H. John 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.

Comparison of 2 laser instruments for measuring axial length.

PURPOSE: To compare axial length (AL), anterior chamber depth (ACD), and keratometric (K) measurements of 2 laser biometers. SETTING: Private practices, Lynwood and Santa Monica, California, USA. METHODS: In this prospective comparative observational study of eyes with cataract and eyes with a clear lens, AL, ACD, and K measurements were performed using an IOLMaster biometer, which uses partial coherence interferometry (PCI), and a Lenstar LS 900 biometer, which uses optical low‐coherence reflectometry (OLCR). Intraocular lens (IOL) power calculation was performed using the Haigis formula. The IOL prediction error was calculated for each eye. RESULTS: The study evaluated 50 eyes with cataract and 50 eyes with a clear lens. There was a good correlation between AL, ACD, and K measurements in the cataractous eyes (r = 0.9993, 0.9667, and 0.9959, respectively) and in eyes with a clear lens (r = 0.9995, 0.8211, and 0.9959, respectively). The OLCR unit measured a slightly longer AL in the cataract group and clear lens group (mean difference 0.026 mm and 0.023 mm, respectively), a deeper ACD (0.128 mm and 0.146 mm, respectively), and a flatter K (−0.107 diopter [D] and −0.121 D, respectively). The differences were statistically significant (P<.0001). The mean absolute error in IOL power prediction was 0.455 D ± 0.32 (SD) with the OLCR unit and 0.461 ± 0.31 D with the PCI unit (P>.1). CONCLUSIONS: Measurements were comparable between the OLCR device and the PCI device. A slight decrease (0.050) in the a0 constant is recommended if the Haigis formula is used. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

A comparison of immersion and contact techniques for axial length measurement.

A prospective study was conducted on 180 eyes to evaluate axial length measurements obtained with both contact and immersion techniques. Each eye was measured with the Ocuscan-DBR (contact), the Ocuscan-400 (immersion), and the Kretz 7200 MA (immersion) units. Axial length measurements obtained by the two methods were highly reproducible. Axial length measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0.24 mm.

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.

The fudged formula for intraocular lens power calculations

The formula presented is a modification of Colenbranders formula. The formula yields a 0.70 diopter stronger power emmetropizing lens for a 23.0 mm eye. It also fudges the axial length by subtracting 0.1 mm for each 1.0 mm longer than 23.0 mm, and by adding 0.1 mm for each 1.0 mm shorter than 23.0 mm. Axial length was measured using immersion A-scan technique. In our prospectively tested group, the adjusted formula predicted implant power within +/- 1.00 diopter in 95% of eyes.

Repeatability and reproducibility of biometry and keratometry measurements using a noncontact optical low-coherence reflectometer and keratometer.

PURPOSE To evaluate the repeatability, reproducibility, or both of the biometry and keratometry measurements obtained by a new optical low-coherence reflectometer and keratometer (Lenstar LS 900, version 1.10; Haag-Streit). DESIGN Prospective, comparative, observational study. METHODS SETTING Private practice, Lynwood, California, and Santa Monica, California. STUDY POPULATION The measurements of the second eye to be operated on of 37 patients between October 2010 and January 2011 were analyzed. OBSERVATION PROCEDURE The axial length, central corneal thickness, aqueous depth, anterior chamber depth, crystalline lens thickness, white-to-white corneal diameter, as well as the keratometric readings at the flattest meridian, the steepest meridian, the average K, the amount of astigmatism, and the minus astigmatic cylinder axis were measured before surgery on the patients first eye and were repeated 1 month later before the second eye surgery. main outcome measures: The repeatability and reproducibility of the biometry and keratometry measurements. RESULTS Intrasession repeatability and intersession reproducibility were excellent with a very low coefficient of variation and high interclass correlation coefficients for all measured parameters. Bland-Altman plots show good correlation for axial length measurements (95% limits of agreement ranging from -0.056 to +0.04 mm), anterior chamber depth (-0.22 to +0.18 mm), crystalline lens thickness (-0.21 to +0.27 mm), corneal diameter (-0.28 to +0.24 mm), average keratometric readings (-0.56 to +0.47 diopters), and amount of astigmatism (-0.58 to +0.40 diopters). CONCLUSIONS The precision of the measurements obtained by the new optical reflectometer and keratometer is extremely high.

