## 4 Discussion
Their structure and chemical composition are very similar to those of natural hydroxy apatite, with appropriate porosity for vascular invasion and a surface chemistry that allows cells to adhere, and it is even characterized by appropriate mechanical properties [19; 28]. The percentage of residual graft particles observed in this study was in a similar range to that observed for other bone substitutes that have been used for ridge preservation procedures. Some biomaterials like demineralized freeze-dried bone allograft exhibited a low percentage of residual graft particles (8.8%), whereas hydroxyapatite-based material shows rather high levels (40.8%) [11]. In this study, the residual graft particles constituted the \(22.0\pm 9.7\%\) of the whole sample and were slightly higher when compared with some other previous studies that used the same biomaterial. Some authors have reported an amount of residual graft material of \(15.99\pm 11.4\%\) or \(15.83\pm 8.70\%\)[19; 29]. This could be explained by the initial application and condensation of the material into the defect and that the buccal bone plate was completely or partially (\(>\)5 mm) lost in the selected cases. The percentage of newly formed bone in this study was \(29.0\pm 9.3\%\), after six months. This was slightly higher when compared to the histomorphometrical results from [PERSON] et al., who in a recent case series on intact sockets used hardening biphasic calcium phosphate revealing an observed \(21.3\pm 9.1\%\) of new bone at a 5 month follow up [21]. That outcome was also in accordance with [PERSON] et al., who in a previous prospective cohort study reported a range of 28.00%-36.75% of new bone formation after four months in extraction with buccal defects grafted with biphasic calcium phosphate particles and a collagen membrane [28]. When compared with results from ridge preservation studies performed with different grafting materials, the percentage of new bone was 24.6% for freeze-dried bone allograft and 47.15% for calcium phosphosilicate [11]. A previous study, performed by [PERSON] et al., used biphasic calcium phosphate in combination with a hydroxyapatite in sinus augmentation procedures; the authors found that the newly formed bone was 28.35% 6-8 months after surgery. Interestingly, the percentage of newly formed bone increased from 6 to 8 months after grafting procedures. This finding could mean that in a biphasic calcium phosphate augmented site, a longer healing period could be beneficial in terms of higher amounts of newly formed bone [30]. Clinical parameters of horizontal and vertical bone loss after dental extraction were widely studied, and it was demonstrated that marked alteration occurred during the first eight weeks of healing. In this interval, intense osteocl activity results in resorption of crestal bone. During the first phase, the bundle bone, the portion of alveolar process that surrounds the teeth, was resorbed and subsequently replaced with a woven bone. On the vestibular side of the alveolar socket, predominantly composed of bundle bone, a substantial vertical (11%-22%) and horizontal (29%-63%) reduction of the buccal crest could be observed during the spontaneous healing without any grafting biomaterials [2; 3]. Clinical measurements from this study showed an alveolar horizontal resorption after ridge preservation (\(\Delta\)AHB) of \(0.5\pm 1\) mm (7.4%) after 6 months. That was in accordance with [PERSON] et al., who reported \(0.79\pm 0.7\) mm of width reduction from CBCT analysis (cone beam computer tomography) [21]. [PERSON] et al. showed slightly different results in an RCT using biphasic calcium phosphate or a bovine-derived xenograft in a post-extraction site: after 8 months of healing, the bucco-lingual dimensions of the alveolar ridge decreased by 1.1 \(\pm\) 1 mm and 2.1 \(\pm\) 1 mm, respectively [16]. That difference could probably be explained with the different follow-up analysis and the inclusion of different sites (incisor area) by the authors.