Zirconia-toughened Alumina Femoral Heads Research Articles

Does Taper Design Affect Taper Fretting Corrosion in Ceramic-on-Polyethylene Total Hip Arthroplasty? A Retrieval Analysis

BackgroundCeramic-on-polyethylene (CoP) implants have exhibited lower fretting and corrosion scores than metal-on-polyethylene implants. This study aims at investigating the effect of taper design on taper corrosion and fretting in modular CoP total hip arthroplasty (THA) systems. MethodsUnder an institutional review board–-approved protocol, a query of an implant retrieval library from 2002 to 2017 identified 120 retrieved CoP THA systems with zirconia toughened alumina femoral heads. Femoral stem trunnions were visually evaluated and graded for fretting, corrosion, and damage at the taper interface. Medical records were reviewed for patient demographics and implant characteristics. Data were statistically analyzed using Spearman correlation and rank-sum tests with a Dunn’s post hoc test, with a significance level of α = 0.05. ResultsFour different taper designs were evaluated: 11/13 (n = 18), 12/14 (n = 53), 16/18 (n = 21), and V40 (n = 28). There were no statistically significant demographic differences between taper groups for duration of implantation, laterality, patient age, and patient sex, but patients with 16/18 tapers had a higher body mass index than V40 tapers (P = .012). Duration of implantation had a weak positive correlation with both trunnion fretting (ρ = 0.224, P = .016) and corrosion (ρ = 0.253, P = .006). Summed fretting and corrosion scores were significantly greater on the V40 and 16/18 tapers compared with the 12/14 tapers (all P ≤ .001). ConclusionTaper fretting and corrosion were observed in CoP THA implants and were greatest with V40 and 16/18 tapers and lowest with 12/14 tapers. Differences in taper design characteristics may lead to greater micromotion at the taper-head interface, leading to increased fretting and corrosion.

Bioceramics are Not Bioinert: The Role of Oxide and Non-Oxide Bioceramics on the Oxidation of UHMWPE Components in Artificial Joints

The following research is aimed at understanding the influence of Zirconia-Toughened Alumina (ZTA) and Silicon Nitride (Si3N4) on Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular liners. Bioceramic femoral heads were systematically tested against UHMWPE in controlled environment according to static/load-free coupling in hydrothermal environment, pin-on-ball wear testing, and hip-simulator wear testing. In addition, a retrieved ZTA femoral head has been analyzed and results have been compared to the simulations. Experimental results from X-ray photoelectron (XPS), cathodoluminescence (CL), Raman and Fourier-Transformed Infrared spectroscopy suggest that, despite conventional notions imply that bioceramics are inert, the surface chemistry of bioceramics was relevant to the oxidation rate of polyethylene liners. Non-biointertness could either be advantageous or disadvantageous toward polyethylene oxidation. The main reason resides in the peculiar chemical interactions between polyethylene and different ceramics, and, more specifically, depends on the direction of oxygen flow at the interface between the ceramic and the polymer. ZTA femoral heads were found to release a non-negligible amount of oxygen moieties from their surfaces, thus accelerating oxidative degradation of polyethylene. Conversely, Si3N4 ceramics exerted a protective role towards the polyethylene liner by scavenging oxygen from the tribolayer. The results of this work provide new insights into the interaction between bioceramics and polymers, which should also be considered when designing the next generation artificial hip joints with significantly elongated lifetimes.

Degradation phenomena occurring in the conical taper of a short-term retrieved ZTA femoral head: A case study

Despite being the most used ceramic biomaterial in hip joint prosthesis, Zirconia Toughened Alumina (ZTA) is liable to undergo several degradation mechanisms in vivo, which might drastically affect its structural performance over time. Our standing point in this investigation was that neither in vitro tests nor finite elements simulations are yet capable to fully reproduce the complexity of the in vivo conditions. Through accurate analyses of retrievals, we attempt to add more pieces to the puzzle of in vivo loading history and to obtain better insight into the ZTA degradation mechanisms. For doing so, we applied a previously established automated scanning protocol to investigate the coupling between a ZTA femoral head and its metal stem counterpart in a short-term (20 months) retrieved hip implant. Metal contamination on the ZTA surface was severe and polymorphic transformation widespread on the whole surface of the taper, though reaching different values depending on the specific zone of the taper. Clear signs of third body abrasive wear and fatigue crack initiation could be found by means of laser and electron microscopies, and X-ray spectroscopy. Highly resolved mapping by Raman micro-spectroscopy revealed very high monoclinic fractions and pronounced residual stress magnitudes despite the relatively short in-vivo lifetime.

Reducing fretting-initiated crevice corrosion in hip simulator tests using a zirconia-toughened alumina femoral head.

