Maya C. Shammas

Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis

PURPOSE To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method. DESIGN Interventional case series. METHODS Retrospective analysis of consecutive cases from clinical practice. Two hundred randomly selected eyes from 200 patients were evaluated before and after LASIK surgery. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (Kpre) and post-LASIK K readings (Kpost). We then calculated for each case the pre- and post-LASIK refraction at the corneal plane and the amount of correction obtained by the refractive surgery (CRc). The cases were divided into two groups. Group I was used to derive the two formulas. The K values were calculated using the history-derived method (Kc.hd) in which Kc.hd = Kpre - CRc. Kc.hd was compared with Kpost. The average difference was 0.23 diopters for every diopter of myopia corrected. This value was used to calculate the corneal power using the refraction-derived method (Kc.rd) where Kc.rd = Kpost -0.23CRc. A regression equation was used to develop a clinically derived method (Kc.cd) where Kc.cd = 1.14Kpost -6.8. The values obtained with the two methods were compared with the Kc.hd values in group II to validate the results. RESULTS Both Kc.rd and Kc.cd values correlated highly with Kc.hd when plotted on a scattergram (P <.001), and there was no statistically significant difference between the mean keratometric values (P >.5). CONCLUSIONS The corneal power measurements for intraocular lens power calculations after LASIK need to be corrected to avoid hypermetropia after cataract surgery by either the history-derived method, the refraction-derived method, or the clinically derived method.

No-history method of intraocular lens power calculation for cataract surgery after myopic laser in situ keratomileusis

PURPOSE: To prospectively evaluate the no‐history method for intraocular lens (IOL) power calculation in 15 cataractous eyes that had previous myopic laser in situ keratomileusis (LASIK) and for which the pre‐LASIK K‐readings were not available. SETTING: Private practice, Lynwood, California, USA. METHODS: The predicted IOL power was calculated in each case. Also calculated were the mean arithmetic and absolute IOL predictor errors, range of the prediction errors, and number of eyes in which the error was within ±1.00 diopter (D). RESULTS: The mean arithmetic IOL prediction error was −0.003 D ± 0.63 (SD), and the mean absolute IOL prediction error was 0.55 ± 0.31 D (range −0.89 to +1.05 D). Fourteen eyes (93.3%) were within ±1.00 D. The results of the Shammas post‐LASIK formula compared favorably to the results obtained with the optimized Holladay 1 (P = .42), Hoffer Q (P = .25), Haigis (P = .30), and Holladay 2 (P = .19) formulas and were better than the results obtained with the optimized SRK/T formula (P = .0005). CONCLUSION: The no‐history method is a viable alternative for IOL power calculation after myopic LASIK when the refractive surgery data are not available.

Scheimpflug photography keratometry readings for routine intraocular lens power calculation

PURPOSE: To prospectively evaluate keratometry (K) values obtained by Scheimpflug photography in eyes scheduled for cataract surgery, compare the results with K values obtained with an autokeratometer (automated K), and evaluate the K values in commonly used intraocular lens (IOL) power calculation formulas for routine cataract surgery. SETTING: Private clinical ophthalmology practice, Lynwood, California, USA. METHODS: The mean simulated K power (simulated K), equivalent K (equivalent K), and true net power (true net K) readings from the Pentacam Comprehensive Eye Scanner were compared with the automated K readings. Automated K, simulated K, and equivalent K values were compared in commonly used IOL power calculation formulas. RESULTS: The mean automated K value was 43.49 diopters (D) ± 1.75 (SD) and the mean simulated K value, 43.49 ± 2.00 D (P>.1). The mean equivalent K value was 43.78 ± 1.97 D and exceeded the mean automated K and simulated K by 0.29 D (P>.1). The mean true net K was 42.31 ± 2.13 D, which was 1.18 D lower than the automated K and simulated K values (P = .015). The IOL prediction mean absolute error was 0.41 ± 0.27 D using the automated K method, 0.50 ± 0.36 D using the simulated K method (difference 0.09 D) (P>.1), and 0.65 ± 0.35 D using the equivalent K method (difference 0.24 D) (P<.01). CONCLUSION: The K values from Scheimpflug photography did not improve accuracy over autokeratometer values for routine IOL power calculation.

