UHMWPE for arthroplasty: past or future?

Single-Site Catalyst for the Synthesis of Disentangled Ultra-High-Molecular-Weight Polyethylene

Disentangled ultra-high-molecular-weight polyethylene (d-UHMWPE) solves the problem of the difficult processing of traditional UHMWPE caused by entanglements between molecular chains. In this review, we look into the innovative realm of nascent disentangled UHMWPE, concentrating on the recent advances achieved through the in situ polymerization of ethylene by single-site catalysts. The effect of single-site catalysts and polymerization conditions on the molecular characteristics is discussed in detail from the perspective of mechanism and DFT calculations. The key factors to low entanglement are revealed, which have instructive implications for the development of new single-site catalytic systems that can generate d-UHMWPE more efficiently and become closer to industrial production. The progress in the preparation for nascent d-UHMWPE with homogeneous and heterogeneous single-site catalysts is systematically reviewed. Rheology and DSC can be used to characterize the degree of entanglement. High-modulus and high-strength biaxial films, tapes, and fibers are obtained by the solid-state processing of these nascent d-UHMWPE.

Diffusion doping of analgesics into UHMWPE for prophylactic pain management

Pain management after total joint arthroplasty is often addressed by systemic delivery of opioids. Local delivery of non-opioid analgesic drugs directly in the joint space from the UHMWPE component of the prosthesis would be highly beneficial to increase the efficacy of the drugs, decreasing the overall side effects and the risk of opioid addiction. It has been shown that effective concentrations of local analgesics can be achieved by eluting from analgesic-blended UHMWPE; however, this approach is limited by the decrease in mechanical properties resulting from the extent of phase separation of the blended drugs from the polymeric matrix. Here we hypothesized that mechanical properties could be maintained by incorporating analgesics into solid form UHMWPE by diffusion as an alternative method. Lidocaine or bupivacaine were diffused in solid form UHMWPE with or without radiation crosslinking. The loaded drug content, the spatial distribution of the drugs and their chemical stability after doping were characterized by FTIR and NMR spectroscopy, respectively. Drug release kinetics, tensile mechanical properties and wear rates were assessed. The results showed that diffusion doping could be used as a promising method to obtain a therapeutic implant material without compromising its mechanical and structural integrity.

Investigation of the effects of irradiation and aging on the tribological behavior of ultra-high molecular weight polyethylene/graphene oxide composites under water lubrication

Ultra-high molecular weight polyethylene/graphene oxide (PE-UHMW/GO) composites have demonstrated potential in artificial joint applications. The tribological behavior of irradiated PE-UHMW/GO composites under water lubrication remained unclear, which limited their application range. In this study, the PE-UHMW/GO composites were gamma irradiated at 100 KGy in a vacuum and subsequently aged at 80 °C for 21 days in air. We assessed their water absorption, and mechanical and tribological properties post-treatment. Notably, gamma irradiation markedly enhanced the mechanical and tribological performance of PE-UHMW/GO composites. Irradiated composites had a 6.11% increase in compressive strength and a 25.72% increase in yield strength compared to unirradiated composites. Additionally, under water lubrication, the irradiated composites showed improved wear resistance and a reduced friction coefficient. The irradiation enhancement can be attributed to the irradiation-induced strengthening of the interface bonding between GO and PE-UHMW. Conversely, accelerated aging led to oxidative degradation, negatively impacting these properties. Aged composites exhibited lower compressive and yield strengths, higher friction coefficients, and diminished anti-wear properties compared to the irradiated composites. The wear mechanism evolved from predominantly fatigue wear in irradiated PE-UHMW/GO to a mix of abrasive and fatigue wear post-aging. While GO and aging influenced water absorption, irradiation had a minimal effect. These insights significantly contribute to the application potential of irradiated PE-UHMW/GO composites in artificial joints.

Updates on Biomaterials Used in Total Hip Arthroplasty (THA)

One of the most popular and effective orthopedic surgical interventions for treating a variety of hip diseases is total hip arthroplasty. Despite being a radical procedure that involves replacing bone and cartilaginous surfaces with biomaterials, it produces excellent outcomes that significantly increase the patient's quality of life. Patient factors and surgical technique, as well as biomaterials, play a role in prosthetic survival, with aseptic loosening (one of the most common causes of total hip arthroplasty failure) being linked to the quality of biomaterials utilized. Over the years, various biomaterials have been developed to limit the amount of wear particles generated over time by friction between the prosthetic head (metal alloys or ceramic) and the insert fixed in the acetabular component (polyethylene or ceramic). An ideal biomaterial must be biocompatible, have a low coefficient of friction, be corrosion resistant, and have great mechanical power. Comprehensive knowledge regarding what causes hip arthroplasty failure, as well as improvements in biomaterial quality and surgical technique, will influence the survivability of the prosthetic implant. The purpose of this article was to assess the benefits and drawbacks of various biomaterial and friction couples used in total hip arthroplasties by reviewing the scientific literature published over the last 10 years.

