July 2010
Volume 51, Issue 7
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Cornea  |   July 2010
The Shape of Posterior Corneal Surface in Normal, Post-LASIK, and Post–epi-LASIK Eyes
Author Affiliations & Notes
  • Lin Zhang
    From the Tianjin Eye Hospital and Eye Institute, Tianjin Medical University, Tianjin, China.
  • Yan Wang
    From the Tianjin Eye Hospital and Eye Institute, Tianjin Medical University, Tianjin, China.
  • Corresponding author: Yan Wang, Tianjin Eye Hospital and Eye Institute, Tianjin Medical University, No 4. Gansu Road, Heping District, Tianjin 300020, China; wangyan7143@vip.sina.com
Investigative Ophthalmology & Visual Science July 2010, Vol.51, 3468-3475. doi:10.1167/iovs.09-4811
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      Lin Zhang, Yan Wang; The Shape of Posterior Corneal Surface in Normal, Post-LASIK, and Post–epi-LASIK Eyes. Invest. Ophthalmol. Vis. Sci. 2010;51(7):3468-3475. doi: 10.1167/iovs.09-4811.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose.: To evaluate the shape of the posterior corneal surface in eyes with normal myopia or myopic astigmatism and the changes after LASIK and epi-LASIK.

Methods.: A total of 152 eyes were included in the study. The posterior corneal elevation was measured with Scheimpflug imaging (Pentacam; Oculus, Wetzlar, Germany), before LASIK or epi-LASIK and at 1 and 6 months after surgery, along the four optical zones (center and 2-, 4-, and 6-mm diameters) as a function of the meridian.

Results.: In normal eyes, the heights were lower in the vertical meridian than in the horizontal meridian and were also lower in the superior hemisphere than in the inferior hemisphere. In post-LASIK eyes, almost all regions protruded 1 month after surgery, except for the 6-mm diameter, which exhibited a slight but not significant (P = 0.731) backward displacement. The various elevations returned to original levels 6 months after LASIK. In the post–epi-LASIK group, there was a significant backward displacement in the center and 2-mm regions after surgery. Moreover, there was no shift at the 4-mm diameter, and the 6-mm diameter showed a progressive forward shift.

Conclusions.: The shape of the normal posterior corneal surface exhibited obvious regional disparities and radial asymmetry. The displacement after LASIK was observed as time-dependent changes that protruded at an early stage but then returned to original levels 6 months after surgery. On the other hand, the displacement after epi-LASIK appeared as region-dependent changes that could be divided into central and peripheral parts that displayed backward and forward shift trends, respectively.

