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A. M. Mahmoud, C. J. Roberts, L. Wang, D. D. Koch; Comparison of Ray-Traced Total Corneal Power vs. Gaussian Total Power and the Influence of Posterior Surface Power. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5089.
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© ARVO (1962-2015); The Authors (2016-present)
To compare ray-traced total corneal power (TCP) and Gaussian equivalent power (GEP) calculated using the thick lens formula with posterior corneal power from topography, assuming incoming parallel rays.
Theoretical spherical surfaces representing both the anterior and posterior cornea were calculated, and rays of light were propagated through both surfaces assuming indices of refraction of air = 1.0, cornea = 1.376, and aqueous = 1.336. The anterior radius of curvature ranged from 6.5 mm to 8.5 mm in 0.25 mm steps. The ratio of posterior to anterior radius of curvature ranged from 0.7 to 0.9 in 0.025 steps. The central pachymetry ranged from 450 µm to 550 µm in 25 µm steps. TCP and GEP were calculated for each posterior/anterior ratio and pachymetry. Also, the effective posterior power (EPP) was determined by calculating the difference between TCP minus anterior ray-traced power. TCP was then compared to GEP for each posterior/anterior ratio and central pachymetry. Also, EPP was compared to the posterior power (GPP) used in the Gaussian formula which assumes incoming parallel rays for the posterior surface.
As posterior/anterior radius of curvature ratio increased, the average difference of TCP - GEP also increased. The average differences in power ranged from 0.64 D (ratio 0.7) to 0.81 D (ratio 0.9). For a constant posterior/anterior ratio of 0.8 and central thickness of 500 µm, the differences TCP - GEP increased from 0.69 D to 0.82 D with increasing anterior curvature. As central thickness increased, the differences TCP - GEP decreased, from 0.83 D at 450 µm to 0.66 D at 550 µm for a constant ratio of 0.8 and constant anterior radius of curvature of 7.5 mm.As posterior/anterior radius of curvature ratio increased, the average difference EPP - GPP also increased. The average differences in power ranged from 0.79 D (ratio 0.7) to 0.93 D (ratio 0.9). For a constant posterior/anterior ratio of 0.8 and central thickness of 500 µm, the difference EPP - GPP increased with increasing anterior curvature, from 0.78 D to 0.98 D. As central thickness increased, the differences EPP - GPP decreased, from 0.94 D at 450 µm to 0.79 D at 550 µm for a constant ratio of 0.8 and constant anterior radius of curvature of 7.5 mm.
Calculating posterior corneal power assuming parallel incoming rays, in the manner done by current corneal topography devices, overestimates the effective power of the posterior surface and introduces error in the calculation of total corneal power using the Gaussian thick lens formula.
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