Biomaterials for Bone, Regenerative Medicine [1 ed.] 9783038134428, 9780878491537

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Biomaterials for Bone, Regenerative Medicine

Dedicated To our beloved Parents and Contributors N. Sooraj Hussain & J. D. Santos

Contents Chapter 1: Skeletal Regenerative Nanobiomaterials .......................... 1 Chapter 2: Silica-based Materials as Precursors of Nanoapatites....................................................................................... 35 Chapter 3: Phytochemicals for Bone Regeneration ........................... 81 Chapter 4: Designed Biomaterial Scaffolds for Bone Regeneration............................................................................... 107 Chapter-5: Engineered Ca-Si Based Ceramics for Skeletal Tissue Reconstruction.......................................................................... 121 Chapter 6: Calcium Phosphate-based Materials for Bone regenerative Medicine ......................................................................... 151 Chapter7: Cell adhesion and proliferation over Zinc-Glass Reinforced Hydroxyapatite Composites (Zn-GRHA)...................... 181

Forward Hench invented series of Bioglass® in 1969 (US Patent), first in the human history, to notify the world that silicate glasses should have close interactions with living cells or organs. In 1970 Carlisle pointed out the importance of silicon in chicken diet for bone framework development of chicken. Both teams independently and almost at the same time showed that properly designed artificial materials stimulate bone-tissue growth. They were nightingales telling the dawn of new era in biomaterials. Within 10 years after, we saw development and commercialization of new silicate ceramics, such as Ceravital®, Bioverit®, Cerabone A-W®, to name a few. Those materials should only symbolize the evolution of modern biomaterials, yet: those materials basically look at tissue repair applications by filling bone-cavity or replacing diseased bone tissue, though complex biological reactions, take place at the interface between one of those materials and the tissue, resulting in the formation of material-bone bonding or bone tissue growth on the material surface. In the past couple of decades, novel concepts have been introduced in therapeutic procedure, like tissue regeneration, reconstruction, and delivery of cell, protein, gene, or drug to the demanded location, and hence, the materials that meet the needs are demanded. Those materials should be optimised in mechanical strength and toughness, chemical and biological reactivity or stability, or pore characteristics (porosity and size) suitable for relevant cells, needless to mention safety and ease of handling. This book is a result of collaborations among active research groups with expertise in the hottest field of bone tissue regeneration and reconstruction. It concisely covers the materials ranging from nano-scale to the ordinary one, and gives comprehensive description for new comers in this field as well as for professionals who might be a little unfamiliar with ceramic materials of those applications. Akiyoshi Osaka, Ph. D. Okayama University, Japan 2009

Preface The aim of the book entitled “Biomaterials for Bone Regenerative Medicine” is to extensively review the latest developments on Biomaterials and their applications on bone regeneration in vivo. Indeed, biomaterials research and their novel applications are essential for the health issues related with the currently aging population. In fact, a wide range of investigation is going on by the eminent scholars in order to further develop innovative materials for worldwide next generation applications. In future, it is expected that a tissue engineering approach, associating the novel biomaterials with stem cells will be proficient for all types of bone defects. Therefore, this book is a reference guide model in bone regenerative medicine for biomedical engineering researchers and physicians. The first chapter addresses the concept of biomimetic approach for processing nano biomaterials, in particular nanostructure biomineralized hydroxyapatite/collagen systems for use in skeletal tissue repair and regeneration, especially bone, presenting experimental examples. The excellent bioactivity of sol-gel glasses makes them potential candidates for osseous tissue regeneration and therefore, the second chapter deals with silica-based materials as precursors of nanoapatites. The chapter three will outline the investigations of several osteogenic phytochemicals for bone regeneration. The chapter four describes an integrated computational design and fabrication approach to enhance bone tissue engineering. A critical review of chapter five deals with calcium silicate based ceramics for skeletal tissue reconstruction. In chapter six presents Bonelike® an innovative modified calcium phosphate that mimics the chemical composition and structure of the mineral part of the human bone and is gradually substituted for new formed bone. Cell adhesion and proliferation of human osteoblastic-like cells over Zinc-Glass Reinforced Hydroxyapatite Composites (ZnGRHA) were reported in chapter seven. Finally, we hope that the chapters of this book reflect the quality and standards of recent advances in biomaterials field for all research community in bone tissue engineering applications.