In recent years, the evolution of modern corneal refractive surgery has refocused attention on the biological properties of the cornea, because an adequate understanding of the corneal shape and its natural variations is important for designing and evaluating corneal refractive surgeries as well as in detecting such corneal diseases as keratoconus at an early stage. It is well known that the radius is not constant over the corneal surface, as the peripheral cornea is aspheric, and the radius of curvature changes from the center to the limbus and does so at different semimeridians. 1,2 Thus, more accurate data on the shape of the cornea are needed. Traditionally, only the topography of the anterior corneal surface has been described. 3 Accurate data on the shape of the posterior surface of the cornea have been rather insufficient, despite the considerable contribution of the posterior surface to total corneal power. 410 The posterior corneal surface compensates for 31% 11 of the anterior corneal surface astigmatism and has a significant effect on the spherical aberration of the cornea. 12 On the other hand, there are two main disadvantages in describing the posterior corneal surface by using existing studies. First, early investigations of corneal topography were confined to gross estimates of the corneal curvature, which has limitations in reconstructing the corneal surface. 13 The axial curvature (global or sagittal curvature) is a referenced parameter that has been used to measure the curvature at a certain point on the corneal surface in the axial direction relative to the center. As a result, the same shape can have many different curvatures based on which axis is used to make the measurement. Instantaneous curvature data (local or tangential curvature) are highly sensitive to local changes in shape. 14 The curvature is usually not measured directly and must be calculated from axial power by taking a derivative, and so the values may exaggerate the noise. Also, the data do not allow direct visualization of the overall corneal location. 15 The shape of the posterior surface can also be expressed in micrometers as the elevation of the actual surface relative to a chosen reference surface (best-fit-sphere [BFS]). The BFS has led to a better understanding of the surface created from the x, y, and z coordinates of the usual representation of data in a 3-D world. 16 Furthermore, data measured in elevation are of particular value in corneal excimer laser surgeries, where the outcome is determined by the depth of tissue removed. The second disadvantage has to do with the regional selection, because most previous studies focused on the central cornea or gave a small number of meridians. As a result, information may have been lost during the calculations. 
After the widespread acceptance of surgical correction of refractive errors, 1719 there have been increasing reports of central posterior corneal ectasia after surgery. However, to the best of our knowledge, most published reports that showed posterior elevation increases were based on topography measurements (Orbscan II; Bausch & Lomb, Tampa, FL) and the accuracy of assessing the posterior corneal measurement has been questioned. 2022 Recently a Scheimpflug imaging–based commercial device, the Pentacam (Oculus GmbH, Wetzler, Germany) has become available. This system allows for calculation of posterior corneal elevation without mathematical reconstruction. 23 The earlier studies with Pentacam have shown no significant changes in the posterior surface of the cornea after refractive surgery. 2431 The results of posterior corneal elevation changes with Pentacam are summarized in Table 1
Table 1.
 
Published Posterior Corneal Elevation Changes after Refractive Surgery with Pentacam*
Table 1.
 
Published Posterior Corneal Elevation Changes after Refractive Surgery with Pentacam*
Source n Surgery Mean ± SD (μm) Range (μm) Follow-up Period
Ciolino and Belin 24 103 LASIK 2.64 ± −4.95 −12 to +14 4–8 weeks
18 PRK 0.88 ± −4.64 −7 to +10 4–8 weeks
Ciolino et al. 25 102 LASIK 0.47 ± −3.48 −7 to +10 13.6 mo (8.8–19.3 mo)
Hashemi and Mehravaran 26 46 LASIK 0.87 ± 6.18 −21 to +13 6 weeks
Vicente et al. 28 10 LASIK 2 ± 4 −9 to +24 11 wk (4–22)
24 Enhancement 5 ± 6 11 wk (4–22)
Pérez-Escudero et al. 29 27 LASIK <8 1.3, 10.8, and 33.4 d
Ha et al. 30 19 PRK 4.60 ± 4.43 1–3 mo
17 PRK 4.55 ± 4.01 3 mo–1 y
Sun et al. 31 30 PRK, LASIK 0.47 ± 4.23 −8 to +9 2.21 ± 1.12 mo
In this study, we presented measurements of the topography by using Pentacam and created a detailed reference database that can be used to determine the shape of the posterior corneal surface in normal myopia and myopic astigmatism. Furthermore, we retrospectively studied the data on posterior corneal elevation changes after LASIK and epi-LASIK and evaluated the changes along four optical zones (center and 2-, 4-, and 6-mm diameters). 
Methods
Subjects
A total of 78 subjects (152 eyes) were recruited at the Tianjin eye hospital, Tianjin Medical University. Experimental protocols were approved by an Institutional Review Board and complied with the tenets of the Declaration of Helsinki. Informed consent was obtained from each subject after a thorough discussion of the benefits and known risks of the procedure. One hundred fifty-two eyes, 75 right and 77 left eyes, had either normal myopia or myopic astigmatism before surgery. The subjects met the inclusion criteria and had no history of ocular disease, ocular surgery, or ocular allergy. Forty-one subjects (79 eyes) with ages ranging from 18 to 36 years (25 ± 5 years) underwent myopic LASIK refractive surgery for intended myopic spherical corrections that ranged from −2.25 to −7.75 D (−4.73 ± 1.17 D; mean ± SD). Cylinder ranged from 0.00 to −3.00 D (−0.60 ± 0.63 D). Seventy-three eyes of 37 subjects with epi-LASIK were included. These patients sought correction of myopia of −4.63 ± 1.27 D of mean manifest refraction spherical equivalent (MRSE), with spherical corrections ranging from 0.00 to −6.75 D (−4.22 ± 1.32 D) and cylinder corrections from 0.00 to −3.00 D (−0.78 ± 0.58 D). Detailed clinical data are shown in Table 2
Table 2.
 
