## 1 Introduction  
Although many procedures are aimed at preserving teeth for as long as possible, poor oral hygiene, periodontal disease, destructive caries lesions and other issues may result in edentulism. The increase in life expectancy of the individual and the consequent growth in the number of totally edentulous individuals with functional and aesthetic needs, especially in cases where more traditional removable prosthetic solutions are not well tolerated, leads to an increased demand for the stabilization of prosthetic restorations by means of implant fixtures [1; 2; 3; 4; 5; 6]. Following the loss of dental elements, what ensues is progressive involution in terms of contraction of the upper jaw and in terms of expansion in the mandible [7; 8; 9]. The consequence, in a transverse direction, is the alteration of inter-maximally skeletal relationships, with the development, even in previously normal-occluded subjects, of a third skeletal class. In the vertical direction, bone loss results in an increase in the inter-arch distance, which causes a progressive reduction in vertical dimension directly proportional to the degree of atrophy [10,11]. The senescent appearance of the patient is further emphasized by the loss of tissue trophism, mainly due to the loss of support of the orbicularis muscle of the lip and the labial and period soft tissues. The downward prominence of the nasal pyramid causes an exacerbation of the peri-oral and nose-to-lip furrows, with progressive reduction of the nasolabial filter [12,13]. From the point of view of the noble structures, the following changes can be seen in the case of bone atrophy in the anterior upper jaw: the nasopalatine canal and the foramen of the same name tend to move closer to the margin of the crest [14]; in the posterior region there is the process of pneumaticization of the maxillary sinus and distalization of the greater palatine nerve, closer to the palatal margin of the bone crest [15]; in the anterior mandible, the mental foramen and the corresponding nerve trend towards superficialization [16], as occurs in the posterior region with the inferior alveolar nerve [17]. In relation to soft tissues, as hard tissues act as spinner expanders, it follows that the process of bone atrophy is associated with a reduction in the proportion of keratinized mucosa [18,19]. Furthermore, when tooth loss occurs, alveolar ridge resorption [20,21] could bring a significant bone atrophy that increases more in the posterior region than in the anterior, also associated with an important bone density decrease [22]. To achieve the necessary bone volume for axial implant placement, several procedures have been suggested to regenerate height and width (e.g., bone osteogenesis distraction, guided bone regeneration (GBR) with or without membrane or titanium mesh, bone blocks, bone substitutes) [23,24]. Given possible complications related to these surgical procedures, dental implants placed in basal bone by exploiting fixture tilting could be considered a viable therapeutic alternative [25, 26, 27]. Although several therapeutic alternatives have been proposed as a solution to the reported problems, in case of low bone height of the posterior sectors and superficializing of the nerve-vessel structures, rehabilitation by means of the All-on-Four protocol could be considered the best choice to exploit the basal bone [28,29], thus avoiding more invasive procedures [30, 31, 32, 33, 34, 35, 36]. The aim of this retrospective clinical study was to evaluate clinically and radiographically full-arch implant prosthetic rehabilitations supported by six axial implants or four implants placed according to the All-on-Four protocol. In detail, implant survival, peri-implant parameters, marginal bone loss and cortical bone thickness were considered, investigating possible muscular, articular and prosthetic complications and the degree of patient satisfaction with the performed work.