Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery

Comparison of Corneal Power Difference Maps with Achieved Myopic Correction Using Scheimpflug Tomography After LASIK, PRK, and SMILE

Purpose

To compare corneal power difference maps (∆maps) obtained from the Pentacam in patients with 1 year follow-up after LASIK, PRK, and SMILE with further stratification to low, moderate, and high myopia.

Patients and methods

This retrospective study was comprised of patients who had preoperative and 1-year postoperative power maps that were obtained-front sagittal (SagF), refractive power (RP), true net power (TNP), and total corneal refractive power (TCRP)-for evaluation. Measurements were recorded and compared at the 4mm, 5mm and 6mm pupil and apex zones. Comparisons were made between each specific power ∆map and the surgically induced refractive change (SIRC). Further analysis of the ∆maps was performed based on degree of myopia (high, moderate, and low). Correlation and agreement were also assessed with regression and limits of agreement (LoA).

Results

There were 172 eyes in the LASIK group, 187 eyes in the PRK group, and 46 eyes in the SMILE group. In the LASIK group, TNP ∆map at 5mm pupil zone had the least absolute mean difference with SIRC (0.007 ± 0.42D). In the PRK group, TNP ∆map at 5mm apex zone was most accurate compared to SIRC (0.066 ± 0.45D). In the SMILE group, TCRP ∆map at 4mm apex zone had the closest absolute value when compared to SIRC (0.011 ± 0.50D). There was good correlation and agreement for all three surgery groups, LASIK: r = 0.975, LoA -0.83D to +0.83D, PRK: r = 0.96, LoA -0.83D and +0.95D, and SMILE: r = 0.922, LoA -0.97 D to +0.99D.

Conclusion

TNP ∆maps most accurately measured corneal power in the LASIK and PRK groups while TCRP ∆maps were most accurate in the SMILE group. The degree of myopia may change which ∆map is most accurate.

Excimer Laser Corneal Refractive Surgery in the Clinic: A Systematic Review and Meta-analysis

Objective

To systematically evaluate the efficacy, safety, recovery speed, and long-term visual quality of excimer laser corneal refractive surgery and to provide evidence-based medicine for the promotion and use of excimer laser corneal refractive surgery.

Methods

Randomized controls on excimer laser refractive surgery in Web of science, PubMed, EMBASE, ScienceDirect, Cochrane Library, China Knowledge Network (CNKI), China VIP Database, Wan Fang Database, and China Biomedical Literature Database (CBM) were searched by the computer. Randomized controlled trial (RCT) data were extracted independently by two researchers, and the risk of bias of each included RCT was assessed according to the Cochrane Handbook 5.1.0 criteria. Meta-analysis of the collected data was performed using RevMan5.4 statistical software.

Results

In the end, 9 high-quality literatures were included, with a total of 4366 samples, and meta-analysis was used. There was no significant difference in uncorrected visual acuity WMD after excimer laser keratorefractive surgery, but there was a statistically significant difference in WMD in the safety of excimer laser keratorefractive surgery. The results of uncorrected visual acuity (close) indicated the following: Chi2 = 13.56, DF = 5, P = 0.02, and I2 = 100%; the results of uncorrected visual acuity (distance) indicated the following: Chi2 = 34.44, DF =5 (P < 00000), and I2 = 85%; the results of best corrected visual acuity (myopia) indicated the following: Chi2 = 0.65, DF = 3, P = 088 > 0.05, and I2 = 0%; the results of best corrected visual acuity (hyperopia) indicated the following: Chi2 = 1.80, DF = 3, P = 0.61 > 0.05, and I2 = 0%.

Conclusion

Excimer laser corneal refractive surgery is safe and effective, with faster recovery and better long-term visual acuity treatment effect. However, more studies and follow-up with higher methodological quality and longer intervention time are needed for further validation.

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.

Comparison of refractive outcomes using Scheimpflug Holladay equivalent keratometry or IOLMaster 700 keratometry for IOL power calculation

PURPOSE

This study aims to compare postoperative refractive error results using Pentacam (Oculus Optikgeräte GmbH) Holladay equivalent keratometry readings (EKR) or IOLMaster 700 (Carl Zeiss Meditec AG) keratometry (K) values in IOL power calculation.