The Measured Parameters in the Normal, LASIK, and Epi-LASIK Groups
Table 2.
 
The Measured Parameters in the Normal, LASIK, and Epi-LASIK Groups
Parameters Normal (n = 152) LASIK (n = 79) Epi-LASIK (n = 73)
Range Mean ± SD Range Mean ± SD Range Mean ± SD
SE, D −7.75 to 0.00 −4.49 ± 1.26 −7.75 to −2.25 −4.73 ± 1.17 −6.75 to 0.00 −4.22 ± 1.32
CE D −3.00 to 0.00 −0.69 ± 0.61 −3.00 to 0.00 −0.60 ± 0.63 −3.00 to 0.00 −0.78 ± 0.58
MRSE, D −9.00 to −1.50 −4.84 ± 1.30 −9.00 to −2.38 −5.03 ± 1.30 −7.25 to −1.50 −4.63 ± 1.27
CCT, μm 481 to 621 538.60 ± 29.70 481 to 621 555.56 ± 24.74 481 to 593 520.25 ± 23
AD, μm 22 to 98 68.04 ± 13.67 40 to 98 72.22 ± 13.56 22 to 86 63.52 ± 12.36
RBT, μm 277 to 445 373.34 ± 29.58 403 to 545 456.73 ± 26.17
Surgical Procedure
All patients were operated on by the same surgeon, and all surgical procedures were performed with topical anesthesia. An excimer laser (model S4 Star; VISX, Inc., Santa Clara, CA) was used in all the eyes. An automated microkeratome (Moria M2; Moria SA, Antony, France) was used to create a hinged corneal flap of 110-μm thickness during LASIK. The optical zone was 6.5 mm with an 8.00-mm blend zone. All eyes that underwent epi-LASIK had the epithelial layer removal by epikeratome (Amadeus II; AMO, Inc., Santa Ana, CA). Chilled balanced salt solution (BSS; Alcon, Fort Worth, TX) was applied to the epithelium, and the epithelial separator was advanced (automatically) across the cornea to create the flap. The epithelial sheet was then removed. Excimer laser treatment was performed in the usual manner with the same nomogram and laser system as was used in LASIK, but with an additional eye-tracking device. A bandage contact lens was placed on the surgically treated eye. 
After surgery, topical ofloxacin 0.3% combined with 0.1% fluorometholone was applied four times daily for 1 month and then tapered over 4 months in the epi-LASIK group. In the LASIK group, it was applied four times daily for 1 week and then tapered over 1 month. 
Pentacam Scheimpflug Imaging
All patients were imaged with the Pentacam (software ver. 1.17r27; Oculus GmbH) in both eyes before surgery, as well as at 1 and 6 months after LASIK or epi-LASIK. The Pentacam is a rotating Scheimpflug camera that measures 25,000 true elevation points to compute the corneal topography. Some studies have explored the reliability of this device in measuring the anterior segment. 3235 According to the manufacturer, this device provides quantitative information about the posterior cornea, as it automatically compensates for the optical and geometric distortion of the system. Furthermore, validation of the accuracy of this system in measuring the posterior cornea has been shown previously. 30 Patients were asked to blink twice and then look at the fixation device. Image acquisition was a 1-second scan of 25 rotational Scheimpflug images through the corneal sighting point, which is the point where the ray of light from the fovea to the fixation device crosses the cornea. All measurements were performed just after a blink, to minimize the effect of tear film alteration on the data. Acceptable maps had at least 10.0 mm of corneal coverage with no extrapolated data in the central 9.0-mm zone. Extrapolated data are indicated by black dots on the color topographic map. 
Data Collection and Analysis
