IOL Calculation after Corneal Refractive Surgery

After corneal refractive surgery the asphericity of the normally prolate cornea is often changed to an oblate shape. If corneal vertex radii are measured by keratometry in such cases, the resulting radii may be too small, resulting in an hyperopic outcome after cataract surgery. In OKULIX this is avoided if the corneal topography is used to extract the vertex radii ⁹. No further user action is required beside loading the topography. Any other data prior to refractive surgery ("clinical history") are not needed. In most cases, it is even not necessary to know about the fact of prior refractive surgery, since all information is based on the actually measured data. In cases with very high alterations of corneal thickness an additional pachymetry should be performed for a more reliable measurement of posterior corneal surface.

Pachymetry

Normally, posterior corneal vertex radii in OKULIX are calculated from anterior vertex radii. In most cases this is sifficiently accurate, because variations of posterior radii have only a small impact on overall accuracy. An error of 0.1mm in anterior corneal vertex radius causes a refractive error in corneal plane of 0.6D for a mean-sized eye, the same error in posterior corneal vertex results in 0.06D. Nevertheless, the error from posterior corneal radius may be non-negligible (0.5-0.75D) in cases of excessive corneal ablation. In such cases spatially resolved pachymetry (25 points) measured by a Tomey pachymeter can be helpful, i.e. posterior corneal surface is calculated from anterior corneal surface measured by topography and corneal thickness measured by pachymetry.