The incision should be made perpendicular to the sclera along the outlined area to reach about two-thirds of the scleral thickness (figure 4a). This will facilitate and ease dissection of an external scleral flap in a controlled manoeuvre.

Figure 4 (a). Triangle trabeculectomy flap

The trabeculectomy flap is then gently separated from the underlying sclera by using side-to-side motion of the spatula blade or the smooth dissection movement with a number 15 blade. The flap is dissected further forward passing the bluish grey zone into the transparent zone of clear cornea. When clear cornea is encountered, the flap is dissected sufficiently far forward (figure 4b).

Figure 4 (b). external scleral flap

Paracentesis

Before preparing the filtering fistula, I make a limbal paracentesis into the anterior chamber. This self-sealing wound permits access into the anterior chamber after the filtering fistula is made. The paracentesis should be placed into the clear cornea near the limbus below the horizontal meridian temporally (figure 5). A disposable 15° super-sharp blade can be used. The blade should be gently, but definitely, advanced through the cornea into the anterior chamber parallel to the iris plane. This will allow a flat, bevelled incision that will be self-sealing and will not require suturing. The paracentesis will allow access to the anterior chamber intraoperatively and postoperatively at the slit-lamp examination.

Figure 5. Limbal paracentesis.

Internal Block Excision (Trabeculectomy)

The fistula into the anterior chamber should be prepared. A rectangular block of internal ostium excision ab externo is outlined with a sharp knife, such as a 15° supersharp blade (figure 6a). Two small vertical cuts are made, each approximately 0.5 mm from the lateral extreme of the scleral bed, starting with the anterior-most clear cornea and extending the incision posteriorly to the level of the blue zone. The internal block is completely excised with a sharp knife in a progressive, small-split, up-cutting man-oeuvre under direct visualisation (figure 6b). If adjunctive mitomycin-C is used, the internal block excision is kept smaller (1 x 1 mm) to lessen postoperative hypotony from overdrainage.

Figure 6 (a). Outlined internal block

Figure 6(b) Internal block excision

Iridectomy

When the iris prolapses into the fistula, radial iridotomy will replace the iris into the anterior chamber. It should be grasped with non-toothed or Jeweller forceps, then pull the iris to the left and use the iridectomy scissors to cut the iris to make an iridotomy incision and pull the iris to the right to complete the iridectomy with the scissors by cutting to the left (figures 7a and b). By doing this, the base of the iridectomy will be broader than the internal fistula so reducing the possibility that the iris will occlude the internal os. The iris will usually reposition spontaneously. If it does not, the overlying cornea should be gently moved towards the centre of the cornea with the heel of the muscle hook or irrigating spatula. This technique will usually disengage the iris from the inner edge of the fistula. The iridectomy should be visible through the clear cornea and the pupil should be round.

Figure 7(a). Iris prolapse

Figure 7(b). Iridectomy