In this study, data on corneal posterior elevation were obtained from the Pentacam software. The reference BFS was determined by the central 8.0-mm zone of the preoperative cornea (i.e., the BFS for both preoperative and postoperative maps were identical and were determined from the preoperative data), so that it was the same across all examinations. The corneal posterior elevation and changes were obtained at the center, paracenter (4 points, 1 mm from the center, at 45°, 135°, 225°, and 315° semimeridians), midperiphery (8 points, 2 mm from the center at 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° semimeridians), and peripheral regions (14 points 3 mm from the center at 15°, 45°, 60°, 90°, 120°, 135°, 165°, 195°, 225°, 245°, 270°, 295°, 315°, and 345° semimeridians). In all mathematical analyses, we set 0° at a point on the right and moved counterclockwise in both eyes. All these points were divided symmetrically into superior and inferior hemispheres by the 0° to 180° meridian, with the exception of the center values. Changes in the posterior surface were determined by subtracting the preoperative elevation data from the postoperative elevation data (difference map). The difference in elevation was determined to be the displacement of the posterior corneal surface. An ectatic change (forward protrusion of the posterior corneal surface) would result in a positive number. 
Statistical Analysis
Descriptive statistical results included the mean, standard deviation, and minimum and maximum values. The normality of all data samples was checked with the Kolmogorov-Smirnov test. Parametric statistics were then applied subsequently. The Student t-test for paired data was used to compare data from the right and left eyes as well as from superior and inferior hemispheres in different optical zones (center and 2-, 4-, and 6-mm diameters). Mauchly's test of sphericity was used before comparing the preoperative corneal posterior elevations with those at 1 and 6 months after surgery. However, because P < 0.05, we used the paired-sample t-test to perform these comparisons. For bivariate correlation analysis, the Pearson correlation coefficient was used in all right eyes because of the bilateral symmetry. P < 0.05 was considered statistically significant (SPSS, ver. 13.0 for Windows; SPSS, Chicago, IL). 
Results
Normal Myopia and Myopic Astigmatism
Figure 1 shows the variations in posterior corneal elevations in various optical zones (center and 2-, 4-, and 6-mm diameters) in both normal myopia and myopic astigmatism as a function of the meridian. The height data on 2-mm diameter rarely deviated from the BFS, and it was slightly lower in the superior than in the inferior hemisphere. From the midperipheral and peripheral elevation distributions, the geometric trend began to change and appeared to have two dips lower than the BFS (the first one occurred at 90° and the second at 270°). Furthermore, the posterior elevation was below the BFS along the vertical meridian but was raised off the BFS along the horizontal meridian (4 mm, vertical, −8.78 ± 8.90 μm; horizontal, 8.07 ± 4.70 μm; t = 25.840, P = 0.000; 6 mm, vertical −22.27 ± 11.38 μm; horizontal 9.69 ± 9.50 μm; t = 31.262, P = 0.000). In addition, a general trend was observed by comparing between the superior and inferior hemispheres in various optical zones (2-, 4-, and 6-mm diameters). The height of the superior hemisphere was significantly lower than that of the inferior hemisphere on the basis of the deviation from the BFS (2 mm, superior 0.80 ± 2.83 μm, inferior 2.78 ± 3.01 μm; t = −7.609, P = 0.000; 4 mm, superior −2.42 ± 10.00 μm, inferior 3.25 ± 6.57 μm; t = −15.188, P = 0.000; 6 mm, superior −7.22 ± 16.82 μm, inferior −0.42 ± 13.06 μm; t = −24.563, P = 0.000). In fact, the geometric characteristic became more obvious as the distance increased from the center. Furthermore, there were no significant differences between the right and left eyes on normal posterior corneal elevations in the various optical zones (center, P = 0.162; 2 mm, P = 0.507; 4 mm, P = 0.071; 6 mm, P = 0.280). 
Figure 1.
 
