Refractive complications of cataract surgery after radial keratotomy

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.

Intraocular lens power calculations using corneal topography after photorefractive keratectomy

PURPOSE

To report two patients (two eyes) with previous photorefractive keratectomy, who subsequently underwent cataract extraction years later.

DESIGN

Case reports.

METHODS

Corneal topography was used to determine corneal power used in intraocular lens power calculations.

RESULTS

In two eyes of two patients, intraocular lens calculations after photorefractive keratectomy were inadequate, which resulted in a hyperopic postoperative refractive error requiring implantation of a piggyback intraocular lens.

CONCLUSION

Corneal topography to determine corneal power in patients with previous photorefractive keratectomy may result in unpredictable intraocular lens power calculations. The clinical history method is the standard to determine corneal power and should be considered in intraocular lens calculations before cataract surgery. We recommend supplying refractive patients with preoperative data for use in future formulas for intraocular lens selection.

Rupture of a radial keratotomy incision after 11 years during clear corneal phacoemulsification

We report a case of rupture of a radial keratotomy (RK) incision that occurred during clear corneal phacoemulsification 11 years after the initial surgery. The RK was done in both eyes for correction of high myopia (>8.0 diopters). This was followed by 2 enhancement procedures at 6 month intervals. The patient presented with diminished vision in both eyes. The diagnosis was nuclear cataract in the right eye, and clear corneal phacoemulsification was done. The intraoperative and postoperative courses were uneventful. Nine months later, clear corneal temporal phacoemulsification was done in the left eye. During surgery, 1 of the radial incisions opened to one third its length. The wound was sutured, and the procedure was completed uneventfully. One month later, best corrected visual acuity was 20/20.

Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis

PURPOSE

To study the accuracy and predictability of intraocular lens (IOL) power calculation in eyes that had laser in situ keratomileusis (LASIK).

SETTING

Gimbel Eye Centre, Calgary, Alberta, Canada.

METHODS

Refractive outcomes in 6 cataract surgery and lensectomy eyes after previous LASIK were analyzed retrospectively. Target refractions based on measured and refraction-derived keratometric values were compared with postoperative achieved refractions. Differences between target refractions calculated using 5 IOL formulas and 2 A-constants and achieved refractions were also compared.

RESULTS

The refractive error of IOL power calculation in postoperative LASIK eyes was significantly reduced when refraction-derived keratometric values were used for IOL power calculation. Persistent residual hyperopia still occurred in some cases; this was corrected by hyperopic LASIK. Refractive results appeared more accurate and predictable when the Holladay 2 or Binkhorst 2 formula was used for IOL power calculation.

CONCLUSION

Hyperopic error after cataract surgery in post-LASIK eyes was significantly reduced by using refraction-derived keratometric values for IOL power calculation. Persistent hyperopic error was corrected by hyperopic LASIK.

Intra-ocular lens calculation status after corneal refractive surgery

The transition from incisional methods such as radial keratotomy (RK) to excimer laser surgery, eg, photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) has dramatically increased the volume of corneal refractive surgery performed worldwide in recent years. As the current younger generation of patients who have undergone refractive surgery ages, we can assume that the presently small number of postrefractive patients requiring cataract surgery and intraocular lens implantation will increase correspondingly. This article addresses the problems encountered with calculating intraocular lens power after corneal refractive procedures. Starting with a description of keratometry in normal eyes, the causes of evident mismeasurements and miscalculation of the corneal power after keratorefractive surgery will be discussed, and different approaches to improving IOL power prediction will be described.

Minimizing radial-keratotomy-induced diurnal variation in vision using contact lenses

A 41-year-old man with 16 radial keratotomy (RK) incisions in each eye reported a paradoxical diurnal variation in vision in both eyes with low Dk/L soft contact lenses. After rk, the patient experienced the conventional diurnal change a morning-to-evening mean (right and left eyes) myopic change of -1.80 diopters (D). However, while wearing low Dk/L contact lenses, a paradoxical morning-to-evening mean hyperopic change of 2.30 D was found. The diurnal variation was minimized (0.50 D) by wearing high Dk/L contact lenses. These results suggest that contact lenses can be used to treat diurnal variation in manifest refraction after RK by inducing appropriate stress.

Accuracy and predictability of intraocular lens power calculation after photorefractive keratectomy

PURPOSE

To investigate the accuracy and predictability of intraocular lens (IOL) power calculation in postoperative photorefractive keratectomy (PRK) eyes.

SETTING

Gimbel Eye Centre, Calgary, Alberta, Canada.

