Journal of Refractive Surgery

Original Article 

Agreement Between Predicted and Measured Ablation Depth After Femtosecond Laser-Assisted LASIK for Myopia

Giacomo Savini, MD; Arthur B. Cummings, MD; Nicole Balducci, MD; Piero Barboni, MD; Jinhai Huang, MD; Marco Lombardo, MD, PhD; Sebastiano Serrao, MD, PhD; Pietro Ducoli, MD

Abstract

PURPOSE:

To investigate agreement between the predicted ablation depth calculated by the EX500 excimer laser (Wavelight Laser Technologie AG, Erlangen, Germany) and the measured ablation depth in eyes that have undergone femtosecond laser-assisted LASIK (FS-LASIK) for myopia.

METHODS:

Corneal thickness was measured with a rotating Scheimpflug camera preoperatively and 3 months postoperatively and the difference between these values was defined as the measured ablation depth. The difference between the predicted and the measured ablation depth was defined as the difference in ablation depth (ΔAD).

RESULTS:

In 85 eyes of 85 patients, no statistically significant difference was detected between the mean predicted ablation depth (66.33 ± 24.15 µm) and the measured ablation depth at the thinnest corneal location (67.04 ± 30.94 µm), the corneal apex (67.52 ± 31.22 µm), or the pupil center (67.73 ± 31.48 µm). Bland–Altman plots revealed moderate agreement for measurements at the thinnest point (95% limits of agreement [LoA]: −25.13 to 23.70 µm), corneal apex (95% LoA: −24.70 to 22.33 µm), and pupil center (95% LoA: −25.30 to 22.51 µm), with a proportional bias between the average ablation depth and ΔAD. The predicted ablation depth was overestimated in eyes with lower correction and underestimated in eyes with higher correction.

CONCLUSIONS:

Moderate agreement between the predicted and measured ablation depth warrants caution when planning myopic FS-LASIK and calculating the residual bed thickness and percent tissue altered. When higher amounts of correction are planned, the laser software may underestimate the predicted ablation depth.

[J Refract Surg. 2016;32(3):164–170.]

Abstract

PURPOSE:

To investigate agreement between the predicted ablation depth calculated by the EX500 excimer laser (Wavelight Laser Technologie AG, Erlangen, Germany) and the measured ablation depth in eyes that have undergone femtosecond laser-assisted LASIK (FS-LASIK) for myopia.

METHODS:

Corneal thickness was measured with a rotating Scheimpflug camera preoperatively and 3 months postoperatively and the difference between these values was defined as the measured ablation depth. The difference between the predicted and the measured ablation depth was defined as the difference in ablation depth (ΔAD).

RESULTS:

In 85 eyes of 85 patients, no statistically significant difference was detected between the mean predicted ablation depth (66.33 ± 24.15 µm) and the measured ablation depth at the thinnest corneal location (67.04 ± 30.94 µm), the corneal apex (67.52 ± 31.22 µm), or the pupil center (67.73 ± 31.48 µm). Bland–Altman plots revealed moderate agreement for measurements at the thinnest point (95% limits of agreement [LoA]: −25.13 to 23.70 µm), corneal apex (95% LoA: −24.70 to 22.33 µm), and pupil center (95% LoA: −25.30 to 22.51 µm), with a proportional bias between the average ablation depth and ΔAD. The predicted ablation depth was overestimated in eyes with lower correction and underestimated in eyes with higher correction.

CONCLUSIONS:

Moderate agreement between the predicted and measured ablation depth warrants caution when planning myopic FS-LASIK and calculating the residual bed thickness and percent tissue altered. When higher amounts of correction are planned, the laser software may underestimate the predicted ablation depth.

[J Refract Surg. 2016;32(3):164–170.]

