- Email: [email protected]
Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery
ARTICLE
Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery Giacomo Savini, MD, Piero Barboni, MD, Vincenzo Profazio, Maurizio Zanini, MD, Kenneth J. Hoffer, MD
PURPOSE: To evaluate corneal power measurements by a rotating Scheimpflug camera (Pentacam, Oculus) in eyes that have had myopic excimer laser surgery. SETTING: Private practice, Bologna, Italy. METHODS: This prospective comparative interventional case series comprised 16 eyes of 16 patients who had myopic excimer laser surgery and for whom all perioperative data were available. Four corneal power measurements obtained with the Pentacam (simulated keratometry, true net power, equivalent K reading, and BESSt formula) were analyzed and compared with values derived using the clinical history method and 2 other formulas for calculating corneal power after refractive surgery (modified keratometric refractive index according to Savini et al. and separate consideration of the anterior and posterior corneal curvatures according to Speicher). RESULTS: Analysis of variance showed a statistically significant difference between all methods (P<.0001). Bonferroni multiple comparison tests showed that the only Pentacam measurements not statistically different from the corneal power values derived using the clinical history method were the equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm and those derived with the BESSt formula; however, considerably large 95% limits of agreement (LoA) were calculated between each of these values and those obtained with the clinical history method. CONCLUSIONS: The Pentacam device gave corneal power measurements that did not statistically significantly had differ from those predicted by the clinical history method in eyes that had previous myopic excimer laser surgery. Wide LoA are a potential source of error in intraocular lens power calculation in such patients. J Cataract Refract Surg 2008; 34:809–813 Q 2008 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and laser-assisted subepithelial keratectomy (LASEK) can correct myopia by decreasing the anterior corneal surface curvature. Because they alter the natural ratio between the anterior and
Accepted for publication January 21, 2008. From a private practice (Savini, Barboni, Profazio, Zanini), Bologna, Italy, and Jules Stein Eye Institute (Hoffer), University of California, Los Angeles, California, USA. No author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Giacomo Savini, MD, Centro Salus, Via d’Azeglio 5, 40123, Bologna, Italy. E-mail: [email protected]. Q 2008 ASCRS and ESCRS Published by Elsevier Inc.
posterior corneal curvatures, these procedures render invalid the conventional keratometric index of refraction (usually 1.3375) used by most topographers and keratometers to convert the measured radius into diopters.1 These instruments thus cannot correctly calculate the corneal power and usually give a measurement that is higher than the actual value. Overestimating corneal power leads to postoperative hyperopia when intraocular lens (IOL) power must be calculated for eyes that will have cataract surgery.2 An increasing number of methods have been devised to overcome the problem of correctly measuring corneal power after excimer laser surgery.3 All rely on several assumptions to arrive at an indirect determination of corneal power. Most also require preoperative data such as the attempted correction or the original corneal power, and none can be reliably adopted for 0886-3350/08/$dsee front matter doi:10.1016/j.jcrs.2008.01.012
809
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CORNEAL POWER MEASUREMENTS WITH PENTACAM AFTER EXCIMER LASER SURGERY
every patient. The ideal solution would be to find a technology that enables the surgeon to measure the correct corneal power directly, without relying on any assumptions. This would necessarily entail measuring the posterior corneal curvature so that the Gaussian optics formula could be used to calculate the corneal power and so the standard keratometric index would no longer be necessary. Two instruments with this ability are currently available: the Orbscan II (Bausch & Lomb) and the Pentacam (Oculus). The former is a horizontal slit-scanning system, whereas the latter is a rotating Scheimpflug camera. Promising results have been reported for the repeatability of several of the Pentacam’s measurements.4–6 The aim of this study was to assess measurements performed with the Pentacam in eyes that had myopic LASIK, PRK, or LASEK and compare the measurements with the values obtained using the gold standard proceduredthe clinical history methoddand 2 additional methods; that is, separate evaluation of the anterior and posterior corneal curvatures and the Savini formula for calculating the postoperative keratometric index on the basis of attempted correction.7–9 PATIENTS AND METHODS Between April and June 2007, 16 consecutive patients who had LASIK, PRK, or LASEK to correct myopia at a private practice (Centro