Videokeratography is the current standard of care in evaluating the
cornea prior to LASIK. I can't claim videokeratography is the
current standard of care prior to contact lens fitting, but it is
the gold standard. Furthermore, the state-of-the-art method to
measure the cornea is a true elevation based system such as Orbscan
II (Bausch &Lomb) or Pentacam (Oculus).
difference between these systems and the others is the ability to
image and view the posterior corneal surface, among other anterior
segment structures. In doing so, a whole new world is opened up for
us to discern. The greatest advantage of these systems is the
ability to screen for abnormally high posterior surface elevation,
which anterior surface topography wouldn't show.
How High is Too High?
cornea is prolate, and thus its anterior and posterior elevation
will rise above the best fit reference sphere. The average amount of
maximum posterior elevation is about 21 μm to 28 μm in non-diseased
eyes. Additionally, in a series of 140 normal eyes examined by Wei
et al (2006), the maximum posterior elevation was never greater than
46 μm. Abnormally high posterior elevation will usually correlate
with a thinner cornea in the area of greatest height, but as Wei et
al has shown it may not correlate well with other measurements such
as corneal curvature. In other words, don't confine screening for
abnormal posterior corneal surface elevation to patients who have
steep corneal curvatures.
Figure 1 displays an example where
anterior corneal topography or keratometry would have lead one
astray when measuring the corneal surface. It displays an average
amount of anterior corneal elevation and curvature.
Figure 1. An average amount of anterior corneal
elevation and curvature with excessive posterior
posterior surface evaluation revealed an excessive amount of
posterior corneal elevation and associated corneal thinning with the
posterior elevation approaching 65 μm! Surprisingly, the patient was
symptom free and a happy soft contact lens wearer. Nonetheless, this
patient is clearly not a candidate for a kerato-refractive ablation
procedure and likely has central keratoconus.
What's the Cutoff?
Clinically, many state that a posterior elevation greater than 50 μm
is clearly out of the range of normal. Others state that a posterior
elevation greater than 40 μm when coupled with positive findings on
other topographic screening programs are cause for concern (Rao S et
Nonetheless, whether screening your patients for refractive surgery
or simply fitting them for contact lenses, know the limits of the
posterior corneal surface to get the most of these state-of-art