MATERIAL AND METHODS

This retrospective study included 54 eyes of 31 patients who underwent cataract surgery. Preoperative biometric measurements of all patients were obtained using IOLMaster 700 followed by Pentacam measurements. IOLMaster 700 K measurements on horizontal (K1) and vertical (K2) axes and EKR measurements on 2 mm (EKR2mm), 3 mm (EKR3mm) and 4.5 mm (EKR4.5 mm) corneal zones were recorded. EKR4.5 mm value and IOLMaster 700 K values were used in Holladay-II, SRK/T, Haigis, and Hoffer-Q formulas to calculate predictive refractive error (PRE). Absolute refractive error (ARE) was calculated as the absolute difference between actual postoperative refractive error (APRE) and PRE values.

RESULTS

Mean age was 72.2 ± 8.3 (51-87) years and mean IOL power was 21.5 ± 2.9 D (18-23 D). There was no significant difference between PRE values when IOLMaster 700 K measurements and EKR4.5 mm K measurements were used in Holladay-II, SRK/T, Haigis, and Hoffer-Q formulas (p = 0.571, p = 0.833, p = 0.165, p = 0.347, respectively). There was no significant difference between APRE and ARE values (p = 0.124). According to mean ARE results, the closest estimate was achieved when the IOLMaster 700 K values were used in the Holladay-II formula (p = 0.271).

CONCLUSION

IOLMaster 700 K measurement and Pentacam EKR4.5 mm measurements can be used interchangeably. IOLMaster 700 K values yielded the most predictive measurement of the refractive result using the Holladay-II formula.

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.

Surgically induced astigmatism in canines following sutured dorsonasal vs dorsotemporal clear corneal incisions

OBJECTIVES

To investigate use of the Pentacam® HR for evaluation of surgically induced corneal astigmatism (SIA) in canines undergoing bilateral phacoemulsification and determine differences between dorsonasal and dorsotemporal clear corneal incisions.

ANIMALS

Client-owned canines undergoing bilateral phacoemulsification.

PROCEDURES

Patients received anterior segment imaging pre-operatively, immediately post-operatively, and 2-4 months post-operatively (follow-up). Total corneal refractive power was used to determine SIA. Surgically induced astigmatism was compared between right and left eyes, representing dorsotemporal and dorsonasal incisions, respectively. Repeated measures analyses were used between time points and paired t test compared SIA between eyes.

RESULTS

Complete imaging series were obtained for seven patients. Follow-up imaging occurred at a median of 112 days (range 60-132 days) post-operatively. For repeated measures analyses, significant differences were found between pre- and immediate post-operative values (P < 0.01), and between immediate post-operative and follow-up values (P < 0.01). There was no significant difference between pre-operative and follow-up values. Surgically induced astigmatism was significantly different between right and left eyes, with values of 2.01 ± 1.24 D and 3.05 ± 1.58 D at 3 mm radius (P < 0.05), and 2.04 ± 1.18 D and 3.06 ± 1.27 D at 4 mm radius (P < 0.05) for dorsotemporal and dorsonasal incisions, respectively.

CONCLUSIONS

Preliminary investigation revealed improvement of corneal SIA 2-4 months post-operatively, but development of significantly more SIA in dorsonasal vs dorsotemporal incisions. This prompts consideration of patient or microscope rotation to create a more dorsotemporal incision when possible.

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.

Myopic Laser Corneal Refractive Surgery Reduces Interdevice Agreement in the Measurement of Anterior Corneal Curvature

OBJECTIVES

To investigate interdevice differences and agreement in the measurement of anterior corneal curvature obtained by different technologies after laser corneal refractive surgery.

METHODS

The prospective study comprised 109 eyes of 109 consecutive patients who had undergone laser-assisted in situ keratomileusis (LASIK). Preoperative and postoperative corneal parameters were measured by Scheimpflug imaging (Pentacam), Placido-slit-scanning (Orbscan) and auto-keratometry (IOLMaster). Preoperative and postoperative anterior corneal curvatures (K readings) were compared between devices. Interdevice agreement was evaluated by Bland-Altman analysis.