Salus, Bologna, Italy) were evaluated. Preoperative data were available for all eyes. The minimum required follow-up was 1 month for LASIK and 3 months for LASEK and PRK. Before being included in the study, all patients were informed of its purpose and gave written consent. The study methods adhered to the tenets of the Declaration of Helsinki for the use of human participants in biomedical research. One eye of each patient was randomly selected and evaluated by videokeratography (TMS-2 topography system, Tomey) and Scheimpflug camera imaging (Pentacam). The surgically induced refractive correction, which is necessary to calculate corneal power according to the clinical history method, was determined after the postoperative refraction was measured under cycloplegia. Cycloplegia was induced by tropicamide 1% instilled twice, 5 minutes apart, as suggested by Hofmeister et al.10 Corneal power was calculated according to 3 methods: the clinical history method, used as the benchmark for comparison; separate evaluation of the anterior and posterior corneal curvatures, as described by Speicher8; and the corrected keratometric index (modified on the basis of attempted correction), as described by Savini et al.9 To avoid miscalculations due to poor videokeratography quality, both preoperative and postoperative examinations were performed immediately after blinking and were carefully inspected before being included in the study. For each eye, only 1 preoperative and 1 postoperative videokeratography with no artifacts (eg, incomplete or irregular corneal rings) were considered for further analysis. The following values provided by the Pentacam device were analyzed and compared with those obtained with the clinical history method: mean simulated keratometry (K);
mean true net power (ie, corneal power calculated with the Gaussian optics formula using the anterior and posterior corneal radii and the corneal thickness); BESSt formula, as described by Borasio et al.11; and equivalent K reading shown in the Holladay report (J.T. Holladay, MD, ‘‘Measuring Corneal Power After Corneal Refractive Surgery,’’ Cataract & Refractive Surgery Today [suppl], January 2006, pages 5–6). Regarding the equivalent K reading, the values given by 2 versions of the Pentacam software were recorded before this study commenced. The first version, 1.14r46, provides only 1 value measured along a ring with a diameter of 4.0 mm. The second version, 1.16, has been available since June 2007 and provides several values measured along rings with diameters of 1.0, 2.0, 3.0, 4.0, 4.5, 5.0, 6.0, and 7.0 mm. (The measurements at 6.0 mm and 7.0 mm were not included in the analysis.) For each eye, only good-quality Scheimpflug images (determined when the quality specification provided by the instrument was ‘‘OK’’) were used. A secondary objective of the study was to compare the estimated preoperative K value (calculated based on the newer version of the Holladay report) and the actual preoperative value. Statistical analyses were performed using GraphPad InStat for Macintosh (version 3a, GraphPad Software). A 1-way analysis of variance (ANOVA) for repeated measures with Bonferroni multiple comparisons was used to compare all corneal power measurements. Preliminary analysis showed that all assumptions required by the ANOVA model were satisfied, including the independence of cases, Gaussian bell-shaped distribution (assessed by KolmogorovSmirnov test), and homogeneity of variance (assessed by Bartlett’s test). A P value less than 0.05 was considered statistically significant. The agreement between the clinical history method and other methods in measuring the mean corneal power was analyzed according to the method described by Bland and Altman,12 who suggest plotting the differences between measurements (y-axis) against their mean (x-axis). The 95% limits of agreement (LoA) were defined as means G2 SD of the differences between the 2 measurement techniques.
RESULTS The mean age of the patients was 37 years G 5.8 (SD). The mean preoperative refractive error was –5.1 G 1.4 diopters. Examinations were performed a mean of 519 G 614 days (range 35 to 2310 days) after surgery. Table 1 shows the corneal power measurements with each method. Overall, the ANOVA P value was significant (P!.0001). The Bonferroni multiple comparisons test did not detect significant differences between the corneal powers determined with the clinical history method, the separate evaluation of the anterior and posterior corneal curvatures, and the corneal power calculated with the Savini et al. formula for correcting the keratometric index of refraction. In contrast, most values provided by the Pentacam camera were statistically significantly different. The mean simulated K reading (which was the same with both software versions) and the mean equivalent K reading given by the older software were significantly higher than the clinical history method–derived values as
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Table 1. Corneal power measured by different methods.
