Abstract: Objective To evaluate the clinical effects and strategy of periodontal therapy on different margin levels of residual tooth to analyze the indications of residual roots reserved．Methods 187 residual roots in 85 cases were included in the study．They were divided into three groups according to PD value,margin level of tooth structure to bone (MTB),and different periodontal therapeutic strategies: (1) Group therapy 1 (T1): 67 residual roots with PD≥3 mm,MTB≥4 mm,receiving gingivectomy; (2) Group therapy 2 (T2): 71 residual roots with PD<2 mm and the MTB< 3 mm,receiving crown lengthening surgery; (3) Group control (C): 49 residual roots with PD<2 mm and MTB <3 mm,receiving gingivectomy.SBI,GI and tooth mobility of different groups before operation,1 week,4 weeks,six months and 1 year after operation were measured.Results The success rate of residual roots was 97.3%.There was significant difference in SBI and GI value when comparing the group T1,T2 with C at different observation points.The tooth mobility ratio of group T1 and T2 decreased at 6 months or 1 year after operation comparing with that before therapy,while that of group C increased at 6 months or 1 year after therapy.The success rates of group T1 and T2 were significant higher than group C at 1 week and 1 year after operation.Conclusions Rational periodontal strategy and approach display significant value on different margin levels of residual tooth,which can significantly improve success rate of residual roots reserved.

Key words: Multi-disciplinary, residual root reserved, crown lengthening surgery, gingivectomy, success rate.