The prediction of surgically induced refractive change from corneal topography

Long-term follow-up of astigmatic keratotomy for corneal astigmatism after penetrating keratoplasty

PURPOSE

To report the long-term stability of paired arcuate corneal keratotomies (AKs) in patients with high regular postpenetrating keratoplasty astigmatism.

METHODS

Retrospective chart review of best-corrected visual acuity, refraction and keratometric values of 41 eyes with AK between 2003 and 2012.

RESULTS

Magnitude of median target induced astigmatism vector was 9.2 dioptres (Dpt). We reached a median magnitude of surgically induced astigmatism vector of 9.81 Dpt and a median magnitude of difference vector of 5.5 Dpt. In keratometry, we achieved a net median astigmatism reduction by 3.3 Dpt. The average correction index was 1.14, showing a slight overcorrection. Irregularity of keratometric astigmatism increased by 0.6 Dpt, and spherical equivalent changed by 1.75 Dpt. Monocular best spectacle corrected visual acuity increased from preoperatively 20/63 (0.5 logMAR) to 20/40 (0.3 logMAR) postoperatively. Median gain on the ETDRS chart was two lines. Long-term follow-up showed a median keratometric astigmatic increase by 0.3 Dpt per year.

CONCLUSION

Arcuate corneal keratotomies is a safe and effective method to reduce high regular corneal astigmatism following penetrating keratoplasty but has limited predictability. The long-term follow-up shows an increase of keratometric astigmatism by 0.3 Dpt/year, equalizing the surgical effect after 10 years.

Corneal morphological changes after myopic excimer laser refractive surgery

PURPOSE

To evaluate the changes in central corneal thickness (CCT) and corneal volume (CV) in eyes that have undergone myopic photorefractive keratectomy (PRK).

METHODS

CCT and CV obtained with an Oculus Pentacam before 1, 3, and 6 months after PRK were analyzed in 84 eyes with a mean preoperative refraction of -4.93 ± 2.23 diopter. The changes were compared with the amount of refractive treatment. The differences were evaluated with the Student t test and the correlations with the Pearson index.

RESULTS

One month after PRK, CCT and CV mean differences were 73.2 ± 31.5 μm (P < 0.001) and 2.2 ± 1.7 mm (P < 0.001), respectively. Three months after PRK, CCT and CV mean differences were 66.6 ± 26.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. Six months after PRK, CCT and CV mean differences were 65.3 ± 25.7 μm (P < 0.001) and 1.4 ± 1.3 mm (P < 0.001), respectively. The effective treatment at each follow-up point was correlated with CCT changes (R = 0.62, 0.71, and 0.73, respectively), but not with CV changes (R = 0.04, 0.04, and 0.01, respectively).

CONCLUSIONS

Our findings support the hypothesis that after myopic PRK, when a series of corneal lamellae are severed centrally, the remaining peripheral segments relax. The squeezing force on the matrix is reduced, and the distance between the lamellae expands.

Comparison of simulated keratometric changes induced by custom and conventional laser in situ keratomileusis after myopic ablation: retrospective chart review

PURPOSE

To determine the relationship between the achieved refractive change and the change in simulated keratometry (K) after myopic laser situ keratomileusis (LASIK) and compare this relationship between custom and conventional treatments.

SETTING

Department of Ophthalmology, University of California, Davis, Sacramento, California, and John A. Moran Eye Center, Salt Lake City, Utah, USA.

METHODS

The change in simulated K and the refractive change induced by custom myopic LASIK and conventional LASIK were determined. The relationship between the variables was analyzed by regression methods.

RESULTS

Custom treatment was performed in 106 eyes and conventional treatment in 224 eyes. Simple linear regression analysis did not fit the clinical observation when the refractive change was less than 2.00 diopters (D) of myopic correction with both treatments. Under the linear model and nonlinear model, each unit of refractive change yielded a greater change in corneal topographic power with custom treatment than with conventional treatment. With both treatments, the rate of change in simulated K was not constant and was much more variable with lower amounts of correction. The relationship was more constant and linear with larger amounts of refractive correction.

