prescribing for astigmatism
Lies That My Keratometer Tells Me
BY PETER D. BERGENSKE, OD, FAAO
As clinicians, we tend to take our trusty instruments for granted -- and often at face value. We may occasionally calibrate, especially if someone has moved or mishandled an instrument, but by and large, we believe the information they give us. When it comes to fitting contact lenses for spherical, toric or irregular corneas, we've come to rely a great deal on the not-always-so-trusty keratometer. Remembering its limitations, however, can allow it to serve us better.
Developing an Understanding
This year we celebrate the 150th birthday of the ophthalmometer, a device invented by Helmholtz in 1854 that allows us to determine corneal power by measuring the size of a reflected image in the cornea.
More specifically, by determining the slope of the corneal surface, we can deduce the radius of curvature of the front surface. Then, applying an assumed index of refraction of the cornea, we calculate corneal power. Although most often this is a pretty accurate estimation of the true corneal power, we should keep in mind the raft of assumptions that we accept in buying this lock, stock and barrel -- that the back surface power of the cornea is standardized, the true index of refraction of the cornea is uniform, the thickness is standard and the optics are more or less spherical.
Not Too Bad After All
Despite all of these potential sources of error, the keratometer is quite reliable at determining corneal radius and power -- at least for the average cornea. But consider the reasons for which the keratometer should be only a guide, not a final authority.
First of all, the radius and power it determines is specific to the region of the cornea measured (a ring 3.0mm in diameter). We have to assume anything central or peripheral to that.
For normal corneas, this usually provides a good estimate of power within the 3.0mm, but for post surgical corneas, this is often not so. Peripheral to the 3.0mm, we can generally assume a gradual flattening of the cornea, however, the variance here even in normal corneas is quite diverse.
For corneas that are abnormally flat or steep, the keratometer estimations become increasingly inaccurate, and for post-surgical corneas, they're down-right misleading (both for power and for a fitting curve we would use on a contact lens).
Another issue, particularly with irregular corneas, is that of elevation. We tend to assume that a steeper curve indicates higher elevation. This can be quite misleading -- even with a corneal topographer-- as true determination of elevation requires a reference plane and information on shape, and reflection-based systems don't do this well. Sagittal depth requires you to take diameter into account -- again something we don't get from these devices.
An Old Stand By
If Helmholtz were alive, I think he'd be surprised that we still depend on his method for determining corneal shape and power. But for all of its shortcomings, the keratometer still provides useful information. And even with the great advantages provided by modern computerized topography, we can still rely on keratometry to provide us with valuable clinical data -- as long as we keep its limitations in mind.
Dr. Bergenske, a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for more than 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at: firstname.lastname@example.org.