Determining corneal power using Pentacam after myopic photorefractive keratectomy

Effect of sodium fluorescein strip application on cornea parameters commonly used in laser-assisted in-situ keratomileusis

To assess the effect of sodium fluorescein (NaF) strip on corneal parameters commonly used in Laser-assisted in-situ keratomileusis (LASIK). Eighty-six subjects (172 eyes) scheduled for LASIK were recruited between January and March 2022. The study and statistical analysis test were conducted in April 2022. Topographic measurements of corneal parameters, including central corneal thickness (CCT), anterior keratometric (K) readings (K1, flat keratometry; K2, steep keratometry), horizontal corneal diameter (white to white, WTW), and corneal asphericity (Q value), were obtained using a Scheimpflug device (Pentacam) before and 10 min after NaF strip treatmentThe Pentacam recorded a small significant increase in CCT (mean 538.88 ± 28.78 μm to 547.90 ± 29.94 μm; p < .001), with no differences in K1 and K2 (mean 42.24 ± 1.35D to 42.24 ± 1.35D, and mean 43.34 ± 1.50D to 43.32 ± 1.51D; P > .05, for all) as well as WTW(mean 11.58 ± 0.32 mm to 11.58 ± 0.32 mm, P > .05) before and after NaF strip intervention. Furthermore, there was no significant difference was observed in Q value (mean - 0.30 ± 0.13 to - 0.30 ± 0.14, P > .05). These results indicate that clinicians should avoid NaF strip application before obtaining precise topographic measurements of cornea parameters using the Pentacam.

Comparing Standard Keratometry and Total Keratometry Before and After Myopic Corneal Refractive Surgery With a Swept-Source OCT Biometer

Background

More recently, the swept-source OCT biometer-IOLMaster 700 has provided direct total corneal power measurement, named total keratometry. This study aims to evaluate whether standard keratometry (SK) and total keratometry (TK) with IOLMaster 700 can accurately reflect the corneal power changes induced by myopic corneal refractive surgery.

Methods

In this study, the biometric data measured with the swept-source OCT biometer—IOLMaster 700 before and 3 months after the myopic corneal refractive surgery were recorded. The changes of biological parameters, including SK, posterior keratometry (PK), and TK, and the difference between SK and TK were compared. In addition, the changes of SK and TK induced by the surgery were compared with the changes of spherical equivalent at the corneal plane (ΔSEco).

Results

A total of 74 eyes (74 patients) were included. The changes of SK, PK, TK, axial length, anterior chamber depth, and lens thickness after refractive surgery were all statistically significant (all p < 0.01), while the change of white-to-white was not (p = 0.075). The difference between SK and TK was −0.03 ± 0.10D before the corneal refractive surgery and increased to −0.78 ± 0.26D after surgery. The changes of SK and the changes of TK induced by the surgery had a good correlation with the changes of SEco (r = 0.97). ΔSK was significantly smaller than ΔSEco, with a difference of −0.65 ± 0.54D (p < 0.01). However, the difference between ΔTK and ΔSEco (0.10 ± 0.50D) was not statistically significant (p = 0.08).

Conclusions

Using SK to reflect the changes induced by the myopic corneal refractive surgery may lead to underestimation, while TK could generate a more accurate result. The new parameter, TK, provided by the IOLMaster 700, appeared to provide an accurate, objective measure of corneal power that closely tracked the refractive change in corneal refractive surgery.

Corneal power measurements by ray tracing in eyes after small incision lenticule extraction for myopia with a combined Scheimpflug Camera-Placido disk topographer

PURPOSE

To evaluate the accuracy of the measurements of corneal power obtained by ray tracing with a combined Scheimpflug camera-Placido disk corneal topographer (Sirius) in eyes after small incision lenticule extraction for myopia (SMILE).

