Mathematical analysis of corneal remodelling after intracorneal ring surgery in keratoconus

AIM：To represent mathematically the intersection between the ectatic corneal geometry and the plane of intracorneal ring implants（ICRS）in order to determine the corneal response to ICRS surgery in keratoconus（KC）.Thereafter,to present the concept and early results of a newly derived topography-guided nomogram for ICRS surgery for the treatment of keratectasia. METHODS：The corneal rings plane intersection was modelled to a conic section. Ring effect was the result of：the ring size,position（steep vs flat）,location（distance from the geometric centre of the cornea）,and the discrepancy between the ring＇s curvature and the tunnel＇s curvature.Femtosecond laser was used to create the tunnels and the incision sites were chosen according to the nomogram in order to place the thickest ring in the steepest portion of the cornea regardless of the astigmatism axis of refraction.RESULTS：The conic section had a more prolate shape in the steep area of the cornea than in the flat area,depending on the corneal sagittal curvature. Equal ring size had more flattening effect in the steep area than in the flat area. Thick segment should be implanted under the steep portion of the cornea regardless of the cylinder axis of refraction. Single segment in the steep area was sufficient in early and moderate cases of KC. The new nomogram provided more topographic regularity with significant reduction of astigmatism and better improvement in uncorrected visual acuity（UCVA）and best-corrected visual acuity（BCVA）than the conventional nomogram. CONCLUSION：The newly derived nomogram can produce better results than the conventional nomogram. Moreover,based on this concept,a new nomogram can be integrated into the femtosecond laser software to create topography-guided,customized,elliptical tunnels with modified focal asphericity that allows for customized focal flattening of the irregularly steepened ectatic cornea.

Histopathologic findings of keratoconus corneas underwent penetrating keratoplasty according to topographic measurements and keratoconus severity

AIM： To investigate the histopathologic and morphological changes of the corneas with keratoconus（KC） undergoing penetrating keratoplasty（PKP） according to topographic findings and severity of KC.METHODS： The corneal tissue of 35 samples with KC was retrospectively evaluated with conventional light microscopy. Topographic and pachymetric parameters of keratoconus corneas by means of Pentacam such as mean keratometry（K） and central corneal thickness（CCT） were recorded. Severity of KC was graded according to Amsler-Krumeich classification. RESULTS： Epithelial thinning and breaks in Bowman＇s layer are the most common findings in keratoconus corneas（94.3% and 82.9% corneas, respectively）. The results revealed statistically significant higher mean K value and lower CCT in the keratoconus corneas that were affected by epithelial thinning, breaks in the Bowman＇s layer, folds in the Descemet＇s membrane, epithelial scars, breaks in Descemet＇s membrane, and stromal scars than those corneas without these findings（P〈0.05）. Moreover, those corneas with epithelial thinning, breaks in the Bowman＇s layer, folds in Descemet＇s membrane, epithelial scars, and stromal scars had significantly more severe disease than those corneas without these findings（P〈0.05）. The presence of the stromal and epithelial scars were associated with the higher KC severity, in which, respectively, 87.5% and 80.0% of the corneas with stromal and epithelial scars had stage 4 of the KC severity. CONCLUSION： It seems that there are some specific patterns in histologic changes of the keratoconus corneas. The presence of pathologic findings was correlatedwith thinner and steeper corneas. Epithelial or stromal scars were associated with the highest disease severity. The description of histopathologic findings of KC may help in elucidating the pathogenesis of the disease and help pathologist in differentiating KC from other corneal diseases.