The effect of scleral flap edge apposition on intraocular pressure control in experimental trabeculectomy

Background: To assess the contribution of scleral flap edge apposition to intraocular pressure (IOP) control in trabeculectomy, using a previously described and validated experimental model of guarded filtration surgery. Materials and methods: Twelve rectangular‐flap trabeculectomy operations each with two apical adjustable sutures were performed on six donor human eyes connected to a constant flow infusion with real‐time IOP monitoring. Three sizes of scleral flap were created: 4 × 4 mm, 16 mm² (n = 4), 3 × 3 mm, 9 mm² (n = 4) or 3 × 2 mm, 6 mm² (n = 4). Sutures were tied tightly to produce high aqueous outflow resistance, and equilibrium IOP established. The lateral and posterior edges of the scleral flap were removed, the sutures tightened again, and the new equilibrium IOP measured. Results: Following flap closure and with intact flap edges, the mean absolute IOP for all flaps (n = 12) was 19.5 ± 3.9 mm Hg (mean ± SD, range 12.4–27 mm Hg) and following flap edge excision 18.7 ± 4.4 mm Hg (range 5.6–27.9 mm Hg), demonstrating no significant difference between flaps with edge apposition compared with those without (P = 0.33). Mean relative IOP (% of baseline) was 68.4 ± 12.1% (range 40.9–94%) with intact flap edges and 65.4 ± 14.5% (range 18.5–97.2%) following flap edge excision (P = 0.31). Flaps measuring 4 × 4 mm and 3 × 3 mm behaved in a similar manner with minimal change in equilibrium IOP following excision of flap edges. Conclusions: In this experimental model, scleral flap edge apposition is not required for generating outflow resistance. Suture tension generated during tight flap closure produces apposition of the underside of the scleral trapdoor to the underlying bed, and it is this apposition, which determines IOP.