Innate Immune Response in Orthopedic Implant Failure

The total joint replacement is recognized as one of the most effective medical arbitrations leading to increased mobility, pain relief, and an overall restored function of the joint. Unfortunately, prosthetic debris accumulates after long-term wear of the implant leading to activation of the innate immune response and periprosthetic osteolysis. Understanding the intricate biological mechanisms underlying the innate immune response to implant debris would support the development of novel pharmacological treatments to prolong the life span of the implant. This article provides a detailed description on the role of the innate immune system in response to implant debris, emphasizing the most recent research and outstanding questions. Furthermore, a critical discussion is presented on the novel pharmacological treatments currently under investigation to prevent implant failure.

Early failure of a highly cross-linked polyethylene inlay after total hip arthroplasty probably due to insufficient irradiation

Highly cross-linked polyethylene (XLPE) is a major advance in total hip arthroplasty (THA), as it suffers from less wear and thus is associated with lower revision rates than standard ultra-high molecular weight polyethylene. Early failures are reported rarely, and associated with specific design or manufacturing issues. We report a case requiring early revision due to adverse reaction to polyethylene particles. Investigations identified insufficient irradiation as the most probable cause of failure. Here are reported the features of a clinical case with determination of the material properties of the retrieved XLPE liner and establishment of the appropriate calibration curves as reference. Periprosthetic joint infection could be ruled out with appropriate sampling as cause for the inflammatory periarticular tumour. Histology identified a massive macrophagic reaction to micrometric polyethylene particles. No component malposition was present, nor any third-body wear. The trans-vinylene index (TVI) indicated insufficient and potentially detrimental irradiation of the polyethylene, while gel content, crystallinity, melting temperature and oxidation index remained within expected ranges. Histologically proven failure of a XLPE THA liner was identifiable despite correct implantation of the components. The cause of failure most probably was an inadequate irradiation, as indicated by determination of the TVI. This case underscores the importance of histologic workup even in aseptic revisions, and of detailed analysis of retrievals. The calibration curves provided are essential for analysis of other retrievals.

Injection‐Molded Ultrahigh Molecular Weight Polyethylene Material with Improved Moldability for Artificial Joint Implants Design

Fabrication of pure ultrahigh molecular weight polyethylene (UHMWPE)‐based artificial joint implants is a bottleneck problem as it is only limited to quasistatic approaches that have low efficiency due to the high density of fusion defects. The main objective of this work is to improve the moldability of UHMWPE using the injection molding technique while maintaining its superior mechanical integrity. Groups of UHMWPE samples are injected at barrel melting temperatures of 300, 320, 340, and 360 °C, at various injecting pressures. The high‐temperature melting (HTM) allows the fabrication of robust UHMWPE samples using injection molding with an improved yield stress of 29.63 ± 3.19, compared to 23.0 ± 0.6 and 17.8 ± 0.75 achieved by HTM compression molding and conventional compression molding, respectively. Moreover, the scanning electron microscopy (SEM) results suggest an almost complete elimination of all fusion defects, and that is supported by the outstanding tensile strain at breaks reaching up to approximately 1300%. The thermochemical properties of the injection‐molded UHMWPE samples are tested as well to investigate the impact of HTM on their characteristics. Moreover, the materials’ biocompatibility and wear are assessed. This study sets up a new approach to fabricate high‐performance injection‐molded UHMWPE joint implants by HTM.

Mechanical in vitro fatigue testing of implant materials and components using advanced characterization techniques

Implants of different material classes have been used for the reconstruction of damaged hard and soft tissue for decades. The aim is to increase and subsequently maintain the patient's quality of life through implantation. In service, most implants are subjected to cyclic loading, which must be taken particularly into consideration, since the fatigue strength is far below the yield and tensile strength. Inaccurate estimation of the structural strength of implants due to the consideration of yield or tensile strength leads to a miscalculation of the implant's fatigue strength and lifetime, and therefore, to its unexpected early fatigue failure. Thus, fatigue failure of an implant based on overestimated performance capability represents acute danger to human health. The determination of fatigue strength by corresponding tests investigating various stress amplitudes is time-consuming and cost-intensive. This study summarizes four investigation series on the fatigue behavior of different implant materials and components, following a standard and an in vitro short-time testing procedure, which evaluates the material reaction in one enhanced test set-up. The test set-up and the applied characterization methods were adapted to the respective application of the implant with the aim to simulate the surrounding of the human body with laboratory in vitro tests only. It could be shown that by using the short-time testing method the number of tests required to determine the fatigue strength can be drastically reduced. In future, therefore it will be possible to exclude unsuitable implant materials or components before further clinical investigations by using a time-efficient and application-oriented testing method.

A tannic acid-reinforced PEEK-hydrogel composite material with good biotribological and self-healing properties for artificial joints

Polyetheretherketone (PEEK) is widely considered as a promising material for joint implants but it still has limitations involving high friction and wear. To mimic the cartilage-subchondral bone structure in natural joints, a polyvinyl alcohol (PVA) hydrogel layer was fabricated on the PEEK substrate to provide a lubrication mechanism. In addition, tannic acid was applied to form dynamic hydrogen bonds with PVA molecules, for the purpose of strengthening the hydrogel layer and endowing it with self-healing ability. Our experimental results demonstrated that the prepared PEEK-hydrogel composite exhibited good biotribological performance with a low average friction coefficient around 0.06 and little wear after the friction test. It also could repair the scratch made by a blade spontaneously at room temperature taking advantage of the reversibility of the hydrogen bonds. The influence of the properties of the PVA hydrogel and the concentration of tannic acid on the frictional and self-healing behavior of the composite structure was investigated and the internal mechanism was discussed. This work presents a facile method to fabricate a PEEK-hydrogel composite possessing outstanding tribological properties and self-healing capacity simultaneously, hopefully promoting its potential in producing artificial joints.

