Comparative In Vitro Evaluation of Tube Resistance in Glaucoma Drainage Devices with Intraluminal Polypropylene Threads

(1) Background: To compare tube resistance among glaucoma drainage devices (GDDs) with different tube geometries under various intraluminal polypropylene (PP) thread conditions and assess agreement between measured and formula-derived pressure–flow profiles. (2) Methods: Four GDDs were evaluated: Ahmed ClearPath Small Tube (ACP-ST), Paul Glaucoma Implant (PGI), Ahmed ClearPath (ACP), and Virna Glaucoma Implant (VGI) (n = 4/device). ACP-ST and PGI were tested under no-thread, 6-0 PP Corza, and 6-0 PP Ethicon conditions, whereas ACP and VGI were tested under no-thread, 4-0 PP Corza, and 3-0 PP Ethicon conditions. Saline was infused at 1.0–5.0 µL/min, and pressure was recorded using a transducer system. Tube and thread dimensions were measured using ImageJ and used to calculate formula-derived pressure from fluid dynamic equations. Comparisons used linear mixed-effects regression. (3) Results: In ACP-ST and PGI, both 6-0 PP threads significantly increased pressure compared with the no-thread condition, with no significant thread manufacturer-related differences. In ACP and VGI, all threaded conditions significantly increased pressure, and the 3-0 PP Ethicon condition produced higher pressure than the 4-0 PP Corza condition. Under threaded conditions, measured and formula-derived pressure values did not differ significantly in ACP-ST but differed significantly in PGI, ACP, and VGI. (4) Conclusions: Intraluminal PP threads increased GDD tube resistance. Manufacturer-related differences had a limited influence on resistance in small-lumen devices. Agreement between measured and formula-derived pressure varied among devices, highlighting the importance of experimental validation. These findings may help guide intraluminal thread selection and flow-restriction strategies during GDD implantation.