Sylvain Meille

Linear elastic properties of 2D and 3D models of porous materials made from elongated objects

Porous materials are formed in nature and by man by many different processes. The nature of the pore space, which is usually the space left over as the solid backbone forms, is often controlled by the morphology of the solid backbone. In particular, sometimes the backbone is made from the random deposition of elongated crystals, which makes analytical techniques particularly difficult to apply. This paper discusses simple two- and three-dimensional porous models in which the solid backbone is formed by different random arrangements of elongated solid objects (bars/crystals). We use a general purpose elastic finite element routine designed for use on images of random porous composite materials to study the linear elastic properties of these models. Both Youngs modulus and Poissons ratio depend on the porosity and the morphology of the pore space, as well as on the properties of the individual solid phases. The models are random digital image models, so that the effects of statistical fluctuation, finite size effect and digital resolution error must be carefully quantified. It is shown how to average the numerical results over random crystal orientation properly. The relations between two and three dimensions are also explored, as most microstructural information comes from two-dimensional images, while most real materials and experiments are three dimensional.

Bone micromechanical properties are compromised during long-term alendronate therapy independently of mineralization.

In the treatment of postmenopausal osteoporosis (PMOP), the use of alendronate (ALN) leads to a decrease in the risk of vertebral and nonvertebral fractures. To explore the possible adverse effects of prolonged ALN therapy, we studied the effects of 8 ± 2 years (6–10 years) of ALN treatment on the iliac cortical bone mineral and collagen quality and micromechanical properties; by design, our study examined these parameters, independent of the degree of mineralization. From six ALN‐treated and five age‐matched untreated PMOP women, 153 bone structural units have been chosen according their degree of mineralization to obtain the same distribution in each group. In those bone structural units, Fourier transform infrared spectroscopy, quantitative microradiography, and nanoindentation were used to assess bone quality. Irrespective of the degree of mineralization, ALN treatment was associated with higher collagen maturity (+7%, p < 0.001, c.v. = 13% and 16% in treated and untreated women, respectively) and lower mineral crystallinity than that observed in the untreated PMOP group (−2%, p < 0.0001, c.v. = 3% in both groups). Bone matrix from ALN‐treated women also had lower elastic modulus (−12%, p < 0.0001, c.v. = 14% in both groups) and, contact hardness (−6%, p < 0.05, c.v. = 14% in both groups) than that of untreated women. Crystallinity (which reflects the size and perfection of crystals) was associated with both elastic modulus and contact hardness in treated women exclusively (r = 0.43 and r = 0.54, p < 0.0001, respectively), even after adjustment for the amount of mineral. We infer that long‐term ALN treatment compromises micromechanical properties of the bone matrix as assessed ex vivo. The strength deficits are in part related to difference in crystallinity, irrespective of the mineral amount and mineral maturity. These novel findings at local levels of bone structure will have to be taken into account in the study of the pathophysiology of bone fragilities associated with prolonged ALN treatment.

Respective roles of organic and mineral components of human cortical bone matrix in micromechanical behavior: an instrumented indentation study.

Bone is a multiscale composite material made of both a type I collagen matrix and a poorly crystalline apatite mineral phase. Due to remodeling activity, cortical bone is made of Bone Structural Units (BSUs) called osteons. Since osteon represents a fundamental level of structural hierarchy, it is important to investigate the relationship between mechanical behavior and tissue composition at this scale for a better understanding of the mechanisms of bone fragility. The aim of this study is to analyze the links between ultrastructural properties and the mechanical behavior of bone tissue at the scale of osteon. Iliac bone biopsies were taken from untreated postmenopausal osteoporotic women, embedded, sectioned and microradiographed to assess the degree of mineralization of bone (DMB). On each section, BSUs of known DMB were indented with relatively high load (~500 mN) to determine local elastic modulus (E), contact hardness (H(c)) and true hardness (H) of several bone lamellae. Crystallinity and collagen maturity were measured by Fourier Transform InfraRed Microspectroscopy (FTIRM) on the same BSUs. Inter-relationships between mechanical properties and ultrastructural components were analyzed using multiple regression analysis. This study showed that elastic deformation was only explained by DMB whereas plastic deformation was more correlated with collagen maturity. Contact hardness, reflecting both elastic and plastic behaviors, was correlated with both DMB and collagen maturity. No relationship was found between crystallinity and mechanical properties at the osteon level.

