For RK, averaging the 0 mmm, 1 mm and 2 mm annular power rings of the Numerical View of the Zeiss Humphrey Atlas topographer (below) will typically a useful estimate of central corneal power.
The Numerical View feature of the Zeiss Humphrey Atlas topographer.
Transient hyperopia following cataract surgery and prior radial
keratotomy
Patients with previous 8-incision radial keratometry will commonly show
variable amounts of hyperopia in the immediate post-operative period
following cataract surgery. This is felt to be due to stromal edema in
the areas of 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, or an optical zone of less
than 2.0 mm. 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 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, at the same time of the day,
are stable. If 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 -0.50 D and -1.00 D
and are designed to make the operative eye more myopic than usual, so
that five to ten years from now, 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.
Formula
