Superficial anterior lamellar keratoplasty (SALK) involves the transplantation of a lamina or anterior corneal layer, generally disc-shaped, respecting the normal posterior stroma and endothelium. The term "superficial" differs from deep techniques that reach a level close to the Descemet’s (DALK). It has received other descriptive descriptions of a certain aspect of the technique such as automated lamellar therapeutic keratoplasty (ALTK), which denotes the use of the microkeratome – an undoubted factor that revitalized this surgery ¹, or anterior lamellar keratoplasty assisted with a femtosecond laser (FALK)^2,3.

The goal of a SALK is, normally, to replace the altered surface tissue with another normal one and with similar dimensions. However, what defines it as such is that the recipient bed is superficial and not necessarily the donor disk. In some cases, this may be thicker than the tissue removed, for reconstructive (in case of corneal thinning) or refractive reasons⁴.

INDICATIONS ACCORDING TO PURPOSE

The possible purposes of a SALK and therefore its indications follow the same general scheme as in the penetrating keratoplasty: (a) optics, in the sense of recovering transparency, (b) tectonic or reconstructive, and (c) refractive, although more than one can coexist in a certain case. The fourth purpose, "therapeutic" in the sense of treating an infection resistant to medication, does not seem applicable to SALK, since these situations usually require a more profound action. Thus, the most common indications include^1,4,11:

- Optical purpose. Anterior corneal opacities (leucomas, nebulae) that affect the visual axis, provided that they are within the appropriate depth limits:

• Anterior corneal dystrophies and degenerations (of the Bowman and the anterior stroma). Those who have undergone this technique the most are the dystrophy of Reis-Bücklers and similar – and often confused – such as that of Thiel-Behnke (Figure 1) or the superficial variant of granular dystrophy.

• Post-traumatic, e.g., the scar after a superficial flap laceration.

• After refractive surgery. Scarring residual opacities after the reaction to laser ablation ("haze", Figure 2), after diffuse lamellar keratitis, or after perforated or irregular flaps.

• Late anterior opacities after penetrating keratoplasty⁵.

• Post-infection: of any origin (bacterial or viral, etc.). It includes those secondary to adenoviral keratitis when they do not respond to other treatments (Figure 3) and after inactive superficial herpes simplex (Figure 4).

- Tectonic or reconstructive purpose

• Losses of superficial substance after infectious or other ulcers. They are often associated with opacity and are partially filled by epithelium (corneal facet). It is important to distinguish to what extent the problem is tectonic, of opacity or of irregular astigmatism.

• After surgery of pterygium resection, superficial tumors or other keratectomies that have affected the visual axis.

• After multiple ablations, being refractive or therapeutic, that have increasingly reduced the thickness (with irregularity and/or persistent opacity).

• Ectasias. Although nowadays they tend to be treated with deep grafts, flat profile epi-keratoplasties can be included here, still an option to be considered in certain cases^6,7.

- Refractive purpose

• Keratophakia and epi-keratophakia involve, in fact, a graft of corneal tissue on a superficial bed, so they can be considered within this section.

• In any case, the importance of the cornea in ocular refraction means that all SALK have a refractive component.

PATIENT SELECTION: ALTERNATIVES AND LIMITS

The selection of a patient for SALK is conditioned by the other treatment options of these situations, such as a rigid permeable contact lens (RPCL), superficial keratectomy and DALK. In cases with dim opacities in which irregularity predominates, the use of RPCL can allow a satisfactory visual recovery. It is advisable in any case to do a test before indicating the surgery.

Anterior lamellar keratoplasty vs. superficial keratectomy

The most superficial lesions can be eliminated by superficial keratectomy, manual (scraping, etc.), mechanized (with microkeratome)^8,9 and nowadays especially with excimer laser (PTK)^10,11. This is possible up to about 100-200 μm of stroma, which implies a minimum depth or superficial limit for the indication of SALK. The obvious advantage is not to need a transplant, and also, in case of laser ablation, it is possible to associate with a refractive treatment. The disadvantages of keratectomy include the possibility of secondary opacities (even applying mitomycin C) and of inducing hyperopia or astigmatism. Since opacities reduce the effectiveness of ablation, when they are dense – as a reference, if they impede visualizing iris details – a microkeratome technique will be preferable to laser.