Corneal Power Measurement SimK by TMS-2 CHM ACPCC Savini SimK by Pentacam EKR 1.0 mm EKR 2.0 mm EKR 3.0 mm EKR 4.0 mm EKR 4.5 mm EKR True net power BESSt formula
Mean G SD (D)
P Value*
38.12 G 0.99 37.69 G 1.09 37.56 G 1.10 37.56 G 1.10 38.38 G 1.18 38.44 G 1.21 37.46 G 1.77 37.58 G 1.66 37.82 G 1.49 38.16 G 1.24 38.40 G 1.13 36.54 G 1.34 38.05 G 1.41
!.05 d NS NS !.001 !.001 NS NS NS !.01 !.001 !.001 NS
Mean Difference Vs CHM (D) 0.42 d 0.14 0.15 0.69 0.75 0.23 0.12 0.13 0.46 0.70 1.16 0.36
95% LoA Vs CHM (D) 0.18 to C1.42 d 0.70 to C0.42 0.65 to C0.34 0.18 to C1.55 0.20 to 1.69 1.93 to C1.47 1.58 to C1.35 0.99 to C1.25 0.35 to C1.27 0.11 to C1.51 2.21 to 0.10 0.68 to C1.40
ACPCC Z separate evaluation of anterior and posterior corneal curvatures; CHM Z clinical history method; EKR Z equivalent K reading; LOA Z limits of agreement; NS Z not significant; SimK Z simulated keratometry * Bonferroni multiple comparisons with the clinical history method
well as the 4.0 mm and 4.5 mm equivalent K readings given by the newer software; the true net power (which was the same with both software versions) was significantly lower than the corneal power determined using the clinical history method; no statistically significant differences were detected between the 1.0 mm, 2.0 mm, and 3.0 mm equivalent K reading values and the values calculated by the BESSt formula and from the clinical history method. The 2.0 mm and 3.0 mm equivalent K reading values showed the smallest difference (0.11 D lower and 0.13 D higher, respectively, than the clinical history method–derived value). Analysis of the 95% LoA according to Bland and Altman showed that the separate evaluation of the anterior and posterior corneal curvatures and the Savini et al. method yielded the highest agreement with the clinical history method, whereas the Pentacam provided measurements that differed considerably (Table 1). Comparison of the estimated preoperative K value and the actual preoperative value showed no statistically significant differences by a paired t test as the mean values were 43.04 G 1.01 D and 43.00 G 1.19 D, respectively. However, the Bland–Altman plot showed only a moderate level of agreement between the 2 values as the 95% LoA ranged from 1.43 to C1.50 D. DISCUSSION Conventional videokeratography is considered an accurate and repeatable technique to measure the curvature and power of the central cornea,13 especially if the
post-blink interval of image capture is standardized to avoid the influence of changes in tear-film stability.14,15 Unfortunately, corneal power measurements by videokeratography are no longer valid after excimer laser surgery.1 Clinical and theoretical studies3,16–18 suggest that the clinical history method is best for calculating corneal power after laser surgery; for this reason, we accepted it as the gold standard (as nothing else had been suggested) and adopted it as the benchmark for comparison in this study. The clinical history method, however, has drawbacks. Its accuracy depends on the availability of perioperative data, and the method shows poor reliability when such data are missing or even simply imprecise. Thus, the clinical history method can be used only in selected cases; moreover, its repeatability has not been studied. The literature in the past decade offers a variety of alternative methods for calculating corneal power or (directly) IOL power when complete perioperative data are not available. The separate evaluation of the anterior and posterior corneal curvatures and the Savini et al. formula for modifying the keratometric index of refraction are 2 options. The former requires the preoperative corneal power only, whereas the latter is based on the surgically induced refractive change.8,9 The lack of a statistically significant difference between the mean corneal powers estimated using these methods and the values obtained with the clinical history method confirm their accuracy, as does the fact that they had the lowest 95% LoA. However, the methods still rely on some preoperative data and thus cannot be used for all patients. The main purpose of this study was to investigate the corneal power measurements provided by the
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Pentacam camera to assess whether they might be reliably entered in IOL calculation formulas when perioperative data are not available. Three of the measurementsdsimulated K, true net power, and equivalent K readingddeserve attention. It was not surprising that the mean simulated K given by the Pentacam device was higher than the mean clinical history method value because simulated K is calculated using the standard keratometric index (1.3375), which is known to overestimate corneal power after refractive surgery.2 Rather, the Pentacam’s potential lies in its ability to measure the posterior corneal curvature, which means the Gaussian optics formula can be used to calculate corneal power. The Pentacam device automatically displays this value as the true net power. Our results agree with those of Borasio et al.11 as the true net power in our sample was significantly lower than the value obtained with the clinical history method. This discrepancy is likely the result of the different refractive indices used by the 2 methods; the clinical history method is still based on the conventional value of 1.3375, whereas the Gaussian optics formula adopts the true refractive indices of air (1), the cornea (1.376), and aqueous humor (1.336). To eliminate the difference between the corneal powers calculated using the Gaussian optics formula and the clinical history method, Borasio et al.11 developed the BESSt formula; we confirmed that their adjustment is useful for this purpose as no statistical difference was detected between values derived with the formula and those derived using the