## 1 Introduction  
Dental agenesis was defined as the absence or failure of formation of a tooth [1]. The prevalence of tooth agenesis was reported to range between 1-3% among different population groups [2, 3]. Missing permanent maxillary lateral incisors represents about 20% of all cases of agenesis; this anomaly was noticed to be more frequently bilateral [3]. Different prevalence percentages were reported in the literature for maxillary lateral incisor agenesis on the basis of race and sex: its prevalence varied across population between 0.8%to 4.5% [4], although studies on North-Western European populations showed a lower congenital prevalence, from 1% to 2% [5]. Missing teeth were often associated with tooth ectopias and/or other abnormal dental conditions, such as smaller or peg-shaped teeth on the contralateral side. In such cases, the canines are often mesial or lingual positioned, and the midline is displaced towards the side affected by agenesis. deciduous maxillary lateral incisors often remain in the dental arch for a time longer than its physiologic falling [6; 7]. The lack of maxillary lateral incisors was reported to cause remarkable esthetic concerns by patients [8]. Several treatment options have been proposed for the rehabilitation of maxillary lateral incisor agenesis, as follows:  
* Conservative approach: this approach should be considered a short-term solution and consists only in an esthetic reshaping of the deciduous lateral incisor with composite resins [9]. * Orthodontic approach: intercepting or orthodontic treatment is used to move the canines into the place of the lateral incisors and the space of the missing tooth is closed by moving the adjacent premolars and molars mesially [10]. * Prosthetic approach: the missing maxillary lateral incisor is replaced by means of an inlay bridge (i.e., Maryland bridge) or a fixed partial denture (metal-ceramic or all-ceramic) sustained by the upper canine and central incisor [11; 12]. The proper position of the canine is a paramount factor, since an adequate space in occlusion is indispensable to guarantee the correct thickness of an inlay bridge. Furthermore, a sufficient space between the upper central incisor and the upper canine is necessary to realize a prosthetic restoration of appropriate size. * Implant approach: osseointegrated implant-supported restoration replaces the missing tooth. Occasionally, a previous orthodontic adjustment of the prosthetic space is indispensable [13; 14]. According to the literature, patients usually prefer the orthodontic approach to close the edentulous space or the implant treatment, since no tooth preparation is required. As reported by several authors, the implant-proshetic rehabilitation of maxillary lateral incisor agenesis or the orthodontic closure of the edentulous space can be considered the most conservative and widespread treatments [15; 16]. Scientific investigations confirmed that the crest bone resorption of submerged and non-submerged implants are comparable [17]. Recent investigations found that clinical outcome with narrow-diameter implants was comparable to standard-diameter implants in the anterior zone [18; 19]. Particularly, the cumulative success rate reported was satisfactory, ranging from 84.2% to 100% (mean of 95.2%). Prosthetic (fracture of the abutment, loosening of the abutment screw, chipping of the veneering ceramic) and biologic (mucositis, peri-implantitis, fistulae, food impaction) complications were the major drawbacks reported [20]. A 10-year study on submerged implant-supported prostheses showed that prosthetic complications represent 18.8% of the drawbacks in the implant-proshetic treatment of missing teeth [21]. The present prospective clinical study aimed at evaluating the marginal bone resorption and the peri-implant soft tissue conditions around Narrow-Neck ITI implants in the implant-proshetic rehabilitation of the maxillary lateral incisor agenesis.