The effect of order on pain of local anesthetic infiltration

To define the relationship between order of injection and pain of infiltration of buffered lidocaine.

Volunteers > or = 18 years of age were enrolled in a prospective, double-blind experimental protocol. Subjects received a 0.5-mL intradermal injection of anesthetic in each forearm. All injections were given by the same individual in the same manner. Immediately following each injection, the subjects rated the pain of infiltration on a standardized 100-mm visual analog pain scale. Both the subjects and the individual giving the injections were blinded to the anesthetic being administered. They were told that the injections could be either plain or buffered lidocaine. They were further informed that any individual subject could receive 2 of the same anesthetics or 1 of each in either order. In reality, all injections were buffered lidocaine for all subjects. Pain scores were converted to a numerical score by making measurements to the nearest millimeter and analyzed by a Wilcoxon signed-rank test with p < 0.05 considered significant.

Fifty subjects were enrolled. The first injection had a median pain score of 13.5 mm, compared with 23.5 mm for the second (p = 0.007).

The second injection of buffered lidocaine was found to be statistically more painful than the first in this protocol that controlled for all variables except for order of injection. Future studies involving paired comparisons should take this information into account.