## 4 Discussion
Most of studies did not report any data about the type of bone defects affecting the teeth to be treated, except two: one case series [11] about orthodontic extrusion, which reported a classification of the different types of osseous defects based on the amount of the residual periodontal attachment, and another study [16] about regenerative surgery, which reported the type of defect (Cadwood and Howell class V defect). In the studies comparingspontaneous tissue healing after extraction with ridge preservation procedures, no details were given about bone defects. Although it has been suggested an association between patient's gingival biotype and the tendency toward gingival recessions after surgical procedures [37; 38; 39], only three studies [11; 17; 22] included information on this subject. The articles reviewed provided detailed information regarding the treatment procedures and materials used. Different methods were employed to apply the extrusive force: NiTi archwire and a more apical bonding of brackets on the teeth to be extruded, SS archive with extrusion steps, SS archive with L-shape loops or elastic bands from orthodontic hooks bonded to the cervical region of teeth to be extruded and a removable appliance. The most used appliance to perform orthodontic extrusion was a multi-bracket system; in only one study [9], a removable appliance was used. The multi-bracket approach was full bimaxillary in three studies [14; 29; 30] and partial in seven studies [5; 7; 8; 10; 11; 12; 13]. In one study [8], a lingual appliance was used; in another study [13], ceramic brackets were used. No clinical study has proved the \"uper'ority of extrusive biomechanics versus another, but it is mandatory to use low and controlled extrusive forces. A low extrusive force (<100 g) was applied in all studies, but five studies [5; 8; 12; 29; 30] did not report data about the intensity of the force used to perform the extrusion. It is important to understand that when tension is applied to the periodontal ligament, periodontal fibers bundles are elongated, and osteoblasts are induced to deposit new bone in the areas where periodontal attachment exists. The same effects on alveolar bone height can be seen when orthodontic treatment is carried out with controlled forces which do not tear apart the periodontal fibers. In other words, a tooth moved into the dental arch by controlled extrusive orthodontic forces will bring alveolar bone, and soft tissues will follow, too [5; 6]. Therefore, using low and controlled extrusive forces is fundamental to enhance hard and soft tissues through orthodontic extrusion. Regarding the extrusion ratio, the extrusion was performed slowly (mean of 1 mm/month) in six studies [7; 8; 9; 10; 11; 30] whereas the other five studies did not report any data. Just two studies [7; 11] reported the amount of tooth extrusion performed. Another important aspect has sufficient anchorage to limit the risk of adverse side effects on the teeth adjacent to those planned to be extruded. Most of the studies used an SS archive as an anchorage during the extrusion; one study [9] used a removable appliance and one study [8] used a composite resin veneer pontic, while three studies did not report information about the anchorage used. The orthodontic extrusion should be performed according to the tooth's long axis to avoid generating any apical compression against the buccal cortical bone, which is likely to cause bone fenestrations. The frequency of follow-up varied from 2 to 4 weeks; during each control visit, hygiene conditions were checked, the orthodontic extrusion device was activated, and progressive occlusal adjustments were performed in order to maintain the teeth extruded out of the occlusal plane. In all studies except one [14], a stabilization period followed the orthodontic extrusion to ensure a complete maturation of the newly formed bone; the retention period lasted from 1.5 to 6 months, depending on each study and retention was ensured in most of cases by the multibracket system, which was left in place.