Annular keratectomy assisted by femto second laser: a case report

Abstract

keratoconus is a multifactorial corneal degeneration. It commonly presents with corneal thinning, irregular astigmatism and poor vision that generally does not improve with glasses and typically requires the use of contact lenses. It appears in pediatric patients, adolescents and young adults and very rarely presents itself in individuals older than 35.1 In developing countries, it is one of the principal indications for a full thickness corneal graft. In these countries, it is generally difficult to obtain tissue, generating long waiting lists for corneal transplants. Fortunately, the number of penetrating keratoplasties for keratoconus has decreased in recent years thanks to earlier detection of the disease and new treatment options. Professor Theo Seiler discovered corneal collagen cross-linking (CXL) at the end of the 1990’s at Dresden Technical University in Germany. The FDA has approved it in the US. It is indicated most in young patients with evidence of progressive keratoconus, the principal purpose of CXL is to avoid progression.2 The intracorneal segments may be very effective in regularizing the cornea and improving the astigmatic component, especially when assisted by the femtosecond laser. However, the spherical component, especially in regards to high degree myopia, is difficult to correct with this technique. Some surgeons combine the use of intracorneal segments with phakic lenses to achieve better UDVA.3 However, if the segments are implanted superficially or when the segments are near to the incision, they can extrude through the anterior corneal surface. In central keratoconus without leukoma or Vogt’s striae, 355° intracorneal segments may be indicated (Keraring, Mediphacos).4 However, in our experience, they remain near to the incision and extrude with greater frequency. For this reason, we reduced the arc to 340°. We do not have enough experience with these shorter segments to make a definitive comment. More than 50years ago, Professor José Ignacio Barraquer demonstrated the law of thickness. Based on this law, there exist two alternatives for flattening the central cornea and correcting myopia, or in this case keratoconus. The first alternative is to augment the peripheral corneal thickness, as the intracorneal segments or lenticule inclusions do in the periphery of the bag created with the femtosecond laser.5 The second alternative is to take out the tissue in the center of the cornea to flatten it.6 Combining the surface ablation with excimer laser (PRK), simultaneously guided by topography and CXL, is called the Athens Protocol. In a very select group of patients, positive results are being reported with keratoconus grade I or II with minor stable refraction at 6 D of spherical equivalent and pachymetry at 400 microns.7 Enrique Graue et al. in Mexico City describe the combination of SMILE and CXL as the “Aztec Protocol” to differentiate it from the Athens Protocol in which SMILE and CXL are combined to treat thwarted keratoconus or irregular corneas, injecting riboflavin inside the bag once the SMILE lens is extracted and applying Ultraviolet A light with a wavelength of 370nm to 3mW/cm2 for 30minutes. Their results suggested that combining SMILE and intrastromal corneal CXL is a promising treatment option for patients for whom conventional laser refractive surgery is contraindicated.8 There are multiple case reports of patients with progressive keratoconus who submitted to SMILE without CXL and it was not a viable alternative.9,10 Some of these proposed techniques are still under investigation and are not appropriate for patients with advanced keratoconus. The semilunar keratectomy or “crescent keratectomy” as described by Dr. José Ignacio Barraquer, has been used for years to improve high astigmatism in patients with pellucid marginal degeneration or in cornea transplants with high astigmatism postoperatively.6 At the moment, this technique can only be performed manually. Its results are not as reproducible and depend on the ability and experience of the surgeon. Recently, Dr. Carriazo published the creation of semilunar and annular resections with a laser. He resumed Dr. Barraquer’s work, but using an excimer laser instead to treat patients with keratoconus with semilunar or annular resections with greater precision. He uses a mask designed for this purpose, making the laser ablation on the mask. He has published his very promising results.11