An experimental study on the aspirating reliability of different types of injection syringes with regard to the formation of punch cylinders

Aspiration before local anaesthesia is essential to avoid an incorrectly placed needle. We tested 142 types of cannula in mucobuccal preparations from 29 cadavers. The cannulas were obtained from 23 different firms and the formation of punch cylinders caused by injection was studied at angles of penetration of 70° and 90°. The force of puncture was measured by a computer-controlled apparatus. In a second series of tests the aspirating ability of 121 cannulas with self-aspirating and actively aspirating syringes was tested in vitro. The results show that false negative aspirations are caused by: using a cannula that is too small (<= 27 Gauge); the cannula lumen becoming clogged with tissue punch cylinders; using a Carpule needle end that is too short to perforate the rubber of the ampoule; containing rubber fragments in the back of the Carpule needle end; and using an injection syringe with too small a viewport when the injection syringe was positioned at a 90°. We recommend the construction of a system in which the distance between the cone and viewport is reduced.