Wear and damage in retrieved humeral inlays of reverse total shoulder arthroplasty-where, how much, and why?

BACKGROUND

Polyethylene (PE) wear and material degradation have been reported as complications in reverse total shoulder replacements (rTSAs). In this regard, scapular notching is associated with more clinical complications. Therefore, the purposes of the study were to quantify the linear and volumetric wear, as a measure for the amount of removed material, and to qualitatively assess the PE damage modes to describe the material degradation in retrieved rTSA humeral PE inlays that contribute to failure of shoulder replacements. Furthermore, this study aimed to evaluate the effect of scapular notching on PE wear and rim damage of the humeral components.

METHODS

The total study population of 39 humeral inlays contains 2 cohorts that were used for the damage mode analysis and for the wear analysis, respectively. The extent and presence of wear damage modes in 5 defined zones were assessed by a grading system for all PE joint replacements. For quantitative wear analysis the most frequent design (n = 17) was chosen. Using a coordinate-measuring machine and postprocessing software, volumetric wear measurements for the retrieved humeral PE inlays were undertaken. Furthermore, prerevision radiographs were analyzed for scapular notching. Finally, retrieval findings were correlated with clinical and radiographic data to consider the effect of notching and to identify risk of failures for these prostheses.

RESULTS

Damage on the rim of the humeral PE inlays was more frequent and severe than on the intended articulation surface. Irrespective of the damage mode, the inferior rim zone sustained the greatest amount of wear damage followed by the posterior zone. Burnishing, scratching, pitting, and embedded particles are most likely to occur in the articular surface area, whereas surface deformation, abrasion, delamination and gross material degradation are predominantly present in the inferior and posterior rim zones. The retrieved inlays exhibited a mean volumetric wear rate of 296.9 mm³/yr ± 87.0 mm³/yr. However, if the notched and non-notched components were compared, a significant higher volumetric wear rate (296.5 ± 106.1 mm³/yr) was found for the notched components compared to the non-notched group (65.7 ± 7.4 mm³/yr). Generally, there was a significantly greater incidence of damage and greater amount of wear if scapular notching occurred.

CONCLUSION

The notched components showed a 5-fold increase in PE wear rate. Therefore, scapular notching has a strong effect on PE damage and wear. If scapular notching can be clinically avoided, the PE wear performance is in a similar magnitude as found for hip and knee replacements.

Influence of radiation conditions on the wear behaviour of Vitamin E treated UHMWPE gliding components for total knee arthroplasty after extended artificial aging and simulated daily patient activities

The long term performance of total knee arthroplasty (TKA) with regards to the bearing materials is related to the aging behaviour of these materials. The use of highly crosslinked materials in hip arthroplasty improved the clinical outcome. Nevertheless, the outcome for these materials compared to conventional UHMWPE (ultra-high molecular weight polyethylene) remains controversial in TKA and alternative bearing materials may be advantageous to improve its outcome in the second and third decade. The aim of this study is the evaluation of the influence of radiation conditions on the wear behaviour of Vitamin E blended UHMWPE gliding components for TKA by simulation of extended aging and high demanding daily patient activities. For a medium radiation dose (30 kGy), the influence of the irradiation type (E-beam or Gamma radiation) and the thermal conditions (room temperature (RT) or heated to 115 °C) are evaluated in comparison to non-irradiated material. Significant influences on the wear behaviour were found for the radiation source and temperature during irradiation. Furthermore, no relevant degradation of the tested materials was observed after extended artificial aging. There was a good correspondence between the wear pattern in this study and retrievals.

Ultra-high molecular weight polyethylene (UHMWPE) for hip and knee arthroplasty: The present and the future

PURPOSE

to review advances and clinical performance of polyethylene in total joint arthroplasty, summing up historical problems and focusing on the latest innovations.

METHODS

search for medical grade Ultra-High-Molecular-Weight-Polyethylene (UHMWPE); Data Sources: PubMed, Scopus, Cochrane Library.

RESULTS

the increasing number of joint arthroplasties and high-activity patients led to progressive developments of bearing surfaces to improve performance and durability. Different strategies such as crosslinking UHMWPE (HXLPE) and the addition of vitamin-E (HXLPE) have been tested to improve wear and oxidation resistance.

CONCLUSION

Recent innovations about UHMWPE showed improvements either for hip and knee, with the potential of long-term survivorship.

Plastics in Total Knee Replacement: Processing to Performance

Polyethylene (PE) is the key component of total knee replacement (TKR). The wear of polyethylene, a common cause of revision surgeries, depends on multiple factors. The mechanical properties, wear characteristics, and oxidative resistance of PE can be manipulated by the techniques of processing, sterilization, and packaging methods. This article describes the making of conventional and cross-linked poly, packaging, sterilization, processing techniques, and a summary of commercially available plastics and their rationale in TKR including the latest advances.