RESULTS

Preoperatively, the difference of K reading for Pentacam-IOLMaster (0.04±0.20 D) was not statistically significant (P=0.059). The differences between Pentacam-Orbscan and Orbscan-IOLMaster were 0.20±0.34 D (P<0.001) and -0.17±0.29 D (P<0.001), respectively. After surgery, no difference was found for Pentacam-Orbscan (-0.05±0.38, P=0.136). The differences between Pentacam-IOLMaster and Orbscan-IOLMaster were 0.13±0.29 D (P<0.001) and 0.19±0.34 D (P<0.001). Preoperative interdevice agreement (95% limit of agreement [LOA]) between Pentacam and Orbscan, Pentacam and IOLMaster, and Orbscan and IOLMaster were 1.31 D, 0.79 D and 1.14 D, respectively. The 95% LOAs decreased to 1.47 D, 1.14 D, and 1.34 D after refractive surgery.

CONCLUSION

Corneal refractive surgery changed the preoperative and postoperative interdevice differences in corneal curvature measurements and reduced interdevice agreement, indicating that the devices are not interchangeable.

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.

Corneal power evaluation after myopic corneal refractive surgery using artificial neural networks

Background

Efficacy and high availability of surgery techniques for refractive defect correction increase the number of patients who undergo to this type of surgery. Regardless of that, with increasing age, more and more patients must undergo cataract surgery. Accurate evaluation of corneal power is an extremely important element affecting the precision of intraocular lens (IOL) power calculation and errors in this procedure could affect quality of life of patients and satisfaction with the service provided. The available device able to measure corneal power have been tested to be not reliable after myopic refractive surgery.

Methods

Artificial neural networks with error backpropagation and one hidden layer were proposed for corneal power prediction. The article analysed the features acquired from the Pentacam HR tomograph, which was necessary to measure the corneal power. Additionally, several billion iterations of artificial neural networks were conducted for several hundred simulations of different network configurations and different features derived from the Pentacam HR. The analysis was performed on a PC with Intel^® Xeon^® X5680 3.33 GHz CPU in Matlab^® Version 7.11.0.584 (R2010b) with Signal Processing Toolbox Version 7.1 (R2010b), Neural Network Toolbox 7.0 (R2010b) and Statistics Toolbox (R2010b).

Results and conclusions

A total corneal power prediction error was obtained for 172 patients (113 patients forming the training set and 59 patients in the test set) with an average age of 32 ± 9.4 years, including 67% of men. The error was at an average level of 0.16 ± 0.14 diopters and its maximum value did not exceed 0.75 dioptres. The Pentacam parameters (measurement results) providing the above result are tangential anterial/posterior. The corneal net power and equivalent k-reading power. The analysis time for a single patient (a single eye) did not exceed 0.1 s, whereas the time of network training was about 3 s for 1000 iterations (the number of neurons in the hidden layer was 400).

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.

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.

Intraocular lens power calculation following laser refractive surgery

Refractive outcomes following cataract surgery in patients that have previously undergone laser refractive surgery have traditionally been underwhelming. This is related to several key issues including the preoperative assessment (keratometry) and intraocular lens power calculations. Peer-reviewed literature is overwhelmed by the influx of methodology to manipulate the corneal or intraocular lens (IOL) powers following refractive surgery. This would suggest that the optimal derivative formula has yet been introduced. This review discusses the problems facing surgeons approaching IOL calculations in these post-refractive laser patients, the existing formulae and programs to address these concerns. Prior published outcomes will be reviewed.

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.

Scheimpflug analysis of corneal power changes after myopic excimer laser surgery

PURPOSE

To assess the ability of corneal power measurements by a rotating Scheimpflug camera to measure the refractive change induced by myopic excimer laser surgery.

SETTING

G.B. Bietti Foundation-IRCCS, Rome, Italy.

DESIGN

Evaluation of diagnostic test.

METHODS

The following corneal power measurements by the Pentacam Scheimpflug camera were analyzed: average keratometry (K), true net power (calculated by Gaussian optics formula), and total corneal refractive power (TCRP) at 2.0 mm, 3.0 mm, and 4.0 mm, calculated by ray tracing on a ring and as the average of the zone inside the ring. The difference between the preoperative and postoperative values was compared with the subjective surgically induced refractive change (SIRC) and with the difference between the preoperative and the postoperative anterior corneal power measured by Placido corneal topography (Keratron).