clinical history method. Nevertheless, according to our findings, the mean corneal power determined with the BESSt formula seems closer to the mean simulated K value given by videokeratography than to the mean clinical history method value and the 95% LoA were wide. Thus, notwithstanding the excellent results reported by Borasio et al., it is likely that the BESSt formula requires further refinement before it can be introduced into clinical practice. To our knowledge, this is the first study to evaluate the equivalent K reading, which is automatically calculated by the Pentacam in the Holladay report. According to Holladay, who developed the value, the equivalent K reading is an adjustment of the corneal power calculated using the Gaussian optics formula (J.T. Holladay, MD, ‘‘Measuring Corneal Power After Corneal Refractive Surgery,’’ Cataract & Refractive Surgery Today [suppl], January 2006, pages 5–6). We had an opportunity to analyze the data from the 16 eyes in our study with 2 versions of the same software. The results with the older version were discouraging as the mean equivalent K reading (measured at 4.0 mm) was significantly higher than the value obtained with the clinical history method and even
higher than the simulated K value obtained by conventional videokeratography. Thus, a considerable degree of hyperopia could be expected after IOL implantation if this value were to be used. Similar results were obtained with the latest software when the mean equivalent K readings at 4.0 mm and 4.5 mm were selected. Conversely, the mean equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm were not statistically significantly different from the values derived with the clinical history method, with the 2.0 mm reading being closest to the benchmark value (mean difference 0.11 D). This may represent the most important result in our study as it seems that we finally have a direct measurement of corneal power that closely matches the clinical history method value without the need for perioperative data. However, caution is required due to the large 95% LoA (range 1.35 to C1.58 D). Finally, as a secondary outcome, we observed that the latest Pentacam software can predict the preoperative corneal curvature with a moderate degree of accuracy. Having this value can be useful where the double-K formulas must be used for IOL power calculations.19 This study had limitations. First, the sample size was small because we had the older version of the Pentacam software for a limited time only and we decided to consider only patients who had been analyzed with both versions of the software. A larger sample is currently being enrolled in a further study that will evaluate only the new software. Second, our comparison might have included many more methods of calculating corneal power after excimer laser surgery; however, our primary intention was to compare the Pentacam’s capabilities vis-a`-vis the clinical history method. Finally, as mentioned, the conclusions of the present study might be biased by the lack of knowledge about the repeatability of the clinical history method. Further studies are required to understand the impact that the variability of this measurement can have on the clinical outcome. We cannot exclude, for example, that the wide LoA in the present study are caused, at least in part, by such variability. (Another source may be represented by the small sample size.) In conclusion, we found that none of the data currently provided by the Pentacam camera can accurately replace those obtained with the clinical history method, either because of a statistically significant difference in the mean values or poor agreement. Further studies are needed to assess whether it is worthwhile to modify the Pentacam software to obtain a value that more closely matches the corneal power value obtained with the clinical history method or whether it might be preferable to modify the current theoretical IOL power calculation formulas (SRK/T, Holladay, Hoffer Q)20–23 to be able to enter the true net power,
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which represents a direct measurement of corneal power and, unlike the clinical history method, does not require any preoperative datum to be calculated. REFERENCES 1. Seitz B, Langenbucher A. Intraocular lens power calculation in eyes after corneal refractive surgery. J Refract Surg 2000; 16:349–361 2. Seitz B, Langenbucher A, Nguyen NX, et al. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999; 106:693–702 3. Savini G, Barboni P, Zanini M. Intraocular lens power calculation after myopic refractive surgery: theoretical comparison of different methods. Ophthalmology 2006; 113:1271–1282 4. Jain R, Dilraj G, Grewal SPS. Repeatability of corneal parameters with Pentacam after laser in situ keratomileusis. Indian J Ophthalmol 2007; 55:341–347. Available at: http://www.ijo.in/ temp/IndianJOphthalmol555341_171807.pdf. Accessed February 1, 2008 5. Chen D, Lam AKC. Intrasession and intersession repeatability of the Pentacam system on posterior corneal assessment in the normal human eye. J Cataract Refract Surg 2007; 33: 448–454 6. Lackner B, Schmidinger G, Pieh S, et al. Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005; 82:892–899 7. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 8. Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17–29 9. Savini G, Barboni P, Zanini M. Correlation between attempted correction and keratometric refractive index of the cornea after myopic excimer laser surgery. J Refract Surg 2007; 23:461–466 10. Hofmeister EM, Kaupp SE, Schallhorn SC. Comparison of tropicamide and cyclopentolate for cycloplegic refractions in myopic adult refractive surgery patients. J Cataract Refract Surg 2005; 31:694–700 11. Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg 2006; 32: 2004–2014