Protocols for Studies of Intraocular Lens Formula Accuracy

M ANY STUDIES HAVE BEEN PUBLISHED ASSESSING the accuracy of intraocular lens (IOL) power calculation. Since the formation of the IOL Power Club in 2005, errors have been noted in the protocols used in these studies of accuracy in the peerreviewed literature. These errors have been seen in articles in most all of our most respected journals. Unfortunately, no methodology standards for authors for these studies have been published since 1981. Many discussions were held along with statistical consultation to agree on a set of protocols. In an attempt to aid authors, 10 recommendations are offered to make a study statistically valid and completely fair in evaluating the accuracy of tested formulas, methods, and instruments. Firstly, the demographics of the study population (ie, sex, age, and ethnicity) should be clearly described at the beginning of the Methods section. These may well have a relevant influence on eye biometric parameters and, therefore, IOL power calculation performance. Optimization through IOL-specific lens constants may depend on these variables as well. More importantly, before comparing the results of the formulas, the mean error (ME) of the study group for each formula should be made to equal zero by changing the lens factor (constant) individually for each formula. This eliminates the bias of the lens factor chosen and is the only proper way to do this so that all the formulas are the same. This can easily be done using the Excel software’s Data/What If Analysis/Goal Seek function. There are other ways to do this if you have the dataset in a database and are able to do stepwise iterations through the lens constant

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.

Precision of biometry, keratometry, and refractive measurements with a partial coherence interferometry–keratometry device

PURPOSE: To evaluate the precision of the axial length (AL), keratometry (K), anterior chamber depth (ACD), astigmatism, and minus astigmatic cylinder axis measurements by a partial coherence interferometry (PCI)–keratometry device. SETTING: Private practice, Lynwood, California. METHODS: This prospective comparative observational study analyzed measurements in the second eye to have cataract surgery. Before surgery in the first eye, AL, K, ACD, astigmatism, and cylinder axis in both eyes were measured with an IOLMaster PCI device. The measurements were repeated approximately 1 month later, before second eye‐surgery. The 2 sets of measurements were compared. RESULTS: The study evaluated 121 eyes of 121 patients. The interclass correlation coefficient (ICC) for AL was 0.999 in all 3 signal‐to‐noise ratio (SNR) categories; the highest difference range was with an SNR below 100. Astigmatism, K, and cylinder axis had a high correlation in flat corneas (K reading <42.0 diopters [D]) (ICC = 0.994, 0.978, and 0.918, respectively) and a poorer correlation with K readings between 42.0 D and 44.0 D (ICC = 0.905, 0.774, and 0.456, respectively) and K readings above 44.0 D (ICC = 0.988, 0.729 and 0.446, respectively). CONCLUSIONS: The precision of the PCI measurements was extremely high for AL with low fluctuations (95% limits of agreement [LoA], 0.06 mm) and was relatively high for K readings with higher fluctuations (95% LoA, 0.55 D) and for ACD (95% LoA, 0.2 mm). The precision of astigmatism and cylinder axis was high in flat corneas and relatively low in steeper corneas. Financial Disclosure: Neither author has a financial or proprietary interest in any material or method mentioned.

Axial length measurement and its relation to intraocular lens power calculations

The axial length was measured in five hundred consecutive cases using the Kretz 7200 MA ultrasound unit and an immersion technique. The reproducibility of the measurements was within +/- 0.2 mm when performed manually, and within +/- 0.05 mm when performed electronically. Intraocular lens power calculations were performed using a modification of Colenbranders formula. We predicted within + 1 diopter in 78.8% of the cases. The accuracy was up to 83% when the axial length ranged between 23.0 and 25.0 mm. Stronger power lenses were implanted in shorter eyes and weaker power lenses in longer eyes, necessitating the use of a fudge factor. Surgeons using an immersion technique for axial length measurements should use formulas yielding stronger power lenses, such as Binkhorsts formula or our modification of Colenbranders formula, with a fudge factor for short and long eyes.