Trabecular bypass stents are one of the surgical techniques classified as minimally invasive glaucoma surgery (MIGS)5). MIGS is performed via an ab interno approach, without incising the conjunctiva, minimizing tissue trauma5).
In open-angle glaucoma, there is increased resistance to aqueous humor outflow at the level of the trabecular meshwork. Trabecular bypass stents penetrate the dysfunctional trabecular meshwork, creating a direct drainage pathway from the anterior chamber to Schlemm’s canal6). This bypasses the trabecular resistance, facilitating aqueous outflow and lowering intraocular pressure.
MIGS is safer than traditional glaucoma surgeries (trabeculectomy or tube shunt surgery), but the intraocular pressure lowering effect is somewhat inferior5). There is a trade-off between safety and efficacy, and traditional surgery is indicated for advanced cases.
MIGS (minimally invasive glaucoma surgery) is a collective term for low-invasive glaucoma surgeries. It uses an ab interno approach, preserving the conjunctiva and minimizing tissue manipulation5). In addition to trabecular bypass stents (iStent, Hydrus), it includes trabeculotomy (Trabectome, gonioscopy-assisted transluminal trabeculotomy) and canaloplasty. Compared to traditional filtration surgery, the risk of complications is lower and visual recovery is faster, but the intraocular pressure lowering effect is modest, so mild to moderate glaucoma is the main indication5).
iStent (Glaukos)
First generation: Snorkel-shaped titanium device (1.0×0.33 mm, lumen diameter 120 µm). Heparin coated. FDA approved in 20125)
Second generation (iStent inject): Contains two conical stents. Four 50 µm openings in the head. FDA approved in 20185)
iStent inject W: An improved version with an enlarged flange2)
Hydrus Microstent (Alcon)
Structure: An 8 mm long crescent-shaped scaffold made of nitinol (nickel-titanium alloy). Occupies approximately 3 clock hours (90 degrees) of Schlemm’s canal6)
Features: Expands Schlemm’s canal to 4–5 times its natural width, preventing collapse. Recovers shape after deformation due to superelasticity
iStent and Hydrus are approved for use in combination with cataract surgery in adults with mild to moderate open-angle glaucoma being treated with topical ocular hypotensive medications5). They are used for primary open-angle glaucoma as well as pigmentary glaucoma and pseudoexfoliation glaucoma1).
They are contraindicated in angle-closure glaucoma and neovascular glaucoma. Use is also not recommended in conditions with elevated episcleral venous pressure (e.g., thyroid eye disease, Sturge-Weber syndrome).
iStent is a small titanium stent that creates a focal bypass at one point in Schlemm’s canal5). Hydrus is an 8 mm nitinol scaffold that dilates and supports approximately 90 degrees (3 clock hours) of Schlemm’s canal6). Because Hydrus covers a wider area of Schlemm’s canal, it is expected to provide access to more collector channel openings. The COMPARE trial showed that Hydrus achieved better intraocular pressure reduction than iStent5).
A glaucoma workup including gonioscopy is necessary. Assess the openness of the angle, presence of peripheral anterior synechiae (PAS), and neovascularization. Evidence suggests that collector channels are located near areas of trabecular pigmentation, which can guide stent placement.
The same clear corneal incision as for cataract surgery is used. Using an intraoperative gonioscope (e.g., Swan-Jacob type), the nasal angle is visualized, and a preloaded inserter is used to insert the stent into Schlemm’s canal through the trabecular meshwork 1). In the second generation (iStent inject), the trabecular meshwork is punctured perpendicularly, and two stents are placed sequentially 5).
Insertion can be performed either before or after cataract surgery. Preoperative insertion provides less corneal edema and better visualization, while postoperative insertion allows for a deeper angle and improved access.
A preloaded inserter is inserted through a 1.5 mm clear corneal incision. The cannula tip is advanced from the trabecular meshwork into Schlemm’s canal, and the microstent is deployed using the tracking wheel.
Topical antibiotics and anti-inflammatory agents (steroids and NSAIDs) are used 1). Glaucoma eye drops may be temporarily discontinued to evaluate stent efficacy. The decision to restart or discontinue eye drops is based on postoperative intraocular pressure.
| Complication | Frequency | Notes |
|---|---|---|
| Hyphema | 2–4% | Usually resolves within one week |
| Stent malposition | 3–17% | May require repositioning |
| Transient intraocular pressure elevation | Reported | Stabilizes within a few days |
Hyphema is often due to blood reflux from Schlemm’s canal through the stent, and may also be a sign of distal outflow patency 1).
