The insertion torque was measured using the manual torque wrench (Straumann Dental Implant System®, Waldenburg, Switzerland) (Fig. 1b), according to three categories: < 15 N cm, 15 to 35 N cm, and > 35 N cm. A healing cap was installed, and the suture was made with nylon 5-0 (Fig. 1c). The patients were prescribed with antibiotics (amoxicillin 500 mg, 8/8 h for 7 days), anti-inflammatory drugs (nimesulide 100 mg, 12/12 h for 4 days), and mouthwash with 0.12% chlorhexidine digluconate for 15 days. The sutures were removed after 1 week.

During the drilling for implant placement, the surgeon used his tactile perception to assess the bone ridge. The surgeon considered the thickness of the cortical layer and the resistance of the trabecular bone to categorize the bone into four types, based on the classification of Lekholm and Zarb [5]: type 1 (large homogeneous cortical bone and little trabecular bone), type 2 (thick cortical layer surrounding a dense trabecular bone), type 3 (thin cortical layer surrounding a dense trabecular bone), and type 4 (thin cortical layer surrounding a sparse trabecular bone). All surgeries were performed by the same previously trained surgical team.

Preoperative diagnostic CT images were acquired in the Digital Imaging and Communications in Medicine (DICOM) protocol, and one single cone beam scanner (i-CAT, Imaging Sciences Intl, Hatfield, PA, USA) and one single multislice scanner (Elscint CT Twin II, Elscint Ltd., Haifa, Israel) were used in this study. The DICOM images were reconstructed with the ImageJ software (version 1.51; National Institute of Health, Bethesda, MD, USA) for bone quality evaluation of the regions of interest (ROIs).

A standardized digital periapical radiograph was obtained after suture removal and used to measure the distance from the actual implant center to the proximal side of the nearest tooth at bone level. Using this reference distance, the ROIs in the 1-mm thick CT slices were manually traced corresponding to the alveolar bone (cortical and trabecular bones) in the axial, coronal, and sagittal sections as follows (Fig. 2):