Calculating Intraocular Lens Geometry by Real Ray Tracing

Abstract

PURPOSE: An implementation of real ray tracing based on Snell's law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs). METHODS: The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae. RESULTS: Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11±0.43 diopters (D), cylinder -0.18±0.52 D, and axis 5.13°±30.19°. Pearson's correlation coefficient was sphere r=0.92, P<.01; cylinder r=0.79, P<.01; and axis r=0.91, P<.01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae. CONCLUSIONS: It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history. [J Refract Surg. 2007;23:393-404.]

Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula

“…Masket and Masket 6 report 30 eyes, Latkany et al 8 21 eyes, Rosa et al 9 19 cases, Einighammer et al 21 12 eyes, Walter et al 7 9 eyes, Shammas and Shammas 3 15 eyes, and Rabsilber et al 23 10 eyes. It seems as though the 22 ME 0.08 G 0.62, 4 ME 0.14 D, 23 and ME À0.15 G 0.29 D. 6 Shammas and Shammas' 3 results (ME 0.003 G 0.63 D, MA 0.55 G 0.31 D) are given for the IOL prediction error; converted to refraction prediction errors, these numbers are even smaller.…”

Comparison of OKULIX ray-tracing software with SRK-T and Hoffer-Q formula in intraocular lens power calculation

“…In the past, IOL power calculations have been performed with analytical formulas based on Gaussian optics, 1 , 2 , 3 , 4 , 5 , 6 which is an approximation in which the sine function in Snell's law is approximated by the argument: sin(α) ≃ α. 7 …”

Customized aspheric intraocular lenses calculated with real ray tracing

“…52 Posterior corneal surface and anterior chamber depth (ACD) were estimated as previously described. 51 An individual eye model was constructed to calculate the characteristics of the IOLs (Figure 1). The corneal surfaces were modeled with a spline-based interpolation method.…”