True Net Power Research Articles

A new central–peripheral corneal curvature method for intraocular lens power calculation after excimer laser refractive surgery

To propose the central-peripheral (C-P) method, which requires no data history to calculate intraocular lens (IOL) powers for eyes that underwent laser in situ keratomileusis (LASIK), and compare the accuracy of the C-P method with other IOL formulas for eyes after LASIK. Sixteen patients with cataract (25 eyes) who underwent myopic LASIK were analysed retrospectively. The C-P method is a modified double-K method using the SRK/T formula, in which the estimated pre-LASIK keratometric power calculated from the post-LASIK peripheral anterior sagittal power (also called the axial power) is used for the Kpre in the double-K method using the SRK/T formula, and the post-LASIK anterior sagittal power is used for the Kpost. We compared the accuracy of the C-P method with other popular IOL calculation formulas for use in eyes after LASIK. The median values of the arithmetic and absolute prediction errors with the C-P method were 0.11 diopter (D) (range, -1.67 to 1.97 D) and 0.55 D (range, 0.02-1.97 D), respectively. The prediction error using the C-P method was within ±0.5 D in 48% of eyes, within -1.0 to +0.5 D in 60% of eyes, and within ±1.0 D in 68% of eyes. The C-P method resulted in a significantly higher percentage of eyes within ±0.5 D than the BESSt formula, Shammas-PL formula, true net power method, double-K method using 43.5 D for Kpre, and Feiz-Mannis method. The C-P method may be a good option for calculating IOL powers in eyes undergoing cataract surgery after LASIK when the preoperative LASIK data are unavailable.

Intraocular Lens Power Calculations for Cataract Surgery After Phototherapeutic Keratectomy in Granular Corneal Dystrophy Type 2

To investigate the predictability of various intraocular lens (IOL) power calculation methods in granular corneal dystrophy type 2 (GCD2) with prior phototherapeutic keratectomy (PTK) and to suggest the more predictable IOL power calculation method. Medical records of 20 eyes from 16 patients with GCD2, all having undergone cataract surgery after PTK, were retrospectively evaluated. Postoperative cataract refractive errors were compared with target diopters (D) using IOL power calculation methods as follows: 1) myopic and 2) hyperopic Haigis-L formula in IOLMaster (Carl Zeiss Meditec); 3) SRK/T formula using 4.5-mm zone Holladay equivalent keratometry readings (EKRs) (single-K Holladay EKRs method); 4) central keratometry power of true net power map in the Pentacam system (Oculus Optikgeräte GmbH); and 5) clinical history, Aramberri double-K, and double-K Holladay EKRs methods. Topographic status of corneal curvature after PTK was evaluated. Fourteen (70%) of 20 eyes showed central island formation after PTK. When central island was present, the mean absolute error (MAE) using the hyperopic Haigis-L formula was 0.25±0.15 D. When central island was not present, the myopic Haigis-L formula showed MAE of 0.33±0.16 D. When central island formation and IOLMaster keratometry underestimation were present, the hyperopic Haigis-L formula showed the least MAE of 0.26±0.08 D when switching the IOL-Master keratometry values equal to 4.5-mm zone Holladay EKRs. In planning for cataract surgery after PTK in GCD2, topographic analysis for central island formation is necessary. With or without central island formation, the hyperopic or myopic Haigis-L formula can be applied. When IOLMaster keratometry shows underestimation, the Haigis-L formula using 4.5-mm zone Holladay EKRs can be considered.

