Effect of flat cornea on visual outcome following lasik: a prospective observational study at lasik centre in north India

Introduction: A trichobezoar is a mass of The relationship between preoperative keratometry (K) and visual outcomes in laser-assisted in situ keratomileusis (LASIK) has been studied in high myopia and hyperopia, but not in moderate myopia. Rao SK et al report increased under-correction in eyes with preoperative spherical equivalent (SE) of −10.0 to −11.9 diopters (D), and in eyes with flat corneas compared with steeper corneas. Williams LB et al, conversely, reported under-correction and loss of best spectacle corrected visual acuity (BSCVA) following hyperopic LASIK in eyes with steep corneas, compared with flat corneas.7Some refractive surgeons have expressed concern that a relatively steep postoperative cornea following hyperopic LASIK may reduce the quality of outcomes. However, published studies examining the association between postoperative keratometry and visual outcomes yield conflicting results. One possible reason for these conflicting findings is the covariance of postoperative keratometry with preoperative sphere. Specifically, higher levels of hyperopic correction typically result in steeper postoperative keratometry, but larger corrections (due to high preoperative sphere values) also tend to result in poorer outcomes, irrespective of keratometry. Therefore, to accurately assess how postoperative keratometry affects visual outcomes, an analysis must differentiate the effect of a large sphere correction from the effect of a steep postoperative cornea. However, studies with limited sample sizes may lack the statistical power to discriminate between these two effects, and most of the available published reports include fewer than 150 eyes. Aims and Objectives: To study the effect of flat cornea on the visual outcome following LASIK. Material and Methods: Our study was a Prospective observational study conducted at the LASIK centre at the Postgraduate Department of Ophthalmology, Govt. Medical College Srinagar. The study was conducted over a period of one and a half year on 89 patients (174 myopic eyes. The pre-LASIK examination included; assessment of uncorrected visual acuity (UCVA) and best corrected visual acuity (BCVA), slit lamp examination, biomicroscopy, non-contact tonometry, indirect opthalmoscospy, specular microscopy for measuring central corneal thickness and corneal topography. Inclusion Criteria: 1. Age >18 years. 2. A stable refraction change of less than 0.5 Diopters(D) in the last year 3. Preoperative cycloplegic spherical refraction between -1.00 D and -8.00 D of myopia and up to -3.00 D of astigmatism. 4. Preoperative best corrected visual acuity (BCVA) > 6/9. 5. Estimated residual thickness of the stroma of at least 250 µm after laser ablation with emmetropia being the goal in all cases. EXCLUSION CRITERIA: 1. Keratoconus or forma frustakeratoconus 2. Central corneal thickness < 450 µm. 3. Unstable refraction 4. Prior ocular and/or corneal surgery 5. Connective tissue disorder 6. Pregnancy and breast feeding 7. Severe dry eye disease. LASIK was done using the Moria One Use Plus micro-keratome (MoriaSurgicals, France) that creates a flap of 9mm with an intended thickness of 100 micro meter with a nasal hinge. Subsequent laser ablation was done with the Carl Zeiss Meditec MEL 80 (Germany) Excimer laser. All eyes were treated using an identical method with optical zone diameter ranging from 6.00 mm to 7.00 mm.Corneal topography was performed on ATLAS (ZIESS) machine. Analysis of the topographic maps was done besides measuring the corneal keratometry (K) in the flat and steep axis. Pre and postoperative average K = (K flat + K steep)/ 2 was calculated.Patient were examined post-LASIK at third month and corneal keratometry was done. Results: Table-3: Effect of keratometry on spherical equivalent Group Mean Pre-LASIK Keratometry No. of Eyes Mean Spherical Equivalent Post Lasik Spherical Equivalent A 40.12 35 -4.00 -0.75 B 44.00 118 -4.25 -0.25 C 45.50 21 -3.75 -0.25 Our study showed that Group A with mean pre-LASIK keratometry of 40.12D had mean postlasik spherical equivalent towards higher myopic side ie 0.-75diopter as compared to Group B and C with residual spherical equivalent of 0.-25diopter. Discussion: Our study showed that pre LASIK mean keratometry influences the visual outcome. The factors influencing the final visual outcome was attributed to the fact that eyes with pre LASIK flatter cornea(mean k 40.12 D), showed a tendency towards under correction as depicted in Table 3. We grouped eyes according to pre- LASIK keratometry and found that the post-LASIK spherical equivalent was towards higher myopic side in group A with mean pre LASIK keratometry of 40.12 D as compared to other groups despite mean pre LASIK spherical equivalent being comparable in all the groups. Our results were supported by many studies, Our study concluded flatter corneas have a tendency towards under-correction