Low-temperature degradation in zirconia with a porous surface

Today there is growing interest in zirconia in the dental field, but its use is still recent. Dental zirconia is mainly found in the form of yttria-stabilized zirconia crowns, bridges and abutments, and several companies are developing zirconia implants as an alternative to the standard biomedical grade titanium. In order to favor bone in-growth and osseointegration of zirconia implants, several strategies are now being explored to process rough and/or porous surfaces. The aim of this paper was to evaluate the resistance to environmental degradation of yttria-stabilized zirconia coated with a porous layer. We show that specific conditions of processing to generate the porous layer at the surface can lead to an accelerated tetragonal-monoclinic transformation of the porous layer in the presence of water. The impact of the transformation was evaluated in terms of structural integrity. Bending strength was not affected but the cohesion of the porous coating and its adhesion with the dense part deteriorated. We show that other processing conditions insure much better stability. Low-temperature degradation resistance of such porous surfaces should therefore be carefully followed and controlled in order to avoid critical problems in the future.

A meta-analysis of the mechanical properties of ice-templated ceramics and metals

Abstract Ice templating, also known as freeze casting, is a popular shaping route for macroporous materials. Over the past 15 years, it has been widely applied to various classes of materials, and in particular ceramics. Many formulation and process parameters, often interdependent, affect the outcome. It is thus difficult to understand the various relationships between these parameters from isolated studies where only a few of these parameters have been investigated. We report here the results of a meta analysis of the structural and mechanical properties of ice templated materials from an exhaustive collection of records. We use these results to identify which parameters are the most critical to control the structure and properties, and to derive guidelines for optimizing the mechanical response of ice templated materials. We hope these results will be a helpful guide to anyone interested in such materials.

In vitro and in vivo evaluation of a polylactic acid-bioactive glass composite for bone fixation devices

Poly(lactic acid) is nowadays among the most used bioabsorbable materials for medical devices. To promote bone growth on the material surface and increase the degradation rate of the polymer, research is currently focused on organic-inorganic composites by adding a bioactive mineral to the polymer matrix. The purpose of this study was to investigate the ability of a poly(L,DL-lactide)-Bioglass® (P(L,DL)LA-Bioglass(®) 45S5) composite to be used as a bone fixation device. In vitro cell viability testing of P(l,dl)LA based composites containing different amounts of Bioglass(®) 45S5 particles was investigated. According to the degradation rate of the P(L,DL)LA matrix and the cytocompatibility experiments, the composite with 30 wt % of Bioglass® particles seemed to be the best candidate for further investigation. To study its behavior after immersion in simulated physiological conditions, the degradation of the composite was analyzed by measuring its weight loss and mechanical properties and by proceeding with X-ray tomography. We demonstrated that the presence of the bioactive glass significantly accelerated the in vitro degradation of the polymer. A preliminary in vivo investigation on rabbits shows that the addition of 30 wt % of Bioglass(®) in the P(L,DL)LA matrix seems to trigger bone osseointegration especially during the first month of implantation. This composite has thus strong potential interest for health applications.

Crystallization processes at the surface of polylactic acid—bioactive glass composites during immersion in simulated body fluid†

We report on the crystallization processes occurring at the surface of PDLLA-Bioglass® composites immersed in simulated body fluid. Composites manufactured by injection molding and containing different amounts (0, 20, 30, and 50 wt %) of 45S5 Bioglass® particles were tested for durations up to 56 days and compared with Bioglass® particles alone. Crystallization processes were followed by visual inspection, X-ray diffraction (with Rietveld analysis) and scanning electron microscopy. Both calcite and hydroxyapatite were formed at the surface of all materials, but their relative ratio was dependent on the Bioglass® content and immersion time. Hydroxyapatite was always the major phase after sufficient immersion time, insuring bioactivity of such composites especially for Bioglass® content higher than 30 wt %. A scenario of crystallization is proposed. Rapid degradation of the composites with 50 wt % was also observed during immersion. Therefore, composites with 30 wt % of Bioglass® particles seem to exhibit the best balance between bioactivity and stability at least during the first weeks of immersion in contact with body fluids.