When planning a refractive treatment, surgeons rely on the laser portal software, which provides the estimated ablation depth based on the amount of intended correction, the optical zone diameter, the preoperative corneal curvature, and the induced spherical aberration.1–3 Knowing the estimated ablation depth is important for two reasons: (1) preoperatively it enables surgeons to calculate the “percent tissue altered” (PTA) and minimize the risk of ectasia4 and (2) postoperatively it may be used to check whether a residual refractive error depends on the ablation depth, if it was different from the preoperative estimated value.

Several studies have investigated the difference between the estimated and the postoperatively measured ablation depth and showed conflicting results.5–10 Some authors reported that the measured tissue ablation was greater than the predicted tissue ablation,5–8 whereas others found it to be lower than the predicted tissue ablation.9,10 Only one study did not detect any systematic difference.11 These differences may depend on several factors, such as the technique used to measure the corneal thickness and the laser model used to correct the refractive error.

The purpose of this study was to assess agreement between the estimated ablation depth calculated by the EX500 excimer laser (Wavelight Laser Technologie AG, Erlangen, Germany) and the measured ablation depth in eyes that underwent femtosecond laser-assisted LASIK (FS-LASIK) for myopia. The measured ablation depth was defined as the difference between the preoperative and the 3-month central corneal thickness values provided by a rotating Scheimpflug camera.

Patients and Methods

Patient Population

This prospective study included all consecutive patients operated on between September 2012 and July 2014 using FS-LASIK for myopia and myopic astigmatism. Each patient was informed of the purpose of the study and gave written consent to participate. The study protocol was approved by the G.B. Bietti Foundation ethics committee. Study methods adhered to the tenets of the Declaration of Helsinki guidelines for research involving human subjects.

Surgical Technique

Each patient underwent bilateral FS-LASIK using the FS200 femtosecond laser (WaveLight Laser Technologie AG) and the EX500 excimer laser. Only patients treated according to the same ablation protocol (wavefront-optimized) were enrolled. The wavefront-optimized ablation protocol applies a precalculated spherical aberration treatment to produce an aspherical non-adjustable profile.2,12,13 Flap thickness was set at 120 µm and the planned flap diameter was 9 mm in all cases. Pulse energy for bed cut and side cut ranged between 0.66 and 0.7 µJ, as recommended by the manufacturer. To achieve emmetropia, the Wellington nomogram was applied, as suggested by the manufacturer and as is the practice of other surgeons.8,14 The nomogram that is issued with the laser is a starting point for surgeons new to the platform and is intended to ensure that they do not get refractive surprises. The nomogram adds −0.25 diopters (D) to corrections of less than 2.00 D. Between −2.25 and −6.00 D, there are typically no adjustments. The nomogram subtracts −0.25 D from the ablation for corrections above −6.00 D, −0.50 D for corrections above −7.00 D, −0.75 D for corrections above −8.00 D, and −1.00 D for corrections above −9.00 D. This trend continues up to −12.00 D, where −1.75 D is subtracted from the refraction (ie, for a −12.00 D correction, −10.25 D would be entered into the laser portal software). The predicted ablation depth, following the nomogram adjustment, was displayed by the laser software and recorded for statistical analysis. For comparative purposes, we also analyzed the predicted ablation depth without any nomogram-based adjustment.

After surgery, patients were prescribed 0.3% netilmycin and 0.1% dexamethasone (Netildex; SIFI, Catania, Italy) to be taken four times a day for the first week, and preservative-free artificial tears to be taken every hour for the first day and then tapered off during the following 3 months.

Assessment of Corneal Thickness

Preoperatively, all eyes underwent subjective manifest and cycloplegic refractions, slit-lamp examination of the anterior segment, intraocular pressure measurement, indirect ophthalmoscopy, and anterior segment imaging with the Pentacam HR rotating Scheimpflug camera (software version 1.20r10; Oculus Optikgeräte, Wetzlar, Germany). Scans were taken in the automatic release mode. Of the different scanning options available, the 25-picture scan was used. Only scans that had an examination quality specification graded by the instrument as “OK” were saved. The postoperative evaluation was performed after 3 months and included manifest and cycloplegic refraction and anterior segment Scheimpflug imaging.