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12. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307–310 13. Buehren T, Collins MJ, Iskander DR, et al. The stability of corneal topography in the post-blink interval. Cornea 2001; 20:826–833 14. Erde´lyi B, Csa´ka´ny B, Ne´meth J. Spontaneous alterations of the corneal topographic pattern. J Cataract Refract Surg 2005; 31:973–978 15. Liu Z, Pflugfelder SC. Corneal surface regularity and the effect of artificial tears in aqueous tear deficiency. Ophthalmology 1999; 106:939–943 16. Latkany RA, Chokshi AR, Speaker MG, et al. Intraocular lens calculations after refractive surgery. J Cataract Refract Surg 2005; 31:562–570 17. Argento C, Cosentino MJ, Badoza D. Intraocular lens power calculation after refractive surgery. J Cataract Refract Surg 2003; 29:1346–1351 18. Gimbel HV, Sun R. Accuracy and predictability of intraocular lens power calculation after laser in situ keratomileusis. J Cataract Refract Surg 2001; 27:571–576 19. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg 2003; 29:2063–2068 20. Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990; 16:333–340; correction, 528 21. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993; 19:700–712; errata 1994; 20:677 22. Zuberbuhler B, Morrell AJ. Errata in printed Hoffer Q formula. J Cataract Refract Surg 2007; 33:2; reply by KJ Hoffer, 3 23. Holladay JT, Prager TC, Chandler TY, et al. A three-part system for refining intraocular lens power calculations. J Cataract Refract Surg 1988; 14:17–24
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First author: Giacomo Savini, MD Private practice, Bologna, Italy
Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery Giacomo Savini, MD, Piero Barboni, MD, Vincenzo Profazio, Maurizio Zanini, MD, Kenneth J. Hoffer, MD
PURPOSE: To evaluate corneal power measurements by a rotating Scheimpflug camera (Pentacam, Oculus) in eyes that have had myopic excimer laser surgery. SETTING: Private practice, Bologna, Italy. METHODS: This prospective comparative interventional case series comprised 16 eyes of 16 patients who had myopic excimer laser surgery and for whom all perioperative data were available. Four corneal power measurements obtained with the Pentacam (simulated keratometry, true net power, equivalent K reading, and BESSt formula) were analyzed and compared with values derived using the clinical history method and 2 other formulas for calculating corneal power after refractive surgery (modified keratometric refractive index according to Savini et al. and separate consideration of the anterior and posterior corneal curvatures according to Speicher). RESULTS: Analysis of variance showed a statistically significant difference between all methods (P<.0001). Bonferroni multiple comparison tests showed that the only Pentacam measurements not statistically different from the corneal power values derived using the clinical history method were the equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm and those derived with the BESSt formula; however, considerably large 95% limits of agreement (LoA) were calculated between each of these values and those obtained with the clinical history method. CONCLUSIONS: The Pentacam device gave corneal power measurements that did not statistically significantly had differ from those predicted by the clinical history method in eyes that had previous myopic excimer laser surgery. Wide LoA are a potential source of error in intraocular lens power calculation in such patients. J Cataract Refract Surg 2008; 34:809–813 Q 2008 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and laser-assisted subepithelial keratectomy (LASEK) can correct myopia by decreasing the anterior corneal surface curvature. Because they alter the natural ratio between the anterior and
Accepted for publication January 21, 2008. From a private practice (Savini, Barboni, Profazio, Zanini), Bologna, Italy, and Jules Stein Eye Institute (Hoffer), University of California, Los Angeles, California, USA. No author has a financial or proprietary interest in any material or method mentioned. Corresponding author: Giacomo Savini, MD, Centro Salus, Via d’Azeglio 5, 40123, Bologna, Italy. E-mail: [email protected]. Q 2008 ASCRS and ESCRS Published by Elsevier Inc.
posterior corneal curvatures, these procedures render invalid the conventional keratometric index of refraction (usually 1.3375) used by most topographers and keratometers to convert the measured radius into diopters.1 These instruments thus cannot correctly calculate the corneal power and usually give a measurement that is higher than the actual value. Overestimating corneal power leads to postoperative hyperopia when intraocular lens (IOL) power must be calculated for eyes that will have cataract surgery.2 An increasing number of methods have been devised to overcome the problem of correctly measuring corneal power after excimer laser surgery.3 All rely on several assumptions to arrive at an indirect determination of corneal power. Most also require preoperative data such as the attempted correction or the original corneal power, and none can be reliably adopted for 0886-3350/08/$dsee front matter doi:10.1016/j.jcrs.2008.01.012
809
810
CORNEAL POWER MEASUREMENTS WITH PENTACAM AFTER EXCIMER LASER SURGERY