CONCLUSIONS

The relationship between the measured change in simulated K and the induced refractive change better fit a nonlinear relationship with smaller amounts of refractive correction in custom LASIK and conventional LASIK. Under all forms of analysis, custom treatments yielded a greater per-unit change in corneal curvature than conventional treatments, especially for refractive corrections of 4.00 D and higher.

Origins of the keratometer and its evolving role in ophthalmology

The keratometer, or ophthalmometer as it was originally known, had its origins in the attempt to discover the seat of accommodation in the eye. Since that early beginning, it has been re-invented a number of times, with improvements and modifications made in the original principles of its design for new applications that arose as ophthalmology advanced. The cornea is not only responsible for the majority of the refraction in the eye, but is also readily accessible for measurement and modification. The keratometer's ability to measure the cornea has allowed it to play a central role in critical advances in ophthalmic history. This review describes the origins and principles of this instrument, the novel applications that led to the keratometer's continued resurgences over its nearly 250-year history, and the modern devices that have borrowed its basic principles and are beginning to replace it in common clinical practice.

Cataract surgery after refractive surgery

PURPOSE OF REVIEW

To review recent contributions addressing the challenge of intraocular lens (IOL) calculation in patients undergoing cataract extraction following corneal refractive surgery.

RECENT FINDINGS

Although several articles have provided excellent summaries of IOL selection in patients wherein prerefractive surgery data are available, numerous authors have recently described approaches to attempt more accurate IOL power calculations for patients who present with no reliable clinical information regarding their refractive history. Additionally, results have been reported using the Scheimpflug camera system to measure corneal power in an attempt to resolve the most important potential source of error for IOL determination in these patients.

SUMMARY

IOL selection in patients undergoing cataract surgery after corneal refractive surgery continues to be a challenging and complex issue despite numerous strategies and formulas described in the literature. Current focus seems to be directed toward approaches that do not require preoperative refractive surgery information. Due to the relative dearth of comparative clinical outcomes data, the optimal solution to this ongoing clinical problem has yet to be determined. Until such data are available, many cataract surgeons compare the results of multiple formulas to assist them in IOL selection for these patients.

Intraocular lens power calculation after previous laser refractive surgery

Methods to attempt more accurate prediction of intraocular lens power in refractive surgery eyes are many, and none has proved to be the most accurate. Until one is identified, a spreadsheet tool is available and can be used. It automatically calculates all the methods for which data are available on a single sheet for the patient's chart. The various methods and how they work are described.

Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia

PURPOSE

To assess the accuracy of Orbscan II slit-scanning videokeratography for intraocular lens (IOL) calculation in eyes with previous photorefractive surgery for myopia.

SETTING

Private practice, St. Louis, Missouri, USA.

METHODS

Corneal power (K) was measured by manual keratometry, Placido-based videokeratography (Atlas), slit-scanning videokeratography (Orbscan II), and contact lens overrefraction in 21 post-photoablation eyes having cataract surgery. Postoperative data collected after phacoemulsification were used to back-calculate corneal power (BCK). The BCK values were statistically compared at 3.0 to 6.0 mm central Orbscan II curvature and power measurements, including total axial power, total tangential power, total mean power, and total optical power. Similar comparisons were made to Atlas curvature at the 0.0 to 10.0 mm zones.

RESULTS

The mean corneal power after refractive surgery based on BCK values using the Holladay 2 formula (BCK H2) was 39.35 diopters (D) +/- 2.58 (SD). The mean manual value (40.52 +/- 1.95 D) and Atlas-based values were statistically higher than BCK H2 values (P<.001). The mean corneal power calculated from historical data was 39.33 +/- 2.70 D (P = .83 to BCK H2; n = 19) and from contact lens overrefraction, 41.38 +/- 3.11 D (P = .19; n = 5). Orbscan II parameters (n = 21) of the total mean power (3.0 mm, 39.10 +/- 2.63 D), total tangential power (3.0 mm, 39.11 +/- 2.60), total axial power (5.0 mm, 39.19 +/- 2.55 D), and total optical power (3.0 mm, 39.08 +/- 2.78 D; 4.0 mm, 39.39 +/- 2.76 D) were statistically similar to both the historical and BCK H2 values (P>.11). If used prospectively, 80.9% of eyes would have been within +/-0.50 D of the targeted refraction using a 4.0 mm total optical power, 76.2% using a 5.0 mm total axial power, and 42.1% using the historical method.

