title: Cone location and correction of keratoconus with rigid gas-permeable contact lenses.
abstract: OBJECTIVE
  To evaluate the influence of cone location and corneal cylinder on RGP corrected visual acuities and residual astigmatism in patients with keratoconus.
  
  
  METHODS
  In this prospective study, 156 eyes from 134 patients were enrolled. Complete ophthalmologic examination including manifest refraction, Best spectacle visual acuity (BSCVA), slit-lamp biomicroscopy was performed and corneal topography analysis was done. According to the cone location on the topographic map, the patients were divided into central and paracentral cone groups. Trial RGP lenses were selected based on the flat Sim K readings and a 'three-point touch' fitting approach was used. Over contact lens refraction was performed, residual astigmatism (RA) was measured and best-corrected RGP visual acuities (RGPVA) were recorded.
  
  
  RESULTS
  The mean age (±SD) was 22.1±5.3 years. 76 eyes (48.6%) had central and 80 eyes (51.4%) had paracentral cone. Prior to RGP lenses fitting mean (±SD) subjective refraction spherical equivalent (SRSE), subjective refraction astigmatism (SRAST) and BSCVA (logMAR) were -5.04±2.27 D, -3.51±1.68 D and 0.34±0.14, respectively. There were statistically significant differences between central and paracentral cone groups in mean values of SRSE, SRAST, flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal cylinder (p-values<0.05). Comparison of BSCVA to RGPVA shows that vision has improved 0.3logMAR by RGP lenses (p<0.0001). Mean (±SD) RA was -0.72±0.39 D. There were no statistically significant differences between RGPVAs and RAs of central and paracentral cone groups (p=0.22) and (p=0.42), respectively. Pearson's correlation analysis shows that there is a statistically significant relationship between corneal cylinder and BSCVA and RGPVA, However, the relationship between corneal cylinder and residual astigmatism was not significant.
  
  
  CONCLUSIONS
  Cone location has no effect on the RGP corrected visual acuities and residual astigmatism in patients with keratoconus. Corneal cylinder and Sim K values influence RGP-corrected visual acuities but do not influence residual astigmatism.
authors.affiliation: Poostchi Ophthalmology Research Center, Department of Ophthalmology, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Mahmood
authors.initials: M
authors.lastname: Nejabat
authors.forename: Mohammad Reza
authors.initials: MR
authors.lastname: Khalili
authors.forename: Cirous
authors.initials: C
authors.lastname: Dehghani
mesh_terms: D001251:Astigmatism / Q000175:diagnosis* / Q000209:etiology / Q000534:rehabilitation*; D003261:Contact Lenses; D005260:Female; D006801:Humans; D007640:Keratoconus / Q000150:complications / Q000175:diagnosis* / Q000534:rehabilitation; D008297:Male; D016896:Treatment Outcome; D055815:Young Adult