The field of keratoplasty seems easy to narrow down. In fact, however, a very varied set of procedures can be included under this term, which makes a precise and comprehensive definition difficult. A classification or, better, a series of classifications will help us understand that variety, while a brief analysis of the etymology of the term keratoplasty and its family of terms, may reveal something of what lies at the origin of these words.

CONCEPT AND DEFINITION

Usually, keratoplasty is a graft or corneal transplant, that is, the partial or total replacement of the same by a donor tissue. This definition corresponds to the subject that is covered mainly in this book, although it only covers part of the concept.

More broadly and as etymology shows, a keratoplasty can be any intervention that intends to shape the cornea. In this sense, beyond the substitution of tissue that involves transplantation, we can include diverse actions, from cuts (keratotomies), ablations (keratectomies), tissue retraction (thermal or "conductive" keratoplasty), to the addition of material, be it grafts (keratophakia), implants or prostheses. The purpose in this group is usually to modify the shape of the cornea and therefore its refraction, but the opposite is true, if we include in this broad concept procedures such as photodynamic reticulation or crosslinking (CXL), which precisely seek to prevent the cornea from changing.

CLASSIFICATION

Classifications are one of the best conceptual tools we have to understand complex realities. In the case of keratoplasties, we can organize them according to different criteria, which allows us to consider them from several points of view. Each classification responds to a fundamental question: What is it for?; What function does it restore?; What type of tissue does it use?; What type of action is carried out?; What instruments are used?; What procedures are carried out the practice?

Teleological classification

The most basic way to classify keratoplasties is according to their purpose. In most cases – including almost all transplants – the purpose will be therapeutic. However, refractive keratoplasty involves a whole field whose aims are generally considered differentiated from the previous group – although the conceptual limits may be debatable –. Finally, we can consider a keratoplasty of cosmetic purpose, which would include corneal tattoos.

Functional classification

This is a subdivision of the therapeutic purpose keratoplasties into 4 types depending on which function of the cornea they intend to restore (Figure 1): (1) the loss of transparency, as in the case of a leukoma or a corneal edema (optic keratoplasty); (2) the loss of optical-refractive quality, as in a keratoconus (refractive keratoplasty, in this case in therapeutic sense); (3) the structural function of the cornea as part of the ocular wall, as in the case of a large loss of substance or perforation (tectonic keratoplasty); and (4) the defensive function of the cornea, in the case of an infection resistant to other treatments (anti-infective keratoplasty, sometimes called "hot" or "therapeutic" in a specific sense). Although many cases actually participate in more than one of these categories, it is a useful classification because of its simplicity.

Technical classification

Based on the techniques and modes of action that can be used on the cornea to mold it (in the broader sense of the term keratoplasty): (1) Substitution, being transplantation or prosthesis; (2) Resection or ablation of tissue: keratectomy, keratomileusis in its different variations, wedge resection, etc.; (3) Addition or implant, being a lenticule of tissue (keratophakia) or artificial, or in another way like with ring segments, etc.; (4) Incisions or keratotomies of different types; (5) Retraction of tissue by thermal effect: thermokeratoplasty or conductive keratoplasty; and (6) Other changes in the properties of the tissue, like hardening it with CXL or even, as previously suggested, modifying its refraction index¹.

Immunological classification

Depending on the origin of the tissue employed as a graft. When it comes from another individual we can have three situations: if the donor is from the same species we talk about homograft – or allograft, which is the same term but with the Greek root instead of Latin –, except in the case of identical twins – oftentimes between lab animals – where we talk about isograft. When it comes from a different species we talk about heterograft or xenograft. If the tissue comes from the same individual we talk about autograft, which at the same time can be homotopic or heterotopic. The first occurs in keratoplasties by rotation or in cases of crown homograft in which the central part comes from the own patient’s cornea. The second includes cases where the tissue comes from the fellow eye – being cornea, limbus or conjunctiva –, as well as in grafts of other origins from the same individual (mucosa, skin, etc.).

Instrumental classification

Depending on the nature of the instrumental media employed, they can be: (1) Mechanical, being manual (scalpel, etc.) or motorized (microkeratome, motorized trephine, etc.); (2) Physical, being thermal (radiofrequency, Holmium laser, microwaves, cryotherapy), photoablative (excimer laser) or photodisruptive (femtosecond laser); and (3) Photochemical, as in CXL.

Nosological classification

Finally, it is worth mention the series of procedures that are used in practice under the broader concept of keratoplasty: (1) Transplantation or corneal graft, being penetrating or lamellar, etc.; (2) Refractive keratoplasty in its different modalities (PRK, LASIK, etc.); (3) Superficial keratoplasty, being manual or with laser (PTK); (4) In crescent keratotomy or keratectomy; (5) Transplantations to restore ocular surface (of conjunctiva or other mucosae, of limbus, of amniotic membrane…); (6) Keratoprosthesis or artificial cornea; and (7) Corneal molding without cutting or ablation, which includes thermokeratoplasty or conductive keratoplasty, ring segments implantation or even CXL, etc.

TYPES OF CORNEAL TRANPLANTATION

If we refer exclusive to transplants or corneal grafts, we can consider a series of modalities based on morphological criteria such as the extension, position, shape or depth.

