Anaesthesia in irreversible pulpitis: why the block fails and which supplementary techniques to use

Why the inferior alveolar nerve block fails so often in the mandibular molar with acute pulpitis, how to verify pulpal anaesthesia before starting, and which supplementary techniques to use when the baseline anaesthetic is not enough.

The inferior alveolar nerve block achieves adequate pulpal anaesthesia in the great majority of teeth with healthy pulp, but the success rate collapses when the mandibular molar is the site of symptomatic irreversible pulpitis: clinical studies report with striking consistency figures around half of cases or a little above. The finding has an implication worth spelling out, because it shapes clinical behaviour: a block that fails in this scenario is in most cases not an execution error to be corrected by repeating the same technique, but the predictable outcome of biological conditions that make the conventional block insufficient in itself. Repeating the identical injection a second time is the least effective response available.

The mechanisms are multiple and concurrent. Tissue pH lowered by inflammation reduces the non-ionised fraction of the anaesthetic, that is the only form able to cross the nerve membrane, but this factor alone does not explain the phenomenon, because the block is deposited at a distance from the inflamed site. Central and peripheral sensitisation weighs more heavily: sensitised nociceptors lower their activation threshold and increase expression of sodium channels resistant to local anaesthetics, particularly the tetrodotoxin-resistant isoforms. To this is added the anatomical variability of mandibular innervation, with accessory contributions from the mylohyoid nerve that the conventional block does not reach.

Verifying pulpal anaesthesia before starting is the single most useful measure and the one most often omitted: a cold test or electric pulp tester on the tooth to be treated, rather than merely asking the patient about lip numbness, identifies inadequate anaesthesia before pain during instrumentation signals it. A pulp test still positive after an adequate latency period indicates the need for a supplementary technique, not for waiting longer.

Among supplementary techniques, the intraligamentary injection is the simplest to perform and requires no dedicated instrumentation, but is limited in duration and often needs repeating during the appointment. Intraosseous injection offers superior efficacy in refractory cases, with the caveat of transient tachycardia when vasoconstrictor-containing solutions are used, which should be anticipated to the patient so as not to alarm them. Intrapulpal injection, feasible only with the chamber open, guarantees profound anaesthesia but involves a moment of acute pain as the needle enters. Supplementary buccal infiltration with articaine, performed in addition to the block already given, has shown in several studies a significant increase in the success rate in the mandibular molar, and for its ease of execution often represents the sensible first step before resorting to more invasive techniques.

At Oralzon you will find local anaesthetics in various formulations and concentrations, syringes and needles for intraligamentary and intraosseous techniques, and the electric pulp testers needed to verify pulpal anaesthesia before beginning instrumentation.