Predicting Postoperative Astigmatism Using Scheimpflug Keratometry (Pentacam) and Automated Keratometry (IOLMaster)

Purpose: To compare keratometry measurements obtained using an automated keratometer (AK, IOLMaster) and a Scheimpflug keratometer (Pentacam) in predicting residual astigmatism after cataract surgery.Methods: Preoperative corneal astigmatism was calculated using preoperative refraction, an AK and a Scheimpflug keratometer (anterior corneal power [ACP] and true net power [TNP]) in 155 eyes of 107 Asian subjects. Phacoemulsification cataract removal and nontoric intraocular lens insertion (Akreos®MI-60™, Bausch & Lomb, Rochester, NY, USA) were performed through a 2.8 mm temporal clear corneal incision. Six months later, postoperative astigmatism (postA) was calculated using manifest refraction. Error angle (EA) and error magnitude (EM) of above keratometries (AK, ACP, and TNP) in prediction of postA was calculated. The correlation between preoperative astigmatism and postA was analyzed using power vectors (J0 and J45).Results: AK resulted in the lowest EM and ACP resulted in the lowest EA. Preoperative astigmatism (preA) measured using AK, ACP, and TNP showed significant correlation with postA in both J0 and J45 components. (AK: rJ0 = 0.554, rJ45 = 0.559, ACP: rJ0 = 0.346, rJ45 = 0.281, TNP: rJ0 = 0.409, rJ45 = 0.231). Preoperative refractive astigmatism showed no significant correlation in any components.Conclusions: PreA determined using AK showed superior performance in prediction of postA than ACP and TNP.

Effects of error in radius of curvature on the corneal power measurement before and after laser refractive surgery for myopia

To investigate the sources of error in corneal power measurement before and after corneal refractive surgery for myopia. The study comprised 28 eyes of six males and eight females with a mean age of 26 (range 18-39 years). The radius of curvature of anterior and posterior corneal surface, Q-Values of anterior and posterior corneal surface and corneal central thickness were measured by rotating Scheimpflug imaging (Pentacam). The true net power F(g), back vertex power F(v), and keratometric power SimK, were calculated respectively at the apex and at a paracentral area on the 3 mm ring. For virgin eyes, the overestimation (0.53 ± 0.11 D) of the corneal power by using a keratometric index of 1.3375 was balanced by the underestimation (-0.21 ± 0.09 D) of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error with a mean value of 0.33 ± 0.11 D. With the Q-value changing from -0.09 to -0.41, the percentage balanced by the error in radius of curvature increased from 16% to 73%. However, for eyes after laser refractive surgery, the radius of curvature error lead to an overestimation (0.54 ± 0.16 D) of the corneal power and the keratometric index of 1.3375 again overestimated (1.59 ± 0.26 D) the corneal power, resulting in a large measurement error with a mean value of 2.12 ± 0.40 D. With the Q-value changing from 0.35 to 1.89, the percentage added by the error in radius of curvature increased from 14% to 32%. For virgin eyes, the overestimation of the corneal power by using a keratometric index of 1.3375 is balanced by the underestimation of the corneal power by the error in the radius of curvature, resulting in a relatively small corneal power error. However, for eyes after laser refractive surgery, the flatter anterior corneal surface means that the use of a keratometric index of 1.3375 significantly overestimates the corneal power and the radius of curvature error now adds to this overestimation and results in a large measurement error.

Visual outcomes and corneal changes after intrastromal femtosecond laser correction of presbyopia

To assess the effect of intrastromal femtosecond laser presbyopia treatment on uncorrected near visual acuity (UNVA) and corneal integrity over an 18-month period. Department of Ophthalmology, International Vision Correction Research Centre, University of Heidelberg, Heidelberg, Germany. Clinical trial. The UNVA (at 40 cm), corneal pachymetry, and true net power were evaluated preoperatively and 1, 3, 6, 12, and 18 months after femtosecond intrastromal presbyopic treatment (Intracor). Endothelial cell density (ECD) was measured preoperatively and 3, 6, and 12 months postoperatively. Data were analyzed with the Wilcoxon test at a P=.01 level of significance. The median UNVA improved significantly from 0.7 logMAR preoperatively to 0.4 logMAR, 0.2 logMAR, 0.2 logMAR, 0.3 logMAR, and 0.2 logMAR at 1, 3, 6, 12, and 18 months, respectively (all P<.001). The median corneal true net power increased significantly by 1.1 diopters (D) to 0.7 D, 0.8 D, 1.0 D, and 0.9 D, respectively (all P<.001); pachymetry showed no significant thinning postoperatively. There was no significant difference in ECD between preoperatively and postoperatively. Intrastromal femtosecond presbyopic treatment yielded a significant and stable gain of UNVA and corneal steepening without significant loss of endothelial cells or corneal thinning up to 18 months postoperatively. No significant regression of visual acuity or further corneal steepening occurred during the follow-up period. Dr. Auffarth and Dr. Holzer received lecture and consulting fees from Technolas Perfect Vision GmbH. No author has a financial or proprietary interest in any material or method mentioned.

