Intraocular lens power calculations using a Scheimpflug camera to measure corneal power K Xu, Y Hao, H Qi Department of Ophthalmology, Peking University Third Hospital, 49 North Garden Road, Haidian District, Beijing, China Accepted November 18, 2013
Abstract We measured corneal power using an Oculus Pentacam® to assess its accuracy for calculating intraocular lens (IOL) power after myopic refractive surgery. A series of corneal power measurements were performed on 22 patients (43 eyes) who had undergone myopic refractive surgery. In 37 of the 43 eyes, phacoemulsification and IOL implantation subsequently were performed. Conventional keratometry and three corneal measurements (mean true net power, central true net power, and 4.5 mm equivalent K reading) obtained using a Pentacam were analyzed and compared to values derived from the clinical history method. Prediction errors of three Pentacam corneal power measurements inserted in third generation IOL formulas also were compared. Analysis of the variance showed that only two Pentacam corneal measurements, mean true net power and central true net power, were not significantly different from those of the clinical history method. Mean true net power was correlated more closely with the clinical history method corneal power than other corneal power values. The one-sample t-test showed that of three Pentacam corneal measurements combined with third-generation formulas, only the mean true net power inserted in the SRK/T implant power calculation formula was not significantly different from zero. The percentages of eyes within 0.50 D and 1.00 D of the refractive prediction error of this method were 67.6% and 86.5%, respectively. Mean true net power inserted in the SRK/T formula can be used to calculate directly IOL power after myopic refractive surgery. Key words: cornea, eyes, IOL calculation, lens, Pentacam, refractive surgery, Scheimpflüg camera Many new methods for improving the accuracy of intraocular lens (IOL) calculations after refractive surgery have been reported (Borasio et al. 2006, Shammas and Shammas 2007, Savini et al.2008, Kim et al. 2009, Wang et al. 2010, Jin et al. 2010, McCarthy et al. 2011, Tay et al. 2011). The primary approach has been to measure the true corneal power after laser refractive surgery, which alters the natural ratio between the anterior and posterior corneal curvatures. Most methods, including the
Correspondence: Yansheng Hao, Department of Ophthalmology, Peking University Third Hospital, 49 North Garden Road, Haidian District, Beijing, China. Tel: 86-13520679323, Fax: 8601082089951, E-mail: [email protected] © 2014 The Biological Stain Commission Biotechnic & Histochemistry 2014, 89(5): 348–354.
DOI: 10.3109/10520295.2013.867532
clinical history method (Holladay 1997), the Feiz-Mannis method (Feiz et al. 2001) and the corneal bypass method (Walter et al. 2006) require historical data that sometimes are unavailable. Also, none of these methods can be used for all patients. The ideal method would be to measure corneal power directly. The instruments that can measure both anterior and posterior corneal power directly include the Orbscan® (Bausch and Lomb, Rochester, NY), Ziemer Galilei™ Pentacam (Oculus Optikgeräte GmbH,Wetzler,Germany), Tomey, and Sirius (Costruzioni Strumenti Oftalmici, Florence, Italy). The Orbscan and Ziemer Galilei use optical, Placido disc, and Scheimpflug imaging technology (Qazi et al. 2007). The other three methods use the rotating Scheimpflug imaging principle. The Pentacam, however, uses many measuring points, 348
which makes it more reliable for calculating posterior corneal power (Ha et al. 2009). Although the Pentacam has been used for measuring corneal power after refractive surgery, the results have been controversial owing to limited sample sizes (Savini et al. 2008, Holladay et al. 2009, Falavarjani et al. 2010). We used the Pentacam to measure directly three corneal characteristics: central and mean values in the true net corneal power map and a 4.5 mm equivalent K reading. We compared these measurements with the clinical history method and analyzed their accuracy with different third-generation formulas for calculating IOL power after myopic refractive surgery.
target refraction were calculated using different combinations (three Pentacam corneal power measurements, K cTNP, K mTNP and EKR, combined with third generation formulas, Holladay 1, SRK/T, and Hoffer Q) (Retzlaff et al. 1990, Hoffer 1994, 2007) . Statistical analyses were performed using SPSS (Version 16; SPSS Inc., Chicago, IL). Bonferroni multiple comparisons in a one-way analysis of variance (ANOVA) were used to compare the five corneal power measurements and the K value was calculated using CHM (KCHM). A one-sample t-test was performed to analyze the accuracy of the IOL calculation methods. Bonferroni multiple comparisons were used to compare mean arithmetic predictive errors using K mTNP with different formulas; p 0.05 was considered statistically significant.
