Hot pulp
Lip numbness is not a reliable sign of pulpal anesthesia.
All subjects with normal pulp (no pain) reported lip numbness after inferior alveolar nerve block, but only 75% had pulpal anesthesia in molar (defined as no response to electric pulp testing) (Dagher et al. 1997).
Lip numbness is even more misleading in symptomatic patients. A similar clinical trial as above study conducted in patients with irreversible pulpitis of a lower molar showing the incidence of pulpal anesthesia was even lower (62%), even though 100% of the subjects reported lip numbness (Cohen et al. 1993). Another clinical study conducted in the same way as Cohen 1993 showing that the result was even worse, only 38% of the subjects had pulpal anesthesia, though all of the subjects had lip numbness (Nusstein et al. 1998)
Why is that?
We believed that unmyelinated C fibers are the most sensitive to anesthetic, followed by the lightly myelinated neurons (A delta fibers), with the heavily myelinated neurouns (A beta fibers) being the least sensitive to these drugs. Under normal conditions, pain perception is mediated by the C and A delta fibers, whereas touch and propioception are mediated by the A beta fibers. As textbooks have suggested that a positive lip sign (lip numbness or lack of touch sensation due to the blockade of A beta fibers) predicts that pulpal pain fibers are anesthetized and the patient is ready for the treatment. However, this conclusion was based on research using rather old technique in 1930s. More recent studies used single fiber recording technique instead of the whole nerve technique as in the old days. With more refined technique, results are different i.e. A delta and A beta fibers can be more easily blocked by anesthetics than C fibers.
Thus, a positive lip sign (i.e. lack of touch sensation) does not necessarily indicate pulpal anesthesia in symptomatic patient. Instead of relying solely on positive lip sign, the offending tooth should be tested directly with CO2 ice test, EPT or even percussion test.
If the lip is numb, but the tooth is not; what else can we do to get the tooth numb?
Practical suggestions for anesthetic failures
Increase the dose – expose greater length of the inferior alveolar nerve (anesthetic effects are cumulative), don’t forget the safety dose limit
Buccal infiltration with Articaine
Second block at higher level – Gow-Gates technique
Considering block mylohyoid nerve – usually it’s motor, but in rare case it can be sensory (just infiltrate on the lingual side of the lower molar)
Intraligamental technique
Intraosseous technique
Intrapulpal technique – back pressure is required to achieve anesthesia
Test the teeth in question with EPT or cold test rather than rely on positive lip sign.
References
Cohen HP, Cha BY, Spangberg LS (1993) Endodontic anesthesia in mandibular molars: a clinical study Journal of Endodontics 19, 370-3.
Dagher FB, Yared GM, Machtou P (1997) An evaluation of 2% lidocaine with different concentrations of epinephrine for inferior alveolar nerve block Journal of Endodontics 23, 178-80.
Nusstein J, Reader A, Nist R, Beck M, Meyers WJ (1998) Anesthetic efficacy of the supplemental intraosseous injection of 2% lidocaine with 1:100,000 epinephrine in irreversible pulpitis Journal of Endodontics 24, 487-91.