To compare two sites of air injection to achieve Descemet's membrane (DM) detachment in big-bubble deep anterior lamellar keratoplasty (DALK).

In this prospective, randomized study, 48 eyes of 48 keratoconus-affected patients who underwent DALK by cornea fellows were enrolled. Each patient was randomly assigned into one of two groups. After trephination to approximately 80% of the corneal thickness, a 27-gauge needle was inserted into the stroma from the trephination site. The needle was moved radially inside the trephination site and advanced to the central or paracentral cornea in group 1. In group 2, the needle was inserted into the deep stroma from the trephination site and advanced into the peripheral cornea to approximately 1.5 mm anterior to the limbus. Air was gently injected into the deep stroma until a big bubble was formed. The rates of DM separation and complications were compared between the two groups.

Big-bubble formation was successful in 79.2% of the eyes in the entire study group. A bare DM was achieved by central injection in 68.0% of group 1 and by peripheral injection in 69.6% of group 2 (P=0.68). This rate was increased to 80.0% and 78.3% in groups 1 and 2, respectively, after the injection site was shifted when injections failed. The study groups were comparable in terms of complications including DM perforation and bubble bursting.

Both injection sites were equivalent in their rates of big-bubble formation and complications. Less experienced surgeons are advised to initially inject air outside the trephination.