Nandyala Sooraj Hussain and José Domingos da Silva Santos

CONTRIBUTORS A Bakr M Rabie Biomedical and Tissue Engineering University of Hong Kong Hong Kong C.L. Flanagan Department of Biomedical Engineering The University of Michigan Ann Arbor, MI USA C.M. Botelho Life and Health Sciences Research Institute (ICVS) School of Health Sciences University of Minho Campus de Gualtar 4710-057 Braga PORTUGAL C.Y. Lin Department of Neurosurgery The University of Michigan Ann Arbor, MI USA Chengtie Wu Biomaterials and Tissue Engineering Research Unit AMME School The University of Sydney Sydney, NSW, 2006 AUSTRALIA E.N. Moffitt Department of Biomedical Engineering The University of Michigan Ann Arbor, MI USA

H. Cheung Department of Mechanical Engineering The University of Michigan Ann Arbor, MI USA Hala Zreiqat Biomaterials and Tissue Engineering Research Unit AMME School The University of Sydney Sydney, NSW, 2006 AUSTRALIA James J. Hickman NanoScience Technology Center University of Central Florida 12424 Research Parkway Orlando, FL 32826 USA Jiang Chang Biomaterials and Tissue Engineering Research Center Shanghai Institute of Ceramics Chinese Academy of Sciences 1295 Dingxi Road, Shanghai 200050 P. R. CHINA J. V. Lobato ICBAS - Instituto de Ciências Biomédicas de Abel Salazar Universidade do Porto Largo Professor Abel Salazar, 2 4099-003 Porto PORTUGAL J.D. Santos DEMM - Faculty of Engineering University of Porto Rua Dr.Roberto Frias 4200-465 Porto PORTUGAL

K. Panduranga Rao Spark Biotech 3720 Sunburst Lane Naperville, IL 60564 USA María Vallet-Regí Departamento de Química Inorgánica y Bioinorgánica. Facultad de Farmacia Universidad Complutense de Madrid 28040- Madrid. SPAIN Masaya Kotaki Department of Advanced Fibro-Science Kyoto Institute of Technology Kyoto 606-8585 JAPAN Marta Santos DEMM - Faculty of Engineering University of Porto Rua Dr.Roberto Frias 4200-465 Porto PORTUGAL M. A.Lopes DEMM - Faculty of Engineering University of Porto Rua Dr.Roberto Frias 4200-465 Porto PORTUGAL M. Gutierres FMUP - Faculdade de Medicina da Universidade do Porto – Hospital de São João Largo Hernâni Monteiro 4200 Porto PORTUGAL

M.H. Fernandes Laboratório de Farmacologia e Biocompatibilidade Celular Faculdade de Medicina Dentária Universidade do Porto Rua Dr. Manuel Pereira da Silva 4200-393 Porto PORTUGAL Nandyala Sooraj Hussain INESC Porto/ Department of Physics Faculty of Sciences University of Porto Rua do Campo Alegre, 687 4169 -007 Porto PORTUGAL P. S. Gomes Laboratório de Farmacologia e Biocompatibilidade Celular Faculdade de Medicina Dentária Universidade do Porto Rua Dr. Manuel Pereira da Silva 4200-393 Porto PORTUGAL P.H. Krebsbach Department of Oral Medicine, Oncology, Pathology The University of Michigan Ann Arbor, MI USA Peter Molnar NanoScience Technology Center University of Central Florida 12424 Research Parkway Orlando, FL 32826 USA

R. Murugan NanoScience Technology Center University of Central Florida 12424 Research Parkway Orlando, FL 32826 USA R. M. Pinto ISCS-N Instituto Superior de Ciências da Saude-Norte R. Central de Gandra, 1317 4585-116 Gandra Prd PORTUGAL R.M. Schek Department of Biomedical Engineering The University of Michigan Ann Arbor, MI USA Ricky W K Wong Biomedical and Tissue Engineering University of Hong Kong Hong Kong S. S. Liao NUS Nanoscience and Nanotechnology Initiative Division of Bioengineering National University of Singapore 117576, SINGAPORE S. Das Department of Mechanical Engineering The University of Michigan Ann Arbor, MI USA S.E. Feinberg Department of Surgery (Section of Oral and Maxillofacial Surgery) The University of Michigan Ann Arbor, MI USA

