Analysis of aggregate surgically induced refractive change, prediction error, and intraocular astigmatism.

PURPOSE: To demonstrate analytical methods for evaluating the results of keratorefractive surgical procedures and emphasize the importance of intraocular astigmatism. SETTING: University of Texas Medical School, Houston, Texas, USA. METHODS: A standard data set, provided by an editor of this journal, comprising the preoperative and postoperative keratometric and refractive measurements of 100 eyes that had keratorefractive surgery was evaluated by 2 methods, vector and spheroequivalent (SEQ) analysis. The individual and aggregate surgically induced refractive changes (SIRCs) and prediction errors were determined from the refractive and keratometric measurements using both methods and then compared. The refraction vertex distance, keratometric index of refraction, and corneal asphericity were used to make the results calculated from refractive data directly comparable to those derived from keratometric data. Doubled-angle and equivalency plots as well as frequency and cumulative histograms were used to display the data. Standard descriptive statistics were used to determine the mean and standard deviation of the aggregate induced astigmatism after converting the polar values (cylinder and axis) to Cartesian (x and y) values. RESULTS: The preoperative SEQ refractive errors were undercorrected by at least 0.25 diopter (D) in most cases (78%). Six percent were corrected within +/- 0.24 D, and 16% were overcorrected by at least 0.25 D SEQ. The mean SEQ was -6.68 D +/- 2.49 (SD) before and -0.61 +/- 0.82 D after surgery, reflecting a SIRC SEQ of -6.07 +/- 2.40 D. The defocus equivalent (DEQ) was 7.41 +/- 2.53 D before and 0.96 +/- 0.74 D after surgery; for a nominal 3.0 mm pupil, this corresponded to an estimated improvement in uncorrected visual acuity (UCVA) from worse than 20/200 to better than 20/25, respectively. The predictability of the treatment decreased as the attempted refractive correction increased. The average magnitude of the refractive astigmatism was 1.46 +/- 0.61 D before and 0.40 +/- 0.38 D after surgery. The centroid of the refractive astigmatism was +0.96 x 87.9 +/- 0.85 D, rho = 0.43 before and +0.11 x 83.1 +/- 0.37, rho = 0.49 after surgery. The decrease in the square root of the centroid standard deviation shape factor (rho1/2) indicated an 8% increase in the amount of oblique astigmatism in the population. The prevalence of preoperative keratometric irregular astigmatism in excess of 0.5 D in this group of patients was 13%. The correlation between keratometric and refractive astigmatism was extremely poor before (r2 = 0.26) and especially after surgery (r2 = 0.02), demonstrating the presence of intraocular astigmatism and the limitations of manual keratometry. The centroid of intraocular astigmatism at the corneal plane was +0.48 x 178 +/- 0.49 D, rho = 0.59, and was compensatory. CONCLUSIONS: The 2 analytical methods are complimentary and permit thorough and quantitative evaluation of SIRCs and allow valid statistical comparisons within and between data sets. The DEQ allows comparison of refractive and visual results. The decrease in refractive predictability with higher corrections is well demonstrated by the SEQ and doubled-angle plots of the SIRC. Doubled-angle plots were particularly useful in interpreting errors of cylinder treatment amount and errors in alignment. The correlation between refractive and keratometric astigmatism was poor for preoperative, postoperative, and SIRC data, indicating the presence of astigmatic elements beyond the corneal surface (ie, intraocular astigmatism). Sources of error in refractive outcome statistics include the use of multiple lens systems in the phoropter, errors in vertex calculations, difficulty in accurately defining the axis of astigmatism, and failure to consider measurement errors when working with keratometric data. The analysis of this particular data set demonstrates the significant clinical benefits of refractive surgery: an 8-fold increase in UCVA, an 11-fold decrease in SEQ refractive error, as well as a 9-fold and nearly a 2 1/2-fold decrease in the magnitude and distribution of astigmatism, respectively.

International Intraocular Lens & Implant Registry.

This is the third annual update of the International Intraocular Lens & Implant Registry. A total of 1307 intraocular lenses (IOLs) or nonoptical implants from 34 manufacturers are available to surgeons in 2001.

Intraocular lens power calculations in patients with extreme myopia.