Tailor-made design, fabrication and validation of SrO doped nanostructured ZTA ceramic Femoral head - Acetabular socket liner assembly

An established commercial grade SrO doped ZTA composition has been considered to design, fabrication through uniaxial pressing followed by sintering and polishing, validation of dimension, 3D surface profile, 3D microstructure, and compressive load bearing capacity of femoral head - acetabular socket liner prototypes for the hip prosthesis. While design and dimensions are concerned, both the steel (HRC60) molds were designed and machined to achieve precious dimensions of femoral head (FH; OD - 30 ± 0.01 mm) and acetabular socket liner (ASL; ID - 30.15 ± 0.01 mm). A close variation in the range of ±0.01 mm was confirmed the precision geometry of polished FH and ASL with consideration of 22 points coordinate measuring method (CMM). 3D surface profile ensures the surface characteristics of R_a = 0.2 ± 0.01 μm and R_q = 0.5 ± 0.01 μm for outer surface of FH and an inner surface of ASL, respectively. Structure integrity and fabrication defects including cracks and pores free bulk structures were confirmed by Micro CT. The compressive load resistance known as burst strength for independent FH and FH-ASL assembly were measured 16.2 KN and 17.6 KN, respectively. The developed ceramic prototypes have an economic advantage and can be adopted as artificial hip prosthesis after extensive in-vitro and in-vivo analysis.

Does vitamin E highly-crosslinked polyethylene convey an advantage in primary total hip replacement? A systematic review and meta-analysis

BACKGROUND

Vitamin E highly cross-linked polyethylene (HXLPE) was developed to reduce wear in total hip replacement (THR). This formal systematic review and meta-analysis aimed to provide independent synthesis of wear characteristics of Vitamin E treated HXLPE compared to HXPLE/UHMWPE. Secondary outcome measures were differences in revision rates and functional scores.

METHODS

We performed a formal systematic review as per PRISMA guidelines; literature searches were conducted on 14 November 2017 (MEDLINE, Embase on Ovid, and the Cochrane Library). We included randomised controlled trials, analyses of joint registries, and case-controlled studies of primary THR comparing cups with a vitamin E HXLPE bearing with bearing surfaces made from other types of polyethylene. Initial screening was performed by 2 independent assessors; disagreement resolved in discussion with a third reviewer. Studies were evaluated using the Cochrane risk of bias tool. Data extraction permitted meta-analysis.

RESULTS

372 studies were identified on initial screening, 5 studies met the eligibility criteria. There was no significant heterogeneity between studies. There was variable risk of bias. At a mean of 35 (range 20-60) months, Vitamin E HXLPE had significant advantages over highly cross-linked polyethylene with regards total femoral head penetration (p = 0.004). Given the RSA measurement errors this may not be clinically significant.There were neither significant differences in revision rates nor Harris Hip Scores (p = 0.06).

CONCLUSION

At a minimum of 3 years follow-up there was reduced total femoral head penetration for vitamin E HXLPE over HXLPE. This bearing surface does not, as yet, have clinically significant advantages in terms of revision rates or patient function over HXLPE.

Design of Wear-Resistant UHMWPE-Based Composites Loaded with Wollastonite Microfibers Treated with Various Silane Coupling Agents

The tribomechanical properties of the wear-resistant ultrahigh molecular weight polyethylene (UHMWPE)-based composites loaded with wollastonite microfibres silanized with various coupling agents (“KH-550”, “Penta-1006”, and “OTS”) were investigated. It was demonstrated that the mechanical properties of UHMWPE-based composites filled with various amounts of wollastonite (7–23 wt. %) increased by 1.3 times (yield strength) and by 1.8 times (elastic modulus), while the wollastonite silanization further improved yield strength by 9% in some cases. It was demonstrated that the composite loaded with 23 wt. % wollastonite silanized with the “KH-550” coupling agent possessed the maximum wear resistance under “moderate” conditions of tribological loading. Under “severe” conditions, the composites containing 23 wt. % wollastonite silanized with the less efficient “OTS” and “Penta-1006” agents showed the greatest wear resistance during dry sliding friction. Wear resistance significantly depended on filler weight fraction and the load–speed mode of the tribological tests. Based on the obtained experimental data on the mechanical (including impact toughness) and tribological properties of the UHMWPE-based composites loaded with wollastonite, the optimal compositions (the filler content and the type of the coupling agent) for two load–speed modes were designed using the developed computer algorithm. The composites provided the predefined high tribomechanical properties for operation in the metal-polymer friction units compared to neat polymer.

Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) as a Promising Polymer Material for Biomedical Applications: A Concise Review

Ultra-High Molecular Weight Polyethylene (UHMWPE) is used in biomedical applications due to its high wear-resistance, ductility, and biocompatibility. A great deal of research in recent decades has focused on further improving its mechanical and tribological performances in order to provide durable implants in patients. Several methods, including irradiation, surface modifications, and reinforcements have been employed to improve the tribological and mechanical performance of UHMWPE. The effect of these modifications on tribological and mechanical performance was discussed in this review.

In vivo wear measurement in a modern total knee arthroplasty with model-based radiostereometric analysis

Aims

A retrospective study was conducted to measure short-term in vivo linear and volumetric wear of polyethylene (PE) inserts in 101 total knee arthroplasty (TKA) patients using model-based radiostereometric analysis (MBRSA).