The rotating Scheimpflug camera measures corneal thickness normal to the anterior surface tangent15 and provides three pachymetry values for measurements taken at the pupil center, corneal apex, and thinnest location. In this study, all three values were investigated. The measured ablation depth was defined as the difference between the preoperative and the postoperative pachymetry values.8,10 The difference between the predicted and the measured ablation depth was defined as the difference in ablation depth (ΔAD).

Statistical Analysis

Because ocular measurements are more alike between fellow eyes than between eyes of different patients and measurements from fellow eyes cannot be treated as if they were independent,16 only one eye per patient was randomly selected and included for statistical analysis. When both eyes were eligible, the more myopic one was chosen. All statistical analyses were performed using GraphPad Instat version 3.05 for Macintosh (GraphPad Software, San Diego, CA). A P value less than .05 was considered statistically significant. The Kolmogorov–Smirnov test was used to assess the normal distribution of data. A paired t test was performed to evaluate the difference between normally distributed data, whereas the Wilcoxon matched pairs test was performed to compare data not normally distributed. Repeated measures analysis of variance (ANOVA) with the Bonferroni posttest was performed to compare mean pachymetry values at different locations, both preoperatively and postoperatively. Moreover, linear regression was used to investigate the relationship between the predicted and measured ablation depth, between the preoperative spherical equivalent and the difference between predicted and measured ablation depth (ΔAD), and between ΔAD and different preoperative parameters such as preoperative keratometry and pachymetry.

Agreement was evaluated using Bland–Altman plots, in which the differences between the measurements (ie, the ΔAD) are plotted against their mean.17 The 95% limits of agreements (LoA) were defined as the mean ± 2 standard deviations of the differences between the two measurement techniques.

Bland–Altman plots were also used to study possible relationships of the discrepancies between the measurements and the true value (ie, proportional bias). The existence of proportional bias indicates that the methods do not agree equally across the range of measurements; that is, the LoA will depend on the actual measurement. To evaluate this relationship formally, the difference between the methods was regressed on the average of the two methods. When a relationship between the differences and the true value was identified (ie, a significant slope of the regression line), regression-based 95% LoA were provided.18

Based on a previous study reporting a within-subject standard deviation of 4.29 µm for the thinnest pachymetry value given by the rotating Scheimpflug camera,19 and using PS version 3.0.12 ( http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/PowerSampleSize, checked on March 27, 2015), it was estimated that a sample size of 40 eyes would be necessary to detect a difference in ablation depth of 2.5 µm with a power of 95% at a significant level of 5%.

Results

Eighty-five eyes of 85 patients (48 women and 37 men; mean age: 34.8 ± 8.2; range: 20 to 59 years) who underwent FS-LASIK for myopia were analyzed preoperatively and 3 months postoperatively. No complications were recorded during or after the surgery. The mean manifest spherical equivalent was −4.10 ± 2.10 D (range: −9.00 to −1.25 D) preoperatively and 0.04 ± 0.20 D (range: −0.50 to 0.50 D) postoperatively (P < .0001, paired t test). The optical zone, which was selected on the basis of the pupil diameter and PTA, was 6 mm in 10 eyes, 6.5 mm in 55 eyes, 7 mm in 15 eyes, 7.5 mm in 1 eye, and 8 mm in 4 eyes. The transition zone ranged between 0.3 and 1.25 mm. The mean preoperative keratometry was 43.46 ± 1.50 D.

Analysis of the Predicted Ablation Depth with Nomogram Adjustment

The mean predicted ablation depth with nomogram adjustment was 66.33 ± 24.15 µm (range: 23.14 to 117.45 µm). Linear regression detected a significant relationship between the preoperative spherical equivalent and the predicted ablation depth with nomogram adjustment (r = −0.9154, P < .0001).