PURPOSE: To determine the variables that might contribute to improved intraocular lens (IOL) power calculations preoperatively in cataract patients with extreme myopia. METHODS: This retrospective study included 50 patients with extreme myopia and axial lengths longer than 27.0 mm. All patients had clear corneal phacoemulsification by the same surgeon and implantation of the Domilens SiFlex 1 IOL (power range -6.0 to +5.0 diopters [D]). The performances of the SRK/T, Hoffer Q, Holladay 1, and Holladay 2 formulas in predicting an IOL power that would meet the target refraction of +/-1.00 D were compared. RESULTS: The formulas tended to suggest underpowered IOLs, more severe in eyes with axial lengths greater than 30.00 mm. These eyes accounted for most of the minus-power IOLs implanted. Back calculations of axial lengths in patients with minus-power IOLs showed that, on average, emmetropia could have been predicted by choosing shorter axial lengths (up to 2.72 mm shorter) than those used in the original IOL power calculations. Preoperative B-scan ultrasonography demonstrated the presence of a posterior pole staphyloma temporal to the optic nerve in several patients who required minus-power IOLs, which suggests that axial length measurement problems were a major source of IOL calculation errors in these patients. CONCLUSIONS: In eyes with axial lengths longer than or equal to 27.0 mm, current third- and fourth-generation lens calculation formulas have a tendency to over minus patients between -1.0 and -4.0 D. The formulas appear to perform better for plus-power IOL implantation than for minus-power IOL implantation. The use of B-scan ultrasonography to locate posterior pole staphylomas may improve the accuracy of IOL calculations in eyes with extreme myopia.

Ultrasonographic measurement of induced myopia associated with capsular bag distention syndrome.

OBJECTIVE: To determine the causes of postoperative myopia associated with capsular bag distention syndrome (CBDS) and characterize the associated findings. DESIGN: Prospective, multicenter, nonrandomized, comparative (self-controlled) trial. PARTICIPANTS: Six eyes from six patients had CBDS develop after phacoemulsification and posterior chamber intraocular lens implantation. METHODS: Keratometric values, axial length, visual acuity, and manifest refraction were assessed in each eye. Using 20-MHz (I3SYSTEM-ABD, Innovative Imaging Inc, Sacramento, CA) and 50-MHz (Ultrasound Biomicroscope, Zeiss Humphrey Systems, Dublin, CA) ultrasonographic probes, images and measurements of the anterior segment were obtained. After neodymium:yttrium-aluminum-garnet (Nd:YAG) capsulotomy, these measures were repeated and correlated with predicted and actual refractive changes. MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), manifest refraction, and anterior chamber depth before and after treatment of CBDS. RESULTS: Although intended postoperative refraction averaged -0.58 diopters (D) (range, -0.12 to -1.63 D), eyes with CBDS had an average spherical equivalent refraction of -2.35 D (range, +0.13 to -4.50 D), P < 0.05 (one-tailed, paired t test). BCVA averaged 20/24 (range, 20/15-20/40-1), but UCVA averaged 20/133 (range, 20/60 to 20/400). Average distance from the corneal surface to the anterior intraocular lens (IOL) optic surface was 3.55 mm in eyes with CBDS, and 4.30 mm after Nd:YAG capsulotomy. Posterior movement of the IOL optic after capsulotomy accounted for 1.23 D of hyperopic shift or 82% of the CBDS-induced myopia. Treatment of CBDS resulted in both improved UCVA and BCVA. Nd:YAG capsulotomy also released the colloidal suspension within the capsular bag posterior to the IOL optic. The incidence of CBDS was 0.3% in one of the practices reviewed. CONCLUSIONS: Capsular bag distention syndrome includes unexpected myopia and poor UCVA after cataract surgery with lens implantation in cases involving a continuous capsulorhexis. A dramatic posterior distention of the posterior capsule is observed, as well as anterior chamber shallowing, tight apposition of the iris to the IOL, and anterior bowing of the iris. A slightly turbid colloidal suspension behind the IOL implant and late posterior capsular fibrosis are also observed. Timely treatment of CBDS can correct unwanted myopia, improve UCVA and BCVA, and restore normal anatomic relationships in the eye.

Comparison of changes in manifest refraction and corneal power after photorefractive keratectomy.