Patients and Methods

Nonweightbearing supine RSA exams were performed postoperatively and at six, 12, and 24 months. Weightbearing standing RSA exams were performed on select patients at 12 and 24 months. Wear was measured both linearly (joint space) and volumetrically (digital model overlap) at each available follow-up. Precision of both methods was assessed by comparing double RSA exams. Patient age, sex, body mass index, and Oxford Knee Scores were analyzed for any association with PE wear.

Results

Linear wear occurred at 0.015 mm/year (supine) and 0.220 mm/year (standing). Volumetric wear occurred at 10.3 mm3/year (supine) and 39.3 mm3/year (standing). Wear occurred primarily on the medial side of the joint. Weightbearing imaging greatly improved the reliability of measurement. Clinical precision of volumetric wear was 34 mm3. No significant associations were found between patient demographics or function scores and measured wear.

Conclusion

In vivo volumetric wear of TKAs can be assessed at short-term follow-up using MBRSA. Cite this article: Bone Joint J 2019;101-B:1348–1355.

Polyethylene in total knee arthroplasty: Where are we now?

Polyethylene (PE) remains the gold standard for the articulating surface in hip and knee arthroplasty. To increase arthroplasty longevity and improve wear resistance, newer versions of PE have been designed with resultantly different wear properties. Highly cross-linked polyethylene (HXLPE) is used in total hip arthroplasty with excellent outcomes; however, its use in total knee arthroplasty (TKA) remains conflicting. This review summarizes biomechanical and wear properties, clinical outcomes, and cost of polyethylene inserts in TKA. Simulation studies have convincingly shown decreased wear and oxidation rates with HXLPE when compared to conventional polyethylene (CPE). Registry results have been conflicting, and short- to midterm clinical studies have not demonstrated a significant difference between HXLPE and CPE. The cost of HXLPE inserts is higher than CPE. Long-term clinical data are lacking and further studies are warranted to evaluate the role of HXLPE in TKA.

Performance analysis of grafted poly (2-methacryloyloxyethyl phosphorylcholine) on additively manufactured titanium substrate for hip implant applications

The incidence of total hip arthroplasty (THA) has been evidently growing over the last few decades. Surface modification, such as polymer grafting onto implant surfaces using poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC), has been gaining attention due to its excellent biocompatibility and high lubricity behaviour resulting in reducing surgical recurrence number and increasing implant lifetime. Investigating thermal stability and mechanical properties of the grafted polymer is, therefore, extremely important as these properties define the failure mechanism of implants. This study focuses on optimising monomer concentration to achieve the best physical, thermal and mechanical properties of the grafted additively manufactured titanium (Ti6Al4V) implants. Three different concentration of monomers, 0.4 M, 0.6 M and 0.8 M, were investigated, and grafted implants were characterised. The results from thermal analysis confirmed that the PMPC polymer is thermally stable for implant applications regardless of the monomer concentrations. A significant reduction in Young's modulus of polymer grafted samples (33.2-42.9%), in comparison with untreated Ti6Al4V samples and consequent improvement of wear resistance and elasticity behaviour, proved the potentiality of polymer films for implant applications. In summary, polymer grafted implant prepared with 0.6 M monomer concentration showed the optimal thermal, physical and wear resistance properties.

Particle analysis of shape factors according to American Society for Testing and Materials

Polyethylene wear is one of the major factors influencing the survivorship of joint replacements. Depending on the number, size and morphology of the polyethylene particles, biological responses of the periprosthetic soft tissue in terms of inflammatory processes can occur, leading to loosening of the implant. Various parameters are used to analyze wear particles, which are usually determined by examining scanning electron microscopy (SEM) images with a particle analysis program. In this study, three different software solutions for particle analysis (self-developed Particleanalyzer_HD, Leica QWin and ImageJ) were compared regarding particle number, size and morphology. These solutions were also compared to the American Society for Testing and Materials (ASTM) F1877-16 specifications regarding particle morphology. SEM image analysis revealed no differences for the equivalent circle diameter (p = 0.969). However, a significant difference was found for the aspect ratio between the Particleanalyzer_HD and the other two software solutions (p < 0.001) and between Leica QWin and the other two software solutions regarding the roundness (p < 0.001). Only the Particleanalyzer_HD showed an excellent agreement with the ASTM standard for both morphology parameters (intraclass correlation = 1.000). Only the Particleanalyzer_HD calculated the two morphology parameters according to the ASTM standard. A comparison of the particle morphology between different studies is barely possible, as different algorithms for particle analysis are used. It is strongly recommended that the calculation according to the ASTM standard is used to improve future comparability of findings from wear analysis studies. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:225-233, 2020.

Hip simulator testing of the taper-trunnion junction and bearing surfaces of contemporary metal-on-cross-linked-polyethylene hip prostheses

Adverse reaction to metal debris released from the taper-trunnion junction of modular metal-on-polyethylene (MoP) total hip replacements (THRs) is an issue of contemporary concern. Therefore, a hip simulator was used to investigate material loss, if any, at both the articulating and taper-trunnion surfaces of five 32-mm metal-on-cross-linked-polyethylene THRs for 5 million cycles (Mc) with a sixth joint serving as a dynamically loaded soak control. Commercially available cobalt-chromium-molybdenum femoral heads articulating against cross-linked polyethylene (XLPE) acetabular liners were mounted on 12/14 titanium (Ti6Al4V) trunnions. Weight loss (mg) was measured gravimetrically and converted into volume loss (mm³ ) for heads, liners, and trunnions at regular intervals. Additionally, posttest volumetric wear measurements of the femoral tapers were obtained using a coordinate measuring machine (CMM). The surface roughness (Sa) of femoral tapers was measured posttest. After 5 Mc, the mean volumetric wear rate for XLPE liners was 2.74 ± 0.74 mm³ /Mc. The CMM measurements confirmed material loss from the femoral taper with the mean volumetric wear rate of 0.045 ± 0.024 mm³ /Mc. The Sa on the worn area of the femoral taper showed a significant increase (p < 0.001) compared with the unworn area. No other long-term hip simulator tests have investigated wear from the taper-trunnion junction of contemporary MoP THRs. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:156-166, 2020.

