The hot tooth that will not go numb - 25 minutes left on the clock
You are 15 minutes into a 45-minute assessed appointment in the simulated clinic. Your patient, a trained simulated patient, has been referred with a four-day history of severe spontaneous pain in the lower right quadrant that wakes him at night and is provoked by cold drinks. Tooth 46 has a deep disto-occlusal amalgam and a fractured distobuccal cusp. Cold testing of 46 produces pain that lingers for more than 30 seconds; the tooth is tender to percussion; there is no swelling, no sinus tract and no palpable lymphadenopathy; probing depths are 3 mm or less. The periapical radiograph shows caries approaching the pulp horn and a 2 mm radiolucency at the mesial apex. The patient reports that a previous dentist gave an inferior alveolar nerve block two days ago and that it 'wore off before he finished'. Maximum mouth opening is 34 mm and he has an active gag reflex. A rubber dam, an emergency kit, 2% lidocaine with 1:100,000 adrenaline and 4% articaine with 1:100,000 adrenaline are available.
1. Explain why the inferior alveolar nerve block failed and set out your anaesthetic plan, including maximum doses for a 70 kg adult.
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Why it failed: in irreversible pulpitis the pulpal tissue is inflamed and acidic, so less of the local anaesthetic exists in the uncharged base form able to cross the neuronal membrane. Nociceptors are sensitised, with upregulation of tetrodotoxin-resistant sodium channels (Nav1.8/1.9) that are poorly blocked by lidocaine, and pulpal blood flow is increased, accelerating washout. The reported success of an inferior alveolar nerve block in irreversible pulpitis is only around 30–40%, versus 80–90% in a healthy pulp. He may also have accessory innervation (mylohyoid nerve, cervical contribution) or anatomical variation in the lingual/mandibular nerve position.
Plan:
- Confirm the diagnosis (symptomatic irreversible pulpitis with symptomatic apical periodontitis on 46) and premedicate with ibuprofen 600 mg plus paracetamol 1 g if not contraindicated - this reduces central sensitisation and improves anaesthetic success.
- Deposit an inferior alveolar nerve block with 2% lidocaine 1:100,000, 1.8 mL (36 mg), plus long buccal infiltration; wait 8–10 minutes.
- Test with cold and gentle probing before starting. If pulpal anaesthesia is inadequate, give a supplemental buccal infiltration of 4% articaine 1:100,000, 1.8 mL (72 mg). This is the best-supported single supplement for a hot mandibular molar.
- If still inadequate, consider an intraligamentary injection (0.2 mL per root with a pressure syringe).
Maximum doses (70 kg): lidocaine 4.4 mg/kg ≈ 300 mg ≈ 8 cartridges of 2% (36 mg each). Articaine 7 mg/kg ≈ 490 mg ≈ 6–7 cartridges of 4% (72 mg each). Count total milligrammes across both agents and document. Intrapulpal injection is a last resort for the moment of extirpation only, not a primary technique.
2. Twenty-five minutes of the assessed appointment remain and pulpal anaesthesia is still inadequate. What do you do, and what do you say to the patient?
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Decision: do not instrument the canal. Debriding an unanaesthetised hot tooth causes avoidable severe pain, damages trust, and is a professional failure in an assessment setting.
What I would do:
- Stop, remove the rubber dam if already placed, and confirm that soft tissue is numb even though the pulp is not - the two can dissociate.
- Do not remove deep caries or attempt a pulpectomy. If soft tissue anaesthesia permits, place a temporary dressing and a well-sealed restoration to reduce stimulation; if not, leave the tooth sealed with a temporary material only.
- Do not prescribe antibiotics: there is no swelling, no systemic involvement and no indication. Prescribe or confirm analgesia (regular ibuprofen/paracetamol) and give written postoperative instructions.
- Rebook a longer appointment, and consider a different strategy: another operator, intraosseous anaesthesia, or referral to an endodontist. Recognising the limit of what can be achieved in the time available is the correct judgement, not persistence.
What I would say: 'The nerve in this tooth is very inflamed, and that makes it much harder than usual to get it fully numb. I could start drilling, but you would feel it, and that is not something I am prepared to do. Nothing has been lost by stopping now. The tooth is sealed, here is what to do about the pain, and I want to see you again within 24 to 48 hours. If you get swelling, fever or the pain becomes uncontrollable, contact us immediately.'
Document the diagnosis, the agents and doses given, the reason for stopping and the safety-netting advice.
3. How will you achieve isolation for the definitive endodontic treatment, given 34 mm opening and an active gag reflex?
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Rubber dam is mandatory for endodontic treatment - for airway protection, isolation from sodium hypochlorite and a dry field. It is not optional because access is awkward.
Practical approach:
- Use a single-tooth isolation with a small clamp (for example a 26 or 27 pattern) placed on 46, avoiding the fractured distobuccal cusp. Floss-ligate the clamp as a safety tie. If no clamp engages securely, use a Wedjets-type cord ligature or a split-dam technique rather than abandoning isolation.
- Punch a small hole and pre-stretch the dam; seat the clamp first, then the frame.
- Reduce the gag reflex: seat the patient more upright, use topical anaesthetic on the palate, avoid over-extending the dam posteriorly, encourage nasal breathing, offer a salt-on-the-tongue distraction, and use a bite block on the opposite side to reduce the need to hold the mouth open.
- Practise placement before the definitive visit if possible; a failed, rushed attempt on the day wastes appointment time.
If isolation genuinely cannot be achieved, do not proceed with chemomechanical debridement. Discuss the options with the patient - referral to a specialist or a service with appropriate adjuncts - and document the reason.
Plan ahead for the restoration: a fractured distobuccal cusp on an endodontically treated molar needs cuspal coverage (onlay or crown). Build-up should preserve as much sound tissue as possible; a post is not indicated merely because the tooth has had root canal treatment.