## 4 Discussion
However, in class 3 furcation lesions, 100% fill was observed only when the width of the furcation entrance was within 4 mm. Therefore, it should not be overlooked that the difference in bone fill between maxillary and mandibular locations was not significant in this study, and the prognosis may vary depending on the morphology of individual teeth. We conducted a comparative analysis of each case and found that, in intrabony defects with function involvement, as well as meso-distal and interproximal defects, deeper defects did not result in a lower defect fill percentage. Consequently, we performed additional experiments during statistical analysis. Anticipating that the outcome might differ depending on the extent of the intraosseous defect, we analyzed the correlation between the initial amount of bone loss and the amount of bone filling. The correlation between the distance (in mm) from the cemento-enamel junction to the lowest point of the bone defect lesion immediately before GTR and the amount of bone mass gain approximately 2 years after GTR was examined. A clear quantitative correlation was found. This supports findings from other previous studies, such as the one conducted by Tonetti et al. in 1993 Tonetti et al. (1993), suggesting that the greater the amount of bone loss, the greater the amount of bone gain. This is likely attributed to the deeper and narrower nature of the bone defect and the larger volumetric measurements of the focal area, resulting in a more abundant supply of surrounding bone tissue and periodontal progenitor cells or originating growth factors, as supported by the literature [24; 25]. Additionally, the vertical depth implies that the height of the marginal bone is higher than the base of the lesion, potentially preventing the collapse of the graft material or membrane itself and improving soft tissue stability for the operator. Therefore, if volumetric stability of the bone graft and primary wound closure can be achieved, bone filling can be explored through GTR even in lesions with deep bone defects. A limitation of this retrospective study is that the prognostic judgment of the surgeon was added to the decision of GTR in each case, and only cases with high predictive value were recruited. Nevertheless, it is noteworthy that similar amounts of bone filling were found in the maxilla, mandible, segmental lesions, and intraosseous defects. As revealed in this study, the type of defect and the alveolar location of the lesion do not significantly impact the differentiation of indications in the surgical treatment of lesions with alveolar bone defects. Therefore, it is worthwhile for clinicians to consider GTR more aggressively without being bound by traditional indication differentiation criteria. When performing the GTR procedure, the choice of membrane is crucial. In this study, absorbable membranes were primarily used, but it is possible that the membrane resorption process is accompanied by a cellular inflammatory phase, which may have an adverse effect on healing. Expanded polytetrafluoroethylene (ePTFE) membranes, once considered the gold standard, or high-density polytetrafluoroethylene (dPTFE) membranes, commonly used in a wide range of hard tissue regeneration, may also be considered. In the case of Capetillo, J.E. et al. [26], GTR of a mixed Hamp grade 1 and 2 wall intrabony defect using a high-density polytetrafluoroethylene (dPTFE) membrane and a mixed graft of mineralized freeze-dried bone allograft and demineralized freeze-dried bone allograft resulted in successful bone regeneration despite the exposure of the membrane. If GTR is performed in large and challenging-to-repair defects, as this study aims to do, a releasing vertical incision will be performed due to the management of the periodontal flap, securing the vision, and efforts for primary closure. This sometimes results in inadequate nutrient or blood supply to the upper part of the flap, leading to necrosis or shrinkage, and ultimately membrane exposure.