Physical Parameters Influencing the Rate of Subretinal Injection: A Microfluidic Analysis.

Purpose: Subretinal injection during vitrectomy has become an important technique owing to the growing number of cell and gene therapy trials for retinal disease. Reducing the injection rate can mitigate trauma or shear forces on the retinal pigment epithelium, yet many surgeons alter the subretinal injection system in ways that could influence this rate. Methods: We created a theoretical model of a standard subretinal injection system by applying the Hagen-Poiseuille equation, assuming steady, incompressible, laminar flow of a water-like Newtonian fluid. The system consisted of the vitrectomy console, plastic tubing, the MedOne Microdose kit, and the MedOne 38G/41G subretinal cannula set. Results: This model generated an open-access web app allowing user modification of parameters including cannula length and diameter, beveling, viscosity, console injection pressure, and intraocular pressure. The internal diameter of the cannula had the most prominent effect on flow, with small increases producing large increases in flow. Increasing console pressure linearly increased tip flow. Trimming the tip by 3 mm modestly increased flow by 18%, whereas beveling the tip or altering intraocular pressure produced negligible changes. Conclusions: With this theoretical model, we were able to identify key parameters that influence the rate of subretinal injection, thus providing important guidance for surgeons. Translational Relevance: This open-access web application enables vitreoretinal surgeons and gene- and cell-therapy trialists to assess how modifications to the subretinal injection system alter flow at the cannula tip, supporting standardized and reproducible delivery that minimizes flow-related trauma to the outer retina.