Michael K. Smolek

Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia

Purpose Because keratometry readings may not accurately reflect the refractive changes after keratorefractive surgery for myopia, better methods for the assessment of corneal curvature in the postsurgical cornea are needed. Methods We developed a procedure to calculate the average central power (ACP) of the cornea within the entrance pupil from videokeratography. Videpkeratograph-derived keratometry-style readings (average K; K) and calculated ACPs, as well as the differences between the two values, were compared in four groups: normal corneas (n = 30), corneas with regular astigmatism (n = 30), post-radial keratotomy corneas (RK, n = 85), and post-excimer laser photorefractive keratectomy corneas (PRK, n = 63). Intraocular lens (IOL) powers calculated by using K or ACP in the Sanders-Retzlaff-Kraff formula were compared. Results In the groups with normal corneas or regular astigmatism, none of the eyes showed a difference between K and ACP >0.25 D. However, six (7%) of the RK eyes and 16 (25%) of the PRK eyes had differences >0.55 D; in these eyes, the disparity between IOL powers calculated by using K and IOL powers calculated by using ACP was >0.5 D. Conclusion These results suggest that basing the calculation of IOL powers on keratometry readings in patients who have undergone RK, PRK, or possibly other refractive procedures may result in a residual refractive error in some eyes. In particular, patients undergoing surgery involving a small optical zone or large attempted correction or both, as well as those with low postoperative keratometry readings, may be at risk for this problem if IOL placement becomes necessary in later years.

Safety and efficacy of cyclosporine 0.05% drops versus unpreserved artificial tears in dry-eye patients having laser in situ keratomileusis.

PURPOSE: To evaluate dry‐eye signs, symptoms, and refractive outcomes in patients with dry‐eye disease having laser in situ keratomileusis (LASIK). METHODS: In this randomized parallel double‐masked prospective clinical trial, 42 eyes of 21 myopic patients (mean spherical equivalent −4.3 diopters [D], range −1.00 to −10.63 D) with dry‐eye disease were treated with unpreserved artificial tears or cyclosporine 0.05% ophthalmic emulsion twice a day beginning 1 month before LASIK. Treatment with the study drug was discontinued for 48 hours post surgery and then resumed for 3 additional months. Both groups used additional artificial tears as needed. Study visits occurred pretreatment (baseline), before surgery, and at 1 week and 1, 3, 6, and 12 months after surgery. RESULTS: Statistically significant increases from baseline were found in Schirmer scores for artificial tears at 1 month (P = .036) and for cyclosporine 0.05% before surgery and 1 week, 1 month, and 6 months after surgery (P<.018). There were no significant differences from baseline or between groups in responses to the Ocular Surface Disease Index questionnaire or best corrected visual acuity (BCVA), nor were there significant between‐group differences in superficial punctate keratitis or uncorrected visual acuity. Mean refractive spherical equivalent in cyclosporine‐treated eyes was significantly closer to the intended target at 3 and 6 months after surgery than in artificial‐tears–treated eyes (P = .007). A greater percentage of cyclosporine eyes was within ±0.5 D of the refractive target 3 months after surgery than artificial tears eyes (P = .015). CONCLUSION: Successful outcomes after LASIK were achieved for dry‐eye disease patients. Treatment with cyclosporine 0.05% provided greater refractive predictability 3 and 6 months after surgery than unpreserved artificial tears.

Corneal topography of excimer laser photorefractive keratectomy

ABSTRACT The application of the 193 nm excimer laser for keratorefractive surgery promises to deliver a higher degree of precision and predictability than traditional procedures such as radial keratotomy. The development and evaluation of keratorefractive surgery have benefited from the parallel advances made in the field of corneal topography analysis. We used the Computed Anatomy Topography Modeling System (TMS‐1) to analyze a Louisiana State University (LSU) Eye Center series of patients who had photorefractive keratectomy for the treatment of myopia with the VISX Twenty/Twenty™ excimer laser system. The excimer ablations were characterized by a relatively uniform distribution of surface powers within the treated zone. In the few cases that exhibited marked refractive regression, corneal topography analysis showed correlative changes. With topographical analysis, centration of the ablations relative to the center of the pupil could be evaluated. Marked improvement in centration occurred in the patients of LSU Series IIB in which the procedure to locate the point on the cornea directly over the pupils center during surgery was refined. Corneal topographical analysis provides objective measures of keratorefractive surgical results and is able to measure the precise tissue removal effect of excimer laser ablation without the uncertainties caused by measuring visual acuity alone. Our observations forecast the need for improved aids to center the laser ablations and for the development of a course of treatment to prevent post‐ablation stromal remodeling.

