Deep anterior lamellar keratoplasty (DALK) requires the dissection of the corneal stroma in its layers near the Descemet’s membrane (DM) and the endothelium. Obtaining good visual quality is linked to leaving as little tissue as possible in the bed. The difficulty lies in achieving that minimum without piercing the residual back sheet. To this end, several dissection techniques have been developed that have had the effect of introducing new concepts in the anatomy of this region.

Among these techniques, the pneumo-dissection by "big bubble" of Anwar (BB) stands out¹. It is based on an injection of air into the deep stroma that infiltrates the parenchyma and gives it a characteristic whitish emphysematous appearance, until finding a plane of cleavage where adherence is lower. From there, a cavity or bubble is formed, that increases progressively from a central or pericentral area, first with difficulty until suddenly an "explosive" separation of the layers is created. When accessing that space, there appears under it a very thin, transparent and smooth membrane, which was believed to be the DM itself. However, when Jafarinasab et al studied the histopathology of operated corneal buttons in which, despite a successful BB, the surgery was converted to penetrating keratoplasty due to rupture of the posterior lamina, they found that in reality the plane of dissection of the BB was at a stromal level somewhat before the DM and not in the own stroma-DM interface².

Successive studies in eye bank corneas confirmed these findings and observed two types of dissection or BB. In the case – by far – more frequent, the cavity is limited by a whitish margin of emphysematous tissue that prevents its advancement. This does not reach more than about 8 mm in diameter (Figure 1, video 5.2.1) and if one insists the bubble may burst. With this type of BB, called type 1, a thin layer of stroma remains in front of the DM (Figure 2)³.

More rarely, said whitish margin is not formed and the bubble progresses progressively and rapidly to the extreme periphery (9-10.5 mm). Then we talk about BB type 2 (Figure 3) and the dissection takes place in the own stroma-DM interface without residual tissue remaining (Figure 4)⁴. This second type possibly corresponds to that already described by Anwar as a more peripheral bubble with a thin and transparent edge⁵. Since its visibility is lower, it is sometimes only detected when the needle is removed, due to a "wave like" recoil effect beyond trepanation (video 5.2.2). In this case the residual layer (DM only) is especially brittle.

The residual stromal layer between type 1 BB and DM, was characterized by Dua et al, and named "Dua’s predescemetic layer" (PDL). This would be an acellular layer, with a thickness of 10.15 ± 3.6 μm, formed by 5 to 8 lamellae of predominantly collagen type 1 fibers, oriented in longitudinal, transverse and oblique directions. The presence of this layer supposes a greater resistance: to burst the BB type 1, 1.46 bar was necessary, while the BB type 2 did it with 0.6 bar⁶. This coincides with the experience of the surgeons on the – at first surprising – resistance of the BB type 1 and the greater fragility of the BB type 2.

Successive studies by the same group, using electron microscopy and immunohistology in donor corneas, showed that PDL also contains type VI collagen continuously to its periphery. From this emerge collagen bundles that are subdivided to reach the bundles of the trabeculum. CD34 cells, typical of the trabeculum, are also found on the periphery of PDL⁷. These relationships would explain a greater adhesion that prevents the extension of type 1 BB beyond 8-8.5 mm in diameter. Another possible explanation invokes the elastic fibers, which form a complex network in the deep peripheral cornea and which, starting from the sclera or limbus, extend towards the center and bifurcate or trifurcate into the pre-descemetic stroma⁸.

The nature of PDL as a differentiated anatomical structure has been controversial. Its supposed acellular character has been questioned in particular. Schlötzer-Schrehardt et al found keratocytes at variable minimum distances from the DM, but generally lower than the thickness described for the PDL. In the central area of ​​3 mm in diameter, keratocytes began to be found between 1.5 and 12 μm of the DM (about 5 μm in average), minimum distance that increases towards the middle periphery (8 mm) to about 3.5-14 μm (about 8 μm on average) and more peripherally at 4.5-18 μm (about 10 μm on average). The number of collagen lamellae between DM and the first keratocytes varied between 2 and 10, with collagen fibrils of 23.5 ± 1.8 nm, not different from the rest of the stroma. At the stroma-DM interface, a thin layer (from 0.5 to 1.0 μm) composed of randomly arranged non-aligned collagen fibers with positivity for type III and VI collagen was observed. The BB maneuver gave rise to layers of thickness between 4.5 and 27.5 μm, very variable – even in the same cornea up to 3 times – and composed of 5 to 11 lamellae of collagen with keratocytes on its anterior face and between the same⁹.

Beyond the discussion in the field of basic science¹⁰, PDL seems an established entity in surgical anatomy, and it has been shown that in its absence (either by excision or by laser ablation) it is not possible to create a BB¹¹. There is also the possibility of obtaining a mixed type or BB type 3. Corresponds to a BB type 1 in which the expanding air has crossed the PDL by some pore and has created a second cavity between the PDL and the DM (Figure 5). This is often eccentric – as described by Anwar⁵ – and its presence is not ideal either because it is difficult to distinguish between the different planes (see chapter 5.8.5).

Thorough knowledge of anatomy is a fundamental requirement for a good surgeon. Although in ophthalmology we are accustomed to the ease with which structures are normally observed under a surgical microscope, the development of techniques such as DALK takes us back to a frontier terrain where we need to familiarize ourselves with increasingly fine details. The incorporation of new technologies such as OCT in surgical microscopes will probably be very useful in this field and may bring new perspectives.