The Effect of Calcium Hydroxide Pastes on Isolated Vital Nerve Fibers

Introduction: Calcium hydroxide pastes (CHPs), commonly used for disinfecting root canals during endodontic treatment, are generally considered safe. However, accidental extrusions result in minimal injuries and little to no discomfort, except when extruded pastes come into contact with nerve bundles, such as the inferior alveolar nerve. Currently, there is a lack of information about the possible role of specific paste vehicles on the extent of nerve injury. The purpose of this study was to compare the role that paste vehicles, such as water or methylcellulose, may play when nerve fibers are exposed to CHP. Methods: Isolated sciatic nerves of Sprague-Dawley rats were exposed to either water-based or methylcellulose-based CHP for varying durations of time (30, 60, or 90 minutes). Histopathological changes, including axonal edema, myelin alterations, and loss of cellular outlines, were assessed, and the degrees of changes were compared using chi -square intraclass correlation coefficient tests. Results: Both groups exposed to the pastes demonstrated varying degrees of histopathologic changes, including axonal edema, myelin changes, and loss of cellular outlines, at different exposure times. The water-based calcium hydroxide paste induced these changes more rapidly than the methylcellulose-based paste. Similar patterns were observed in the scanning electron microscopic findings. Exposure time emerged as an important difference in the effects of the 2 pastes. In each of these tests, all observations of water-based paste exposure were rated as moderate to severe, whereas the observed cellular changes (axonal, myelin, and intact cellular outline) were rated as mild to moderate after exposure to methylcellulose-based paste for the same exposure durations. The chi -square tests indicated a statistically significant association between the material and each of the outcomes (axonal changes: chi(2)(15) = 81.0, P < .001; myelin changes: chi(2)(15) = 81.0, P < .001; intact cellular outline, chi(2)(15)= 81.0, P < .001). The intraclass correlation coefficient value was 0.93. Conclusions: The study demonstrates that axonal and myelin damage increase with longer exposure times, with water-based CHP causing more damage than methylcellulose-based CHP at each time point. (J Endod 2024;50:355-361.)