Endothelial keratoplasty currently has two main techniques. In the first, the endothelium and the Descemet’s membrane (DM), are accompanied by a more or less thin stroma sheet and therefore in this work the term "endothelial lamellar keratoplasty" (ELK) is proposed. This corresponds to the English terms "Descemet’s Stripping (Automated) Endothelial Keratoplasty" (DSEK/DSAEK). In the second, endothelium is exclusively transplanted and DM without stroma, which in English has been called "Descemet's Membrane Endothelial Keratoplasty" (DMEK) and can be translated as "Descemet-endothelial keratoplasty" (DEK). Both techniques share the tissue preparation in the recipient, but that of the donor is different. For the best understanding of this surgery, there is a series of anatomical details of the posterior third of the cornea that the clinician must know.

THE POSTERIOR CORNEAL STROMA

The posterior stroma of the cornea is the place where the donor tissue is cut in the ELK/DSAEK. The structure of the deep stroma has a series of differences with respect to the anterior one, with surgical implications. The fibrils of the different types of entangled collagen that constitute the corneal stroma are organized in the form of lamellae in which they are arranged orthogonally. The inter-fibrillary space increases from the center to the corneal periphery, while they become more compact when the stroma is deepened¹. The lamellae in the posterior third of the cornea are thicker, wider and more organized than in the more superficial stroma. This arrangement allows a manual dissection of the deep stroma easier to maintain the plane than in the more superficial layers. However, the mechanized cutting of the disc or donor lenticule for ELK with a microkeratome allows obtaining a smoother surface than with manual dissection and in a more repeatable manner (Figure 1).

The introduction of the femtosecond laser predicted the obsolescence of microkeratomes. However, the separation of the tissue that the laser obtained in deep stroma has not achieved a smooth surface as it does in the anterior third or comparable to that obtained with microkeratome (Figure 2). It is believed that the absence of interlamellar fibers at this level results in the cavitation of the laser creating a plane of cleavage between lamellae and a more irregular plane through it. This fact, together with the folds that tend to cause the absence of tensile forces in the posterior lamellae, results in surfaces of inferior quality to those obtained mechanically (see chapter 6.4.3)².

It has been proposed the existence of a structure just in front of the DM, the so-called pre-descemetic layer of Dua (PDL), which would be acellular, have a higher density of fibers, and would occupy only the central 8-9 mm³. Although its admission as a differentiated histological entity is controversial¹, it has obtained notable acceptance in surgical anatomy, since it explains certain behaviors of the tissue, especially its types of response to pneumo-dissection (see chapter 5.2). In any case, the existence of different possible dissection planes at that depth has implications also in the surgery of endothelial transplants.

The posterior stroma also presents a lower density of keratocytes¹. This contributes to the fact that the antigenic load introduced with an ELK-type transplant is even lower – and therefore the risk of rejection – compared with a penetrating keratoplasty (PK).

CORNEAL INNERVATION

The nerve trunks of the cornea are located mainly in the superficial third of it. The sub-basal and subepithelial plexus collect the most anterior information, and the stromal nerves mainly the middle third. Only a small percentage of the latter can be found in deeper layers⁴. Consequently, endothelial transplants maintain the corneal innervation entirely. The excision in the recipient of only the DM-endothelium complex (DMEC) avoids the neural damage that occurs in the rest of keratoplasties. However, corneal nerves are dynamic structures that can be affected remotely by ELK surgery. Structural and functional alterations, generally minimal, have been observed and they recover their basal levels in approximately 4 months. On the other hand, the pathologies in which these techniques are indicated – such as Fuchs' dystrophy – already tend to present an anomalous nervous structure, with fewer and more tortuous nerve bundles⁵. In general, these changes do not alter the corneal sensitivity measured with conventional methods.

DESCEMET’S MEMBRANE

DM corresponds to the basal membrane of the corneal endothelium – or to the accumulation of successive layers of basement membrane material –. Its thickness increases progressively from birth (about 3 μm) to adulthood (10-13 μm). In it, 3 layers are distinguished: the anterior and finest is an interfacial matrix (IFM), which serves as a transition with the posterior stroma (0.3 μm); it follows an anterior layer with bands (2-4 μm) that forms during the fetal period; and finally, the posterior layer without bands (4-10 μm) or postnatal, which grows throughout life (Figure 3)⁶.

DM is intimately related to the corneal stroma, and its adhesion to it is mediated by (a) an amorphous matrix between the two layers, with intimate junctions with the anterior zone of the band layer and the stromal collagen fibers, (b) ) randomly distributed collagen fibers, which project perpendicularly or at an acute angle and penetrate 0.5 μm into DM, and (c) filaments of proteoglycans that connect the stromal collagen fibers with the IFM⁷.

The manual dissection of the DMEC, either by means of the descemetorhexis maneuver in the receiver (both in the ELK/DSAEK and the DEK/DMEK) or those used in the preparation of the donor tissue ("roll") of the second, almost always follows a cleavage plane between the IFM and the posterior stromal collagen lamellae. This way, a smooth surface in a high percentage of cases is achieved in a simple and manual way⁷. However, there may be interindividual variations in the union of the DMEC with the posterior stroma that condition incomplete separations. Peg-like focal irregularities or increases in adhesion glycoproteins in the IFM may be the cause of incomplete dissections of the DMEC⁶.

The DMEC extracted from the donor for the realization of a DEK always rolls with the endothelium out, because the elastic tension exerted by the DM is greater than that of the endothelial layer. DM of older donors are thicker and more rigid, and therefore their capacity to roll is lower than in young people. Therefore, donors older than 50 years are preferable for this technique, in order to facilitate their deployment in the anterior chamber⁸.

THE CORNEAL ENDOTHELIUM

The cellular monolayer that covers the posterior surface of the cornea or corneal endothelium consists of flattened polygonal cells, which form a generally hexagonal mosaic. It is a population of about 3,500 cells/mm² in youth, which decreases progressively with age⁹. Its mission is to maintain corneal transparency through continuous dehydration of the stroma, thanks to the action of the ionic pumps at its level⁹. The adhesion without sutures of an endothelial keratoplasty will depend on its correct functioning. However, hydrostatic forces, surface tension and the adhesiveness of proteoglycans of the exposed posterior stroma also participate in the initial moments after graft application¹⁰.

The ideal endothelial mosaic is hexagonal and of identical cells. However, in practice cell size varies and its shape may have more or less sides. An increased degree of variability of sizes or polymegatism is an indicator of endothelial dysfunction. Variability in shape or pleomorphism is measured by the percentage of hexagonality and its decrease is related to the cicatricial response of the endothelium. These parameters allow to assess, along with the cell population density, the quality of the donor tissue. However, it should not be forgotten that these are morphological parameters, which only indirectly give information on endothelial function.