CONCLUSION

The Orbscan II 5.0 mm total axial power and 4.0 mm total optical power can be used to more accurately predict true corneal power than the history-based method and may be particularly useful when pre-LASIK data are unavailable.

Anterior Chamber Depth Measurement before and after Photorefractive Keratectomy

PURPOSE

To measure the anterior chamber depth (ACD) with 2 different devices before and after photorefractive keratectomy (PRK).

DESIGN

Noncomparative case series.

PARTICIPANTS

One hundred forty-three eyes of 143 patients who had undergone PRK with refractive errors ranging from -13.13 diopters (D) to +7 D (mean, -3.67+/-3.58) were analyzed.

METHODS

The ACD values preoperatively and at 1, 3, and 6 months postoperatively were measured with the Orbscan II and IOL Master. The results were analyzed using the Pearson correlation.

MAIN OUTCOME MEASURE

Anterior chamber depth.

RESULTS

The instruments showed good agreement between the measurements before and after surgery. A significant decrease between the preoperative and 1-month postoperative measurements was found in the ACD measured from the epithelium with Orbscan II (P<0.01) and IOL Master (P<0.01). A nonsignificant decrease with both IOL Master (P>0.01) and Orbscan II (P>0.01) was found between 3 and 6 months after surgery. The ACD measured from the endothelium using the Orbscan II showed a significant difference only between the 3- and 6-month follow-up data (P<0.01).

CONCLUSIONS

The 2 devices showed good agreement, and the changes detected postoperatively seem to be related not only to corneal thinning but also to anterior segment remodeling.

Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery

PURPOSE

To test the reliability of a corneal radius correcting factor (R factor) in calculating intraocular lens (IOL) power in eyes that developed cataract after refractive surgery and compare it with the clinical history (CHM) and double-K (DKM) methods.

SETTING

Department of Ophthalmology, Second University of Naples, Naples, Italy.

METHODS

Nineteen eyes from the literature that underwent cataract extraction and IOL implantation after refractive surgery were used to compare actual postoperative and expected refractive errors utilizing the R factor, CHM, and DKM. Intraocular lens powers were calculated with 3 formulas: SRK/T, Hoffer Q and Holladay 1. The differences were evaluated with the Wilcoxon test and Spearman correlation.

RESULTS

With the R factor SRK/T and Holladay 1 formulas gave the best results; 16 (84.2%) and 17 (89.5%) eyes were within +/-2 diopters (D) of emmetropia. With CHM, the best results were obtained using the SRK/T and Holladay 1 formulas; with both formulas 12 (63.2%) eyes were within +/-2 D of emmetropia. With DKM, the best results were obtained using SRK/T and Holladay 1 formulas; with both formulas 10 eyes (52.63%) were in the range of +/-2 D from emmetropia.

CONCLUSIONS

The R factor can be used with the SRK/T or Holladay 1 formula because this method seems comparable or superior to DKM and CHM.

Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK

OBJECTIVE

To compare methods of calculating intraocular lens (IOL) power for cataract surgery in eyes that have undergone myopic LASIK.

DESIGN

Noncomparative case series.

PARTICIPANTS

Eleven eyes of 8 patients who had previously undergone myopic LASIK (amount of LASIK correction [+/-standard deviation], -5.50+/-2.61 diopters [D]; range, -8.78 to -2.38 D) and subsequently phacoemulsification with implantation of the SA60AT IOLs (Alcon Surgical, Inc., Fort Worth, TX) were included (refractive error after cataract surgery, -0.61 +/- 0.79 D; range, -2.0 to 1.0 D).