PURPOSE: To determine which corneal curvature values most closely correlate to change in manifest refraction after excimer laser photorefractive keratectomy. METHODS: In a prospective study at the Cullen Eye Institute, excimer laser photorefractive keratectomy was performed on 27 eyes of 27 patients (mean age, 38.07+/-6.65 years). Preoperative refractive errors ranged from -2.25 diopters to -8.75 diopters (mean, -5.74+/-2.09 diopters). Preoperatively and 1 month postoperatively, we determined the spherical equivalent of the subjective manifest refraction (corrected for a 12-mm vertex distance) and measured corneal power using standard keratometry (Bausch & Lomb Keratometer; Rochester, New York) and computerized videokeratography (EyeSys Corneal Analysis System; Premier Laser Systems Inc, Houston, Texas). We collected 15 corneal values: standard keratometry and 14 computerized videokeratography values calculated using the axial, instantaneous, and refractive formulas. All calculations were performed with 1.3375 and 1.376 for the refractive index of the cornea. For each of the corneal values, we subtracted the change in corneal power from the change in manifest refraction and calculated for this difference the means, SDs, correlations, and regressions. RESULTS: Mean differences between change in refraction and change in corneal power were lower when for a refractive index of 1.376 than for 1.3375, were lowest for the most central measurement points, and displayed a high SD. A value of 1.408 for the refractive index would be required to optimize the correlation between change in manifest refraction and effective refractive power of the central 3 mm of the cornea. CONCLUSIONS: For individual patients who have undergone photorefractive keratectomy, changes in corneal values determined by computerized videokeratography or by standard keratometry do not reliably predict change in manifest refraction.

International Intraocular Lens & Implant Registry 2000.

This article updates the International Intraocular Lens Registry published in January 1999. A total of 936 intraocular lenses (IOLs) and nonoptical implants are available to surgeons in 2000. Sixty-seven new implants were added to the registry, and 25 existing IOLs required a change in the lens constant. The cooperation of the companies has been excellent, and comments from implant surgeons have been very gratifying.

Analysis of edge glare phenomena in intraocular lens edge designs.

PURPOSE: To determine the image and relative intensity of reflected glare images from 4 commonly used intraocular lens (IOL) edge designs to assess the potential for noticeable postoperative edge glare. SETTING: University of Texas Medical School, Houston, Texas, USA. METHODS: The interaction of light rays from 4 common IOL edge designs were examined in an eye model using the OptiCAD 3-D radiometric ray-tracing program (Opticad Corp.). Comparison of the potential of the 4 edge designs to produce visual sensations was derived from plots of the spatial location and energy distribution of rays forming the retinal image. RESULTS: Edge designs with no anterior and posterior dioptric powers at the lens periphery (lenticular) and rounded corners distributed the edge glare rays over a large retinal area. Edge designs with sharp edges formed by "cropping" the anterior and posterior optic zones focused edge glare rays into distinct arc-shaped images. The peak intensity of the arc-shaped image was 8 to 10 times stronger than the peak intensity of the diffuse image formed by lenses with rounded edges. CONCLUSIONS: Rounded IOL edges distribute reflected glare image over a significantly greater area than sharp edges. Rounded edges reduce the potential for edge glare phenomena that appear to the patient as a thin crescent or partial ring.

Evaluation of relationships among refractive and topographic parameters.

PURPOSE: To examine the relationships among several refractive and topographic parameters. SETTING: Cullen Eye Institute Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA. METHODS: Using computerized videokeratography (EyeSys Corneal Analysis System), 287 corneas of 150 patients were retrospectively analyzed. The Holladay Diagnostic Summary (HDS) refractive maps were used to evaluate relationships among variables of the HDS and refractive error. RESULTS: Myopic spherical equivalent refraction (P = .0003) and more negative asphericity (Q-values) (P = .0119) were correlated with steeper corneas. The Q-values were less negative in eyes with moderate myopia (2.0 to 6.0 diopters [D]) than in those with hyperopia (1.0 D or greater). The Q-values below -0.3 were correlated with less favorable values for predicted corneal acuity and corneal uniformity index values. Mean corneal curvature measurements obtained by computerized videokeratography and standard keratometry showed a strong degree of correlation (P = .0001). CONCLUSION: As the degree of myopia and negative asphericity increased, the corneal radius of curvature decreased. Corneal Q-values less than -0.3 were associated with reduced optical performance of the cornea.

Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography.

PURPOSE: To demonstrate the functional vision and corneal changes following laser in situ keratomileusis (LASIK) determined by contrast sensitivity, glare testing, and corneal topography. SETTING: University of Texas Medical School, Houston, Texas, USA. METHODS: Seven patients ranging in age from 20 to 61 years who had bilateral LASIK were evaluated preoperatively and 1 day, 1 week, and 1 and 6 months postoperatively. Visual acuity, using letters on the Baylor Visual Acuity Testor (BVAT) at 98% (standard acuity) and 13% contrast, and the contrast threshold were determined at 3 light levels (darkness, medium brightness acuity testor [BAT], high BAT). Pupil sizes were measured at each level, and corneal topography was performed at each visit. RESULTS: The greatest changes were found 1 day postoperatively: The contrast threshold worsened by a mean of 0.6 lines +/- 1.0 (SD) (P = .05) in darkness, 0.4 +/- 0.7 lines (P = .05) at medium BAT, and 0.8 +/- 0.7 lines (P = .002) at high BAT. The 98% contrast acuity decreased a mean of 1.4 +/- 1.6 lines (P = .01) in darkness, 1.0 +/- 2.0 lines (P = .09) at medium BAT, and 0.8 +/- 2.3 lines (P = .22) at high BAT. The 13% contrast acuity decreased a mean of 2.2 +/- 2.6 lines (P = .01) in darkness, 1.3 +/- 1.9 lines (P = .02) at medium BAT, and 1.4 +/- 2.5 lines (P = .07) at high BAT. The predicted corneal acuity (PCA) obtained from corneal topography decreased by a mean of 3.3 +/- 3.1 lines (P = .002), and the asphericity (Q-value) increased by an average of +0.35 +/- 0.67 (P = .07). All values returned to the preoperative levels by 1 week except PCA, asphericity, visual acuity at 13%, and contrast threshold in darkness, which improved slightly but had not returned to baseline by 6 months. The 98% contrast acuity at medium BAT improved by 0.2 +/- 1.0 lines (P = .34) and 0.3 +/- 0.8 lines (P = .16) at high BAT at 1 month. The 98% contrast acuity values remained 0.3 lines over baseline through 6 months. Corneal topography showed that all corneas became oblate after LASIK to a mean Q-value of +0.47 +/- 0.40 (P = .0001) and PCA was decreased by 1.6 +/- 1.1 lines (P = .0002) at 6 months. CONCLUSIONS: Functional vision changes do occur after LASIK. The optical quality of the cornea is reduced and the asphericity becomes oblate. Changes in functional vision worsen as the target contrast diminishes and the pupil size increases. These findings indicate that the oblate shape of the cornea following LASIK is the predominant factor in the functional vision decrease.

Photorefractive keratectomy with an ablatable mask and rail system for simultaneous correction of myopia and astigmatism: preliminary results.

BACKGROUND: Alternative methods to correct myopic astigmatism with excimer laser surgery are being examined in an attempt to increase efficacy and reliability. One approach that is developing interest is the incorporation of a polymethylmethacrylate mask into the laser procedure. We examined the efficacy and safety of the Summit Apex Plus laser using a new ablatable mask and rail system. METHODS: Retrospective study based on follow-up of patients at 1 month (59 eyes), 3 months (37 eyes) and 6 months (24 eyes) at an eye laser centre in southern Ontario. Patients were recruited from 19 ophthalmic practices located primarily in the Niagara region and in western New York. Selected patients with naturally occurring myopia and astigmatism were treated with the Summit Apex Plus laser with the mask and rail system. Patients were evaluated for changes in refraction and visual acuity, and complications (including glare, haloes, haze and lines lost of best corrected visual acuity) were assessed. We calculated the surgically induced refractive change using the Holladay 10-step method. RESULTS: At 6 months the average residual sphere for the low myopia group (-7.00 dioptres or less) was -0.60 D and for the high myopia group (greater than -7.00 D) -0.61 D. This result was found to be both stable and accurate over the follow-up period. The cylinder component was reduced irrespective of the amount of preoperative myopia. Vector analysis showed that in most patients the correction of astigmatism was accurate, although slight undercorrection was observed. Complications were minimal, with few residual adverse effects at 6 months. INTERPRETATION: We found the mask and rail technique to be both effective and safe. Refractive change was stable and accurate, although astigmatism was slightly undercorrected. The procedure resulted in few complications, most of which resolved by 6 months.

