Radostina Alexandrova, PhD BONE IMPLANTS – OLD PROBLEMS AND NEW PERSPECTIVEES Radostina Alexandrova, PhD Institute of Experimental Morphology, Pathology AND Anthropology with Museum, Bulgarian Academy of Sciences

Collagen type I fibers (95%) Bone tissue Organic matrix Collagen type I fibers (95%) Proteoglicans and numerous non-collagenous proteins (5%)

structure of osseoum tissue

CELLS Cell type Origin Differentiation Function and phenotype OSTEOBLASTS (Four categories) pluripotential mesenchymal stem cells Highly differentiated !. Active osteoblasts - mononuclear cells with cuboidal shape; rich in alkaline phosphatase; synthesize and secrete collagen type I and glycoproteins (osteopontin, steocalcin), cytokines, and growth factors into a region of unmineralized matrix (osteoid) between the cell body and the mineralized matrix; produce calcium phosphate minerals extra- and intracellularly within vesicles; 2. Osteocytes – mature osteoblasts which have become trapped within the bone matrix and are responsible for its maintenance; 3. Bone-lining cells - found along the bone surfaces that are undergoing neither bone formation nor resorption, inactive cells that are believed to be precursors osteoblasts; 4. Inactive osteoblasts - elongated cells, undistinguishable morphologically from the bone-lining cells. OSTEOCLASTS hematopoietic stem cells (monocytes) Polynuclear cells responsible for bone resorption (by acidification of bone mineral leading to its dissolution and by enzymatic degradation of demineralized extracellular bone matrix; important for growth and development CHONDROCYTES mesenchymal stem cells Cells found in cartilage that produce and maintain the cartilaginous matrix MESENCHYMAL STEM CELLS AND OSTEOPROGENITOR CELLS Adult mesenchimal stem cells can be isolated from bone marrow, adipose tissues, or amniotic membrane; non differentiated with self-renewal capacity; multipotent cells able to repopulate all the appropriate differentiation lineages (osteoblastic, myoblastic, adipocytic, chondrocytic, endothelial, and neurogenic). For the osteogenic lineage, mesenchimal stem cells sustain a cascade of differentiation steps as described by the following sequence: Mesenchimal stem cell → immature osteoprogenitor → mature osteoprogenitor → preosteoblast → mature osteoblast → osteocyte or lining cell or apoptosis. In bone marrow osteoprogenitor cells represent a very small percentage (e.g. < 0.005%) of nucleated cell types in healthy adult bone. Osteoprogenitor stem cell differentiation is controlled by the “osteogenic mastergene” Cbfa1/Osf2 that intervenes in skeleton and tooth mineralization.

Hormones (parathyroid hormone (PTH) and 1α, 25(OH)2 vitamin D3) Cytokines Hormones (parathyroid hormone (PTH) and 1α, 25(OH)2 vitamin D3) Growth factors (IGFs, PDGF, FGFs, VEGFs, tTGFs and bone morphogenic proteins (BMPs)

F U N C T I O N Involved in body protection, support and motion; It is a protection and production site for specialized tissues (bone marrow, heart, lungs) Supports structurally the mechanical action of soft tissues, like the construction of muscles or extension of lungs. It is a mineral reservoir whereby endocrine system regulate the level of calcium and phosphate ions in the circulating body fluids

Mechanical properties

Congenitive disorders Diseases Trauma Congenitive disorders Cancer, ostoeporosis, Rheumatoid arthritis, etc Fractura, Burning BONE DEFFECTS

Techniques used to repair damaged bones Autografts Allografts Limited availability Foreign body reactions Infections Limited amount Artificial materials

B I O M A T E R I A L S

DEFINITION OF BIOMATERIALS A biomaterial can be defined as any material used to make devices to replace a part or a function of the body in a safe, reliable, economic, and physiologically acceptable manner. A variety of devices and materials is used in the treatment of disease or injury.

biocompatibility

Приложение на тъканното инженерство

BIOMATERIAL SHORT CHARACTERISTIC ADVANTAGES DISADVAMTAGES CERAMICS Based mainly on hydroxyapatite, since this is the inorganic compound of bone Able to form bone apatite-like material or carbonate hydroxyapatite on their surface, enhancing their osseointegration; Able to bind and concentrate cytokines, as in the case of natural bone Brittleness and slow degradation rates METALS Mainly stainless steel and titanium alloys (i.e. Ti-6Al-4V) Excellent mechanical properties, which makes them the most widely applied implant material used in bone surgical repairs The lack of tissue adherence and the low rate of degradation results either in a second surgery to remove the implant or in permanent implantation in the body with the related risks of toxicity due to accumulation of metal ions due to corrosion NATURAL POLYMETS Collagen and glycosaminoglycans Silk-based biomaterials Biocompatibility and biodegradability, since they compose the structural materials of tissues Low mechanical strength and high rates of degradation (they are used in composites or in chemical modification by cross-linking. These changes make cause cytotoxic effects and reduce compatibility). Biocompatibility, excellent mechanical properties, long-standing use of silk as sature material. SYNTHETIC POLYMERS The versatility of chemically synthesized polymers enables the fabrication of scaffolds with different features (forms, porosities and pore size, rates of degradation, mechanical properties) to match tissue specific applications COMPOSITES Each individual material has advantages for osteogenic applications, each also has drawbacks associated in certain properties (i.e. brittleness of ceramics) that can be overcome by combining different materials.

Orthopedic implants may fail owing to different reasons: poor osseointegration at the tissue-implant interface generation of wear debris stress and imbalance between implant and surrounding tissues infections The average lifetime of the current bone biomaterials is less than 15 years

TISSUE ENGINEERING STEM CELLS SCAFFOLD

Osteogenesis occurs by seeding the scaffolds before implantation with cells that will establish new centers for bone formation, such as osteoblasts and mesenchymal cells that have the potential to commit to an osteoblastic lineage Genetically transduced cells that express osteoinductive factors can also be used

Scaffolds for bone regeneration should meet certain criteria to serve this function: Mechanical properties - Biocompatibility and biodegradability Osteoconductivity Dlivery vehicles for cytokines such as BMPs, IGFs and TGFs – similar to those of the bone repair site at a rate commensurate with remodeling. the phenomenon of new bone formation on the surfaces of biomaterial that transform recruited precursor cells from the host into bone matrix producing cells, thus providing osteoinduction.

Cytokines Growth factors MSC protocol

3D scaffold

Micrograph three-dimensional and inconsistency in pore sizes and surfaces of a cubic scaffold produced

1.Activation: preosteoclasts are stimulated and differentiate under the influence of cytokines and growth factors into mature active osteoclasts 2. Resorption: osteoclasts digest mineral matrix (old bone) 3. Reversal: end of resorption 4. Formation: osteoblasts synthesize new bone matrix 5. Quiescence: osteoblasts become resting bone lining cells on the newly formed bone surface

The complexity of architecture and the variability of properties of bone tissue (e.g. porosity, pore size, mechanical properties, mineralization of mineral density, cell type and cytokine graduated features) Differences in age, nutritional state, activity (mechanical loading and disease status of individuals a major challenge in fabricating scaffolds and engineering bone tissues that will meet the needs of specific repair sites in specific patients

Poly(lactic-coglycolic acid scaffold

Acknowledgement: This study was supported by Grant DTK-02-70/2009, National Science Fund, Bulgaria

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