METHODS

Retrospective cases study includes 50 eyes of 50 patients who underwent myopic SMILE. Mean value of simulated keratometry (K_post), mean pupil power (MPP) (ray tracing, diameter of the entrance pupil range 3-6 mm), anterior and posterior corneal radius, and corneal thickness were obtained with Sirius topographer preoperatively and three months postoperatively, as well as cycloplegic refraction. True net power, equivalent keratometry readings, and Haigis equivalent power formula were calculated, and these measurements, MPP and K_post, were compared with the corneal power calculated with the clinical history method (CHM).

RESULTS

Corneal power measurements obtained with all methods were significantly different from CHM (P < 0.001), except the value of MPP obtained at 5.5 mm (P = 0.927). A good direct correlation was found between CHM and all measurements. The distribution of differences as compared with the CHM showed that the lowest difference corresponded to the value of MMP at 5.5 mm (- 0.002 ± 0.6). The Bland-Altman plots for the MPP at 5.5 mm showed 95% limits of agreement between - 1.1787 D and 1.1741 D.

CONCLUSIONS

MPP obtained by ray tracing within a diameter of entrance pupil of 5.5 mm could predict corrected corneal power derived from the CHM in eyes following SMILE surgery.

Comprehensive evaluation of total corneal refractive power by ray tracing in predicting corneal power in eyes after small incision lenticule extraction

PURPOSE

To assess the prediction accuracy of four variations of total corneal refractive power (TCRP) by the ray tracing method in determining corneal power in eyes after myopic small incision lenticule extraction (SMILE).

METHODS

Forty eyes of forty patients who had undergone myopic SMILE were enrolled in this prospective study. Manifest refraction and Pentacam HR were performed preoperatively and three months or more postoperatively. Mean keratometry (Km), true net power (TNP), equivalent keratometry readings (EKR) and 4 subtypes of TCRP (pupil centered or apex centered within a ring or a zone)-TCRPpupil,ring, TCRPpupil,zone, TCRPapex,ring and TCRPapex,zone-were recorded and compared to the theoretical postoperative keratometry value using the clinical history method (CHM).

RESULTS

The only keratometric values that showed no statistically significant differences from the CHM were 4.0 mm and 4.5 mm EKR, 6.0 mm TCRPpupil,zone and TCRPapex,zone. Pearson's correlation test revealed that 4.0 mm TCRPpupil,zone exhibited the highest correlation coefficient (r = 0.974) followed by TCRPapex,zone 4.0 mm (0.972) and EKR 4.5 mm (0.970). The 95% limits of agreement (LOA) of the 4.0 mm EKR and CHM, the 4.5 mm EKR and CHM, the 6.0 mm TCRPpupil,zone and CHM, the 6.0 mm TCRPapex,zone and CHM were (-1.27 to 1.22 D), (-1.04 to 0.98 D), (-1.39 to 1.08 D) and (-1.38 to 0.96 D), respectively, while the modified 4.0 mm TCRPpupil,zone (TCRPpuil,zone + 0.70 D) and TCRPapex,zone (TCRPapex,zone+0.70 D) yielded the narrowest 95% LOA of (-0.96 to 0.95 D) and (-0.96D, 1.05 D).

CONCLUSIONS

Total corneal refractive power using the ray tracing method could predict corrected corneal power derived from the CHM in eyes following SMILE surgery after simple modification.

Corneal power measurement with a new aberrometer/corneal topographer in eyes after small incision lenticule extraction for myopia

PURPOSE

To assess corneal power measurements obtained by the OPD SCAN III Topographer in eyes with prior myopic small incision lenticule extraction (SMILE) surgery.

METHODS

Sixty untreated myopic eyes of sixty subjects and forty previous myopic SMILE surgery eyes of forty subjects were consecutively enrolled in the present study. Manifest refraction, OPD SCAN III and Pentacam HR were performed. Keratometric measurements assessed by OPD SCAN III-simulated keratometry, average pupil power and effective central corneal power (ECCP) were compared with mean keratometry (Km) obtained by Pentacam HR in the untreated group and the clinical history method (CHM) in the treated group.

