## 4 Bone Grafts and Substitutes for Bone Repair
### Natural Bone Grafts and Substitute Materials
#### 4.1.2 Allografts  
Allografts are natural materials that come from an individual of the same species and can be obtained from a compatible living donor or from cadaveric bone sources. Allograft materials can be prepared in three main forms--fresh, frozen or freeze-dried [32]. Fresh and frozen homologous materials have superior osteoinductive properties but are rarely used today due to the increased risk of a host immune response, limited vitality and increased risk of disease transmission [33]. Allografts are commonly used in the United States, being preferred by orthopedic surgeons, and there are currently four bone tissue banks that deal with the procurement and processing of osteochondral tissue [34]. The disadvantages of using allografts are generally reduced by lyophilization but also by other methods of tissue processing, such as mechanical debridement, ultrasonic washing and especially sterilization using gamma radiation [35; 36]. In Europe, increased regulatory restrictions on the use of materials from other people have led to a shift in the frequency of use from these materials to synthetic augmentation materials [37]. Allografts have good histocompatibility and are available in various forms, from whole bone segments, cortical-spongious segments and cortical pieces, to small pieces in the form of bone chips, powder and demineralized bone matrices. They can be produced in custom forms to meet the requirements of the receiving sites [38]. Homologous bone blocks from bone banks have been commonly used to rehabilitate bone supply in cases of severe atrophy of the alveolar process; they allow a sufficient volume of bone to be obtained for implant placement. Although their use compared to autologous materials avoids the existence of a second operating field, the integration time to be used in implant surgery exceeds 12 months [39]. The use of fragmented bone in the form of small pieces of spongy or cortical bone is indicated with encouraging results due to the increased osteeconductive potential, especially in the case of larger defects in the posterior maxillary area that require performing operations to lift the sinus membrane. These small fragments of spongy and cortical bone are usually used in a mixture with each other or with other categories of augmentation materials due to the increased risk of resorption of the spongy bone [40]. Demineralized bone matrix (DBM) is a decalcified product for which obtaining an acidic solution is used to remove mineral components, leaving behind collagen, other proteins, bone morphogenic proteins (BMP), variable percentages of calcium phosphate and a small percentage of cell debris. After decalcification, BMPs are released from the surrounding mineral structure and can fully exercise their osteoinductive potential, while the remaining collagen proteins in the matrix can provide a 3D configuration for the growth of host tissue capillaries, perivascular tissue and osteoprogenitor cells. In the meantime, the original cells and any bacteria in the allogeneic bone are removed, which could reduce the risk of immune rejection and infection [41; 42]. The release of BMP-type osteoinductive growth factors have cause demineralized bone tissue allografts to be considered a gold standard in periodontal regeneration surgical techniques [43; 44]. DBM-based augmentation materials have been used successfully both to improve the bone substrate prior to dental implant insertion [45] and for peri-implant repair surgery [46]; however, they are especially used as a supportive material for a number of biologically active substances [47]. Moreover, the increasing use of DBM in dental applications is related to its use as a transport vehicle for a number of added excipients, such as glycerol, starch, hyaluronic acid or saline, which allows for good maneuverability and improved adaptability [48].