On the molecular interaction between femoral heads and polyethylene liners in artificial hip joints: phenomenology and molecular scale phenomena

In vitro & in vivo investigation of the silicon nitride ceramic hip implant’s safety and effectiveness evaluation

Background

With regard to the ceramic hip joint implant, given the concerns in ceramic about the alumina brittleness and zirconia instability, is there any alternative material solution for the orthopedic implant? Beyond the metastable oxide ceramics, along the echelon of advanced technical ceramics, looking at the non-oxide ceramic, the silicon nitride could be an excellent candidate for the joint implant’s application. The purpose of this study is to investigate the safety, effectiveness and to demonstrate the potential of this silicon nitride hip implant.

Methods

According to the related ISO (International Organization for Standardization) standards, a series of in vitro (nine) & in vivo (five) tests, which had been accomplished for the aforementioned aim. Especially, the total hip replacement in pigs had been achieved, as per the authors’ knowledge, this is the first time to apply the THA (Total Hip Arthroplasty) in the big animal.

Results

Refer to the ISO 6474-2, in comparison with the current monopolized German product, this silicon nitride ceramic hip implant has high strength, high hardness, excellent fracture toughness, lower density, better wear resistance, good biocompatibility, inherent stability, corrosion resistance and bioactivity, bone integration capability.

Conclusions

This silicon nitride ceramic will be an admirable alternative solution with superior comprehensive property that can withstand the toughest conditions in the most demanding applications like in orthopedic and beyond.

Factors Influencing the Outcomes of Artificial Hip Replacements

Hip replacement is one of the most successful surgeries in the clinic for the removal of painful joints. Hip osteoarthritis and femoral head necrosis are the 2 main reasons for hip replacement. Several factors are associated with the outcomes of surgery. Nonsurgical factors include gender, age, body mass index, prosthetic material, and risk factors. Surgical factors are anesthesia, postoperative complications, and rehabilitation. Considering the increasing demand for hip arthroplasty and the rise in the number of revision operations, it is imperative to understand factor-related progress and how modifications of these factors promotes recovery following hip replacement. In this review, we first summarize recent findings regarding crucial factors that influence the outcomes of artificial hip replacement surgery. These findings not only show the time-specific effect for the treatment and recovery from hip arthroplasty in the clinic, but also provide suitable choices for different individuals for clinicians to consider. This, in turn, will help to develop the best possible postoperative program for specific patients.

Mechanisms induced by transition metal contaminants and their effect on the hydrothermal stability of zirconia-containing bioceramics: an XPS study

Zirconia containing bioceramics suffer from low temperature degradation in biological and hydrothermal environments, and the presence of transition metal contamination has been shown to greatly affect the zirconia stability in different materials. In this paper, X-ray photoelectron spectroscopy was used to investigate the compositional and structural variations of different zirconia containing hip-joint bioceramics with and without transition metal stains in hydrothermal environments. Non-stained and stainless-steel-stained femoral head samples of 3 mol% Y2O3 doped tetragonal zirconia polycrystals (3Y-TZP) and zirconia-toughened alumina (ZTA) subjected to isothermal treatments in water vapor were investigated with quantifying their respective compositional XP lines. The outputs of these spectroscopic experiments revealed a significant difference in the off-stoichiometric reactions taking place at the surface of zirconia-containing ceramics in the presence and absence of transition metal contamination. The complex off-stoichiometric chemistry that occurred in the presence of metal contaminants could be interpreted in terms of defect-related chemical reactions among metal, water vapor, and oxide lattice, with a crucial contribution of the alumina phase in the transformation kinetics of ZTA.

