Imminent Replacements for Trabeculectomy

21 Imminent Replacements for Trabeculectomy

Paul Palmberg

Minimally invasive glaucoma surgery (MIGS) may soon be obsolete, as operations that appear to be as safe and are far more effective should soon be available in the United States. MIGS was intended to achieve an adequate intraocular pressure (IOP) lowering while reducing the complications associated with the gold standard, mitomycin C (MMC) trabeculectomy. Although MIGS is a safe procedure, the IOP control it offers has been less than optimal, and it as not reduced the need for supplemental medications.

Fortunately, new filtering operations now in Food and Drug Administration (FDA) clinical trials have been shown in studies outside the United States to match the effectiveness of a MMC trabeculectomy without the risk of hypotony maculopathy, and with a greatly reduced risk of bleb leaks or infection.

Both the AqueSys XEN (AqueSys, Aliso Viejo, CA; Figs. 21.1 and 21.2) and the InnFocus MicroShunt (InnFocus, Miami, FL; Figs. 21.3, 21.4, 21.5, 21.6) replace the scleral flap of a trabeculectomy with a small-diameter tube that sets a predictable transscleral pressure gradient at normal aqueous flow. This was achieved by employing the Hagen-Poiseuille law to calculate the internal diameter needed to produce the desired pressure gradient from one end of the tube to the other at normal aqueous flow. (The pressure gradient is proportional to the length of the tube, the viscosity of the aqueous, and the rate of aqueous flow, and inversely proportional to the tube internal diameter to the fourth power.)¹ Indeed, neither device has produced persistent hypotony or hypotony maculopathy in any of several hundred cases in studies done outside the United States. At 2.5 µL/min flow, the AqueSys XEN, with a 45- to 55-µm internal diameter and 6-mm length, sets a pressure gradient of ~ 7 mm Hg,¹ and the InnFocus MicroShunt, with a 62- to 70-µm internal diameter and 8.5-mm length sets a pressure gradient of ~ 6 mm Hg (in part due to hydrophobic effects). The AqueSys XEN is made of cross-linked porcine collagen, and the InnFocus MicroShunt is made of SIBS, a polystyrene plastic.

Fig. 21.1 The AqueSys XEN implant shown in relation to an Ahmed Implant (a) and an ultrasound biomicroscopy (UBM) image (b) showing the path of the flexible implant tube connecting the anterior chamber to the subconjunctival space. (AqueSys is not approved for sale in the United States; it has Investigational Device Exemption [IDE]-approved investigative status.) (© Copyright 2012. AqueSys and the AqueSys logo are registered trademarks of AqueSys, Inc.)

Fig. 21.2 Operative photo of the 27-g needle tip emerging under the conjunctiva 3 mm behind the superior nasal limbus, with the tip of the implant visible in the bevel just prior to delivering it ab interno. A portion of the inserter in which the needle is mounted is seen in the anterior chamber. The chamber is maintained with viscoelastic during the insertion, and is then rinsed. (AqueSys is not approved for sale in the United States; it has Investigational Device Exemption [IDE]-approved investigative status.) (© Copyright 2012. AqueSys and the AqueSys logo are registered trademarks of AqueSys, Inc.)

The two devices share the additional advantage of being placed through a 3-mm needle track in the sclera that avoids chamber shallowing during placement and therefore obviates the need for an iridectomy. The external portion of the tube extends an additional 2 to 3 mm behind the limbus, so that aqueous drains quite posteriorly, producing diffuse blebs.

In addition to the difference in the material of which these devices are composed, the devices also differ in that the AqueSys XEN is inserted ab interno across the anterior chamber, thus also eliminating the conjunctival flap, and the InnFocus MicroShunt is inserted ab externo after making a fornix-based conjunctival flap. The XEN inserter has a 27-gauge needle mounted at the tip, in which is loaded the dry implant. After advancing the device through an inferior temporal corneal incision across the anterior chamber toward the superior nasal angle and penetrating to the scleral surface under the conjunctiva (Fig. 21.2), the surgeon advances a slider on the handle that, due to an ingenious dual cam, simultaneously extrudes the implant and withdraws the needle tip, leaving the implant in position, with ~ 1 mm in the anterior chamber, 2 to 3 mm in the scleral needle track, and 2 to 3 mm in the sub-Tenon’s space. The InnFocus MicroShunt is placed through a 25-g needle track from the outside (Fig. 21.4), with 2 to 3 mm inside the anterior chamber, 2 to 3 mm in the scleral needle track, and 4 mm in the sub-Tenon’s space.

Both devices depend on the use of MMC for their maximum effectiveness. The XEN is on the market in Europe and Canada, with MMC used optionally in the majority of cases as a preoperative subconjunctival injection of 10 to 30 µg. Data collection for the XEN FDA trial for refractory cases should be completed soon.

The InnFocus MicroShunt is using Mitosol on sponges in the U.S. trial, limited to the FDA-approved MMC dose of 0.2 mg/mL, although trials conducted outside the U.S. suggested that an MMC dose of 0.4 mg/mL yielded even better efficacy, without the risk of hypotony. The MicroShunt study has completed phase I, is scheduled to be completed in 2016–2017, and is a randomized trial of the MicroShunt versus a MMC trabeculectomy in primary cases.

The devices use different strategies for fixation to avoid migration. The XEN is inserted dry, and when in place it imbibes fluid and swells, causing it to fit tightly in the needle track. The MicroShunt has a small fin 3 mm from the exterior end that seats snuggly in a 1-mm triangular pocket at the exterior needle entry site. Neither device requires a patch graft to avoid tube erosion to the surface because they exit 3 mm behind the superior limbus, well behind the excursion of the upper lid, and indeed no tube exposures have occurred in that location.