RESULTS

In the untreated group, no statistically significant differences were revealed between all corneal power measurements obtained with OPD SCAN III and Km. In the treated group, all the corneal power measurements were statistically different from the CHM except for the Haigis method and the Shammas method, while ECCP had a statistically but not clinically significant overestimation of 0.42 D with 95% limit of agreement (LOA) of - 0.81 D to 1.64 D. The three modified ECCP had better prediction performance with narrower 95% of LOA lying in (- 1.20, 1.20 D) (- 1.22, 1.23 D) and (- 0.90, 1.00 D), respectively.

CONCLUSIONS

The ECCP provided with OPD SCAN III could be used as an alternative option for the CHM after specific modifications in eyes with previous myopic SMILE surgery when the preoperative data are unavailable considering the narrowest agreement between the modified ECCP and the CHM. Otherwise, caution must be raised considering the wide LOA.

Assessment of total corneal power after myopic corneal refractive surgery in Chinese eyes

PURPOSE

To develop a new regression formula based on the Gaussian thick lens formula and to verify the accuracy of the regression formula.

METHODS

In this prospective study, 207 eyes of 207 myopic subjects and 133 eyes of 67 postoperative subjects were included. For the 133 postoperative eyes, 127 eyes underwent laser-assisted in situ keratomileusis, and 6 eyes underwent photorefractive keratectomy. Subjective refraction and Pentacam HR were performed preoperatively and postoperatively, and IOLMaster was performed in the postoperative group. SimK, keratometry based on the Gaussian optic formula (K_GOF), K_CHM obtained using the clinical history method, and the regression formulas K_RF1 and K_RF2 were calculated.

RESULTS

(1) A statistically significant difference (t = 155.164, P = 0.000) between SimK and K_GOF of 1.24 ± 0.12 D was observed, and there was a good correlation between SimK and K_GOF (r = 0.996, P = 0.000). The first regression formula (K_RF1 = 0.351 + 1.021 × K_GOF) was obtained using linear regression. (2) Statistically significant differences (t = 19.114, - 25.184, 4.702, and all P = 0.000) between SimK and K_CHM, K_GOF and K_CHM and K_RF1 and K_CHM of 0.75 ± 0.45 D, 0.96 ± 0.44 D and 0.18 ± 0.43 D, respectively, were obtained. Good correlations between SimK and K_CHM, K_GOF and K_CHM and K_RF1 and K_CHM (all r ≧ 0.977, all Ps = 0.000) were also observed. The regression formula (K_RF2 = - 1.204 + 1.027 × K_RF1) was obtained using linear regression. (3) Six methods were used for the prediction of IOL power in the postoperative group. The highest results were obtained from the Shammas formula (without preoperative data) combining K_m (obtained by IOLMaster) followed by the K_CHM and K_RF2 combining Haigis formula. The third was obtained from the K_CHM and K_RF2 combining Hoffer Q formula; and the smallest was the K_m combining Haigis formula.

CONCLUSION

The IOL power predicted by K_RF2 in eyes after myopic CRS may be accurate.

Comparison of Different Corneal Power Readings From Pentacam in Post-laser In Situ Keratomileusis Eyes

OBJECTIVES

To compare the various Pentacam-measured K-readings with the clinical history method (CHM) in eyes that have undergone myopic laser in situ keratomileusis (LASIK).

METHODS

In this prospective study, Pentacam examination was performed in 71 eyes 1 month after myopic LASIK. The true net power (TNP) 4 mm, total corneal refractive power (TCRP) 4 mm, equivalent K-reading (EKR) 4.0 mm, and EKR 4.5 mm obtained from the same scan were compared with the K derived from CHM.

RESULTS

The average baseline spherical equivalence was -5.44±2.38 D. After LASIK, the mean KCHM was 37.67±2.13 D, TCRP4mm was 37.14±1.79 D, TNP4mm was 36.88±1.76 D, EKR4.0mm was 37.58±1.94 D, and EKR4.5mm was 37.51±1.94 D. TCRP4mm, TNP4mm, and EKR4.5mm showed a statistically significant deviation from the KCHM, with the mean error being 0.53 D, 0.79 D, and 0.16 D, respectively (P<0.05). Only the EKR4.0mm showed no statistically significant difference from the KCHM (mean error 0.09 D, P=0.23). The EKR4.0mm also had the narrowest 95% limits of agreement (LoA) (-1.10 to +1.28 D), whereas both TCRP4mm and TNP4mm had a wider LoA (-0.88 to +1.95 D and -0.62 to +2.20 D, respectively). All four Pentacam K-readings had a strong and statistically significant correlation with the KCHM.