Material and methods Results Our research followed the tenets of the Declaration of Helsinki and was approved by the Eye Center of Peking University Third Hospital. We obtained written informed consent from all subjects involved in the study. Our research also From June 2009 to May 2012, 22 consecutive patients with a history of myopic refractive surgery and subsequent phacoemulsification and IOL implantation in our eye center were enrolled in our study. The mean patient age was 49.35 8.0 years. All 22 patients (43 eyes) had a history of myopic refractive surgery (LASIK, LASEK or photorefractive keratectomy [PRK]). The corneal power of each eye was evaluated using an Auto Keratometer (Topcon,Tokyo, Japan), IOLMaster (Carl Zeiss Meditec, Dublin, CA), and Pentacam (Oculus). The following Pentacam values were measured: central true net power (cTNP), mean true net power (mTNP) and 4.5 mm equivalent K reading (EKR). Only quality images (according to quality specifications for the Pentacam) were used for each eye. The clinical history method was used to calculate corneal power for 17 patients (33 eyes) with available preoperative data. Axial length (AL) was measured using an immersion ultrasonic A scan (OcuScan®, Alcon Inc., Fort Worth, TX). Subsequently, 22 patients (37 eyes) underwent standard phacoemulsification surgery and all IOLs were placed in the capsular bag. The IOL power was calculated using mTNP, then combined with the SRK/T formula, and the final IOL power was determined by the surgeon. The same surgeon performed all surgeries and no surgical complications were reported. The final refraction was obtained 12 weeks after cataract surgery. Mean arithmetic and absolute differences between the actual postoperative refraction and
Corneal data from 43 eyes (22 patients) were analyzed. Twenty-six eyes were treated by LASIK, two eyes had LASEK and 15 had PRK. The mean pre-keratorefractive surgery refraction was 11.39 3.96 diopters. The mean pre-cataract surgery refraction was 8.62 6.61 diopters. The mean axial length was 29.52 2.12 mm and ranged from 25.72 to 33.41 mm. Subsequently, 22 patients (37 eyes) had cataract surgery . Table 1 shows the corneal power measurements; overall, the differences were significant (p 0.05). The Bonferroni multiple tests showed that Kauto and Kmaster were significantly higher than KcTNP, KmTNP, and KCHM, and the differences between them and EKR were not significant. With regard to the three Pentacam corneal measurements (KcTNP, KmTNP and EKR), the difference between any two measurements was significant and their mean value increased one by one (KcTNP 33.5 3.26 D, KmTNP 35.47 2.73 D, EKR 37.42 2.48 D). Table 1 also shows that compared to KCHM, only the KcTNP and KmTNP values were not significantly different (p 0.067 and 1.000, respectively). KmTNP (Pearson r 0.573, p 0.001) correlated with KCHM more closely than other values (Table 2). The mean differences and 95% CIs between the corneal power measured by different methods and KCHM are shown in Fig. 1. Figure 2 is a box plot of the IOL predictive errors from the various methods studied. Table 3 shows the mean arithmetic and absolute IOL predictive errors. Of the nine IOL calculation methods (three Pentacam measured corneal powers inserted into three third-generation formulas), only KmTNP inserted into SRK/T was not significantly different from zero; differences among the other eight IOL power calculations to measure corneal power 349
Table 1. Corneal power measured by different methods Corneal power measurement K AUTO Kmaster KCHM KcTNP KmTNP EKR
Mean ⴞ SD (D)
p*
Mean difference vs. KCHM (D)
38.33 2.18 38.18 2.23 35.32 2.68 33.58 3.26 35.47 2.73 37.42 2.48
0.000 0.000 – 0.067 1.000 0.009
3.01 2.86 – 1.74 0.15 2.10
95% CI of the difference vs. KCHM (D) 1.20 1.04 – 3.54 1.65 0.31
– 4.82 – 4.68 – 0.06 – 1.95 – 3.90
CI, confidence interval; K AUTO, K-value of autokeratometry; Kmaster, K-value of IOL master; KCHM, K-value from clinical history method; KcTNP, central K-value in true net power map of Pentacam; KmTNP, mentral K-value in true net power map of Pentacam; EKR, equivalent K-reading from the Pentacam *Bonferroni multiple comparisons to K CHM
methods were significant (p 0.05 with a one-sample t-test). The percentages of eyes within 0.50 D and 1.00 D of the refractive predictive error of the different methods are shown in Table 4. In KmTNP combined with Hoffer Q, Holladay 1 and SRK/T methods with relatively higher percentage values, ANOVA analysis showed no significant differences in their mean absolute errors. The Bonferroni multiple test of the mean arithmetic predictive errors showed that KmTNP-SRK/T and KmTNP-Hoffer Q were significantly lower than KmTNP-Hollady1 (Table 5).
