Not enough pre-LASIK data?

A new mathematical ratio could be helpful in providing better visual results for certain cataract surgery patients who previously had LASIK.
The ratio may be useful for LASIK patients without pre-LASIK corneal data, which sometimes is a fact of life. With the new ratio, an estimation of a patient’s pre-LASIK corneal data may be taken and used in what is known as the double-K technique, a formula to calculate IOL power for patients after LASIK.
“Different methods have been suggested to assess the corneal power in patients who have had LASIK,” according to lead study author Arthur C.K. Cheng, M.R.C.S., F.C.Ophth.(H.K.), Department of Ophthalmology, Guy Hugh Chan Refractive Surgery Center, Hong Kong Sanatorium & Hospital, Hong Kong. “Some of these methods do not require preoperative information to determine the true corneal power after LASIK. However, with the double-K method, information before LASIK (K1) is needed to allow ELP [effective lens position] estimation.”
Thanks to the new research, K1—the necessary pre-op corneal data for the double-K technique—may be estimated accurately.

Testing the formula

Dr. Cheng and colleagues first analyzed 192 eyes from 192 LASIK patients. In these patients, pre-op corneal data was obtained—specifically, the radius of curvature.
“Radius of curvature (ROC) of the corneal surfaces was obtained by choosing the Orbscan [Bausch & Lomb, Rochester, N.Y.] elevation map, user-defined function with alignment at apex, and fit zone diameter adjusted to 10 mm for the anterior corneal surface and 7–10 mm for peripheral fitting for the posterior corneal surface,” Dr. Cheng reported. “The ratio of the anterior and posterior radius of curvature (RROC) was obtained for each individual patient using preoperative (preoperative anterior central 10-mm corneal curvature and posterior peripheral 7–10-mm corneal curvature) data only.”
In a completely different set of 98 eyes of 98 LASIK patients, Dr. Cheng set to work estimating the pre-op corneal data using in part the RROC data already obtained.
First, he calculated the anterior corneal radius (AntROC) as follows: AntROC = RROC× post-LASIK posterior peripheral 7–10-mm corneal curvature
Then, he estimated total corneal power (TCPest) as follows: TCPest = (1.3375 – 1) × 100/ AntROC
This total corneal power estimation could then be compared with the true pre-op keratometric value (SimK) for these patients.
When actual values were plugged in, RROC from the first group based on measurements was found to be 1.166.
“Using the ratio of 1.166 on the second data set, the mean TCPest was 43.42 +/–2.21 D, whereas the measured preoperative SimK was 43.40 +/–2.64 D,” Dr. Cheng reported.
The mean difference between the estimated total corneal power and measured pre-op SimK was only 0.02 +/–2.65 D.
“In summary, the preoperative corneal power can be estimated using our RROC (1.166) to work out K1 with post-LASIK data alone in patients after uncomplicated LASIK,” Dr. Cheng concluded. “The results of this study can help improve future IOL power calculations using the double-K method in patients after uncomplicated LASIK but without pre-LASIK data.”
Mohan Rajan, M.D., medical director, Rajan Eye Care Hospital, Chennai, India, commented that this new formula should be helpful and add to the list of available formulas to choose from to determine corneal power.
Dr. Rajan and colleagues are working on a CD to be released soon through the All India Ophthalmological Society that will help ophthalmologists choose which IOL power calculation formula is right for them. “Suppose you have no information at all about a past history of LASIK,” Dr. Rajan said. “We can help you choose the best formula for your patient.”
Currently, Dr. Rajan said, there are 14 different IOL power calculation formulas available.
As for this new formula by Dr. Cheng and colleagues, Dr. Rajan said it should be helpful for patients who have had LASIK in centers that did not provide proper documentation or perhaps patients who had LASIK abroad.
Dr. Rajan estimated that 5% to 10% of LASIK patients do not have pre-LASIK corneal power data available to them.

Editors’ note: Dr. Cheng has no financial interests related to this study. Dr. Rajan has no financial interests related to his comments.

Contact information

Cheng: artcheng@netvigator.com
Rajan: +65 6254 6330, rajaneye@vsnl.com