International intraocular lens registry.

To ensure the highest quality of care for patients, surgeons must have the most up-to-date information on the intraocular lenses (IOLs) they implant. This registry presents the lens constants (A constant, anterior chamber depth, and surgeon factor) for all commercially available IOLs--869 lenses from 22 manufacturers. The constants will be updated every January.

Relationship of the actual thick intraocular lens optic to the thin lens equivalent.

PURPOSE: To theoretically derive and empirically validate the relationship between the actual thick intraocular lens and the thin lens equivalent. METHODS: Included in the study were 12 consecutive adult patients ranging in age from 54 to 84 years (mean +/- SD, 73.5 +/- 9.4 years) with best-corrected visual acuity better than 20/40 in each eye. Each patient had bilateral intraocular lens implants of the same style, placed in the same location (bag or sulcus) by the same surgeon. Preoperatively, axial length, keratometry, refraction, and vertex distance were measured. Postoperatively, keratometry, refraction, vertex distance, and the distance from the vertex of the cornea to the anterior vertex of the intraocular lens (AV(PC1)) were measured. Alternatively, the distance (AV(PC1)) was then back-calculated from the vergence formula used for intraocular lens power calculations. RESULTS: The average (+/-SD) of the absolute difference in the two methods was 0.23 +/- 0.18 mm, which would translate to approximately 0.46 diopters. There was no statistical difference between the measured and calculated values; the Pearson product-moment correlation coefficient from linear regression was 0.85 (r2 = .72, F = 56). The average intereye difference was -0.030 mm (SD, 0.141 mm; SEM, 0.043 mm) using the measurement method and +0.124 mm (SD, 0.412 mm; SEM, 0.124 mm) using the calculation method. CONCLUSION: The relationship between the actual thick intraocular lens and the thin lens equivalent has been determined theoretically and demonstrated empirically. This validation provides the manufacturer and surgeon additional confidence and utility for lens constants used in intraocular lens power calculations.

Evaluating and reporting astigmatism for individual and aggregate data.

PURPOSE: To demonstrate the proper method for evaluating and reporting astigmatism for individual and aggregate data. SETTING: University of Texas Medical School and Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. METHODS: The surgically induced refractive change (SIRC) was determined for three data sets of patients who have had keratorefractive (photorefractive keratectomy) or cataract surgery. To make changes in refraction comparable, vertex distances for the refractions and keratometric index of refraction were considered. Doubledangle plots and single-angle plots were then used to display the data. Polar values (cylinder and axis) were converted to a Cartesian (x and y) coordinate system to determine the mean value of the induced astigmatism for each data set. RESULTS: Doubled-angle plots clearly demonstrated the trends of induced astigmatism for each data set, and the mean value for induced astigmatism agreed exactly with the intuitive appearance of the plot. CONCLUSIONS: Converting astigmatism data to a Cartesian coordinate system allowed the correct computation of descriptive statistics such as mean values, standard deviations, and correlation coefficients. Using doubled-angle plots to display the data provides the investigator with the best method of recognizing trends in the data.

Near vision contrast sensitivity after photorefractive keratectomy.

BACKGROUND: To evaluate near vision contrast sensitivity as a measure of visual performance after photorefractive keratectomy (PRK). SETTING: LSU Eye Center, New Orleans, Louisiana. METHODS: Using Holladay Contrast Acuity Test cards, near (reading) vision for five levels of contrast sensitivity was evaluated in a cross section of 53 eyes of 31 patients 25 to 732 days after PRK. Twenty-four normal eyes of 22 myopic patients served as controls. RESULTS: Near contrast sensitivity decreased at all tested contrast levels for approximately 7 months after PRK and then returned to baseline. This phenomenon paralleled the fluctuation in best corrected distance Snellen acuity. CONCLUSIONS: These preliminary results indicate that Snellen visual acuity and near contrast sensitivity returned to baseline within 1 year after PRK.

Corneal topography using the Holladay Diagnostic Summary.

