Intraocular lens power calculation after corneal refractive surgery

Post-small incision lenticule extraction phacoemulsification with multifocal IOL implantation: A case report

A 54-year-old patient presented with cataract, 5 years after undergoing SMILE for high myopia in both eyes. He was motivated in achieving spectacle free vision and his post SMILE-induced aberrations were minimal, due to which he was found suitable for a trifocal IOL implant. Of the various methods considered, the IOL power predicted by a novel combined telecentric keratometry and swept source OCT-based method was finally selected. One month post-operatively, the patient achieved a binocular UDVA of 20/20p and near vision of N.6, suggesting that newer IOL formulae could be superior in providing satisfactory outcomes in post refractive patients.

Corneal Power Measurement Obtained by Fourier-Domain Optical Coherence Tomography: Repeatability, Reproducibility, and Comparison With Scheimpflug and Automated Keratometry Measurements

PURPOSE

To assess the repeatability and reproducibility of corneal power values obtained by a Fourier-domain optical coherence tomography (FD-OCT) system (RTVue) and to compare them with the values obtained by a Scheimpflug camera system (Pentacam HR) and by automated keratometry (IOL Master).

METHODS

Thirty-two eyes from 32 healthy subjects were included in this prospective study. Two experienced observers measured each eye 3 consecutive times with the Pentacam, IOLMaster, and RTVue centered on either the pupil or corneal vertex. The conventional keratometry equivalent (CKE) and anterior (Ka), posterior (Kp), and net (Kn) corneal power values were determined.

RESULTS

The corneal power values obtained by the RTVue showed high repeatability (all intraclass correlation coefficient >0.96) and reproducibility (coefficient of variation <1.0%). Pupil-centered FD-OCT performed slightly better than corneal vertex-centered FD-OCT. Mean corneal values had higher reproducibly than any of the individual values. CKE, Ka, Kp, and Kn obtained by FD-OCT were 0.62 to 0.68 diopters (D), 0.70 to 0.76 D, 0.11 to 0.13 D, and 0.93 to 0.94 D higher than those obtained by the Pentacam HR, respectively. CKE and Ka obtained with the RTVue were also 0.60 to 0.74 D higher than those obtained with the IOLMaster, respectively.

CONCLUSIONS

The corneal power measurements obtained by the RTVue FD-OCT system showed high repeatability and reproducibility. Measurements obtained by FD-OCT with pupil centration were more reproducible than those obtained by FD-OCT with corneal vertex centration. We recommend that pupil-centered FD-OCT be used in clinical applications. Neither RTVue versus Pentacam HR nor RTVue versus IOLMaster can be used interchangeably.

Cataract surgery outcome in eyes with keratoconus

BACKGROUND

To review the refractive outcome of cataract surgery in eyes with keratoconus.

METHODS

We retrospectively reviewed the medical records of 64 consecutive patients (92 eyes) who underwent cataract surgery with implantation of a spherical intraocular lens (IOL). We recorded the method of refractive correction and the effect of the keratometry (K) on the biometry prediction error (BPE).

RESULTS

35 eyes had mild keratoconus (mean K<48 dioptres (D)), 40 had moderate keratoconus (mean K 48 D to 55 D) and 17 had severe keratoconus (mean K>55 D). Actual K values were used in all eyes with mild or moderate keratoconus with a target refraction of approximately -1.0 D in mild keratoconus and -1.5 D in moderate keratoconus that resulted in a mean BPE of 0.0 D and +0.3 D, respectively. The actual K values were used in eight of the 17 eyes with severe keratoconus with a mean target refraction of -5.4 D, which resulted in a mean BPE of +6.8 D. In the remaining nine eyes, a standard K value of 43.25 D was used with a mean target refraction of -1.8 D, which resulted in a mean BPE of +0.6 D.

CONCLUSIONS

Using the actual K values with a target of low myopia is a suitable option for spherical IOL selection for eyes with a mean K of ≤55 D. When there is severe keratoconus, the use of actual K values can result in a large hyperopic error and the use of standard K value in these eyes should be considered.

Confirmation and refinement of the Diehl-Miller nomogram for intraocular lens power calculation after laser in situ keratomileusis

PURPOSE

To validate the Diehl-Miller equation for achieving emmetropia after cataract surgery in post-laser in situ keratomileusis (LASIK) eyes and to present a refined equation.

SETTING

Jules Stein Eye Institute, Los Angeles, California, USA.

DESIGN

Cohort study.

METHODS

Preoperative and postoperative refractive and cataract surgery data were collected from the medical records of post-LASIK patients. The IOL power data and postoperative refractive information were used to calculate the true target refractive error that should have been chosen to achieve emmetropia. Regression analysis of the combined data from Diehl-Miller and the current study was used to develop a refined equation. Diehl-Miller relates the manifest refraction spherical equivalent (MRSE) change induced by LASIK to an adjusted target postoperative refractive error to be used in intraocular lens (IOL) power calculations.