METHODS

We evaluated the accuracy of various combinations of: (1) single-K versus double-K (in which pre-LASIK keratometry is used to estimate effective lens position) versions of the IOL formulas; the Feiz-Mannis method was also evaluated; (2) 4 methods for calculating corneal refractive power (clinical history, contact lens overrefraction, adjusted effective refractive power [EffRP(adj)], and Maloney methods); and (3) 4 IOL formulas (SRK/T, Hoffer Q, Holladay 1, and Holladay 2). The IOL prediction error was obtained by subtracting the IOL power calculated using various methods from the power of the implanted IOL, and the F test for variances was performed to assess the consistency of the prediction performance by different methods.

MAIN OUTCOME MEASURES

Mean arithmetic IOL prediction error, mean absolute IOL prediction error, and variance of the IOL prediction error.

RESULTS

Compared with double-K formulas, single-K formulas predicted lower IOL powers than the power implanted and would have left patients hyperopic in most cases; the Feiz-Mannis method had the largest variance. For the Hoffer Q and Holladay 1 formulas, the variances for EffRP(adj) were significantly smaller than those for the clinical history method (0.43 D2 vs. 1.74 D2, P = 0.018 for Hoffer Q; 0.75 D2 vs. 2.35 D2, P = 0.043 for Holladay 1). The Maloney method consistently underestimated the IOL power but had significantly smaller variances (0.19-0.55 D2) than those for the clinical history method (1.09-2.35 D2; P<0.015). There were no significant differences among the variances for the 4 formulas when using each corneal power calculation method.

CONCLUSIONS

The most accurate method was the combination of a double-K formula and corneal values derived from EffRP(adj). The variances in IOL prediction error were smaller with the Maloney and EffRP(adj) methods, and we propose a modified Maloney method and second method using Humphrey data for further evaluation.

Reliability of the IOLMaster in measuring corneal power changes after photorefractive keratectomy

PURPOSE

To test the accuracy of the IOLMaster (Carl Zeiss) in detecting corneal power changes after photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, 2nd University of Naples, Naples, Italy.

METHODS

Two hundred twenty-five consecutive eyes that had PRK (mean -5.13 diopters [D] +/- 2.98 [SD] [range +0.25 to -16.25 D]) were analyzed. The data included preoperative and postoperative (1, 3, and 6 months) subjective refraction and computerized keratometry. Statistical analysis was performed to determine the correlation between the changes in the subjective refraction at the corneal plane and the changes in keratometry.

RESULTS

The mean difference between the changes in refraction and the measured corneal changes was 0.75 +/- 1.13 D (range -3.84 to +7.68 D) at 1 month, 0.92 +/- 1.10 D (range -0.87 to +7.93 D) at 3 months, and 0.75 +/- 0.98 D (range -1.70 to +3.85 D) at 6 months. The difference was significant (P<.001).

CONCLUSION

Automated keratometry provided by the IOLMaster did not accurately reflect the effective refractive changes after PRK, particularly in eyes that had a high dioptric treatment.

Correlation of Changes in Refraction and Corneal Topography After Photorefractive Keratectomy

PURPOSE

To establish which corneal power evaluation measured with corneal topography correlates best with refractive changes after photorefractive keratectomy (PRK) for myopia.

METHODS

Two hundred fifty-one consecutive eyes of 171 patients who had PRK for myopia ranging from -14.80 to -0.50 D (mean -5.43 +/- 2.978 D), calculated at the corneal plane, were included in the analysis. Data included preoperative and postoperative (1, 3, and 6-mo) subjective refraction and videokeratography with a Keratron Scout (Optikon 2000). Statistical analysis was performed to determine the correlation between the change in subjective refraction at the corneal plane and changes in six corneal power measurements: best fit sphere, simulated keratometry (Sim K), corneal apex, and center of the pupil (last two evaluated for axial and meridional curvatures).

RESULTS

The closest correlation between subjective refraction change and corneal power measurement during the three follow-up evaluations was found with Sim K (R2 = 0.904; 0.889; 0.854) and best fit sphere (R2 = 0.919; 0.909; 0.872), whereas the other measurements showed poor correlation with the different curvatures.

