Anesthesia · patient case question
An intrapulpal injection is given to Patient A. What principally accounts for its effect?
Patient case: A hot molar and a failing post crown
Patient: Two patients
Chief Complaint: A: “This tooth has been keeping me awake and the injection is not touching it.” B: attends for a routine review.
Background: A, 34-year-old male: healthy. Tooth #30 with deep caries, prolonged lingering response to cold, tender to percussion. Two inferior alveolar nerve blocks and a buccal infiltration with articaine have not produced pulpal anaesthesia; the access is now open and the pulp remains exquisitely sensitive. B, 61-year-old female: healthy, no symptoms.
Findings: B: full mouth radiographs show a cast post and core with a crown on tooth #8. A well-defined radiolucent gap of about 3 mm separates the apical end of the post from the remaining root filling. No periapical radiolucency at present.
Show answer and explanation
Correct answer: B. Back pressure generated by injecting into a confined space
- A. Nerve trunk blockade is what the two inferior alveolar blocks already attempted and failed to achieve — which is why supplemental technique is required.
- B. CORRECT. The intrapulpal injection works largely by PRESSURE rather than pharmacology. The needle is wedged firmly into the pulp chamber or canal orifice so that solution is forced into a confined, non-compliant space under high BACK PRESSURE. Two consequences follow directly from that mechanism. It is briefly but sharply PAINFUL — warn the patient before giving it. And it only works if a tight seal is obtained, so a wide-open chamber that lets solution escape produces no anaesthesia at all. Onset is essentially immediate but duration is short, roughly 15 to 20 minutes, which is enough to complete pulp extirpation.
- C. Systemic absorption plays no part in producing local anaesthesia and is only relevant to toxicity.
- D. Diffusion to the periapex describes the mechanism of periapical infiltration and intraosseous techniques, where solution reaches the nerve through cancellous bone.
Note: The escalation ladder for the hot mandibular molar, in order: conventional inferior alveolar block → buccal infiltration with 4% articaine → intraligamentary (periodontal ligament) injection → intraosseous injection → intrapulpal as the last resort once the chamber is open. Pre-operative NSAIDs improve block success in irreversible pulpitis, and the inflamed pulp's lowered pH and altered sodium channels are why it is so difficult to anaesthetise in the first place.