The effect of needle gauge, needle type, and needle orientation on the volume of a drop

Objective: The purpose of this study was to determine impact of needle gauge, type, and orientation on average volume of drop dispensed. Procedure: Five needle gauges (22G, 23G, 25G, 27G, and 30G) were examined. For each gauge, volume of drop delivered was determined for standard sharp beveled tip, blunt tip, and after breaking off of the sharp needle from the hub. Vertical and horizontal orientation of the needle was tested for effect on drop volume for 22-G and 30-G sharp beveled needles. Results: Mean drop volume was affected by needle gauge, needle orientation, and whether the needle had been broken off from its hub. Mean drop volume scaled directly with needle diameter with drop volumes of 25.0 μL (±20.2) and 83.9 μL (±16.5) being found for 30-gauge and 22-gauge needles, respectively. Intermediate gauges (27, 25, 23G) yielded intermediate drop volumes. Blunt needles tended to produce larger drop volumes compared to sharp beveled needles, but these differences did not reach statistical significance. Breaking off of the needle from the hub produced substantially larger drop volumes with little difference being found between needle gauges. Average volumes of 1 drop from a 22-G vertical, 22-G horizontal, 30-G vertical, and 30-G horizontal sharp beveled needle were 20.2, 9.1, 10.1, and 3.3 μL, respectively. Conclusions: These findings have relevance for controlled delivery of topical ophthalmic medications to patients.