Tooth Pulp Inflammation Reduces the Efficacy of Local Anesthetics

Even when proper dental anesthetic injection technique is used, failure to obtain complete or profound anesthesia in patients undergoing root canal therapy remains problematic. We hypothesize that local anesthetic (LA) failure in inflamed tooth pulp is due to the change in subunit expression and sensitivity of voltage-gated Na+ channels (VGSC) in pulpal afferents. Objective: The goal of this study is to provide behavioral evidence that pulp inflammation alters sensitivity of pulp afferents to LA treatment in rats. Methods: We studied the jaw-opening reflex (JOR) by electrically stimulating the maxillary incisor with a bipolar electrode (1Hz, 0.2ms duration, 5s pulse interval) at 3 times threshold in naïve control animals (n=5) and an experimental group of animals with pulp inflammation (n=5). Tooth inflammation was induced by exposing the incisor pulp three days prior to recording. Changes in digastric EMG amplitude were considered to reflect changes in the sensitivity of pulpal afferents. A transorbital approach was used to expose the infraorbital and superior alveolar nerves. The exposed nerves were superfused with phosphate buffer (pH 7.3). After the baseline JOR was measured, 1ml of 50µM lidocaine was applied to the exposed nerves for one minute and EMG was recorded for 10 minutes. Results: Lidocaine treatment produced a transient reduction in the digastric EMG which recovered within 10 minutes. The average reduction of EMG by lidocaine was 49% in control rats, and only 13% in inflamed rats (p<.05, t-test). Conclusion: Inflammation of the tooth or its nerves is associated with deceased efficacy of lidocaine and may contribute to anesthetic failure in patients undergoing root canal therapy or other dental treatments. (Supported by DE06027 and T32DE007309; NIH, NIDCR.)