The implants in this group use subconjunctival filtration to establish a nonphysiological route for aqueous outflow akin to traditional trabeculectomy and aqueous shunt surgery. Therefore, they require subconjunctival mitomycin C (MMC) injection pre-insertion to optimise bleb function and survival.
XEN Gel Stent
Subconjunctival filtration represents a familiar route for aqueous outflow, and is the basis of trabeculectomy and aqueous shunt procedures. The XEN gel stent (Allergan, Dublin, Ireland) is an ab interno gelatin stent that is implanted via a clear corneal incision, avoiding conjunctival dissection. It is 6 mm in length and composed of porcine gelatin crosslinked with glutaraldehyde. Three models have been evaluated with inner diameters of 45, 63 and 140 μm , with 45 μm being recommended by the manufacturer (Fig. 3). Due to Poiseuille’s law of laminar flow, the length and inner diameter of the tube determine the rate of flow, and thus the flow resistance—which prevents hypotony. Preclinical tests established that the implant does not occlude inside the lumen and the implant material does not cause a tissue reaction in the eye .
There are few published studies on the XEN 45 implant. Two pilot studies [19, 20] have been published. One study investigated the insertion of one XEN implant (the 63 or 140 model) +MMC combined with cataract surgery, and demonstrated a reduction of IOP from 22.4 (±4.2) mmHg to 15.4 (±3.0) mmHg at 12 months; there was a reduction in glaucoma medications from 2.5 ± 1.4 to 0.9 ± 1.0 .
In another pilot study using XEN 140 model implantation alone +MMC (n = 49 eyes), 40% had unqualified success at 12 months (IOP ≤18 mmHg and ≥20% reduction in IOP), 89% had qualified success, and most cases involved a previous failed trabeculectomy .
In those studies, there were no serious adverse events such as tube erosion or prolonged hypotony, but some patients had injection of ophthalmic viscosurgical device into the anterior chamber, particularly in the XEN 140 group.
A small prospective interventional study of 13 cases (XEN 45 +MMC)—mostly combined procedures (n = 10), with a follow-up period of 12 months—was performed recently . IOP dropped from 16 ± 4 mmHg pre-op to 9 ± 5, 11 ± 6, 12 ± 5, 12 ± 4 and 12 ± 3 mmHg at 1 week, 1, 3, 6 and 12 months (p = 0.004, 0.026, 0.034, 0.01 and 0.01, Wilcoxon signed-rank test) consecutively. Mean number of medications dropped from 1.9 ± 1 preoperatively to 0.3 ± 0.49 (p = 0.003) at 1 year. 42% of eyes achieved complete success (IOP reduction of at least 20% from baseline without medications) and 66% achieved qualified success. Complications included choroidal detachment in 2 eyes and implant extrusion in 1 eye, and 2 eyes underwent trabeculectomy.
Obviously, larger studies are required to evaluate long-term efficacy and late complications. Results of the phase 4 APEX trial using XEN 45 will be available later this year.
The InnFocus microshunt (Santen Pharmaceutical Company Ltd, Osaka, Japan) is an ab externo drainage device. It involves more steps akin to trabeculectomy compared to other MIGS. The material is SIBS (polystyrene-block-isobutylene-block-styrene) which has been developed specifically for medical implants. It is a biocompatible and biostable thermoplastic elastomer (Fig. 4).
In one study (n = 23 eyes) of microshunt +MMC, 80% had IOP ≤14 mmHg at 3 years, but some cases were standalone and others were combined procedures. The mean IOP for the entire group was 10.7 ± 1.5 mmHg at 3 years; the qualified success rate was 95%, with a reduction in medications from 2.6 ± 0.9 to 0.8 ± 1.2 .
Transient hypotony and transient choroidal effusion occurred in 13% and 8.7%, respectively, but with spontaneous resolution. There were no serious long-term adverse events, leaks, erosions, migrations or infections.
The results of the RCT of InnFocus microshunt versus trabeculectomy are awaited.