Discussion IOL implantation after refractive surgery is a challenging procedure, because standard IOL power formulas can cause significant unintended postoperative refractive errors. IOL prediction errors after refractive laser surgery are mainly from two sources. One is the inaccurate corneal power determination by keratometers and corneal topography systems due to the altered relationship between the anterior and posterior corneal curvatures and the inability to detect the central value based on the Placido disc theory (Masket 2006, Hamilton and Hardten 2003, Randleman et al. 2002, Patel et al. 2000). The second source of error is incorrect Table 2. Pearson correlation coefficients between corneal powers measured by different methods and KCHM Corneal power measurement K AUTO Kmaster KcTNP KmTNP EKR KCHM
350
r
p
0.493 0.537 0.545 0.573 0.570 –
0.004 0.002 0.001 0.000 0.001 –
estimation of the effective lens position (ELP) in the third-generation IOL formula, which uses flattened corneal power (Haigis 2008, Aramberri 2003, Koch and Wang 2003). Methods for improving IOL prediction accuracy generally are aimed at obtaining the true corneal power and IOL formula adjustment. Many of these methods require preoperative information such as clinical history (Holladay 1997), Feiz-Mannis (Feiz et al. 2001) and corneal bypass (Walter et al. 2006), or they require several assumptions to arrive at an indirect corneal power determination such as the Maloney method, modified Maloney method and the Shammas no-history method (Smith et al. 1998, Wang et al. 2004). Improvements in the calculations include double-K adjustment (Aramberri 2003) and the fourth-generation formula (Holladay 2 and Haigis), which require extensive data including horizontal white-to-white distance, phakic anterior chamber depth and phakic lens thickness. We evaluated a direct and convenient corneal power measurement for calculating IOL power after myopic refractive surgery. Unlike most topography, which is based on the Placido disc theory, the Pentacam uses a Scheimpflug rotating imaging system and transforms elevation data into corneal curvature. The Pentacam method can obtain more accurate values for both anterior and posterior corneal power as well as the central area. True net power comes from the Gaussian optics formula for thick lenses (Olsen 1986). The mean value calculated from the 3.0 mm zone in the Pentacam true net power map is called mean true net power and the central value is called central true net power. The equivalent K reading, which is calculated automatically by the Pentacam in the Holladay report, is an adjustment of the corneal power calculated from the Gaussian optics formula (Holladay et al. 2009). Version 1.16 of Pentacam software can provide values of
Biotechnic & Histochemistry 2014, 89(5): 348–354
Fig. 1. Mean differences and 95% confidence intervals between corneal powers measured using different methods.
diameters from 1.0 to 7.0 mm. According to the findings of Holladay et al. (2009), 4.5 mm EKR gives a relatively accurate estimate of the central corneal power following refractive surgery. Consequently, we chose 4.5 mm EKR as one corneal power measurement for our study.
It was not surprising that Kauto and Kmaster were significantly higher than KCHM; it confirms the fact that conventional keratometry overestimates corneal power and leads to hyperopic outcomes after myopic corneal refractive surgery. We showed that in the three Pentacam corneal power measurements
Fig. 2. Box plot of the difference between actual postoperative refractions and target refractions using different methods.
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Table 3. Mean difference between actual postoperative refractions and target refractions with different methods Mean difference between actual postoperative refractions and target refractions (D) Arithmetic Method
Absolute
Eye (n)
mean ⴞ SD
range
mean ⴞ SD
range
37 37 37 37 37 37 37 37 37
0.52 0.94* 0.11 0.82 0.42 1.11* 1.26 1.10* 1.79 1.11* 2.3 1.25* 2.4 1.12* 1.64 0.93* 1.58 1.2*
1.38 – 1.35 2.25 – 2.81 2.54 – 3.00 3.4 – 1.72 4.47 – 1.28 4.31 – 1.31 0.22 – 5.09 0.54 – 4.44 0.54 – 4.39
0.88 0.60 0.55 0.62 0.88 0.79 1.4 0.9 1.88 0.95 2.36 1.11 2.4 1.12 1.67 0.87 1.61 1.15
0.14 – 3.35 0.01 – 2.81 0.03 – 3.00 0.01 – 3.4 0.26 – 0.47 0.21 – 4.31 0.22 – 5.09 0.08 – 4.44 0.03 – 4.39
KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KcTNP-Holladay1 KcTNP-SRK/T KcTNP-Hoffer Q EKR-Holladay1 EKR -SRK/T EKR-Hoffer Q
*Significantly different from zero; p 0.05 with one-sample t-test
(KcTNP,KmTNP, and 4.5 mm EKR), KmTNP had the greatest correlation with KCHM. This result differs from results reported by Savini et al. (2008) and Falavarjani et al. (2010), who found that the mean true net power in Pentacam was significantly different from KCHM. We also showed that 4.5 mm EKR was significantly higher than KCHM, which was consistent with the report by Savini et al. (2008), but different from the report by Falavarjani et al. (2010), who found that 4.5 mm EKR was highly correlated with KCHM. One reason for this difference may be that the patients recruited for the latter study had a lesser degree of myopia (mean SE 3.46 1.16 D, range 1.5 to 6.00 D). Savini et al. (2008) compared the EKR in eight patients (16 eyes) after refractive surgery with KCHM. The small sample size may contribute to the difference from our study. Other investigators have reported that corneal power overestimation in eyes Table 4. The percentages of eyes within 0.50 D and 1.00 D of refractive prediction error of different methods Percentage Method KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KcTNP-Holladay1 KcTNP-SRK/T KcTNP-Hoffer Q EKR-Holladay1 EKR -SRK/T EKR-Hoffer Q