Variability of axial length, anterior chamber depth, and lens thickness in the cataractous eye

PURPOSE: To review and evaluate the biometry measurements in 750 eyes (first eye developing cataract) of 750 consecutive patients with no retinal pathology. SETTING: Private practice, Lynwood, California, USA. METHODS: All measurements were performed with the I3 system A‐scan (Innovative Imaging, Inc.) using an immersion technique. The axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) measurements were evaluated in relation to each other and in relation to age, sex, and keratometric readings. RESULTS: The mean AL was 23.46 mm ± 1.03 (SD), the mean ACD was 2.96 ± 0.45 mm, and the mean LT was 4.93 ± 0.56 mm. Men presented for surgery at an earlier age than women (mean 73 ± 9.41 years versus 75 ± 8.55 years) with a longer AL (23.76 ± 1.00 mm versus 23.27 ± 1.01 mm). The AL tended to be longer in younger patients (r = −0.127; P<.001); the ACD tended to be deeper in younger patients (r = −0.250; P<.001) and in longer eyes (r = 0.423; P<.001). The LT tended to be thicker in older patients (r = 0.385; P<.001) and in shorter eyes (r = −0.179; P<.001), with large scatter in the distribution. CONCLUSIONS: There was a positive correlation between AL and ACD and an inverse correlation between AL and LT. Also, AL was inversely correlated with age and corneal power.

Variation in corneal hysteresis and central corneal thickness among black, hispanic and white subjects

Purpose:  To determine whether differences in corneal hysteresis (CH) and central corneal thickness (CCT) between black, Hispanic and white subjects exist independently of one another.

Improving the Second-Eye Refractive Error in Patients Undergoing Bilateral Sequential Cataract Surgery

PURPOSE To assess the refractive error in the second eye to undergo surgery when the intraocular lens (IOL) power was modified to correct 50% of the error from the first eye when such an error exceeded 0.50 diopter (D). DESIGN Prospective, observational case series. PARTICIPANTS Two hundred fifty patients with bilateral, sequential cataract surgery. METHODS Two hundred fifty consecutive patients who underwent the first-eye cataract operation 1 to 3 months earlier were scheduled for cataract surgery in the second eye. When choosing the IOL power for the second eye, the calculations were adjusted to correct 50% of the first-eye refractive error (FERE). The adjusted second-eye refractive error (aSERE) was evaluated 6 to 8 weeks after surgery. It was compared with the FERE, with a potential nonadjusted SERE, and with a potential fully adjusted SERE. MAIN OUTCOME MEASURES Postoperative refractive error. RESULTS The median aSERE was significantly lower in the second eye compared with the median FERE in the 47 cases in which the FERE ranged from -0.50 to -1.00 D (-0.12 vs. -0.66 D), in the 15 cases in which the FERE exceeded -1.00 D (-0.12 vs. -1.25 D), in the 24 cases in which the FERE ranged from 0.50 to 1.00 D (-0.03 vs. 0.65 D), and in the 11 cases in which the FERE exceeded 1.00 D (-0.29 vs. 1.19 D). The difference was statistically significant in all categories (P<0.00001). CONCLUSIONS In patients undergoing bilateral sequential cataract surgery and in cases in which the FERE exceeded 0.50 D, the refractive error of the second eye can be improved by modifying the IOL power to correct up to 50% of the error from the first eye. FINANCIAL DISCLOSURE(S) The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Post-LASIK IOL Power Calculations: Where Are We in 2012

Calculating the correct intraocular lens (IOL) after refractive surgery, whether it is PRK, LASIK, LASEK, or RK, can be a challenge. This review will aim to give an understanding of the errors and how it affects the formulas used in IOL power calculations. This will aid ophthalmologists in choosing the best IOL for their patients.