The influence of scleral flap position and dimensions on intraocular pressure control in experimental trabeculectomy

To assess the effect on intraocular pressure (IOP) of varying the length of the side incisions of the scleral flap during trabeculectomy.

Trabeculectomy operations were performed with adjustable sutures on 8 donor human eyes connected to a constant flow infusion with real-time IOP monitoring, using either a large (4 x 4 mm, 16 mm, n=8) or a small (3 x 2 mm, 6 mm, n=8) scleral flap. For each flap the side incisions began 1 mm behind the limbus and extended to the posterior edge of the flap. The side incisions were extended sequentially in 0.5-mm steps up to the limbus, then each flap dissected 1 mm further into clear cornea.

Mean IOP after sclerostomy fashioning was 0.84 mm Hg (range 0 to 2.7 mm Hg). After flap closure, with side incisions extending to 1 mm behind the limbus, mean IOP was 21.6 mm Hg (79.5% of baseline) and 23.03 mm Hg (79.2% of baseline) for large and small flaps (P=0.26). In each size group, extending flap side incisions to the limbus produced a small nonsignificant fall in mean IOP, whereas flap extension 1 mm into clear cornea led to a significantly lower mean IOP relative to baseline of 43.2% (P<0.05) for large flaps and 35.4% for small flaps (P<0.01).

Using this adjustable suture technique, IOP is well maintained for both flap sizes if the flap and side incisions do not extend beyond the limbus. Excessive forward dissection of a scleral flap into the clear cornea, anterior to the sclerostomy may result in increased aqueous outflow and lower IOP.