Extracting at a time the respective plural books in every tooth kind the plural teeth which extract a tooth as retention impossible as the object, the loss dental root membrane area (S L) it calculates sample attachment level of each tooth which it measured (M) with, the entire dental root membrane area (S) of on the basis, the occlusion force factor which differs furthermore depending upon only each tooth kind which reflects the occlusion power of each tooth (B) and, the loss dental root membrane area when losing of each tooth (S LB) with statistics it processes relationship in every each tooth kind, in this case utilizing the fact that it is the relationship in the straight line in bothDoing, the loss dental root membrane area when the losing occlusion force factor (B) and the tooth (S LB) with it draws up the formula in the straight line which shows relationship, it seeks the loss dental root membrane area when losing of the aforementioned tooth (S LB) of on the basis (the remains dental root membrane area which adds occlusion power (S SB)/the entire dental root membrane area (S))The 100 (%) it calculates the tooth lap the tooth which displays the advance extent of the flame lap as flame index BPI.保存不可能として抜歯された複数の歯を対象として、歯種毎にそれぞれ複数本づつを抽出して測定した各歯のサンプルアタッチメントレベル(M)と、総歯根膜面積(S)に基づき喪失歯根膜面積(SL)を算出し、さらに各歯の咬合力を反映した各歯種のみによって異なる咬合力係数(B)と、各歯の喪失時の喪失歯根膜面積(SLB)との関係を各歯種毎に統計処理し、この際両者に直線的関係があることを利用して咬合力係数(B)と、歯の喪失時の喪失歯根膜面積(SLB)との関係を示す直線の式を作成し、前記歯の喪失時の喪失歯根膜面積(SLB)に基づき求める(咬合力を加味した残存歯根膜面積(SSB)/総歯根膜面積(S))×100(%)を歯周炎の進行度合いを表す歯周炎指標BPIとして算出する。