The preoperative and postoperative mean pachymetry values are summarized in Table 1. Preoperatively, repeated measures ANOVA detected a statistically significant difference among corneal thicknesses at the three locations (P < .0001) because the pachymetry value was significantly lower at the thinnest point than at the corneal vertex (P < .001) and the pupil center (P < .001), with no statistically significant difference between locations at the corneal vertex and the pupil center. The same results were observed postoperatively.

Mean Preoperative and Postoperative Pachymetry (µm)

Table 1:

Mean Preoperative and Postoperative Pachymetry (µm)

Table 2 shows the mean values of the predicted and measured ablation depth. The statistical analysis disclosed a high correlation between them (P < .0001 at any location, Figure 1), without any significant difference (ANOVA, P = .5080). However, the Bland–Altman plots revealed only moderate agreement between the predicted and measured ablation depth for measurements at any location. In the Bland–Altman analysis, linear regression disclosed a proportional bias between the average ablation depth and ΔAD (measurements at the thinnest point: r = −0.5664, P < .0001; measurements at the corneal vertex: r = −0.6106, P < .0001; measurements at the pupil center: r = −0.6230, P < .0001). An overestimation of the predicted ablation depth was evident in eyes with lower intended tissue ablation and an underestimation in eyes with higher intended tissue ablation. Figure 2 shows the regression-based limits of agreement (16.30 to 0.25 × ± 20.06); the line indicating the mean of the differences had a slope of −0.255 per average µm. Consistently, ΔAD was related to the preoperative spherical equivalent when corneal thickness was measured at the thinnest point (r = 0.2321, P = .0325), corneal apex (r = 0.3159, P = .0032), and pupil center (r = 0.3358, P < .0017). On the other hand, ΔAD was not statistically correlated to the preoperative keratometry or pachymetry.

Predicted Ablation Depth With Nomogram Adjustment and Measured Ablation Depth in the Whole Sample

Table 2:

Predicted Ablation Depth With Nomogram Adjustment and Measured Ablation Depth in the Whole Sample

Linear regression analysis revealed a significant correlation (r = 0.9311, P < .0001) between the measured (Y) and the predicted ablation depth with nomogram adjustment (X), with reference to the thinnest corneal point (Y = −12.076 + 1.193*X).

Figure 1.

Linear regression analysis revealed a significant correlation (r = 0.9311, P < .0001) between the measured (Y) and the predicted ablation depth with nomogram adjustment (X), with reference to the thinnest corneal point (Y = −12.076 + 1.193*X).

Regression-based 95% limits of agreement between predicted and measured ablation depth.

Figure 2.

Regression-based 95% limits of agreement between predicted and measured ablation depth.

We therefore divided the whole sample into two subsets (based on the 50th percentile): eyes with a predicted ablation depth less than 63.49 µm (n = 42) and eyes with a predicted ablation depth greater than 63.49 µm (n = 43). As shown in Table 3, the predicted ablation depth was higher than the measured ablation depth at each location in the less than 63.49 µm group and lower in the greater than 63.49 µm group.

Predicted and Measured Ablation Deptha

Table 3:

Predicted and Measured Ablation Depth

To provide surgeons with a practical reference, we also looked for cut-off values below or above which the predicted ablation depth, on average, is neither overestimated or underestimated. We found that when the predicted ablation depth was less than 50 µm, an overestimation greater than 10 µm was found in 42% of eyes (an underestimation < −10 µm in 8% of eyes). Conversely, when the predicted ablation depth was greater than 87 µm, an underestimation less than −10 µm was found in 40% of eyes (an overestimation > 10 µm in 10% of eyes). In eyes with a predicted ablation depth between 50 and 87 µm, the cases with overestimation or underestimation by more than 10 µm were almost the same (10% vs 18%).

