The necrotic tooth and, even more so, the tooth previously treated endodontically, by definition, lack the pulp nerve fibers, and from a theoretical point of view, performing a procedure to obtain “pulpal” anesthesia appears to be a contradiction.
In endodontic therapy of the necrotic tooth, clinical practice and knowledge of the progressive course of tissue inflammation often lead to anesthetic infiltration, even in the absence of a positive response from the pulp chamber , in the fear that vital pulp residues may have remained in the canal.
In the same necrotic tooth and also in the reprocessing of the previously devitalized tooth (without residual pulp but in a state of apical suffering), there is also the risk of painful stimulation following the accidental invasion of the periapical space during negotiations or instrumentation of the root canals.
In the literature, there is broad agreement in the indication to end the root canal treatment at the apical constriction and in defining the electronic apical detector as the most predictable and reliable tool (accuracy between 80 and 90%) in determining the length of work.
On this basis, some authors have questioned the universal indication for the administration of local anesthesia in the necrotic tooth or already endodontically treated.
A particularly interesting clinical finding on the subject under consideration is provided by the Kfir study, published in 2017 in the International Endodontic Journal. The work evaluated in vivo the electronic length, i.e., the length determined by the detector, comparing it with the periodontal length, i.e., the length to which the first reported sensation corresponds in contact with the periapical tissue. The experimenters instead set the length of the work at 0.5 less than the electronic one.
The periodic length was higher than an electronic length in 96% of the 80 evaluated root canals (equally distributed between necrotic elements and undergoing reprocessing in the presence of the periapical lesion in both cases). The mean difference between the two lengths is 0.63 (±0.15) and 0.78 (±0.11) mm respectively. In the cases of necrosis and reprocessing, none of which has been evaluated as statistically significant, no the patient reported an actual painful symptom.
In conclusion, the authors interpreted these results on a clinical level as a support to the possibility of avoiding anesthetic administration in the necrotic tooth or in the case of reprocessing, a possibility which is subordinate to the use of the electronic apical detector.
At least, they consider it incorrect to equate the painful symptom associated with the instrumentation with the need for painless positioning of the rubber dam clamp or the patient’s requests. On the other hand, it should be remembered that the injection procedure itself has biological costs linked more to the discomfort of the conventional technique than to the pharmacological aspects.