In-vitro characterisation of pressure control with stent sutures: implications for non-valved glaucoma drainage device titration

Abstract

Purpose : A Baerveldt implant is one of the most commonly used aqueous shunts to treat glaucoma. It lacks a mechanism of early flow control due to its unrestricted large lumen diameter (300 µm). Stent sutures can be inserted inside the tube with/without external ligatures to control the outflow facility and intraocular pressure. Limited studies have been done to characterise the relationship between the length of sutures in the tube, external ligations and the pressure drop.

Methods : Silicone tubes (inner diameter: 300 µm, outer diameter: 640 µm, Dow corning) were immersed in a bath at 35.7±0.9 °C. Aqueous proteinaceous solution of bovine serum albumin at 0.25mg/mL was injected into the tube at a 2.0±0.1 µL/min flow rate using a microfluidic set-up. A variety of sutures were introduced along the lumen to a distance of 10 mm with the end outside the tube attached to the shaft of a stepper motor retracting the suture in a linear motion of 3 s/mm. Pressure drop and flow rate were recorded at 50 Hz (or every 7 µm). Each experiment was repeated in triplicate (Figure 1). Separately, a 6-0 Prolene suture (spacer) was inserted into a tube, and the tube was externally ligated using 8-0 Prolene at 2 locations. The suture was then slowly retracted from the tube and the pressure drop recorded as the suture passed each ligation (Figure 2).

Results : Monofilament sutures presented lower flow resistance compared to braided sutures of the same size. It was observed that larger sutures had higher flow resistance than smaller sutures for the same material. Dragging the suture along the lumen resulted in a non-linear decrease of the pressure drop, contrary to linear decrease using metal wires, showing the complex interaction between suture filaments and the tube. A decrease in pressure drop was observed with each ligation suture demonstrating flow titration.

Conclusions : Stenting techniques can be studied in detail using a microfluidic set-up combined with a stepper motor. The decrease of flow resistance when retracting the suture was non-linear. There is a complex interaction between flow control and suture placement/material within silicone tubes. The impact of the study may help standardise methods for early flow control when using the Baerveldt implant.

This is a 2020 ARVO Annual Meeting abstract.

 

Pressure drop of different suture size, material and length inside tube.

 

Pressure drop using a spacer and multiple external ligatures.