CONCLUSIONS

The best fit sphere corneal topography parameter correlated best with the refractive changes, primarily for low treatment amounts, whereas it showed a clear-cut underestimation in eyes that had undergone high dioptric treatments.

Predicted corneal visual acuity in keratoconus as determined by ray tracing

PURPOSE

To evaluate the optical quality of the central anterior corneal surface in normal eyes and in eyes with keratoconus, and to investigate the accuracy of the predicted corneal visual acuity (PCVA) index as determined by ray tracing analysis.

METHODS

Twenty keratoconus eyes with contact lens-corrected visual acuity (VA) of 20/20 or better (11 patients, group A) and 20 eyes of 15 normal subjects (group B) were evaluated. After a detailed eye examination including measurement of pupil diameter, keratometry, topography and pachymetry, each subject eye was evaluated using ray tracing analysis with the Technomed C-scan colour ellipsoid topometer, using basic software (Technomed GmbH, Baesweiler, Germany). The PCVA was determined for each patient, and the results were analysed comparatively using two-sample t-test, regression analysis and Pearson correlation analysis.

RESULTS

The average best spectacle-corrected VA was measured as 0.2 +/- 0.2 logMAR (20/32) in group A and -0.1 +/- 0.1 logMAR (20/16) in group B. The average PCVA measurements derived from ray tracing analysis for 3.0 mm, 3.5 mm and 4.0 mm pupil diameters were 0.06 +/- 0.12 logMAR, 0.14 +/- 0.13 logMAR and 0.21 +/- 0.17 logMAR, respectively, in group A, and -0.14 +/- 0.08 logMAR,-0.11 +/- 0.09 logMAR and -0.09 +/- 0.11 logMAR, respectively, in group B. There was good correlation between best corrected VA and PCVA in both groups for all pupil diameters measured (p < 0.007).

CONCLUSION

Predicted corneal visual acuity as determined by ray tracing analysis is useful for estimating best spectacle-corrected VA in normal corneas and the effect of irregular corneal astigmatism on VA in eyes with mild to moderate keratoconus. Further studies are required to evaluate the efficacy of ray tracing in evaluation of aberrations of the optical system of the eye.

Autorefractometry after laser in situ keratomileusis

PURPOSE

To correlate cycloplegic subjective refraction with cycloplegic autorefractometry in eyes that have had laser in situ keratomileusis (LASIK).

SETTING

Vlemma Eye Institute, Athens, Greece.

METHODS

Subjective refraction and autorefractometry under cycloplegia were performed in 73 eyes of 46 patients 1, 6, and 12 months after LASIK to correct myopia or myopic astigmatism. The preoperative subjective refraction and autorefractometry under cycloplegia in the same eyes served as controls.

RESULTS

A statistically significant difference between subjective refraction and autorefraction was found in the sphere and cylinder at all postoperative times. No statistically significant difference was found in the axis. There was no statistically significant difference in the control eyes.

CONCLUSIONS

Automated refractometry in eyes that had had LASIK was reliable in the axis only. Retreatments after LASIK should always be based on subjective refraction.

A New Method of Calculating Intraocular Lens Power After Photorefractive Keratectomy

PURPOSE

To find a method of calculating intraocular lens (IOL) power that may be independent of preoperative data, in eyes that have developed a cataract after refractive surgery.

METHODS

Prior to and 1 month after PRK, the SRK/T formula was used to calculate IOL power in 88 eyes of 65 patients with a preoperative spherical equivalent refraction between -16.25 to +0.25 D (mean -5.39 +/- 3.19 D). IOL power was calculated by utilizing the spherical equivalent refraction as target both before and after PRK. Utilizing the postoperative corneal radius measurement (R2), an underestimation of the IOL power was found. For this reason, the mean postoperative corneal radius (R3) that gave the same IOL power found before surgery was calculated for each patient. The R3/R2 ratios were plotted against the axial eye length and a linear regression formula was used to calculate R2 correcting factors that gave the new corneal radius (R4). Patients were divided into classes according to axial eye length, and the mean R3/R2 ratios for each class were calculated and used to recalculate the new mean radius (R5). IOL power for emmetropia was calculated in all patients by utilization of R3, R4, R5, the historical method, and the "true corneal power" method.