Oxide ceramic femoral heads contribute to the oxidation of polyethylene liners in artificial hip joints

Experimental evidence demonstrates that a loss of stoichiometry at the surface of oxide bioceramic femoral heads enhances the oxidation rate of polyethylene acetabular liners in artificial hip joints. Contradicting the common notion that ceramics are bioinert, three independent experiments confirmed substantial chemical interactions between the ceramic femoral heads and their polyethylene counterparts. The experiments reported herein included hydrothermal tests, frictional tests, and hip-simulator experiments. It was discovered that oxide and non-oxide femoral heads differently affected the oxidation processes at the surface of the polyethylene liners, all other testing parameters being equal. Analytical data from X-ray photoelectron (XPS), cathodoluminescence (CL), Fourier-transform infrared (FTIR), and Raman spectroscopies unequivocally and consistently showed that the oxidation rate of polyethylene liners was greater when coupled with oxide as opposed to non-oxide ceramic heads. XPS analyses of O-Al-O bond fractions at the surface of a zirconia-toughened alumina (ZTA) short-term (20 months in vivo) femoral heads retrieval showed a ~50% reduction in favor of oxygen vacancy O-Al-V_O and hydroxylated Al-O-H bonds. Off-stoichiometry drifts were confirmed in vitro under both static and dynamic conditions. They triggered oxidation and tangibly affected an advanced highly cross-linked sequentially irradiated and annealed ultra-high molecular weight polyethylene (UHMWPE) liner (increase in oxidation index up to ΔOI~1.2 after 5 × 10⁵ cycles under dynamic swing conditions). Second-generation UHMWPE liners infused with vitamin E were also affected by the free flow of oxygen from the oxide femoral heads, although to a lesser extent. The fundamental findings of this study, which were also confirmed on retrievals, call for revised standards in material design and testing. Adopting these new criteria will provide an improved understanding of the importance of off-stoichiometry at the head/liner interface and may lead to significant extensions in artificial joint lifetimes.

Raman spectroscopy of biomedical polyethylenes

With the development of three-dimensional Raman algorithms for local mapping of oxidation and plastic strain, and the ability to resolve molecular orientation patterns with microscopic spatial resolution, there is an opportunity to re-examine many of the foundations on which our understanding of biomedical grade ultra-high molecular weight polyethylenes (UHMWPEs) are based. By implementing polarized Raman spectroscopy into an automatized tool with an improved precision in non-destructively resolving Euler angles, oxidation levels, and microscopic strain, we become capable to make accurate and traceable measurements of the in vitro and in vivo tribological responses of a variety of commercially available UHMWPE bearings for artificial hip and knee joints. In this paper, we first review the foundations and the main algorithms for Raman analyses of oxidation and strain of biomedical polyethylene. Then, we critically re-examine a large body of Raman data previously collected on different polyethylene joint components after in vitro testing or in vivo service, in order to shed new light on an area of particular importance to joint orthopedics: the microscopic nature of UHMWPE surface degradation in the human body. A complex scenario of physical chemistry appears from the Raman analyses, which highlights the importance of molecular-scale phenomena besides mere microstructural changes. The availability of the Raman microscopic probe for visualizing oxidation patterns unveiled striking findings related to the chemical contribution to wear degradation: chain-breaking and subsequent formation of carboxylic acid sites preferentially occur in correspondence of third-phase regions, and they are triggered by emission of dehydroxylated oxygen from ceramic oxide counterparts. These findings profoundly differ from more popular (and simplistic) notions of mechanistic tribology adopted in analyzing joint simulator data. Statement of Significance This review was dedicated to the theoretical and experimental evaluation of the commercially available biomedical polyethylene samples by Raman spectroscopy with regard to their molecular textures, oxidative patterns, and plastic strain at the microscopic level in the three dimensions of the Euclidean space. The main achievements could be listed, as follow: (i) visualization of molecular patterns at the surface of UHMWPE bearings operating against metallic components; (ii) differentiation between wear and creep deformation in retrievals; (iii) non-destructive mapping of oxidative patterns; and, (iv) the clarification of chemical interactions between oxide/non-oxide ceramic heads and advanced UHMWPE liners.