Superficial anterior lamellar keratoplasty (SALK) is an established technique for the treatment of opacities and other superficial corneal pathologies. It is attractive because of its simplicity, the shortage of complications and the ease of replacement in conditions that tend to recur as dystrophies. However, most experts admit that the visual results are only moderately good.

RESULTS IN LITERATURE

The specific information on SALK results is not abundant. A recent search in PubMed with the terms «superficial» «anterior» «lamellar» and «keratoplasty» gives 93 entries, but mostly referring to the deep technique or other types of keratoplasty. If it is asked to look for the complete sentence, 4 works are obtained, of which only two include analysis of visual results. With the term "SALK keratoplasty" you get another, and two more when you add "femtosecond".

Patel et al. studied 9 eyes in 8 patients with superficial opacities after penetrating keratoplasty: 3 due to recurrence of dystrophy, 2 due to haze scarring after PRK and 4 due to scarring after stromal melting. After SALK performed with microkeratome and superimposed sutures, they found an improvement in visual acuity with correction (VACC) in all of them in the last control. 7 of the 9 eyes reached 20/40 or better in the first month. Astigmatism improved an average of 0.7 diopters (D)¹. Fogla and Knyazer describe a technique in two phases with a microkeratome at 140 μm and without sutures in 4 eyes of 2 patients with Reis-Bücklers dystrophy. In the last control at 19 months on average they find a VACC of 20/30². Agarwal et al. compare two groups of 6 eyes each, operated on of SALK with microkeratome at 200 μm and without sutures, with or without associated cataract surgery (phacoemulsification), followed for 6 months. The VACC improved respectively 8.8 ± 3.4 lines and 6.8 ± 5.2 lines, which did not mean a statistically significant difference between these groups but indicates improvement of the corneal intervention itself³.

Using femtosecond lasers and sutureless technique, Shousha et al. report 13 patients followed between 12 and 69 months (mean 31). 54% obtained a VACC of 20/30 or better at 12 months. 2 patients lost 1.5 VACC lines, one due to haze after PRK and another due to recurrence of corneal dystrophy in the new corneal button. At 5 weeks, 83% had reached a VACC less than 2 lines different other than the measurement at 24 months⁴.

OUR OWN RESULTS

We performed a retrospective study of 36 consecutive cases, operated of SALK at the Centro de Oftalmología Barraquer between 1996 and 2014. It includes the three techniques: 12 hand-carved, 19 with ACS microkeratome, and 5 with femtosecond laser (Figure 1), which will allow us to compare them.

The sample presents an equal sex distribution (50%/50%) and an average age of 30 ± 7 years. In all cases there was involvement of the superficial layers of the cornea secondary to various causes such as infectious keratitis, trauma, etc. The most frequent group (41%) was that of leucomas of unspecified cause, followed by opacities after refractive surgery (Figure 2).

Both preoperatively and postoperatively, we collected the following data: visual acuity without correction (VASC) and VACC in decimal, refraction; corneal keratometry, pachymetry, topography and OCT; biometry. Examinations of the anterior segment and fundus were also performed. The preoperative VACC was not an inclusion criterion. Diameters (in mm) and cutting thickness (in mm) of donor and recipient were also collected (table 1).

When the TCL was removed at one week, the complete epithelialization of the graft was confirmed in all cases. Some presented a superficial punctate keratopathy that resolved with intense lubrication and topical corticosteroids. The most relevant complication was the presence of residual opacities, either due to incomplete resection or excessive scarring at the graft-recipient interface. It is not easy to differentiate both, since the greatest scarring usually occurs where the preoperative opacities were. In one patient, the recurrence of his original disease was observed. The statistical analysis was carried out with the help of the statistical program SPSS, with the level of significance established at a value of p <0.05.

Regarding the mean preoperative and postoperative VACC, they were respectively: in the manual dissection group, 0.19 ± 0.26 and 0.21 ± 0.24 (p = 0.168); in the microkeratome group, 0.35 ± 0.22 and 0.57 ± 0.24 (p = 0.001); and in the femtosecond laser group, 0.21 ± 0.14 and 0, 19 ± 0.11 (p = 0.68). That is, there was only significant improvement, of two lines on average, in the group operated with a microkeratome (Figure 3, Table 3).

The preoperative mean spherical equivalent (SE) for the manual dissection group was -1.77 ± 2.7 D (-5.0 / + 1.37). In the postoperative period, a tendency to hyperopia was observed, with an average SE of +1.11 ± 4.2 D (-4 / + 8). In the microkeratome group, the mean preoperative SE was -2.31 ± 6.7 D (-21 / + 7.0). In the postoperative period, a slight reduction of up to -1.19 ± 6.07 D (-23 / + 4.0) was observed. Finally, in the femtosecond laser group, the mean preoperative SE was -1.4 ± 5.5 D (-8.5 / + 6.5) and in the postoperative period it changed to a mean of -2.25 ± 3.2 D (-6.5 / +1.2), apparently due to a certain reduction in the hyperopic range. The differences between the mean preoperative and postoperative SEs were not statistically significant in any of the techniques. This may be due to the breadth of the ranges; it would be necessary to make a stratified analysis by refractive groups, since opposite changes in high myopic and high hyperopic may be cancelled in the averages.

In general, there was a certain increase in astigmatism measurable by refraction at the last control, of the order of 0.9 D on average. We compared this data between the different techniques using an ANOVA test (Figure 4, Table 4), after verifying that the sample followed a normal distribution (Shapiro-Wilk p <0.05). No significant differences were observed between these values ​​(p = 0.810), which were -3.86 ± 3.5 D (-11/0) in the group with manual dissection, -3.14 ± 2.8 D ( -10/2.5) in the group with microkeratome and -3.80 ± 2.1 D (-6.5/-1.2) in the group with femtosecond laser. This change is not clinically relevant, since the intervention seeks above all to improve transparency and irregular astigmatism.

The results have been generally satisfactory, especially in the SALK with microkeratome in which the VACC improved 2 lines on average, and considerably more in some patients. The poorer results with manual technique fall within the known, although the reconstructive nature of some of these interventions must be remembered.

In the 5 cases of SALK with femtosecond laser, the visual results have been somewhat disappointing. It is possible that this is due to the learning curve. In some cases, we found that the indication had not been correct, as irregular astigmatism prevailed over an opacity that was not very central. We also observed a tendency towards the formation of opacities at the interface in these patients, which may require more intense or prolonged corticoid postoperative treatment. In comparison with the literature – and with the cases of microkeratome in this series, it seems confirmed that to obtain visual improvements with a SALK, the depth of 200 μm should not be exceeded. Deeper lesions should be directed toward deep anterior lamellar keratoplasty.