Recent updates for biomaterials used in total hip arthroplasty

Background

Total hip arthroplasty (THA) is probably one of the most successful surgical interventions performed in medicine. Through the revolution of hip arthroplasty by principles of low friction arthroplasty was introduced by Sir John Charnley in 1960s. Thereafter, new bearing materials, fixation methods, and new designs has been improved. The main concern regarding failure of THA has been the biological response to particulate polyethylene debris generated by conventional metal on polyethylene bearing surfaces leading to osteolysis and aseptic loosening of the prosthesis. To resolve these problems, the materials of the modern THA were developed since then.

Methods

A literature search strategy was conducted using various search terms in PUBMED. The highest quality articles that met the inclusion criteria and best answered the topics of focus of this review were selected. Key search terms included ‘total hip arthroplasty’, ‘biomaterials’, ‘stainless steel’, ‘cobalt-chromium’, ‘titanium’, ‘polyethylene’, and ‘ceramic’.

Results

The initial search retrieved 6921 articles. Thirty-two articles were selected and used in the review.

Conclusion

This article introduces biomaterials used in THA and discusses various bearing materials in currentclinical use in THA as well as the newer biomaterials which may even further decrease wear and improve THA survivorship.

Initial Response of Human Bone Marrow-Derived Stem Cells after Contact with Ultrahigh-Molecular-Weight Polyethylene (UHMWPE) Material: An In Vitro Study on Cell Viability and Interleukin-6 Expression

Introduction:

Ultrahigh-molecular-weight polyethylene (UHMWPE) is a thermoplastic polymer useful in biomaterial applications, especially in orthopedic field. Yet, little is known concerning its initial effect on human bone marrow stem cells (hBMSCs) after implantation.

Materials and Methods:

A cytotoxicity analysis was performed with a 3-(4,5-dimethylthiazol 2-yl)-2,5-diphenyltetrazolium assay after 24, 48, and 72h of incubation of hBMSC culture. Expression of interleukin-6 (IL-6) was measured using enzyme-linked immunosorbent assay. Cell viability was measured with Inhibitory concentration 50% (IC₅₀) formula.

Results:

All treatment groups showed a cell viability of >50% ranging from 78% to >100%. Lower expression of IL-6 of hBMSC compared to control group was found in 48h of incubation period.

Conclusion:

hBMSC showed high cell viability after initial contact with UHMWPE material. Modulation of IL-6 expression was present at the initial stage as a response to foreign material.

Wear Behaviours and Oxidation Effects on Different UHMWPE Acetabular Cups Using a Hip Joint Simulator

Given the long-term problem of polyethylene wear, medical interest in the new improved cross-linked polyethylene (XLPE), with or without the adding of vitamin E, has risen. The main aim of this study is to gain further insights into the mutual effects of radiation cross-linking and addition of vitamin E on the wear performance of ultra-high-molecular-weight polyethylene (UHMWPE). We tested four different batches of polyethylene (namely, a standard one, a vitamin E-stabilized, and two cross-linked) in a hip joint simulator for five million cycles where bovine calf serum was used as lubricant. The acetabular cups were then analyzed using a confocal profilometer to characterize the surface topography. Moreover; the cups were analyzed by using Fourier Transformed Infrared Spectroscopy and Differential Scanning Calorimetry in order to assess the chemical characteristics of the pristine materials. Comparing the different cups’ configuration, mass loss was found to be higher for standard polyethylene than for the other combinations. Mass loss negatively correlated to the cross-link density of the polyethylenes. None of the tested formulations showed evidence of oxidative degradation. We found no correlation between roughness parameters and wear. Furthermore, we found significantly differences in the wear behavior of all the acetabular cups. XLPEs exhibited lower weight loss, which has potential for reduced wear and decreased osteolysis. However, surface topography revealed smoother surfaces of the standard and vitamin E stabilized polyethylene than on the cross-linked samples. This observation suggests incipient crack generations on the rough and scratched surfaces of the cross-linked polyethylene liners.

Biological effects of metal degradation in hip arthroplasties

Metals and metal alloys are the most used materials in orthopedic implants. The focus is on total hip arthroplasty (THA) that, though well tolerated, may be associated with local and remote adverse effects in the medium-long term. This review aims to summarize data on the biological consequences of the metal implant degradation that have been attributed predominantly to metal-on-metal (MoM) THA. Local responses to metals consist of a broad clinical spectrum ranging from small asymptomatic tissue lesions to severe destruction of bone and soft tissues, which are designated as metallosis, adverse reactions to metal debris (ARMD), aseptic lymphocytic vasculitis associated lesion (ALVAL), and pseudotumors. In addition, the dissemination of metal particles and ions throughout the body has been associated with systemic adverse effects, including organ toxicity, cancerogenesis, teratogenicity, and immunotoxicity. As proved by the multitude of studies in this field, metal degradation may increase safety issues associated with THA, especially with MoM hip systems. Data collection regarding local, systemic and long-term effects plays an essential role to better define any safety risks and to generate scientifically based recommendations.