RESULTS

In 36 consecutive eyes, the average K significantly underestimated the SIRC as determined by subjective refraction (-4.47 diopters [D] ± 1.81 [SD]) and corneal topography (-4.38 ± 1.81 D). The 3.0 mm and 4.0 mm ring total corneal refractive power significantly overestimated the SIRC. The remaining values did not show statistically significant differences with respect to the SIRC. The 3.0 mm zone TCRP and the 2.0 mm ring TCRP provided the lowest median difference compared with the SIRC (-0.07 D and -0.17 D, respectively) and the closest agreement.

CONCLUSIONS

The corneal power values provided by the Scheimpflug camera accurately reflected the SIRC after myopic excimer laser surgery. The best options seem to be the 3.0 mm zone TCRP and the 2.0 mm ring TCRP.

Orbscan II and double-K method for IOL calculation after refractive surgery

BACKGROUND

Precise IOL calculation in post-refractive surgery patients is still a challenge for the cataract surgeon. The purpose of this study is to test whether adding Orbscan II values into the double-K method improves IOL calculation in this group of patients.

METHODS

A prospective study with 43 eyes previously submitted to refractive surgery that underwent cataract extraction. IOL calculation was performed with double-K method. Post-K value was derived from Orbscan total-mean power map. The average corneal curvature of the general population (43.8D) was used as the pre-K value. Refraction results 30 days after surgery were compared with refraction that would be obtained if we used: (1) post-K values from keratometry, (2) post-K values from topography, and (3) pre-K values from Orbscan total-mean power. Anterior chamber depth measures obtained with the IOL Master and Orbscan II were compared.

RESULTS

Mean postoperative spherical equivalent (SE) was -0.25 ± 1.10 D in eyes submitted to radial keratotomy , -1.04 ± 1.42 D in eyes previously submitted to myopic Lasik, and +0.05 ± 1.76 D in those submitted to hyperopic surgeries. Had we inputted post-K values derived from keratometer and from topography, we would have obtained significantly higher postoperative refractive errors in eyes previously submitted to myopic refractive surgery (p < 0.05). Refractions using pre-K derived from the central 8 mm Orbscan instead of 43.8 D were similar in all studied groups (p > 0.05). Anterior chamber depth measured with IOL Master or Orbscan were similar.

CONCLUSIONS

Orbscan measurements used as the post-K values into the double-K method provide a precise IOL calculation, especially in post myopic refractive surgery patients.

Scheimpflug camera measurement of anterior and posterior corneal curvature in eyes with previous radial keratotomy

PURPOSE

To compare the anterior and posterior corneal curvature in eyes with previous radial keratotomy (RK) to normal unoperated eyes.

METHODS

In this retrospective observational case series, 29 eyes from 29 consecutive patients were analyzed and compared to a control group of 71 unoperated eyes. Corneal imaging was obtained by a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH). Anterior and posterior corneal curvature radii were measured at the 3-mm zone.

RESULTS

The mean anterior and posterior corneal radii were 9.54 ± 0.89 and 8.54 ± 1.01 mm, respectively, both values being significantly higher than in the control group (7.81 ± 0.28 and 6.40 ± 0.24 mm, respectively, P<.0001). The mean anterior-to-posterior corneal curvature ratio was 1.12 ± 0.07, a value significantly lower than in the control group (1.22 ± 0.03, P<.0001). Mean corneal flattening was more evident in the posterior (33.44%) than in the anterior (22.15%) corneal curvature. The mean keratometric index, as calculated with the Gullstrand equation for thick lenses, was 1.3319 ± 0.0026, a value significantly higher than in the control group (1.3281 ± 0.0011, P<.0001). Linear regression detected a significant and directly proportional relationship between the number of radial incisions and flattening of both corneal surfaces (P<.0001).

CONCLUSIONS

After RK, both corneal surfaces flatten but do not deform in parallel as commonly accepted, as shown by the fact that the anterior-to-posterior corneal curvature ratio decreases. This finding invalidates the standard keratometric index and thus has relevant implications for intraocular lens power calculation in RK eyes.