PURPOSE: To describe new software developments in videokeratography and illustrate their application in patients with corneal disease or patients who have had keratorefractive surgery. METHODS: The Holladay Diagnostic Summary provides 4 maps and 15 corneal parameters for the clinician. The maps include 2 refractive power maps on standard and auto scales, a profile difference map for determining the corneal shape relative to normal asphericity, and a distortion map to display the optical quality of the cornea. The 15 corneal parameters provide quantitative information about the cornea for a 3.0 mm pupil; e.g., effective refractive power, regular astigmatism, asphericity, and predicted corneal acuity. The maps and parameters are used to analyze a calibration ball and five clinical examples. RESULTS: Patients with diseases such as keratoconus or patients who have had keratorefractive surgery have characteristic changes in the 4 maps and the 15 corneal parameters. CONCLUSION: When true refractive power maps, a profile difference map, a distortion map, and 15 corneal parameters are analyzed, characteristic changes are found that can help clinicians recognize and monitor corneal disease and refractive surgery and correlate these changes with patients' visual symptoms.

Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations.

PURPOSE: To provide a method and values that facilitate standardization of constants for ultrasonic biometry, keratometry, and intraocular lens (IOL) power calculations. SETTING: University of Texas Medical School, Houston, Texas, USA. METHODS: Keratometry and ultrasonic biometry provide the two measured input variables for the six variable vergence equations used to calculate the appropriate IOL power for a specific patient with a cataract. A review of the literature reflecting the past 156 years of research and development reveals the appropriate index of refraction to be used with the keratometer for net optical corneal power, the location of the principal planes of the cornea, the nominal value for retinal thickness, and the appropriate velocities for ultrasonic measurement of the axial length. The relationship of the thick IOL to the thin IOL is derived along with the physical location of the thick lens. Two methods are described that provide the best IOL constant to be used by a manufacturer to minimize the prediction error for a surgeon using the lens for the first time. The formulas for phakic IOLs and secondary piggyback IOLs are also derived and applied to methods described above for standard IOLs. RESULTS: Using a standardized net index of refraction of 4/3 for the cornea eliminates a variability of 0.56 diopter (D) in the predicted refraction. Using a standardized 1532 m/s velocity for axial length measurements and adding a value of 0.28 mm reduces the tolerance of axial length measurements to +/-0.03 mm for any length eye. The physical location of the thick IOL's secondary principal plane must be anterior to the thin lens equivalent by approximately the separation of the principal planes of the thick lens. For biconvex poly(methyl methacrylate) IOLs, the separation in the principal planes is approximately 0.10 mm. Using these relationships, the physical position of the thick lens within the eye can be used to confirm the lens constant for any IOL style. CONCLUSIONS: Standardizing the constants for keratometry, ultrasonic biometry, and IOL power calculations can significantly improve the predictability of refractive outcomes. Back-calculating and physically measuring the position of the lens within the eye can provide surgeons with an initial lens constant known to have a standard error of the mean of +/-0.05 mm (+/-0.10 D). Other parameters such as the cardinal points of a lens, the shape factor, the lens-haptic plane, and the center lens thickness would allow further refinement of IOL power calculations.

Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses.

PURPOSE: To examine the refractive results and limitations of current intraocular lens power formulas when implanting two posterior chamber lenses in-the-bag to achieve emmetropia in extremely short eyes. METHODS: Preoperative measurements (corneal diameter, axial length, keratometry, anterior chamber depth, and lens thickness) and postoperative measurements (refraction, corneal vertex to iris depth, and iris to front anterior lens surface) were taken in six eyes from three patients, with axial lengths ranging from 15.09 to 19.95 mm. These data were used to calculate the prediction error for three current third-generation formulas (Holladay, Hoffer Q, SRK/T) and two older formulas (SRK2 and SRK1). RESULTS: None of the formulas accurately predicted the refractions using the optimized lens constants for normal eyes. The third-generation formulas were not different (P > or = 0.602) and averaged 5 diopters (D) of absolute error (Hoffer Q = 4.64 +/- 1.57 D; Holladay = 5.07 +/- 1.28 D; SRK/T = 5.12 +/- 1.43 D). The older formulas were significantly worse (P = 0.0006), with average mean absolute errors of 10.93 +/- 5.09 D for the SRK2 and 13.33 +/- 5.09 D for the SRK1. When the formulas were optimized for these six eyes, the mean absolute errors were Holladay = 1.33 +/- 1.25 D; SRK/T = 2.10 +/- 1.31 D; Hoffer Q = 4.54 +/- 2.00 D; SRK2 = 4.71 +/- 1.94 D; and SRK1 = 4.71 +/- 1.94 D. The Holladay and SRK/T formulas were statistically better (P = 0.0068) than the Hoffer Q and the two older formulas. CONCLUSION: Current third-generation formulas are better than older formulas for extremely short eyes, but still are not acceptable for the desired clinical accuracy. Newer formulas that will use additional anterior segment measurements (corneal diameter, anterior chamber depth, and lens thickness) will be required for improved accuracy, because the anterior segment often is not proportional to the axial length.