352
within ⴞ 0.50 D
within ⴞ 1.00 D
29.7 67.6 45.9 16.2 8.1 8.1 2.7 10.8 16.2
67.6 86.5 67.6 37.8 13.5 8.1 8.1 21.6 37.8
that had undergone myopic keratorefractive surgery is directly correlated with the amount of refractive change and may not occur after a small amount of correction (Hamed et al. 2002, Seitz et al. 1999). We found that among the three Pentacam corneal power measurements (KcTNP, KmTNP and 4.5-mm EKR), the mean true net power (3 mm) estimates were closest to KCHM, and the percentage values of 0.50 D and 1.00 D of the predictive error of KmTNP combined with Hoffer Q, Holladay 1 and SRK/T methods were relatively higher. KcTNP was relatively lower and, when inserted into the three third generation formulas, yielded a myopic prediction error. Interestingly, the 4.5 mm EKR was relatively higher (p 0.05) compared to Kauto and Kmaster and, when inserted into the three third-generation formulas, yielded a hyperopic prediction error. The difference indicates the size of the measured area of corneal power. In eyes that had PRK or LASIK, circular zones of different sizes have different mean dioptric values, because laser energy may vary with the ablation depth. As the assessed zone increases, the value
Table 5. Bonferroni multiple test of mean arithmetic prediction error between KmTNP combined different formulas Method pair KmTNP-SRK/T KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KmTNP-Holladay1 KmTNP-Hoffer Q
Biotechnic & Histochemistry 2014, 89(5): 348–354
p 0.05 (0.018) 0.05 (0.552) 0.05 (0.000)
becomes similar to the conventional K value and there is a greater probability of overestimating the corneal power in the myopic eye. In nine Pentacam corneal power and third-generation formula combinations, only KmTNP inserted into SRK/T was not significantly different from zero and the percentages of eyes within 0.50 D and 1.00 D of the refractive prediction errors of this method were 67.6 and 86.5%, respectively. The percentage values of KcTNP inserted in SRK/T were 8.1 and 13.5%, respectively. These findings differ from the those of Kim et al. (2009) where 30 eyes were analyzed using the single central true net power combined with SRK/T; the percentages of eyes within 0.50 D and 1.00 D of the refractive prediction error were 70 and 93%, respectively. It is not clear what caused the differences between results report by Kim et al. (2009) and our results; however, we believe that the corneal apex might shift after photo-ablation and the determination of the visual quality is a 3 mm pupil size. We used an immersion ultrasonic A scan to measure axial length; nearly all axial length values exceeded 26 mm. It has been reported, however, that some physicians prefer to measure axial length using the IOLMaster™ (Andrew et al. 2001). Although the IOLMaster is reliable for axial length and anterior chamber depth measurements, it may not be accurate for patients who have poor vision with weak fixation ability (Verhulst and Vrijghem 2001). Therefore, we used the traditional ultrasonic A scan and compared the predictive errors of KmTNP combined with SRK/T, Holladay 1 and Hoffer Q. The Holladay 1 method tends to predict hyperopic outcomes (mean arithmetic predictive error of 0.52 0.94 D) and the Hoffer Q method tends to predict myopic outcomes (mean arithmetic predictive error of 0.42 1.11 D). This is consistent with the findings of McCarthy et al. (2011), who compared the three formulas with CHM in eyes 27 mm. Third generation IOL formulas (SRK/T, Holladay 1 and Hoffer Q) depend on the corneal curvature to estimate the ELP. By entering the flatter post-LASIK K-values, the formula underestimates ELP. After corneal refractive surgery, the error produced by the ELP underestimation and the error produced by the corneal power overestimation or underestimation are cumulative and yield hyperopic or myopic outcomes. When we entered three different Pentacam K values (KcTNP, KmTNP and 4.5 mm EKR) in these three formulas, we found that KcTNP combined with the three formulas all predicted myopic outcomes (the mean arithmetic predictive errors were all negative values) and 4.5 mm EKR combined with the three formulas all predicted hyperopic outcomes (the mean arithmetic predic-
tive errors were positive values). There were both positive and negative mean arithmetic predictive errors in KmTNP combined with the three formulas. This finding was consistent with the difference we found for the three Pentacam corneal power measurements (KcTNP KmTNP 4.5 mm EKR). All of our patients had myopic refractive surgery and almost all axial lengths exceeded 26 mm. Further studies of hyperopic surgical history and other axial length measurements for the universal use of Pentacam mean true net power to calculate IOL power after refractive surgery. We compared several Pentacam corneal power measurements and their accuracy in calculating IOL power after refractive surgery for myopia. Although the third-generation IOL formula has drawbacks, the mean true net power inserted in the SRK/T formula provides a direct IOL calculation method that does not require preoperative data or assumptions. Declaration of interests: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this paper.
