Failed mandibular block in an acutely inflamed molar, then a spreading swelling

Why both blocks failed. Failure is common in an acutely inflamed pulp (often quoted at 10–20%) and is usually multifactorial rather than a single technical error.

• Deposition site. The IANB relies on solution pooling near the inferior alveolar and lingual nerves just before they enter the mandibular foramen on the anteromedial ramus. A needle that contacts bone too low, too anterior or too far lateral deposits solution away from the nerves; the absence of lingual numbness suggests the lingual nerve was not bathed either.
• Accessory and variant innervation. The mylohyoid nerve (a branch of the inferior alveolar nerve) can carry pulpal fibres to mandibular molars, and bifid mandibular canals or retromolar canals carry fibres that a conventional block misses.
• Inflammation. Acidosis in inflamed tissue reduces the fraction of uncharged local anesthetic base that crosses the axonal membrane, and upregulated sodium channels lower the threshold for firing. This is a pharmacological problem, not simply a volume problem.

Options, in order. A Gow-Gates or Vazirani-Akinosi block targets the same nerves more proximally and is a reasonable first switch. A buccal infiltration of 4% articaine 1.8 mL often supplements mandibular molar anesthesia because articaine diffuses through buccal cortical bone. Intraligamentary injection (about 0.2 mL per root with a pressure syringe) or intraosseous delivery gives direct pulpal access; an intrapulpal injection under pressure completes anesthesia once access is gained.

Dose check. She has already received 36 mg lidocaine and 72 mg articaine, plus 0.036 mg epinephrine - well inside adult limits for a 70 kg patient (articaine 7 mg/kg = 490 mg). A further articaine cartridge (total 144 mg) is safe, but repeating identical blocks without changing technique is not.

Anatomical route. The apices of mandibular molar roots lie below the mylohyoid line, so pus perforating the lingual cortex drains into the submandibular space, which is bounded above by mylohyoid, medially by the hyoglossus and geniohyoid, laterally by the platysma and skin, and inferiorly by the deep cervical fascia. The sublingual space sits above mylohyoid; a root apex above the mylohyoid attachment drains there instead. The combination of submandibular and sublingual involvement on the same side, and especially bilaterally, constitutes Ludwig angina. Spread posteriorly runs to the pterygomandibular and then parapharyngeal spaces, and from there to the retropharyngeal space and mediastinum.

Findings demanding immediate referral. The raised floor of the mouth and trismus of 25 mm already indicate involvement of more than one space. Add any of: dysphagia, odynophagia, muffled 'hot potato' voice, drooling, inability to swallow saliva, stridor, rapidly increasing swelling over hours, systemic upset (fever, tachycardia, hypotension), or a compromised airway - any one mandates urgent same-day maxillofacial assessment for airway protection, intravenous antibiotics and incision and drainage. Trismus alone is not the discriminator; the floor of the mouth and airway are.

What not to do. Do not attempt definitive endodontic or surgical treatment of the tooth in a primary-care setting before the airway has been assessed, and do not prescribe oral antibiotics as a substitute for drainage. Incision and drainage plus removal of the causative tooth remains the definitive treatment once the airway is secure.

Lidocaine. A 2% solution is 20 mg/mL, so 1.8 mL contains 1.8 × 20 = 36 mg. For a 70 kg adult the maximum recommended dose with a vasoconstrictor is 7 mg/kg = 490 mg (4.4 mg/kg = 308 mg without vasoconstrictor). She has used 36 mg, roughly 7% of the limit.

Articaine. A 4% solution is 40 mg/mL, so 1.8 mL contains 1.8 × 40 = 72 mg. The usual adult maximum is 7 mg/kg = 490 mg for 70 kg, so she has used 72 mg, about 15% of the limit. Cumulative articaine after one more cartridge would be 108–144 mg, still well under the ceiling.

Epinephrine. A 1:100,000 concentration is 0.01 mg/mL, so each 1.8 mL cartridge contains 0.018 mg. Two cartridges delivered 0.036 mg. A healthy adult limit is around 0.2 mg; in significant cardiovascular disease the practical limit is about 0.04 mg, so she is already close to the conservative cardiac ceiling despite being healthy.

Practical implication. The failure was not due to under-dosing - the volumes were adequate and the doses trivial against the limits. Adding more of the same block is therefore low-yield. The correct move is to change the technique (Gow-Gates, infiltration, intraligamentary or intraosseous) or, if anesthesia still fails, to accept that a 'hot' tooth may need pulpectomy with supplemental anesthesia rather than repeated cartridges. Note also that articaine maximum doses are sometimes quoted lower in some jurisdictions, so the local formulary should be checked before exceeding three or four cartridges.