Determining Corneal Power after Radial Keratotomy.
Unlike ablative forms of myopic keratorefractive surgery (LASIK and
PRK) in which
the ratio between the posterior : anterior corneal radii is
decreased, for eyes that
have previously undergone radial keratotomy, the ratio between the
posterior : anterior
corneal radii is increased. This allows for a direct estimation of
the central corneal
power using elevation data of the central 4.0 mm, if carried out in a
Image above: Zeiss Atlas Topographer,
Annular Ring Power from the Numerical View feature. Use the average
of the 1 mm, 2 mm, 3 mm and 4 mm annular power values.
For eyes with prior radial keratotomy, averaging the 1 mm, 2 mm, 3 mm
and 4 mm annular power rings of the Numerical View of the 995, 994, and
993 Zeiss Atlas topographer (right) will typically give a useful
estimate of central corneal power.
For the Zeiss Atlas
9000 topographer, you would take average of the 1 mm, 2 mm, 3 mm
and 4 mm ring (not zone) values. If the Zeiss
Atlas topographer is not available, then the adjusted effective refractive
power (EffRPadj) from the Holladay Diagnostic Summary of the EyeSys Corneal
Analysis System can be used.
Above image: Axial curvature map from the Zeiss Atlas 9000 topographer displaying the ring values. For prior RK, the 1 mm, 2 mm, 3 mm and 4 mm ring values are used to estimate the central corneal power.
The key concept here is that we are looking to discover
the corneal power at its center. Instruments such as manual keratometers,
autokeratometers, or simulated keratometry using a standard topographer
will typically over-estimate the central corneal power, resulting in
a post-operative hyperopic surprise.
Of course, correctly estimating the central corneal power following RK is
only half of the exercise. The calculated IOL power must also be adjusted to
prevent the artifact of a very flat central corneal power from having the formula
underestimate IOL power. Follow this link: 2-variable
IOL Power Formulas for a summary of why this is so and how this
is carried out.
Transient hyperopia following cataract surgery and
prior radial keratotomy
Patients with previous 8-incision radial keratotomy will commonly show variable
amounts of transient hyperopia in the immediate post-operative period following
cataract surgery. This is felt to be due to stromal edema around the radial
incisions, producing a temporary enhancement of central corneal flattening.
While this central corneal flattening is usually transient, it can be as much
as +4.00 D, and is further accentuated by greater than eight incisions, an
optical zone of less than 2.0 mm, or incisions that extend all the way to the
If a patient exhibits any of the above, significant unanticipated hyperopia
may be seen in the immediate post-operative period, which should gradually
resolve after eight to twelve weeks. Sometimes, due to a lack of corneal stability,
the post-operative refraction can continue to slowly shift myopic over a several
month period. We have seen several patients with myopic shifts as large a -5.00
D over a 12-week period.
If the final post-operative refractive objective remains elusive, plans for
an IOL exchange, or a piggyback IOL, should not be made until at least two
months have passed and two consecutive refractions, two weeks apart (at the
same time of the day), are stable (the "rule of twos.").
more than six months passes before cataract surgery is required for the fellow
eye, the corneal measurements should be repeated due to the fact that additional
corneal flattening frequently occurs over time following radial keratotomy.
For this reason, IOL power calculations are usually targeted for between -0.75
D and -1.00 D and are designed to make the operative eye more myopic than usual,
so that five to ten years from surgery, the post-cataract surgery refractive
error does not drift into hyperopia. This also helps to avoid hyperopic refractive
results, which are quite common, in spite of every precaution being taken.
Of all the various forms of keratorefractive surgery, we have had the best
overall accuracy following radial keratotomy using the above technique.