Aspiration in local anesthesis. Comparison between disposable self-aspirating and usual syringes.

The positive aspirations obtained with a self-aspirating syringe (Astra, Sweden) were compared with those obtained when aspiration was performed manually (Uniject syringe, Hoechst, West Germany). The subjects were 1186 army recruits, to whom 2348 injections of local anesthetic were given in connection with dental treatment. When aspiration was performed manually, the results were positive in 4.7% of the cases; with the self-aspirating syringe, they were positive in 1.1% of the cases. The total frequency of positive aspirations was significantly higher (P less than 0.001) with the manual aspiration technique than with the self-aspirating syringe was due to the fact that the value did not include aspiration occurring when the injection needle ruptured small blood vessels.