The conventional technique of block anesthesia of the lower arch, mandibular inferior alveolar block, is widely discussed in the scientific literature with reference to failure rates, which, from what has been documented, in fact, can be rather prohibitive (up to 20-25%).
The technique is also subject to other criticalities; consider, for example, the aspects related to the onset of the anesthetic effect. Once the injection procedure has been completed, even though it has been carried out by Lege Artis, it is often necessary to wait a few minutes in order to ascertain the effect. The clinical definition of onset is commonly the appearance of the symptom of numbness at the level of the ipsilateral hemilabrum. Some patients report this subjective symptom even after 15-20 minutes of administration of the drug.
Since soft tissue anesthesia is directly detectable, lip numbness is used as an empirical indicator of pulpal anesthesia, which is the one that most interests the clinician and, in practice, is not generally checked by electrical testers. Secondly, the lower alveolar blockage affects the soft tissues circulating the dental arch; on this point, it seems reductive to imagine that all the areas – soft tissues and vestibular periosteum distal to the mental foramen , soft tissues and lingual periosteum and oral floor, 2/3 anterior to the hemilingual by involvement of the lingual nerve – are affected homogeneously.
On this basis, it seems interesting to ask how the anesthetic is distributed in the different tissue sites and, on the other hand, whether one of these, in a particular way, can act as a predictable indicator of pulpal anesthesia.
In an interesting study published in Cho, colleagues have subjected to lower alveolar block of a pool of 59 students (in Korea, as in many other countries in the world, the student-to-student is a common educational model to the procedure in question).
The effectiveness of soft tissue anesthesia was tested 15 minutes after the injection with the aid of a pressure algometer (only successful in the case of non-response to the maximum force, equal to 700 g) at 4 different sites: angle of the vermilion of the lower hemicabre (LL), adherent vestibular gum at the level of the lateral incisor (BGI), the first molar (BGM), and lingual gingiva (LG).
After waiting another 5 minutes, the same lateral incisor and first molar were subjected to evaluation of pulpal anesthesia by an electric tester.
The highest success rate at tissue level was recorded in the lingual area (93.9%; from a clinical point of view, this makes any lingual supplement unnecessary) followed by the lip (79.6%) and the incisor (53.1%); the very low success rate distal to the mental canal (BGM: 14.3%) was reliable.
The analysis of the positive predictive value has effectively evaluated the symptom of numbness of the lip as a bad indicator of real pulpal anesthesia (incisor and molar); on the other hand, by virtue of the high negative predictive value, it can be said that the lack of this clinical finding indicates the lack of effectiveness at the pulpal level with almost absolute certainty.