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Biotechnic & Histochemistry 2014, 89(5): 348–354
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Abstract We measured corneal power using an Oculus Pentacam® to assess its accuracy for calculating intraocular lens (IOL) power after myopic refractive surgery. A series of corneal power measurements were performed on 22 patients (43 eyes) who had undergone myopic refractive surgery. In 37 of the 43 eyes, phacoemulsification and IOL implantation subsequently were performed. Conventional keratometry and three corneal measurements (mean true net power, central true net power, and 4.5 mm equivalent K reading) obtained using a Pentacam were analyzed and compared to values derived from the clinical history method. Prediction errors of three Pentacam corneal power measurements inserted in third generation IOL formulas also were compared. Analysis of the variance showed that only two Pentacam corneal measurements, mean true net power and central true net power, were not significantly different from those of the clinical history method. Mean true net power was correlated more closely with the clinical history method corneal power than other corneal power values. The one-sample t-test showed that of three Pentacam corneal measurements combined with third-generation formulas, only the mean true net power inserted in the SRK/T implant power calculation formula was not significantly different from zero. The percentages of eyes within 0.50 D and 1.00 D of the refractive prediction error of this method were 67.6% and 86.5%, respectively. Mean true net power inserted in the SRK/T formula can be used to calculate directly IOL power after myopic refractive surgery. Key words: cornea, eyes, IOL calculation, lens, Pentacam, refractive surgery, Scheimpflüg camera Many new methods for improving the accuracy of intraocular lens (IOL) calculations after refractive surgery have been reported (Borasio et al. 2006, Shammas and Shammas 2007, Savini et al.2008, Kim et al. 2009, Wang et al. 2010, Jin et al. 2010, McCarthy et al. 2011, Tay et al. 2011). The primary approach has been to measure the true corneal power after laser refractive surgery, which alters the natural ratio between the anterior and posterior corneal curvatures. Most methods, including the
Correspondence: Yansheng Hao, Department of Ophthalmology, Peking University Third Hospital, 49 North Garden Road, Haidian District, Beijing, China. Tel: 86-13520679323, Fax: 8601082089951, E-mail: [email protected] © 2014 The Biological Stain Commission Biotechnic & Histochemistry 2014, 89(5): 348–354.
DOI: 10.3109/10520295.2013.867532
clinical history method (Holladay 1997), the Feiz-Mannis method (Feiz et al. 2001) and the corneal bypass method (Walter et al. 2006) require historical data that sometimes are unavailable. Also, none of these methods can be used for all patients. The ideal method would be to measure corneal power directly. The instruments that can measure both anterior and posterior corneal power directly include the Orbscan® (Bausch and Lomb, Rochester, NY), Ziemer Galilei™ Pentacam (Oculus Optikgeräte GmbH,Wetzler,Germany), Tomey, and Sirius (Costruzioni Strumenti Oftalmici, Florence, Italy). The Orbscan and Ziemer Galilei use optical, Placido disc, and Scheimpflug imaging technology (Qazi et al. 2007). The other three methods use the rotating Scheimpflug imaging principle. The Pentacam, however, uses many measuring points, 348
which makes it more reliable for calculating posterior corneal power (Ha et al. 2009). Although the Pentacam has been used for measuring corneal power after refractive surgery, the results have been controversial owing to limited sample sizes (Savini et al. 2008, Holladay et al. 2009, Falavarjani et al. 2010). We used the Pentacam to measure directly three corneal characteristics: central and mean values in the true net corneal power map and a 4.5 mm equivalent K reading. We compared these measurements with the clinical history method and analyzed their accuracy with different third-generation formulas for calculating IOL power after myopic refractive surgery.
target refraction were calculated using different combinations (three Pentacam corneal power measurements, K cTNP, K mTNP and EKR, combined with third generation formulas, Holladay 1, SRK/T, and Hoffer Q) (Retzlaff et al. 1990, Hoffer 1994, 2007) . Statistical analyses were performed using SPSS (Version 16; SPSS Inc., Chicago, IL). Bonferroni multiple comparisons in a one-way analysis of variance (ANOVA) were used to compare the five corneal power measurements and the K value was calculated using CHM (KCHM). A one-sample t-test was performed to analyze the accuracy of the IOL calculation methods. Bonferroni multiple comparisons were used to compare mean arithmetic predictive errors using K mTNP with different formulas; p 0.05 was considered statistically significant.