RESULTS

Twenty-three eyes of 18 patients were evaluated. The target refractive error calculated by Diehl-Miller differed from the true target refractive error by an absolute mean of 0.481 diopter (D) ±0.376 (SD). Regression analysis of the combined data from Diehl-Miller and the current study yielded the following 2nd-order polynomial equation: Target refractive error = -0.0198 (MRSE change)(2) +0.170 (MRSE change)-0.0079. The calculated target refractive error fell within ±0.50 D of the true target in 49% of patients and within ±1.00 D in 93% of patients.

CONCLUSIONS

This study validated that the Diehl-Miller equation accurately predicts the target refractive error to achieve emmetropia in post-LASIK eyes. The slightly refined Diehl-Date-Miller equation and associated lookup tables can be used to avoid postoperative refractive surprises.

Determining corneal power using Pentacam after myopic photorefractive keratectomy

PURPOSE

To assess the accuracy of Pentacam Scheimpflug camera for corneal power measurement in eyes with previous photorefractive keratectomy for myopia.

METHODS

In this comparative interventional case series, 35 eyes of 35 patients who had myopic photorefractive keratectomy were studied. Corneal power was measured by conventional topography and Pentacam Scheimpflug camera, and equivalent keratometry readings (EKR) in different central corneal rings (0.5 to 4.5 mm), true net power and simulated keratometry (K) measurements as well as those obtained using Shammas no-history, Koch-Maloney and Haigis methods were compared with clinical history method.

RESULTS

All corneal power measurements except for the topography simulated K and true net power values were statistically similar to the clinical history values. Simulated keratometry and 4.5-mm EKR values were more closely correlated with clinical history method. Shammas formula, Pentacam simulated K and 3-, 4- and 4.5-mm EKR provided a 95% confidence interval within +/-0.50 D of the mean clinical history method value, among these, the width of the 95% limits of agreement (LoA) was narrower for Shammas and Pentacam simulated K and 4.5-mm EKR values; however, considerably large 95% LoA were found between each of these values and those obtained with the clinical history method. Estimated preoperative keratometry was statistically similar to the preoperative measurement; however, estimated refractive change was different from actual value.

CONCLUSIONS

The Pentacam 4.5-mm EKR and simulated keratometry may be used as an alternative to clinical history method to predict corneal power when pre-keratorefractive surgery data are unavailable; however, wide LoA should be considered in the calculations.

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

AIM

To evaluate the accuracy of a practical method (the Actual K(a+p) method) of corneal power measurement for post-LASIK eyes undergoing cataract surgery.

METHODS

Ten eyes of 7 patients (4 male, 3 female, average age 50.10±4.01 years, with -11.01±3.55D mean refraction before LASIK), underwent post-LASIK phaco+IOL cataract surgery. We used the posterior corneal curvature as measured by the Pentacam in a method we named Actual K(a+p) to calculate the post-LASIK corneal power for IOL calculation. The refractive outcomes after cataract surgery were evaluated. The Actual K(a+p) was compared with the back- calculated corneal power (BCK), which was thought to be the benchmark of true corneal power. The corneal power estimated by other published methods, including Maloney, Shammas, Koch-Maloney, Savini, and McCulley, together with the true net power and equivalent K reading (EKR) as found by the Pentacam were also compared with the BCK.

RESULTS

All eyes achieved satisfied refractive status after cataract surgery. The difference between the postoperative refraction and the target refraction was 0.04±0.40D, range from -0.63D and +0.85D. Among all the methods we studied, although the Bonferroni multiple comparison tests did not detect significant differences between any two of them, the Actual K(a+p) yielded the highest agreement with the BCK, with 80% of the eyes falling within ±0.5D and 100% within ±1.0D from the BCK values.

CONCLUSION

The Actual K(a+p) method can provide encour- aging results in post-LASIK eyes undergoing cataract surgery.

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.

New frontiers for the perioperative data method for IOL calculation following corneal refractive surgeries

PURPOSE

To evaluate the efficiency of the perioperative data method for intraocular lens (IOL) calculation after correction of myopia and hyperopia with different techniques, including reoperated cases.

METHODS

Thirty-five eyes (26 patients) that developed cataract after corneal refractive procedures were evaluated retrospectively. They were categorized according to initial error of refraction into myopes and hyperopes and according to types of refractive procedures into ablative, incisional, both, or others. Reoperated cases were also considered. Number of refractive procedures was noted. Time interval between the first procedure and cataract extraction was indicated. Perioperative method was used to calculate the K value. SRK/T formula was used to calculate IOL power. Difference between intended and finally achieved manifest refraction was an indicator for efficiency of the calculation.

RESULTS

Postoperatively, 77.2% of cases had manifest refraction +/-1.5 D of intended refraction. There was no difference between myopes and hyperopes in terms of final manifest refraction, best-corrected visual acuity, and difference between intended and finally achieved manifest refraction. Similarly were groups of different types of surgeries. Efficiency of the method decreased with high axial lengths and low IOL powers. Neither the number of refractive surgeries nor time interval between surgeries affected efficiency of the method.

CONCLUSIONS

The perioperative data method is equally effective for myopes and hyperopes. The types, numbers of refractive procedures, as well as the time interval between refractive surgery and cataract extraction do not alter the credibility of the method. In high degrees of myopia, the method gives less accurate results.