In dental treatment, there is a “worst-case scenario” everyone fears: despite administering local anesthesia, the moment drilling starts, a shooting, jump-out-of-the-chair pain erupts. This is “local anesthesia failure.” The problem is especially pronounced in endodontic patients who require root canal therapy. Why does pain persist even after careful anesthetic injection?

Behind this lies a state called hyperalgesia. Inflammatory changes essentially “turn the pain-volume knob to maximum,” so stimuli that would normally be imperceptible are transmitted to the brain as severe pain. The molecular culprit behind this stubborn phenomenon was elucidated by Dr. Jason E. Wells and colleagues (Wells et al., 2007). They identified a specific sodium channel in neurons, Nav1.9, as a key perpetrator of anesthetic failure.

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Surprising Fact 1: In painful teeth, the “pain generators” triple

Why is the inflamed dental pulp so hypersensitive? Wells and colleagues compared the expression of the Nav1.9 channel in pulpal tissue from symptomatic teeth and asymptomatic, healthy teeth. A decisive “smoking-gun” result emerged:

  • Axons in asymptomatic pulp: 9.20%
  • Axons in symptomatic pulp: 27.22%

In severely painful teeth, Nav1.9 expression surged to roughly three times higher. This “upregulation” means cells responding to inflammation have massively increased the number of “pain gates.” The proliferation of these gates markedly heightens neuronal excitability, forming the basis for the intense pain experienced during treatment.

Surprising Fact 2: A “resistance” trait that makes common anesthetics (lidocaine) less effective

What makes Nav1.9 particularly troublesome is not only its increased number but also its lower susceptibility to common dental local anesthetics such as lidocaine. The channel is tetrodotoxin (TTX)-resistant. TTX is the well-known potent neurotoxin found in pufferfish, yet Nav1.9 resists even this toxin.

Crucially, compared with other channels, Nav1.9 is about four times less susceptible to blockade by local anesthetics like lidocaine. Furthermore, Nav1.9 carries a persistent current. Unlike typical channels that close rapidly, Nav1.9 closes slowly and incompletely. As a result, a small “leak” of current continuously escapes across the neuronal membrane, keeping the neuron in a persistent “on” (depolarized) state, always primed to fire.

As the paper states:

“Nav1.9 channels increase neuronal excitability and have low sensitivity to blockade by local anesthetics.”

Recent research suggests that another TTX-resistant channel, Nav1.8, may be comparatively more sensitive to lidocaine, pointing to Nav1.9 as the more likely primary driver when anesthesia “doesn’t take.”

Surprising Fact 3: Small cells in the trigeminal ganglion—direct to the brain—hold the key

Nav1.9’s effects are not confined to the tooth. The research team also examined the trigeminal ganglion, the relay center that transmits facial sensation to the brain. They focused on the signal-to-noise ratio (SNR), an index indicating how strongly Nav1.9 is expressed relative to background noise.

They found that Nav1.9 expression was strongest in small-sized neurons within the trigeminal ganglion. This correlation was statistically supported, with significant relationships between cell size and SNR across three specimens (coefficients of determination r^2 = 0.039, 0.056, 0.137). These small neurons are “elite pain units,” often expressing nociceptive markers such as CGRP (calcitonin gene-related peptide) and enkephalin. Upregulated Nav1.9 in the inflamed tooth effectively arms these dedicated pain neurons, enabling powerful signals to punch through anesthesia and reach the brain.

Surprising Fact 4: Hope for future therapies—and the intriguing effect of clove oil

What if we could selectively inhibit Nav1.9 where it wreaks havoc—in the brain’s gateway and in the dental pulp? This research strongly suggests the potential for next-generation anesthetics targeting Nav1.9.

There is also a clinically interesting hint: eugenol, the main component of clove oil long used in dentistry for analgesia. Wells and colleagues referenced prior work (Park et al.) indicating that eugenol can effectively suppress TTX-resistant sodium currents that lidocaine struggles to block. The possibility that a traditional natural compound may work against an opponent that challenges modern synthetic anesthetics (lidocaine, bupivacaine) offers a valuable clue for tackling anesthetic failure in the future.

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Conclusion: Channel research opening the future of dental care

In the past, when anesthesia “didn’t work” during dental treatment, it was sometimes dismissed as the patient’s imagination or purely a technical issue. The study by Wells et al. (2007) introduced a clear scientific basis—Nav1.9. We now understand that inflammation can build a “defensive wall” that resists local anesthetics at the neuronal level, and research to overcome this is steadily advancing. If a novel anesthetic that specifically blocks Nav1.9 becomes practical, patients could receive treatment in the dental chair with comfort, no matter how severe their pain. A future where fear of the dentist becomes a thing of the past may well lie on the other side of this small molecule’s story.

Reference

Wells, J. E., Bingham, V., Rowland, K. C., & Hatton, J. (2007). Expression of Nav1. 9 channels in human dental pulp and trigeminal ganglion. Journal of endodontics, 33(10), 1172-1176.

 

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Specialist in Periodontology and Endodontics

Certified by the University of Gothenberg, Sweden in 1996

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