In-vitro optimisation of frequencies in endodontometry, apical constriction diameter free

Objectives: accuracy of electronic apex locators using impedances ratio criteria depends on frequencies (IADR 00, 01) and Apical Constriction Diameter (ACD) (IADR 06). The aim of this in-vitro study was to determine the association of frequencies allowing the determination of working length, insensitive to ACD, sensitive to catheterism.

Methods: 4 resin models, (ACD from 10/100^e to 40/100^e, 10/100 ^e step), inserted in CuSO₄ gel, were catheterized from 2mm above Apical Constriction (AC), down to 2mm beyond (5/100^e step), using a 10/100 ^e K-file conducted by a micrometric displacement table. 1456 ratios Q_ij (Q_ij=Z_fj/Z_fi; fi: reference frequency from 20Hz to 100kHz; fj: working frequency from f_i+1 to 100kHz) were computed, using Z_f (mean of 3 impedances). A new criterion, C_ij, issued from graphic study of 52 3D-curves (Q_ij versus catheterism and ACD), was defined:

C_ij = mean (max Q_ij – min Q_ij) _{[-0.5, AC, +0.5]} /[mean(Q_ij) _[-0.5] - mean ( Q_ij) _[AC]].

Results: about reference frequency (fi), it appears, according to Kruskal-Wallis test (p=0.05) that there was no significant difference between C_ij. Nevertheless, the study of C_ij values and Q_ij curves at AC, -0.5 and +0.5, versus ACD recommends us these frequencies: 20Hz as reference (fi) and 100kHz as working (fj). Moreover, with these frequencies, Q_ij are linearly correlated to catheterism from -0.5 to +0.5 (r²=0.993) and do not depend on ACD (-0.5, AC, +0.5).

Conclusion: ratio method in electronic endodontometry, using 20Hz and 100kHz, gives in-vitro the best results to determine working length and catheterism (-0.5 to +0.5), whatever AC diameter (10/100^e to 40/100^e).