METHODS

The results in 5 cataract surgery eyes that had had PRK were analyzed retrospectively. Target refractions based on actual and refraction-derived keratometric values were compared with postoperative achieved refractions. The target refractions calculated using 5 IOL formulas and 2 A-constants were also compared with the achieved refractions.

RESULTS

In postoperative PRK eyes, the power calculation was more accurate and predictable when the smaller of either the actual or refraction-derived keratometric value was used to calculate the IOL power. The difference between target and achieved refractions appeared smaller when the Binkhorst formula was used. No significant hyperopic shift was observed after cataract surgery.

CONCLUSION

The smaller of the actual or the refraction-derived keratometric value is recommended for calculating IOL power in post-PRK eyes.

Intraocular Lens Power Calculation in Eyes After Corneal Refractive Surgery

PURPOSE

The purpose of this review article is to discuss the major reasons for postoperative hyperopia after cataract surgery following radial keratotomy (RK) and photorefractive keratectomy (PRK) and to illustrate potential methods for improvement of intraocular lens (IOL) power prediction after keratorefractive surgery based on exemplary model calculations.

METHODS

We previously performed model calculations in eyes after PRK for myopia (-1.50 to -8.00 D, mean -5.40 +/- 1.90 D) using keratometry readings as measured by the Zeiss keratometer and the TMS-1 topography unit and as calculated using the "clinical history method" (spherical equivalent refraction change) and change in anterior surface keratometry readings.

RESULTS

We found that after PRK, mean measured keratometry readings were significantly greater than respective calculated values considering the preoperative to postoperative change of anterior corneal surface (P < .001), which itself was significantly greater than calculated keratometry readings considering the preoperative to postoperative change of spherical equivalent refraction (P < .001). IOL power underestimation correlated significantly with the difference between preoperative and postoperative spherical equivalent refraction (P = .001).

CONCLUSIONS

For correct assessment of keratometric readings to be entered into more than one modern third-generation IOL power calculation formula (but not a regression formula), the clinical history method should be applied whenever refraction and keratometric diopters before the keratorefractive procedure are available to the cataract surgeon. If preoperative keratometric diopters and refraction are not known, average central power on the postoperative videokeratograph may be used after RK, but refined calculation of keratometric diopters from radius of anterior and posterior corneal surface should be used after PRK and/or LASIK.

Intraocular lens calculations status after corneal refractive surgery

With the increasing number of keratorefractive surgical procedures, an increasing number of cataract surgeries in eyes after keratorefractive surgery is anticipated within a few decades. Although cataract extraction seems to be feasible without major technical obstacles, intraocular lens (IOL) power calculation turned out to be problematic. Insertion of the measured average K-readings (= "central corneal power" = keratometric diopters) after myopic radial keratotomy (RK), photorefractive keratectomy (PRK), or laser in situ keratomileusis (LASIK) into standard IOL power-predictive formulas commonly results in substantial undercorrection and postoperative hyperopic refraction or anisometropia. In this article, the major reasons for IOL power miscalculations (which are different for RK versus RRK/LASIK) are discussed based on model calculations and based on case series of cataract surgeries, methods for improved assessment of keratometric diopters as the major underlying problem are exemplary illustrated, and finally a clinical step-by-step approach to minimize IOL power miscalculations status after corneal refractive surgery is suggested. The "clinical history method" (i.e., subtraction of the spherical equivalent [SEQ] change after refractive surgery from the original K-reading) should be applied whenever refraction and K-reading before the keratorefractive procedure are available to cataract surgeons. In addition, more than one modern third-generation formula (e.g., Haigis, Hoffer Q, Holladay 2, or SRK/T) but not a regression formula (e.g., SRK I or SRK II) should be applied and the highest resulting IOL power should be used for the implant.

Refractive error in cataract surgery after previous refractive surgery

Bilateral cataract extraction with posterior chamber intraocular lens (IOL) implantation was performed in a patient after previous photorefractive keratectomy, radial keratotomy (RK) combined with astigmatic keratotomy, and retreatment of RK. Significant hyperopic error was observed after cataract surgery, and the IOLs were eventually exchanged in both eyes. A review of this case found that the refractive error was smaller when a refraction-derived keratometric value was selected for IOL power calculation. Nevertheless, hyperopic error still occurred.

Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity

PURPOSE

To assess the accuracy of corneal power measurement by contact lens overrefraction in patients with normal corneas and to determine the suitability of this method for use in intraocular lens (IOL) calculations.

SETTING

General ophthalmology clinic at a public hospital (Ben Taub General Hospital, Houston, Texas, USA).