Based on extension

The majority of transplants are partial, in the sense that one does not substitute the entire cornea, but typically only the central area. The contrary, very much rare, is a total transplant, which reaches the limbus or beyond to become a corneoscleral or sclerocorneal graft (Figure 2).

Based on position

The majority of corneal transplants are central, but there are special situations that require peripheral grafts.

Based on shape

Almost all corneal transplants are circular, but in some occasions there have been cases with very diverse shapes, especially in peripheral grafts which can be horseshoe (Figure 3), crown, or fusiform shaped, etc.

Based on depth

Classically we have distinguished between penetrating and lamellar keratoplasties. The latter have been subdivided first in anterior and posterior, and later based on the different histological layers or levels. Thus, in the anterior group deep anterior lamellar keratoplasty (DALK, see section 5) has become particularly important, and by exclusion one can talk about a superficial type (SALK, see section 4), although epithelial keratoplasty and that of Bowman's layer have also been proposed.

Among the posterior keratoplasties, which have in common the inclusion of the endothelial layer, and during their historical evolution, multiple variants were devised with the consequent proliferation of terms and acronyms. Currently, two stand out, as they incorporate a layer of deep stromal tissue together with the endothelium, which we can call endothelial lamellar keratoplasty (DSEK or DSAEK, see section 6) and the Descemet-endothelial keratoplasty (DMEK, see section 7), which does not include stroma.

TERMINOLOGY

The word "keratoplasty" originates from the union of two Greek roots: kératos (κέρατος), "horn" or "of horn", joined to plastikè (πλαστική), "ability to model, plastic art". From the first, by Latin evolution derives the anatomical term "cornea". In Spanish, the use of the spelling "kerato" has at least the virtue of preserving the phoneme "ke", which would have been distorted into sound "ze" having gone to use a "c" as in "cinesis" (originally kinesis) or in "triceratops", from the same root as the cornea – or as with Portuguese “ceratoplastia”. The application of the second root is not so clear, since it does not directly mean a graft or transplant. It is true that "plasty" is widely used in surgery as a synonym for graft, but the meaning of "modeling" is what allows naming keratoplasty to very different things apart from corneal grafts.

Keratojargon

From the word keratoplasty, a whole family of terms (or "keratojargon")² have been generated, such as keratotomy, merging kératos with tomé (τομή), “to cut”, or with tomaios (τομαιος), "cut"; keratectomy, with ektomias (εκτομιας), "amputation, castration"; keratoprosthesis, with pros-thesis (πρόσ-θεσις), «action of putting in front, application», and so on. The term corneoplasty has also been proposed to encompass all corneal molding techniques that work without cutting or ablating. But the Latin root "cornea" actually means exactly the same as the Greek "kératos", so trying to create a different term (with a more restricted scope than keratoplasty) is a bit forced.

Special cases represent "keratophakia" and "keratomileusis", terms coined by José I. Barraquer to describe his techniques of refractive keratoplasty.³ The first arises from uniting kératos with fakós (φακός) «lentil» to mean «putting a lens inside the cornea". The second should perhaps be originally "keratosmileusis" (with one “s” in the middle), with the ending derived from smíle (σμίλη), "cobbler's blade, cutter", giving a term that means "to cut or carve the cornea". It is curious the coincidence of that Greek word with an acronym recently invented for another procedure that comes to be a recreation of the keratomileusis. Another possible origin would be myle (μυλη), "mill" or myleides (μυλο-ειδής), "like a mill", but it does not seem that "grinding the cornea" is a good description of what refractive surgery is proposed for.

The acronyms

Today’s widespread use of acronyms makes it inevitable to include them in a text like this one, although with caution. The appeal of summarizing in a few letters what a whole phrase would require, especially if it has to be repeated often, is undeniable. But the same reason has led to its proliferation, and the abuse of them can make the uninitiated to get lost in this "alphabet soup". And unlike the Greco-Roman root neologisms, which are "international" and do not favor any particular living language, acronyms derive in general from English. Mixtures of multiple origins have been perpetrated, such as LASIK, an acronym that combines another acronym ("L" from LASER) with words from English ("A" from assisted), from Latin ("I" and "S" from in situ) and a Greek neologism ("K" from keratomileusis).

Laminar or lamellar?

Finally, while it is customary in English to borrow terms directly from Latin, it is not clear that these can be passed as they are to Spanish, especially if in our lexicon there are adequate words. For example, when a layer or sheet of the cornea is grafted, in English it is called "lamellar keratoplasty". In the dictionary of the RAE (The Spanish Royal Academy of the language) we find «laminar», which means «in the form of a sheet», but there is no «lamellar» or «lamella». In fact, in Latin there is also the word "lamina", of which "lamella" is a diminutive. This would be translated into Spanish, therefore, as "laminilla" and with it we could create the adjective "laminillar", which perhaps would be preferable to "lamelar", despite the widespread use among specialists. Although the corneal stroma is made up of microscopic collagen lamellae, in general partial grafts comprise macroscopic sheets of tissue, so it does not seem necessary to resort to a diminutive. For purposes of unification, in this work we will preferably use the term "laminar", which seems more correct to us.