Topographic determination of corneal asphericity and its lack of effect on the refractive outcome of radial keratotomy.

PURPOSE The normal human cornea flattens peripherally. The amount of flattening, or asphericity, has traditionally been calculated from multiple keratometric measurements. We devised a mathematical technique for determining asphericity from computed corneal topography. We then determined whether asphericity affects the refractive outcome of radial keratotomy. METHODS One eye each of 41 patients who underwent four- or eight-incision radial keratotomy and preoperative computed corneal topography was identified retrospectively and analyzed. The asphericity, P, of each cornea was calculated by fitting Baker's equation (y2 = 2r0x-Px2) to each meridian of the topographic map. For each patient, we calculated the difference between the refractive outcome in diopters for radial keratotomy and the prediction of a quadratic least-squares best-fit model involving optical zone size and age. RESULTS Aspericity could be calculated from the topographic maps in all 41 patients and ranged from 0.33 to 1.28, with mean +/- S.D. of 0.82 +/- 0.21. Aphericity varied among the meridians of a cornea, with an average standard deviation among meridians of 0.17. No statistical correlation was found between calculated asphericity and refractive outcome. CONCLUSIONS Corneal asphericity can be calculated from corneal topographic maps. Asphericity is not constant in the different meridians of a normal cornea. Corneal asphericity is not useful in predicting the refractive outcome of radial keratotomy.

[1]  J. F. Fleming.  Corneal Asphericity and Visual Function After Radial Keratotomy , 1993, Cornea.

[2]  M. Lynn,et al.  Factors affecting outcome and predictability of radial keratotomy in the PERK Study. , 1987, Archives of ophthalmology.

[3]  S D Klyce,et al.  A new reconstruction algorithm for improvement of corneal topographical analysis. , 1989, Refractive & corneal surgery.

[4]  G O Waring,et al.  Accuracy and precision of keratometry, photokeratoscopy, and corneal modeling on calibrated steel balls. , 1989, Archives of ophthalmology.

[5]  G O Waring,et al.  Reproducibility of normal corneal power measurements with a keratometer, photokeratoscope, and video imaging system. , 1990, Archives of ophthalmology.

[6]  J. Holladay,et al.  Prospective evaluation of radial keratotomy. Photokeratoscope corneal topography. , 1988, Ophthalmology.

[7]  T. Y. Baker,et al.  Ray tracing through non-spherical surfaces , 1943 .

[8]  M Guillon,et al.  CORNEAL TOPOGRAPHY: A CLINICAL MODEL , 1986, Ophthalmic & physiological optics : the journal of the British College of Ophthalmic Opticians.

[9]  W A Douthwaite,et al.  Corneal asphericity and refractive error , 1989, Ophthalmic & physiological optics : the journal of the British College of Ophthalmic Opticians.

[10]  R. Mandell,et al.  The enigma of the corneal contour , 1992 .

[11]  The central‐peripheral radius of the normal corneal curvature , 1985, Acta ophthalmologica.

[12]  G Smith,et al.  Meridional Variations of Corneal Shape , 1984, American journal of optometry and physiological optics.

[13]  S. Klein,et al.  A Corneal Topography Algorithm that Produces Continuous Curvature , 1992, Optometry and vision science : official publication of the American Academy of Optometry.

[14]  Barry A. J. Clark,et al.  MEAN TOPOGRAPHY OF NORMAL CORNEAS , 1974 .

[15]  P. Kiely,et al.  The Mean Shape of the Human Cornea , 1982 .

[16]  J. F. Fleming.  Corneal Topography and Radial Keratotomy , 1986 .

[17]  J. F. Fleming.  Should refractive surgeons worry about corneal asphericity? , 1990, Refractive & corneal surgery.

[18]  D. Sanders,et al.  Factors affecting predictability of radial keratotomy. , 1985, Ophthalmology.

[19]  J T Holladay,et al.  The relationship of visual acuity, refractive error, and pupil size after radial keratotomy. , 1991, Archives of ophthalmology.