METHODS

Using contact lens overrefraction (CLO), and standard keratometry, the corneal power in 33 eyes of 20 normal patients and patients scheduled for cataract extraction was prospectively measured. The eyes were divided into 3 groups based on their best spectacle-corrected visual acuity: (1) 20/20 to 20/40, (2) 20/50 to 20/70, and (3) 20/80 to 20/400. For each group, the means (absolute and arithmetic), standard deviations, and ranges of differences in corneal power as measured by CLO and keratometry were calculated. These values were used to estimate the induced variance in refractive outcome for IOL calculations.

RESULTS

The mean absolute differences in corneal power by group were 0.35 diopter (D) +/- 0.18 (SD), 0.54 +/- 0.33 D, and 0.77 +/- 0.28 D, respectively. The mean arithmetic differences in corneal power were -0.05 +/- 0.39 D, +0.37 +/- 0.51 D, and +0.17 +/- 0.80 D, respectively.

CONCLUSIONS

In eyes of patients with good visual acuity (20/20 to 20/40), corneal power measurements by CLO and keratometry were similar. The accuracy of the CLO-derived value decreased with increasing media opacity but was still acceptable with acuity of 20/70. Contact lens overrefraction may be a viable alternative to refractive history and videokeratography for estimating true corneal power in patients with surgically altered or irregular corneas.

Effect of astigmatic keratotomy on spherical equivalent: results of the Astigmatism Reduction Clinical Trial

PURPOSE

To determine the effect of astigmatic keratotomy on spherical equivalent, as measured by the coupling ratio and a new quantity, coupling constant.

METHODS

In a prospective multicenter study, subjects underwent arcuate keratotomy at a 7-mm optical zone by means of the Lindstrom nomogram for correction of astigmatism. One hundred fifty-seven eyes of 95 patients who had a follow-up examination 1 month postoperatively were studied. Mean preoperative refractive cylinder +/- SEM was 2.82 +/- 1.17 diopters. Coupling ratio was defined as the ratio of the flattening of the incised meridian to the steepening of the opposite meridian. Coupling constant was defined as the ratio of the change in spherical equivalent to the magnitude of the vector change in astigmatism. Coupling ratio, coupling constant, and change in spherical equivalent were calculated on the basis of change in refraction and keratometry.

RESULTS

On the basis of change in refraction, coupling ratio was 0.95 +/- 0.10 (mean +/- SEM) and coupling constant was -0.01 +/- 0.03, consistent with a minor shift in the spherical equivalent of -0.03 +/- 0.07 diopter. On the basis of change in keratometry, coupling ratio was 0.84 +/- 0.05 and coupling constant was -0.04 +/- 0.02, consistent with minor postoperative keratometric steepening of -0.10 +/- 0.04 diopter. Coupling ratio based on change in refraction was not statistically different from the coupling ratio predicted by the Gauss' law for inelastic domes (P = .370). Incision length and number, amount of achieved cylinder correction, age, and sex had no statistically significant effect on coupling ratio, coupling constant, and change in spherical equivalent.

CONCLUSIONS

Cornea behaved as an inelastic surface in response to arcuate keratotomy performed with the Astigmatism Reduction Clinical Trial study nomogram. On average, astigmatic keratotomy had a minimal effect on spherical equivalent refraction. There was variability, however, in coupling ratio, coupling constant, and change in spherical equivalent from eye to eye after astigmatic keratotomy. Caution is therefore advised when simultaneous correction of cylinder and spherical equivalent is planned.

Intraocular lens power calculation after decentered photorefractive keratectomy

A 59-year-old patient who had photorefractive keratectomy (PRK) to correct high unilateral myopia developed a progressive nuclear cataract. Phacoemulsification and intraocular lens (IOL) implantation were performed. However, determination of IOL power using automated keratometry and computerized videokeratography was not successful in this case of high axial myopia because of a decentered ablation zone, resulting in too-steep keratometric readings. Postoperative hyperopia could only be corrected by an IOL exchange. Because it may not be possible to determine the exact keratometric values for IOL calculation after PRK, subtracting the change in refraction induced by PRK from the preoperative keratometric readings might have been more accurate in this patient.

