Corneal power after refractive surgery for myopia: contact lens method.
OBJECTIVE
To clarify the theoretical background of the rigid contact lens overrefraction (CLO) method to determine corneal power after corneal refractive surgery.


METHODS
University Eye Clinic, University of Würzburg, Würzburg, Germany.


METHODS
Using paraxial geometrical optics, the measurement situation for the contact lens method was analyzed and the definitions of corneal refractive power were reviewed. Based on the theoretical Gullstrand eye, model eyes were constructed, representing 1 emmetropic and 2 myopic eyes (primary refraction -5.21 diopters [D] and -10.25 D, respectively) before and after photorefractive keratectomy and laser in situ keratomileusis. In these eyes, the application of the CLO was mathematically simulated using Gaussian thick-lens optics and commercial ray-tracing software.


RESULTS
The CLO method measured neither the equivalent (total) power nor the vertex (back) power of the cornea but rather the quantity 336/R(1C) (R(1C) = anterior corneal radius). Based on these results and the Gullstrand eye, new formulas are proposed to derive the equivalent power and vertex power of the cornea by the CLO method.


CONCLUSIONS
Depending on whether intraocular lens calculation formulas are based on equivalent (total) corneal power or vertex corneal power, the respective new formulas for the CLO method should be applied in patients after corneal refractive surgery. An increase in prediction accuracy of the refractive outcome is expected.
Haigis
Wolfgang
W
University Eye Clinic, University of Würzburg, Würzburg, Germany. w.haigis@augenklinik.uni-wuerzburg.de
D003198:Computer Simulation; D003261:Contact Lenses; D003315:Cornea / Q000503:physiopathology*; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer; D008954:Models, Biological; D009216:Myopia / Q000503:physiopathology* / Q000601:surgery*; D009885:Ophthalmology / Q000295:instrumentation / Q000379:methods*; D018903:Photorefractive Keratectomy; D011184:Postoperative Period; D012029:Refraction, Ocular