XEN: the evolution of the stent and technique
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Introduction: Microincisional or minimally invasive glaucoma surgeries (MIGS) can be categorized into internal drainage and subconjunctival drainage procedures. The XEN Gel Stent (Allergan, Irvine, CA, USA) was FDA cleared in 2016, making it the first subconjunctival MIGS device approved in the USA.
Description: The XEN Gel Stent is a hydrophilic tube constructed from porcine-derived gelatin crosslinked with glutaraldehyde and designed using the principles established by the Hagen-Poiseuille equation. Several implant models 6 mm in length with varying internal lumen sizes were developed. As their names suggest, the XEN140 had the largest internal diameter of 140 μm. XEN63 had a smaller lumen at 63 μm and XEN45 was the smallest at 45 μm.
Surgical technique: All steps pertaining to implanting the XEN45 should be performed with the following guiding principles: control inflammation, minimize bleeding, and reduce conjunctival resistance, which are described in detail later in the chapter.
Postoperative management: Management decisions are often nuanced and are made depending on intraocular pressure (IOP) level and bleb morphology, fully discussed in the chapter. A well-timed and executed bleb needling has been shown to be effective in lowering IOP long term.
Alternative approaches: More surgeons are now opting to place the XEN implant using the transconjunctival
technique and open conjunctival implantation; advantages and disadvantages will be addressed.
Updated injector: An updated injector was introduced to enhance the ergonomics of the delivery system for improved control over stent placement.
Conclusions: The XEN Gel Stent brings science to the art of glaucoma surgery by harnessing the principles of fluid dynamics. Applying the Hagen-Poiseuille equation in the design of the device has greatly reduced the risk of clinically significant hypotony, improving the safety of subconjunctival drainage procedures while maintaining effectiveness. Furthermore, reduced conjunctival manipulation may help limit proinflammatory factors predisposing to fibrosis.
New Concepts in Glaucoma Surgery Series: Volume 1, pp. 167-179 #12
Edited by: John R. Samples and Iqbal Ike K. Ahmed
© Kugler Publications, Amsterdam, The Netherlands
Video 1. Transconjunctival XEN implantation
Video Description: Video 1. Transconjunctival XEN implantation. The conjunctiva is marked at 3 mm from the limbus. The needle of the injector should enter the conjunctiva at least 7 mm posterior to the limbus. The needle is
advanced anteriorly under the conjunctiva and around the 3 mm marks it engages the sclera, tunneling through against countertraction. At the surgical limbus, the needle approach is steepened to enter the anterior chamber parallel to the iris plane. A gonioscopy lens can be used at this point to confirm needle entry in the anterior chamber, away from the iris and cornea. Once position is appropriate, the blue slider of the injector is pushed forward until the needle is retracted and the implant is released. No forward bias is needed during this step. After deployment, final positioning is confirmed, ideally following the “1-2-3” rule, as recommended by the manufacturer. Adjustment with toothless forceps can be done to the implant, as needed. A subconjunctival injection of MMC after stent placement is performed, which allows targeting fluid placement at the distal end of the implant. Video courtesy of Jonathan Myers.
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