the contact lens exam
Determining Posterior Elevation
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).
The 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?
A normal 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.
The 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 al, 2002).
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 devices.
Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.