Current Scenario of Bone Grafting

Bone grafting is a surgical method that repairs or replaces the defective bones with healthy bones with the aid of materials or substances either from the patient's own body or from synthetic sources. The materials used for bone grafting are known as bone graft materials or bone grafts. They are widely used in orthopedic surgery, plastic surgery, oral and maxillofacial surgery, and dental surgery. It should be noted that bone is the second most transplanted tissue in humans. Bone grafts not only fill defective gaps, but also eventually provide mechanical and structural support; thereby enhancing bone tissue growth. By this method, bone defect-healing time gets reduced and new bone formation strengthens the defective area by bridging implanted bone grafts with host bone. There is a variety of bone grafting methodologies available, including autografting, allografting, xenografting, and alloplastic or synthetic bone grafting, each varying with their own advantages and disadvantages [21-26]. A diagrammatic representation of typical bone grafting in humans is shown in Figure 13.4. The selection of the grafting method is purely dependent on the nature and complication of the bone defects as well as the choice of available bone grafts and, of course, the key decision depends on the surgeon's own experience.

Autografting is a class of method in which bone tissue is transplanted from one site to another site of the same individual. The concept is basically the patient's own bone collected from a donor site and transplanted to another site of the same body that requires bone regeneration. It is commonly harvested from the patient's iliac crest in the form of cancellous bone. Mowlem clinically proved it in 1944 [27], and he had successfully treated about 75 critical cases using cancellous bone from the

Figure 13.4. A scheme for bone grafting strategies.

iliac crest. Another recently explored source of autogenic graft is the osteoblastic stem cells found in bone marrow, also familiarly known as bone marrow aspirate (BMA). These cells can be harvested from the patient's own body with the help of a syringe needle and transplanted to the defective site without complicated surgery as in the above cases. BMA extensively provides osteogenic and osteoinductive factors necessary for fast bone regeneration. They can also be transplanted by blending or mixing with other bone graft materials in a composite form [28,29]. The latter method is a good choice because there is a chance of migration of cells from the implanted site if we deliver them without a suitable carrier system.

So far, in general, the best clinical success rate has been achieved by this auto-grafting method because it provides osteogenic cells as well as essential osteoin-ductive factors for the activation of bone growth; thereby it is clinically considered as a gold standard [30,31]. This method is also recognized as the 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 a few disadvantages in several clinical situations, including (1) insufficient amount of grafts, particularly in children and when dealing with large bone defects; (2) significant postoperative risk of morbidity at the donor site; and (3) complexity in making the required shape.

Allografting is another kind of bone grafting method, which can be defined as tissue transplantation between individuals of the same species but of nonidentical genetic composition. Lexer carried out the first clinical use of allograft in 1908 [32]. The materials used as allografts are mostly cancellous, cortical, or a combination of each. The bone banks stock this type of grafts, which are usually harvested from cadavers. Typically, they are frozen or freeze-dried bones. However, after sterilization, most of them seem to lose much of their strength and, of course, they will not be resorbed absolutely after implantation; therefore they often remain as 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 a 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 because the bone growth cells and proteins are removed during the cleansing and disinfecting process [33,34]. This method is, therefore, of particular importance only if the bone defect is considerably large or the defect is not amenable to repair by autografting owing to insufficient quantity of the graft materials.

Xenografting is a method of transplanting tissue from one species to another; bone from animal to human, for example. It comes from the Greek word xenos, meaning strange. The feasibility of using animals as a source of bone tissue for transplantation has been the focus of research for many decades. Most of the bone transplantations using xenografts were performed in the early 1960s. The bone banks usually stock an abundant quantity of xenografts because of their easy harvesting compared to bone from humans. Currently, multiple xenografts are being processed, including frozen calf bone, freeze-dried calf bone, decalcified ox bone, deproteinized bovine bone, and anorganic bone [35]. Kiel bone is one

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Figure 13.5. Evolution of bone grafts.

of the few commercially available xenografts, consisting of deproteinized bone of freshly sacrificed calves [36]. Xenografting is not very successful compared to autografting or allografting, probably due to the peril of antigenicity. However, the shortage of organs has prompted continued research in this field and thus extensive research is still ongoing throughout the world.

As an alternative to the above three types of bone grafts, synthetic substances are gaining much interest in 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 synthetic substances used in bone grafting are called synthetic bone grafts. Over the past four decades a variety of synthetic bone grafts had been developed with the aim of eliminating or minimizing the above said complications associated with autografts, allografts, and xenografts [37-40]. Evolution of bone grafts with their past, present, and future trends are schematically illustrated in Figure 13.5 for the easy understanding of readers and for the comparison of their tremendous growth during the last few decades.

The compiled data imply that a variety of synthetic materials has been exploited for bone grafting. It is should be noted that each material has different characteristic functions either in vitro or in vivo or both; thereby it is quite difficult to judge which is the best system for bone grafting. Each of them has many advantages as well as disadvantages with respect to a specific need. In general, synthetic grafts eliminate some of the shortcomings of autografts or allografts associated with the donor shortage and the chance for rejection or transmission of infectious disease, respectively; therefore they are considered a good choice for bone grafting. 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 tissue in many aspects, which is mainly due to the lack of appropriate designing technique. In this regard, the so-called biomimetic approach is considered one of the promising methods in designing such a bone-resembling graft. Now, research is progressing in this direction and the next decade may witness many breakthroughs in the fascinating field of biomimetics.

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