GIÁ TRỊ CỦA PHƯƠNG PHÁP TÍNH TAM SUẤT TRONG XÁC ĐỊNH CHIỀU DÀI LÀM VIỆC TRÊN PHIM X QUANG CHỤP THEO KỸ THUẬT ĐƯỜNG PHÂN GIÁC KHI TRÂM Ở NHỮNG VỊ TRÍ KHÁC NHAU TRONG ỐNG TỦY

Abstract

Background: In 1970, Grossman introduced a proportional basic rule used on periapical radiographs taken with the bisecting angle technique to calculate the working length in endodontics and this rule was widely used until now. Objectives: Determine the value of Grossman's proportional basic rule in determining the working length from measurements on periapical film captured by bisecting technique when endodontic file at different positions in the canal and cone at different vertical angles. Materials and methods: We selected 20 extracted human single-root premolars, measured the actual tooth lengths and took periapical radiographs by bisecting technique with the tip of the endodontic file at apical third and middle third position of the root and with the cone head positioned at correct, higher (+ 10⁰) and lower (-20⁰) vertical angles. The differences between the tooth lengths calculated using Grossman's formula and the actual tooth lengths were recorded. The Wilcoxon test was used to test the significance of these differences when the file tip was at different positions and cone was positioned at different vertical angles. Results: The mean of differences between the calculated tooth lengths and the actual tooth lengths when the cone at correct vertical angle and the file tip at apical third of the root was lowest (0.531 ± 0.272 mm); this value was highest in case tip of file at the middle third and cone at higher angle (2,002 ± 1,181 mm). At different positions of file in the root canal,  the difference of length means was not statistically significant  with correct cone angle (p = 0.179) but statistical significant with  higher and lower vertical cone angle (p = 0.05 ; p = 0.033 respectively).Conclusion: The Grossman’s proportional basic rule in determining the endodontic working length through x-ray film taken by bisecting technique has high validity when the cone is set at correct angle and the more the file tip is located close to apex of the root the more accuracy the calculated working length.