every patient. The ideal solution would be to find a technology that enables the surgeon to measure the correct corneal power directly, without relying on any assumptions. This would necessarily entail measuring the posterior corneal curvature so that the Gaussian optics formula could be used to calculate the corneal power and so the standard keratometric index would no longer be necessary. Two instruments with this ability are currently available: the Orbscan II (Bausch & Lomb) and the Pentacam (Oculus). The former is a horizontal slit-scanning system, whereas the latter is a rotating Scheimpflug camera. Promising results have been reported for the repeatability of several of the Pentacam’s measurements.4–6 The aim of this study was to assess measurements performed with the Pentacam in eyes that had myopic LASIK, PRK, or LASEK and compare the measurements with the values obtained using the gold standard proceduredthe clinical history methoddand 2 additional methods; that is, separate evaluation of the anterior and posterior corneal curvatures and the Savini formula for calculating the postoperative keratometric index on the basis of attempted correction.7–9 PATIENTS AND METHODS Between April and June 2007, 16 consecutive patients who had LASIK, PRK, or LASEK to correct myopia at a private practice (Centro Salus, Bologna, Italy) were evaluated. Preoperative data were available for all eyes. The minimum required follow-up was 1 month for LASIK and 3 months for LASEK and PRK. Before being included in the study, all patients were informed of its purpose and gave written consent. The study methods adhered to the tenets of the Declaration of Helsinki for the use of human participants in biomedical research. One eye of each patient was randomly selected and evaluated by videokeratography (TMS-2 topography system, Tomey) and Scheimpflug camera imaging (Pentacam). The surgically induced refractive correction, which is necessary to calculate corneal power according to the clinical history method, was determined after the postoperative refraction was measured under cycloplegia. Cycloplegia was induced by tropicamide 1% instilled twice, 5 minutes apart, as suggested by Hofmeister et al.10 Corneal power was calculated according to 3 methods: the clinical history method, used as the benchmark for comparison; separate evaluation of the anterior and posterior corneal curvatures, as described by Speicher8; and the corrected keratometric index (modified on the basis of attempted correction), as described by Savini et al.9 To avoid miscalculations due to poor videokeratography quality, both preoperative and postoperative examinations were performed immediately after blinking and were carefully inspected before being included in the study. For each eye, only 1 preoperative and 1 postoperative videokeratography with no artifacts (eg, incomplete or irregular corneal rings) were considered for further analysis. The following values provided by the Pentacam device were analyzed and compared with those obtained with the clinical history method: mean simulated keratometry (K);
mean true net power (ie, corneal power calculated with the Gaussian optics formula using the anterior and posterior corneal radii and the corneal thickness); BESSt formula, as described by Borasio et al.11; and equivalent K reading shown in the Holladay report (J.T. Holladay, MD, ‘‘Measuring Corneal Power After Corneal Refractive Surgery,’’ Cataract & Refractive Surgery Today [suppl], January 2006, pages 5–6). Regarding the equivalent K reading, the values given by 2 versions of the Pentacam software were recorded before this study commenced. The first version, 1.14r46, provides only 1 value measured along a ring with a diameter of 4.0 mm. The second version, 1.16, has been available since June 2007 and provides several values measured along rings with diameters of 1.0, 2.0, 3.0, 4.0, 4.5, 5.0, 6.0, and 7.0 mm. (The measurements at 6.0 mm and 7.0 mm were not included in the analysis.) For each eye, only good-quality Scheimpflug images (determined when the quality specification provided by the instrument was ‘‘OK’’) were used. A secondary objective of the study was to compare the estimated preoperative K value (calculated based on the newer version of the Holladay report) and the actual preoperative value. Statistical analyses were performed using GraphPad InStat for Macintosh (version 3a, GraphPad Software). A 1-way analysis of variance (ANOVA) for repeated measures with Bonferroni multiple comparisons was used to compare all corneal power measurements. Preliminary analysis showed that all assumptions required by the ANOVA model were satisfied, including the independence of cases, Gaussian bell-shaped distribution (assessed by KolmogorovSmirnov test), and homogeneity of variance (assessed by Bartlett’s test). A P value less than 0.05 was considered statistically significant. The agreement between the clinical history method and other methods in measuring the mean corneal power was analyzed according to the method described by Bland and Altman,12 who suggest plotting the differences between measurements (y-axis) against their mean (x-axis). The 95% limits of agreement (LoA) were defined as means G2 SD of the differences between the 2 measurement techniques.
RESULTS The mean age of the patients was 37 years G 5.8 (SD). The mean preoperative refractive error was –5.1 G 1.4 diopters. Examinations were performed a mean of 519 G 614 days (range 35 to 2310 days) after surgery. Table 1 shows the corneal power measurements with each method. Overall, the ANOVA P value was significant (P!.0001). The Bonferroni multiple comparisons test did not detect significant differences between the corneal powers determined with the clinical history method, the separate evaluation of the anterior and posterior corneal curvatures, and the corneal power calculated with the Savini et al. formula for correcting the keratometric index of refraction. In contrast, most values provided by the Pentacam camera were statistically significantly different. The mean simulated K reading (which was the same with both software versions) and the mean equivalent K reading given by the older software were significantly higher than the clinical history method–derived values as
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Table 1. Corneal power measured by different methods.
