Post Keratorefractive Surgery.
Determining Corneal Power after RK, ALK, PRK and LASIK.
The true corneal power following RK, ALK, PRK and LASIK is difficult to measure by any form of direct measurement, such as keratometry, or corneal topography.
Keratometry and topography assume a normal relationship between the anterior and posterior corneal curvatures, and measure the anterior corneal radius.
Incisional keratorefractive surgery for myopia flattens both the anterior
corneal radius and the posterior corneal radius. Ablative keratorefractive
surgery for myopia flattens the anterior corneal radius but leaves the posterior
corneal radius mostly unchanged.
Standard keratometry measures an intermediate area and extrapolates the
central power based on some very broad assumptions. For this reason,
keratometry, autokeratometry and simulated keratometry by topography
will typically over-estimate central corneal power, following
keratorefractive surgery for myopia. Failure to keep this important
fact in mind will often result in an unexpected and unpleasant
post-operative hyperopic surprise.
"DOUBLE K" FORMULA CORRECTIONS:
Learn how 2-variable IOL power calculation formulas may be another source of
errors following keratorefractive surgery. Lacking the Holladay 2 formula, instead
you can use the "Double K"
correction method in conjunction with the SRK/T, Hoffer Q or Holladay 1 formulas.
Keep in mind that the above methods will give an estimation of the true central corneal power and may not be exact. At present, these techniques represent the best clinical methods available. Hopefully, in the future we may have a more accurate, and less time-consuming, method for measuring these challenging eyes.