Validation of Orbscan II posterior corneal curvature measurement for intraocular lens power calculation

Abstract

Aims To validate the use of Orbscan II slitscanning topography for measuring posterior corneal curvature, by comparing corneal power calculations using this value with the standard keratometric method of corneal power calculation. Method Corneal measurements were taken from both eyes of 15 normal subjects using the Javal-Schiotz keratometer and the Orbscan II topographer. Corneal power was calculated using standard keratometric indices of 1.337.5 or 1.331.5 and Javal-Schiotz keratometry. Corneal power was then recalculated using the thick-lens formula, with anterior corneal curvature from the Javal-Schiotz or Orbscan II and posterior measurements from the Orbscan II; in addition, the 3.0 mm 'Mean Power' value from the Orbscan II software was noted. Six comparisons were then made using meandifference plots. Results The smallest difference and therefore the most predictable agreement was between Javal-Schiotz keratometry using a refractive index of 1.331.5 and the thick-lens formula using Javal-Schiotz anterior curvature and Orbscan II posterior curvature. The mean difference was 0.27 D with a confidence interval of 0.02-0.52 D. Conclusions In normal eyes, data on posterior corneal curvature from the Orbscan II can be used to calculate corneal power in close agreement with the standard keratometric method. This suggests the use of the Orbscan II in eyes that have previously undergone refractive surgery, for calculation of intraocular lens power prior to cataract surgery.

Orbscan computerized topography: Attributes, applications, and limitations

“…112 Studies of Orbscan assessment of total corneal optical power in normal and surgically altered corneas show close agreements and improved repeatability in manifest refraction. [113][114][115] Orbscan measurements appear similar to those of the IOLMaster (Carl Zeiss Meditec) in the determination of ACD but not as reliable as the IOLMaster in measurement of white-to-white corneal diameter. 116,117 Orbscan has also recently been used to assess the cornea after cataract surgery 118,119 ; assess PMD 120 ; and indicate the effect of curvature on endothelial cell measures, 121 corneal aberrations, 122,123 and contact lens-induced edema.…”

“…[106][107][108][109][110][111][112][113][114][115] A notable exception is a study comparing Orbscan topography to the EyeSys system, which suggests that Orbscan curvature topography may be as accurate and precise as the Placido-based EyeSys system. 106 Comparison of Orbscan pachymetry with US techniques continues to provide data that reveal similar or slightly thinner measurements when derived from normal corneas yet significantly underestimates when measuring post-LASIK corneal thickness (0.92 acoustic factor adjustment included).…”

Validity of the keratometric index: Evaluation by the Pentacam rotating Scheimpflug camera

“…1,2 For this algorithm to be valid, the anterior and posterior corneal curvatures are presumed to have a constant and linear relationship. 3 The commonly used keratometric indices include 1.3375, which is built into many keratometers, 1,4 and 1.3315, which is derived from the Gullstrand schematic eye 5 and independently recommended by Olsen. 2 In various schematic eyes, the ratio of the radius of the anterior corneal curvature to the radius of the posterior corneal curvature (AP ratio) ranged from 1.132 (Gullstrand schematic eye) to 1.2 (Le Grand full theoretical eye, Lotmar finite schematic eye, Kooijman finite schematic eye).…”

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

“…After laser correction for myopia, the ratio increases; therefore, keratometry overestimates postoperative corneal power and underestimates the surgically induced refractive change. 4,5 This can cause a problem in subsequent cataract surgery. The usual keratometry-based intraocular lens (IOL) power formula calculates an IOL with insufficient power, which results in unintended hyperopia after cataract surgery.…”

“…The usual keratometry-based intraocular lens (IOL) power formula calculates an IOL with insufficient power, which results in unintended hyperopia after cataract surgery. [4][5][6][7][8] Direct measurement of powers of both corneal surfaces avoids making these assumptions and may be more accurate in post-LASIK eyes. Optical coherence tomography (OCT) 9 can measure both corneal surfaces and is a possible approach for direct corneal power measurements.…”