Analysis of the Predicted Ablation Depth Without Nomogram Adjustment

When the predicted ablation depth was considered without the nomogram-based adjustment, the differences between the predicted and measured ablation depth were lower. In the less than 63.49 µm group, the predicted ablation depth decreased from 45.82 ± 11.71 to 44.06 ± 13.32 µm; it was still higher than the measured ablation depth measured at any location, but the difference was no longer statistically significant. In the greater than 63.49 µm group, the predicted ablation depth increased from 86.37 ± 14.19 to 88.81 ± 17.41 µm; it was still lower than the measured ablation depth at any location, but the difference was statistically significant only when compared to measurements at the pupil center (P = .0239).

Agreement between the predicted ablation depth without nomogram adjustment and the actual ablation depth slightly improved across the whole sample. The 95% LoA ranged between −24.55 and +23.86 µm at the thinnest corneal point, between −23.94 and +22.30 µm at the corneal apex, and between −24.44 and +22.38 µm at the pupil center. A proportional bias between the average ablation depth and ΔAD could still be observed, but was weaker (measurements at the thinnest point: r = −0.3087, P = .0040; measurements at the corneal vertex: r = −0.3471, P = .0011; measurements at the pupil center: r = −0.3641, P = .0006).

Discussion

This study examined the relationship between the predicted and measured ablation depth in eyes that had undergone FS-LASIK for myopia using the EX-500 excimer laser. On average, the 3-month measured value, calculated as the difference between preoperative and postoperative pachymetry by the rotating Scheimpflug camera, was highly correlated with the predicted ablation depth and did not show any statistically significant difference within the sample as a whole. A similar result was recently reported for the same laser platform by Kanellopoulos et al.11

However, a more detailed analysis of our data reveals that agreement between the predicted ablation depth (according to the nomogram-based adjustment) and the measured ablation depth is only moderate and that the EX500 laser tends to underablate the corneal tissue in eyes with lower myopic correction, and overablate it in eyes with higher myopic correction. These results are in accordance with those reported by Labiris et al., who investigated the ablation depth resulting from surgery with the Allegretto 200-Hz (WaveLight) laser using the same wavefront-optimized ablation profile as in the current study and observed that the mean predicted ablation depth was lower than the measured value, this difference being more marked in eyes with higher myopic correction.8 Our results are also in partial agreement with those reported by Kanellopoulos et al., although these authors stated that the difference between the predicted and measured ablation depth did not depend on their magnitude.11 A careful look at Figure 1 of their study actually reveals a trend toward overestimation by the laser software for lower amounts of ablation depth and toward underestimation for higher amounts of ablation depth. This trend is less evident in their sample than in ours, as demonstrated by the different slope of the line indicating the mean difference (−0.054 in their study, −0.255 in ours).

Finding an explanation for the discrepancy between the predicted and measured ablation depth is not easy. We do not believe that it depends on the inaccurate measurements by the Scheimpflug camera, because these have been shown to be highly repeatable after LASIK20,21 and with no significant difference compared to those provided by ultrasound pachymetry.22 Moreover, a different Scheimpflug camera (Sirius; CSO, Firenze, Italy) yielded the same results on the same sample (unpublished data) and we think that it is extremely unlikely that two different devices would give the same result with the same error. Finally, even in the event of a measurement error related to the Scheimpflug camera, it would be logical to expect a fixed and not a proportional bias, such as the one found in the current study. However, a confirmatory study based on a different technology, such as optical coherence tomography, might be an interesting project.

We cannot ascribe the above-mentioned discrepancy to epithelial remodeling and hyperplasia, which have been shown to occur after LASIK.9 This phenomenon, which has been previously suggested as an explanation for the discrepancy between predicted and measured ablation depth,10,23 seems unlikely in our sample. Epithelial thickening should have led to an increased central corneal thickness and subsequent overestimation of the predicted ablation depth with nomogram adjustment in the eyes of the greater than 63.49 group, but we found the opposite result (ie, a mean underestimation in eyes with higher degrees of myopic correction).

