Camera system's EKRs inaccurate for some calculations

Equivalent keratometer readings (EKRs) from a novel rotating Scheimpflug camera system (Pentacam, Oculus Inc.), calculated by version 1.16r04 software, are still inaccurate for eyes with a history of LASIK, PRK, or RK. The EKR measures steeper than true corneal power.

Key Points

Chicago-Holladay equivalent keratometer readings (EKRs), provided by a novel rotating Scheimpflug camera system (Pentacam, Oculus Inc.), are inaccurate for calculating IOL power in eyes that have undergone refractive surgery. Despite inclusion of the manufacturer's latest generation of software, additional refinements are still needed, reported Qiong-yan Tang, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

Dr. Tang and colleagues conducted a retrospective/prospective study to determine the accuracy of the Holladay EKRs report from the rotating Scheimpflug camera system in IOL power calculation after patients had undergone LASIK, PRK, or RK.

Software version 1.16r04 was used to measure the EKR at the 4.5-mm optical zone, Dr. Tang explained. She is an international fellow at the Jules Stein Eye Institute, University of California, Los Angeles.

Making comparisons

Each patient was asked to return to the practice to undergo three measurements on the same day: Holladay EKR measurement using the Holladay EKRs, a careful refraction, and anterior chamber depth measurement by optical pachymetry.

"Based on axial length measured preoperatively by ultrasound, effective lens position measured by optical pachymetry, the power of the implanted IOL, and the postoperative refraction, we back-calculated the true corneal K value for each patient using the original Hoffer formula. We then compared the Holladay EKR with the back calculation," Dr. Tang explained.

Differing changes

Dr. Tang pointed out that in the subgroup of patients who underwent RK, corneal changes were different from the changes in patients who underwent LASIK or PRK. Therefore, the investigators separated patients who underwent RK from those who underwent the other two procedures.

"For the patients who underwent LASIK or PRK, the [rotating Scheimpflug camera] corneal power was a mean of +1.84 D (range, +0.66 to +4.94 D) higher than the back-calculated K. For the patients who underwent RK, we found that the [rotating Scheimpflug camera] corneal power was +2.17 D higher, on average (range, +0.48 to +3.09 D)," she reported.

"Based on our results, we find that [rotating Scheimpflug camera] Holladay EKR, calculated using version 1.16r04 software, is still inaccurate for eyes that had undergone LASIK, PRK, or RK. The [rotating Scheimpflug camera] EKR consistently measures a steeper power than the true corneal power. We believe that additional refinements of the [rotating Scheimpflug camera] will be necessary to improve its clinical usefulness," Dr. Tang concluded.