Evaluation of the Parameters of a Constitutive Mode1 for b.c.c. Metals Based on Thermal Activation

L'objet de cet article consiste à modéliser le comportement mécanique d'un tantale et à travers celui-ci des métaux à structure cubique centrée en se basant sur un formalisme physique proposé par J.R. Klepaczko[l]. La gamme de vitesse de déformation étudiée est comprise entre et 10-4 et 103 s-1. Nous présentons ici une procédure expérimentale et analytique qui permet de déterminer les paramètres du modèle. A partir du modèle ainsi déterminé, nous simulons, en bon accord avec les résultats expérimentaux, des essais à vitesse constante ainsi que des sauts de vitesse. Une discussion sur les micro-mécanismes de déformation permet une validation théorique des coefficients du modèle.The aim of this paper consists in modelling the mechanical behaviour of tantalum as an example for b.c.c. metals. The range of strain rates is between et 10-4 et 103 s-1. A formalism proposed by J.R. Klepaczko [l] and based on physical relations has been adopted. A methodology based on experiments and analysis is presented, which enables to determine the parameters. Tests at constant strain rates and jump tests are simulated showing a good agreement with the experiments. A discussion on deformation mechanisms gives a theoretical validation of the model's coefficients

Short (15 Minutes) Bone Morphogenetic Protein-2 Treatment Stimulates Osteogenic Differentiation of Human Adipose Stem Cells Seeded on Calcium Phosphate Scaffolds In Vitro

A one-step concept for bone regeneration has been postulated in which human adipose stem cells (hASCs) are harvested, triggered to differentiate, seeded on carriers, and implanted in the same operative procedure. Toward this goal it was investigated whether short (minutes) incubation with bone morphogenetic protein-2 (BMP-2) suffices to trigger osteogenic differentiation of hASCs seeded on calcium phosphate carriers. hASCs were treated with or without BMP-2 (10 ng/mL) for 15 min, and seeded on β-tricalcium phosphate granules (β-TCP; sized 0.7 mm) or biphasic calcium phosphate (BCP; 60%/40% or 20%/80% hydroxyapatite/β-TCP). Attachment was determined after 10-30 min. Proliferation (DNA content) and osteogenic differentiation (alkaline phosphatase activity, gene expression) were analyzed up to 3 weeks of culture. hASC attachment to the different scaffolds was similar, and unaffected by BMP-2. It stimulated gene expression of the osteogenic markers core binding factor alpha 1, collagen-1, osteonectin, and osteocalcin in hASCs seeded on BCP and β-TCP. Downregulation of osteopontin expression by BMP-2 was seen in BCP-seeded cells only. BMP-2 treatment inhibited expression of the adipogenic marker peroxisome proliferator-activated receptor gamma. In conclusion, 15 min BMP-2 preincubation of hASCs seeded on BCP/β-TCP scaffolds had a long-lasting stimulating effect on osteogenic differentiation in vitro. These results strongly support a one-step clinical concept for bone regeneration

Human Maxillary Sinus Floor Elevation as a Model for Bone Regeneration Enabling the Application of One-Step Surgical Procedures

Bone loss in the oral and maxillofacial region caused by trauma, tumors, congenital disorders, or degenerative diseases is a health care problem worldwide. To restore (reconstruct) these bone defects, human or animal bone grafts or alloplastic (synthetic) materials have been used. However, several disadvantages are associated with bone graft transplantation, such as limited bone volume, donor-site morbidity, surgical and immune rejection risks, and lack of osseo-integration. Bone tissue engineering is emerging as a valid alternative to treat bone defects allowing the regeneration of lost bony tissue, thereby recovering its functionality. During the last decades, the increasing aged population worldwide has also raised the prevalence of maxillary atrophy. Maxillary sinus floor elevation (MSFE) has become a standard surgical procedure to overcome the reduced amount of bone, thus enabling the placement of dental implants. MSFE aims to increase the bone height in the posterior maxilla, by elevating the Schneiderian membrane and placing the graft material into the surgically created space in the maxillary sinus floor. Importantly, oral bone regeneration during MSFE offers a unique human clinical model in which new cell-based bone tissue engineering applications might be investigated, since biopsies can be taken after MSFE before a dental implant placement and analyzed at the cellular level. New approaches in oral bone regeneration are focusing on cells, growth factors, and biomaterials. Recently, adipose tissue has become interesting as an abundant source of mesenchymal stem cells, which might be applied immediately after isolation to the patient allowing a one-step surgical procedure, thereby avoiding expensive cell culture procedures and another surgical operation. In this new cell-based tissue engineering approach, stem cells are combined with an osteoconductive scaffold and growth factors, and applied immediately to the patient. In this review, MSFE is discussed as a valid model to test bone tissue engineering approaches, such as the one-step surgical procedure. This procedure might be applied in other regenerative medicine applications as well

A histomorphometric and micro-computed tomography study of bone regeneration in the maxillary sinus comparing biphasic calcium phosphate and deproteinized cancellous bovine bone in a human split-mouth model

Objective The gain of mineralized bone was compared between deproteinized bovine bone allograft (DBA) and biphasic calcium phosphate (BCP) for dental implant placement. Study Design Five patients with atrophic maxillae underwent bilateral sinus elevation with DBA (Bio-Oss) and BCP (Straumann BoneCeramic). After 3 to 8 months, 32 Camlog implants were placed, and biopsies were retrieved. Bone and graft volume, degree of bone mineralization, and graft degradation gradient were determined using micro-computed tomography, and bone formation and resorption parameters were measured using histomorphometry. Implant functioning and peri-implant mucosa were evaluated up to 4 years. Results Patients were prosthetically successfully restored. All but one of the implants survived, and peri-implant mucosa showed healthy appearance and stability. Bone volume, graft volume, degree of bone mineralization, and osteoclast and osteocyte numbers were similar, but BCP-grafted biopsies had relatively more osteoid than DBA-grafted biopsies. Conclusions The BCP and DBA materials showed similar osteoconductive patterns and mineralized bone, although signs of more active bone formation and remodeling were observed in BCP- than in DBA-grafted biopsies