Translimbal Collagen Implant: AqueSys XEN Gel Stent

IID Transconjunctival Filtration Procedures


37 Translimbal Collagen Implant: AqueSys XEN Gel Stent


Herbert A. Reitsamer, Markus Lenzhofer, Melchior Hohensinn, Arsham Sheybani, Iqbal Ike K. Ahmed, and Vanessa Vera


Case Presentation


A 58-year-old Caucasian man with progressive primary open-angle glaucoma treated with topical prostaglandin analogue and a beta-blocker presented with increasing intraocular pressure (IOP) in his left eye with questionable adherence to the therapy. The decision to recommend surgery was supported by the patient’s increasing discomfort with various prostaglandin analogues. Preoperative IOPs were 19 mm Hg OD and 26 mm Hg OS. Gonioscopy revealed a medium deep anterior chamber with a Shaffer angle grade 2 (Fig. 37.1a [bottom]), and visual acuity was 20/20 in both eyes. Fig. 37.2 shows the visual field data and the optic disk topography both preoperatively and 5 years after surgery for both eyes.


In a day-clinic setting, surgery with the XEN Gel Stent (AqueSys Inc., Aliso Viejo, CA) was performed with local anesthesia. Two weeks before the surgery, the patient started treatment with preservative-free topical corticosteroids t.i.d. and topical glaucoma therapy was replaced by systemic carbonic anhydrase inhibitor (250 mg o.d.). Before implantation of the XEN Gel Stent, an off-target-site injection of 10 µg mitomycin C (0.1 mL solution) into the sub-Tenon’s space was performed. To remain as nontraumatic as possible, the mitomycin depot was repositioned from the injection site into the target zone by directional massage with a cotton swab. The stent was then implanted into the superior nasal quadrant without complications (Fig. 37.1b–d). The patient was released on the same day of surgery and returned the next day for the first follow-up visit. No cycloplegia/pupil dilation was used in the postsurgical regimen. The day after surgery, the visual acuity was 18/20 and the IOP was 8 mm Hg. During the first 3 months, the highest IOP value was 14 mm Hg, which was followed by a consistent decrease until month 12. Final pressure after 5 years was between 11 and 12 mm Hg, with a beta-blocker (preservative free) and stable visual fields in both eyes. Visual acuity was stable at 20/20 from the first week after surgery on (Fig. 37.1).


The Procedure


The AqueSys XEN Gel Stent was introduced to provide a new form of filtration surgery for lowering IOP that meets the highest standards of safety and efficacy for the treatment of early as well as advanced glaucoma. Both glaucoma only as well as combined cataract–glaucoma procedures are within the field of application of the XEN Gel Stent. At present, more than 2,000 stents were implanted in patients with early to advanced glaucoma and the results are promising. Postsurgical follow-up is available up to 5 years with excellent safety and efficacy data.1,2 So far, the XEN Gel Stent is the only ab interno device available that utilizes subconjunctival filtration for aqueous drainage.


Rationale Behind the Procedure


The XEN Gel Stent is a hydrophilic tube composed of a porcine gelatin and cross-linked with glutaraldehyde (Fig. 37.3). It decreases the IOP by creating a permanent outflow pathway from the anterior chamber to the subconjunctival space through which the aqueous humor can flow. From the subconjunctival space, the aqueous humor has numerous potential drainage pathways, including diffusion through the conjunctiva, diffusion into the venous system of the sclera and conjunctiva, as well as potential lymphatic pathways (Fig. 37.3).


During the implantation procedure, the stent is rigid and retains its straight form (which allows it to be implanted into sub-Tenon’s space). Once in place, the stent immediately hydrates to become soft and highly flexible. As aqueous humor is introduced, the implant also swells to its final dimensions. This causes an increase in diameter, which prevents migration inside the implantation channel. Clinical findings demonstrate that the stent retains its position after implantation. The cross-linked collagen is durable and permanent, which provides a variety of desirable characteristics. The material has an extensive track record for medical use in a variety of geographic regions, including the European Union, the United States, Japan, and Canada. The XEN Gel Stent is made with gelatin that meets the compendium requirements of the European Pharmacopeia. The biocompatibility properties of gelatin are well established, and early clinical trials of the XEN Gel Stent also show a remarkable lack of foreign-body reactions in the human eye.3


The XEN Gel Stent has several key advantages when compared with other minimally invasive glaucoma therapies, as well as when compared with more traditional filtration procedures:


• The stent is implanted with a minimally invasive procedure with an excellent safety profile that reduces risk to the patient and minimizes damage from the surgery.


• The ab interno approach eliminates the need to surgically opening the conjunctiva and the need to perform scleral flap, thereby reducing postoperative inflammation and scarring.


• The procedure entails minimal damage to the conjunctiva and tissues, and multiple and repeatable implantations over the lifetime of the patient are feasible, if necessary.


• The procedure bypasses the trabecular meshwork, Schlemm’s canal, and the collector channels entirely, thus avoiding the major structures of resistance for aqueous humor.


• The procedure entails low and diffuse outflow into intact tissue anatomy and drainage pathways in the conjunctiva, giving maximum efficacy pressure reduction.


• The current model, which has an inner diameter of 45 to 55 µm, avoids postoperative hypotony.





The laminar flow through a tube is calculated using the Hagen–Poiseuille equation. Using this equation, AqueSys has created three different versions of the XEN Gel Stent. The equation is as follows:



images

where Φ = volumetric flow rate; ΔP = pressure difference between the two ends of the tube; η = dynamic fluid viscosity; L = length of the tube; R = internal radius of the lumen of the tube.


