Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia

Abstract

The Orbscan II 5.0 mm total axial power and 4.0 mm total optical power can be used to more accurately predict true corneal power than the history-based method and may be particularly useful when pre-LASIK data are unavailable.

Orbscan II and double-K method for IOL calculation after refractive surgery

“…Pre-K value used in the double-K method was 43.8 D, as the average value for the general population [14]. Post-K values were assessed from Orbscan II average of total-mean power from the central 2 mm for eyes submitted to myopic correction, and from the central 4 mm for eyes submitted to hyperopic surgery [4,5,[9][10][11][12].…”

“…In non-operated corneas, this ratio is approximately 82%; that is, posterior curvature radio is 1.2 mm less than the anterior, which leads to a corneal refractive index of 1.3375. After refractive surgery, this relationship is not constant anymore, which leads to a change in corneal refractive index [4][5][6]. Furthermore, these formulas consider the corneal curvature to calculate the effective lens position, and flat corneal measurements erroneously estimate the effective lens position [4].…”

“…Keratometers and Placido ring topographers usually generate a measure that is steeper than the real central corneal curvature, because they calculate an average of the inner reflected ring on the cornea, not reflecting the real central corneal curvature [5,6]. Adjusted keratometry, contact lens and clinical history methods are other alternatives, but also with poor predictability [1-3, 8, 9].…”

“…Corneal tomography with Orbscan II (Bausch & Lomb, USA), Pentacam (Oculus, Germany) or Galilei (Ziemer Group, Switzerland), on the other hand, have been shown to be more precise for evaluating central corneal curvature after refractive surgery, because they analyze corneal elevation, including the central area [4][5][6][9][10][11][12][13]. Furthermore, the total corneal power assessed by these equipments includes data from the corneal thickness, from both corneal surfaces instead of only from the anterior surface, and physiologic refraction indices instead of a fictitious keratometric refractive index [9,12,13].…”

Anterior Eye Imaging with Optical Coherence Tomography

“…Slit-scanning tomography, rotating-slit Scheimpflug camera, and dual-Scheimpflug systems have all been used for this purpose [95][96][97]. However, the axial resolutions of these devices are between 50 and 100 mm, much lower than the resolution offered by current OCT technology.…”