In a prospective case series of 78 eyes by Kozera et al., no serious intraoperative complications were observed with iStent implantation combined with cataract surgery 1). Early postoperative microhyphema and corneal edema were observed in 7 eyes, all resolving within one week 1).
UGH syndrome: A syndrome of recurrent uveitis, glaucoma, and hyphema after iStent implantation has been reported 4). It was hypothesized that zonular weakness due to pseudoexfoliation syndrome and nighttime positional changes promote contact between the iris and the stent, causing microtrauma 4). Symptoms resolved after stent removal 4).
Endophthalmitis: Acute endophthalmitis caused by Staphylococcus epidermidis has been reported after cataract surgery combined with iStent inject 3). It was treated with vitreous tap and intravitreal antibiotics, and vision recovered to 20/20 without stent removal 3). Cataract surgery combined with MIGS is associated with approximately three times the risk of endophthalmitis compared to cataract surgery alone (0.12% vs 0.04%) 3).
Device malfunction: A case has been reported where a trocar shaft breakage during iStent inject W surgery caused two stents to be ejected connected together 2). Re-implantation was successful with a different injector. Device inspection is recommended when stent ejection is faulty 2).
Hydrus has been reported to cause slightly greater corneal endothelial cell loss compared to cataract surgery alone (14% from baseline at 24 months vs 10%). This loss occurs mainly within the first 3 months postoperatively.
Most cases of early postoperative hyphema are due to blood reflux from Schlemm’s canal and may be a good sign indicating that the distal outflow pathway is functioning. It usually resolves spontaneously within one week and requires no additional treatment. However, persistent or recurrent hyphema should raise suspicion of stent malposition or iris contact (UGH syndrome), and evaluation with gonioscopy or anterior segment OCT is recommended 4). If recurrent hyphema occurs with positional changes, stent removal should be considered 4).
In open-angle glaucoma, changes in the extracellular matrix of the trabecular meshwork increase aqueous outflow resistance 1). The normal aqueous outflow pathway is: anterior chamber → trabecular meshwork → Schlemm’s canal → collector channels → episcleral veins. The resistance in this pathway mainly exists in the trabecular meshwork and the inner wall of Schlemm’s canal.
The iStent penetrates the trabecular meshwork and places a lumen in Schlemm’s canal, creating a direct drainage route that bypasses the resistance of the trabecular meshwork 6). The Hydrus expands approximately 3 clock hours of Schlemm’s canal with a scaffold, preventing lumen collapse and improving aqueous access to a wider area of collector channel openings 6).
The intraocular pressure (IOP) lowering effect of trabecular meshwork bypass stents is limited by the resistance of the distal outflow pathway beyond the collector channels and the episcleral venous pressure (EVP) 5). EVP is typically 6–9 mmHg, which sets the theoretical lower limit for IOP reduction.
Cataract surgery itself is known to lower IOP through microstructural changes in the trabecular meshwork and widening of the angle structures. Ultrasound energy and inflammation during phacoemulsification induce remodeling of the trabecular meshwork. Combining stent implantation with cataract surgery provides additive IOP reduction 1).
In a prospective case series by Kozem et al. (78 eyes, Polish population), iStent combined with cataract surgery reduced mean IOP from 18.5 mmHg to 16.1 mmHg (−2.7 mmHg) at 24 months, and the mean number of glaucoma medications decreased from 1.8 to 0.4 1). 68% of eyes were medication-free 1). The complete success rate (IOP ≤15 mmHg, no medications) at 24 months by Kaplan-Meier analysis was 35.1%, and the qualified success rate (IOP ≤15 mmHg, with or without medications) was 51.9% 1).
The HORIZON trial reported long-term outcomes of Hydrus combined with cataract surgery, confirming significant IOP reduction and medication reduction compared to cataract surgery alone 5).
Future Challenges:
- Kozera M, Konopinska J, Mariak Z, Rekas M. Treatment of open-angle glaucoma with iStent implantation combined with phacoemulsification in Polish Caucasian population. Clin Ophthalmol. 2021;15:473-480.
- Shimada A, Ichioka S, Ishida A, Kaidzu S, Tanito M. A case of two connected stents deployed during iStent inject W surgery. BMC Ophthalmol. 2023;23:206.
- Siedlecki A, Kinariwala B, Sieminski S. Uveitis-glaucoma-hyphema syndrome following iStent implantation. Case Rep Ophthalmol. 2022;13:82-88.
- American Academy of Ophthalmology. Primary Open-Angle Glaucoma Preferred Practice Pattern®. 2020.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma, 6th Edition. Br J Ophthalmol. 2025.