CONCLUSIONS

Using the CHM as reference, the EKR4.0mm demonstrated the closest agreement when compared with the EKR4.5mm, TNP4mm, and TCRP4mm obtained from the same scan.

Comparison of Holladay equivalent keratometry readings and anterior corneal surface keratometry measurements in keratoconus

PURPOSE

To compare the accuracy of the anterior corneal simulated keratometry (SimK) and the Holladay equivalent keratometry reading (EKR) provided by a Scheimpflug camera (Pentacam HR) with the keratometry (K) provided by a Placido system (T-Cone topography) in keratoconus and control eyes.

METHODS

This prospective study included 40 consecutive patients with keratoconus and 40 voluntary participants with no ocular complaints. Any patients with corneal scar, corneal trauma, history of corneal surgery or contact lens usage were excluded from the study. Mean SimK and Holladay EKR measurements were taken with Pentacam HR in the 2, 3, and 4.5 mm corneal zones, and these values were compared with the T-Cone mean K value with the Placido topography system attachment on the Lenstar LS 900. Statistical analysis was performed using the paired Student's t test and the Bland-Altman analysis.

RESULTS

A statistically significant difference was determined between the Placido K and the Scheimpflug EKR 2, 3, 4.5 mm and SimK values in the keratoconus group (p < 0.05). In the analyses which showed a difference between the SimK and Holladay EKR, it was observed that as the diameter of the corneal zone increased, the 95% LoA values were extended. No statistically significant difference was determined between the SimK and EKR 2 mm values (p > 0.05). In the control group, there was no statistically significant difference between any of the keratometric values.

CONCLUSION

In diseases which affect the posterior corneal surface, such as keratoconus, it is thought that because of the asymmetrical peripheral placement of the corneal apex, as the corneal diameter increases there could be an error increase of 1-3 mm in keratometric systems evaluating the anterior surface.

Comparison of Corneal Power and Astigmatism between Simulated Keratometry, True Net Power, and Total Corneal Refractive Power before and after SMILE Surgery

Purpose. To compare the mean corneal power (Km) and total astigmatism (Ka) estimated by three methods: simulated keratometry (simK), true net power (TNP), and total corneal refractive power (TCRP) before and after femtosecond laser small incision lenticule extraction (SMILE) surgery. Methods. A retrospective, cross-sectional study. SimK, TNP, and TCRP from a Scheimpflug analyzer were obtained from 144 patients before and 6 months after SMILE surgery. Km and Ka were recorded as the mean of individual paracentral rings of 1.0 to 8.0 mm (R1 to R8). The surgically induced changes in Km (delta-simK, delta-TNP, and delta-TCRP) and Ka (delta-simKa, delta-TNPa, and delta-TCRPa) were compared to the changes in spherical equivalent of the cycloplegic refraction (delta-SE) and astigmatism (delta-RA). Results. Preoperatively, astigmatism values were greatest with simKa from R1 to R5 and greatest with TCRPa from R6 to R8. Astigmatism values were smallest with TNPa from R1 to R7. Postoperatively, astigmatism values were greatest with simKa from R1 to R5 and greatest with TCRPa from R6 to R8. Delta-TCRP₃ and Delta-TCRP₄ matched delta-SE most closely, and delta-TCRPa₃ matched delta-RA most closely. Conclusions. TCRP proved to be the most accurate method in estimating corneal power and astigmatism both before and after SMILE surgery.

Evaluation of Equivalent Keratometry Readings Obtained by Pentacam HR (High Resolution)

Purpose

To assess the repeatability of Equivalent Keratometry Readings (EKRs) obtained by the Pentacam HR (high resolution) in untreated and post-LASIK eyes, and to compare them with the keratometry (K) values obtained by other algorithms.

