DEVELOPMENT

Lamellar keratoplasty originated simultaneously with penetrating keratoplasty; in fact, the first successful cornea transplant in a human, carried out by von Hippel in 1866, was that of a full-thickness rabbit cornea applied to the lamellar bed in a woman¹. And despite the famous penetrating homograft of Zirm in 1905, at least until the 1930s most of the (unusual) keratoplasties were lamellar.

As the development of eye banks, asepsis and microsurgery made penetrating keratoplasty viable in routine practice, the lamellar was progressively relegated due to its worse visual results, despite the essential advantage of respecting the integrity of the globe and its lower risk of rejection.

Technological advances were needed, such as those linked to the development of lamellar corneal refractive surgery – from the inventions and techniques of José I. Barraquer (microkeratome, keratomileusis, etc.)² to the femtosecond laser, so that this field was revitalized to the point of changing the paradigm. Demonstrating that a lamellar keratoplasty, could have visual results as good as a penetrating one, led to the logical conclusion that the exclusive replacement of the layers affected by the pathology is preferable, respecting the others. This explains the rise in lamellar keratoplasty since the turn of the century^3,4.

CONCEPT

Lamellar keratoplasty can be defined – negatively with respect to penetrating keratoplasty, as any replacement or delivery of a sheet of corneal tissue that does not involve resection of the recipient cornea in its full thickness. Since the cornea is a structure composed of a series of layers of tissue (epithelium, Bowman, stroma, Descemet, endothelium) and in turn the stroma by a series of lamellae, the possible types of lamellar grafts are very varied, depending on the various combinations of layers or levels (Figure 1)⁵.

We can distinguish in practice three possible courses of action: anterior, posterior and interlamellar. For each of them, there are two options in turn: tissue replacement (the actual transplant), and its contribution or addition. The latter may have a reconstructive or refractive purpose (as in a keratophakia). Substitution and addition can coexist, as in the cases in which a thinned tissue is replaced by another of normal thickness. Even if we applied the meaning of keratoplasty in its broader etymological sense of "shaping the cornea", we could add here the subtractive lamellar techniques, be they therapeutic (PTK) or refractive (PRK, LASIK, SMILE, etc.).

TYPES

Among the anterior lamellar grafts, historically there was hardly any distinction according to their depth. However, since the middle of the last century it was observed how the best visual results were obtained with the deepest ones. It has been the development of techniques that allow the dissection of the entire stroma to a plane close to the Descemet’s membrane, which has led to the current distinction between Deep Anterior Lamellar Keratoplasty (DALK) and Superficial Anterior Lamellar Keratoplasty (SALK). Because of its importance, we dedicate a section of this book to each of these two variants. SALK can be performed with three general types of techniques: (a) manual dissection with blades or spatulas; (b) mechanized lamellar cutting devices (microkeratomes), and (c) by femtosecond laser.

In the superficial limit, we can consider the transplants of epithelium, which belong to the field of the ocular surface, object a few years ago of another Official Report of the Spanish Ophthalmological Society⁶. In any case, the deepest limit of what is considered a SALK does not reach the level where it would begin to be considered the DALK; between both is a range of depths in which a third technique could theoretically be defined, apart from those of interlamellar type. The margin for innovation is evident with techniques such as the Bowman transplant, to which we devote a chapter. And apart from the usual disc-shaped grafts, other forms also fit in (horseshoe, crown, crescent, fusiform, etc.).

The grafts on the posterior surface, have as a general purpose the transplantation of endothelium, and are differentiated according to the amount of support tissue that accompanies it. For historical reasons that are reviewed in another chapter, from the term "endothelial keratoplasty", procedures appear whose denomination includes a reference to the maneuver of peeling of Descemet’s devised by Melles or descemetorrexis⁷. The English terminology "Descemet-Stripping (Automated) Endothelial Keratoplasty" (DSEK/DSAEK) does not describe, in fact, the type of graft itself – beyond including endothelium – but that aspect of receptor preparation. This was done to distinguish this method from the previous ones, now obsolete, in which they were trepanning or otherwise dissecting a disc from the recipient's posterior stroma. For techniques that include a more or less thin layer of stromal tissue together with the endothelium, we propose a more generic denomination as Endothelial Lamellar Keratoplasty (ELK). The reference to the endothelium, the object of the transplant, seems to us more appropriate and clearer than those that use the term "posterior", which can lead to some confusion with the anterior deep techniques.

The most recent procedures such as the "Descemet Membrane Endothelial Keratoplasty" (DMEK) or its variant the "Pre-Descemetic Endothelial Keratoplasty" (PDEK), minimize the accompanying tissue to the donor endothelium. They continue to use the peeling of the receiver's Descemet although they no longer refer to it in their denomination. These techniques are the subject of another section of the book, differentiated from the group of endothelial transplantation with stromal delivery (ELK).