Analysis of changes in crystalline lens thickness and its refractive power after laser in situ keratomileusis.

To evaluate changes in the anterior chamber depth (ACD), crystalline lens thickness (LT) and its refractive power after laser in situ keratomileusis (LASIK). In all cases, the preoperative and postoperative central ACD which were measured with Pentacam, Orbscan, IOL-Master and A-scan ultrasonography, central corneal true net power which was measured with the Pentacam, Orbscan and IOL-Master, axial length (AL) which was measured with IOL-Master and LT which was measured with the A-scan ultrasonography were compared using the paired sample t test. Ocular refractive errors and lens refractive power at corneal plane were calculated and their correlations were also evaluated before and after LASIK. At 1 week after LASIK, LT and crystalline lens refractive power at corneal plane (Dlens) which were associated with the IOL-Master and Pentacam increased significantly (P≤0.005), ACD decreased significantly (P≤0.001), but no significant increase was observed in the Dlens which was associated with the Orbscan (P=0.261). Significant correlations between the changes in the ocular refractive errors and Dlens which were associated with the Pentacam were observed at 1 week and 6 months after LASIK (P=0.028; P=0.001). LT increased significantly after LASIK, and this might partially lead to ACD decrease, Dlens increase and a small quantity of myopic regression.

Corneal Power After DSAEK Using Microkeratome-Prepared Tissues

To investigate changes of corneal power after descemet stripping automated endothelial keratoplasty (DSAEK). Anterior, posterior, and true-net (anterior + posterior) power of post-DSAEK and contralateral transparent corneas were measured, using the Pentacam rotating Scheimpflug camera (Oculus Wetzlar, Germany), 6 months after surgery in 23 consecutive patients who had undergone unilateral DSAEK at the same medical center. Moreover, in each cornea, Pentacam pachimetry was measured at the vertex and at 3 mm from the vertex, to calculate corneal thickness increase (CTI). In post-DSAEK corneas, the anterior power on average decreased by -0.24 ± 0.61 diopters (D), the negative posterior power increased by -0.96 ± 0.42 D, and the true-net power decreased by -1.19 ± 0.74 D (range: +0.17 to -2.23 D). Differences vs. contralateral transparent corneas did not reach statistical significance for anterior (p = 0.09) but were significant (p < 0.001) for posterior and true-net corneal power. CTI at 3 mm from the vertex was statistically larger (p < 0.0001) in post-DSAEK corneas (mean values 0.27 ± 0.07 vs. 0.12 ± 0.05); CTI values showed a statistically significant (p < 0.05) correlation with negative posterior corneal power values in post-DSAEK (r = -0.55) and in contralateral corneas (r = -0.67). In comparison with contralateral corneas, post-DSAEK corneas show a moderate reduction of the true-net corneal power measured by Pentacam. Knowledge of true-net corneal power changes might be useful in adjusting K values and calculating intraocular lens power, in eyes scheduled for DSAEK combined with intraocular lens implantation/exchange.

