iStent MIGS device

INTRODUCTION

The iStent is the smallest device ever implanted in the human body. This minimally invasive glaucoma surgery (MIGS) device is devised to bypass the obstructed trabecular meshwork (TM) and allow flow of aqueous from the anterior chamber into the Schlemm’s canal. The instrument was devised by the Glaukos company in the USA and one of the earliest published report was by Spiegel et al. in 2007. The first generation iStent was 1 mm in length and 0.3 mm in height, making it the smallest device approved for implantation by FDA in 2012. This stent is shaped like a snorkel with retention ridges. It is made from an implant grade, heparin-coated titanium alloy and is inserted in the TM manually. The stent is implanted by an ab interno approach guided by gonioscopy.

Subsequently, in 2018 the company came up with the iStent Inject. This device comes with two stents within the injector, as opposed to the first-generation device that had only one stent. Similarly, it is also composed of implant-grade titanium and is 360 μm in length and 230 μm in width, which is one-third the size of the first-generation iStent. Unlike the snorkel shaped first generation iStent, the second generation stent contains a central inlet and multiple outlets designed to provide multidirectional flow through Schlemm’s canal.

Studies regarding these devices have usually been conducted utilizing combined cataract surgery with iStent implantation. Most of the studies have shown significant reduction in intra-ocular pressure (IOP) and medication usage. One of the major studies regarding this device has been the US iStent Study Group. The study compared combined cataract surgery with a single iStent implantation versus phacoemulsification alone. 70% patients in the combined group achieved IOP <22 mmHg without medications compared to 50% in the phacoemulsification alone group (p <0.001). At 12 months, the number of patients requiring medications in the combined group was 15% compared to 35% in the phacoemulsification alone group (p=0.001). These effects were also found in a follow-up study conducted at 24 months.

Studies on solo iStent implantation have also demonstrated >20% IOP lowering until 24-months of follow up. Studies have compared 1,2 and 3 iStent implantations. It was found that 2 and 3 iStent implantation led to nearly 41% IOP reduction, compared to 31% by single stent use.

Therefore, it can be concluded that iStent is a useful trabecular bypass MIGS device which can be effective in IOP lowering as a solo or combined procedure.

SURGICAL TECHNIQUE

The surgical steps for iStent implantation are available in detail elsewhere (See link). Here, the steps are given in brief. In the first generation iStent, the anterior chamber is filled with viscoelastic. A temporal clear-corneal incision is made. If combined with cataract surgery the same incision as the one for phacoemulsification can be used. The iStent on its injector is pushed into the AC and using a gonioscope to guide, is directed towards the nasal angle in such a way that the snorkel is facing downwards. The stent enters the TM and with a sideways motion of the hand the device is pushed deep inside. Subsequently, the inserter button is pressed, allowing the device to settle into the position. Small adjustments are made by tapping the end of the snorkel by the inserter. Finally, the inserter is taken out of the eye and the AC cleared of the viscoelastic.

The iStent Inject comes with 2 pre-loaded implants. The surgeon enters the eye from the temporal clear-corneal incision and using the gonioprism reaches the TM. Then the device is brought into approximation at the desired position and the button pressed without any side to side movement of the hand to allow the implant to be delivered into the desired spot. The second stent is then implanted similarly 2-3 clock hours away.

https://www.youtube.com/watch?v=LCDE0NB6S2Y

COMPLICATIONS

Most studies did not report any complication from the iStent per se and most were attributed to cataract surgery. A few studies have reported on stent occlusion or malposition occurring in 2.6-18% patients. The new iStent Inject provides multiple openings to minimize the possibility of occlusion. Mild bleeding during implantation of the iStent is common, but usually resolves spontaneously.

A study was conducted in Canada regarding cost effectiveness of the device.  The potential savings of the iStent compared to a 2-drug regimen over a 6-year period was CA$1272 and CA$2124 when using 3 drugs.

CONCLUSION

 The iStent is a useful addition to our choice of MIGS devices. It is technically easy to use, efficient and safe.