S.J. Hollister Department of Biomedical Engineering The University of Michigan Ann Arbor, MI USA Seeram Ramakrishna NUS Nanoscience and Nanotechnology Initiative Division of Bioengineering National University of Singapore 117576, SINGAPORE Z. M. Huang School of Aerospace Engineering and Applied Mechanics Faculty of Engineering Tongji University Shanghai 200092 P. R. CHINA

Table of Contents Dedicated Contents Forward Preface Contributors

CHAPTER 1 Skeletal Regenerative Nanobiomaterials 1. Introduction 2. Basics of Bone Biology 3. Current Scenarios of Bone Grafting 4. Concept of Biomimetics in Skeletal Regeneration 5. Mechanism of Biological Mineralization 6. Biomimetic Mineralization – Rationale and Benefits 7. Processing of Biomineralized Nanobiomaterial Systems 8. Biomineralization of Electrospun Nanofibers – A New Approach Conclusions and Future Challenges Glossary References

3 4 7 9 10 11 18 23 26 28 31

CHAPTER 2 Silica-Based Materials as Precursorsof Nanoapatites 1. Bioactive glasses 2. Sol-Gel Glasses: Components of Mixed Materials 3. Organic-Inorganic Hybrids to Expand the Clinical Application of Bioactive Glasses 4. Star Gels Bioactive Materials 5. Silica Based Ordered Mesoporous Materials 6. Synthesis of Templated Glasses 7. Considerations on Materials Eligible for Bone Regeneration Duties References

37 52 56 58 59 66 68 71

CHAPTER 3 Phytochemicals for Bone Regeneration 1. Introduction 2. Material and Methods 3. Results 4. Discussion 5. Conclusion References

83 87 91 92 97 98

CHAPTER 4 Designed Biomaterial Scaffoldsfor Bone Regeneration 1. Introduction 2. Scaffold Design 3. Effective Mechanical and Permeability Properties of Designed Scaffolds 4. Bone Regeneration on Designed Scaffolds

109 110 112 114

b

Biomaterials for Bone, Regenerative Medicine

5. Discussion References

117 118

CHAPTER 5 Engineered Ca-Si Based Ceramics for Skeletaltissue Reconstruction 1. Introduction

2. Ca-Si Based Bioactive Glass and Glass-Ceramics 3. Ca-Si Based Binary Oxide System Bioactive Ceramics 4. Ca-Si-Mg Bioactive Ceramics 5. Future Trends. References

123 124 125 142

CHAPTER 6 Calcium Phosphate-Based Materials for Boneregenerative Medicine 1. Introduction 2. Bioactive Glasses and Glass-Ceramics 3. Silicon-Substituted Apatites 4. Calcium Phosphate-Based Materials 5. Bonelike® Medical Applications References

153 154 156 157 158 172

CHAPTER 7 Cell Adhesion and Proliferation over Zinc-Glassreinforced Hydroxyapatite Composites (Zn-GRHA) 1. Introduction 2. Materials and Methods 3. Physicochemical and Morphological Analysis of the Zn-GRHA Composites 4. In Vitro Biocompatibility of the Zn-GRHA Composites 5. Conclusions. References

183 185 187 190 193

CHAPTER 1

Skeletal Regenerative Nanobiomaterials R. Murugan1, S. S. Liao2, Seeram Ramakrishna2*, Peter Molnar1, Z. M. Huang3, Masaya Kotaki4, K. Panduranga Rao5, and James J. Hickman1 1

2

NanoScience Technology Center, University of Central Florida, 12424 Research Parkway, Orlando, FL 32826, USA.

NUS Nanoscience and Nanotechnology Initiative, Division of Bioengineering, National University of Singapore, Singapore 117576, Singapore. 3

4

School of Aerospace Engineering and Applied Mechanics, Faculty of Engineering, Tongji University, Shanghai 200092, P. R. China.

Department of Advanced Fibro-Science, Kyoto Institute of Technology, Kyoto 606-8585, Japan. 5

Spark Biotech, 3720 Sunburst Lane, Naperville, IL 60564, USA. *[email protected]

Table of Contents 1.

Introduction ............................................................................................. 3

2.

Basics of Bone Biology ................................................................................. 4 2.1.

Components of Bone Tissue ........................................................... 4

2.2.

Cellular Functions of Bone Tissue ..................................................... 5

2.3.