Regular and irregular refractive powers of the front and back surfaces of the cornea

The refractive status of the posterior corneal surface has not been well studied. The purpose of this study is to quantitatively evaluate the regular and irregular astigmatism of the posterior corneal surface. In 50 normal human eyes, topography of the anterior and posterior corneal surface was measured with the scanning-slit videokeratoscope. Using Fourier series harmonic analysis, dioptric data on a mire ring were decomposed into spherical component, regular astigmatism, asymmetry (tilt or decentration), and higher order irregularity. The obtained values for the central 3.0 mm of the posterior corneal surface were -6.55+/-0.32 D (spherical component), 0.18+/-0.16 D (regular astigmatism), 0.40+/-0.22 D (asymmetry), and 0.02+/-0.02 D (higher order irregularity). The posterior to anterior ratios of these parameters were 13.6+/-0.6%, 35.0+/-41.3%, 45.8+/-56.9%, and 39.9+/-39.8%, respectively. The ratio of the spherical component was significantly lower than the other three parameters (P<0.001, Wilcoxon signed-rank test), indicating that non-spherical components (regular and irregular astigmatism) of the posterior corneal surface are not negligible in the optical quality of the cornea. The current results can serve as the control data and reference for the future clinical studies of optical characteristics of the cornea as a whole.

Dehiscence of a radial keratotomy incision during clear corneal cataract surgery

We report a case of dehiscence of a radial keratotomy (RK) incision caused by clear corneal cataract surgery. The patient had eight-incision RK in both eyes 9 months previously with enhancement surgery in the left eye 1 month later. Cataract surgery through a clear corneal incision was performed in the right eye and 1 month later, in the left. Surgery in the right eye was uneventful. However, during surgery in the left eye, dehiscence of one radial incision occurred. The wound dehiscence was closed with interrupted sutures, and the patient achieved 20/20, uncorrected visual acuity 1 week after surgery.

The prediction of surgically induced refractive change from corneal topography

PURPOSE

To develop a method to predict the refractive power of the cornea from corneal topography.

METHODS

We reviewed preoperative and postoperative cycloplegic refraction, keratometry, and corneal topography in 40 eyes of 40 patients who had undergone photorefractive keratectomy, radial keratotomy, myopic keratomileusis in situ, or hyperopic lamellar keratoplasty. For each axial dioptric power map, we calculated the aspheric ellipsoid that best fit that map. Central corneal points were weighted more heavily than peripheral points, based on the Stiles-Crawford effect. The equation of the best-fit ellipsoid yielded the spherical and astigmatic power and axis for each cornea preoperatively and postoperatively.

RESULTS

The preoperative corneal spherical and astigmatic powers measured by the best-fit method were consistent with the spherical and astigmatic powers measured by keratometry and simulated keratometry. The change in corneal spherical power predicted by the best-fit method was significantly (P < .05) more accurate at predicting the change in spherical equivalent refraction than change either in spherical equivalent keratometry or in spherical equivalent simulated keratometry. The prediction of the astigmatic change was less precise than that of the spherical, but the best-fit method was the most accurate.

CONCLUSIONS

The best-fit method is more accurate than simulated keratometry and standard keratometry are in evaluating corneal refractive power after refractive surgery. An improved method of calculating corneal refractive power may facilitate subjective refraction after refractive surgery, improve the accuracy of intraocular lens power calculation for eyes that have had previous refractive surgery, and improve ablation profiles for excimer laser refractive surgery.

Intraocular Lens Power Calculation for Cataract Surgery after Photorefractive Keratectomy for High Myopia

OBJECTIVE

To assess intraocular lens (IOL) power calculations in patients undergoing cataract surgery after excimer laser photorefractive keratectomy (PRK) for myopia.

METHODS

Four eyes of two patients underwent phacoemulsification with IOL implantation after PRK for myopia. The estimated refractive error that would have been induced had the IOL predicted for emmetropia been implanted was calculated using SRK-II, SRK/T, Holladay, and Binkhorst formulas. Manual keratometry and videokeratography-simulated keratometry values measured before surgery were used. Keratometry values calculated by subtracting the refractive change induced by the excimer laser PRK from the manual keratometry or videokeratography-simulated keratometry values measured before PRK were also used. Both spectacle and corneal plane calculations were performed.

RESULTS

Manual keratometry and videokeratography-simulated keratometry values underpredicted the IOL power. Corneal plane manual or videokeratography refraction-derived keratometry calculations were most accurate using the SRK/T formula, while spectacle plane calculations were most accurate using the SRK-II formula. In both methods the calculated refractive error was within 0.52 diopters (D) for the emmetropic lens power predicted. Statistical analysis was not performed.

CONCLUSIONS

Refraction-derived keratometric values provided the most accuracy in calculating IOL powers. Our results suggest the SRK/T formula was the most accurate for corneal plane calculations, while the SRK-II formula was the most accurate for spectacle plane calculations.

Computerized videokeratography and keratometry in determining intraocular lens calculations

PURPOSE

To compare the accuracy of standard keratometry and computerized videokeratography (CVK) in determining intraocular lens (IOL) power calculations.

