## 4 Discussion  
The aim of this study was to analyze the clinical outcomes of complete arch restored with complete-arch prostheses that were screwed onto four NDIs and loaded immediately. The main limitations of this study were the lack of a control group and its retrospective nature. Nevertheless, 120 NDIs were positioned, restoring 30 complete arches. Only one implant did not achieve osteointegration after the healing period, and it was successfully replaced, resulting in an ISR of 99.2% after an observation period ranging from one to eleven years. Several studies investigated the clinical performance of NDIs [22; 23; 24]. Two systematic reviews by [PERSON] et al. [10] and [PERSON] et al. [11] reported that the clinical outcomes of NDIs can be comparable to the standard dental implant outcomes, especially when significative bone augmentation procedures are needed to place standard implants. The meta-analysis showed an overall NDI ISR of 97.80% over 3.5 years of follow up, while the ISR for implants placed with horizontal bone augmentation procedures reported an overall ISR of 97.22% after 3.22 years of follow-up [11]. In recent years, NDIs were investigated in clinical scenarios of partially edentulous patients when supporting fixed prostheses in the aesthetic and posterior zones [9; 15; 25], providing high ISR values and a low incidence of biologic and/or prosthetic complications. Moreover, NDIs may be useful when wide bone augmentation procedures are required, such as after trauma, malformation, neoplasms, the use of removable prostheses, and periodontal disease. Reconstructive surgeries are always related to greater morbidity, leading to multiple surgeries and higher economic costs for the patients. In complete edentulous patients, NDIs were mostly used to support two-implant mandibular overdentures, providing encouraging clinical results in terms of ISR, marginal bone loss (MBL), and patient satisfaction [16; 17; 18; 26]. The main limitation of this treatment option is the hybrid support of the prostheses. The mandibular overdenture can be attached to two implants and can have a mucosa support. In this sense, the comfort for the patients could decrease because of the load on the posterior region of the mucosa in the mandible. For fixed prostheses, four implants are considered to be the minimum number to give support and to reduce overload. One study conducted by [PERSON] [27] reported clinical data of 67 NDIs supporting fixed complete-arch prostheses in combination with standard dental implants, with a maximum of 2 years of follow-up. In these terms, the present study reported clinical data of 30 complete arches from 1 to 11 years of use restored with complete-arch prostheses screwed onto four NDIs. Complete-arch restorations with four implants could be considered a valid treatment option in the medium to long term. As a matter of fact, the last systematic review with a meta-analysis by [PERSON] et al. [11] indicated that \"NDIs do not seem suitable for 'all-on-four' rehabilitations due to the increased load stress during occlusal function\". The present study goes the opposite way and shows favorable results for the all-on-four technique with NDIs. The incidence of implant-prosthetic failures and biological and/or prosthetic complications were mostly related to smoking and/or the frequency of maintenance sessions [1; 3; 4; 5]. In the present study, the overall treatment success was 94.1% due to three biological complicationstions and four prosthetic complications that were successfully solved. The low incidence of complications could be related to the high motivation of the patients included in this retrospective study. Moreover, the reinforcement of oral hygiene instructions was constantly delivered to each patient at each professional hygiene session. Biological complications can occur in every implant-based rehabilitation, from a single tooth gap to complete arches.