Fig. 8.1
iStent. This is a very small L-shaped device in non-ferromagnetic titanium, coated in heparin to assist self-priming. The most recent model is 1 mm long, 0.33 mm thick and weighs 60 μg
Fig. 8.2
Correct positon of the iStent and its mechanism of action. The foot plate of the iStent is positioned in the Schlemm Canal (Fig. 8.2). The distal part (shaped like a snorkel) is positioned in the AC, is 0.25 mm long with an opening of diameter 120 μm. The aqueous humor passes through the iStent (blue arrow), by-passing the trabeculate, flowing from the AC to the Schlemm Canal. It is successively drained by the collector canals into the episcleral veins, especially in the quadrant containing the by-pass
Fig. 8.3
The iStent preloaded in an injector. There are two types of iStents (Fig. 8.3). Both are preloaded in a sterile injector, one for the right eye and one for the left eye
Surgical Technique
Even though this operation is easier to perfome compared to other glaucoma procedures, we would recommend local anesthesia (retrobulbar or peribulbar) for the first implants performed by learning surgeons. Preoperative instillation of miotic eyedrops (1 or 2% pilocarpine) is recommended to open the angle. For this type of procedure, the surgeon sits in a specific position that has already described in Chap. 8 about the goniotomy procedure: he sits on the opposite side to the portion of the angle he intends intervening on; the patient’s head is rotated slightly to the side opposite the surgeon. The operating microscope is tilted 30°–45° to allow optimized vision of the angle.
The surgical steps are as follows:
Step 1: creation of a 1.5 mm incision in clear cornea in a temporal position parallel to the iris. The incision must be diametrically opposite the chosen position for the implant and must be close to the limbus.
Step 2: introduction of low resting molecular weight cohesive VES into the AC to encourage the successive entrance of the handle: creation of air bubbles in the AC should be avoided as these can reduce the visualization of the camerular angle.
Fig. 8.4
Control of the vision of the camerular angle (step 3). Now the surgeon proceeds by placing gel on the cornea, positioning the goniolens and controlling the vision of the angle (Fig. 8.4). Normally a modified Swan-Jacobs lens is applied: high magnification is recommended (10–12×). The gonioprism is generally held in the non-dominant hand; light pressure is applied to maintain a uniform meniscus of gel on the cornea. Excessive pressure applied in this phase can cause the formation of corneal folds that can ostacle the angle visualization. In the event the surgeon cannot identify the anatomical landmarks, he may attempt to induce blood reflux in the Schlemm Canal by applying light pressure to the perilimbal sclera. As we mentioned before, two types of iStent are available. Both are preloaded in a sterile injector, one for the right eye and one for the left eye: this facilitates the insertion in the infero-nasal quadrant. The surgeon identifies the trabeculate localized between the scleral spur and the Schwalbe line: this is the point where the device is inserted. The surgeon should always identify the most suitable site for the implant: the most pigmented area of the trabeculate is where there is greater drainage and a greater density of the collector canals. Generally it is localized in the inferior nasal quadrant. After identifying the surgical landmarks (the trabeculate localized between the scleral spur and the Schwalbe line), the goniolens is removed and the surgeon concentrates on the implantation of the iStent
Fig. 8.5
Progression of the implant in the AC in a nasal direction (step 4). The implant is inserted in the AC parallel to the iris to avoid engaging it. The implantation procedure proceeds in a nasal direction inside the AC (Fig. 8.5) and the goniolens is repositioned
Fig. 8.6
Manoeuvre for the correct positioning of the iStent in the Schlemm Canal (step 5). Holding the goniolens in the non-dominant hand, the iStent preloaded in the injector is held in the dominant hand; it penetrates beyond the pupil margin as far as the angle, more precisely till the anterior 1/3 of the trabeculate, at a 15° angle
Fig. 8.7
To ensure that the iStent approaches the Schwalbe Line correctly, the surgeon must perform a slight rotation to allow it to lie perpendicular to the angle: in this maneuver, the surgeon will perceive a sensation similar to wet tissue paper. During the correct insertion, considering that the IOP drops, a small amount of blood reflux will be observed in the Schlemm Canal. If the surgeon perceives considerable resistance, the posterior portion of the Schlemm Canal may have been engaged: in this event, the iStent should be withdrawn and re-implanted 1–2 h later
