There are unique benefits to an ab externo placement of the Xen Gel Stent, which have led many surgeons to include this technique to their surgical armamentarium. The first benefit is to expand the targeted area of implantation. When the stent is placed ab interno, it is typically left in the superonasal quadrant as it is difficult to pivot the injector in the main incision and move further temporal. An ab externo approach provides easy accessibility to the entire superior quadrant. The ab externo technique is especially advantageous in patients who have had previous trabeculectomy in the superior or superonasal areas, as the Xen Gel Stent can be placed much further from pre-existing scar tissue than with an ab interno approach.
A second benefit to an ab externo approach is that it avoids maneuvers inside the anterior chamber and obviates the need for corneal incisions and the use of viscoelastic. Absence of maneuvers inside the anterior chamber may eliminate reported and potential complications, such as corneal wound leaks, retained viscoelastic and consequent IOP spikes, lens or endothelial cell touch, among others . In phakic eyes, the ab externo approach essentially eliminates the risk of inadvertent damage to the lens capsule. This benefit is especially important in phakic patients with significant lens rise and a shallow chamber where concomitant phacoemulsification may not be indicated.
Third, the ab externo technique may be the easiest transition technique for glaucoma surgeons new to the use of the Xen Gel Stent, as it most closely utilizes existing surgical skills and does not involve any new angle-surgery skills. Opening and closing of the conjunctiva is similar to trabeculectomy, and the insertion technique with the Xen injector needle is very similar to the creation of a needle track for the insertion of tube shunts.
Finally, as mentioned above, the open conjunctiva approach allows the surgeon to dissect or move the Tenon’s capsule away and to easily make micro-adjustments of the stent by grasping it directly. The stent can be pulled out or pushed further into the anterior chamber to ensure that it is positioned properly.
Open Conjunctiva, Ab Externo Placement Technique
A 7.0 Vicryl (polyglactin 910; Johnson & Johnson Vision) traction suture may be placed in the superior cornea to rotate the eye looking down. A peritomy (around 2 mm for RC and 1 clock hour for DY), ideally at 12 o’clock, followed by a blunt dissection to break the adhesions between the conjunctiva and the Tenon’s at the limbus, is more than sufficient to allow for delivery of the device (Fig. 9).
As with trabeculectomy, the vessels can be cauterized, if desired. Next, a Tenon’s incision is made to enter sub-Tenon’s space and expose the bare sclera (Fig. 10).
An optional tenectomy can be performed in cases of significant Tenon’s thickening due to race, inflammation, scarring from previous surgery or other risk factors (Fig. 11). Performing a limited tenectomy may decrease the risk of stent obstruction/early failure. However, this may potentially lead to a higher risk of erosion through the conjunctiva. The Xen injector is then placed bevel up in the sclera, 2.0–2.5 mm from the limbus (Fig. 12); the original traction suture can be released and used for counter traction as the applicator needle is advanced through the sclera.
Advance the needle superficially until the tip is at the surgical limbus. The needle is then tilted downwards by about 30° and advanced until it enters the anterior chamber. The most common problems encountered during the delivery process are amputation of the stent or pulling the stent back out of the anterior chamber due to the tendency of the needle to flick upwards. These can be avoided by rotating the injector needle 90° or beveling down prior to deploying the stent and ensuring there is no upwards or side pressure on the injector. One option to avoid any issues when deploying the stent is to pull the injector back slightly (the sleeve of the injector should be about 1 mm from the scleral needle entry point) prior to pushing the blue slider forward. There is no need for forward pressure for the needle to stay in place within the scleral tunnel unlike with an ab interno delivery approach.
Once the stent is fully released, the flow of aqueous can be confirmed. As this technique does not create an entry into the anterior chamber, the anterior chamber remains very stable without the use of viscoelastic. At this point, a slow but steady flow of aqueous through the device can usually be visible. If no flow is seen, it is often due to peri-tubular flow. One of the authors (D.Y.) uses a pre-placed 10.0 nylon encircling suture (before the injector enters the sclera) approximately 1 mm posterior to the limbus to reduce peri-tubular filtration and the risk of early hypotony (Fig. 13). The Xen injector is then placed bevel up 1 mm posterior to the pre-placed scleral suture (Fig. 14). Beading of aqueous will frequently start as soon as an encircling suture is tied (Fig. 15) in these cases.
Once flow has been confirmed, the length and position of the Xen Gel Stent can be adjusted with tying forceps so that it is 1 mm in the anterior chamber (Fig. 16).
If the tip of the Xen Gel Stent is lifting off the wall of the sclera, it may be advantageous to secure the tip of the stent back against the scleral wall with a 10.0 nylon suture. This will reduce the risk of conjunctival erosion, especially in patients where a tenectomy was necessary.
The Tenon’s layer and conjunctiva are then pulled anteriorly and closed (in two steps or both layers at once) with Vicryl sutures (Fig. 17) or 10-0 Nylon, creating a watertight closure with two wings or a running suture technique, as per surgeon preference. Care should be taken when pulling the conjunctiva forward to avoid dragging the Xen into the anterior chamber. Ideal placement with this technique is the Xen Gel Stent tip to be either directly under the conjunctiva (when generous tenectomy is performed) or tucked inferior underneath Tenon’s capsule.
XEN Ex open Conjunctiva (MP4 67809 kb)
Open Conjunctiva XEN implantation via ab externo (no forward bias) (MP4 39220 kb)
Open Conjunctiva XEN implantation via ab externo (injector pulled back) (MP4 113191 kb)
Open Conjunctiva XEN implantation via ab externo placement (MP4 111191 kb)