Scheimpflug camera useful as no-history method for determining corneal power in post-LASIK eyes

August 15, 2008

Images from a rotating Scheimpflug camera (Pentacam, Oculus) provide less accurate information on corneal power in post-LASIK eyes than methods involving adjustment based on the amount of laser correction, but the imaging device represents a useful tool when clinical history data are not available, according to the results of a retrospective study.

Key Points

Analyses were based on 26 eyes with a history of myopic LASIK or PRK that underwent cataract surgery with IOL implantation; 18 eyes had complete pre-and post-LASIK historical data available, said Dr. Hsiao, resident, Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas.

For each eye, the post-cataract surgery refraction was assumed to be the target refraction. Then, the corresponding IOL power for each eye was calculated using two Scheimpflug camera-based K values-the Holladay equivalent keratometer reading (EKR) and the proprietary BESSt formula (EB eye Ltd.)-along with seven other methods for determining corneal power. Then, the calculated IOL powers from each method were compared with the actual IOL power implanted to derive absolute IOL power error.

'Growing challenge'

"IOL power calculation for patients with prior keratorefractive surgery presents a growing challenge as more and more patients undergo these vision correction procedures," Dr. Hsiao said. "Our study reinforces that for these patients, pre-and post-LASIK history data are important for improving the accuracy of IOL calculation after myopic keratorefractive surgery.

"Regarding the [Scheimpflug camera], the results achieved using its measurements can be considered good, but by themselves they do not seem to reflect accurately the corneal refractive power after myopic LASIK," he said. "Perhaps large studies to optimize [the imaging device] results based on regression analysis might be promising."

The device offers the ability to accurately determine the anterior and posterior corneal curvature along with the pachymetry to calculate a true corneal power. In the study presented, the other methods for determining the K value included the clinical history method, the average central corneal power (ACCP) adjusted, Sim K adjusted, Sim K regression no history, and the ACCP regression no history. In addition, the Masket and Masket-modified methods were included to represent direct IOL adjustment methods.

Dr. Hsiao explained that the ACCP adjusted and Sim K regression methods were developed by ophthalmologists at UT Southwestern and are described in articles in press in the Journal of Cataract and Refractive Surgery.

The ACCP adjusted method and Sim K regression method are based on a proprietary topography system (TMS, Tomey), Dr. Hsiao said, adding that the Sim K regression method is similar to the modified Maloney formula. The ACCP adjusted method averages the central 3 mm of the cornea and is very similar to the effective refractive power adjusted, or EffRP adjusted, for a corneal topography unit by another company (EyeSys), he said.

IOL power calculated

For each corneal power estimation method excluding the Masket and Masket-modified methods, a corresponding IOL power was calculated using the axial length, A constant, and double-K values with the double-K modified Holladay I formula. In eyes with no history, 43.86 D was used to calculate the double-K value. For the Masket and Masket-modified methods, an adjusted IOL power was calculated using the single-K Holladay 1 formula.

"With the Masket and Masket-modified methods, the final adjustment takes care of both the keratometry and effective lens position estimation errors that occur in post-refractive surgery eyes," Dr. Hsiao explained.

Lowest absolute IOL error

Considering only eyes with full history information, the ACCP adjusted method and the Masket-modified method yielded the lowest absolute IOL error, and they were significantly superior compared with use of the BESSt formula, the EKR, and the clinical history method. Assuming no history information was available, calculations performed for all 26 eyes using the BESSt, EKR, ACCP regression, and Sim K regression data showed the BESSt formula was associated with a little higher IOL error than the other three methods. The outcomes were not significantly different comparing the BESSt or EKR with each other or any of the other methods, however.

Analyses also were conducted to compare outcomes based on proportions of eyes with an IOL error exceeding 1 or 1.5 D. Considering data for all eyes, the proportions of eyes with an IOL error exceeding 1 D or exceeding 1.5 D was approximately two-fold higher with use of the BESSt formula compared with use of the EKR or ACCP regression values. Results for analyses including only eyes with preoperative history also demonstrated that the BESSt formula was associated with the poorest outcomes compared with the ACCP adjusted, Masket-modified, and EKR methods.

Dr. Hsiao conducted this research in collaboration with Wayne Bowman, MD; James P. McCulley, MD; and Shady T. Awwad, MD, in the Department of Ophthalmology.

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