Goodness-of-prediction of Zernike polynomial fitting to corneal surfaces.

PURPOSE: To determine the goodness‐of‐prediction of the fitting routine by measuring the difference between topographic corneal surfaces and their Zernike reconstructions as a function of polynomial order and optical zone size for various corneal conditions. SETTING: Corneal research laboratory in a university eye center. METHODS: Corneal topography maps (N = 253) were obtained from the Louisiana State University Eye Center. A variety of corneal conditions were used: normals; astigmatism; laser in situ keratomileusis, photorefractive keratoplasty (PRK), and radial keratotomy (RK) postoperative cases (myopic spherical corrections); keratoconus suspect; mild, moderate, and severe keratoconus; pellucid marginal degeneration; contact lens–induced corneal warpage; and penetrating keratoplasty. The root‐mean‐square (RMS) error of the goodness‐of‐prediction of the Zernike representation of corneal surface elevation was extracted for 4, 6, and 10 mm optical zones, whereas Zernike radial orders were varied from 3 to 14 in 1‐order steps. The mean ± SEM of the RMS error was plotted as a function of Zernike order and compared with criteria for normal surface fits. RESULTS: Fitting accuracy improved as more Zernike terms were included, but some conditions showed significant errors (when compared with normal surfaces), even with many added terms. For a 6 mm optical zone, the normal cornea group had the lowest RMS error and did not require terms above the 4th order to achieve <0.25 μm RMS error. Astigmatism met the 0.25 μm threshold at the 5th order, whereas keratoconus suspect required 7 orders. Laser in situ keratomileusis and PRK met the 0.25 μm threshold at the 8th order, whereas RK required 10 orders. Contact lens–induced corneal warpage and mild keratoconus needed 12 orders to meet the 0.25 μm threshold, whereas pellucid marginal degeneration, moderate and severe keratoconus, and keratoplasty categories were not well fitted even at 14 orders. CONCLUSIONS: A 4th‐order Zernike polynomial appeared reliable for modeling the normal cornea, but using a 4th‐order fitting routine with an abnormal corneal surface caused a loss of fine‐detail shape information. As more Zernike terms were added, the accuracy of the fit improved, and the result approached the minimum error found with normal corneas. Unless sufficient higher‐order Zernike terms are included when analyzing irregular surfaces, some diagnostic applications of Zernike coefficients may not be rigorous. This conclusion also suggests that wavefront shape analysis is similarly dependent on the number of orders used. Current surgical corrections based on normal‐eye wavefronts may fail to capture all visually relevant aberrations in abnormal eyes, such as those having laser retreatments or experiencing corneal warpage from contact lens wear. A clinical goodness‐of‐fit or goodness‐of‐prediction index would indicate whether the number of terms in use has fully accounted for all of the visually significant aberrations present in the eye.

Advantages and Disadvantages of the Zernike Expansion for Representing Wave Aberration of the Normal and Aberrated Eye