Corneal Power Measurement SimK by TMS-2 CHM ACPCC Savini SimK by Pentacam EKR 1.0 mm EKR 2.0 mm EKR 3.0 mm EKR 4.0 mm EKR 4.5 mm EKR True net power BESSt formula
Mean G SD (D)
P Value*
38.12 G 0.99 37.69 G 1.09 37.56 G 1.10 37.56 G 1.10 38.38 G 1.18 38.44 G 1.21 37.46 G 1.77 37.58 G 1.66 37.82 G 1.49 38.16 G 1.24 38.40 G 1.13 36.54 G 1.34 38.05 G 1.41
!.05 d NS NS !.001 !.001 NS NS NS !.01 !.001 !.001 NS
Mean Difference Vs CHM (D) 0.42 d 0.14 0.15 0.69 0.75 0.23 0.12 0.13 0.46 0.70 1.16 0.36
95% LoA Vs CHM (D) 0.18 to C1.42 d 0.70 to C0.42 0.65 to C0.34 0.18 to C1.55 0.20 to 1.69 1.93 to C1.47 1.58 to C1.35 0.99 to C1.25 0.35 to C1.27 0.11 to C1.51 2.21 to 0.10 0.68 to C1.40
ACPCC Z separate evaluation of anterior and posterior corneal curvatures; CHM Z clinical history method; EKR Z equivalent K reading; LOA Z limits of agreement; NS Z not significant; SimK Z simulated keratometry * Bonferroni multiple comparisons with the clinical history method
well as the 4.0 mm and 4.5 mm equivalent K readings given by the newer software; the true net power (which was the same with both software versions) was significantly lower than the corneal power determined using the clinical history method; no statistically significant differences were detected between the 1.0 mm, 2.0 mm, and 3.0 mm equivalent K reading values and the values calculated by the BESSt formula and from the clinical history method. The 2.0 mm and 3.0 mm equivalent K reading values showed the smallest difference (0.11 D lower and 0.13 D higher, respectively, than the clinical history method–derived value). Analysis of the 95% LoA according to Bland and Altman showed that the separate evaluation of the anterior and posterior corneal curvatures and the Savini et al. method yielded the highest agreement with the clinical history method, whereas the Pentacam provided measurements that differed considerably (Table 1). Comparison of the estimated preoperative K value and the actual preoperative value showed no statistically significant differences by a paired t test as the mean values were 43.04 G 1.01 D and 43.00 G 1.19 D, respectively. However, the Bland–Altman plot showed only a moderate level of agreement between the 2 values as the 95% LoA ranged from 1.43 to C1.50 D. DISCUSSION Conventional videokeratography is considered an accurate and repeatable technique to measure the curvature and power of the central cornea,13 especially if the
post-blink interval of image capture is standardized to avoid the influence of changes in tear-film stability.14,15 Unfortunately, corneal power measurements by videokeratography are no longer valid after excimer laser surgery.1 Clinical and theoretical studies3,16–18 suggest that the clinical history method is best for calculating corneal power after laser surgery; for this reason, we accepted it as the gold standard (as nothing else had been suggested) and adopted it as the benchmark for comparison in this study. The clinical history method, however, has drawbacks. Its accuracy depends on the availability of perioperative data, and the method shows poor reliability when such data are missing or even simply imprecise. Thus, the clinical history method can be used only in selected cases; moreover, its repeatability has not been studied. The literature in the past decade offers a variety of alternative methods for calculating corneal power or (directly) IOL power when complete perioperative data are not available. The separate evaluation of the anterior and posterior corneal curvatures and the Savini et al. formula for modifying the keratometric index of refraction are 2 options. The former requires the preoperative corneal power only, whereas the latter is based on the surgically induced refractive change.8,9 The lack of a statistically significant difference between the mean corneal powers estimated using these methods and the values obtained with the clinical history method confirm their accuracy, as does the fact that they had the lowest 95% LoA. However, the methods still rely on some preoperative data and thus cannot be used for all patients. The main purpose of this study was to investigate the corneal power measurements provided by the
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Pentacam camera to assess whether they might be reliably entered in IOL calculation formulas when perioperative data are not available. Three of the measurementsdsimulated K, true net power, and equivalent K readingddeserve attention. It was not surprising that the mean simulated K given by the Pentacam device was higher than the mean clinical history method value because simulated K is calculated using the standard keratometric index (1.3375), which is known to overestimate corneal power after refractive surgery.2 Rather, the Pentacam’s potential lies in its ability to measure the posterior corneal curvature, which means the Gaussian optics formula can be used to calculate corneal power. The Pentacam device automatically displays this value as the true net power. Our results agree with those of Borasio et al.11 as the true net power in our sample was significantly lower than the value obtained with the clinical history method. This discrepancy is likely the result of the different refractive indices used by the 2 methods; the clinical history method is still based on the conventional value of 1.3375, whereas the Gaussian optics formula adopts the true refractive indices of air (1), the cornea (1.376), and aqueous humor (1.336). To eliminate the difference between the corneal powers calculated using the Gaussian optics formula and the clinical history method, Borasio et al.11 developed the BESSt formula; we confirmed that their adjustment is useful for this purpose as no statistical difference was detected between values derived with the formula and those derived using the clinical history method. Nevertheless, according to our findings, the mean corneal power