Automated keratometry in routine cataract surgery: Comparison of Scheimpflug and conventional values

To compare the mean keratometry (K) readings obtained with a conventional automated keratometer (IOLMaster) and a Scheimpflug keratometer (Pentacam) in eyes having preoperative assessment for routine cataract surgery. Epsom and St. Helier University Hospitals, London, United Kingdom. Evaluation of diagnostic technology. Mean K values were obtained with the conventional and Scheimpflug keratometers. The following Scheimpflug readings were evaluated: anterior K, true net power, and Holladay equivalent K measured at 1.0 to 7.0 mm corneal diameters. Mean readings for each type of keratometry were compared. Bland-Altman plots were used to determine the 95% limits of agreement (LoA) for the conventional and Scheimpflug keratometers. The mean conventional K was statistically significantly greater than the mean Scheimpflug K for true net power and equivalent K at 1.0 mm, 2.0 mm, 3.0 mm, and 4.0 mm corneal diameters. The mean conventional K was significantly less than the equivalent K at 5.0 mm, 6.0 mm, and 7.0 mm. The smallest mean difference was for equivalent K at 4.5 mm (0.02 diopters [D]). The smallest 95% LoA were -0.68 to 1.16 D (equivalent K at 5.0 mm) and -0.91 to 0.95 D (equivalent K at 4.5 mm). Overall, the equivalent K at 4.5 mm had the closest match with the conventional K values. The degree of interdevice variability with the conventional keratometer was lowest for the equivalent K at 4.5 mm and 5.0 mm, although this variability would be sufficient to influence intraocular lens power selection.

Practical method to calculate post-LASIK corneal power: the Actual K(a+p) method.

To evaluate the accuracy of a practical method (the Actual K(a+p) method) of corneal power measurement for post-LASIK eyes undergoing cataract surgery. Ten eyes of 7 patients (4 male, 3 female, average age 50.10±4.01 years, with -11.01±3.55D mean refraction before LASIK), underwent post-LASIK phaco+IOL cataract surgery. We used the posterior corneal curvature as measured by the Pentacam in a method we named Actual K(a+p) to calculate the post-LASIK corneal power for IOL calculation. The refractive outcomes after cataract surgery were evaluated. The Actual K(a+p) was compared with the back- calculated corneal power (BCK), which was thought to be the benchmark of true corneal power. The corneal power estimated by other published methods, including Maloney, Shammas, Koch-Maloney, Savini, and McCulley, together with the true net power and equivalent K reading (EKR) as found by the Pentacam were also compared with the BCK. All eyes achieved satisfied refractive status after cataract surgery. The difference between the postoperative refraction and the target refraction was 0.04±0.40D, range from -0.63D and +0.85D. Among all the methods we studied, although the Bonferroni multiple comparison tests did not detect significant differences between any two of them, the Actual K(a+p) yielded the highest agreement with the BCK, with 80% of the eyes falling within ±0.5D and 100% within ±1.0D from the BCK values. The Actual K(a+p) method can provide encour- aging results in post-LASIK eyes undergoing cataract surgery.

Corneal power measurement with a rotating Scheimpflug imaging system after Descemet-stripping automated endothelial keratoplasty

To evaluate alterations in corneal power parameters after Descemet-stripping automated endothelial keratoplasty (DSAEK) using rotating Scheimpflug imaging. University of Texas, Southwestern Medical Center, Dallas, Texas, USA. Eyes that had DSAEK were evaluated with a rotating Scheimpflug imaging system (Pentacam). The outcome parameters were mean anterior and posterior keratometry (K), mean anterior radius of curvature and posterior radius of curvature, anterior and posterior astigmatism, central corneal thickness (CCT), true net power, corneal volume, keratometric power deviation, and mean zonal-equivalent K readings. These values were compared with those in a control group of age and sex-matched normal eyes. The DSAEK group comprised 32 eyes (28 patients) and the control group, 32 eyes. The mean anterior K, posterior K, and true net power were 42.53 diopters (D) +/- 1.52 (SD), -6.80 +/- 0.55 D, and 40.55 +/- 1.79 D, respectively, in the DSAEK group and 43.60 +/- 1.62 D, -6.20 +/- 0.31 D, and 42.49 +/- 1.57 D, respectively, in the control group. The posterior radius of curvature, anterior and posterior K values, and true net power were statistically significantly lower in DSAEK eyes than in control eyes (P<.01). The equivalent K readings were significantly lower in all zones in DSAEK eyes than in control eyes (P<.01). The mean CCT was significantly higher in DSAEK eyes than in control eyes (628 +/- 46 mum versus 553 +/- 35 mum) (P<.01). The true net power of the cornea decreased significantly after DSAEK, mainly because of an increase in posterior curvature. No author has a financial or proprietary interest in any material or method mentioned.

