Root bifurcation lesions are those that involve the root bifurcation area of multiple teeth after periodontitis has progressed to a more advanced stage. The prevalence is highest in the mandibular first molars and lowest in the maxillary bicuspids, and the incidence increases with age. The extent of the lesion can be judged according to the probe and X-ray film, and is divided into the following four degrees according to the extent of the lesion: 1, degree I: it belongs to the early stage of the lesion, the depth of the periodontal pocket reaches the root bifurcation, and the root bifurcation can be detected, but the bone resorption of the alveolar bone is slight, and the X-ray shows no bone resorption, and the doctor can probe the root bifurcation with the probe, but cannot probe into it horizontally; 2, degree II: there is bone resorption in the bifurcation area of multiple teeth, but it is not connected to the opposite side, and the X-ray shows only the widening of the periodontal membrane. X-ray shows that only the periodontal membrane is widened, or the bone density is reduced, the doctor can partially reach into the root bifurcation with the probe, but the whole root bifurcation is not connected; 3, Ⅲ degree: all the alveolar bone between the roots is resorbed, the probe can pass through, covered by the soft tissue of the periodontal pocket, not directly exposed to the oral cavity, X-ray shows that the bone in this area disappears, translucent, the probe can pass through the root bifurcation horizontally, but it is impossible to see the root bifurcation directly in the oral cavity 4, Ⅳ degree: the root bone interval is completely destroyed, gingival recession, root bifurcation area is completely open and can be seen directly, the X-ray film is similar to the Ⅲ degree lesion, the root bifurcation can be seen directly with the eye, and the root bifurcation can be completely passed with the probe, at this time, it indicates that the alveolar bone and gingival atrophy are serious.