Abstract
Purpose: :
To validate the use of Orbscan II slit–scanning topography (Bausch & Lomb) for measuring post–LASIK keratometry, by comparing corneal power calculations using this value with the standard clinical history method for corneal power calculation.
Methods: :
A total of 42 eyes of 21 consecutive patients who underwent LASIK for myopia (sphere: –3.8 ± 1.6 diopters) or myopic astigmatism (sphere: –3.7 ± 1.5 D; cylinder : –1.5 ± 0.4 D) were included in this reprospective clinical study. Subjective refraction, standard automated keratometry and Orbscan II topography were performed before and 3 months after LASIK. Average values of all points of keratometric–mean, total–mean and total–optical power maps were assessed using Orbscan II statistical analysis device for 6 central areas with 0.5, 1.0, 2.0, 3.0, 4.0 and 4.5 mm diameters. A composite value was determined from the Gaussian optics formula based on postoperative Orbscan II anterior and posterior surface power and central ultrasound pachymetry. Keratometry values derived from Orbscan II were compared with those obtained using the clinical history method (His–K) using the paired sample t test and Pearson correlation coefficient. Factors related to wrong corneal power measurement were evaluated.
Results: :
The keratometry values obtained by the clinical history method had very good Pearson correlation with the keratometric–mean (r = 0.78, p < 0.01), total–optical (r = 0.85, p < 0.01), total–mean (r = 0.87, p < 0.01) power, and Gaussian optics formula (r = 0.83, p < 0.01). The smallest difference and therefore the most predictable agreement was between His–K and 2 mm total–mean power (mean difference: 0.13 ± 0.53 D; p=0.42) and 3 mm total–optical power (mean difference: –0.10 ± 0.47 D; p=0.41).
Conclusions: :
After myopic LASIK, actual corneal power is overestimated by standard manual or automated keratometry when deduced from the anterior surface radius and keratometric refractive index, which, in turn, results in underestimation of intraocular lens power. The higher the intended refractive correction, the greater is the error. In our study, Orbscan II total–mean, total–optical power maps and keratometry derived from the Gaussian optics formula accurately assess the corneal power after myopic LASIK independent of preoperative data, and should improve intraocular lens calculation prior to cataract surgery. In corneas that have had refractive surgery, these maps should theoretically be more accurate than maps that analyze the anterior surface alone.
Keywords: refractive surgery: corneal topography • refractive surgery: LASIK • imaging/image analysis: clinical