Methods

In this prospective study, 100 untreated eyes and 71 post-LASIK eyes were included. In the untreated group, each eye received 3 consecutive scans using the Pentacam HR, and EKR values in all central corneal zone, the true net power (K_net) and the simulated K (SimK) were obtained for each scan. In the post-LASIK group, each eye received subjective refraction and 3 consecutive scans with the Pentacam HR preoperatively. During the 3-month post-surgery exam, the same examinations and the use of an IOLMaster were conducted for each eye. The EKRs in all zone, the K_net, the mean K (K_m) by IOLMaster and the K values by clinical history method (K_CHM) were obtained. The repeatability of the EKRs was assessed by the within-subject standard deviation (S_w), 2.77S_w, coefficient of variation (CV_w) and intraclass correlation coefficient (ICC). The bonferroni corrected multiple comparisons were performed to analyze the differences among the EKRs and K values calculated by other algorithms within the 2 groups. The 95% limits of agreement (LoA) were calculated.

Results

The EKR values in all central corneal zone were repeatable in both the untreated group (S_w≦0.19 D, 2.77S_w≦0.52 D, CV_w≦1%, ICC≧0.978) and the post-LASIK group (S_w≦0.22 D, 2.77S_w≦0.62 D, CV_w≦1%, ICC≧0.980). In the untreated group, the EKR in 4mm zone was close to SimK (P = 1.000), and the 95% LoA was (-0.13 to 0.15 D). The difference between K_net and SimK was -1.30±0.13 D (95% LoA -1.55 to -1.55 D, P<0.001). In the post-LASIK group, all the EKRs were significantly higher than K_CHM (all P<0.001). The differences between the EKR in 4mm zone and K_CHM, the EKR in 7mm zone and K_CHM, K_net and K_CHM, K_m and K_CHM, SimK and K_net were 0.64±0.50 D (95% LoA, -0.33 to 1.62 D), 1.77±0.88 D (95% LoA, 0.04 to 3.51 D), -0.98±0.48 D (95% LoA, -1.92 to -0.04 D), 0.64±0.53 D (95% LoA, -0.40 to 1.68 D), and 1.73±0.20 D (95% LoA, 1.33 to 2.13 D), respectively.

Conclusions

The EKRs obtained by the Pentacam HR were repeatable in both untreated eyes and post-LASIK eyes. Compared to the total corneal power obtained by the clinical history method, the EKR values generally overestimated the total corneal power in post-LASIK eyes. So, further calibrations for the EKR values should be conducted, before they were used for the total corneal power assessment in post-LASIK eyes.

Precision (Repeatability and Reproducibility) and Agreement of Corneal Power Measurements Obtained by Topcon KR-1W and iTrace

Purpose

To evaluate the repeatability and reproducibility of corneal power measurements obtained by Topcon KR-1W and iTrace, and assess the agreement with measurements obtained by Allegro Topolyzer and IOLMaster.

Methods

The right eyes of 100 normal subjects were prospectively scanned 3 times using all the 4 devices. Another observer performed additional 3 consecutive scans using the Topcon KR-1W and iTrace in the same session. About one week later, the first observer repeated the measurements using the Topcon KR-1W and iTrace. The steep keratometry (Ks), flat keratometry (Kf), mean keratometry (Km), J0 and J45 were analyzed. Repeatability and reproducibility of measurements were evaluated by the within-subject standard deviation (Sw), coefficient of variation (CoV), test-retest repeatability (2.77Sw), and intraclass correlation coefficient (ICC). Agreements between devices were assessed using Bland-Altman analysis and 95% limits of agreement (LoA).

