title: Surgically induced astigmatism after laser in situ keratomileusis for spherical myopia.
abstract: OBJECTIVE
  To study risk factors for surgically induced astigmatism (SIA) after laser in situ keratomileusis (LASIK).
  
  
  METHODS
  In a retrospective case control study of 104 eyes (52 patients) that underwent LASIK for myopia (spherical ablation alone), two groups were studied: 42/104 eyes with SIA, and controls (62/104 eyes). The main variables studied were preoperative refraction, corneal thickness, preoperative keratometric power, amount of ablation, ablation zone diameter, flap thickness, flap size, and the presence of complications. The effect of SIA on visual performance was also evaluated.
  
  
  RESULTS
  The mean myopia for which LASIK was undertaken was -4.50 +/- 2.04 D. Mean scalar astigmatism induced was 0.35 +/- 0.50 D at 1 month, 0.33 +/- 0.40 D at 3 months, and 0.16 +/- 0.60 D at 6 months. SIA based on refractive cylinder was 0.66 +/- 0.29 D at 1 month, 0.54 +/- 0.32 D at 3 months, and 0.49 +/- 0.34 D at 6 months. Mean axis of vector induced astigmatism was 82.5 degrees +/- 57 degrees at 1 month, 98.86 degrees +/- 52.4 degrees at 3 months, and 113.9 degrees +/- 62.6 degrees at 6 months. Risk factors associated with the occurrence of SIA were preoperative keratometric power of >44 D [OR (95% CI); 1.97 (0.62 to 6.26)], ablation zone diameter of <6 mm [OR (95 % CI) 2.76; (0.6 to 12.6)], and suction ring diameter of 8.5 mm [OR (95% CI) 12.46; (2.0 to 77.38)]. The occurrence of SIA had no significant effect on uncorrected Snellen high contrast visual acuity, contrast sensitivity, and glare in comparison with controls.
  
  
  CONCLUSIONS
  Surgically induced astigmatism was more likely to occur with the use of smaller suction rings of 8.5 mm and in ablation zones less than 6 mm. Parameters for visual performance were not affected by the presence of surgically induced astigmatism.
authors.affiliation: Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Namrata
authors.initials: N
authors.lastname: Sharma
authors.forename: Mayank S
authors.initials: MS
authors.lastname: Pangtey
authors.forename: Rasik B
authors.initials: RB
authors.lastname: Vajpayee
authors.forename: Tanuj
authors.initials: T
authors.lastname: Dada
authors.forename: Tushar
authors.initials: T
authors.lastname: Aggarwal
authors.forename: Vijay K
authors.initials: VK
authors.lastname: Dada
authors.forename: Ravindra M
authors.initials: RM
authors.lastname: Pandey
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000209:etiology* / Q000517:prevention & control; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000009:adverse effects*; D053685:Laser Therapy; D008297:Male; D009216:Myopia / Q000601:surgery*; D011183:Postoperative Complications; D012029:Refraction, Ocular; D012044:Regression Analysis; D012189:Retrospective Studies; D012307:Risk Factors; D013524:Surgical Flaps; D016896:Treatment Outcome; D014792:Visual Acuity