By what mechanism does an INTRAPULPAL injection produce anesthesia?

Correct answer: A. Primarily by back-pressure within the confined pulp chamber

• A. CORRECT. The intrapulpal injection works chiefly through PRESSURE. The needle is wedged tightly into the access opening so that solution can only be expressed under considerable resistance, and the resulting BACK-PRESSURE within the rigid, non-compliant pulp chamber produces immediate anesthesia. The evidence for this is that saline injected under equivalent pressure gives comparable relief. The pharmacological block contributes, but pressure is the primary mechanism.
• B. Blocking PDL sodium channels describes the intraligamentary injection, which is a different supplemental technique delivered into the periodontal ligament space.
• C. Vasoconstriction reduces bleeding and prolongs the local effect but is not the anesthetic mechanism.
• D. Diffusion out through the apical foramen to the nerve trunk is neither the route nor fast enough; the onset of an intrapulpal injection is essentially instantaneous.

Note: Two practical consequences follow from the pressure mechanism. First, the injection is briefly and sharply PAINFUL — warn the patient. Second, it only works if the needle achieves a tight seal against resistance; if solution flows freely out of the chamber, no pressure develops and no anesthesia results. Duration is short, roughly 15–20 minutes, so proceed at once.