This video demonstrates placement of a Xen implant using an external approach transconjunctivally to enhance supra-Tenon’s placement. To ensure adequate length of the Xen implant within the subconjunctival space, pull the injector back by 2mm from the scleral entry site as the slider is advanced forward. This typically leaves approximately 3mm of the Xen subconjunctival, 2mm intrascleral and 1mm in the anterior chamber. Both “ab-interno” and “transconjunctival ab-externo” Xen placement may have a place in surgical approaches.
Surgeon: Dr. Ike K. Ahmed, University of Toronto, Canada
This video describes a transconjunctival Xen implantation approach. 7-0 vicryl suture is placed in the superior corner for traction control, and a mark is made 2.5 millimeters back from the limbus, and a second mark will be made 2.5 millimeters back from that. These are important landmarks that we’ll discuss. Remember, with the current Xen injector, which is designed to be placed ab interno, once the implant is fully deployed, and the needle is fully retracted, only 1 millimeter of the implant will be visible from the distal end of that cannula. And therefore if it’s placed ab externo, if you place the cannula at the scleral exit point, and we do not withdraw the cannula, only 1 millimeter of the implant would be visible. So when we start the implantation, the needle will enter the limbus, about 2.5 millimeters from the back. That’s where the cannula will be at the initial entry point. At the initiation of the deployment. However, once we finish the deployment, we wish the cannula to be back another 2.5 posterior to the initial mark, which will leave about 2.5 back from that scleral exit point. These are important landmarks to remember, as the cannula position in relationship to the scleral entry point will be important in terms of how much of the Xen will be visible in the subconj space. The needle enters the subconjunctival space about 7+ millimeters back from the limbus. We typically start with a tangential approach, being careful to ensure the needle is just visible under conjunctiva, skimming under the conjunctiva, staying above Tenon’s. Once we reach 2.5 millimeters back from the limbus, we will enter the sclera, and at this point, we will turn the cannula and the needle slightly downward, about 20-degree downward angle, to enter the anterior chamber. We ideally want to enter the anterior chamber at the level of the TM or slightly above it. It’s important not to get too deep, which is easy to do in a blind approach like this. Once we see the needle bevel, we will then allow the implant to be pushed forward just slightly. Maybe about a quarter push of the slider. At this point, stop. Pull the entire injector back about a millimeter. And then continue to go about halfway to the travel distance of the slider. At this point, the entire Xen is deployed. We can pull back another millimeter, and then finally we can push the slider all the way forward, to ensure the needle is withdrawn in the cannula. This ideally will create about a 2.5 millimeter length of that implant from the exit of the sclera. A pair of Fechtner forceps can be used to adjust the positioning. We do want to make sure we have at least 2 millimeters exiting the sclera, to ensure we avoid entrapment. We see a nice mobile and a free implant, which is ideally to show us in the supra-Tenon’s space. Here’s our exit point in the anterior chamber, showing that we’re above the TM, just below Schwalbe’s line. Again, just to show the facility of the implant, as you move it back and forth, to show that we have a nice free and mobile implant, which is ideally seen in supra-Tenon’s. Priming the bleb. This is optional here. Viscoelastic is optional as well. But we see a nice bleb forming here to show that we primed it well. At this point, we typically inject mitomycin C after deployment. This allows us to do it afterwards. Doing it before may make the visibility of the implantation procedure a bit difficult. Typically, we’re using about 0.4 milligrams per CC. About 0.1CC is injected into the subconj space. Keeping the mitomycin away from the limbus is important to avoid limbal avascularity. The transconjunctival Xen implantation technique may have some advantages, to ensure the implant sits above Tenon’s. However, this injector is designed for ab interno delivery. Therefore, it’s important to know when to push the slider forward, as well as pull the entire injector back to ensure we have an adequate length in the subconj space.
November 18, 2019