Clinical and Patient Outcomes of 4 mm Ultra-Short vs. 8 mm Implants With Bone Augmentation in the Back Upper Jaw

inclusion criteria: * Inform consent signed. * Adult patients (≥18 years old). * No pregnant women or without the desire to stay. * Periodontal and peri-implant health in a pristine or reduced periodontium. * Full mouth plaque score \< 20% (O'Leary, 1972) * Maxillary multiple posterior maxillary healed sites at least 4 months after extraction (with antagonist teeth with a good prognosis (Lindhe et al. 2008)) presenting ≥ 6.5 mm of bone width and 5-6 mm in height from the bone crest to the floor of the sinus. * Need for a multiple splinted implant supported restoration with distal adjacent teeth or distal free-end in the posterior maxilla exclusion criteria * Patients with uncontrolled systemic diseases. * Smoker patients (≥10 cig/day). * Patients taking medications that affect bone metabolism or immunologic disorders. * Patients allergic to penicillin. * Patients presenting acute or chronic maxillary sinus lesions. * Advanced vertical crestal bone atrophy in the posterior maxilla (Type V and IV, Cawood-Howell Classification) * Vertical space of \< 6 mm for a screw-retained implant restoration. * Patients referring allergy to titanium or to any component of the implant/restoration. * No previous attempts of implant installation in the same surgical site. * Pregnant or lactating women. * Patients willing to start or in course of an orthodontic therapy.