Abstract
Purpose :
The optimal syringe to use for intravitreal injections remains under debate. We investigated the relationship between the force applied to the syringe and volume output.
Methods :
Empty pre-filled syringes of aflibercept and ranibizumab as well as 1 mL tuberculin (TB) syringes, used for bevacizumab injections, were filled with 50 µL of deionized water. Syringes were attached to a 32-gauge needle and held in an apparatus with weights being placed in 50 g (0.49 N) increments until the plunger was completely depressed and fluid ceased exiting the needle. Following this equilibrium point, additional weight was added serially to increase the force on the plunger. The mass of water expelled at each force was measured and converted to volume (1 mg water = 1 µL water). Six trials of each syringe were performed, and differences were compared using an ANOVA test. Pairwise differences were calculated with t-tests.
Results :
The mean total excess volume with 400 g (3.92 N) being added after equilibrium was reached was 8.58 µL for aflibercept (95% CI: 8.04-9.13), 5.70 µL for ranibizumab (5.13-6.27), and 0.40 µL for the TB syringe (0.14-0.66). One-way ANOVA revealed that the mean total excess output was significantly different between the three syringes (F = 495.6, p = 2e-14) and t-tests found significant differences between all syringe pairs (p < 0.0001).
Conclusions :
Ideally, 50 µL is injected with intravitreal injections, but with only 400 g of extra weight applied to the depressed plunger the output volume increased by 17% from the aflibercept syringe and 11% from the ranibizumab syringe. Under the same conditions, the TB syringe expressed only 0.8% more volume. Design elements of the syringes, including the domed shape, compressibility of the stopper, short push length, and large diameter of the plunger likely account for the differences. Given the small volume of intravitreal injections, the effect of force on syringe output should be minimized to prevent improper dosing of medication.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.