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Introduction
Anwar's pneumodissection or 'big-bubble' technique of deep anterior lamellar keratoplasty (DALK) has become popular since it was first published in 2002. 1 It requires no special equipment, is relatively quick to perform and has been shown to yield visual results as good as those for penetrating keratoplasty. 2
Two distinctly different types of big-bubble can be seen clinically. The most common form was described by Anwar in his original paper. 1 He described a 'white, semi-opaque disk with a near circular outline' occurring with successful bubble formation ( figure 1A ). Since the original description, most authors have continued the assumption that this is a Descemet membrane (DM)-baring procedure. 2
He later described another less common type of bubble which has a 'clear outline ... hence the white band is not seen.' 3 It is a distinctly different form of bubble that has an appearance similar to that of an air bubble in the anterior chamber ( figure 2A ), the difference being that the latter is mobile. This bubble often forms eccentrically, and Anwar suggested it may represent an intra-DM bubble forming between the two zones of DM. Most studies of big-bubble DALK have not made a distinction between the two types, despite their potentially different dissection levels.
Recently, a study examined three keratoconic eyes that underwent big-bubble DALK and required intraoperative conversion to penetrating keratoplasty. 4 Histological examination found that all three eyes had a residual stromal layer covering the excised DM. Despite this, the assumption that big-bubble DALK routinely bares DM continues. 2
The aim of our study was to further investigate whether residual stroma remains on the posterior lamella in big-bubble DALK.
Materials and methods
Corneoscleral discs deemed unsuitable for transplantation due to reasons other than corneal disease were obtained from the Manchester CTS eye bank. Consent had been obtained for all corneas to be used in research, and ethical approval was obtained from the local ethics committee.
Corneas had been stored in organ culture and were transferred to dextran prior to use to decrease stromal oedema. For the procedure, they were mounted on an artificial anterior chamber (Coronet, Ripon, UK) and connected to a bag of balanced salt solution. Big-bubble dissection was then performed as per Anwar's original...