PURPOSE Zernike expansion has been selected for use in describing wavefront aberrations in the human eye. The advantages and limitations of this approach are assessed for eyes with varying degrees of aberration. METHODS Corneal topography examinations were taken with the Nidek OPD-Scan topographer/aberrometer. These higher data density corneal topography examinations were converted to height data and subsequently to wavefront representations. System noise was evaluated with a 2D frequency analysis of 43-D test balls. Both Zernike polynomials and 2D Fourier transforms were used to evaluate fidelity in the presentation of the point spread function. A display format for potential clinical use was developed based upon Zernike decomposition. RESULTS Systematic noise from the corneal topographer was found to be minimal and, when eliminated, produced small changes in the point spread function. Using Zernike decomposition up to the 30th order failed to preserve the higher frequency aberrations present in aberrated eyes. Use of a Zernike decomposition display with a fixed micron scale presented only clinically significant details of spherical aberration, coma, trefoil, irregular components above third order and total higher-order aberrations (above second order). CONCLUSIONS Zernike polynomials excel in extracting the low-order optical characteristics of visual optics. Zernikes accurately represent both low- and high-order aberrations in normal eyes where high-order aberrations are clinically insignificant. For eyes after corneal surgery or eyes with corneal pathology such as keratoconus that have significant higher-order aberrations, the Zernike method fails to capture all clinically significant higher-order aberrations.

Corneal hydration and central islands after excimer laser photorefractive keratectomy

Purpose: To determine whether uneven corneal surface hydration during excimer laser photorefractive keratectomy (PRK) is related to postoperative occurrence of central islands. Setting: LSU Eye Center, New Orleans, Louisiana, USA. Methods: A retrospective study reviewed intraoperative videotapes and postoperative videokeratography of 49 eyes of 49 patients who had excimer laser PRK for myopia. The uniformity of corneal hydration within the photoablation zone, particularly the frosty appearance of the ablated zone, was characterized. The presence or absence of a topographic central island (steepening of at least 3.0 diopters and 1.5 mm in diameter) was determined from the 1 month postoperative videokeratographs. Results: Twelve eyes (24,5%) developed central islands postoperatively. A statistically significant association was observed between the uneven surface hydration (central accumulation of fluid) within the ablation zone intraoperatively and the formation of central islands postoperatively (P < .001, Kruskal‐Wallis test; Kendall, r rank correlation = 0.534; P < .001). Conclusion: Nonuniform fluid distribution during photoablation was a risk factor for central island formation after PRK. Intraoperatively, the presence of excess fluid in the central cornea appeared as a shiny area. This mirror‐like surface layer may reduce the rate of central ablation by reflecting and absorbing a significant amount of the incident excimer laser light.

Variability of subjective classifications of corneal topography maps from LASIK candidates

PURPOSE To evaluate the variability of subjective corneal topography map classification between different experienced examiners and the impact of changing from an absolute to a normative scale on the classifications. METHODS Preoperative axial curvature maps using Scheimpflug imaging obtained with the Pentacam HR (Oculus Optikgeräte, Wetzlar, Germany) and clinical parameters were sent to 11 corneal topography specialists for subjective classification according to the Ectasia Risk Scoring System. The study population included two groups: 11 eyes that developed ectasia after LASIK and 14 eyes that had successful and stable LASIK outcomes. Each case was first reviewed using the absolute scale masked to the patient group. After 3 months, the same cases were represented using a normative scale and reviewed again by the same examiners for new classifications masked to the patient group. RESULTS Using the absolute scale, 17 of 25 (68%) cases had variations on the classifications from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 60%. Using the normative scale, the classifications from 11 of 25 (44%) cases varied from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 61%. Eight examiners (73%) reported statistically higher scores (P < .05) when using the normative scale. Considering all 550 topographic analyses (25 cases, 11 examiners, and two scales), the same classification from the two scales was reported for 121 case pairs (44%). CONCLUSION There was significant inter-observer variability in the subjective classifications using the same scale, and significant intra-observer variability between scales. Changing from an absolute to a normative scale increased the scores on the classifications by the same examiner, but significant inter-observer variability in the subjective interpretation of the maps still persisted.

Intervention with vitamins in patients with nonproliferative diabetic retinopathy: a pilot study.

