Without certain identification of Schlemm’s canal, this procedure can produce either no IOP effect when ablation occurs in the cornea or create arterial bleeding with large hyphemas and dangerous postoperative IOP. If the TM is nonpigmented, then induction of hypotony by wound gape is an effective means to visualize the TM (Fig. 16.2).

Under gonioscopic view, the trabectome tip is moved toward the trabecular meshwork (TM). It is easier to initiate ablation at a point that is more toward the left than exactly at the opposite site of the chamber. This offers a better angle of engagement because the tip is now pointed toward, and not parallel to, the TM (Fig. 16.3).

The TM is approached with the tip at a 45° angle aimed upward (Fig. 16.4), from just inferior to the TM at the scleral spur. The spur will keep the space open and resist collapse of Schlemm’s canal which occurs when the trabectome tip is held parallel and pushed outward when engagement is attempted directly across from the incision. Once the footplate is behind the TM, a slight inward pull should be exerted to counteract the tendency to push outward. This tenting of the TM changes the visibility of the ablation tip and confirms proper insertion. Ablation is started toward the surgeon’s nondominant hand.

Ablation can be carried out starting at 0.8 mW and titrated up. Power should be decreased if visible blackening of the upper and lower edges of the TM occurs. This is possible although heat transfer through the footplate to the outer wall is only 1.2°C. As discussed, the bottle height should be at maximum. The trabectome foot pedal can be tapped (black ball switch) for continuous, gravity-fed irrigation, then depressed to position 1 for continuous, nonlinear aspiration and position 2 for continuous and constant ablation power. Aspiration has a maximum flow rate of 10 ml/min [2].

During ablation, it is pertinent to avoid any outward pressure, ideally preventing touching the outer wall entirely. The outer wall of Schlemm’s canal is lined with 50-μm orifices of the collector system that easily scar over when the lining endothelium is damaged (Fig. 16.5). Yet because beginning surgeons are focused on trying to enter Schlemm’s canal and engaging the TM, this mistake is extremely common leading to disappointing pressure results either early on or after several weeks as a response to endothelial damage.

Aiming to ablate 180° of trabecular meshwork increases the chance of obtaining successful access to viable collector channels. Contrary to common belief, aqueous flow is not circumferential (Fig. 16.6). Studies ranging from injecting intracameral tracer [5] to anterior segment optical coherence tomography (OCT) [6] have shown that aqueous outflow through Schlemm’s canal is segmental, increasing near collector channels. For instance, Schlemm’s canal was shown to be 33 % wider nasally than temporally [7], and actual septae have been imaged in 2-μm resolution 3d micro-computerized tomography scans [8]. Adequate ablation can be confirmed by the white appearance of the outer wall of Schlemm’s canal. The first 60° can usually be ablated continuously without many adjustments. As visualization is turned to the remaining 30°, the goniolens is rotated in the same direction the ablation is occurring, and the eye is tilted closer to the endpoints to increase visibility (Fig. 16.7).

Although the goal should be to ablate a full 90°, the final 10–15° cannot always be visualized for ablation. Once the full extent of visualization is reached, the tip is retracted from Schlemm’s canal by pulling inward. It is rotated with one hand and under continuous gonioscopic view, tip down, by a full 180° and reinserted into Schlemm’s canal where the original ablation was started.

The handpiece is rotated between the index finger, ring finger, and thumb while the wrist is increasingly pronated during the ablation of the remaining 90° (Fig. 16.8). The metal sleeve of the trabectome can be rested firmly against the proximal, straight edge of the goniolens during the 180° rotation into the opposite direction. Visualization of the wall of Schlemm’s canal is confirmed as ablation continues. It is advisable not to force the tip ahead but instead stop and retract the tip. This will unwrinkle the TM or retract the tip from the outer wall and collector openings, and then ablation can be continued. After ablation is complete, the handpiece is withdrawn completely from the TM. The handpiece can then be removed from the eye.

Once the ablation is complete, viscoelastic is injected directly along the ablation arc at the root of the iris. This will displace non-coagulated as well as coagulated blood and tamponade Schlemm’s canal and collector channels. We compared five different viscoelastic substances for this purpose and recommend a premixed viscoelastic with both cohesive and dispersive agents in one syringe (e.g., DiscoVisc, Alcon, Fort Worth, TX). The dispersive properties allow the viscoelastic to flow easily into the chamber angle including the orifices of the collector channels, while the cohesive properties allow to maintain the space and prevent displacement. The entire anterior chamber can be pressurized while the patient’s head is still rotated, or the head can be rotated back and the chamber then fully filled and pressurized with the remaining viscoelastic. The patient and microscope are now rotated back until they are again perpendicular to the floor.

To continue with cataract surgery, a standard keratome is partially inserted planar to the iris into the main incision (Fig. 16.9), the tip is carefully advanced past the inner lip of Descemet’s membrane, and then the heel is moved all the way down against the conjunctiva before the keratome is carefully inserted further, aided by small rotating movements around the axis of the handle if necessary. This will create a partial tri-planar incision that has a slight inverted U-shape like a scleral tunnel in ECCE and provide self-sealing features on the left and right aspects of the smaller, initial trabectome keratome incision. To conclude the enlargement of the incision, the blade is now held parallel to the iris again and the keratome moved forward to complete the incision.

Capsulorrhexis and the remaining phacoemulsification steps are done in standard fashion. Just before lens insertion, viscoelastic is injected again to prepare the capsule for implantation, and a second viscoelastic crescent is injected directly along the root of the iris again with the goal to tamponade and displace refluxing blood. Following lens implantation, viscoelastic can be partially removed and the incision is hydrated. We recommend retaining at least 30 % of viscoelastic in the eye to create a positive pressure gradient from the anterior chamber to the outflow system. This will avoid postoperative hypotony and hyphema with better vision early on, and may also provide better early pressures by reducing the phacocytotic load on the remaining trabecular meshwork from red blood cells while minimizing blood clotting along the unroofed canal. It is especially important to maintain postoperative IOP because evidence suggests that fluctuations in IOP, even the brief hypotony induced following a blink, can lead to flow from the ocular surface through the clear corneal incision and into the anterior chamber [9, 10]. OCT has shown full-thickness gaping of a clear corneal wound when the IOP was at 5 mmHg [11]. The wound is hydrated but a suture is not needed and may in fact cause gaping hypotony and instant hyphema. Although the viscoelastic mentioned above has a tendency to cause considerable postoperative hypertension after regular cataract surgery if not carefully removed, the unroofing of Schlemm’s canal in trabectome surgery will allow for improved evacuation similar to glaucoma drainage device surgery.

The patient is seen the following day, at 1 week, at 1 month, and finally at 3 months. All glaucoma medications are discontinued the day of surgery. The postoperative regimen for the first week consists of a fourth-generation fluoroquinolone, prednisolone acetate 1 %, and pilocarpine 1 or 2 %, all of which are started four times daily on the day of surgery. The fluoroquinolone is continued for 7 days. The prednisolone is tapered weekly by one drop for a 4-week course. Pilocarpine is usually maintained at 4 times daily for about 4 weeks, then three times per day 4 more weeks. Flattening the iris out using pilocarpine is thought to reduce the chance of forming peripheral anterior synechiae [12]. If trabectome surgery is combined with phacoemulsification, then a proper refraction cannot be obtained until pilocarpine is stopped.