Corneal asphericity following excimer laser photorefractive keratectomy. Summit PRK Topography Study Group.

BACKGROUND AND OBJECTIVE: To analyze corneal asphericity following excimer laser photorefractive keratectomy (PRK) and its influence on clinical outcomes. PATIENTS AND METHODS: A computer program (Holladay Diagnostic Summary, EyeSys Laboratories, Houston, TX) was used to qualitatively and quantitatively analyze the corneal asphericity of 132 patients 1 year following PRK for correction of myopia. Color maps depicting actual corneal asphericity as compared to the normal expected asphericity were reviewed, and quantitative values of asphericity were evaluated for associations with clinical outcomes of uncorrected visual acuity and spectacle corrected visual acuity, achieved refractive correction, a subjective glare/halo index, and subjective patient satisfaction, as well as standard corneal topography patterns and optical zone decentration following PRK. RESULTS: Following PRK, all corneas exhibited a positive central asphericity, changing from a prolate (negative asphericity) to an oblate optical contour. There was a trend toward higher positive asphericity measurements with improving spectacle corrected visual acuity which was not statistically significant; such a relationship was not found with uncorrected visual acuity. A significant association was found between greater achieved refractive correction and increased postoperative positive asphericity. No association was found between postoperative asphericity and the glare/halo index, subjective patient satisfaction, topography pattern, or optical zone decentration. CONCLUSION: Asphericity may be a useful quantitative descriptor of corneal optical contour following PRK. Greater positive central corneal asphericity is found with greater degrees of refractive correction. Further understanding of both the pre- and post-operative corneal contour and the consequent optical effects should aid in a better understanding of the optical outcomes of PRK.

Corneal optical irregularity after excimer laser photorefractive keratectomy. The Summit Photorefractive Keratectomy Topography Study Group.

PURPOSE: To assess the influence of corneal surface microirregularities on objective and subjective visual performance after photorefractive keratectomy (PRK). SETTING: Multicenter clinical trial. METHODS: The alpha version of the Potential Corneal Acuity (PCA) computer program, currently under development, was used to qualitatively and quantitatively analyze the corneal surface of 176 eyes of 176 patients 1 year after PRK. Color maps of corneal surface irregularities were reviewed and quantitative values (PCA) predicting best spectacle-corrected visual acuity (BSCVA) as limited by the cornea were evaluated for associations with qualitative topography patterns, optical zone decentration, and clinical outcomes of BSCVA, uncorrected visual acuity (UCVA), subjective patient satisfaction, and a subjective glare/halo index. RESULTS: Qualitatively, corneas after PRK were generally characterized by a ring of optical irregularity at the juncture of the ablation zone and untreated cornea. Standard corneal topography maps graded as irregular after PRK had a significantly higher PCA value than those graded as regular. There was a trend toward higher PCA values with greater optical zone decentration that was not statistically significant. Actual BSCVA was identical to that which the PCA value predicted in 32% of patients and was within one Snellen line in 71%, within two lines in 89%, and within three lines in 94%. The correlation between the PCA and the glare/halo index and with subjective patient satisfaction was statistically significant. The relationship between PCA and UCVA was not significant. CONCLUSIONS: A ring of optical microirregularity of the corneal surface can appear at the juncture of the treated and untreated cornea after PRK, indicating that the optical zone edge might affect objective and subjective postoperative visual outcomes. Further understanding of corneal surface topography and refinement of the PCA program should help explain visual outcome after PRK.