## 1 Introduction  
In the last 30 years, Guided Bone Regeneration (GBR) has become one of the most popular techniques to achieve bone regeneration of horizontal and vertical alveolar ridges, allowing implant rehabilitation [1; 2]. The key role of GBR is played by the barrier, which protects a bone graft from non-osteogenic cells and creates a suitable biological environment for osteoprogenitor cells to colonize the bone defect, favoring angiogenesis and bone regeneration [3]. The first-generation barriers were non-resorbable membranes made of PTFE and e-PTFE [4]. Although clinically effective, non-resorbable membranes were also prone to complications, such as membrane exposure, which often led to the impairment of the regenerative procedure [5; 6; 7]. Recently, a publication describing the management of 80 complications in vertical and horizontal ridge augmentation cases treated with d-PTFE found that 70% of complications appeared 2 months after surgery, 43.75% of complications were in the anterior maxilla, and 20% were in the lower left mandible [8]. These issues, together with the need for a second surgery for their removal, push the research toward the development of resorbable barriers. Among resorbable barriers, collagen membranes are the most commonly used thanks to their biocompatibility, ease of handling, and active role in recruiting cells into the wound area, thus modulating the overall osteogenic process [9; 10]. One of the limitations of collagen membranes is the lack of rigidity and the resorption time, which may not be suitable for large, three-dimensional defects [10]. Due to these limitations, in the last decade, a new class of bioresorbable barriers was developed: the cortical bone laminas. These bone sheets are made of collagen-preserved,partially demineralized cortical bone of porcine [11; 12; 13] or equine origin (Flex Cortical Sheet--FCS) [14; 15; 16]. Differently from pure collagen membranes, the thin layer of the remaining hydroxyapatite confers a slight rigidity to the cortical sheet and long-term protection of the bone graft/bone defect (more than 4 months) [14; 17]. Multiple formats are available and suitable for horizontal ridge augmentation as well as for vertical ridge augmentation where the anatomy is favorable [11; 12; 13; 18; 19]. Another advantage of a cortical bone sheet is that, even in the case of accidental exposure, it is simply hydrolyzed by collagenases while soft tissue granulates on top of the cortical layer without significantly affecting the success of bone regeneration [12]. The bone sheet has also been used successfully in other cranial applications like the reconstruction of the orbital floor and the reparation of the sinus wall combined with zygomatic implants [20; 21; 22]. Recently, a new generation of FCS has been introduced to the market, with a machine-made thickness of 0.2, 0.5, and 0.9 mm and increased flexibility and adaptability to ridge defects [23]. In most cases, horizontal and vertical ridge augmentation requires a bone graft to achieve the proper volumetric augmentation [2]. Although the autologous bone graft is considered the \"gold standard\", there are some drawbacks relative to its use. Indeed, autologous grafts involve the opening of a second surgical site with an increased risk of intra- and post-operative complications, including donor-site pain, abnormal gait (in the case of the iliac crest as the harvesting site), and numbness [24]. Moreover, the supporting clinical evidence for the use of allografts, xenografts, and synthetic biomaterials [2; 24; 25; 26; 27] decreased the use of autologous grafts alone. This case report highlights the clinical findings on the combined use of a new generation of FCS with a collagen-preserved bone substitute of equine origin in the reconstruction of a three-dimensional defect in the lower-posterior mandible.