Material and methods Results Our research followed the tenets of the Declaration of Helsinki and was approved by the Eye Center of Peking University Third Hospital. We obtained written informed consent from all subjects involved in the study. Our research also From June 2009 to May 2012, 22 consecutive patients with a history of myopic refractive surgery and subsequent phacoemulsification and IOL implantation in our eye center were enrolled in our study. The mean patient age was 49.35 8.0 years. All 22 patients (43 eyes) had a history of myopic refractive surgery (LASIK, LASEK or photorefractive keratectomy [PRK]). The corneal power of each eye was evaluated using an Auto Keratometer (Topcon,Tokyo, Japan), IOLMaster (Carl Zeiss Meditec, Dublin, CA), and Pentacam (Oculus). The following Pentacam values were measured: central true net power (cTNP), mean true net power (mTNP) and 4.5 mm equivalent K reading (EKR). Only quality images (according to quality specifications for the Pentacam) were used for each eye. The clinical history method was used to calculate corneal power for 17 patients (33 eyes) with available preoperative data. Axial length (AL) was measured using an immersion ultrasonic A scan (OcuScan®, Alcon Inc., Fort Worth, TX). Subsequently, 22 patients (37 eyes) underwent standard phacoemulsification surgery and all IOLs were placed in the capsular bag. The IOL power was calculated using mTNP, then combined with the SRK/T formula, and the final IOL power was determined by the surgeon. The same surgeon performed all surgeries and no surgical complications were reported. The final refraction was obtained 12 weeks after cataract surgery. Mean arithmetic and absolute differences between the actual postoperative refraction and
Corneal data from 43 eyes (22 patients) were analyzed. Twenty-six eyes were treated by LASIK, two eyes had LASEK and 15 had PRK. The mean pre-keratorefractive surgery refraction was 11.39 3.96 diopters. The mean pre-cataract surgery refraction was 8.62 6.61 diopters. The mean axial length was 29.52 2.12 mm and ranged from 25.72 to 33.41 mm. Subsequently, 22 patients (37 eyes) had cataract surgery . Table 1 shows the corneal power measurements; overall, the differences were significant (p 0.05). The Bonferroni multiple tests showed that Kauto and Kmaster were significantly higher than KcTNP, KmTNP, and KCHM, and the differences between them and EKR were not significant. With regard to the three Pentacam corneal measurements (KcTNP, KmTNP and EKR), the difference between any two measurements was significant and their mean value increased one by one (KcTNP 33.5 3.26 D, KmTNP 35.47 2.73 D, EKR 37.42 2.48 D). Table 1 also shows that compared to KCHM, only the KcTNP and KmTNP values were not significantly different (p 0.067 and 1.000, respectively). KmTNP (Pearson r 0.573, p 0.001) correlated with KCHM more closely than other values (Table 2). The mean differences and 95% CIs between the corneal power measured by different methods and KCHM are shown in Fig. 1. Figure 2 is a box plot of the IOL predictive errors from the various methods studied. Table 3 shows the mean arithmetic and absolute IOL predictive errors. Of the nine IOL calculation methods (three Pentacam measured corneal powers inserted into three third-generation formulas), only KmTNP inserted into SRK/T was not significantly different from zero; differences among the other eight IOL power calculations to measure corneal power 349
Table 1. Corneal power measured by different methods Corneal power measurement K AUTO Kmaster KCHM KcTNP KmTNP EKR
Mean ⴞ SD (D)
p*
Mean difference vs. KCHM (D)
38.33 2.18 38.18 2.23 35.32 2.68 33.58 3.26 35.47 2.73 37.42 2.48
0.000 0.000 – 0.067 1.000 0.009
3.01 2.86 – 1.74 0.15 2.10
95% CI of the difference vs. KCHM (D) 1.20 1.04 – 3.54 1.65 0.31
– 4.82 – 4.68 – 0.06 – 1.95 – 3.90
CI, confidence interval; K AUTO, K-value of autokeratometry; Kmaster, K-value of IOL master; KCHM, K-value from clinical history method; KcTNP, central K-value in true net power map of Pentacam; KmTNP, mentral K-value in true net power map of Pentacam; EKR, equivalent K-reading from the Pentacam *Bonferroni multiple comparisons to K CHM
methods were significant (p 0.05 with a one-sample t-test). The percentages of eyes within 0.50 D and 1.00 D of the refractive predictive error of the different methods are shown in Table 4. In KmTNP combined with Hoffer Q, Holladay 1 and SRK/T methods with relatively higher percentage values, ANOVA analysis showed no significant differences in their mean absolute errors. The Bonferroni multiple test of the mean arithmetic predictive errors showed that KmTNP-SRK/T and KmTNP-Hoffer Q were significantly lower than KmTNP-Hollady1 (Table 5).