The Hagen-Poiseuille equation describes the relationship between the diameter and length of a tube and the resistance it develops when a fluid with certain properties passes through. This formula describes the hydrodynamic principle that the XEN Gel Stent’s design is based on.


Table 37.1 shows different versions of the XEN Gel Stent. The XEN 140 and XEN 63 models entailed postoperative episodes of hypotony, but this does not occur with the latest version of the stent—the XEN 45.


Patient Selection


There are two important selection criteria for patients being considered for XEN Gel Stent implantation: (1) a chamber angle of Schaffer grade 2 or wider; and (2) the conjunctiva can accommodate bleb formation, which means that target zones with conjunctival scarring are not suitable for implantation. In combined cataract–glaucoma surgeries, the chamber angle is less of an issue because the lens is extracted before the XEN Gel Stent is implanted. A prospective clinical trial with narrow-angle patients has not yet been performed. However, patients with narrow angles have been implanted, and the early results suggest that the stent may be indicated in such cases; further studies will be required to confirm this. Until now, not many patients with uveitic glaucoma were treated with the XEN Gel Stent, and further experience with this group of patients also needs to be gained.


With the exception of the considerations mentioned above, any patient with a conjunctiva allowing for bleb formation is well suited for surgery with the XEN Gel Stent. This essentially means that if the conjunctiva and/or Tenon’s capsule is scarred over due to previous surgeries or missing due to chemical burns or other trauma, the patient is not a candidate for filtration surgery. However, because the XEN Gel Stent gives rise to smaller blebs than does classic trabeculectomy, the exclusion criteria might be less strict.


The XEN implantation is performed in a day-clinic setting, and patients are discharged the same day of surgery. It is recommended that patients come in for a follow-up visit the next day, either at the day clinic or at their physician’s office.


Surgical Technique


The implantation is performed from the inside of the anterior chamber (ab interno approach). This surgical approach is fundamentally different from other filtration surgeries currently used. The major advantage of an ab interno approach is conjunctiva preservation compared with approaches from outside the eye (ab externo), in which the conjunctiva needs to be dissected and displaced. The expectation from the minimally invasive XEN procedure is reduced scarring and reduced numbers of bleb failures.


The mechanism of action of the XEN procedure is fundamentally consistent with other full-thickness surgical treatments for glaucoma such as valved and nonvalved tube shunts and trabeculectomies (which, like XEN, bypass all potential outflow obstructions). However, it also mitigates several of the limitations of those technologies. The XEN Gel Stent maintains a micro-fistula between the anterior chamber and the subconjunctival space while the tissues surrounding the implant heal naturally. There is no need to create an iridotomy, and by introducing only a minimal amount of trauma, subsequent inflammation and fibrosis is minimized and many of the complications associated with more invasive procedures such as trabeculectomy and tube shunt implantation can be avoided.


Table 37.1 XEN Implant Model Variations by Internal Lumen Size

























Name and Inner Diameter (ID)


Stent Length


Photo of the Stent


XEN 140 (~ 140 µm ID)


6 mm


images


XEN 63 (~ 63 µm ID)


6 mm


images


XEN 45 (~ 45 µm ID)


6 mm


images


Note: During the development of the XEN Gel Stent, three different designs were used in clinical studies. Originally it was thought that higher diameters might serve better in advanced glaucoma, delivering lower intraocular pressures due to larger openings. The major disadvantage of the larger diameters (140 µm and 63 µm) is that over an implant length of 6 mm, the larger diameters do not add enough resistance to aqueous humor flow to avoid postoperative hypotony. For this reason, the two models were replaced by the XEN 45 Gel Stent, which is currently implanted worldwide. The XEN 45 adds enough resistance to aqueous flow to avoid hypotony after implantation in patients with roughly normal aqueous humor production (2–2.5 µL/min). Any local therapy that decreases aqueous production consequently lowers the pressure drop along the XEN Gel Stent, hence lowering intraocular pressure even further.


During the course of development, the injector of the XEN Gel Stent underwent several changes in function and design; the current version is shown in Fig. 37.4. It can be operated with a single hand and the implantation can be performed with the surgeon positioned temporally (Fig. 37.4a) or superiorly (Fig. 37.4b) to the patient’s head. A sliding mechanism moves various internal parts including the implantation needle to provide proper placement and handling of the implant. The device utilizes a small 27-gauge needle, and the injector is designed to both protect the XEN stent and to accurately place the implant into the correct anatomic location. It reaches from the anterior chamber (0.5 to 1.0 mm), through the angle, exits the sclera 2.5 to 3.5 mm posterior to the limbus, and extends into the subconjunctival and sub-Tenon’s space (2.0 to 2.5 mm).


The preloaded/single-use injector comes individually packaged and sterile. After standard ophthalmic preparation is completed, the surgeon inserts the injector into the peripheral cornea and directs the needle across the anterior chamber to the angle to achieve the ideal scleral length of 3 mm (Fig. 37.5a). The entry zone of the angle is a broad and forgiving area, giving the surgeon the flexibility of deciding whether to use gonioscopy during the procedure, in contrast to other minimally invasive glaucoma surgery (MIGS) procedures in which a specific target tissue landmark in the angle must be achieved. The needle can enter the angle anywhere from the Schwalbe’s line to the scleral spur (Fig. 37.5b). The needle then goes through the sclera and into the subconjunctival/sub-Tenon’s space, creating a slit as it cuts through the tissue.


Oct 29, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Translimbal Collagen Implant: AqueSys XEN Gel Stent
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