Immunological Responses to Total Hip Arthroplasty

The use of total hip arthroplasties (THA) has been continuously rising to meet the demands of the increasingly ageing population. To date, this procedure has been highly successful in relieving pain and restoring the functionality of patients’ joints, and has significantly improved their quality of life. However, these implants are expected to eventually fail after 15–25 years in situ due to slow progressive inflammatory responses at the bone-implant interface. Such inflammatory responses are primarily mediated by immune cells such as macrophages, triggered by implant wear particles. As a result, aseptic loosening is the main cause for revision surgery over the mid and long-term and is responsible for more than 70% of hip revisions. In some patients with a metal-on-metal (MoM) implant, metallic implant wear particles can give rise to metal sensitivity. Therefore, engineering biomaterials, which are immunologically inert or support the healing process, require an in-depth understanding of the host inflammatory and wound-healing response to implanted materials. This review discusses the immunological response initiated by biomaterials extensively used in THA, ultra-high-molecular-weight polyethylene (UHMWPE), cobalt chromium (CoCr), and alumina ceramics. The biological responses of these biomaterials in bulk and particulate forms are also discussed. In conclusion, the immunological responses to bulk and particulate biomaterials vary greatly depending on the implant material types, the size of particulate and its volume, and where the response to bulk forms of differing biomaterials are relatively acute and similar, while wear particles can initiate a variety of responses such as osteolysis, metal sensitivity, and so on.

Oscillatory device for use with linear tribometer, for tribological evaluation of biomaterials

Orthopedic implants still have limitations regarding their durability, despite being in use for over fifty years. Particles arising from wear due to the relative motion of their surfaces remain responsible for aseptic failure. This paper presents a device to be coupled with a reciprocal linear tribometer to reproduce the ex vivo wear of biomaterials, allowing the measurement of force and coefficient of friction. The device consists of a structure connected to the tribometer that transforms its reciprocal linear motion into one that is oscillatory for the mechanical assembly that contains the samples to test the desired biomaterials. The tribological pair used for testing consisted of Ultra High Molecular Weight Polyethylene (UHMWPE) in conjunction with the austenitic stainless steel AISI 316L in dry lubrication. The results showed that the values of the coefficient of friction in the linear mode and oscillatory mode and the UHMWPE life curve in the oscillatory mode were consistent with those cited in the literature for tests in a dry lubrication environment. Moreover, the UHMWPE sample life curve showed a reduction in the wear rate that can be explained by the preponderance of a wear mechanism over the others. The volumetric wear showed an increase with the number of cycles.

All dual mobility cups are not the same

PURPOSE

Although the natural history of dual mobility has been exclusively borne for 20 years by a single company (due to industrial ownership), the concept has undeniably been very widely popularised with nearly 40 cups on the French market which should be regarded as a weight bearing surface, both broadly and in their own right. However, within the same original idea, these implants are not all identical (design, material, fixation ect.).The aim of this work is to propose a classification of different dual mobility cups by distinguishing between thegeneral characteristics of a conventional cup and those particular to this type of implant.

METHODS

By comparison with a standard metal-back cup, dual mobility is based on at least one additional interface corresponding to the mobility of the polyethylene insert in the concavity of the acetabular cup called the outersurface. Design, constitutive material, fixation of the cup and characteristics of the retentive insert are analysed through the published results.

RESULTS

The complications associated, in particular, the intraprosthetic dislocation and to a lesser extent fixation failures undoubtedly condemned the dissemination of the dual mobility concept, as witnessed by the fact that despite the precedence of this 40-year old concept, the overriding majority of publications (more than 95%) have only appeared in the last ten years.

CONCLUSION

The latest generation of dual mobility cups combines: 1) a cast chrome-cobalt alloy cup covered with a bilayer coating of porous titanium and hydroxyapatite for long-term press-fit fixation to 2) an insert designed to eliminate all of the risks of intraprosthetic dislocation, whilst keeping all of the elasticity properties of the polyethylene, which has demonstrated its medium and long term effectiveness on preventing instability by overcoming other complications.

Wear performance of neat and vitamin E blended highly cross-linked PE under severe conditions: The combined effect of accelerated ageing and third body particles during wear test

