The purpose of this randomized controlled clinical trial is to compare the effectiveness of advanced platelet-rich fibrin (A-PRF+) combined with xenograft versus a collagen membrane combined with xenograft in the treatment of degree II furcation involvements in mandibular first molars. Adult patients with periodontitis will randomly receive either the A-PRF+ treatment or the collagen membrane treatment during periodontal flap surgery. The study will evaluate clinical periodontal healing, bone regeneration using 3D imaging (CBCT), patient comfort (pain levels and satisfaction), and changes in the TWEAK biomarker in gingival crevicular fluid over a 6-month period.