Local anesthesia for lacerations: pain of infiltration inside vs outside the wound

To compare the pains of infiltration of a local anesthetic for simple lacerations when used from within the wound vs through intact skin.

A randomized, prospective, single-blind, experimental protocol was conducted on a convenience sample of adults with simple lacerations. Subjects received two 0.5-mL injections of buffered lidocaine in each of the sites being compared. Immediately following each injection, pain was measured using a visual analog pain scale. Pain scores were converted to a numerical score and analyzed by a Wilcoxon signed-rank test. Subjects also were asked which injection hurt more, the first or the second (analyzed by a chi2 test).

A total of 63 subjects were enrolled. The first injection was within the wound and the second injection through intact skin (group 1) for 32 patients, and the order was reversed for 31 patients (group 2). Median pain scores for all inside-the-wound injections (14 mm) were lower than those with intact skin injections (37 mm; p < 0.0001). In group 1, 4 subjects reported the first injection (within wound) hurt most, 23 thought the second (intact skin) hurt most, and 5 found no difference. In group 2, 18 subjects reported the first injection (intact skin) hurt most, 5 believed the second injection (within wound) hurt most, and 8 found no difference, p < 0.0001.

Local anesthesia is less painful when injected from within a laceration as compared with intact skin.