradiation thermally-treated, highly cross-linked retrievals, regardless of initial lipid levels or oxidation, showed oxidative degradation, demonstrated by subsurface oxidative peaks (MOI=0.30–2.63), increased hydroperoxides (3–5X), and decreased cross-link density (−34–90%). In contrast, vitamin E-stabilised retrievals showed below MOI<0.2 with no significant loss of cross-link density. Never-implanted controls for each material type showed no oxidative changes after accelerated aging. Discussion/Conclusion. Accelerating aging after in vivo service has shown oxidative instability characterised by high oxidation and material property loss in the three highly cross-linked materials without an incorporated antioxidant. This oxidative degradation took place regardless of post-operative oxidation levels, indicating that even without detectable oxidation the material had undergone changes during in vivo service, as compared to the lack of oxidative response in never-implanted controls. These findings also suggest that the presence of an antioxidant may be able to slow down and/or stabilise in vivo mechanisms compromising long-term oxidative stability and increase the longevity of highly cross-linked UHMWPE materials

INTRODUCTION. Growth-guidance constructs are an alternative to growing rods for the surgical treatment of early onset scoliosis (EOS). In growth-guidance systems, free-sliding anchors preserve longitudinal spinal growth, thereby eliminating the need for surgical lengthening procedures. Non-segmental constructs containing ultra-high molecular weight polyethylene (UHMWPE) sublaminar wires have been proposed as an improvement to the traditional Luque trolley. In such a construct, UHMWPE sublaminar wires, secured by means of a knot, serve as sliding anchors at the proximal and distal ends of a construct, while pedicle screws at the apex prevent rod migration and enable curve derotation. Ideally, a construct with the optimal UHMWPE sublaminar wire density, offering the best balance between providing adequate spinal fixation and minimizing surgical exposure, is designed preoperatively for each individual patient. In a previous study, we developed a parametric finite element (FE) model that potentially enables preoperative patient-specific planning of this type of spinal surgery. The objective of this study is to investigate if this model can capture the decrease in range of motion (ROM) after spinal fixation as measured in an experimental study. MATERIALS AND METHODS. In a previous in vitro study, the ROM of an 8-segment porcine spine was measured before and after instrumentation, using different instrumentation constructs with a sequentally decreasing number of wire fixation points. In the current study, the parametric FE model of the thoracolumbar spine was first validated relative to ROM values reported in the literature. The rods, screws, and sublaminar wires were implemented, and the model was subsequently used to replicate the in vitro tests. The experimental and simulated ROM”s for the different instrumentation conditions were compared. RESULTS. Good agreement between in vitro biomechanical tests and FE simulations was observed in terms of the decrease in ROM for the complete construct with wires at each level. The stepwise increase in total ROM with decreasing number of wires at the construct ends was less prominent in silico in comparison to in vitro. CONCLUSION. Important first steps in the implementation and validation of a growth-guidance construct for EOS patients in a patient-specific FE model of the spine have been made in this study. The parametric nature of the FE model allows for rapid personalization. Although further improvements to the model will be necessary to better distinguish between different spinal instrumentation constructs, we conclude that the model can well capture essential aspects of spinal motion and the overall effect of instrumentation

Introduction. The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator. Methods. A six-station lumbar spine simulator (Simulation Solutions, UK) was used to test ProDisc-L TDR components (Synthes Spine, USA) under the ISO 18192-1 standard inputs and with the addition of an AP load of +175 and −140N. Wear particles were isolated at 2 and 5 mc using a modified alkaline digestion protocol [4]. Particles were collected by filtration and imaged by high resolution FEGSEM. Particle number and volume distributions were calculated as described previously [4] and were compared statistically by one way ANOVA (p<0.05). Results. Similar particle morphologies were observed under 4DOF and 5DOF inputs, including flakes, fibrils and granules. No significant differences were observed between the size and volume distributions under 4DOF and 5DOF when comparisons were made at the same time point, although there was a trend towards larger particles being generated under 5DOF inputs. The mode of the frequency distribution was in the submicron size range with the volume distributions showing greater variability. Discussion. This study represents the first comprehensive wear particle analysis comparing 4DOF (ISO conditions) to 5DOF inputs on a single TDR device. Vicars et al. [5] reported no significant differences in the wear volume of ProDisc-L TDR components under 4 and 5DOF inputs. The present study has shown that the particle size distributions and particle morphologies were not significantly affected by the addition of AP shear. The wear particles were similar in size and morphology to those previously reported for total hip replacements and total knee replacements [6], indicating that wear debris produced by TDR may have a similar potential for osteolysis

