###### Abstract  
The aim of this study was to evaluate the clinical application of fixed screw-retained complete-arch rehabilitation supported by four narrow-diameter implants (NDIs). The records of patients treated with complete-arch prostheses screwed onto four NDIs treated with an immediate loading protocol between 2010 and 2020 with at least 1 year of follow-up after the positioning of the definitive restoration were reviewed. The implants were placed according to the final prosthetic design and were immediately loaded. The interim prostheses were replaced after the healing period by definitive acrylic resin titanium-supported prostheses. Patients were followed to evaluate treatment success, the implant survival rate (ISR), and the prosthetic survival rate (PSR). A total of 121 NDIs were positioned in 30 patients to restore 30 complete arches (18 maxilla and 12 mandible). One implant did not achieve osseointegration, resulting in an overall ISR of 99.2%. No prosthetic or implant failures occurred during the 1 to 11 years of follow-up. Three biological and four prosthetic complications occurred, resulting in a treatment rehabilitation survival of 94.1% and a PSR of 86.7%. Despite the limitations of the present retrospective study, such as the use of one single type of dental implant and patients treated in a single rehabilitation center, complete-arch rehabilitation with fixed prostheses supported by four NDIs seems to be a reliable treatment in the medium to long term.  
narrow-diameter implants; immediate loading; complete arch; fixed prostheses 20220  
procedures should be performed only by expert clinicians to minimize possible intraoperative complications and postoperative morbidity. The combination of several augmentation procedures with dental implant placement may influence the overall surgery outcomes, such as early graft or membrane exposure [15; 16; 17; 18; 19; 20]. Narrow-diameter dental implants (NDIs) were introduced to reduce or eliminate the need for horizontal bone augmentation procedures, reducing the implant diameter in the most coronal part [21; 22; 23; 24]. NDIs can be classified in three categories according to the ITI consensus report [25]:  
1. Implants < 2.5 mm \(\O\) (\"Mini-implants\");
2. Implants 2.5 mm to <3.3 mm \(\O\);
3. Implants 3.3 mm to 3.5 mm \(\O\).  
In certain clinical scenarios, NDIs provide prosthetic flexibility in patients with a reduced mesiodistal width or reduced bone thickness. Moreover, NDIs can be used for single-tooth rehabilitation, such as lateral incisors in the upper jaw or incisors in the lower jaw [26; 27; 28; 29]. However, reducing the implant diameter may increase the risk of implant or component fracture in patients with heavy chewing function or parafunction. NDIs positioned to replace missing teeth in the posterior zone are still not well investigated in the literature. In these cases, it is mandatory to use caution in patients with parafunctional habits and malocclusion. As a matter of fact, for NDIs in the anterior zone there are different biomechanical forces that can overload the implants compared to the posterior zone [25; 28; 30; 31; 32; 33].  
The aim of this retrospective study was to evaluate the clinical survival of fixed screw-retained complete-arch rehabilitations supported by four NDIs.