determined with the BESSt formula seems closer to the mean simulated K value given by videokeratography than to the mean clinical history method value and the 95% LoA were wide. Thus, notwithstanding the excellent results reported by Borasio et al., it is likely that the BESSt formula requires further refinement before it can be introduced into clinical practice. To our knowledge, this is the first study to evaluate the equivalent K reading, which is automatically calculated by the Pentacam in the Holladay report. According to Holladay, who developed the value, the equivalent K reading is an adjustment of the corneal power calculated using the Gaussian optics formula (J.T. Holladay, MD, ‘‘Measuring Corneal Power After Corneal Refractive Surgery,’’ Cataract & Refractive Surgery Today [suppl], January 2006, pages 5–6). We had an opportunity to analyze the data from the 16 eyes in our study with 2 versions of the same software. The results with the older version were discouraging as the mean equivalent K reading (measured at 4.0 mm) was significantly higher than the value obtained with the clinical history method and even
higher than the simulated K value obtained by conventional videokeratography. Thus, a considerable degree of hyperopia could be expected after IOL implantation if this value were to be used. Similar results were obtained with the latest software when the mean equivalent K readings at 4.0 mm and 4.5 mm were selected. Conversely, the mean equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm were not statistically significantly different from the values derived with the clinical history method, with the 2.0 mm reading being closest to the benchmark value (mean difference 0.11 D). This may represent the most important result in our study as it seems that we finally have a direct measurement of corneal power that closely matches the clinical history method value without the need for perioperative data. However, caution is required due to the large 95% LoA (range 1.35 to C1.58 D). Finally, as a secondary outcome, we observed that the latest Pentacam software can predict the preoperative corneal curvature with a moderate degree of accuracy. Having this value can be useful where the double-K formulas must be used for IOL power calculations.19 This study had limitations. First, the sample size was small because we had the older version of the Pentacam software for a limited time only and we decided to consider only patients who had been analyzed with both versions of the software. A larger sample is currently being enrolled in a further study that will evaluate only the new software. Second, our comparison might have included many more methods of calculating corneal power after excimer laser surgery; however, our primary intention was to compare the Pentacam’s capabilities vis-a`-vis the clinical history method. Finally, as mentioned, the conclusions of the present study might be biased by the lack of knowledge about the repeatability of the clinical history method. Further studies are required to understand the impact that the variability of this measurement can have on the clinical outcome. We cannot exclude, for example, that the wide LoA in the present study are caused, at least in part, by such variability. (Another source may be represented by the small sample size.) In conclusion, we found that none of the data currently provided by the Pentacam camera can accurately replace those obtained with the clinical history method, either because of a statistically significant difference in the mean values or poor agreement. Further studies are needed to assess whether it is worthwhile to modify the Pentacam software to obtain a value that more closely matches the corneal power value obtained with the clinical history method or whether it might be preferable to modify the current theoretical IOL power calculation formulas (SRK/T, Holladay, Hoffer Q)20–23 to be able to enter the true net power,
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which represents a direct measurement of corneal power and, unlike the clinical history method, does not require any preoperative datum to be calculated. REFERENCES 1. Seitz B, Langenbucher A. Intraocular lens power calculation in eyes after corneal refractive surgery. J Refract Surg 2000; 16:349–361 2. Seitz B, Langenbucher A, Nguyen NX, et al. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999; 106:693–702 3. Savini G, Barboni P, Zanini M. Intraocular lens power calculation after myopic refractive surgery: theoretical comparison of different methods. Ophthalmology 2006; 113:1271–1282 4. Jain R, Dilraj G, Grewal SPS. Repeatability of corneal parameters with Pentacam after laser in situ keratomileusis. Indian J Ophthalmol 2007; 55:341–347. Available at: http://www.ijo.in/ temp/IndianJOphthalmol555341_171807.pdf. Accessed February 1, 2008 5. Chen D, Lam AKC. Intrasession and intersession repeatability of the Pentacam system on posterior corneal assessment in the normal human eye. J Cataract Refract Surg 2007; 33: 448–454 6. Lackner B, Schmidinger G, Pieh S, et al. Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005; 82:892–899 7. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203 8. Speicher L. Intra-ocular lens calculation status after corneal refractive surgery. Curr Opin Ophthalmol 2001; 12:17–29 9. Savini G, Barboni P, Zanini M. Correlation between attempted correction and keratometric refractive index of the cornea after myopic excimer laser surgery. J Refract Surg 2007; 23:461–466 10. Hofmeister EM, Kaupp SE, Schallhorn SC. Comparison of tropicamide and cyclopentolate for cycloplegic refractions in myopic adult refractive surgery patients. J Cataract Refract Surg 2005; 31:694–700 11. Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg 2006; 32: 2004–2014
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First author: Giacomo Savini, MD Private practice, Bologna, Italy