Hierarchical Tactics of Bone Tissue................................................... 6

3.

Current Scenarios of Bone Grafting...................................................... 7

4.

Concept of Biomimetics in Skeletal Regeneration ............................... 9

5.

Mechanism of Biological Mineralization ............................................ 10

6.

Biomimetic Mineralization – Rationale and Benefits ........................ 11 6.1.

The Key Nucleation Sites of Collagen Mineralization ................. 13

6.2.

The Hierarchical Assembly of Mineralized Collagen .................. 14

6.3.

Advantages of Mineralized Collagen System............................... 16

7.

8.

Processing of Biomineralized Nanobiomaterial systems ................... 18 7.1.

SBF Immersion Technique ........................................................... 18

7.2.

Co-Precipitation Self-Assembly Method ...................................... 20

7.3.

Molecular Self-assembly Process ................................................. 22

Biomineralization of Electrospun Nanofibers – A New Approach .. 23

Conclusions and Future Challenges ............................................................... 26 Glossary ............................................................................................................ 28 References ......................................................................................................... 31

Materials Science Foundations Vol. 62 (2010) pp 3-3 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSFo.62.3

1.

INTRODUCTION

Skeletal tissue regeneration is a subject of intensive research in orthopedics, because skeletal defects, especially bone defects, acutely influence quality and length of human life. Bone is a complex and a highly specialized form of connective tissue pertaining to the formation of the skeleton of the body. It is a good example of a dynamic tissue in the human body since it has a unique capability of strong regenerative potential throughout the life without leaving a scar. However, skeletal defects resulting from trauma, tumor, or bone abnormality often call for bone grafting procedures. Each year, millions of people suffer from bone defects and several die due to insufficiency of ideal bone graft and/or failure of implanted bone regenerative system. With reference to statistical reports [1-3], about 6.3 million fractures occur every year in the United States of America (USA) itself, of which about 550,000 cases require some kind of bone grafting. It was also noticed that the fractures occur at an annual rate of 2.4 per 100 populations. The most frequently occurring fractures are, in decreasing order, hip, ankle, tibia, and fibula fractures. It is reported that the total number of hip replacements was about 152,000 in the year of 2000, which is an increase of about 33% compared to the year of 1990 in the USA alone and it is expected to increase to about 272,000 by the year of 2030 [4], indicating that there is a great need for synthetic bone grafts. According to a reported data [5], bone graft sales was found to exceed US$980 million in 2001 in the USA and about US$1.16 billion in 2002, which is also expected to double by 2006. The need for synthetic bone graft materials depends on the extent of the bone defects. For example, if the defect is minor, bone has its own capability to self-regenerate within a few weeks. Therefore, surgery is not required. In the case of severe defects and loss of volume, bone would not heal by itself and thus grafting procedure is required to restore normal function without damaging living tissues. There are several methods available for the treatment of bone defects. Most classical methods are based on the transplantation of autologeous bone tissue (also called Gold standard), but the supply of autograft is limited. On the other hand, allogenic and xenogenic bone tissues are less preferred because of their clinical complications associated with immunological reactions and transfer of pathogens like viruses and prions. Therefore, there is a great need of engineering synthetic bone graft materials, mimicking natural bone tissue in all aspects. Numerous synthetic bone graft materials have been developed and are being used in the clinical practice with good results. Although there is a good progress in bone grafting using these materials, their long-term performance is generally not satisfactory, and the way in which they execute their functions in-vivo is also quite different from one another. The characteristics of the currently available synthetic bone graft materials are drastically different from that of natural bone tissue in many aspects, in particular molecular interaction between minerals and organic components. This is often due to lack of appropriate methods to process bone-resembling graft materials. Processing of bone grafts that mimic the chemical composition, phase structure, and functionality of natural bone is a good option for