Scheimpflug keratometry versus conventional automated keratometry in routine cataract surgery

To evaluate keratometry (K) readings obtained with an automated keratometer (IOLMaster) and Scheimpflug imaging (Pentacam) in eyes having routine cataract surgery and to compare the predicted and actual refractive outcomes. Epsom/St. Helier University Hospitals, London, United Kingdom. In this retrospective study, the mean absolute prediction errors (MAEs) were obtained for automated keratometry and Scheimpflug keratometry: true net power, anterior K, and equivalent K [corrected] values for 1.0 to 7.0 mm corneal diameters. Eyes were divided into lower delta K (mean 1.15 diopters [D]) and higher delta K (mean 2.13 D) groups and lower preoperative astigmatism (mean 0.83 D) and higher preoperative astigmatism (mean 2.55 D) groups to determine notable trends. The study evaluated 29 eyes. The lowest MAE was 0.424 D +/- 0.421 (SD) for Scheimpflug equivalent [corrected] K at 3.0 mm; the second lowest was 0.452 +/- 0.359 D for automated keratometry, which had the smallest SD overall. The difference was not statistically significant. In the lower delta K and astigmatism groups, the automated keratometer had the lowest MAE and smallest standard deviation. In the higher groups, there was a trend toward increased accuracy for the Scheimpflug equivalent [corrected] K values at 3.0 mm. In this small study, Scheimpflug imaging was not superior to automated keratometry overall, but the data suggest a trend toward increased accuracy of Scheimpflug equivalent [corrected] K values in eyes with more irregular corneas. No author has a financial or proprietary interest in any material or method mentioned.

Determining corneal power using Pentacam after myopic photorefractive keratectomy

To assess the accuracy of Pentacam Scheimpflug camera for corneal power measurement in eyes with previous photorefractive keratectomy for myopia. In this comparative interventional case series, 35 eyes of 35 patients who had myopic photorefractive keratectomy were studied. Corneal power was measured by conventional topography and Pentacam Scheimpflug camera, and equivalent keratometry readings (EKR) in different central corneal rings (0.5 to 4.5 mm), true net power and simulated keratometry (K) measurements as well as those obtained using Shammas no-history, Koch-Maloney and Haigis methods were compared with clinical history method. All corneal power measurements except for the topography simulated K and true net power values were statistically similar to the clinical history values. Simulated keratometry and 4.5-mm EKR values were more closely correlated with clinical history method. Shammas formula, Pentacam simulated K and 3-, 4- and 4.5-mm EKR provided a 95% confidence interval within +/-0.50 D of the mean clinical history method value, among these, the width of the 95% limits of agreement (LoA) was narrower for Shammas and Pentacam simulated K and 4.5-mm EKR values; however, considerably large 95% LoA were found between each of these values and those obtained with the clinical history method. Estimated preoperative keratometry was statistically similar to the preoperative measurement; however, estimated refractive change was different from actual value. The Pentacam 4.5-mm EKR and simulated keratometry may be used as an alternative to clinical history method to predict corneal power when pre-keratorefractive surgery data are unavailable; however, wide LoA should be considered in the calculations.