Introduction and Objective. Local cartilage defects in the knee are painful and mostly followed by arthritis. In order to avoid impaired mobility, the osteochondral defect might be bridged by a synthetic compound material: An osteoconductive titanium foam as an anchoring material in the subchondral bone and an infiltrated polymer as gliding material in contact with the surrounding natural cartilage. Materials and Methods. Titanium foam cylinders (Ø38 mm) with porosities ranging from 57% to 77% were produced by powder metallurgy with two different grain sizes of the space holder (fine: 340 ± 110 μm, coarse: 530 ± 160 μm). The sintered titanium foam cylinders were infiltrated with UHMWPE powder on one end and UHMWPE bulk at the other end, at two different temperatures (160 °C, 200 °C), using a pressure of 20 MPa for 15 minutes. Smaller cylinders (Ø16 mm) were retrieved from the compound material by water jet cutting. The infiltration depths were determined by optical microscopy. The anchoring of the UHMWPE was measured by a shear test and the mechanical properties of the titanium foam were verified by a subsequent compression test. The tribological behaviour was investigated in protein containing liquid using fresh cartilage pins (Ø5 mm) sliding against a UHMWPE disc with or without a notch to simulate the gap between the implant and the surrounding cartilage. Friction coefficients were determined in a rotation tribometer and the cartilage wear in a multidirectional six-station tribometer from AMTI (load 10 – 50 N, sliding speed 20 mm/s, 37 °C). Results. UHMWPE could be infiltrated into titanium foam by 1.1 – 1.3 mm with fine pores and by 1.5 – 1.8 mm with coarse pores. The infiltration was neither dependent on the type of UHMWPE (powder or bulk) nor on the temperature. The polymer was so well anchored inside the titanium foam pores that the shear forces for the compounds exceeded the shear strength obtained for a UHMWPE-cylinder. This effect was due to the increased stiffness of the compound plug. Uniaxial compression of the titanium foams after the shear-off of the polymer revealed yield strengths ranging from 50 – 88 MPa for porosities of 62 – 73%. The Ø16 mm samples yielded beyond physiological loads in the knee (≥ 10x body weight) and behaved in a strain hardening and fully ductile manner, reaching deformations of at least 50 % of their initial height without the appearance of macroscopically visible cracks. For smaller plug diameters down to Ø8 mm, however, the lower porosity / higher strength foam should be used to limit elastic deformation of the compound to < 0.1 mm. Pore size did not significantly influence the strength and stiffness values. The elevated coefficient of friction between cartilage and UHMWPE of about 1 was not negatively affected by the presence of the gap. The height loss of the cartilage pin after 1 hour (respectively after 3600 reciproque wear cycles) was 0.2 ± 0.1 mm using a flat disc. For discs with a 1 mm wide V-notch, the wear increased to 0.9 ± 0.3 mm. Conclusions. The tested titanium foams are well suited to act as an anchoring material in the subchondral bone as mechanical properties can be tailored by choosing the adequate porosity and as bone ingrowth has previously been demonstrated for the used pore sizes. UHMWPE is not an ideal gliding partner against cartilage because the friction coefficients of frictions were high. The presence of a V-notched gap was detrimental for cartilage wear. More hydrophilic polymers like PCU should be tested as potential gliding materials