Taper fretting corrosion is considered a potentially limiting factor for total hip arthroplasty longevity. Recently, attention has been focused on new materials for ceramic femoral heads, for example, zirconia-toughened alumina (ZTA), since they have an alternative bearing surface that can improve the wear resistance. Moreover, ceramics have high chemical stability and corrosion resistance. In this study, we evaluated the effects of ZTA and Co-Cr-Mo alloy femoral heads on their taper fretting and/or corrosion characteristics under a controlled hip simulator test. After the test, less fretting and corrosion were observed in the taper surface of the trunnion against the ZTA femoral head than for that against the Co-Cr-Mo alloy femoral head. In addition, corrosion damages were only observed in the lateral-distal taper surface (noncontact area) of the trunnion in the Co-Cr-Mo alloy femoral head group. The ZTA femoral head group also eliminated the potential for Co ion release into the lubricants from taper corrosion, reducing the possibility of adverse local tissue inflammatory responses. In conclusion, ZTA femoral heads showed markedly less fretting corrosion compared to Co-Cr-Mo alloy femoral heads and have a lower potential for metal ion release. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2815-2826, 2018.

Mechanistic Study of the Wear of Ceramic Heads by Metallic Stems in Modular Implants

Titanium-based and cobalt–chrome alloys as well as some ceramics have been widely used in orthopaedic applications as these materials can significantly enhance the quality of human life as implant materials. However, the in vivo performance of some large-diameter metal-on-metal joints is unsatisfactory, and concerns have been expressed over the wear behaviour of the materials and released metal ions affecting local tissue and more distant organs. The longevity of these materials is highly influenced by their mechanical properties and this has driven the development of alternative ceramic components with greatly improved tribological performance. Even these novel materials are not immune to damage, for instance in some devices alumina-based ceramic components articulate with titanium alloy counterfaces (e.g. in the taper connections of titanium alloy stems and zirconia-toughened alumina femoral heads in modern modular designs) and damage has been reported of the harder ceramic surface by the softer titanium alloy component. In such contacts, the chemically inert ceramic component is not expected to corrode, so the electrochemical damage mechanisms often suggested for metal–metal contacts are not appropriate. This study attempts to understand why this wear might occur by investigating bulk and surface mechanical properties (such as hardness and Young’s modulus) of a number of hip implants and test samples using a Hysitron Triboindenter. AFM images were also obtained to determine the contact area and hence, pile-up correction factors for the metallic material. It was found that the alumina ceramic heads were generally subject to chemomechanical softening after exposure to water for an extended period whilst titanium alloy oxidised preferentially generating a hard oxide surface which was not softened by water. Furthermore, the oxidised titanium showed significantly higher hardness values therefore damaging the chemomechanically softened alumina material.

Reconciling in vivo and in vitro kinetics of the polymorphic transformation in zirconia-toughened alumina for hip joints: I. Phenomenology

Exploitation of the toughening effect induced by polymorphic phase transformation of zirconia in zirconia-toughened alumina (ZTA) requires the composite being properly designed and carefully manufactured. A sound algorithm for predicting phase stability along with strict control over manufacturing steps are required in order to prevent possible in vivo surface degradation or implant fracture. This paper is the first in a series of three monographs, which aim at: (i) statistically comparing the in vitro/in vivo phenomenology of surface-metastability for currently marketed ZTA femoral heads; (ii) refining pre-existing theoretical models for predicting in vivo zirconia phase metastability via the use of accelerated in vitro ageing experiments; and, (iii) providing a rationale for the mechanism(s) involved with the observed in vivo surface metastability. This initial paper of a series of three, which specifically deals with item (i), shows discrepancies between the levels of polymorphic phase transformation detected in ZTA retrievals and in vitro predictions, and attempts a phenomenological analysis of the reasons behind such discrepancies. Moreover, marked inhomogeneities are also found among as-manufactured components through different years of production. The phenomenology of retrievals' data suggests key roles for both the presence of metallic stain and the initial value of monoclinic volume fraction.