Results

Intraobserver repeatability and interobserver and intersession reproducibility of the Ks, Kf and Km showed a CoV of no more than 0.5%, a 2.77Sw of 0.70 D or less, and an ICC of no less than 0.99. However, J0 and J45 showed poor intraobserver repeatability and interobserver and intersession reproducibility (all ICCs not greater than 0.446). Statistically significant differences existed between Topcon KR-1W and IOLMaster, Topcon KR-1W and iTrace, Topcon KR-1W and Topolyzer, iTrace and Topolyzer, iTrace and IOLMaster for Ks, Kf and Km measurements (all P < 0.05). The mean differences between Topcon KR-1W, iTrace, and the other 2 devices were small. The 95% LoA were approximately 1.0 D to 1.5 D for all measurements.

Conclusions

The Ks, Kf and Km obtained by Topcon KR-1W and iTrace showed excellent intraobserver repeatability and interobserver and intersession reproducibility in normal eyes. The agreement between Topcon KR-1W and Topolyzer, Topcon KR-1W and IOLMaster, iTrace and Topolyzer, iTrace and IOLMaster, Topcon KR-1W and iTrace were not so good, they should not be interchangeable in clinical application. Given that the intraobserver repeatability and interobserver and intersession reproducibility of corneal astigmatism measurements obtained by Topcon KR-1W and iTrace were poor, it should be cautious that Topcon KR-1W and iTrace were applied for the preparation of toric lens implantation.

Keratometric index obtained by Fourier-domain optical coherence tomography

PURPOSE

To determine the keratometric indices calculated based on parameters obtained by Fourier-domain optical coherence tomography (FD-OCT).

METHODS

The ratio of anterior corneal curvature to posterior corneal curvature (Ratio) and keratometric index (N) were calculated within central 3 mm zone with the RTVue FD-OCT (RTVue, Optovue, Inc.) in 186 untreated eyes, 60 post-LASIK/PRK eyes, and 39 keratoconus eyes. The total corneal powers were calculated using different keratometric indices: Kcal based on the mean calculated keratometric index, K1.3315 calculated by the keratometric index of 1.3315, and K1.3375 calculated by the keratometric index of 1.3375. In addition, the total corneal powers based on Gaussian optics formula (Kactual) were calculated.

RESULTS

The means for Ratio in untreated controls, post-LASIK/PRK group and keratoconus group were 1.176 ± 0.022 (95% confidence interval (CI), 1.172-1.179), 1.314 ± 0.042 (95%CI, 1.303-1.325) and 1.229 ± 0.118 (95%CI, 1.191-1.267), respectively. And the mean calculated keratometric index in untreated controls, post-LASIK/PRK group and keratoconus group were 1.3299 ± 0.00085 (95%CI, 1.3272-1.3308), 1.3242 ± 0.00171 (95%CI, 1.3238-1.3246) and 1.3277 ± 0.0046 (95%CI, 1.3263-1.3292), respectively. All the parameters were normally distributed. The differences between Kcal and Kactual, K1.3315 and Kactual, and K1.3375 and Kactual were 0.00 ± 0.11 D, 0.21 ± 0.11 D and 0.99 ± 0.12 D, respectively, in untreated controls; -0.01 ± 0.20 D, 0.85 ± 0.18 D and 1.56 ± 0.16 D, respectively, in post-LASIK/PRK group; and 0.03 ± 0.67 D, 0.56 ± 0.70 D and 1.40 ± 0.76 D, respectively, in keratoconus group.

CONCLUSION

The calculated keratometric index is negatively related to the ratio of anterior corneal curvature to posterior corneal curvature in untreated, post-LASIK/PRK, and keratoconus eyes, respectively. Using the calculated keratometric index may improve the prediction accuracies of total corneal powers in untreated controls, but not in post-LASIK/PRK and keratoconus eyes.

