Fig. 25.1
Options of refractive surgery in keratoconus
These techniques are normally used in patients with forme fruste keratoconus or stable keratoconus [13] and are not choice in progressive keratoconus, although there are some studies along these lines [15, 19, 22].
We have always followed the same stability criteria in our work. If one (or several) of the circumstances described later occurs in an interval of less than 1 year, keratoconus is considered progressive and we consider it is not appropriate to apply some of these techniques :
Increase in astigmatism of 1.0 D or more.
Significant changes in the orientation of refractive axes.
Increase of 1.0 D or more in the optical power of the steepest corneal meridian.
Decrease of 25 μm or more in corneal thickness.
25.1 Phakic Intraocular Lenses (PIOL)
Several studies have reported significant visual improvements with good safety and efficacy indices for phakic intraocular lens insertion in keratoconus [23, 24].
The use of phakic lenses has advantages such as efficient and stable ametropia compensation, preservation of accommodation, centered and rotational stability not dependent on corneal morphology, retinal image magnification in patients with high myopia, and reversibility in the event of failure.
Negative effects of phakic lens implantation include accelerated endothelial cell loss, cataract formation, pupil ovalization, lens rotation or decentration, photic phenomena, and retinal detachment in isolated cases.
25.1.1 Implantation Criteria
Based on our experience and that described in medical literature, we believe the criteria in Table 25.1 should be met for phakic lens implantation in keratoconus:
Table 25.1
Criteria for phakic lens implantation in keratoconus
Indications | Relative indications | Contraindications |
|---|---|---|
Corneal topography stable and age > 35–40 years | Keratoconus, stable following CCL (topography and refraction) | Progressive keratoconus, unstable corneas |
Good spectacle-corrected visual acuity (>0.6; 20/30) | Following DALK or PKP | Young patients (<25 years-old?) |
Stable refraction for about 2 years. Spherical equivalent greater than −2.75 D | Moderately good vision but with poor tolerance to glasses or contact lenses | Highly aberrated eyes (total HOA >3 μm) with poor best spectacle corrected visual acuity (<0.5) |
Absence of clinically significant irregular astigmatism. We consider irregular astigmatism to be clinically significant when there is a difference greater than one line of corrected vision between the vision obtained with glasses and the vision obtained with hard contact lenses | Anisometropia with adequate refractive anatomy and stable refraction |
According to a recent study by our group with the largest number of case studies on phakic lens implantation in keratoconus reported to date [13], the safety of this procedure in visual terms is high (post-CDVA/pre-CDVA = 1.19 ± 0.29). It is also an effective operation (post-UDVA/pre-CDVA = 0.90 ± 0.26). In fact, many patients stopped wearing glasses after phakic lens implantation. These results are roughly consistent with those reported by other authors [8–12].
It is also interesting to note that, based on the results of this study, implantation of a foldable lenses can be as safe and effective as implantation of a hard lens. The following models were studied: foldable Artiflex lens (Ophtec, Netherlands) and the ICL (implantable collamer lens, STAAR Surgical, United States). This result is particularly interesting, because flexible lenses can be inserted and removed through smaller incisions thanks to their flexibility.
We recommend that, whenever possible, implantation be performed using this type of microincision surgery (with incision of 1.8–2.00 mm) to ensure that the ectatic process is not affected as a result of the operation. It has been widely demonstrated that use of this type of incision causes minimal alternations in the cornea [25, 26].
25.2 Photorefractive Keratectomy (PRK)
This is a technique that ablates the stroma without performing the corneal flap. The cornea is reshaped using the excimer laser onto Bowman’s layer and the anterior stroma. The epithelium can be removed mechanically (by brush, blade, or epikeratome), chemically (most often with approximately 20 % alcohol), or by laser.
PRK for the treatment of keratoconus has been discussed by several authors. While the precision obtained in ablating the corneal surface with laser ablation may be very high in normal corneas, it is unclear whether the same occurs with keratoconic patients due to preexisting corneal asymmetry and to the reduced reliability and repeatability of keratometry measurements in such patients [27]. In addition, ablating corneal tissue in patients whose corneal biomechanics are already altered is a risk factor to bear in mind. Obviously, unlike phakic lenses, this technique cannot be used to treat high levels of myopia (which are frequent in keratoconus), because the ablation is so deep in these cases. Furthermore, this technique is not reversible.
Exclusion Criteria
Advanced or progressive keratoconus.
Keratometry >56 D.
Pachymetry less than 440 μm.
Large displacement of the apex.
Presence of scars or rupture of the Bowman membrane.
Our experience shows that it is a safe technique (1.03 ± 0.08) with a high degree of efficacy (0.91 ± 0.18), but we have only been able to confirm these results in the short term. Moreover, we detected some cases in which the ectasia became progressive. Hence the need for combined use with cross-linking and for more cases studied with longer follow-up.
Interestingly, regarding the use of this surgery on keratonic eyes, some authors like Vinciguerra et al. [28] suggest using corneal topography data before epithelial removal to avoid the masking effect over the stroma that may occur in keratoconus. This is obviously an inconvenient practice for the patient but a well-founded one in our opinion.
Lastly, in these cases, the ablation should not exceed 50 μm [29] and regarding the ablation profile to use, we think it is better to use wavefront-guided ablation as opposed to topography-guided ablation. This is because with the latter technique the operation is conducted based on the aberrometric profile of the anterior corneal surface . As has been widely reported in the literature, high internal astigmatism is common in keratoconic eyes due to large disparities between the anterior and posterior corneal surfaces [30]. For this reason, we believe it is better to use a customized wavefront profile.
In any case, as stated earlier, we do not consider PRK on its own to be the technique of choice in keratoconus, despite the high efficacy rates obtained (in a small series of only 21 eyes), due to doubts concerning long-term stability [31].
