Innovative material helps implant lower IOP safely, effectively

Take-home message: The XEN Gel Stent is a micro-fistula implant designed to lower IOP without relying on physiologic outflow pathways – it connects the anterior chamber to the subconjunctival space. Prospective data from more than 900 surgeries from around the world support the safety and efficacy of this device.

By Arsham Sheybani, MD, Special to Ophthalmology Times

The XEN Gel Stent (AqueSys Inc.) is a micro-fistula implant for the treatment of glaucoma that connects the anterior chamber to the subconjunctival space. Conceptually, it lowers IOP by allowing fluid to drain to the subconjunctival space in a similar yet distinctly different method as trabeculectomy and tube shunt surgery.

The goal of the XEN Gel Stent is to lower IOP without relying on physiologic outflow pathways (TM and uvealscleral) while offering a simpler procedure and a favorable safety profile compared to traditional subconjunctival filtration surgeries.

The injector system for the XEN Gel Stent allows for subconjunctival filtration to be performed without creating a conjunctival incision. This reduces tissue trauma and the chance of early postoperative wound leaks. The injector holds the implant in a 27-gauge, dual-beveled needle. (Photo courtesy of AqueSys)Although there are no trials comparing the gel stent to trabeculectomy or tube shunt surgery, prospective data from more than 900 surgeries from around the world support the safety and efficacy of this device for the surgical treatment of glaucoma. 

The XEN Gel Stent is made of porcine gelatin that is cross-linked with glutaraldehyde. As it is hydrophilic, the device hydrates, swelling to secure itself into scleral tissue to limit movement without requiring additional surgical fixation methods such as flap creation, suture, or fibrin glue. The high level of crosslinking prevents degradation of the implant in order to achieve permanence of the device when implanted. The material is essentially inert, as histologic studies show no fibrosis in contact with the implant in canine eyes, unlike the fibrotic reaction formed around silicone-derived materials.¹

The implant material also is highly malleable when compared to silicone tubing used in tube shunt surgery. This flexibility means that it not only bends easily but also conveys less force against tissue once implanted. This, along with the relatively small size of the device, limits erosion of the device without the need for graft or donor tissue reinforcement.

The XEN Gel Stent comes in three models that vary in internal lumen diameter. The XEN 45 implant has around a 45-µm inner lumen (the exact lumen size is proprietary). As it is 6 mm in length, the combination of length and internal lumen diameter provide roughly 6-8 mm Hg of resistance to flow, as calculated by the Hagen-Poiseuille equation and verified by laboratory flow testing.²This provides hypotony protection without the use of a valve or need for subjective suture tension adjustments (as in trabeculectomy and Ex-Press shunt surgeries).

The combination of material and implant design account for the safe IOP-lowering ability of the device. But the implant itself is only part of the ingenuity. The implant injector system is what allows for subconjunctival filtration to be performed without creating a conjunctival incision. This dramatically reduces tissue trauma and the chance of early postoperative wound leaks.

The injector houses the implant in a 27-gauge dual beveled needle. The injector system allows for device deployment through a clear corneal incision in such a manner that leaves the implant in the desired location in the subconjunctival space while retracting the needle into the injector. Importantly, the surgeon only needs one hand on the injector in order to perform this maneuver.

The surgical procedure

The surgical procedure starts with identifying the surgical limbus and marking the conjunctiva with surgical ink approximately 2.5 mm - 3.0 mm posterior to the limbus in the area of desired implantation. The surgeon can perform the procedure from a temporal or superior position. A paracentesis into clear corneal is made (it can also be placed through a cataract surgery wound) approximately 1.2 mm - 1.6 mm in width. The anterior chamber is then filled with a cohesive viscoelastic.

Some surgeons prefer the use of a gonioscopy mirror to identify the TM. The injector housing the implant is advanced through the clear corneal incision and directed to the scleral spur. A second instrument or even viscoelastic cannula through the paracentesis incision provides counter traction while the injector needle is driven through the scleral spur. The surgeon then directs the needle through sclera at a trajectory to ensure that it arises through the sclera and under the conjunctiva at the previously marked distance from the limbus.

The injector is then rotated toward the surgeon slightly to facilitate implant placement along the sclera. The surgeon now advances the thumb slide along the injector, which advances the implant to the subconjunctival space while retracting the needle, leaving the implant in the desired location with part of the implant in the subconjunctival space, sclera, and anterior chamber. Waiting 30 seconds before evacuating the viscoelastic allows for the device to swell in order to hold itself in place. After verifying placement in the angle with the gonioscopy mirror, the surgeon exchanges the viscoelastic for balanced salt solution (BSS). At that time, a bleb should form over the device in the subconjunctival space once the BSS from the anterior chamber moves through the implant. There is no flap to secure, no patch graft to place, no conjunctiva to suture. Usually hydration of the clear corneal incision is enough, but some surgeons prefer suture closure of the wound.

His perspective

My perspective with the device is unique in that I was able to place the implant while in fellowship with Ike Ahmed, MD, as well as for the U.S. FDA trial. While in Canada, we injected mitomycin (MMC) in the area of desired implantation. To use it without MMC is possible, but results in a higher fibrosis and needling rate.

The XEN implant fits the mold of micro-invasive surgery but it has the potential to achieve lower IOPs than canal surgery. Compared to traditional subconjunctival filtration surgery, the procedure is less invasive, the implant profile is smaller, and the subjective nature of flow regulation by suture tension as in trabeculectomy is eliminated. I am excited to have the XEN Gel Stent in our armamentarium for glaucoma surgery.

References

• Lewis RA. Ab interno approach to the subconjunctival space using a collagen glaucoma stent. J Cataract Refract Surg. 2014;40:1301-1306.

• Sheybani A, Reitsamer H, Ahmed II. Fluid Dynamics of a Novel Micro-Fistula Implant for the Surgical Treatment of Glaucoma. Invest Ophthalmol Vis Sci. 2015;56:4789.

Arsham Sheybani, MD

Dr. Sheybani is an AqueSys consultant and principle investigator for its FDA trial.