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.

Intraocular lens power calculation after laser refractive surgery: corrective algorithm for corneal power estimation

PURPOSE

To evaluate an algorithm for corneal power estimation in intraocular lens (IOL) power calculation after myopic laser refractive surgery using direct corneal measurements.

SETTING

International Vision Correction Research Centre, University of Heidelberg, Heidelberg, Germany.

METHODS

Corneal parameters in normal eyes and eyes of refractive surgery cases were evaluated by rotating Scheimpflug imaging. Corneal optical power (K(optical)) calculated by a Gaussian optics formula was simplified as K(optical) = K(anterior) + K(2) (K(anterior) = anterior corneal power; K(posterior) = posterior corneal power; K(2) = K(posterior)--K(anterior) x K(posterior) x corneal thickness/1.376). The variation and change in K(2) induced by refractive surgery were analyzed. A corrective algorithm to calculate K(optical) using mean K(2) (-6.10 diopters [D]), K(corrective) = 1.114 x measured K - 6.10, was derived based on statistical analysis, which was in accordance with the modified Maloney method. The IOL power after refractive surgery was calculated using K(corrective).

RESULTS

The mean K(2) of normal and post-refractive corneas was -6.10 +/- 0.23 D and -6.16 +/- 0.17 D, respectively (P = .17). The mean refractive surgery-induced change in K(2) was -0.06 +/- 0.10 D. The variations in K(2) were small (95% confident interval, -6.55 to -5.65 [normal cornea]; -6.48 to -5.70 [pre-refractive]; - 6.49 to -5.83 [post-refractive)]. Using K(corrective) for IOL power calculation in post-refractive cases yielded mean absolute prediction errors of 0.58 +/- 0.52 D (Haigis), 0.59 +/- 0.49 D (double-K Hoffer Q), and 0.58 +/- 0.47 D (double-K SRK/T).

CONCLUSION

The algorithm that induced low error in corneal power estimation was relatively reliable in IOL calculation after myopic laser refractive surgery.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery

PURPOSE

To present the prospective application of the Orbscan II central 2-mm total-mean corneal power obtained by quantitative area topography in intraocular lens (IOL) calculation after refractive surgery.

METHODS

Calculated and achieved refraction and the difference between them were studied in 77 eyes of 61 patients with previous radial keratotomy (RK), RK and additional surgeries, myopic LASIK, myopic photorefractive keratectomy (PRK), or hyperopic LASIK who underwent phacoemulsification without complications in 3 eye centers. All IOL calculations used the average from the central 2-mm Orbscan II total-mean power of maps centered on the pupil without the use of previous refractive data. Six IOL styles implanted within the bag were used.

RESULTS

Using the SRK-T formula, the overall calculated refraction was -0.64+/-0.93 diopters (D). The overall achieved spherical equivalent refraction (-0.52+/-0.79 D; range: -3.12 to 1.25 D; 95% confidence interval [CI]: -0.70/-0.34 D) was +/-0.50 D in 53% of eyes, +/-1.00 D in 78% of eyes, and +/-2.00 D in 99% of eyes. The overall difference between the calculated and achieved refraction (0.12+/-0.93 D, P=.27; range: -2.18 to 2.62 D; 95% CI: 0.09/0.33 D) was +/-0.50 D in 39% of eyes, +/-1.00 D in 77% of eyes, and +/-2.00 D in 96% of eyes. This difference was +/-1.00 D in 77% of eyes with RK (P=.70), 82% of eyes with myopic LASIK (P=.34), and 90% of eyes with myopic PRK (P=.96). In eyes with RK followed by LASIK, a trend toward undercorrection was noted (P=.03). In eyes with hyperopic LASIK, a trend toward overcorrection was noted (P=.005).

CONCLUSIONS

In eyes with previous corneal refractive surgery, IOL power calculation can be performed with reasonable accuracy using the Orbscan II central 2-mm total-mean power. This method had better outcomes in eyes with previous RK, myopic LASIK, and myopic PRK than in eyes with hyperopic LASIK or RK with LASIK.

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.