Quantitative Interocular Comparison of Total Corneal Surface Area and Corneal Diameter in Patients With Highly Asymmetric Keratoconus.

Keratoconus is a progressive corneal disorder which is frequently asymmetric. The aetiology of keratoconus remains unclear, and the concept of keratoconus as an ectatic disorder has been challenged recently. We carried out a retrospective study in 160 eyes of 80 patients, to evaluate and compare interocular differences in corneal diameter and surface area in patients with unilateral/highly asymmetric keratoconus (UHAKC). Calculations were performed using raw topographic elevation data derived from topographic measurements using Orbscan II, and we extrapolated surface areas up to measured corneal diameter. We also evaluated inter-eye correlation, and correlation between corneal surface area, corneal diameter and keratoconus severity. Our results showed a statistically significant but not clinically important greater corneal diameter (12.14mm and 12.17 mm; p=0.04), and corneal surface area (paired t-test, p<0.0001; p=0.0009 respectively) in more affected eyes. Inter-eye comparison revealed corneal diameter, anterior chamber depth, and corneal surface area were strongly correlated between eyes. Corneal surface area remained strongly correlated, and Bland-Altman analysis also showed strong inter-ocular agreement. Our results show that in patients with UHAKC the interocular difference in corneal diameter and corneal surface area is clinically insignificant, and are consistent with a redistribution, rather than increase, of corneal surface area with keratoconus progression.

Keratorefractive Surgery Outcomes in Keratoconus Suspect Patients

Purpose. To examine the outcomes of keratorefractive surgeries in keratoconus suspect patients. Methods. This study included 192 keratoconus suspects (351 eyes), treated with photorefractive keratectomy (PRK) (211 eyes), Lasik (96 eyes), and Femto-Lasik (44 eyes) surgeries in an eye clinic. The best spectacle-corrected visual acuity (BSCVA) and subjective refraction were evaluated preoperatively and postoperatively (three months and five years after the procedure). The Orbscan II topography system was also used preoperatively and five years after the procedure. Results. The patients’ mean age was 31.6 ± 5.49 years (range: 21–47 years) in the last follow-up. There was no significant difference between the preoperative and postoperative mean values of BSCVA in any of the surgical methods ( P = 0.49). The mean spherical equivalent, cylindrical power, corneal curvature, the thinnest corneal thickness, and the central corneal thickness decreased significantly in the last follow-up ( P < 0.001). Four patients (3.50%) in the PRK group and one patient (1.85%) in the Lasik group needed glasses in the last follow-up. Eleven cases of corneal ectasia were detected in the last follow-up (six eyes of four patients [2.84%] and five eyes of four patients [5.20%] in the PRK and Lasik groups, respectively). Conclusion. In the present study, the high risk of postoperative ectasia was detected in keratoconus suspects following PRK and Lasik surgeries. According to the results, it can be concluded that Femto-Lasik surgery provides better outcomes than Lasik and PRK. Overall, the surgical criteria are suggested to be evaluated case by case, and the patients must be followed up over time to assess the corneal topography and refraction stability.

The Impact of Changes in Corneal Back Surface Astigmatism on the Residual Astigmatic Refractive Error following Routine Uncomplicated Phacoemulsification

Purpose. To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification. Methods. Astigmatism was evaluated by autorefractometry and subjective refraction and at both the corneal surfaces with Orbscan II™ (Bausch & Lomb) over central 3 mm and 5 mm optical zones at 1, 2, and 3 months after routine phacoemulsification in 103 patients implanted with monofocal nontoric intraocular lenses (IOLs, one eye/patient). Data were subjected to vector analysis to determine the actual change (Δ) in astigmatism (power and axis) for the refractive and Orbscan II findings. Results. The number of cases that attended where ΔRA was ≥0.50 DC between 1 and 2 months was 52 by autorefractometry and 36 by subjective refraction and between 2 and 3 months was 24 by autorefractometry and 19 by subjective refraction. Vector analysis revealed significant correlations between ΔFSA and ΔRA for data obtained by autorefractometry but not by subjective refraction. At all times, ΔBSA was greater than ΔFSA (p<0.01). Key findings for ΔBSA values over the central 3 mm zone were between (A) the sine of the axis of ΔRA (y) and sine of the axis of ΔBSA (x) for the data obtained by autorefractometry (between 1 and 2 months, y = 0.749 − 0.303x, r = 0.299, n = 52, p=0.031) and subjective refraction (between 2 and 3 months, y = 0.6614 − 0.4755x, r = 0.474, n = 19, p=0.040) and (B) ΔRA (y) and ΔBSA (x) powers between 2 and 3 months postoperatively for the data obtained by autorefractometry (ΔRA = 0.118 ΔBSA + 0.681 r = 0.467, n = 24, p=0.021) and subjective refraction (ΔRA = 0.072 ΔBSA + 0.545 r = 0.510, n = 19, p=0.026). Conclusion. Changes in the ocular residual refractive astigmatic error after implanting a monofocal nontoric IOL are associated with changes in astigmatism at the back surface of the cornea within the central optical zone.

Group Analysis of Q Values Calculated with Tangential Radius of Curvature from Human Anterior Corneal Surface

Objective. To calculate the Q values from the human anterior corneal surface with the tangential radius of curvature and analyze its distribution characteristics in different age and refractive status groups. Methods. Tangential power maps of the anterior cornea from Orbscan II were acquired for 201 subjects’ right eyes. They were divided into groups of adults and children and then divided further into subgroups according to the refraction status. The Q values of each semimeridian were calculated by the tangential radius with a linear regression equation. The Q value distribution in both the nasal cornea and temporal cornea were analyzed. Results. The mean temporal Q values of the emmetropia group of adults and all children’s groups were significantly different from the mean nasal Q value. The mean nasal corneal Q values were more negative in children. The adult group showed differences only in the low myopia group. The mean Q value of the nasal cornea among different refractive groups of children was significantly different, and so was the temporal cornea between the adult myopia and emmetropia group. Conclusion. The method using the tangential radius of curvature combined with linear regression to obtain anterior surface Q values for both adults and children was stable and reliable. When we analyzed the anterior corneal Q value, area division was necessary.

Comprehensive Evaluation of Anterior Corneal Change in Asphericity Calculated by the Tangential Radius of Curvature after LASIK

To evaluate the change in the anterior corneal asphericity (ΔQ) comprehensively calculated using the tangential radius (rt) after LASIK. Forty-two right eyes were evaluated using the Orbscan II corneal topographer. The pre- and postoperativeQ-values of the flat principal semimeridians calculated by the sagittal radius were compared to those by the tangential radius. TheQ-value of each semimeridian in the horizontal region was calculated byrt. Fourier fitting was used to model the 360-semimeridional variation ofQ-values and to fit theQ-values in the vertical region before and after surgery. There were significant differences inQ-values between the two methods before (P<0.001) and after surgery (P=0.003). A significant increase in postoperativeQ-value was detected compared to preoperativeQ-value (P<0.001) calculated byrt. The 360-semimeridional variation of theQ-values was well fitted with a third- and fourth-degree Fourier function before and after surgery. The ΔQ-value distribution presented double valley variation, with the amount of ΔQbeing lowest in the near-vertical regions and highest in the near-horizontal regions. Calculating theQ-value withrtcombined with Fourier fitting, we evaluated 360 ΔQ-values’ variation of semimeridians of the entire anterior corneal surface and then displayed true and complete anterior corneal shape after LASIK.