## 4 Discussion  
In this study, the influence on the periodontal health of the distal surface of the mandibular second molar after third-molar extraction was evaluated. The split-mouth, randomized clinical trial study demonstrated that there was no significant difference in PD, CAL, and BG between the dPTFE group and the silk mat group at T1 and T2 (Tables 1 and 2; \(p>0.05\)). This is the first study of GBR with silk mat in clinical trials. The presence of a mandibular third molar is considered a threat to the periodontal health of the second molar [2]. Even a non-symptomatic mandibular third molar is recommended for surgical removal to prevent periodontal pathology in the second molar [2]. Age [8,13], sex [34], oral hygiene [20], and anatomical position [20] are considered predisposing factors for periodontal pathology. Accordingly, patients older than 26 years, or with more than 3 mm of an attachment level on the distal surface of the second molar are advised to have their mandibular third molar removed [8,13]. Approximately 40% of patients show increased bone loss after third-molar extraction among patients with more than 3 mm of bony defect in the distal surface of the mandibular second molar [34]. Accordingly, membrane application groups in this study included only patients with more than 3 mm of bony defect in the distal surface of the mandibular second molar preoperatively. To address deep mesioangular impaction, the preoperative PD should be at least 7.5 mm [14]. In this study, PD at T0 was 9.5 \(\pm\) 1.3 mm and 9.3 \(\pm\) 1.3 mm for the dPTFE group and the silk mat group, respectively (Table 1). To prevent postoperative periodontal pathology, flap design, suture technique, and periodontal care for the second molar were considered, but there is controversy about their effect on prevention [16]. GBR with a collagen membrane or PTFE membrane is a reliable method and beneficial in reducing PD compared with a non-regenerative/non-graft procedure [35]. There was no significant difference in PD reduction between the collagen membrane group and the PTFE membrane group after mandibular third-molar extraction [19]. In the case of an impacted third molar, surgical extraction only can increase alveolar bone height on the distal surface of the mandibular second molar [36]. PD at the distal surface of the mandibular second molar is reduced approximately 0.6 mm every three months after surgical removal of a mandibular third molar until PD becomes 2.6 mm [37]. The reduction in PD for the non-regenerative/non-graft procedure group was 1.3 mm at three months [14] and 1.93 mm at 12 months [5]. When the PTFE group was compared with the non-regenerative/non-graft procedure group, a significant benefit from the PTFE membrane was demonstrated after mandibular third-molar extraction [20,35]. In cases of collagen membrane application, the amount of PD reduction at one year after extraction was 5.3 \(\pm\) 1.9 mm [19]. In this study, the amount of PD reduction at six months was 5.9 \(\pm\) 1.7 mm and 6.4 \(\pm\) 1.7 mm for the silk mat and dPTFE groups, respectively (Table 1). Mean gain of attachment at T2 was 5.23 \(\pm\) 1.46 and 5.69 \(\pm\) 1.42 mm for the dPTFE and silk mat groups, respectively (Table 2). When GBR plus bone graft is compared to GBR only, there is little additional value in reducing PD after extraction [35]. Silk mat consists of fibroin and sericin protein. Silk fibroin has been widely studied for bone regeneration [28]. Sericin has been considered industrial waste, but its beneficial effect was illuminated recently [38,39]. Because sericin is hydrophilic protein and easily fragmented in aqueous solution, silk mat can be considered a drug carrier-releasing sericin [25]. All silk mats in this study came from the middle layer of the silkworm cocoon. In previous studies, the amount of fragmented sericin release was smallest in the middle layer [25,26]. The expression level of tumor necrosis factor-alpha (TNF-\(\alpha\)) from macrophages depends on the sericin concentration [25].