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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 patient.29 The establishment of the U.S. Navy Tissue Bank in 1990 was a significant influenc- ing factor for the wide use of bone allografts. The use of allografts has continued to increase since then.30 Properties The properties of allograft material are directly related to its processing and its precedence.31 Allogeneic block grafts may be prepared as fresh, frozen and freeze-dried. Nowadays, the vast majority of grafts are carefully screened, harvested, processed and distributed, and this is governed by the American Association of Tis- sue Banks. The risk of disease transmission is often minimized through the above process- es.32, 33 In addition, during graft preparation, the antigenic components are carefully removed to eliminate any potential host immune response.32 Fresh or frozen allografts retain both osteo- inductive and osteoconductive capacities, allow- ing a slightly faster bone turnover than that of freeze-dried allografts. However, the risks of disease transmission and host reactions are slightly increased,34 whereas the immune re- sponse is reduced in freeze-dried allografts.34 This is due to the elimination of the cells by em- bedding the graft in antibiotic wash twice for 1 h and then storing it at -70 °C to dry up to 5% of the water.35, 36 Another issue to bear in mind is that, because of the drying, mechanical proper- ties are weakened. Hence, microfracture of the grafts might easily occur. Consequently, for this type of block allograft, rehydration is suggested prior to placement in order to regain some of the mechanical properties.37 Currently, Zimmer Biomet Dental (Carlsbad, Calif., U.S.) has pat- ented its suitable preparation sequence (Fig. 2). This is the Tutoplast process, which includes cleaning and ultrasonic lipidization in acetone, an osmotic and later oxidative treatment, ending with dehydration in sequential acetone baths and gamma irradiation.38 The result of this pro- cess is a greater preservation of the minerals and collagen matrix, leading to rapid bone turn- over.39 Clinical outcomes Bone block allografts are a relatively novel al- ternative to autogenous grafts for horizontal and/or vertical bone augmentation of the atro- phic maxilla (Table 1). In 1999, the first case of using an allogeneic block bone graft for bone regeneration was reported. In that case, dental implants for oral rehabilitation were successful- ly placed three months after the grafting pro- cedure.18 Since then, multiple prospective human clinical trials have been published demonstrat- ing proof of principle for this human allograft block usage.40–56 From our clinical experience and others’, when the human allograft is exposed to the oral cavity, it often leads to graft failure.42, 57 More- over, it has much higher failure rate in the man- dible than in the maxilla owing to dificulty in flap advancement and a thinner soft-tissue bio- type.58 Failure of a block graft generally occurs in the early stages of graft healing.41, 45, 52, 55 In addition, bone graft resorption occurs during healing, which is the same as with autogenous grafts. However, greater bone loss occurs at six months after placement compared with autoge- nous bone harvested from the mandibular ramus (52.00 ± 25.87% vs. 25.00 ± 12.73%, respec- tively).46 A recent systematic review found promising results on the use of allogeneic bone grafts for horizontal bone augmentation in max- illae.59 It was shown that not only high graft and implant survival rates had been achieved (98.0% and 96.9%, respectively), but also that a weighed mean of 4.79 mm of horizontal bone had been gained over a mean follow-up period of 23.9 months. Histological and histomorphometric outcomes Indeed, allogeneic block grafts do not behave like autogenous bone from the cellular stand- point because of the lack of osteogenic potential; notwithstanding, respecting a proper healing time (more than six months), this biomaterial results in similar clinical healing to that of native bone40–56 (Figs. 3a–c & 4). Acocella et al. showed that, after nine months, a high number of empty osteocyte lacunae were still present and that more fibrous tissue was present than in the sam- ples taken previously.40 Additionally, newly formed bone (61.96 ± 11.77%) was surrounded by nonvital bone with empty osteocyte lacunae. At the same time after healing, Contar et al. demonstrated a lamellar arrangement around Haversian canals interspersed with osteocytes in lacunae.43 They also observed that the central portions of the grafts showed osteocytes with a higher number of empty lacunae. When histological results are compared between groups (allogeneic vs. autogenous), behavioral dissimilarities are displayed. Lumetti et al. showed that, after six months of healing, osteocyte lacunae were mostly empty for the 22 Volume 3 | Issue 1/2017 Journal of Oral Science & Rehabilitation