JAN 13, 2009
Study investigators developed a new ratio to help calculate the preoperative central keratometry value or K in post-LASIK eyes without preoperative data. By calculating an estimated preoperative K value, the double-K method can be used to calculate the optimal IOL power for post-LASIK eyes lacking preoperative data that later undergo cataract surgery.
The investigators retrospectively reviewed preoperative Orbscan corneal tomography data from 192 right eyes of 192 myopic LASIK patients. The ratio of the radius of curvature from the anterior central 10-mm zone versus the posterior peripheral 7- to10-mm zone was found to average 1.166.
Another set of data from 98 right eyes of 98 myopic LASIK patients was retrospectively reviewed. The investigators back-calculated the anterior radius of curvature (AntROC) by multiplying the three-month post-LASIK Orbscan data of the radius of curvature from the posterior peripheral 7- to10-mm zone by the average radius of curvature of 1.166. The authors used peripheral posterior cornea as opposed to central cornea data because the accuracy of Orbscan measurements of posterior corneal curvature through the central post-LASIK cornea is questionable. The total corneal power (TCP) was then estimated using the result of the above calculation and the following standard formula:
TCPest = (1.3375 - 1) x 1000/AntROC
The estimated pre-LASIK total corneal power in the three-month post-LASIK eyes was found to be very close to the simulated K values from the pre-LASIK Orbscan measurements of the same eyes (43.42 versus 43.40). The authors therefore suggested that pre-operative K values can be calculated using the ratio of 1.166 and the above equation. They noted, however, that their data is limited by the large standard deviation in their calculated radius of curvature ratio and the potential lack of transferability of their ratio to other tomographers.
Dr. Pandit receives lecture fees from Vistakon Pharmaceuticals, LLC.