BACKGROUND AND RATIONALE The XEN gel stent (Allergan inc.) is a gelatin stent that has been developed for the surgical treatment of medically uncontrolled glaucoma and allows the filtration of aqueous humour from the anterior chamber to the subconjunctival space. The ab interno placement and outcomes of the XEN implant has been described (1,2,3). We describe an ab externo approach to implant the XEN microstent; ab externo closed conjunctiva implantation. This approach has many potential advantages over the ab interno technique, notably a quicker procedure, a potentially better subconjunctival placement of the implant and cost effectiveness due to absent need to rely on an operating room to perform the procedure as it can be done in an office setting.

Ab interno technique Approximately 6 mm long and as wide as a human hair, this soft gelatinous stent is implanted via an ab interno surgical approach through a small, self-sealing corneal incision, to facilitate aqueous humor drainage from the anterior chamber to the subconjunctival space. The gelatin imparts hydrophilic properties that allow the Gel Implant to conform to the ocular tissue by expanding when hydrated upon contact with the aqueous humor. This expansion might minimize issues such as migration, erosion, and corneal endothelial damage observed in stents with synthetic materials. The ab interno surgical approach for XEN is less invasive than traditional glaucoma surgeries and has several other advantages, including minimal trauma to the conjunctiva, maintenance of the natural drainage pathways, reduced scarring response, and the ability to accurately place the device under direct visualization of anatomy. Minimizing damage to the conjunctiva allows the possibility of future glaucoma surgery. Ab externo technique The transconjunctival ab externo technique is modification of the ab interno procedure. Topical anesthesia with tetracaine 0.5% eye drops is performed before the ocular surface is disinfected with 5% povidone iodine drops. A full blade speculum was used to retract the patient's eyelid, and the patient was positioned at the slit lamp. While the patient gazed down toward the nose, a mixture of an antimetabolite, lidocaine, and epinephrine was injected in the exposed surgical site of the superotemporal quadrant. The antimetabolite is mitomycin C (MMC). The mixture contained either 0.1 cc of MMC (0.2 mg/ml) and 0.05 cc of 2% lidocaine with epinephrine, for a total volume of 0.15 cc. The speculum was then removed. After 5 to 7 minutes, topical anesthesia and disinfection were repeated, and the speculum was secured again. An inserter with a preloaded gel stent is directed to enter the conjunctiva approximately 2 to 3 mm lateral to the intended scleral entry point, and 6 to 7 mm from the limbus, using an aseptic no-touch technique. The inserter was then directed downward to enter the sclera 2.5 mm posterior to the limbus. Once the tip of the inserter needle was visualized in the AC, the plunger of the inserter was advanced while the inserter needle was slowly and simultaneously withdrawn until the gel stent was fully released. The ideal target position of the stent is approximately 3 mm within the subconjunctiva, 2 mm within the sclera, and 1 mm within the AC.

The positioning of the stent was considered successful if a connection was made by the stent between the subconjunctival space and the AC, and a bleb formed. Gonioscopy was performed to confirm proper seating of the gel microstent and slit lamp biomicroscopy was performed to confirm a formed bleb.