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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. [ABSTRACT FROM AUTHOR]

In this paper, an updated analytical model for the kinetics of the tetragonal to monoclinic ( t → m ) polymorphic transformation in ZTA composites for hip joints is proposed and discussed. The model builds upon the so-called Mehl-Avrami-Johnson (MAJ) description of transformation kinetics, which combines two overlapping processes: nucleation of monoclinic sites, and their successive growth. Dependencies on two specific factors are introduced in the model, namely the initial fraction of monoclinic polymorph as received from the manufacturer, and the presence of different types of transition-metal stains ( e.g. , Ti, CoCr, and Fe) on the ZTA surface. These two factors were studied because clear indications of their potential roles on the environmental stability of implantable ZTA components were found in previous phenomenological analyses of retrievals. Nucleation and growth of monoclinic domains are two key processes whose interplay decides the actual kinetics of the overall transformation process according to two main parameters: an apparent activation energy value function of time and temperature, and a nuclei growth exponent. These parameters were clearly altered by the presence of transition metal contamination, whose effect was incorporated into the model to explain exacerbations of surface degradation. In accordance with a general analytical description of transformation kinetics for isothermal or isochronal evolutions in terms of time and temperature, the modified model of the MAJ description assesses the effect of the initial monoclinic fraction. The updated model has been validated using systematic in vitro experiments, and appears to partly reconcile in vitro and in vivo data of t → m transformation discrepancies in ZTA hip components. [ABSTRACT FROM AUTHOR]

Abstract A natural polysaccharide scaffold, referred to as "freeze-dry nanogel-crosslinked-porous" (FD-NanoCliP) gel, was tested in comparison with an atelocollagen scaffold with respect to osteogenesis versus the mouse mesenchymal progenitor cell line KUSA-A1. The amphiphilic polysaccharide network, engineered in its structure to fit chemically crosslinked nanogels as building blocks into a physically crosslinked porous gel, revealed a superior osteointegrative performance as compared to the soluble atelocollagen network and a peculiar c -plane orientation growth of apatite crystallites, which resembled the structure of natural enamel. Besides evaluating osteogenesis in the FD-NanoCliP gel scaffold, an additional purpose of this study was to assess its chemical composition at the nanoscale and, through its knowledge, to interpret the osteogenic response of mesenchymal cells. In addition to conventional (optical and electron) microscopy and biological evaluation kits, the peculiar chemistry of the FD-NanoCliP gel scaffold and the formation of apatite on it were characterized by means of several independent analytical probes at the molecular scale, which included Raman, cathodoluminescence, energy dispersive X-ray, and X-ray fluorescence spectroscopies. This body of information consistently provided evidence for a peculiar chemistry developed in osteogenesis at the polysaccharide scaffold surface. Such chemistry is not available in soluble atelocollagen and it is key in the superior bioactivity found in the polysaccharide network. Highlights • Pullulan nanogel scaffold could induce a strong osteogenetic impulse to mesenchymal progenitor cells KUSA-A1; • The osteogenetic performance was clearly improved with respect to a more conventional atelocollagen scaffold in terms of bone tissue formation; • Methods of Raman spectroscopy were successfully employed to analyze the structure of the scaffold; • The spectroscopic procedures developed in this study could be useful in studies on producing derivatives of polysaccharides. [ABSTRACT FROM AUTHOR]

Understanding the intrinsic reason(s) for the enhanced tetragonal to monoclinic ( t → m ) polymorphic phase transformation observed on metal-stained surfaces of zirconia-toughened alumina (ZTA) requires detailed knowledge of off-stoichiometry reactions at the molecular scale. In this context, knowledge of the mechanism(s) for oxygen vacancy creation or annihilation at the material surface is a necessary prerequisite. The crucial aspect of the surface destabilization phenomenon, namely the availability of electrons and holes that allow for vacancy creation/annihilation, is elucidated in this paper. Metal-enhanced alterations of the oxygen sublattice in both Al 2 O 3 and ZrO 2 of the ZTA composite play a decisive role in accelerating the polymorphic transformation. According to spectroscopic evidences obtained through nanometer-scale analyses, enhanced annihilation of oxygen vacancies triggers polymorphic transformation in ZrO 2 near the metal stain, while the overall Al 2 O 3 lattice tends to dehydroxylate by forming oxygen vacancies. A mechanism for chemically driven “reactive metastability” is suggested, which results in accelerating the polymorphic transformation. The Al 2 O 3 matrix is found to play a key-role in the ZrO 2 transformation process, with unambiguous confirmation of oxygen and hydrogen transport at the material surface. It is postulated that this transport is mediated by migration of dissociated O and H elements at the surface of the stained transition metal as they become readily available by the thermally activated surrounding. [ABSTRACT FROM AUTHOR]

Using a simple and innovative sandblasting process, disks of monolithic biomedical silicon nitride (β-Si 3 N 4) were texturized with a matrix of regular, discrete square trenches with a total depth in the range of hundreds of microns. The process consisted of sandblasting Si 3 N 4 substrates through a stainless-steel wire-mesh (150 or 200 μm) using abrasive silicon carbide powders (α -SiC, ∼40 μm) under 1,034 kPa (150 psi) of gas pressure. The depth of the porosities could be controlled varying both the treatment time and the distance from the surface. Part of the samples were then filled with 45S5 Bioglass® powders to improve the osteointegration and stimulate the production of bone tissue. Due to the increased macroscopic and microscopic roughness, biological testing using human osteosarcoma cells (SaOS-2) showed improved cell proliferation and greater production of both mineral (hydroxyapatite) and organic (collagen) phases on the patterned surfaces compared to untreated β-Si 3 N 4 or to the biomedical titanium control samples. Both of these effects were further enhanced when the porosities were filled with Bioglass®. • Textured sandblasting has been used for the first time on the surface of silicon nitride. • A steel wire mesh was used to mask the silicon nitride surface during processing. • The textured surface proved to be more osteoinductive compared to polished silicon nitride. • Addition of 45S5 bioglass further enhanced the production of bone tissue by SAOS-2 osteosarcoma cells. [ABSTRACT FROM AUTHOR]

The surface chemistry of silicon nitride plays an important role in stimulating osteoblasts to proliferate and produce bone tissue with improved efficiency. This property, which is advantageous in spinal fusion surgery has a chemical origin and is a direct consequence of the cleavage of covalent S N bonds in an aqueous environment. Building upon a wealth of published research on the stimulation of osteoblastic activity by silicon, the aim of this paper is to explore the role of nitrogen and, more specifically, the N/Si atomic ratio on the osteogenic response of Si 3 N 4. The surface stoichiometry of Si 3 N 4 was gradually altered toward a silicon-rich composition by systematically treating the Si 3 N 4 surface with a high-power pulsed laser in an Ar gas atmosphere (i.e., operated at different pulse times, spot sizes, and voltages). Different analytical probes were used to characterize the surface including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). Osteoconductivity was tested in vitro using SaOS-2 osteosarcoma cells, and samples with different surface stoichiometry were compared for their osteogenic response. These experiments clearly indicated a fundamental role for nitrogen off-stoichiometry in osteogenesis, and showed that both cell proliferation and growth of bone tissue diminished with decreasing nitrogen content. Unlabelled Image • Silicon nitride surfaces were successfully treated using a ND:YAG pulsed laser. • The pulsed laser treatment resulted in a Si-enriched material. • Metallic silicon domains are formed on the surface of silicon nitride after treatment. • Si-rich areas showed poorer osteogenic capabilities. [ABSTRACT FROM AUTHOR]