A 27-year-old female presented for follow-up for keratoconus (KC) after collagen cross-linking in both eyes 6 months previously. Her best-corrected visual acuity (BCVA) on the Snellen chart was 20/50 in both eyes with spectacles. Manifest refraction was –9.75 cyl –5.75 × 60 and –1.25 cyl –2.75 × 150, respectively. Central cornea thicknesses were 486 and 502 microns, respectively. A slit-lamp examination revealed the presence of a Fleischer ring in her right eye and a normal examination in her left eye.
Corneal tomography was obtained via the Pentacam Comprehensive Eye Scanner (OCULUS Optikgeräte GmbH, Wetzlar, Germany). Her right eye Fig. 38.1 showed a moderate central hyperprolate (nipple-type) KC (level 4 as per Amsler-Krumeich classification) with a K max 55.62 diopters (D) and over 5 D of irregular astigmatism. The thinnest pachymetry reading was abnormal at 486 microns, and the inferior-superior (I-S) index was at 5.26 D. The anterior and posterior corneal elevation maps were abnormal Fig. 38.2 . The Belin-Ambrósio Enhanced Ectasia Display and all tomographic indices for KC were positive. The anterior elevation map in her left eye showed an abnormal elevation, with a peak of 15 microns in the 3-mm zone Fig. 38.3 . This area of posterior elevation coincided with the location of corneal thinning.
The decision was made to implant a circular intrastromal ring (MyoRing DIOPTEX GmbH, Linz, Austria) with a diameter of 5 mm and thickness of 320 μm in the right eye Fig. 38.4 . A corneal pocket was created with the femtosecond laser WaveLight FS 200 (WaveLight GmbH) at a depth of 300 µm and with a diameter of 8.5 mm. The procedure was without complications.
At 12 months of follow-up, the uncorrected visual acuity (UCVA) and BCVA had improved in her right eye to 20/50 and 20/25, respectively. The manifest refraction was +0.75 cyl −1.25 × 75. K max by Pentacam was reduced for 7.2 D, and central corneal thickness remained unchanged at 475 microns.
KC often leads to high myopia and astigmatism with significant visual impairment. ,
If spectacle or contact lens correction is not feasible in cases of advanced KC, surgical options include deep anterior lamellar keratoplasty, intracorneal ring segments (ICRSs), or penetrating keratoplasty. Although penetrating keratoplasty is associated with a high success rate, there can be significant complications including immune rejection, secondary cataract, glaucoma, microbial keratitis, and graft dehiscence. Moreover, visual rehabilitation or recovery of visual acuity after penetrating keratoplasty may be slow and unsatisfactory.
ICRS implantation represents another option. These segments are made of polymethyl methacrylate (PMMA) and are implanted within the corneal stroma to induce a change in the geometry and the refractive power of the corneal tissue.
ICRS implantation induces flattening of the central portion of the anterior corneal surface, and the peripheral area adjacent to the ring insertion is displaced forward.
Theoretical models based on finite element analysis have proven that the flattening observed after ICRS implantation is directly proportional to the thickness of the implanted segment and inversely proportional to the corneal diameter of the implantation site. This means that the thicker and the smaller the ICRS diameter, the higher the corneal flattening effect.
The short arc length (90 degrees, 120 degrees, and 130 degrees) ICRS can correct low-to-moderate compound myopic astigmatism safely, whereas larger arc length (210 degrees, 340 degrees, 355 degrees, and 360 degrees) ICRS can result in higher spherical correction.
The advantage of corneal rings is that they can improve vision in these patients and may delay or obviate the need for a corneal transplant.
In oval cones, asymmetric ring segments or even single ring segments placed inferiorly based on the topographic profile may induce greater regularization. However, in cases of central cones, the use of long arc rings is likely to produce a maximum flattening effect. Circular rings were proposed for such cases in 2008, and a variety of studies have shown efficacy and safety of these rings with no loss of BCVA related to implantation. ,
It should be noted, however, that implantation of corneal rings does not stop the progression of KC. Vega-Estrada et al. have shown at 5 years postoperatively after ICRS implantation a significant regression of the positive effect, which negatively affected the vision of young patients.
It is for this reason that corneal cross-linking (CXL) as an adjunct to ICRS implantation may be necessary to halt the disease.
The application of CXL significantly increases corneal rigidity immediately after treatment, with an 80% increase of Young’s modulus and a 450% increase in the thinner human cornea at 6% strain. , , ,
In this case study, we have a patient with advanced and progressive KC (grade 4).
Our approach was to perform the cross-linking first, followed in 6 months by ICRS implantation. Once stabilization after cross-linking is achieved, rings can safely be placed. We have not had difficulty implanting corneal rings after cross-linking either with the use of a femtosecond laser or with the manual technique.
In this case, we chose a circular ring because it better corrects central KC and achieves a more significant flattening effect.