Variations in component positioning of total hip replacements can lead to edge loading of the liner, and potentially affect device longevity. These effects are evaluated using ISO 14242:4 edge loading test results in a dynamic system. Mediolateral translation of one of the components during testing is caused by a compressed spring, and therefore the kinematics will depend on the spring stiffness and damping coefficient, and the mass of the translating component and fixture. This study aims to describe the sensitivity of the liner plastic strain to these variables, to better understand how tests using different simulator designs might produce different amounts of liner rim deformation. A dynamic explicit deformable finite element model with 36mm Pinnacle metal-on-polyethylene bearing geometry (DePuy Synthes, Leeds, UK) was used with material properties for conventional UHMWPE. Setup was 65° clinical inclination, 4mm mismatch, 70N swing phase load, and 100N/mm spring. Fixture mass was varied from 0.5-5kg, spring damping coefficient was varied from 0-2Ns/mm. They were changed independently, and in combination. Maximum separation values were relatively insensitive to changes in the mass, damping coefficient, or both. The sensitivity of peak plastic strain, to this range of inputs, was similar to changing the swing phase load from 70N to approximately 150N – 200N. Increasing the fixture mass and/or damping coefficient increased the peak plastic strain, with values from 0.15-0.19. Liner plastic deformation was sensitive to the spring damping and fixture mass, which may explain some of the differences in fatigue and deformation results in UHMWPE liners tested on different machines or with modified fixtures. These values should be described when reporting the results of ISO14242:4 testing. Acknowledgements. Funded by EPSRC grant EP/N02480X/1; CAD supplied by DePuy Synthes

Ultra-high molecular weight polyethylene (UHMWPE) is a commonly used as bearing material in joint replacement devices. UHMWPE implants can be hard to see on a standard X-ray because UHMWPE does not readily attenuate X-rays. Radiopaque UHMWPE would enable direct imaging of the bearing both during and after surgery, providing in vivo assessment of bearing position, dislocation or fracture, and potentially a direct measure of wear. The X-ray attenuation of UHMWPE was increased by diffusing an FDA approved contrast agent (Lipiodol) into UHMWPE parts (Zaribaf et al, 2018). The aim of this study was to evaluate the optimal level of radiopacity for a UHMWPE bearing. Samples of un-irradiated medical grade UHMWPE (GUR 1050) were machined into 4mm standard medium Oxford Unicompartmental bearings. Samples were immersed in Lipiodol Ultra Fluid (Guerbert, France) at elevated temperatures (85 °C, 95 °C and 105 °C) for 24 h to achieve three different levels of radiopacity. A phantom set-up was used for X-ray imaging; the phantom contained two perspex rods to represent bone, with the metallic tibial tray and polyethylene bearing fixed to the end of one rod and the metallic femoral component fixed to the other rod. Radiographs of the samples were taken (n=5) with the components positioned in full extension. To ensure consistency, the images of all the samples were taken simultaneously alongside an untreated part. The results of our ongoing study demonstrate that the radiopacity of UHMWPE can be enhanced using Lipiodol and the parts are visible in a clinical radiographs. The identification of the optimal treatment from a clinical perspective is ongoing; we are currently running a survey with clinicians to find the consensus on the optimal radiopacity taking into account the metallic components and alignment. Future work will involve a RSA study to assess the feasibility of measuring wear directly from the bearing