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engineering bone tissues. Although natural bone is chiefly made of nano phase hydroxyapatite (HA) and collagen fibers, processed synthetic bone grafts that consist of these components are substantially different from the natural ones in terms of structure and function. This may be due to the absence of peculiar self-organizing interaction between those two phases and their inability to initiate biomineralization in cellular environment immediately after implantation. Recently, biomimetic concept has been introduced in processing nanobiomaterials, in particular nanostructure biomineralized HA/collagen system, which is perceived to be beneficial for skeletal regeneration than conventionally-processed ones owing to their mimicry of biological mechanisms. Further, the biomimetically-derived materials can easily be adapted to bodily environment, leading to faster tissue formation at the bone-implant interface. Biomimetic design or biomimetically inspired design is a methodology that takes inspiration from biological systems to design synthetic alternatives capable of functioning in the bodily environment quite similar to biological ones. This chapter introduces the concept of biomimetic approach in processing nanobiomaterials, in particular nanostructure biomineralized HA/collagen systems for use in skeletal tissue repair and regeneration, especially bone, with experimental examples. The chapter also discusses some of the critical issues and scientific challenges that might be faced in further research and development of this emerging field, bone biomimetics. For the benefit of readers, basics of bone biology, current scenario of bone grafting, and key mechanisms of biological mineralization are also described. The authors hope that it could be useful for readers to gain state-of-the-art in-sights on how to process nanostructure biomineralized systems as a new class of bone grafts for the regeneration of skeletal tissues. 2. 2.1

BASICS OF BONE BIOLOGY COMPONENTS OF BONE TISSUE

The design strategy of biomimetic bone grafts is not straightforward without understanding at least the rudiments of bone composition, structure, and the way in which it is organized. Bone is a well-organized connective tissue made of several building blocks at multiple levels, from nano to macro, that consist of, in decreasing proportions, minerals, collagen, water, noncollagenous proteins, lipids, vascular elements, and cellular components. An overall composition of the bone is given in Table 1 [6]. Bone, in general, is composed of approximately 70% of minerals and 30% of proteins. The bone minerals are chiefly enriched with nanophase HA and the bone proteins mainly consist of collagen nanofibers. Collagen acts as a structural framework in which nanocrystals of HA is embedded to strengthen the bone tissue. The bone collagen has a typical fibrous structure, whose diameter varies from 100 to 2000 nm. Similarly, HA in the bone mineral is in the form of plate-like nanocrystals with dimensions of about 4 nm by 50 nm by 50 nm. The bone minerals are also enriched with few trace elements for various metabolic functions, which include carbonate, citrate, sodium, magnesium, fluoride, chloride, potassium, and iron. The prime role of minerals is to provide

Materials Science Foundations Vol. 62 Skeletal Regenerative Nanobiomaterials

toughness and rigidity to the bone, whereas collagen provides tensile strength and flexibility. It is really amazing to know, how the nature built extremely hard and tough bone using such soft (collagen) and brittle (HA) ingredients. Bone, not only provides mechanical support for the organism but also elegantly serves as a reservoir for minerals, particularly calcium and phosphate. A complete biological mechanism involved in the bone building strategy is still unclear and thus research progresses still in this direction. It is believed that the key to the strength of the bone is the complex structural hierarchy into which it is organized in a selfassembling mode. It is important to note that the minerals are not directly bound to collagen, but bound through non-collagenous proteins. The process involved in this strategy is often called as biological mineralization or biomineralization. The non-collagenous proteins make up approximately 3 to 5% of the bone, which provide active sites for biomineralization. Lipids are also play an important role during biomineralization. It is worth stating that, in general, biomineralization typically begins only 10 days after the organic matrix, particularly collagen, is laid-down. The key mechanism involved in the biological mineralization is briefly discussed in Section 5. Table 1. Composition of bone tissue [6]. Inorganic phase (wt.%)

Organic phase (wt.%)

Hydroxyapatite  60

Collagen  20

Carbonate

 4

Water

 9

Citrate

 0.9

Non-collagenous proteins  3

Sodium

 0.7

(osteocalcin, osteonectin, osteopontin,

Magnesium

 0.5

thrombospondin, morphogenetic proteins,

Other traces:

sialoprotein, serum proteins),

Cl-, F-, K+ Sr2+, Pb2+, Zn2+,

Other traces:

Cu2+, Fe2+

Polysaccharides, lipids, cytokines. Primary bone cells: osteoblasts, osteocytes, osteoclasts.

