- The iStent, a small titanium device which is implanted under gonioscopic control in the trabecular meshwork through a small
corneal incision, has shown encouraging results in patients undergoing combined cataract and glaucoma surgery. One of the
more intriguing aspects of this device is its ability to be inserted easily through a clear cornea incision created during
cataract surgery. Although the implantation needs some manual dexterity and experience with using a direct goniolens, it is
not an insurmountable learning curve for an average cataract surgeon, claims Professor Antonio Fea.
Interest in Schlemm's canal has grown in recent years as glaucoma surgeons continue to look for techniques that will provide
sustainable intraocular pressure (IOP) reduction with safe outcomes.
Large prospective studies have told us the risk of glaucomatous progression is substantially reduced with lowered IOP. In
a study published in the Archives of Ophthalmology in 2003, each mmHg reduction in IOP was associated with about a 10% decrease in the risk of progression.1 This study also confirmed that the lower the IOP at follow-up, the lower the risk of progression.
Attempts to control IOP through medical therapy tend to be first-line treatment although patient compliance and ocular surface
integrity remain a concern.2
Glaucoma specialists would probably consider surgical intervention no longer as a "last-ditch" if procedures carried lower
risks and did not imply para-surgical interventions on the filtering bleb. Safe trabeculectomies3, 4 and non-penetrating procedures5, 6 have been devised, but the former do not solve the need for caring of the filtering bleb while the latter can be limited
by inconsistent IOP-lowering effects and technical difficulties.7,8
In the last few years, several methods to reduce the trabecular resistance to outflow (such as trabeculotomy, trabecular aspiration
and surgical implants) have been introduced. The major advantage of all these methods is the use of microincisions and the
sparing of the conjunctiva; however, these surgical therapies have a high rate of complications. In an attempt to reduce the
high complications rate and lack of sustained efficacy of available surgical therapies, Glaukos Corporation developed the
iStent; recent studies have shown the efficacy of the iStent in improving trabecular outflow in implanted eyes.9
What is the iStent?
The iStent is an L-shaped titanium device, designed to be inserted into the eye through a 1.0 to 1.5 mm clear corneal incision,
which creates a long-lasting patent bypass between the anterior chamber and Schlemm's canal. The device itself is small —
about 1 mm by 0.5 mm. It is implanted into the trabecular meshwork under gonioscopic control using a "device" similar to grasping
vitrectomy forceps (Figure 1).
Figure 1: The iStent.
Results from a European trial of the iStent in patients undergoing combined glaucoma and cataract surgeries showed the iStent
provided a mean IOP reduction of 4.4 mmHg along with a reduction of 1.2 medications. These reductions were maintained on the
study cohort through the 12-month follow-up.10,11,12
Glaukos is currently enrolling patients in a Phase III, randomized, comparative, controlled, parallel-assignment FDA trial,
and is also conducting Phase IV trials in Europe in phakic and pseudophakic eyes.
How to implant the iStent
Some technical abilities are necessary for the stent implantation procedure to be successful. The surgical protocol necessitates
that the surgeon be familiar with gonioscopy and the angle appearance, and must correctly visualize the angle structures before
attempting the implantation. An accurate knowledge of the angle anatomy greatly facilitates implantation; when the stent is
implanted in the correct position, no resistance is felt as it enters the trabecular meshwork and Schlemm's canal. I found
it helpful to analyze carefully with a slit lamp the trabecular meshwork of all the patients I have implanted.
I have implanted the iStent primarily in conjunction with cataract surgery using a temporal approach. The stent is implanted
in the nasal quadrant and preserves the conjunctiva of the superior quadrants for eventual future surgical options.
Because the procedure also uses the patient's pressure-dependent, steady-state physiological outflow system, the potential
for hypotony is virtually non-existent.
I have personally implanted the iStent in conjunction with cataract surgery in 10 patients with a preoperative IOP higher
than 18 mmHg if the patient is on one medication or with any IOP if on more than one medication.