Selective etching of injection molded zirconia-toughened alumina: Towards osseointegrated and antibacterial ceramic implants

Due to their outstanding mechanical properties and excellent biocompatibility, zirconia-toughened alumina (ZTA) ceramics have become the gold standard in orthopedics for the fabrication of ceramic bearing components over the last decade. However, ZTA is bioinert, which hampers its implantation in direct contact with bone. Furthermore, periprosthetic joint infections are now the leading cause of failure for joint arthroplasty prostheses. To address both issues, an improved surface design is required: a controlled micro- and nano-roughness can promote osseointegration and limit bacterial adhesion whereas surface porosity allows loading and delivery of antibacterial compounds. In this work, we developed an integrated strategy aiming to provide both osseointegrative and antibacterial properties to ZTA surfaces. The micro-topography was controlled by injection molding. Meanwhile a novel process involving the selective dissolution of zirconia (selective etching) was used to produce nano-roughness and interconnected nanoporosity. Potential utilization of the porosity for loading and delivery of antibiotic molecules was demonstrated, and the impact of selective etching on mechanical properties and hydrothermal stability was shown to be limited. The combination of injection molding and selective etching thus appears promising for fabricating a new generation of ZTA components implantable in direct contact with bone. STATEMENT OF SIGNIFICANCE Zirconia-toughened alumina (ZTA) is the current gold standard for the fabrication of orthopedic ceramic components. In the present work, we propose an innovative strategy to provide both osseointegrative and antibacterial properties to ZTA surfaces: we demonstrate that injection molding allows a flexible design of surface micro-topography and can be combined with selective etching, a novel process that induces nano-roughness and surface interconnected porosity without the need for coating, avoiding reliability issues. These surface modifications have the potential to improve osseointegration. Furthermore, our results show that the porosity can be used for drug delivery and suggest that the etched surface could reduce bacterial adhesion.

Strain rate influence on human cortical bone toughness: A comparative study of four paired anatomical sites

Bone fracture is a major health issue worldwide and consequently there have been extensive investigations into the fracture behavior of human cortical bone. However, the fracture properties of human cortical bone under fall-like loading conditions remains poorly documented. Further, most published research has been performed on femoral diaphyseal bone, whereas it is known that the femoral neck and the radius are the most vulnerable sites to fracture. Hence, the aim of this study is to provide information on human cortical bone fracture behavior by comparing different anatomical sites including the radius and the femoral neck acquired from 32 elderly subjects (50 - 98 y.o.). In order to investigate the intrinsic fracture behavior of human cortical bone, toughness experiments were performed at two different strain rates: standard quasi-static conditions, and a higher strain rate representative of a fall from a standing position. The tests were performed on paired femoral neck, femoral, tibial and radius diaphyseal samples. Linear elastic fracture toughness and the non-linear J-integral method were used to take into account both the elastic and non-elastic behavior of cortical bone. Under quasi-static conditions, the radius presents a significantly higher toughness than the other sites. At the higher strain rate, all sites showed a significantly lower toughness. Also, at the high strain rate, there is no significant difference in fracture properties between the four anatomical sites. These results suggest that regardless of the anatomical site (femur, femoral neck, tibia and radius), the bone has the same fracture properties under fall loading conditions. This should be considered in biomechanical models under fall-like loading conditions.

The in vitro evolution of resorbable brushite cements: A physico-chemical, micro-structural and mechanical study

The mechanisms by which calcium phosphate bone substitutes evolve and are resorbed in vivo are not yet fully known. In particular, the formation of intermediate phases during resorption and evolution of the mechanical properties may be of crucial interest for their clinical efficiency. The in vitro tests proposed here are the first steps toward understanding these phenomena. Microporous Dicalcium Phosphate Dihydrate (DCPD) samples were immersed in tris(hydroxymethyl)aminomethane (TRIS) and Phosphate Buffered Saline (PBS) solutions, with or without daily refresh of the medium, for time-points up to 14days. Before and after immersion, samples were extensively characterised in terms of morphology, chemistry (XRD coupled with Rietveld analysis), microstructure (X-ray tomography, SEM observations) and local mechanical properties (instrumented micro-indentation). The composition of the immersion solutions was monitored in parallel (pH, elemental analysis). The results show the influence and importance of the experimental set-up and protocol on the formation of apatite and octacalcium phosphate concurrently to DCPD dissolution; moreover, strong inter-correlations between physico-chemistry, microstructure and mechanics are demonstrated. STATEMENT OF SIGNIFICANCE Ideally, the resorption kinetics of biodegradable bone substitutes should be controlled to favor the healing processes of bone. Although biodegradable bone grafts are already used in surgeries, their resorption process is still partially unknown. The present work studies these resorption phenomena, their kinetics and mechanisms and their consequences on the properties of a calcium phosphate resorbable material. The original in vitro approach developed in this work couples for the first time physico-chemical, micro-structural and mechanical assessments. The dissolution of the CaP phase in body fluids and the reprecipitation of more stable phases are studied on a local scale, which has permitted to evidence and monitor the development of a gradient of properties between the surface and the core of the samples.