The objective of this study is to evaluate the effects of third-body particles on the in vitro wear behaviour of three different sets of polyethylene acetabular cups after prolonged testing in a hip simulator and accelerated ageing. Vitamin E-blended, cross-linked polyethylene (XLPE_VE), cross-linked polyethylene (XLPE) and conventional polyethylene (STD_PE) acetabular cups were simulator tested for two million cycles under severe conditions (i.e. by adding third-body particles to the bovine calf serum lubricant). Micro-Fourier Transform Infrared and micro-Raman spectroscopic analyses, differential scanning calorimetry, and crosslink density measurements were used to characterize the samples at a molecular level. The STD_PE cups had twice mass loss than the XLPE_VE components and four times than the XLPE samples; statistically significant differences were found between the mass losses of the three sets of cups. The observed wear trend was justified on the basis of the differences in cross-link density among the samples (XLPE>XLPE_VE>STD_PE). FTIR crystallinity profiles, bulk DSC crystallinity and surface micro-Raman crystallinity seemed to have a similar behaviour upon testing: all of them (as well as the all-trans and ortho-trans contents) revealed the most significant changes in XLPE and XLPE_VE samples. The more severe third-body wear testing conditions determined more noticeable changes in all spectroscopic markers with respect to previous tests. Unexpectedly, traces of bulk oxidation were found in both STD_PE (unirradiated) and XLPE (remelting-stabilized), which were expected to be stable to oxidation; on the contrary, XLPE_VE demonstrated a high oxidative stability in the present, highly demanding conditions.

Failure analysis of retrieved PE-UHMW acetabular liners

Ultra-high molecular weight polyethylene (PE-UHMW) acetabular liners have a limited lifespan in a patient's body. There are many factors affecting the performance of the implant and furthermore the properties of the polymeric material are changing after implantation. In this work material changes according to structure and morphology and their implication on mechanical properties are in focus. The physical and mechanical properties of ten crosslinked (xL) PE-UHMW and nine conventional (conv) gamma-sterilized PE-UHMW hip components, used as sliding surface in total hip joint replacement, with different in-vivo times are compared. The evaluation of the retrieved acetabular liners is performed in view of crosslinking and conventional gamma-sterilization but also in terms of the influence of gender concerning alteration in properties. The oxidative degradation in the PE-UHMW is investigated by means of Fourier Transformed Infrared Spectroscopy (FTIR). The characterization of the morphology is carried out via differential scanning calorimetry (DSC). A depth profile of the micro-hardness and elastic modulus is taken over the cross-section of the components in order to find the influence of chemical constitution and morphology on the micro-mechanical properties. It could be shown that crosslinking and oxidative degradation influence the degree of crystallinity of the polymer. Oxidation occurs for both types of the material due to in-vivo time. Higher degree of crystallinity can be correlated to higher hardness and indentation modulus. No unequivocal superiority of crosslinked over conventional liners can be observed. The influence of sex concerning alteration of the evaluated properties matters but need to be further investigated.

Nanomechanical and surface properties of rMSCs post-exposure to CAP treated UHMWPE wear particles

Wear debris generated by ultra-high molecular weight polyethylene (UHMWPE) used in joint replacement devices has been of concern due to reductions of the implant longevity. Cold atmospheric plasma (CAP) has been used to improve the wear performance of UHMWPE. Our aim was to investigate the elastic and adhesive properties of rat mesenchymal stem cells (rMSCs), through AFM, after exposure to UHMWPE wear debris pre- and post-CAP treatment. The results indicated that the main changes in cell elasticity and spring constant of MSC exposed to wear particles occurred in the first 24 h of contact and the particle concentration from 0.5 to 50 mg/l did not play a significant role. For UHMWPE treated for 7.5 min, with progression of the wear simulation the results of the CAP treated samples were getting closer to the result of untreated samples; while with longer CAP treatment this was not observed.

From the Clinical Editor

Joint replacements are now common clinical practice. However, the use of ultra-high molecular weight polyethylene (UHMWPE) still poses a concern, due to the presence of wear debris. The authors here investigated the effects of wear debris after cold atmospheric plasma treatment on rat mesenchymal stem cells. The positive results provided new strategies in future design of joint replacement materials.

A 3D finite element model to investigate prosthetic interface stresses of different posterior tibial slope

PURPOSE

Posterior tibial slope that is created during proximal tibial resection in total knee arthroplasty has emerged as an important factor in the mechanics of the knee joint and the surgical outcome. But the ideal degree of posterior tibial slope for recovery of the knee joint function and preventions of complications remains controversial and should vary in different racial groups. The objective of this paper is to investigate the effects of posterior tibial slope on contact stresses in the tibial polyethylene component of total knee prostheses.

METHODS

Three-dimensional finite element analysis was used to calculate contact stresses in tibial polyethylene component of total knee prostheses subjected to a compressive load. The 3D finite element model of total knee prosthesis was constructed from the images produced by 3D scanning technology. Stresses in tibial polyethylene component were calculated with four different posterior tibial slopes (0°, 3°, 6° and 9°).

RESULTS

The 3D finite element model of total knee prosthesis we presented was well validated. We found that the stress distribution in the polythene as evaluated by the distributions of the von Mises stress, the maximum principle stress, the minimum principle stress and the Cpress were more uniform with 3° and 6° posterior tibial slopes than with 0° and 9° posterior tibial slopes. Moreover, the peaks of the above stresses and trends of changes with increasing degree of knee flexion were more ideal with 3° and 6° posterior slopes.

CONCLUSIONS

The results suggested that the tibial component inclination might be favourable to 7°-10° so far as the stress distribution is concerned. The range of the tibial component inclination also can decrease the wear of polyethylene. Chinese posterior tibial slope is bigger than in the West, and the current domestic use of prostheses is imported from the West, so their demands to tilt back bone cutting can lead to shorten the service life of prostheses; this experiment result is of important clinical significance, guiding orthopaedic surgeon after the best angle to cut bone.