Effect of age on changes in anterior chamber depth and volume after laser in situ keratomileusis

To evaluate the effect of age on anterior chamber depth (ACD) and anterior chamber volume (ACV) after laser in situ keratomileusis (LASIK) for myopia. Department of Ophthalmology, Keio University School of Medicine, and a private clinic, Tokyo, Japan. In this retrospective study, patients who had uneventful LASIK for myopia or myopic astigmatism were divided into 2 groups: younger (younger than 40 years) and older (40 years and older). Central corneal thickness, central corneal true net power, ACV, and central ACD were measured using a rotating Scheimpflug camera preoperatively and 1 month postoperatively. These parameters were compared between the 2 groups. The study evaluated 161 eyes of 83 patients (mean age 34.5 years +/- 8.3 [SD]). The decreases in central ACD and ACV after LASIK were statistically significant in the younger group (P<.0001 and P = .0050, respectively) but not in the older group. Central ACD and the ACV decreased significantly after LASIK in younger patients (<40 years) but not in older patients, indicating that age influences changes in ACD and ACV after LASIK.

Agreement Between Pentacam and Videokeratography in Corneal Power Assessment

To investigate agreement between a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH) and two corneal topographers (TMS-2 Topography System, Tomey; and Keratron Scout, Optikon 2000 SpA) in measuring the corneal power of normal eyes. The mean corneal powers calculated by simulated keratometry (SimK) with each topographer were compared to those provided by the Pentacam in 71 patients. Specifically, the corneal power values of the Pentacam included in this analysis were the SimK (calculated using the measured anterior corneal radius and standard keratometric index of 1.3375) and the True net power (calculated using the anterior and posterior corneal curvatures and Gaussian optics formula for thick lenses, where the actual refractive index of the air, cornea, and aqueous humor are entered). Bland-Altman plots were used to investigate agreement and analysis of variance (ANOVA) was performed to detect statistical differences. Although ANOVA did not disclose a statistically significant difference among the mean SimK values (TMS-2: 43.20 +/- 1.51 diopters [D], Keratron Scout: 43.29 +/-1.48 D, Pentacam: 43.25 +/- 1.53 D), the 95% limits of agreement between the TMS-2 and Pentacam and between the Keratron Scout and Pentacam were wide (-1.05 to +0.94 D and -0.95 to +1.02 D, respectively). Agreement was even poorer when considering the mean True net power (42.00 +/- 1.54 D), which was significantly lower than the mean Pentacam SimK (P < .001). Although corneal topography and the Pentacam provide similar SimK values, their data should not be used interchangeably as only moderate agreement exists between them. Corneal power values calculated by the True net power are significantly lower than any SimK and cannot be entered into intraocular lens power formulas.

Use of the Pentacam True Net Corneal Power for Intraocular Lens Calculation in Eyes After Refractive Corneal Surgery

To assess the accuracy and validity of true net corneal power of the Pentacam system to provide a keratometry reading for calculating intraocular lens (IOL) power in postoperative refractive surgery eyes. Refraction, an automated keratometry reading, and true net corneal power were measured for 30 eyes that required cataract surgery and had previously undergone refractive surgery. Target refraction values calculated with the SRK/T formula using true net corneal power were compared with postoperative manifest refraction values. Using true net corneal power, the mean deviation from the desired postoperative cataract refractive outcome was 0.47 +/- 0.56 diopters (D); the actual refraction was within +/- 0.50 D of the intended refraction for 70% of eyes (21/30) and within +/- 1.00 D for 93% of eyes (28/30). The true net corneal power can be used as a keratometry reading for eyes with previous refractive surgery requiring cataract surgery.

Scheimpflug photography keratometry readings for routine intraocular lens power calculation

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. Private clinical ophthalmology practice, Lynwood, California, USA. 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. 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). The K values from Scheimpflug photography did not improve accuracy over autokeratometer values for routine IOL power calculation.

The Comparison of Central and Mean True-Net Power (Pentacam) in Calculating IOL-Power After Refractive Surgery

PurposeTo compare the accuracy of central true net corneal power (cTNP) and mean true net corneal power (mTNP) of the Pentacam system to give a keratometry (K) reading for calculating IOL (intraocular lens) power in eyes following refractive surgery.MethodsRefraction, an automated K-reading (Km), cTNP and mTNP were measured for 15 eyes that required cataract surgery and had previously undergone refractive surgery. The difference between postoperative manifest refraction values and target refraction values calculated with the SRK/T formula using cTNP were compared with the one using mTNP.ResultsThe mean deviation from the desired post-cataract refractive outcome was 0.60 diopter (D) ±0.47 (standard deviation) using cTNP; 0.75±0.54 using mTNP (p=0.386). The actual refraction was within ±0.50D of the intended refraction for 60% (cTNP) and 33.3% (mTNP) of eyes, and within ±1.00D for 93% (cTNP) and 66.7% (mTNP) of eyes.ConclusionsAlthough not statistically significant, the cTNP showed better accuracy than mTNP to give a keratometry (K) reading for post-refractive surgery eyes requiring cataract surgery.

Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery

To evaluate corneal power measurements by a rotating Scheimpflug camera (Pentacam, Oculus) in eyes that have had myopic excimer laser surgery. Private practice, Bologna, Italy. This prospective comparative interventional case series comprised 16 eyes of 16 patients who had myopic excimer laser surgery and for whom all perioperative data were available. Four corneal power measurements obtained with the Pentacam (simulated keratometry, true net power, equivalent K reading, and BESSt formula) were analyzed and compared with values derived using the clinical history method and 2 other formulas for calculating corneal power after refractive surgery (modified keratometric refractive index according to Savini et al. and separate consideration of the anterior and posterior corneal curvatures according to Speicher). Analysis of variance showed a statistically significant difference between all methods (P<.0001). Bonferroni multiple comparison tests showed that the only Pentacam measurements not statistically different from the corneal power values derived using the clinical history method were the equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm and those derived with the BESSt formula; however, considerably large 95% limits of agreement (LoA) were calculated between each of these values and those obtained with the clinical history method. The Pentacam device gave corneal power measurements that did not statistically significantly had differ from those predicted by the clinical history method in eyes that had previous myopic excimer laser surgery. Wide LoA are a potential source of error in intraocular lens power calculation in such patients.

No Forward Shifting of Posterior Corneal Surface in Eyes Undergoing LASIK

To evaluate structural changes in the cornea and anterior chamber (AC) after LASIK for myopia. Retrospective nonrandomized study. One hundred sixty-one eyes of 83 patients (mean age, 34.5+/-8.3 years) who underwent uneventful LASIK for myopia and myopic astigmatism. The preoperative refractive error (spherical equivalent) and corneal thickness were -6.02+/-2.10 diopters (D) and 549.9+/-29.3 mum, respectively. The AC volume (ACV), AC depth (ACD), corneal thickness, central corneal true net power, and posterior corneal tangential curvature were measured using a rotating Scheimpflug camera before and 1 week and 1 month after surgery. In 84 eyes of 42 cases, anterior and posterior corneal elevations and corneal thicknesses also were measured by scanning-slit topography before and 1 month after surgery. Time course of the ACV, ACD (central, midperiphery, periphery), corneal thickness, central corneal true net power, posterior corneal tangential curvature using the Scheimpflug camera, and amount of forward shifting of the posterior corneal surface at the center of the difference map using scanning-slit topography. Preoperative and 1-month postoperative mean ACVs were 198.1 mm3 and 196.4 mm3, respectively, and preoperative and postoperative mean ACDs (center, midperiphery, periphery) were 3.24, 2.65, and 1.89 mm, and 3.21, 2.63, and 1.87 mm, respectively. The corneal thickness within the optical zone, subjective refraction, and central corneal true net power significantly changed by tissue subtraction after LASIK (P<0.0001). There were no significant differences in the ACV, ACDs (center, midperiphery, periphery), peripheral corneal thickness, and posterior corneal tangential curvature from preoperatively to 1 month after LASIK. However, using scanning-slit topography, the posterior corneal surface displayed a mean forward shift of 29.0+/-19.0 microm 1 month after surgery. The posterior corneal curvature, peripheral corneal thickness, ACDs, and ACV were consistent. These observations indicated that neither forward shifting of the central posterior corneal surface (bulging) nor backward shifting of the peripheral posterior corneal surface due to corneal swelling after ablation occurred after LASIK.