It might be postulated that the discrepancy between the predicted and measured ablation depth is related to a difference between the axis along which the Scheimpflug camera took the measurements and the axis used by the excimer laser to center the treatment. However, even this explanation seems unlikely because we took measurements at three different points (thinnest point, corneal apex, and pupil center) and the results did not change.

Hence, the most reasonable explanation is that the ablation depth value predicted by the laser software does not correspond to the actual ablation depth, probably because of the increased efficacy in laser energy with higher corrections. This hypothesis is supported by the fact that the manufacturer distributes a nomogram (known as the Wellington nomogram) to avoid unintentional undercorrection in patients with low myopia (where the predicted ablation depth is overestimated) and overcorrection in patients with high myopia (where the predicted ablation depth is underestimated). We postulate that the reason for the apparent increased efficacy in laser energy for higher corrections is related to two factors. First, as the treatment progresses, the cornea becomes more dehydrated and the laser ablation rate increases over the time of treatment.24 Second, with a myopic treatment, the central cornea becomes flatter as the treatment progresses and again the cosine effect is reduced and laser ablation efficacy is increased.

Experience has taught surgeons to develop nomograms, such as the one we followed, to increase the refractive predictability of the laser and improve patient outcomes. When the Wellington nomogram is applied to higher corrections, a lower degree of treatment is entered in the laser portal software (eg, −10.25 D to achieve −12.00 D of refractive change) and, as a consequence, the predicted ablation depth is reduced. However, because the refractive defect is fully corrected, as demonstrated by the refractive outcome of our sample, the amount of corneal tissue that is removed should correspond to the one calculated for the full and not for the adjusted correction. Thus, when planning a treatment and seeking to respect the residual corneal bed thickness, the laser's ablation depth should not be taken at face value because it is based on the nomogram quantum. Our data suggest that the ablation depth would be more accurately predicted if the actual refractive change were entered without any nomogram-based adjustment. When we did so, the difference between the predicted and measured ablation depth lost its statistical significance in both groups (with the only exception of pupil-centered measurements in the greater than 63.49 µm group). Unfortunately, even this approach is not perfect, as shown by Bland–Altman analysis.

In any case, it should be highlighted that, notwithstanding the relatively large LoA between the predicted and the measured ablation depth, all eyes had a refractive outcome within 0.50 D of emmetropia. Therefore, the discrepancy between predicted and measured ablation depth does not interfere with the desired visual outcome in the case of the excimer laser investigated in this study.

The lack of a statistically significant difference across the whole sample between the measured and predicted ablation depth with nomogram adjustment suggests that Scheimpflug imaging can be considered a valuable technology for assessing corneal thickness changes after FS-LASIK. However, the relatively large LoA between the predicted and measured ablation depth suggest that caution is warranted. In eyes with higher amounts of correction, the laser software may underestimate the predicted ablation depth with nomogram adjustment. Because the latter parameter is used to calculate the risk of postoperative corneal ectasia,4 underestimating the ablation depth may lead surgeons to underestimate the risk of post-LASIK ectasia. Relying on the predicted ablation depth for the full correction of the refractive defect (and not the nomogram-based adjusted correction) reduces the difference compared to the measured ablation depth and is a safer approach.

