Sergio Kwitko, Diane Marinho, Samuel Rymer Ophthalmology Department - Hospital de Clínicas de Porto Alegre Federal University of Rio Grande do Sul - Porto Alegre - Brazil The authors have no financial interest in the subject matter of this poster Scanning-Slit Topography and Double-K Method for IOL Calculation After Refractive Surgery

Introduction One of the great challenges in performing cataract surgery in post-refractive surgery patients is obtaining an accurate refractive outcome, especially in previously myopic patients. One of the great challenges in performing cataract surgery in post-refractive surgery patients is obtaining an accurate refractive outcome, especially in previously myopic patients. Intraocular lens (IOL) calculation is less reliable in these cases, generally inducing hyperopic errors after cataract surgery. Intraocular lens (IOL) calculation is less reliable in these cases, generally inducing hyperopic errors after cataract surgery. The main two sources of biometric error are the IOL formulas and the inaccuracy of post-refractive surgery central corneal curvature measurements. The main two sources of biometric error are the IOL formulas and the inaccuracy of post-refractive surgery central corneal curvature measurements.

Purpose To test whether adding Orbscan II calculated values into the double-K method improves IOL calculation in post refractive surgery patients. To test whether adding Orbscan II calculated values into the double-K method improves IOL calculation in post refractive surgery patients.

Methods A prospective study was undergone with 33 eyes previously submitted to refractive surgery. Post refractive surgery corneal curvature (post-K values) were obtained from the average total-mean power of central 2.0 mm in eyes submitted to myopia correction, and from the central 4.0 mm Orbscan II total mean-power in eyes submitted previously to hyperopia surgery(Bausch&Lomb, Germany). Post refractive surgery corneal curvature (post-K values) were obtained from the average total-mean power of central 2.0 mm in eyes submitted to myopia correction, and from the central 4.0 mm Orbscan II total mean-power in eyes submitted previously to hyperopia surgery (Bausch&Lomb, Germany). Axial length was obtained from the IOL Master (Zeiss, Germany). The average corneal curvature of the general population (43.8D) was used as the pre-K value into the double-K method. Refraction results 30 days after surgery were compared with refraction that would be obtained if we used: 1) post-K values derived from keratometry; 2) post-K values derived from topography; 3) pre-K average values derived from the central 8 mm area by Orbscan II, and 4) anterior chamber depth measures obtained with the IOL Master and Orbscan II.

Results Mean post-cataract surgery spherical equivalent was ± 1.17 D (range, D to D) in eyes submitted to myopic surgery, and ± 1.77 (range, to D)in those submitted to hyperopic surgeries. Mean post-cataract surgery spherical equivalent was ± 1.17 D (range, D to D) in eyes submitted to myopic surgery, and ± 1.77 (range, to D) in those submitted to hyperopic surgeries.

Results Had we used post-K values derived from keratometer or from topography we would have obtained significantly higher post- operative refractive errors, in eyes previously submitted to myopic refractive surgery (p<0.01). Had we used post-K values derived from keratometer or from topography we would have obtained significantly higher post- operative refractive errors, in eyes previously submitted to myopic refractive surgery (p<0.01). In eyes previously submitted to hyperopic correction, however, there was no statistically significant difference in using keratometry, topography or Orbscan II derived measurements as post-K values. Simulation of results showed there would be less standard deviation of refractive results using topography simulated K values as post-K values in these eyes. In eyes previously submitted to hyperopic correction, however, there was no statistically significant difference in using keratometry, topography or Orbscan II derived measurements as post-K values. Simulation of results showed there would be less standard deviation of refractive results using topography simulated K values as post-K values in these eyes.

Results Refraction using pre-K average values obtained from the central 8 mm area by Orbscan II were similar to those using the average corneal curvature of 43.8D. Refraction using pre-K average values obtained from the central 8 mm area by Orbscan II were similar to those using the average corneal curvature of 43.8D. Anterior chamber depth measured with IOL Master or Orbscan II did not change the final IOL calculation. Anterior chamber depth measured with IOL Master or Orbscan II did not change the final IOL calculation.

Table 1. Patients data * from 2.0 mm central area in eyes previously submitted to myopic refractive surgery, and of 4.0 mmcentral area in previously hyperopic corrected eyes SE = spherical equivalent RK = radial keratotomy M-Lasik = myopic Lasik H-Lasik = hyperopic Lasik H-PRK = hyperopic photorefractive keratotomy

Table 2. Simulation of results IOL = intraocular lens * from 2.0 mm central area in eyes previously submitted to myopic refractive surgery, and of 4.0 mmcentral area in previously hyperopic corrected eyes SE = spherical equivalent

Conclusion Average total-mean power of central 2.0 mm Orbscan II used as the post-K values into the double-K method provides a precise way of IOL calculation in post myopic refractive surgery patients.Average total-mean power of central 2.0 mm Orbscan II used as the post-K values into the double-K method provides a precise way of IOL calculation in post myopic refractive surgery patients.

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