Discussion IOL implantation after refractive surgery is a challenging procedure, because standard IOL power formulas can cause significant unintended postoperative refractive errors. IOL prediction errors after refractive laser surgery are mainly from two sources. One is the inaccurate corneal power determination by keratometers and corneal topography systems due to the altered relationship between the anterior and posterior corneal curvatures and the inability to detect the central value based on the Placido disc theory (Masket 2006, Hamilton and Hardten 2003, Randleman et al. 2002, Patel et al. 2000). The second source of error is incorrect Table 2. Pearson correlation coefficients between corneal powers measured by different methods and KCHM Corneal power measurement K AUTO Kmaster KcTNP KmTNP EKR KCHM
350
r
p
0.493 0.537 0.545 0.573 0.570 –
0.004 0.002 0.001 0.000 0.001 –
estimation of the effective lens position (ELP) in the third-generation IOL formula, which uses flattened corneal power (Haigis 2008, Aramberri 2003, Koch and Wang 2003). Methods for improving IOL prediction accuracy generally are aimed at obtaining the true corneal power and IOL formula adjustment. Many of these methods require preoperative information such as clinical history (Holladay 1997), Feiz-Mannis (Feiz et al. 2001) and corneal bypass (Walter et al. 2006), or they require several assumptions to arrive at an indirect corneal power determination such as the Maloney method, modified Maloney method and the Shammas no-history method (Smith et al. 1998, Wang et al. 2004). Improvements in the calculations include double-K adjustment (Aramberri 2003) and the fourth-generation formula (Holladay 2 and Haigis), which require extensive data including horizontal white-to-white distance, phakic anterior chamber depth and phakic lens thickness. We evaluated a direct and convenient corneal power measurement for calculating IOL power after myopic refractive surgery. Unlike most topography, which is based on the Placido disc theory, the Pentacam uses a Scheimpflug rotating imaging system and transforms elevation data into corneal curvature. The Pentacam method can obtain more accurate values for both anterior and posterior corneal power as well as the central area. True net power comes from the Gaussian optics formula for thick lenses (Olsen 1986). The mean value calculated from the 3.0 mm zone in the Pentacam true net power map is called mean true net power and the central value is called central true net power. The equivalent K reading, which is calculated automatically by the Pentacam in the Holladay report, is an adjustment of the corneal power calculated from the Gaussian optics formula (Holladay et al. 2009). Version 1.16 of Pentacam software can provide values of
Biotechnic & Histochemistry 2014, 89(5): 348–354
Fig. 1. Mean differences and 95% confidence intervals between corneal powers measured using different methods.
diameters from 1.0 to 7.0 mm. According to the findings of Holladay et al. (2009), 4.5 mm EKR gives a relatively accurate estimate of the central corneal power following refractive surgery. Consequently, we chose 4.5 mm EKR as one corneal power measurement for our study.
It was not surprising that Kauto and Kmaster were significantly higher than KCHM; it confirms the fact that conventional keratometry overestimates corneal power and leads to hyperopic outcomes after myopic corneal refractive surgery. We showed that in the three Pentacam corneal power measurements
Fig. 2. Box plot of the difference between actual postoperative refractions and target refractions using different methods.
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Table 3. Mean difference between actual postoperative refractions and target refractions with different methods Mean difference between actual postoperative refractions and target refractions (D) Arithmetic Method
Absolute
Eye (n)
mean ⴞ SD
range
mean ⴞ SD
range
37 37 37 37 37 37 37 37 37
0.52 0.94* 0.11 0.82 0.42 1.11* 1.26 1.10* 1.79 1.11* 2.3 1.25* 2.4 1.12* 1.64 0.93* 1.58 1.2*
1.38 – 1.35 2.25 – 2.81 2.54 – 3.00 3.4 – 1.72 4.47 – 1.28 4.31 – 1.31 0.22 – 5.09 0.54 – 4.44 0.54 – 4.39
0.88 0.60 0.55 0.62 0.88 0.79 1.4 0.9 1.88 0.95 2.36 1.11 2.4 1.12 1.67 0.87 1.61 1.15
0.14 – 3.35 0.01 – 2.81 0.03 – 3.00 0.01 – 3.4 0.26 – 0.47 0.21 – 4.31 0.22 – 5.09 0.08 – 4.44 0.03 – 4.39
KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KcTNP-Holladay1 KcTNP-SRK/T KcTNP-Hoffer Q EKR-Holladay1 EKR -SRK/T EKR-Hoffer Q
*Significantly different from zero; p 0.05 with one-sample t-test
(KcTNP,KmTNP, and 4.5 mm EKR), KmTNP had the greatest correlation with KCHM. This result differs from results reported by Savini et al. (2008) and Falavarjani et al. (2010), who found that the mean true net power in Pentacam was significantly different from KCHM. We also showed that 4.5 mm EKR was significantly higher than KCHM, which was consistent with the report by Savini et al. (2008), but different from the report by Falavarjani et al. (2010), who found that 4.5 mm EKR was highly correlated with KCHM. One reason for this difference may be that the patients recruited for the latter study had a lesser degree of myopia (mean SE 3.46 1.16 D, range 1.5 to 6.00 D). Savini et al. (2008) compared the EKR in eight patients (16 eyes) after refractive surgery with KCHM. The small sample size may contribute to the difference from our study. Other investigators have reported that corneal power overestimation in eyes Table 4. The percentages of eyes within 0.50 D and 1.00 D of refractive prediction error of different methods Percentage Method KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KcTNP-Holladay1 KcTNP-SRK/T KcTNP-Hoffer Q EKR-Holladay1 EKR -SRK/T EKR-Hoffer Q
