## 4 Discussion
Several studies, including that of [PERSON] et al., have reported that a bone width of 2 mm or more is required on the buccal side at the platform level of the implant for the long-term stability of hard and soft tissues around the implant. If the bone width remains below 2 mm, it may cause tissue retraction and aesthetic loss, and implant failure may also occur in severe cases [23; 24; 25; 26]. However, [PERSON] et al. reported that the prognosis of GBR using a resorbable membrane is worsened, especially for lateral extrabony defects around the implant that do not exist in the envelope of adjacent bone [11]. In this study, we attempted to evaluate hard tissue gain in the non-contained horizontal defects of a 3-D preformed Ti mesh by measuring HG and HGR at the implant platform. In all cases, the average BA immediately after the first surgery was 3.7 \(\pm\) 1.1 mm, and the hard tissue gain 6 months after the second surgery (HG) was averaged at 3.1 \(\pm\) 1.3 mm. The HG of Groups 1, 2, and 3 were 3.2 \(\pm\) 1.7 mm, 3.1 \(\pm\) 1.2 mm, and 3.0 \(\pm\) 1.2 mm, respectively; hard tissue gains of 2 mm or more were observed in all the groups. This was consistent with the findings of previous studies that performed guided bone regeneration simultaneously with implant placement using a 3-D preformed Ti mesh; these studies reported buccal hard tissue gains of more than 2 mm [18; 27]. [PERSON] et al. reported that when only resorbable collagen membrane and bone graft material were used, approximately 40% of horizontal graft loss occurred immediately after suturing [28]. According to a study by [PERSON] et al., the displacement of the graft and the membrane after 9 weeks in the case of GBR using only the resorbable collagen membrane and graft in the buccal dehiscence region significantly worsens the prognosis of bone regeneration [29]. The growth of excellent hard tissue in the buccal extrabony defects of the implant during guided bone regeneration using a 3-D preformed Ti mesh may be attributed to the secure anti-pressure effect at the platform level, which is fixed by the implant fixture, unlike for resorbable membranes, for which it is difficult to maintain space due to the compression of soft tissues [30]. However, the 3-D preformed Ti mesh and the conventional titanium mesh can penetrate soft tissues owing to the macropore structure of the membrane [31; 32; 33]. This soft tissue layer is also called pseudo-periostream; the histological component is a dense connective tissue, and it has low cellularity and is non-mineralized [32; 34]. Therefore, radiolucency was observed below the titanium mesh in the radiographic images. The average bone graft resorption was 0.67 (\(\pm\) 0.92) mm in all 129 implants, 1.10 (\(\pm\) 0.71) mm in Group 1, 0.62 (\(\pm\) 0.98) mm in Group 2, and 0.55 (\(\pm\) 0.79) mm in Group 3. Based on the results of this study, the formation of pseudo-periostream can be reduced when a resorbable collagen membrane, regardless of whether it is cross-linked, is used. This was consistent with the findings of the study by [PERSON] et al., who reported that the formation of pseudo-periostream was inhibited when a resorbable collagen membrane was used with a titanium mesh [35]. The hard tissue gain rate (HGR) was 81.2% in the study by [PERSON] et al., who previously measured HGR using only a conventional titanium mesh, 85% in the study by [PERSON] et al., and 83.3% in the study by [PERSON] et al. [36; 37; 38]. In animal experiments by [PERSON] et al., an improved hard tissue acquisition was observed in the group using the Ti mesh with a resorbable collagen membrane compared with the group using the Ti mesh alone [39]. In the study by [PERSON] et al., when a resorbable collagen membrane was additionally used, HGR was 87.3%, which was higher than that reported in a study using only the conventional titanium mesh [40]. In this study, the average HGR of the 129 bone graft sites was approximately 82.4 \(\pm\) 22.4%, which was similar to those reported in previous studies [39; 38]. studies.