The effect of syringe design and cannula dimensions on time-force curve in intravitreal injection across different drug viscosities: area under the curve and peak injection force.

BACKGROUND: New intravitreal medications can offer longer treatment duration by using higher drug concentrations, which, in turn, increase viscosity. There is also a trend toward having smaller-gauge cannulas, which require a higher injection force. While the ophthalmologist cannot control viscosity, the choice of syringe may affect the force needed to administer the drug. This study compared two standard 1-ml syringes (BD Plastipak™ and Braun Injekt®-F) with a syringe featuring a reduced plunger diameter (Zero Residual™ 0·2 mL silicone oil-free syringe) for injecting a variety of fluids with viscosity that mimics commonly used intravitreal formulations. METHODS: Injection force over time was measured for each combination of the three syringes, four fluids, and two cannulas (30 G and 33 G), and the results were compared using a two-way ANOVA, followed by Tukey’s post-hoc test. Plungers of the 1ml syringes had a diameter of approximately 4.70mm, while the smaller syringe had a plunger diameter of 2.48mm. Of the 1ml syringes, one was silicone oil lubricated (BD Plastipak™). The tested fluids were chosen to simulate bevacizumab (Balanced Salt Solution), pegcetacoplan (13.8 mg/0.1 ml of PEG-40,000), a high-viscosity solution (40 mg/0.1 ml of PEG-400), and a very high-viscosity solution (Tween 80 diluted with BSS). RESULTS: The Zero Residual syringe required significantly lower injection forces, as quantified by both the area under the curve (AUC) and peak force. These effects became more pronounced when using smaller-diameter cannulas and increasingly viscous fluids. The BD Plastipak needed lower forces than the Braun Injekt-F. CONCLUSIONS: While silicone-oil lubrication reduced injection force, its effect was modest compared with that of plunger diameter, which was the primary determinant of the injection force required for a given viscosity and cannula diameter.