The consistent pattern of passage of air is indicative of the architecture and microanatomy of the corneal stroma where collagen lamellae are orthogonally arranged centrally and as a circular annulus at the periphery. The novel peripheral fenestrations explain the peripheral commencement of a type-2BB and the escape of air into the anterior chamber during DALK.

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Purpose Dua’s layer (DL) has considerable relevance to DALK procedure and the hitherto clinically observed but unexplained formation of Types 1, 2 and mixed BB. In this study we explored the dynamics of the formation of different types of BB Methods 50 human sclerocorneal discs were injected with air under BSS and recorded. Videos were studied for leakage of air and formation of BB. Height and diameter of BB were measured. Specimens were subjected to electron microscopy and immunohistology for collagens & matricellular proteins. Age ranged from 3wk‐80yr Results Air injected in the cornea preferentially reached the limbus and moved circumferentially in a clockwise & anticlockwise direction as bands of 2‐3mm till they met, irrespective of direction of needle tip. The air then migrated centripetally to fill the stroma. Air leaked from the TM area at one or more points. Small bubbles formed in the centre and coalesced into a Type‐1BB. This rapidly expanded to attain a height of 5mm and a diameter of ≤9mm. The anterior stomal wall of Type‐1BB showed multiple 'holes' through which air leaked to lift DL. DL was impervious to air. Type‐2BB always started at the periphery. Distinct pores were seen in the peripheral stroma of DL. Most of these were located distal to attachment of DM and explained the leakage of air from TM. Some were located centrally to the attachment and explained formation of Type‐2BB. More than 80% of BB were Type‐1 Immunohistology did not offer an explanation for DL being impervious to air Conclusion DL is a distinct part of the surgical anatomy of the cornea. Identification of pores in DL periphery is novel and explains the formation of a Type‐2BB and the clinical observation of appearance of air in AC during DALK. Leakage is not through the TM

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SummaryDeep anterior lamellar keratoplasty (DALK) has too many techniques to achieve, one of them and most reproducible is the big bubble technique. The steep learning curve known to this operation some times considered a barrier for many surgeons to try and master. However in my presentation I will try to go through the operation step by step giving my own personal tips and tricks for executing the perfect big bubble DALK. Special emphasis will be put on the following important steps: (1) Trephine adjustment and partial thickness trephination of the recipient cornea. (2) Technique of air injection and big bubble formation, which is considered the key step of this technique. (3) Safe opening of big bubble roof. (4) Cruciate incision and cutting of the reaming stroma after lamellar dissection of superficial corneal layers. (5) Descemet's membrane staining and stripping from donor cornea. Moreover, I will put a focus on the new anatomical pre‐ Descemet's layer (Dua's layer), describing and demonstrating by many video clips the different types (type 1, type 2 and mixed type) of big bubbles based on this new anatomical layer. The aim of my presentation is to allow the participants to perform big bubble DALK in a safe and reproducible way with very high percentage of success in obtaining the big bubble.