Keratometry after corneal refractive surgery

In their article, Park and coauthors¹ report the keratometric power before and after laser thermal keratoplasty. They found the postoperative K-value by manual keratometry is smaller than that measured by topography.

In an earlier report, Rosa and coauthors² discuss the reliability of K-values measured by the IOLMaster after photorefractive keratectomy. They conclude that the IOLMaster values do not reflect the refractive change produced by the treatment.

What both groups of authors do not understand is that keratometers do not measure corneal power (K) but rather the local slope of the cornea at a diameter of about 3.0 mm by means of Purkinje images. The slope values are first converted to the anterior corneal radius under the assumption the cornea is spherical. The corneal is, however, aspherical—usually prolate. Thus, the radius is larger (flatter) than at the center. This error is small, however. The next step is to calculate the K-value. This requires knowledge of the posterior radius and the thickness and refractive index of the cornea. Usually, the values of Gullstrand are assumed; that is, the anterior radius is multiplied by the ratio 6.8 to 7.7 to obtain the posterior radius, the thickness is set to 0.5 mm, and the refractive index is considered to be 1.376. The conversions from measured slope to K are done by software internal to the instrument or in older instruments, by engraved scales against which the value can be read. The same applies to K-values from topographers.

After corneal refractive surgery, the assumptions of the conversion no longer prevail. In particular, the ratio between the posterior radius and anterior radius is changed. Thus, the K-value output cannot be correct. To obtain correct K-values, one must measure both anterior and posterior radii and do the calculations using these. The exact thickness is less important but could be included if measured. The posterior radius can be measured by the Orbscan, even though its accuracy has been questioned. The posterior radius could be accurately determined from Scheimpflug photographs,³ but this requires proper correction of the images by ray-tracing calculations. (Similar calculations are performed by Orbscan software.)

It is important that one understands the underlying principles of different types of measurement to be able to judge their reliability, depending on the situation at hand. Sound skepticism toward high-tech “black boxes” is, in general, well advised. You never know what goes on inside these boxes.

Sverker Norrby PhD

Leek, The Netherlands