2.2

CELLULAR FUNCTIONS OF BONE TISSUE

Bone formation is accomplished by synchronized multicellular actions and hence studying the functions of cellular components is significant in the contest of bone metabolism, which further helps to understand their roles in biological mineralization. There are five distinct types of cells associated with the bone tissue with regard to their functions, which are called as osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts, and bone-lining cells. Bone, like other connective tissues in the embryo, is derived from mesenchymal cells. These cells have the ability to proliferate and differentiate into bone cells, which are known as osteoprogenitor cells. They are also called as bone-precursor cells. Osteoblasts are the cells that are

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responsible for the formation of new bone tissue. They start with secreting collagen followed by coating non-collagenous proteins, which are similar to glue that has the ability to bind the minerals, mostly calcium and phosphate, from the bloodstream. Osteocytes are matured cells derived from the osteoblasts that are responsible for the maintenance of the bone tissue. They function as transporting agents of minerals between bone and blood. Osteoclasts are the large cells that are found at the surface of the bone mineral next to the resorbing bone. They are responsible for the resorbtion of the bone tissue. They use acids or enzymes to dissolve the minerals as well as collagen from the matured bone. The dissolved minerals then re-enter the bloodstream and are carried to different parts of the body. Bone-lining cells are found along the surface of the matured bone, which are responsible for regulating the transportation of minerals in and out of the bone tissue. They also respond to hormones by making some exclusive proteins that activate the osteoclasts. These five types of cells are together responsible for building the bone matrix with hierarchical self-assembly, maintenance, and remodeling as required. All these processes must be in equilibrium to ensure a healthy bone tissue. 2.3

HIERARCHICAL TACTICS OF BONE TISSUE

Bone is considered as an assemblage of hierarchical structural units or building blocks elegantly designed at many length scales, macro to nano and so, to meet multiple functions. When bone is initially laid down, it is structurally weak and unorganized. But within a few days, the primary bone remodels to become lamellar bone and gradually get matured, which has a perfect alignment of collagen fibers and minerals. At the macrostructural level, the matured bone can be distinguished into two types, namely, spongy bone and compact bone. As their names imply, they radically vary in density. They are organized with multi-level pores, macro to nano, for the establishment of multiple functions, including transportation of nutrients, oxygen, and body fluids. The dimensiondependent hierarchical structure of the bone tissue, from nanoscale building blocks to macro structure, is shown in Fig. 1 [6]. The spongy bone occupies about 20% of the total bone. It is also often called as trabecular or cancellous bone. It is lighter and lesser dense than compact bone. It has high porosity and higher concentration of blood vessels compared to the compact bone. The porous architecture is easily visible under the lower power microscopes and/or even to the naked eyes if the pores are very large. The diameter of the pores may be from few micrometers to millimeters. On the other hand, compact bone is much denser than the spongy bone. It is also called as cortical or dense bone. It occupies about 80% of the total bone. It has less porosity and thus it has less concentration of blood vessels. Its porous architecture is not visible to naked eyes owing to less porosity. They may be perhaps 10-20 m in diameter and mostly separated by intervals of 200-300 m. The compact bone functions mechanically in tension, compression, and torsion, whereas spongy bone functions mainly in compression. At the microstructural level, the repeated structural unit of compact bone is mostly of osteon or Haversian system, which acts as weight-bearing pillars. In contrast, spongy bone contains

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no such osteon units, but they are made of an interconnecting framework of trabeculae. The trabeculae have three types of cellular structures: plate/plate-like, plate/bar-like, and bar/barlike.

Fig. 1 Bone hierarchical structure, from macro- to nano-assembly [6]. At the nanostructural level, the bone tissue chiefly consists of collagen nanofibers and nanocrystals of bone minerals, particularly HA. Although several structural levels of bone have been identified, a complete understanding of how the mineral-matrix interactions are related to their mechanical reliability at the so far identified hierarchical levels of the bone tissue is still limited. Further, rationales of cell-matrix and cell-cell interactions are also important aspects to be considered at atomic or molecular level. 3.

CURRENT SCENARIOS OF BONE GRAFTING

Bone grafting is a surgical procedure that repair or replace a defective bone with help of naturally-derived or synthetic functional components (also called bone grafts). Bone grafts, in general, provide mechanical or structural support, fill defective gaps, and enhance bone formation. There are many ways in which bone grafting can be done, which include autografting, allografting, xenografting, and alloplastic or synthetic bone grafting, each with their own advantages and disadvantages. Autografting is a surgical method in which tissue or organ is transplanted from one site to another site of the same individual. The concept is basically patient’s own bone collected from a donor site and transplanted to another site of the same body, which requires bone regeneration. It is commonly harvested from the patient’s iliac crest in the form of cancellous bone. Mowlem clinically proved that the autogenous bone harvested from intraoral or extraoral sites is the finest osteogenic graft for bone tissue regenerative applications in 1944, and he had successfully treated about 75 critical cases using cancellous bone of iliac crest [7]. Another source of autogenous graft, which is recently explored, is the osteoblastic stem cells found in bone marrow, also familiarly known as BMA. These cells can be harvested from the patient’s own body with the help of a needle and transplanted to the defective site without a

