Document Detail

Topographic changes in corneal asphericity and effective optical zone after laser in situ keratomileusis.
MedLine Citation:
PMID:  12036634     Owner:  NLM     Status:  MEDLINE    
PURPOSE: To determine the relationship between the spherical refractive change after myopic excimer laser surgery and the effective optical zone (EOZ) and corneal asphericity determined by corneal topography. SETTING: Baylor College of Medicine, Houston, Texas, USA. METHODS: Preoperative and postoperative topographies along with refractions were evaluated in all patients who had had laser in situ keratomileusis since January 1999 and had at least 6 months of follow-up. The VISX Smoothscan S2 excimer laser and the Hansatome microkeratome (Bausch & Lomb) were used in all cases. Because optical zones are oval with astigmatic treatments with the VISX laser, only patients with spherical refractions and treatments were included. Thirty-nine cases met the criteria; their treatments ranged from -1.50 to -18.00 diopters (D). The preoperative and postoperative corneal asphericities (Q-values) were taken directly from the Holladay Diagnostic Summary Report on the EyeSys 2000, version 4.0. The mean diameter of the optical zone was measured on the local radius of curvature map using the outer edge of the yellow zone, which corresponds to 2 color changes or approximately 0.50 D steepening from the mean central radius (green). RESULTS: The EOZ decreased as the amount of treatment increased. The decrease was slightly nonlinear, decreasing slightly more rapidly at higher treatments. For an "intended" 6.0 mm optical zone, the nominal EOZs from the least-squares second-order polynomial regression were 6.0 mm for -1.5 D, 5.4 mm for -5.0 D, 4.6 mm for -10.0 D, 3.8 mm for -15.0 D, and 3.2 mm for -18.0 D. The least-squares second-order polynomial regression yielded a standard error of the estimate (SEE) of +/- 0.22 mm (R(2) = 0.90). The asphericity increased nonlinearly in a positive direction (oblate) with the amount of treatment, indicating greater amounts of correction produced progressively more oblate corneal surfaces. The least-squares second-order polynomial regression yielded an SEE of +/- 0.42 (R(2) = 0.55). CONCLUSIONS: The EOZ decreased and the Q-value increased with the amount of myopic excimer laser treatment. The optical zone was approximately 4.3 mm with a spherical treatment of -12.0 D. These findings may explain the clinical studies that indicate high myopic treatments (above -12.0 D) are associated with poor visual outcomes.
Jack T Holladay; Joseph A Janes
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Publication Detail:
Type:  Comment; Journal Article    
Journal Detail:
Title:  Journal of cataract and refractive surgery     Volume:  28     ISSN:  0886-3350     ISO Abbreviation:  J Cataract Refract Surg     Publication Date:  2002 Jun 
Date Detail:
Created Date:  2002-05-30     Completed Date:  2002-07-29     Revised Date:  2004-11-17    
Medline Journal Info:
Nlm Unique ID:  8604171     Medline TA:  J Cataract Refract Surg     Country:  United States    
Other Details:
Languages:  eng     Pagination:  942-7     Citation Subset:  IM    
Baylor College of Medicine, Houston, Texas, USA.
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MeSH Terms
Cornea / pathology*,  physiopathology*
Corneal Topography*
Keratomileusis, Laser In Situ*
Myopia / physiopathology,  surgery*
Postoperative Care
Preoperative Care
Refraction, Ocular
Comment On:
J Cataract Refract Surg. 2002 Jun;28(6):948-53   [PMID:  12036635 ]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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