Agreement between clinical history method, Orbscan IIz, and Pentacam in estimating corneal power after myopic excimer laser surgery

The purpose of this study was to investigate the agreement between the clinical history method (CHM), Orbscan IIz, and Pentacam in estimating corneal power after myopic excimer laser surgery. Fifty five patients who had myopic LASIK/PRK were recruited into this study. One eye of each patient was randomly selected by a computer-generated process. At 6 months after surgery, postoperative corneal power was calculated from the CHM, Orbscan IIz total optical power at the 3.0 and 4.0 mm zones, and Pentacam equivalent keratometric readings (EKRs) at 3.0, 4.0, and 4.5 mm. Statistical analyses included multilevel models, Pearson’s correlation test, and Bland-Altman plots. The Orbscan IIz 3.0-mm and 4.0 mm total optical power, and Pentacam 3.0-mm, 4.0-mm, and 4.5-mm EKR values had strong linear positive correlations with the CHM values (r = 0.90–0.94, P = <0.001, for all comparisons, Pearson’s correlation). However, only Pentacam 3.0-mm EKR was not statistically different from CHM (P = 0.17, multilevel models). The mean 3.0- and 4.0-mm total optical powers of the Orbscan IIz were significantly flatter than the values derived from CHM, while the average EKRs of the Pentacam at 4.0 and 4.5 mm were significantly steeper. The mean Orbscan IIz 3.0-mm total optical power was the lowest keratometric reading compared to the other 5 values. Large 95% LoA was observed between each of these values, particularly EKRs, and those obtained with the CHM. The width of the 95% LoA was narrowest for Orbscan IIz 3.0-mm total optical power. In conclusion, the keratometric values extracted from these 3 methods were disparate, either because of a statistically significant difference in the mean values or moderate agreement between them. Therefore, they are not considered equivalent and cannot be used interchangeably.

Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery in the absence of clinical history

BACKGROUND AND OBJECTIVE

To identify the most accurate combination of Pentacam's equivalent keratometry readings (EKR) and intraocular lens power formula when the clinical history is unavailable.

PATIENTS AND METHODS

A total of 18 patients underwent cataract surgery after refractive surgery. The Pentacam 4.5- and 3.0-mm EKR were combined with the SRK II, SRK/T, Hoffer-Q, and Holladay I and II formulas.

RESULTS

The smallest deviation from the predicted value was achieved by combining the 4.5 EKR with the Holladay II formula (mean arithmetic deviation, -0.2 ± 0.4 dpt).

CONCLUSION

The 4.5-mm EKR + Holladay II formula can accurately calculate intraocular lens power in patients with previous refractive surgery.

Evaluation of the Pentacam ray tracing method for the measurement of central corneal power after myopic photorefractive keratectomy

PURPOSE

The study evaluated the ray tracing method [total corneal refractive power (TCRP)] in a Pentacam apparatus (Oculus, Wetzlar, Germany) for postoperative keratometry measurement after myopic photorefractive keratectomy (PRK).

METHODS

Manifest refraction (MR) and Pentacam analyses were performed preoperatively and at 6 months postoperatively after the PRK (STAR S4 IR CustomVue; Abbott Medical Optics/Visx) in 49 right eyes from 49 patients (age, 25.42 ± 3.51 years). Postoperative corneal power was calculated using the clinical history method (CHM) and compared with postoperatively measured simulated keratometry (simK), true net power (TNP) at 3 mm, and pupil-centered TCRP at the center, 1, 3, and 4 mm (TCRP0, TCRP1, TCRP3, and TCRP4). Vertex-distance-adjusted refractive change (delta-MR) at the corneal plane was also compared with various keratometric changes (delta-K).

RESULTS

Postoperative TCRP0, TCRP1, TCRP3, and TCRP4 showed no significant difference compared with that of the CHM. Postoperative simK was significantly higher than that of the CHM, whereas the TNP was significantly lower compared with that of the CHM. The delta-Ks measured by simK, TNP, and TCRPs were significantly smaller than delta-MR, and delta-TCRP4 showed the least difference [mean ± SD, 0.28 ± 0.55 diopters (D)] with delta-MR. The 95% limit of agreement between delta-MR and delta-TCRP4 was -0.85 to 1.31 D. The difference between delta-TCRP4 and delta-MR was <0.5 D in 55.1% and <1.0 D in 87.8% of the eyes studied.

CONCLUSIONS

Although postoperative TCRPs showed no significant difference with CHM, delta-MR was still underestimated after myopic PRK.