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complicated surgery. They extensively provide osteogenic and osteoinductive factors necessary for faster bone regeneration. They can also be transplanted by blending or mixing with other bone graft materials in a composite form. The later method is a good choice because there is a chance of the migration of cells from the implanted site if we deliver them without a suitable carrier system. So far, the best clinical success rate has been achieved by this autografting method because it provides osteogenic cells as well as essential osteoinductive factors for the activation of faster bone regeneration; thereby it is clinically considered as a gold standard. This method is also recognized as safest transplant owing to histocompatiblility of the grafts and thus there is no chance for rejection. The microstructural features of the graft perfectly match with host tissue. However, it has few disadvantages in several clinical situations, which includes (i) insufficient amount of grafts, particularly in children and when dealing with large bone defects; (ii) significant postoperative risk of morbidity at the donor site; and (iii) complexity in making required shape. Allografting can be defined as tissue transplantation between individuals of the same species but of non-identical genetic composition. Lexer carried out the first clinical use of allografts in 1908 [7]. The materials used as allografts are mostly cancellous, cortical, or a combination of each. Bone banks stock this type of grafts, which are usually harvested from the cadavers. Typically, they are frozen or freeze-dried. However, after sterilization, most of them seem to lose much of their strength and, of course, they won’t be resorbed completely after implantation; thereby they often remain as a dead tissue or act as a foreign body. The dead portion then gradually becomes brittle and gives further medical complications with surrounding tissues. The advantages of allografting are the elimination of harvesting surgical site, the related postoperative pain and the added expense of a second operative procedure. The disadvantages are the slight chance of disease transmission such as hepatitis B, C, and acquired immune deficiency syndrome (AIDS) and reduced effectiveness since the bone growth cells and proteins are removed during the cleansing and disinfecting process. This method is of particular interest only if the bone defect is considerably large or the defect is not amenable for repair by autografting owing to insufficient quantity of the graft materials. Xenografting is a surgical procedure of transplanting tissue from one species to another (e.g., bone from animal to human). It comes from the Greek word xenos, meaning strange. The feasibility of using animals as a source of tissue or organ for transplantation has been the focus of research over many decades. Most of the bone transplantations using xenografts were performed in the early 1960s. Bone banks usually stock abundant quantity of xenografts because of their easy harvesting compared to the bone from human. Currently, multiple xenografts are being processed, which includes frozen calf bone, freeze-dried calf bone, decalcified ox bone, deproteinized bovine bone, and anorganic bone [7]. Kiel bone is one of the few commercially available xenografts, consisting deproteinized bone of freshly sacrificed calves. The xenografting method is not very successful compared to autografting or allografting, probably due to the peril of antigenicity. However, the shortage of organs has

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prompted continued research on this field and thus research is still ongoing around the world to find the ways to eliminate the problems associated with antigenicity and graft rejection. As an alternative to the above three types of biological bone grafting systems, synthetic substances (also called biomaterials) are gaining much interest for use as bone graft materials. A surgical method that uses synthetic substances to repair or regenerate defective bone tissue is known as alloplastic or synthetic bone grafting. The benefits of synthetic grafts include availability, reproducibility, shapeability, sterility, cost-effectiveness, and reduced morbidity. The synthetic grafts eliminate some of the shortcomings of autografts or allografts associated with donor shortage and the chance for rejection or transmission of infectious disease. However, the selection of grafting procedure is purely dependent on the nature and extent of the bone defects as well as choice of available bone graft systems. 4.