Background. When reversing the hard-soft articulation in inverse shoulder replacement, i.e. hard inlay and soft glenosphere, the tribological behaviour of such a pairing has to be tested thoroughly. Therefore, two hard materials for the inlay, CoCr alloy and alumina toughened zirconia ceramic (ATZ) articulating on two soft materials, conventional UHMWPE and vitamin E stabilised, highly cross-linked PE (E-XLPE) were tested. Methods. The simulator tests were performed analogue to standardised gravimetric wear tests for hip prosthesis (ISO 14242-1) with load and motion curves adapted to the shoulder. The test parameters differing from the standard were the maximum force (1.0 kN) and the range of motion. A servo-hydraulic six station joint simulator (EndoLab, Rosenheim) was used to run the tests up to 5 times 106 cycles with diluted calf serum at 37° C as lubricant. Results. The wear rates measured in the simulator when the CoCr alloy inlay articulated on UHMWPE and E-XLPE were respectively 32.50 +/− 3.48 mg/Mcycle and 10.65 +/− 2.36 mg/Mcycle. In comparison, when the ATZ inlay articulated on UHMWPE and E-XLPE the wear rates were 20.34 +/− 1.14 mg/Mcycle and 5.99 +/− 0.79 mg/Mcycle respectively. Conclusions. The simulator wear rate of the standard articulation CoCr – UHMWPE is similar to that found in the corresponding pairing for hip endoprosthesis. Replacing UHMWPE by E-XLPE, the wear rate is reduced to about 1/3 for both hard counterparts, CoCr and ZTA, respectively. Replacing the CoCr inlay by a part made from ZTA lowers wear by about 37 % in articulation against UHMWPE and about 44 % against E-XLPE. The lowest wear rate, with a reduction of about 80 % compared to the standard CoCr – UHMWPE, exhibits the pairing of both advanced materials, ZTA – E-XLPE. However, long-term clinical follow-up will confirm if this in-vitro wear reduction leads to longer in-vivo survival. Level of evidence. Laboratory test on sample implants. Study financed by Mathys Orthopaedie GmbH

Summary Statement. Antioxidant containing UHMWPE particles induced similar levels of in vitro macrophage proliferation and in vivo inflammation in the mouse air pouch model as UHMWPE particles alone. Benefit of antioxidant in reducing wear particle induced inflammation requires further investigation. Introduction. Wear particles derived from UHMWPE implants can provoke inflammatory reaction and cause osteolysis in the bone, leading to aseptic implant loosening. Antioxidants have been incorporated into UHMWPE implants to improve their long term oxidative stability. However it is unclear if the anti-inflammatory property of the antioxidant could reduce UHMWPE particle induced inflammation. This study evaluated the effect of cyanidin and vitamin E on UHMWPE induced macrophage activation and mouse air pouch inflammation. Methods. Four types of UHMWPE were used: (1) compression molded (CM) conventional GUR1020 (PE); (2) CM GUR1020 blended with 300 ppm cyanidin (C-PE); (3) CM GUR1020 blended with 1000 ppm α-tocopherol (BE-PE); and (4) CM GUR1020, gamma irradiated at 100kGy, diffused with α-tocopherol, and sterilised at 30kGy (DE-PE). Particles were generated by cryomilling. Particle count, size, and aspect ratio were determined using SEM and Image Pro. Each particle group was cultured with RAW264.7 macrophage cells at four concentrations (0.625, 1.25, 2.5, and 5 μg/mL) in a standard medium for 4 days. Cell numbers were quantified using MTT assay. Cytokine expression (IL-1β, TNFα, and IL-6) was measured using RT-PCR and ELISA. Particles were also suspended in PBS at 2 concentrations (0.2 or 1 mg) and injected into subcutaneous air pouches in BALB/c mice. Control animals were injected with PBS alone. Six days post-injection air pouches were harvested, half of which were fixed for histology to measure membrane thickness and inflammatory cell quantity. Remaining air pouches were frozen and analyzed by ELISA for cytokine production. Data were analyzed using one-way ANOVA with post hoc testing. P<0.05 was considered significant. Results. All 4 materials showed similar particle characteristics after cryomilling. Particle size ranged from 1 to 19 μm with 33% of particle population smaller than 2 μm. All particle groups supported macrophage proliferation, showing an inverse correlation between proliferation rate and particle dose. Gene expression of IL-1β and TNFα also showed an inverse correlation with particle dose. Expression of IL-1β, TNFα, and IL-6 appeared lower in cells cultured with C-PE than the other 3 materials. The accumulative protein productions of IL-1β and TNFα were significantly lower while IL-6 production was moderately lower in C-PE, BE-PE and DE-PE when compared to PE. Injection of polyethylene particles increased the air pouch membrane thickness significantly compared to the PBS control in all particle types and doses. Higher particle dose induced thicker membrane in all 4 materials. A similar trend was also observed in the percentage of inflammatory cell infiltration in the pouch membrane. C-PE and DE-PE particles at low dose and C-PE particles at high dose induced lower levels of IL-1β and TNFα than PE. IL-6 production was similar between PE and other 3 groups. Discussion/Conclusion. Antioxidant incorporated in UHMWPE did not alter the level of macrophage proliferation and air pouch inflammation induced by UHMWPE particles, although it reduced cytokine gene expression. Future investigation in a synovial joint environment is desired to evaluate the chronic inflammation response to antioxidant containing UHMWPE wear particles and to verify the effect of antioxidant in UHMWPE properties