RESULTS

Within +/-0.50 D from the IOL power calculated with R3, R4 gave 35 (39.3%) IOLs, while R5 gave 40 (45.5%) IOLs; the clinical history method gave 24 (27.3%) IOLs and "true corneal power" gave 23 (26.1%) IOLs, with a statistically significant difference P<.001).

CONCLUSIONS

Our theoretical method, based on correlation between axial eye length and corneal radius correcting factors, may represent an effective method of calculating IOL power after PRK, especially if the history of the patient is unknown.

Correlation between refractive and measured corneal power changes after myopic excimer laser photorefractive surgery

PURPOSE

To determine the correlation between the refractive and measured corneal power changes after myopic photorefractive surgery.

SETTING

Department of Ophthalmology, National Taiwan University Hospital, Taipei, Taiwan.

METHODS

Eighty-six eyes that had myopic photorefractive surgery were analyzed. The data included preoperative and 1-year postoperative subjective refraction, standard automated keratometry, and computerized videokeratography. Statistical analysis was performed to determine the relationship between the changes in subjective refraction in the corneal plane (Delta SEQco) and in 4 corneal power measurements including the power measured by automated keratometry (Delta Auto K), topographic-simulated keratometric power (Delta Sim K), the power of the first photokeratoscopic ring on videokeratography (Delta Central K), and the average videokeratographic power on the pupil margin (Delta Pupil K).

RESULTS

The measured corneal power always underestimated the Delta SEQco, with Delta SEQco > Delta Central K > Delta Sim K > Delta Pupil K > Delta Auto K. All the changes in measured corneal power could predict the Delta SEQco with more than 90.00% (90.19% to 92.31%) reliability at 1 year as calculated by the regression formulas (P <.001). The underestimation of measured corneal power changes was correlated with the amount of myopic correction, especially the Auto K (all P <.001).

CONCLUSIONS

Direct corneal power measurements using automatic keratometry underestimated the actual corneal flattening after photorefractive surgery, which could be adjusted by a linear regression formula. Measuring the power of the first photokeratoscopic ring on videokeratography might provide a better estimation of actual corneal flattening after photorefractive surgery.

Correlation Between Refractive and Corneal Topographic Changes After Photorefractive Keratectomy for Myopia

PURPOSE

To compare videokeratographic and refractive data obtained before and after photorefractive keratectomy (PRK) for myopia.

METHODS

Seventy-four eyes underwent PRK for myopia ranging from -2.50 to -17.00 D (mean, -7.76 +/- 3.17 D). All patients had videokeratography with the EyeSys instrument before, and 1 and 6 months after PRK, and the changes in three corneal power measurements (center of the ablation, apex, and effective refractive power) were compared with refractive changes.

RESULTS

Changes obtained in the three corneal power measurements at 1 and 6 months were well correlated with manifest refraction (Pearson's coefficient ranged from 0.71 to 0.84).

CONCLUSION

Power measurements obtained with corneal topography, as described above, are a reliable and objective method for the evaluation and follow-up of PRK, provided addition of an approximate 25% correcting factor.

Manual keratometry and videokeratography after photorefractive keratectomy

PURPOSE

To examine the relative accuracy of manual keratometry and videokeratography in eyes treated by photorefractive keratectomy (PRK).

SETTING

Eye Clinic, Cantonal Hospital, Lucerne, Switzerland.

METHODS

Results of manual keratometry and videokeratography were compared with those of subjective refraction in 128 eyes before and 6 months after PRK.

RESULTS

Six months after PRK, the mean subjective refraction of all eyes decreased more than the mean corneal dioptric power measured with videokeratography (P <.0001). The change in the mean subjective refraction compared with the corresponding difference in the mean manual keratometry of all eyes was also significant (P <.0001).