Background The purpose of this study was to determine whether a combination of vitamins B6, B9, and B12 is an effective intervention for reducing the signs and symptoms of nonproliferative diabetic retinopathy. Methods Ten subjects with type 2 diabetes mellitus (n = 20 eyes) with clinically diagnosed mild to moderate nonproliferative diabetic retinopathy were recruited from a private practice ophthalmology clinic for this open-label, uncontrolled, prospective six-month study. Metanx® vitamin tablets (containing 3 mg L-methylfolate calcium, 35 mg pyridoxal-5′-phosphate, and 2 mg methylcobalamin) were administered at a dosage of two tablets daily. Primary outcome indicators were the percent change in mean retinal sensitivity threshold measured by macular microperimetry and the percent change in mean central retinal thickness measured by spectral-domain optical coherence tomography. Results Three subjects were lost to follow-up. In the remaining seven subjects, two of 14 eyes had foveal edema that prevented microperimetry measurements due to poor fixation. The remaining 12 eyes showed a nonlinear improvement in mean threshold retinal sensitivity (P < 0.001). Overall change in mean central retinal thickness in 14 eyes was linear (R2 = 0.625; P = 0.034), with a significant reduction between one and six months (P = 0.012). Conclusion In this pilot study, the Metanx intervention appeared to have some beneficial effects with respect to reducing retinal edema and increasing light sensitivity in subjects with nonproliferative diabetic retinopathy.

Topographic assessment of irregular astigmatism after photorefractive keratectomy

Objective: To correlate new quantitative topographic indexes of corneal irregular astigmatism to best spectacle‐corrected visual acuity (BSCVA) following excimer laser photorefractive keratectomy (PRK). Setting: Department of Ophthalmology, LSU Eye Center, and Refractive Surgery Center of the South, Ear, Nose & Throat Hospital, New Orleans, Louisiana; Manhattan Eye, Ear and Throat Hospital, New York, New York, USA. Methods: Videokeretography data (TMS‐1) were obtained preoperatively and 1, 3, 6, 12, 18, and 24 months postoperatively from 100 eyes having PRK forlow to mild myopia. Algorithms measured fine local Irregularity with the surface regularity index (SRIp), varifocality with the coefficient of variation of corneal power (CVPp), and central islands with the elevation/depression magnitude (EDM). Results: The SRIp and CVPp increased after surgery and remained significantly higher than the preoperative levels throughout the 24 month follow‐up (P < .05). The increase in EDM was significant from 1 to 6 months (P < .05) but not thereafter. Multiple regression analysis revealed that variables having a statistically significant relationship with postoperative BSCVA were CVPp and EDM at 1 month, CVPp at 3 months, and CVPp, haze, and age at 6 months. No statistically significant correlation between any measures of irregular astigmatism and BSCVA was found after 1 year of follow‐up. Conclusion: The quantitative measures used in this study are sensitive methods by which irregular astigmatism after keratorefractive procedures can be classified, evaluated, and compared.

Method for expressing clinical and statistical significance of ocular and corneal wave front error aberrations.

Purpose: The significance of ocular or corneal aberrations may be subject to misinterpretation whenever eyes with different pupil sizes or the application of different Zernike expansion orders are compared. A method is shown that uses simple mathematical interpolation techniques based on normal data to rapidly determine the clinical significance of aberrations, without concern for pupil and expansion order. Methods: Corneal topography maps (TOMEY, Inc, Nagoya, Japan) from 30 normal corneas were collected, and the corneal wave front error was analyzed by Zernike polynomial decomposition into specific aberration types for pupil diameters of 3, 5, 7, and 10 mm and Zernike expansion orders of 6, 8, 10, and 12. Using this 4 × 4 matrix of pupil sizes and fitting orders, the best-fitting 3-dimensional functions were determined for the mean and standard deviation of the root-mean-square error for specific aberrations. The functions were encoded into a software application to determine the significance of data acquired from nonnormal cases. Results: The best-fitting functions for 6 types of aberrations were determined: defocus, astigmatism, prism, coma, spherical aberration, and all higher-order aberrations. A clinical screening method of color coding the significance of aberrations in normal, postoperative laser in situ keratomileusis, and keratoconus cases having different pupil sizes and different expansion orders is demonstrated. Conclusions: A method to calibrate wave front aberrometry devices using a standard sample of normal cases was devised. This method could be potentially useful in clinical studies involving patients with uncontrolled pupil sizes or in studies that compare data from aberrometers that use different Zernike fitting-order algorithms.