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within ⴞ 0.50 D
within ⴞ 1.00 D
29.7 67.6 45.9 16.2 8.1 8.1 2.7 10.8 16.2
67.6 86.5 67.6 37.8 13.5 8.1 8.1 21.6 37.8
that had undergone myopic keratorefractive surgery is directly correlated with the amount of refractive change and may not occur after a small amount of correction (Hamed et al. 2002, Seitz et al. 1999). We found that among the three Pentacam corneal power measurements (KcTNP, KmTNP and 4.5-mm EKR), the mean true net power (3 mm) estimates were closest to KCHM, and the percentage values of 0.50 D and 1.00 D of the predictive error of KmTNP combined with Hoffer Q, Holladay 1 and SRK/T methods were relatively higher. KcTNP was relatively lower and, when inserted into the three third generation formulas, yielded a myopic prediction error. Interestingly, the 4.5 mm EKR was relatively higher (p 0.05) compared to Kauto and Kmaster and, when inserted into the three third-generation formulas, yielded a hyperopic prediction error. The difference indicates the size of the measured area of corneal power. In eyes that had PRK or LASIK, circular zones of different sizes have different mean dioptric values, because laser energy may vary with the ablation depth. As the assessed zone increases, the value
Table 5. Bonferroni multiple test of mean arithmetic prediction error between KmTNP combined different formulas Method pair KmTNP-SRK/T KmTNP-Holladay1 KmTNP-SRK/T KmTNP-Hoffer Q KmTNP-Holladay1 KmTNP-Hoffer Q
Biotechnic & Histochemistry 2014, 89(5): 348–354
p 0.05 (0.018) 0.05 (0.552) 0.05 (0.000)
becomes similar to the conventional K value and there is a greater probability of overestimating the corneal power in the myopic eye. In nine Pentacam corneal power and third-generation formula combinations, only KmTNP inserted into SRK/T was not significantly different from zero and the percentages of eyes within 0.50 D and 1.00 D of the refractive prediction errors of this method were 67.6 and 86.5%, respectively. The percentage values of KcTNP inserted in SRK/T were 8.1 and 13.5%, respectively. These findings differ from the those of Kim et al. (2009) where 30 eyes were analyzed using the single central true net power combined with SRK/T; the percentages of eyes within 0.50 D and 1.00 D of the refractive prediction error were 70 and 93%, respectively. It is not clear what caused the differences between results report by Kim et al. (2009) and our results; however, we believe that the corneal apex might shift after photo-ablation and the determination of the visual quality is a 3 mm pupil size. We used an immersion ultrasonic A scan to measure axial length; nearly all axial length values exceeded 26 mm. It has been reported, however, that some physicians prefer to measure axial length using the IOLMaster™ (Andrew et al. 2001). Although the IOLMaster is reliable for axial length and anterior chamber depth measurements, it may not be accurate for patients who have poor vision with weak fixation ability (Verhulst and Vrijghem 2001). Therefore, we used the traditional ultrasonic A scan and compared the predictive errors of KmTNP combined with SRK/T, Holladay 1 and Hoffer Q. The Holladay 1 method tends to predict hyperopic outcomes (mean arithmetic predictive error of 0.52 0.94 D) and the Hoffer Q method tends to predict myopic outcomes (mean arithmetic predictive error of 0.42 1.11 D). This is consistent with the findings of McCarthy et al. (2011), who compared the three formulas with CHM in eyes 27 mm. Third generation IOL formulas (SRK/T, Holladay 1 and Hoffer Q) depend on the corneal curvature to estimate the ELP. By entering the flatter post-LASIK K-values, the formula underestimates ELP. After corneal refractive surgery, the error produced by the ELP underestimation and the error produced by the corneal power overestimation or underestimation are cumulative and yield hyperopic or myopic outcomes. When we entered three different Pentacam K values (KcTNP, KmTNP and 4.5 mm EKR) in these three formulas, we found that KcTNP combined with the three formulas all predicted myopic outcomes (the mean arithmetic predictive errors were all negative values) and 4.5 mm EKR combined with the three formulas all predicted hyperopic outcomes (the mean arithmetic predic-
tive errors were positive values). There were both positive and negative mean arithmetic predictive errors in KmTNP combined with the three formulas. This finding was consistent with the difference we found for the three Pentacam corneal power measurements (KcTNP KmTNP 4.5 mm EKR). All of our patients had myopic refractive surgery and almost all axial lengths exceeded 26 mm. Further studies of hyperopic surgical history and other axial length measurements for the universal use of Pentacam mean true net power to calculate IOL power after refractive surgery. We compared several Pentacam corneal power measurements and their accuracy in calculating IOL power after refractive surgery for myopia. Although the third-generation IOL formula has drawbacks, the mean true net power inserted in the SRK/T formula provides a direct IOL calculation method that does not require preoperative data or assumptions. Declaration of interests: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this paper.
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