## 1 Introduction  
Periodontal disease is characterized by the destruction of periodontal tissues due to various local and systemic factors, such as infection and trauma. The body's immune response to periodontal pathogens results in the loss of periodontal tissues and alveolar bone. This, in turn, impacts oral hygiene management and impedes healing. Therefore, the classification of the anatomical morphology of alveolar bone defects before the initiation of surgical periodontal interventions is crucial for treatment success. Cortellini et al. (2000), Becker et al. (1986), and Kim et al. (2004) previously categorized the treatment prognosis of deep intrabony defects based on the type of surrounding bone tissues. Three-wall bony defects and two- and three-wall combined defects exhibited superior healing compared to one- or two-wall bony defects [1; 2; 3]. Conventional non-surgical periodontal therapy is often insufficient for addressing periodontal tissue destruction extending from the multi-molar to the function region. Peri-odontal disease typically arises from the unchecked accumulation of pathogenic substances,such as calculus, on tooth surfaces. Involvement of the furcation area is common due to the narrow width of this region, which not only facilitates its occurrence but also complicates the treatment process. Consequently, incipient furcation involvement establishes a vicious cycle leading to the rapid progression of periodontitis. Ross et al. (1980) previously reported a higher incidence of furcation involvement in multi-rooted teeth, with rates three times higher among maxillary molars compared to mandibular molars (Bower et al., 2017). Bower et al. examined furcation anatomy and noted that over half of the maxillary furcation entrances in their study were narrower than the width of a hand curette, contrasting with the situation in the mandible (Bower et al., 2017). Complete calculus removal poses challenges with non-surgical periodontal therapy, resulting in a particularly poor treatment prognosis for periodontal lesions extending into the periapical region. The treatment of furcation involvement has long been recognized as challenging, often prompting periodontitis to recommend extraction as the preferred therapeutic measure (Bower et al., 2017). However, since preserving natural teeth is a fundamental goal in periodontal therapy, various treatment approaches for furcation involvement have been developed, including guided tissue regeneration (GTR), tunneling, and root resection. Previous studies indicate that multi-rooted teeth affected by segmental lesions can be retained for extended periods with adequate patient cooperation and effective surgical intervention. Clinical studies also suggest that furcation involvement may not be as serious a complication as initially suspected, particularly if caries development at the furcation site can be prevented (Bower et al., 2017). Instead, relatively straightforward periodontal treatment measures can be employed to maintain the functionality of these teeth over the long term. Various surgical treatment measures for furcation involvement have been proposed over time. Farrar et al. were pioneers in 1884, reporting the resolution of a periodontal abscess in a multi-rooted tooth through root excision (Bower et al., 2017). Another technique, tunneling, involves exposing the affected furcation and was developed to enhance oral hygiene management. Longitudinal studies have shown its particular effectiveness in mandibular molars, with no associated loss of attachment or periapical or proximal alveolar bone reported (Bower et al., 2017). However, this method is less commonly used nowadays due to its association with complications such as root caries (Bower et al., 2017). In 1986, Gottlow et al. proposed GTR, aiming to achieve balanced attachment of bone, dentin, connective, and epithelial tissues through periodontal tissue regeneration (Gottlow et al., 2018; Gottlow et al., 2018).