CONCEPT OF BIOMIMETICS IN SKELETAL REGENERATION

Regeneration of skeletal tissues is among the most promising areas of tissue engineering, and is providing a broad spectrum of potential clinical applications, including bone grafting. Over the past five decades a variety of synthetic materials had been developed for bone grafting applications, with the aim of eliminating or minimizing the above said complications associated with autografts, allografts, and xenografts [8-19]. It should be noted that each material has different characteristic functions at either in-vitro or in-vivo or both; thereby it is quite difficult to judge which is the best system for bone grafting. Each system has its own advantages and disadvantages, and thus the clinicians must know which system is a right choice to specific applications. Synthetic grafts, in general, eliminate some of the shortcomings of conventional biological bone grafts associated with the donor shortage and the chance for rejection or transmission of infectious disease; thereby they are considered as a good choice for bone grafting, and as alternative to those conventional bone grafting methods. In addition, synthetic grafts are abundantly available, reproducible, shapeable, sterilizable, and cost-effective. Although, synthetic grafts have many desirable characteristics, currently none resembles natural bone in terms of structure and function, which is mainly due to the lack of appropriate processing method. These facts suggest the necessity of introducing a new approach for the processing of bone-mimicking graft materials. Biomimetic approach has been recently introduced in skeletal tissue engineering and is considered as one of the promising methods in processing synthetic bone graft materials that closely mimic the structure and function of natural bone. Biomimetics is a new field of biosciences that studies how Nature designs, processes and assembles molecular level building blocks to fabricate high performance hybrid systems, essential of tissue functions, and then replicates these strategies to design new bone graft materials with unique properties. Now, more researches progressing in this direction and the next decade may witness many breakthroughs in the fascinating field of bone biomimetics. For the better understanding of bone biomimetics, one has to know the basics of biological mineralization and how does it

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occur in bone formation. The following section describes the mechanism involved in the biological mineralization process. 5.

MECHANISM OF BIOLOGICAL MINERALIZATION

Biological mineralization or biomineralization is defined as a process by which living organisms generate biominerals that crystallize and mature in a precisely well-organized way for the growth of bone tissue. The biominerals provide the structural strength to bone. The field of biomineralization is a multidisciplinary subject that combines the basic sciences of physics, chemistry, and biology in association with modern sciences of structural biology and nanotechnology, towards the development of new generation biomaterials for bone regenerative applications. With reference to available data reported in various literatures during the past decade, biomineralization is a promising as well as primary process for the fabrication of novel biomaterials. The process of biomineralization usually involves in the nucleation and growth of biominerals within a matrix of either cellular or extracellular organisms in a fashion, which occurs only under mild and ambient conditions at low temperature. It should also be noted that biomineralization does not occur without a matrix, which acts as a mineral nucleating agent. The matrix is thought to be important in binding various ions at anionic or cationic sites. Calcium and phosphates are the primary biominerals involved in the various stages of biomineralization of human bone tissue. In general, biomineralization process occurs in two steps. In the first step, phosphate accumulates within the matrix through the enzymatic hydrolysis of phyrophosphates, a kind of calcium phosphate-based inorganic phase. The accumulated phosphates then react with calcium ions and subsequently produce apatite crystals, particularly HA, in the case of bone tissue growth. It is believed that anions (phosphate, for example) assist nucleation by providing a negatively charged surface that interacts electrostatically with cations (calcium, for example). Certain ions have a strong attraction for electrons in oxygen-containing anions (phosphate, for example) which further reduces the attraction of water molecules, leading to HA formation as follows: 10Ca2+ + 6HPO42 + 2H2O  Ca10(PO4)6(OH)2 + 8H+. In the second step, the crystal within the matrix grows and is exposed to the cartilage matrix fluid. Notice that cartilage is a precursor of bone or earlier stage in bone formation. The apatite exposed to the matrix fluid then gradually matures owing to the supersaturated fluid containing apatite crystals. The highly matured apatite crystals grow within the collagen fibrils in such a way that their c axes are oriented along the long axes of the fibrils. The distribution of the anions in the matrix may provide a steriochemical template for adsorbing a layer of cations, thereby controlling both the phases and orientation of the nuclei as well as their location (see Fig. 2) [20]. The collagen matrix spatially or temporally defines the space with bound side chain groups and free calcium and/or phosphate ions (Pi) as shown in the Fig. 2. It is also believed that the anions in matrix fluid can influence crystal shape by selective adsorption onto certain faces. The anionic groups exposed in matrix fluid tend to concentrate the inorganic cations, resulting in local ionic supersaturation that stimulate nucleation of bone

Materials Science Foundations Vol. 62 (2010) pp 11-17 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSFo.62.11



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