## 2 Materials and Methods
### Follow-Up  
Follow-up visits were performed 1 week after surgery, at 3 and 6 months, and then once a year for the next 4 years. Each patient was placed in a professional oral hygiene program that would allow both for limiting complications [27; 28] and for monitoring and interception of any complications.  
1. _Implant survival rate._ The implant survival rate was dependent on the number of implants lost during the follow-up period due to mobility associated with progressive marginal bone loss due to peri-implantitis. Implant loss was classified according to the period: if it occurred within 6 months of fixture placement, it was called early failure; after 6 months, it was called late failure. Early failure was usually intercepted at the reopening stage, when there was a lack of osseeintegration of the implant. In the case of late failure, there were signs of peri-implantitis, implant mobility, radiolucent areas around fixtures, mucosal supuration, and/or pain during the follow-up period.
2. _Marginal bone loss (MBL)._ The MBL was evaluated via digital phosphor intra-oral radiography performed for each patient using the parallel cone technique at 6, 12, 24, 36, and 48 months. To assess marginal bone trends, measurements were performed only after image calibration. Digora 2.5 software (Soredex, Tuusula, Finland) was used as an analysis platform, making use of the specific measurement tool contained therein. As a first step, calibration (pixels/mm) of the instrument was performed, using the implant diameter of the survey site as the known unit. Next, any changes in the height of the peri-implant marginal bone in relation to the most coronal part of the implant and the point of contact between the implant and marginal ridge were measured. To evaluate bone resorption, a line passing over the shoulder of the implant was considered as a reference point for measurement from which a straight line was drawn parallel to the long axis of the implant to the most coronal point where the bone met the fixture both mesially and distally. The software automatically provided, in relation to the calibration, the distance between the two points measured in millimeters. To reduce human error, this measurement was performed by three operators, and the average of the three measurements was considered. To evaluate the marginal bone level, first the mesial and distal measurements were taken, then the averages of the mesial, of the distal, and between the two values of a single implant site (MBL, marginal bone level) were calculated, as reported in Section 3. Marginal bone levels detected were divided into two categories according to the implant position, whether mesial (Implant 1/I1) or distal (Implant 2/I2). The first group included only axial implants; the second group also included tilted fixtures, always placed distally and in association with a mesial axial implant (Implant 1/I1). The data thus obtained were then statistically investigated.
3. _Surgical complications._ Surgical complications were divided according to the surgical procedure.
4. _Prosthetic complications._ These included fracture of the provisional prosthesis, unscrewing of temporary crowns and/or abutments (Group C), unscrewing of final crowns and/or abutments (Group C), and chipping.