Wear and surface degradation of commercial ZTA femoral heads under boundary lubrication conditions

The effect of frictional sliding on the surface degradation of commercially available zirconia-toughened alumina (ZTA) femoral heads was studied using a pin-on-ball wear tester under three different lubricating conditions: dry, water, and squalene. Water and squalene were employed under boundary lubrication regimes. Despite the unique (non-standard) character of this apparatus, standard loading conditions could be applied, which effectively determined dynamic friction coefficients as basic material properties. Two types of surface degradation were observed: (i) the polymorphic tetragonal-to-monoclinic (t→m) transformation of the zirconia (ZrO2) dispersoids, and (ii) the off-stoichiometry drift caused by oxygen vacancy formation within the alumina matrix. Scanning laser microscopy (SLM), Raman spectroscopy (RS), scanning electron microscopy (SEM), cathodoluminescence (CL), and X-ray photoelectron spectroscopy (XPS) were utilized to evaluate the fractions of transformed zirconia phase and the stoichiometric evolution of the oxygen sub-lattice at the surface of wear-tested ZTA components. Wear tracks on the surface of the femoral heads were only detected under dry conditions. Dry wear also showed the highest frictional forces and the most pronounced formation of oxygen vacancies among the tested conditions. Conversely, equivalent or greater amounts of the t→m transformation were observed with water and squalene lubrication when compared to the dry wear condition.

Chemically driven tetragonal-to-monoclinic polymorphic transformation in retrieved ZTA femoral heads from dual mobility hip implants

Two short-term (two and nine months) retrieved zirconia-toughened alumina (ZTA) femoral heads and nine pristine femoral heads from the same manufacturer have been investigated with respect to their surface stability by means of confocal Raman spectroscopy. Quantitative estimations of monoclinic volume fraction have been carried out in both non-wear and main wear zones of the retrieved heads, which invariantly showed high volume fractions of monoclinic polymorph. In-depth (sub-surface) profiles, non-destructively collected in the main wear zones with the Raman probe in confocal configuration, indeed confirmed that polymorphic transformation was extended down to 100μm below the bearing surface of the femoral heads. Acceleration of tetragonal-to-monoclinic transformation rate leads to unexpectedly high fractions of monoclinic phase within very short-term in-vivo exposures. Phase transformation in-vivo is much more marked than what one could actually predict according to simply simulating a hydrothermal environment in-vitro and could not be simply ascribed to the mechanical stress fields generated during normal service at the bearing surface. Instead, the chemical consequences of metal contamination on the ZTA femoral head surface are shown to play the most detrimental role in phase destabilization.

Wear of novel ceramic‐on‐ceramic bearings under adverse and clinically relevant hip simulator conditions

Further development of ceramic materials for total hip replacement aim to increase fracture toughness and further reduce the incidence of bearing fracture. Edge loading due to translational mal positioning (microseparation) has replicated stripe wear, wear rates, and bimodal wear debris observed on retrievals. This method has replicated the fracture of early zirconia ceramic-on-ceramic bearings. This has shown the necessity of introducing microseparation conditions to the gait cycle when assessing the tribological performance of new hip replacement bearings. Two novel ceramic matrix composite materials, zirconia-toughened alumina (ZTA) and alumina-toughened zirconia (ATZ), were developed by Mathys Orthopädie GmbH. In this study, ATZ-on-ATZ and ZTA-on-ZTA bearing combinations were tested and compared with alumina-on-alumina (Al2O3-on-Al2O3) bearings under adverse microseparation and edge loading conditions using the Leeds II physiological anatomical hip joint simulator. The wear rate (±95% confidence limit) of ZTA-on-ZTA was 0.14 ± 0.10 mm(3)/million cycles and that of ATZ-on-ATZ was 0.06 ± 0.004 mm(3)/million cycles compared with a wear rate of 0.74 ± 1.73 mm(3)/million cycles for Al2O3-on-Al2O3 bearings. Stripe wear was evident on all bearing combinations; however, the stripe formed on the ATZ and ZTA femoral heads was thinner and shallower that that formed on the Al2O3 heads. Posttest phase composition measurements for both ATZ and ZTA materials showed no significant change in the monoclinic zirconia content. ATZ-on-ATZ and ZTA-on-ZTA showed superior wear resistance properties when compared with Al2O3-on-Al2O3 under adverse edge loading conditions.

Comparative ageing behaviour of commercial, unworn and worn 3Y-TZP and zirconia-toughened alumina hip joint heads

There is a trend today to develop zirconia toughened alumina (ZTA) composites for orthopaedic applications. So far, there is limited data concerning their sensitivity to ageing, especially considering tests performed on implants produced on an industrial scale. Here, complementary tools were used to assess the ageing resistance of ZTA femoral heads. The results were compared to femoral heads processed under the same industrial process with monolithic 3Y-TZP. As expected, monolithic 3Y-TZP implants exhibited significant ageing. In contrast ZTA femoral head showed no sign of degradation even over a period equivalent to that of a human life. The potential impact of coupling effects between wear and ageing is assessed. Monolithic zirconia and ZTA femoral heads were thus first worn in a hip joint simulator, and then aged in autoclave. The kinetics of transformation of the worn monolithic zirconia implants is accelerated while that of worn ZTA femoral heads remains almost unchanged.