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B i o m a t e r i a l s f o r o n l a y b o n e g r a f t s allogeneic block graft group46 and that newly formed bone contained viable osteocytes. In these samples, bone-forming osteoblasts were detected. Dense connective tissue with the pres- ence of inflammatory cells and eroded areas were also reported. Minimal diferences were shown for the autogenous block graft group, in which no connective tissue was found and the presence of inflammatory cells was low. However, Spin-Neto et al. found major difer- ences between groups.55 The following histo- logical characteristics were found to be associ- ated with allogeneic bone block grafts: (a) large segments of necrotic bone with empty osteocyte lacunae and little osteoclastic activity; (b) blood vessels invading the Haversian canals of the material—no direct contact was found between remodeled and grafted bone; and (c) some os- teoclastic activity surrounded by connective tissue with no presence of inflammatory cells by newly formed bone failed to invade the graft. On the contrary, autogenous block grafts pre- sented small areas of necrotic bone with a higher number of osteocytes and a smoother junction between the graft and host bed. Therefore, from the cellular standpoint, allogeneic block grafts in the early stages of healing behave in a difer- ent manner to autogenous block grafts. However, the long-term outcome and diferenc- es remain to be determined. 2. Xenogeneic block grafts Xenografts, which are derived from a genetical- ly diferent species than the host, represent another potential alternative to autogenous block grafts for bone augmentation. Similar to human allografts, the lack of osteogenic capac- ity makes them less predictable in terms of graft incorporation into host bone. In addition, lack of human cells turns xenografts into scafolds with no osteoinductive potential. Despite its novel applicability as block grafts for augmenting se- verely atrophied bone, this type of biomaterial has been widely used as particulate bone graft, showing excellent outcomes by means of space maintenance.60–62 Thus far, there is a scarcity of literature regarding this biomaterial for onlay grafts, and xenogeneic block grafts have been used more commonly as inlay grafts. As men- tioned above, vascularity for this biomaterial is even more critical for success and, consequent- ly, a three-wall defect (as displayed by host bone for inlay grafts) often makes this approach more reliable. However, an advantage of using xeno- geneic biomaterial is that, owing to its slow rate of resorption, space is better maintained over the long term (Fig. 5).63, 64 Currently, two types of xenografts are avail- able as blocks for bone augmentation: bovine and equine. While deproteinized bovine bone relies on its acceptability by clinicians, equine bone has shown to be less fragile to fracture.65 However, as mentioned, more studies are need- ed to verify the viability of this type of biomate- rial in comparison to autogenous or allogeneic block grafts. Properties In contrast to human-derived bone, xenogeneic grafts do not have osteoinductive potential. Therefore, they are used only as scafolds for space maintenance and cell migration guidance. Geistlich Bio-Oss (Geistlich Pharma, Wolhusen, Switzerland), a bovine-derived biomaterial, is the most widely used xenogeneic graft. This biomaterial is claimed to have all organic mate- rial removed, so is nonantigenic. A modified Geistlich Bio-Oss block that contains more col- lagen components for improving its manage- ability has also been introduced.66 Equine bone blocks have recently been introduced and have shown to provide an improved scafold for cases of severe atrophy owing to this bone’s natural trabecular structure.67 Xenogeneic biomaterials, albeit not posing osteoinductive potential, are claimed to serve as slow-resorption scafolds capable of promot- ing bone formation.68, 69 Nonetheless, more studies on this material are still needed to better understand its overall properties and long-term results. Clinical outcomes As mentioned before, studies on xenogeneic block grafts are limited.8 At this point, only a few in vivo studies have been carried out on this bio- material.66, 67, 70–72 The xenogeneic block graft has been advocated for bone augmentation. Steig- mann presented the first human case report that used this biomaterial for horizontal bone aug- mentation in the maxillary anterior region.85 Li et al. successfully used Geistlich Bio-Oss blocks for horizontal bone augmentation via a subperi- osteal tunneling approach.70 This might repre- sent an alternative approach for placing this specific biomaterial owing to the success rate it achieved. Despite these preliminary results, we still need more evidence to support the use of xenogeneic materials for onlay block grafting. 26 Volume 3 | Issue 1/2017 Journal of Oral Science & Rehabilitation