25.3 Combined Treatments
In recent years, refractive surgery of keratoconus has evolved combining different techniques to achieve the following objectives: improve visual quality and stop the progression of the disease. Currently, both PRK and PIOL are combined with CXL and ICRS.
In the combined treatment with CXL, the following principles must be taken into account:
Indications :
Contraindications :
Regarding combined treatments with ICRS , we must take a number of preoperative indications into account [40], in order to increase the likelihood of attaining the best possible postoperative outcomes for the patient:
Corrected distance visual acuity (CDVA) < 0.9.
Stable cases. Patients with refractive and topographic stability, confirmed in the past 12 months.
Aligning of refractive and keratometric axes. The least curved meridian of the cornea (K1) should be aligned with the refractive cylinder axis (expressed as a negative value). When the meridian and the axis form an angle of between 0° and 15° they are considered properly aligned.
Internal astigmatism <3 D .
Corneal pachymetry in tunnel area > 300 μm (Ferrara); > 450 μm (Intacs); > 250 μm (Keraring), with these being minimum thicknesses depending on the thickness of each intracorneal segment.
Absence of corneal leukoma .
25.3.1 Photorefractive Keratectomy
25.3.1.1 PRK and CXL
The rationale for this surgical approach is to halt the progression of keratoconus and to provide better refractive, topographic, and HOA results than with CXL treatment alone by reshaping the cornea and flattening and regularizing the anterior corneal surface, while ensuring stable outcomes over time in terms of nonprogression of the disease. It is very important to know that the CXL significantly increases corneal rigidity immediately after treatment, with a 71.9 % increase of Young’s modulus in pig corneas and a 328.9 % increase in human corneas [41] and therefore, a cross-linked cornea may withstand low tissue ablations.
There are two possibilities for this combination:
Sequential treatment (PRK several months after CXL).
Kanellopoulos and Binder [14] described a case with bilateral progressive keratoconus treated on one eye with topography-guided PRK and who had undergone CXL on both eyes 12 months before. PRK was attempted to improve the post-CXL refractive error; a significant clinical improvement and apparent stability was successfully achieved after two surgeries. At the 18 months follow-up visit, the uncorrected visual acuity (UCVA) and corrected distance visual acuity (CDVA) were improved from 20/100 to 20/20 and from 20/50 to 20/20, respectively, and there was no evidence of topographic progression.
Simultaneous treatment (PRK + CXL).
Kymionis et al. [16] reported promising results in study about customized topography-guided PRK followed by immediate CXL in 12 patients with keratoconus. The preoperative mean (logMAR) UCVA was 0.99 ± 0.81, which increase postoperatively to 0.16 ± 0.15.
Other studies have also shown good results with simultaneous PRK and CXL [16, 18, 42]. It improved in SE, UDVA, CDVA, and KMAX and did not show evidence of topographic progression at 12 months follow-up visit.
Sequential vs. Simultaneous .
Kanellopoulos [17] compared these techniques. For this he used different sequence and timing which were evaluated in consecutive keratoconus cases. This study included a total of 325 eyes with keratoconus divided into two groups:
The sequential group had topo-guided PRK 6 months after the CXL procedure (127 eyes).
The simultaneous group had a CXL procedure immediately after the MMC topo-guided PRK (198 eyes).
The simultaneous group demonstrated statistically superior results in CDVA, spherical equivalent (SE) reduction, maximum keratometry (Kmax) reduction, and corneal haze score. Furthermore, it was not observed progression of ectasia in either group after a mean follow-up of 36 months.
Concluding remarks.
Low ablation of tissue seems to be safe and effective in keratoconic cross-linked corneas according to the current evidence.
Topo-guided PRK provides refractive error correction and HOAs decrease.
Simultaneous PRK+CXL has shown better outcomes than the sequential strategy.
Therefore, PRK+CXL is a promising tool in keratoconus that needs further investigation to assess its stability in the long term and to establish the indications.
25.3.1.2 PRK and CXL + ICRS
Not many research groups have studied about this type of multisurgery. Coskunseven et al. [43] performed a prospective case series (16 eyes with progressive keratoconus). All patients underwent topography-guided transepithelial PRK after Keraring ICRS implantation followed by CXL treatment. They applied a three-step sequence where the time interval between each of the three operations was 6 months. The results are promising: mean SE was significantly decreased from −5.66 ± 5.63 to −0.98 ± 2.21 D. Regarding visual results: the preoperative mean (LogMAR) UDVA and CDVA were significantly improved from 1.14 ± 0.36 to 0.25 ± 0.13 and 0.75 ± 0.24 to 0.13 ± 0.06, respectively. In other study [44], observed cases with moderate keratoconus who had previous Intacs ICRS implantation with the Intralase laser at least 6 months before PRK-CXL. They found that it satisfactorily changed the UDVA, CDVA, and central K values from 0.20 ± 0.12 to 0.55 ± 0.15, from 0.58 ± 0.13 to 0.77 ± 0.17, and from 50.91 ± 5.50 to 46.61 ± 4.52 D, respectively.
Two different studies have performed Intacs implantations followed by same-day PRK and CXL in patients with keratoconus. The results are satisfactory: UDVA, CDVA, and sphere and cylinder refractive measurements all significantly improved after surgery, with most eyes gaining two lines of CDVA or more [45, 46].
25.3.2 Phakic IOL
In progressive cases of keratoconus, surgical alternatives to corneal transplantation have been suggested. These include phakic IOL implantation, CXL, and ICRS. Once the keratoconus has stabilized, refractive solutions can be considered. A perfect power calculation of intraocular lens is essential to achieve good visual results, because we are in cases with high curvature and corneal irregularity.
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