References

  1. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg. 1988;14:46–52. doi:10.1016/S0886-3350(88)80063-4 [CrossRef]
  2. Mrochen M, Donitzky C, Wüllner C, Löffler J. Wavefront-optimized ablation profiles: theoretical background. J Cataract Refract Surg. 2004;30:775–785. doi:10.1016/j.jcrs.2004.01.026 [CrossRef]
  3. Manns F, Ho A, Parel J-M, Culbertson W. Ablation profiles for wavefront-guided correction of myopia and primary spherical aberration. J Cataract Refract Surg. 2002;28:766–774. doi:10.1016/S0886-3350(01)01322-0 [CrossRef]
  4. Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87–95. doi:10.1016/j.ajo.2014.04.002 [CrossRef]
  5. Durairaj VD, Balentine J, Kouyoumdjian G, et al. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology. 2000;107:2140–2143. doi:10.1016/S0161-6420(00)00407-3 [CrossRef]
  6. Erie JC, Hodge DO, Bourne WM. Confocal microscopy evaluation of stromal ablation depth after myopic laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg. 2004;30:321–325. doi:10.1016/j.jcrs.2003.09.058 [CrossRef]
  7. Lackerbauer CA, Gruterich M, Ulbig M, Kampik A, Kojetinsky C. Correlation between estimated and measured corneal ablation and refractive outcomes in laser in situ keratomileusis for myopia. J Cataract Refract Surg. 2009;35:1343–1347. doi:10.1016/j.jcrs.2009.03.030 [CrossRef]
  8. Labiris G, Sideroudi H, Giarmoukakis A, Koukoulas S, Pagonis G, Kozobolis VP. Evaluation of the difference between intended and measured ablation and its impact on refractive outcomes of the wavefront optimize profile and the S001 Wellington nomogram in myopic spherocylindrical corrections. Clin Experiment Ophthalmol. 2012;40:127–133. doi:10.1111/j.1442-9071.2011.02633.x [CrossRef]
  9. Reinstein DZ, Archer TJ, Gobbe M. Corneal ablation depth read-out of the MEL 80 excimer laser compared to Artemis three-dimensional very high-frequency digital ultrasound stromal measurements. J Refract Surg. 2010;26:949–959. doi:10.3928/1081597X-20100114-02 [CrossRef]
  10. Arbelaez MC, Vidal C, Arba-Mosquera S. Central ablation depth and postoperative refraction in excimer laser myopic correction measured with ultrasound, Scheimpflug, and optical coherence pachymetry. J Refract Surg. 2009;25:699–708. doi:10.3928/1081597X-20090707-04 [CrossRef]
  11. Kanellopoulos AJ, Georgiadou S, Asimellis G. Objective evaluation of planned versus achieved stromal thickness reduction in myopic femtosecond laser-assisted LASIK. J Refract Surg. 2015;31:628–632. doi:10.3928/1081597X-20150820-09 [CrossRef]
  12. Kohnen T. Classification of excimer laser profiles. J Cataract Refract Surg. 2006;32:543–544. doi:10.1016/j.jcrs.2006.02.002 [CrossRef]
  13. Stojanovic A, Wang L, Jankov MR, Nitter TA, Wang Q. Wavefront optimized versus custom-Q treatments in surface ablation for myopic astigmatism with the WaveLight ALLEGRETTO laser. J Refract Surg. 2008;24:779–789.
  14. Bohac M, Biscevic A, Koncarevic M, Anticic M, Gabric N, Patel S. Comparison of Wavelight Allegretto Eye-Q and Schwind Amaris 750S excimer laser in treatment of high astigmatism. Graefes Arch Clin Exp Ophthalmol. 2014;252:1679–1686. doi:10.1007/s00417-014-2776-2 [CrossRef]
  15. Villavicencio O, Belin MW, Ambrósio R Jr, Steinmueller A. Corneal pachymetry: new ways to look at an old measurement. J Cataract Refract Surg. 2014;40:695–701. doi:10.1016/j.jcrs.2014.04.001 [CrossRef]
  16. Katz J, Zeger S, Liang KY. Appropriate statistical methods to account for similarities in binary outcomes between fellow eyes. Invest Ophthalmol Vis Sci. 1994;35:2461–2465.
  17. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1:307–310. doi:10.1016/S0140-6736(86)90837-8 [CrossRef]
  18. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8:135–160. doi:10.1191/096228099673819272 [CrossRef]
  19. Aramberri J, Araiz L, Garcia A, et al. Dual versus single Scheimpflug camera for anterior segment analysis: Precision and agreement. J Cataract Refract Surg. 2012;38:1934–1949. doi:10.1016/j.jcrs.2012.06.049 [CrossRef]
  20. Huang J, Pesudovs K, Yu A, et al. A comprehensive comparison of central corneal thickness measurement. Optom Vis Sci. 2011;8:940–949. doi:10.1097/OPX.0b013e31821ffe2c [CrossRef]
  21. Jain R, Dilraj G, Grewal SP. Repeatability of corneal parameters with Pentacam after laser in situ keratomileusis. Indian J Ophthalmol. 2007;55:341–347. doi:10.4103/0301-4738.33819 [CrossRef]
  22. Park SH, Choi SK, Lee D, Jun EJ, Kim JH. Corneal thickness measurement using Orbscan, Pentacam, Galilei, and ultrasound in normal and post-femtosecond laser in situ keratomileusis eyes. Cornea. 2012;31:978–982. doi:10.1097/ICO.0b013e31823d03fc [CrossRef]
  23. Reinstein DZ, Srivannaboon S, Gobbe M, et al. Epithelial thickness profile changes induced by myopic LASIK as measured by Artemis very high-frequency digital ultrasound. J Refract Surg. 2009;25:444–450. doi:10.3928/1081597X-20090422-07 [CrossRef]
  24. Dougherty PJ, Wellish KL, Maloney RK. Excimer laser ablation rate and corneal hydration. Am J Ophthalmol. 1994;118:169–176. doi:10.1016/S0002-9394(14)72896-X [CrossRef]

