## 4 Discussion  
The treatment of maxillary lateral incisor agenesis requires a multidisciplinary approach. Previous orthodontic therapy is frequently a mandatory requirement to achieve a correct surgical space to place the implant. The implant-prosthetic treatment often offers the best cost/benefit ratio and is considered a biological conservative approach for maxillary lateral incisor agenesis. Several authors suggested placing the implant into the bone at the end of the cranium-facial growth, at about 21 for males and 17 for females [26,27]. After the surgical placement of the implant and the recovery screw, a period of healing of about 4-6 months is needed to obtain the osseointegration [23]. It is well-known that a good aesthetic result cannot be achieved without respecting the peri-implant soft tissues and the papillae. In the present prospective study, both AGC on titanium abutment and all-ceramic crowns onto zirconia abutment have shown good marginal bone level stability and peri  
Figure 10: Kaplan–Meier graph showing cumulative survival rates in relation to time. Two different curves as reported for patients wearing AGC and all-ceramic crowns, respectively. implant soft tissues response in the long-term, since 86% of the restorations showed satisfactory morphology of the papillae after 16 years of function. The presence of either titanium or zirconia abutment seems not to influence peri-implant soft tissue stability, confirming the data previously published by other authors [28]. It can be assumed that other factors, such as proper 3D implant positioning, a conservative design of the surgical flaps, and the correct application of temporary acrylic resin restorations, may have played an important role in obtaining this result. Similar investigations highlighted the stability of the marginal bone level and soft tissues around the dental implants in the medium-term [29; 30]. In the present prospective study, healthy and stable peri-implant soft tissues were evidenced in the long-term, probably thanks to the appropriate design of the final restorations and the maintenance of good oral hygiene. All the periodontal parameters considered to evaluate the peri-implant soft tissues conditions confirmed these data. Several studies recognize smoking, together with a history of periodontitis, as a great risk factor to develop peri-implantitis [31; 32]. The long-term health of peri-implant tissues evidenced in the present investigation may also be explained with the high percentage of non-smoker patients (79.2%) included in the study. In the present study, after 16 years of prosthetic loading, high global survival (95%) and success rates (85%) were found, and no statistically significant differences in functional or aesthetic outcomes emerged between AGC crowns and all-ceramic crowns (\(p>0.05\)). High cumulative success (87.1%) and survival (95.7%) rates emerged in the all-ceramic crowns group. The few minor complications were abutment unscrewing (1.42%), chipping of the veneering ceramic (1.42%), 1 decementation (1.42%), and mucositis (4.28%). In none of these cases was an invasive surgical treatment or replacement of the prosthetic restoration necessary. After 10 years of serviceability, a patient with bilateral agenesis returned to the clinic, complaining about sensitivity and mobility of his implants. After peri-implantitis was clinically and radiographically assessed by the clinician, a guided bone regeneration (GBR) procedure was required to restore function and maintain the implant-prosthetic restorations. A high number of cigarettes (15-20 per day) and low daily standards of hygiene of the patient may explain this case. Another patient wearing a zirconia crown required a new restoration after 9 years of function, due to a catastrophic core fracture which occurred while he was eating. Similarly, in the AGC group, satisfactory cumulative success (80%) and survival (93.3%) rates were reported in the present investigation. The minor complications reported were mucositis (10%) and loss of retention (3.33%).