Anterior superficial lamellar keratoplasty vs. deep

The main advantage of a SALK over a DALK is in being less invasive, simpler and faster. It can even be done without sutures¹² or, if used, they can be removed early. This makes possible a fast-refractive surgery if necessary, whether intrastromal by lifting the graft or superficial (PRK). In cases of dystrophies, recurrences can be treated easily, changing the graft with a new one.

Although both techniques respect the integrity of the eyeball due to the fact that they are not penetrating, the degree of biomechanical involvement and the problems of graft-receptor healing are clearly lower with the superficial one. In this there is no risk of perforating the Descemet’s membrane and the stromal rejection is minimal.

Most SALK cases reach 200-250 μm deep in the receiver. This limit would be established by the risk of ectasia if the residual bed is less than about 250-300 μm. And from that level it is difficult to avoid the irregularity of the bed or other effects that degrade the optical quality of the graft, except when we get very close to the Descemet’s¹³.

When the injury affects deep areas of the stroma or there is considerable corneal thinning, a DALK is clearly preferable. This includes in the current practice almost all cases of ectasias, opacities or defects beyond the aforementioned limit, descemetocele or imminent perforation.

PREOPERATIVE EVALUATION

The indication of the type of keratoplasty is conditioned by the clinical conditions of each patient, their visual needs, the availability of tissue, the appropriate instruments, without forgetting the experience of the surgeon. In the case of SALK, in addition to the clinical history, background and main diagnosis, it is important to study the characteristics of the corneal lesion: its extension, optical density, depth, if there is loss of substance and its impact on the regularity of the corneal surface.

Diagnostic imaging tools such as corneal topography and tomography are especially useful. In particular, optical coherence tomography (OCT) allows determining the depth of the opacities or loss of substance, as well as the thickness of the residual bed, in order to decide the most appropriate technique and plan its parameters (microkeratome, femtosecond laser) (Figure 5). It will also show changes in epithelial thickness that can fill defects or irregularities of the superficial stroma. The presence of shadows in the OCT image, under dense opacities or calcified plaques, will indicate in which cases the lasers (femtosecond, or excimer lasers for a PTK) become ineffective and a mechanized cut will be preferable.

It is very important to assess optical irregularity or irregular astigmatism. For this purpose, we must perform a corneal topography (or Scheimpflug tomography) and measure vision with RPCL. When the irregularity is evident in the topography – and there is little opacity in the visual axis – a superficial graft will be ineffective since it will reproduce approximately the original topography. This is especially true for femtosecond laser cutting, somewhat less with microkeratome and can be avoided with manual dissection, although this usually generates irregularity. An ablation technique guided by topography will be indicated if the pathology is very superficial, or else it will be necessary to change to a deep graft (DALK).

In many cases opacities or leucomas and irregularity coexist due to loss of substance more or less filled by epithelium or scars that make prominence on the corneal surface. It is not always easy to distinguish between the two sources of visual degradation. For this, the RPCL test is the most useful, as it largely eliminates irregular astigmatism. If with it the patient improves little, the main problem (excluding non-corneal causes) will probably be opacity; if it can be eliminated with a SALK, a clear visual improvement is to be expected. If, on the other hand, the vision improves a lot with the lens – especially if the leucoma is not very central and dense, the problem will be above all, one of irregularity, and we will have to act as indicated above.

The topographic test (and some OCT) will offer us the pachymetric map, of great value for surgical planning. Endothelial study is also essential. Although SALK does not affect these cells, there are situations in which a superficial pathology may be secondary to an endothelial, such as, for example, a band degeneration as a consequence of endothelial dystrophy.