CONCLUSIONS

This study confirmed an earlier observation that there is a disparity between the change in refraction and the reduction in corneal power measured by videokeratography and with the manual keratometer. Topographical changes from PRK and the subsequent wound-healing processes are likely to falsify objective measurements. The keratometric value in the center of the cornea, since it is not measured by manual keratometry and videokeratography, may actually be lower.

Quantitative topographic irregularity as a predictor of spectacle-corrected visual acuity after refractive surgery

PURPOSE

To evaluate a new topographic index called topographic irregularity as a quantitative predictor of corrected vision after refractive surgery.

METHODS

We defined topographic irregularity as the summed difference at all points between a topographic refractive corneal power map and its best-fit spherocylinder. We prospectively studied 107 eyes of 107 patients 3 months after a variety of refractive procedures. Topographic irregularity was calculated from topographic maps, and the correlation between topographic irregularity and spectacle-corrected visual acuity was determined using both high-contrast and low-contrast acuity charts. This correlation was compared with correlations for the surface regularity index and the surface asymmetry index. Next, we analyzed 54 of these topographic maps to create a regression scale relating surface regularity index, surface asymmetry index, and topographic irregularity to predict spectacle-corrected visual acuity. This scale was then used to predict spectacle-corrected visual acuity on the remaining 53 postoperative patients.

RESULTS

The correlation of topographic irregularity with spectacle-corrected visual acuity (R(2) =.36) was comparable to the correlation for the surface regularity index (R(2) =.36) and stronger than for the surface asymmetry index (R(2) =.11) when spectacle-corrected visual acuity was measured with high-contrast eye charts. Topographic irregularity correlated more strongly with spectacle-corrected visual acuity (R(2) =.42) than either the surface regularity index (R(2) =.28) or the surface asymmetry index (R(2) =.14) when spectacle-corrected visual acuity was measured with low-contrast eye charts. Using the regression scale, prediction of high-contrast and low-contrast spectacle-corrected visual acuity from topographic irregularity was superior to or comparable to predictions using the surface regularity index and the surface asymmetry index.

CONCLUSIONS

Topographic irregularity has a closer correlation with spectacle-corrected visual acuity than existing topographic indexes. Topographic irregularity is also an accurate predictor of spectacle-corrected visual acuity and may be a more sensitive tool for evaluating postoperative visual performance than current topographic measures.

Diurnal stability of refraction after implantation with intracorneal ring segments

PURPOSE

To investigate diurnal changes in visual acuity and refraction in myopic eyes implanted with intracorneal ring segments (ICRS).

SETTING

University of California San Diego Shiley Eye Center, La Jolla, California, and Emory University Vision Correction Center, Atlanta, Georgia, USA.

METHODS

This prospective study involved 2 groups of patients who had ICRS (Intacs) implantation and a follow-up of at least 6 months. The first group included 102 eyes of 51 bilaterally treated patients; the second group, 32 eyes of 16 unilaterally treated patients. Examinations including visual acuity, manifest refraction, and videokeratography were done in the morning and evening at least 9 hours apart on a single day. Refractive changes were analyzed by power vectors; multivariate statistics were used to determine the significance of change in any component of the spectacle prescription.

RESULTS

In the bilateral treatment group, 97 eyes (95%) were within 1 line of spectacle-corrected visual acuity from morning to evening. The mean change in manifest refraction was -0.14 +0.08 x 4 and in spherical equivalent, -0.10 diopters (D) (sigma = 0.3; range -0.750 to +0.875 D). Ninety-six eyes (94%) had a change in refraction within 0.50 D of spherical equivalent. There was no significant change in corneal power (P =.20). In the unilateral treatment group, there was no significant difference between treated and untreated eyes in changes in spectacle-corrected visual acuity, manifest refraction, or corneal power and toricity (P.05).

CONCLUSION

No clinically significant diurnal variation in visual acuity or manifest refraction was observed after ICRS implantation or in untreated paired eyes. Moreover, the data suggest less diurnal change in visual acuity and refraction after ICRS implantation.

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.