METHODS

Using the EyeSys Corneal Analysis System, we prospectively obtained CVK maps on 75 eyes of 69 patients scheduled to have phacoemulsification with implantation of a posterior chamber intraocular lens. Using manifest refraction obtained at 6 weeks postoperatively, we optimized the calculations for the Hoffer Q, Holladay, and SRK/T formulas for standard keratometric and the following six CVK values: average curvatures at the 1 mm, 2 mm, and 3 mm zones, the keratometric equivalent at the 3 mm zone, and the Stiles-Crawford weighted averages over the 3 mm and 6 mm zones. The accuracy of these parameters was determined by calculating the mean absolute error and percentage of patients with accuracy within < or = 0.5 diopter (D), < or = 1.0 D, and < or = 2.0 D.

RESULTS

Keratometrically derived data were slightly more accurate than the CVK-derived values. The average difference in mean absolute error between the keratometric and CVK values was 0.13 D for the Hoffer Q formula, 0.11 D for the Holladay, and 0.08 D for the SRK/T.

CONCLUSIONS

In this population of patients, we found the CVK-derived corneal curvature values to be slightly less accurate than standard keratometry in predicting IOL power. However, CVK provides important corneal curvature data for IOL calculations in patients with abnormal or surgically altered corneal surfaces.

Calculation of intraocular lens power after radial keratotomy with computerized videokeratography

PURPOSE

Because standard methods to determine intraocular lens power are not adequate in eyes that have had radial keratotomy, we undertook this study to evaluate the corneal power derived from computerized videokeratography for use in intraocular lens power calculations.

METHODS

We examined four eyes of three patients who had radial keratotomy and who underwent phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens. We used a computerized videokeratography-derived corneal curvature value in the Holladay formula for intraocular lens calculations. We determined the ideal intraocular lens power and the keratometric value that would have led to the ideal intraocular lens power from the postoperative refraction at 6.1 +/- 1.1 months after cataract extraction. The ideal keratometric value was compared with the keratometric values derived from computerized videokeratography, standard keratometry, contact lens overrefraction, and refractions before and after radial keratotomy.

RESULTS

The postoperative refraction at approximately six months averaged -0.32 +/- 0.63 diopter (range, -0.88 to +0.75 diopter) different than the aim. The mean power in ring 3, which was the closest keratometric value to the ideal, disclosed only 0.09 +/- 0.73 diopter and -0.10 +/- 0.72 diopter of deviation from the ideal keratometric and intraocular lens powers, respectively. One to two weeks after phacoemulsification cataract surgery with implantation of a posterior chamber intraocular lens, the videokeratographic differential map disclosed steepening at the wound site with variable regression by six months in all patients.

CONCLUSION

Results suggest that, after radial keratotomy, using the keratometric value derived from computerized videokeratography in intraocular lens calculations is more accurate than using keratometric values measured by routine methods.

Intraocular Lens Power Calculation for Eyes After Refractive Keratotomy

BACKGROUND

Calculating the intraocular lens (IOL) power for an eye that has previously had refractive keratotomy is a problem because of the difficulty of accurately measuring the central power of the cornea using standard keratometers.

METHODS

Three methods are proposed to better estimate this parameter. The clinical history method involves subtracting the change in myopia induced by the refractive keratotomy from the average corneal power measured before the keratotomy. The contact lens method determines the difference between the manifest refraction with and without a plano hard contact lens of known base curve and subtracts this difference from that base curve. Videokeratography measures the central corneal power inside the approximately 3-mm zone measured by keratometry, and therefore gives a more accurate power to use in IOL calculation formulas, especially with newer software applications becoming available.

RESULTS

Published reports have demonstrated that standard keratometers do not accurately measure corneal power after refractive keratotomy and that regression formulas are less accurate than modern third-generation theoretic formulas for eyes that have flatter corneas from refractive surgery.

CONCLUSION

For eyes that have had refractive surgery, the corneal power derived from clinical history, contact lens refraction, or videokeratography should be used in a third-generation theoretic formula (Hoffer Q, Holladay, SRK/T) to calculate the intraocular lens power used during cataract surgery.

Phacoemulsification with intraocular lens implantation after excimer photorefractive keratectomy: a case report

We report the first case, to our knowledge, of phacoemulsification with lens implantation in a patient with previous photorefractive keratectomy for myopia. Intraocular lens calculations were performed using manual and automated keratometry. The surgical and postoperative course were uneventful with a good visual outcome. Standard intraocular lens calculation and surgical technique appear to be successful for cataract extraction after photorefractive keratectomy.