Different techniques and grafting materials have been used for the horizontal reconstruction of deficient alveolar processes before implant placement, resulting in different degrees of predictability and clinical outcomes [1]. Among the grafting materials, particulated xenogeneic materials have been extensively studied in both experimental and clinical studies and when combined with porcine-derived natural collagen membranes have resulted in predictable clinical and histological outcomes [2].This combined treatment has shown to be safe and efficacious in horizontal ridge augmentations resulting in regenerated bone with similar implant survival rates when compared to implants placed in pristine bone and demonstrating a low degree of morbidity and a low rate of post-operative complications [3, 4]. However, in clinical situations with severe bone resorption of the alveolar process, which results in non-self-containing bone defects, the use of particulate bone replacement grafts with its inherent weak volumetric stability may limit the predictability of the regenerative therapy [5, 6]. In these cases, dental implants are usually placed staged to the lateral bone augmentation procedure and autogenous bone blocks have been the state of the art procedure, resulting in superior results in mean horizontal bone gains when compared with guided bone regeneration with particulate bone grafts [4, 7, 8]. The use of bone blocks, however, has been associated with increased surgical time, morbidity [9, 10], and a higher frequency of post-operative complications [11, 12]. Moreover, the availability for harvesting intraoral bone blocks is limited [13, 14] and these autologous bone grafts may suffer a high degree of bone resorption during healing [15].

To overcome these limitations, the use of xenogeneic bone grafts as an alternative to autogenous bone blocks has been proposed [16]. Recently, a new equine-derived collagen containing xenogeneic bone block (CCXBB) was evaluated in preclinical studies [17, 18], demonstrating to be safe and adequate for ridge augmentation and better graft integration when compared to other xenogeneic bone blocks. Its performance in humans has been recently tested on 10 patients where these xenogeneic bone blocks were placed in single-tooth alveolar bone defects [19]. Clinically, a mean horizontal gains of 3.88 ± 1.75 mm was reported and the histological outcomes resulted in a homogeneous new bone formation within the CCXBB. These results were concordant with a recent histological study also reporting that equine bone grafts were biocompatible and underwent advanced remodelling at the time of implant placement [20]. Although this preliminary evidence on the performance and histological behaviour of equine bone blocks seems promising, there is still limited information when used in staged horizontal bone augmentation of large osseous defects. It was, therefore, the aim of this prospective study to evaluate the histological outcomes of CCXBB blocks used for lateral bone augmentation in large alveolar horizontal defects of at least two adjacent missing teeth.