In 2011, approximately 1.6 million total hip arthroplasties (THAs) were conducted in 27 of the 34 member countries in the Organization for Economic Cooperation and Development (OECD) However, approximately 10–15% of patients still require revision surgery every year. Therefore, new technologies are required to increase the life-spam of the prosthesis from the current 10–15 years to at least 20–30 years. Our strategy focuses on surface modification of the bearing materials with a hydrophilic coating to improve their wear behaviour. These coatings are biocompatible, with high swelling capacity and antifouling properties, mimicking the properties of natural cartilage, i.e. wear resistance with permanent hydrated layer that prevents prosthesis damage. Clear beneficial advantages of this coating have been demonstrated in different conditions and different materials, such as UHMWPE, PEEK, CrCo, Stainless steel, ZTA and Alumina. Using routine tribological experiments, the wear for UHMWPE substrate was decreased by 75% against alumina, ZTA and stainless steel. For PEEK-CFR substrate coated, the amount of material lost against ZTA and CrCo was at least 40% lower. Further experiments on hip simulator adding abrasive particles (1-micron sized aluminium particles) during 3 million cycles, on a total of 6 million, showed a wear decreased of around 55% compared to uncoated UHMWPE and XLPE. In conclusion, CIDETEC‘s coating technology is versatile and can be adapted to protect and improve the tribological properties of different types of surfaces used for prosthesis, even in abrasive conditions

Background. Stress shielding and wear induced aseptic loosening cause failure in arthroplasty surgery. To improve survivorship, the use of a low modulus, low wearing biomaterial may be a suitable alternative to hard bearing prostheses, such as cobalt chromium (CoCr). There has been considerable research interest in the use of polyetheretherketone (PEEK) based on observed clinical success especially in spinal surgery. This study investigated the wear performance of PEEK, carbon reinforced PEEK (CFR-PEEK) and acetal as bearing materials in an all polymer total knee arthroplasty (TKA) using a unidirectional pin on plate test. Methods. The following material combinations were tested: PEEK vs. UHMWPE, CFR-PEEK vs. UHMWPE, PEEK vs. PEEK, CFR-PEEK vs. PEEK, CoCr vs. UHMWPE, PEEK vs. XLPE, CFR-PEEK vs. CFR-PEEK, PEEK vs. Acetal, Acetal vs. XLPE and CoCr vs. XLPE.Tribological couples tested (Pin vs. Plate) Using a previously validated modification of ASTM F732, 20mm diameter spherically ended pins with a radius of 25mm were articulated against 40mm diameter plates. A load of 1000N was applied to generate a contact stress of about 70MPa similar to contact stresses previously reported in the knee. The lubricant used was 25% newborn calf serum containing 0.3% sodium azide to retard bacteria growth and 20mM EDTA to prevent calcium deposition. Three repeats of pin on plate combinations (including 2 passive soak controls) were tested for 2 million cycles at a cycle frequency of 1Hz and a stroke length of 10 mm. Gravimetric wear was analysed every 250,000 cycles and results converted to volumetric wear using material density. Results. All CFR-PEEK articulations were stopped due to excessive wear of the counter-surfaces. Results showed a linear wear rate of UHMWPE and XLPE plates over the test period. PEEK vs. XLPE showed similar wear rate to metal on polyethylene (MoP) bearings. Conclusion. At stresses representative of the knee, PEEK pins when articulated against XLPE plates generated volumetric wear similar to that noted in MoP bearings