Mean Preoperative and Postoperative Pachymetry (µm)

ParameterPreoperativePostoperative
Thinnest point548.16 ± 29.73481.12 ± 45.71
Vertex552.69 ± 29.65485.18 ± 46.02
Pupil center552.55 ± 29.53484.82 ± 45.95

Predicted Ablation Depth With Nomogram Adjustment and Measured Ablation Depth in the Whole Sample

ParameterPredicted Ablation Depth (µm)Measured Ablation Depth

Thinnest PointCorneal VertexPupil Center
Mean value ± SD (µm)66.33 ± 24.1567.04 ± 30.9467.52 ± 31.2267.73 ± 31.48
Correlation coefficient (r)0.93110.94150.9414
95% limits of agreement−24.99 to +24.10−24.70 to +22.33−25.30 to +22.51

Predicted and Measured Ablation Deptha

Parameter< 63.49 µm Group> 63.49 µm Group
Predicted ablation depth (µm)45.82 ± 11.7186.37 ± 14.19
Measured ablation depth (µm)
  Thinnest point41.38 ± 15.95 (P = .0078)92.16 ± 19.09 (P = .0029)
  Corneal vertex41.83 ± 16.93 (P =.0013)92.60 ± 19.14 (P = .0009)
  Pupil center41.79 ± 17.30 (P = .0156)93.07 ± 18.97 (P = .0003)
Authors

From Fondazione G.B. Bietti IRCCS, Rome, Italy (GS, ML, SS, PD); Wellington Eye Clinic, Dublin, Ireland (ABC); Studio Oculistico d'Azeglio, Bologna, Italy (NB, PB); San Raffaele Scientific Institute, Milan, Italy (PB); and the School of Optometry and Ophthalmology and Eye Hospital, Wenzhou Medical University, Wenzhou, Zhejiang, People's Republic of China (JH).

Supported by the Italian Ministry of Health, Fondazione Roma.

Dr. Cummings is a consultant for Alcon Laboratories, Inc., and WaveLight. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (GS, ABC, PB, JH, ML, SS, PD); data collection (NB); analysis and interpretation of data (GS, NB, ML, SS, PD); writing the manuscript (GS, NB, ML, SS); critical revision of the manuscript (ABC, PB, JH, PD); supervision (PD)

Correspondence: Giacomo Savini, MD, Fondazione G.B. Bietti, IRCCS, Via Livenza, 3 Rome, Italy. E-mail: giacomo.savini@alice.it

Received: September 13, 2015
Accepted: December 03, 2015

10.3928/1081597X-20160121-03

Sign up to receive

Journal E-contents