In the submerged group, second-stage surgeries for the placement of healing abutments were carried out after 4 months in the mandible and 6 months in the maxilla. This procedure was performed by a midcrestal minimal incision, slightly larger than the coronal diameter of the implant. No secondary surgical manipulation of the soft tissue was performed. Once the healing screw was inserted, suturing was not necessary. Each submerged implant received a titanium healing abutment in height varying from 2 to 4 mm, so as to obtain an overall mucosa emergence of the complex implant/healing abutment, as similar as possible to that of the controlateral non-submerged implant, and in any case not greater than 2 mm.

Prosthetic restorations were delivered after 5 months for implants in the mandible and 7 months for implants in the maxilla. All restorations were screw-retained, and the abutment type was consistent within the same patient, full titanium or hybrid zirconia, depending on the availability of the prosthetic laboratory.

Radiographs were taken using a film holder at the time of data collection by means of a long cone technique. For the radiograph procedure, an individualized acrylic resin device was fixed to the residual dentition and a radiograph holder was constructed for each patient. This technique ensured that the same position of the radiograph film could be reproduced at each visit and the angle of the radiograph would not deviate. Radiographs were performed immediately at implant placement (BSL), at the delivery of definitive crowns (T0), and each year after loading (T1, T2, T3). The radiographs were taken in high-resolution mode (Vista Scan Durr Dental, Durr Dental Italy S.r.l, Italy) with a dental x-ray machine (TM 2002 Planmeca Proline CC, Planmeca Group Helsinki, Finland) equipped with a long tube that operated at 70 Kw/7.5 mA. Specialized software (DBSWIN software, Durr Dental Italy S.r.l, Italy) was used for linear measurements of marginal bone changes.