Variation in normal posterior corneal elevation in the various optical zones (0-, 2-, 4-, and 6- diameters) as a function of the meridian. The height data on the 2-mm diameter rarely deviated from the BFS, and the superior hemisphere was slightly lower than the inferior hemisphere. At the 4- and 6-mm diameters, the posterior elevation appeared to have two dips lower than the BFS. The first occurred at 90° and the second at 270°. Both were below the BFS along the vertical meridian but were raised off the BFS along the horizontal meridian. The superior hemisphere was significantly lower than the inferior hemisphere.
Figure 1.
 
Variation in normal posterior corneal elevation in the various optical zones (0-, 2-, 4-, and 6- diameters) as a function of the meridian. The height data on the 2-mm diameter rarely deviated from the BFS, and the superior hemisphere was slightly lower than the inferior hemisphere. At the 4- and 6-mm diameters, the posterior elevation appeared to have two dips lower than the BFS. The first occurred at 90° and the second at 270°. Both were below the BFS along the vertical meridian but were raised off the BFS along the horizontal meridian. The superior hemisphere was significantly lower than the inferior hemisphere.
Figures 2 and 3 show the mean and SD for the posterior corneal elevation as a function of optical zone (center and 2-, 4-, and 6-mm diameters) in eyes with normal myopia or myopic astigmatism. As shown in Figure 2, the normal posterior corneal elevation was above the BFS in the central zone, but below it in the peripheral region. The value (as well as the degree of deviation from the BFS) gradually increased to 2 mm from the center and then declined toward the 4-mm diameter (to the point of being negative in the periphery; Fig. 3). The most significant variance occurred in the 6-mm diameter optical zone, where the average standard deviation (±SD) was ±15.44 μm (range, −55.00 to −88.00 μm). The quantitative values of normal posterior corneal elevations in the various optical zones (center and 2, 4, and 6 mm) are displayed in Table 3
Figure 2.
 
Normal mean corneal posterior elevation and SD as a function of the optical zone (0-, 2-, 4-, and 6-mm diameters). The normal corneal posterior elevation was above the BFS in the central zone, but it was below it in the peripheral region.
Figure 2.
 
Normal mean corneal posterior elevation and SD as a function of the optical zone (0-, 2-, 4-, and 6-mm diameters). The normal corneal posterior elevation was above the BFS in the central zone, but it was below it in the peripheral region.
Figure 3.
 
Boxplots of the normal corneal posterior elevation as a function of the optical zone. Shown is the degree of variation in the normal posterior corneal elevation. The most significant variance occurred in the 6-mm diameter optical zone.
Figure 3.
 
Boxplots of the normal corneal posterior elevation as a function of the optical zone. Shown is the degree of variation in the normal posterior corneal elevation. The most significant variance occurred in the 6-mm diameter optical zone.
Table 3.
 
Normal Posterior Corneal Elevation in Various Optical Zones
Table 3.
 
Normal Posterior Corneal Elevation in Various Optical Zones
District Mean ± SD (μm) Range (μm)
Center 1.09 ± 2.56 −4.00–8.00
2 mm 1.79 ± 3.09 −7.00–11.00
4 mm 0.41 ± 8.92 −40.00–22.00
6 mm −3.82 ± 15.44 −55.00–88.00
Laser In Situ Keratomileusis
As seen in Figure 4, the posterior elevation in the various optical zones (center and 2, 4, and 6 mm) essentially showed similar values before LASIK, as well as at 1 and 6 months after surgery as a function of the meridian. Meanwhile, the degree of posterior elevation, when displayed as a function of the optical zones (center and 2, 4, and 6 mm) before surgery as well as at 1 and 6 months after surgery, showed some changes after the procedure. Except for the 6-mm optical zone, where it exhibited a slight but not significant (P = 0.731) backward displacement, the posterior elevation in almost all points protruded slightly 1 month after surgery (Fig. 5; Table 4). Significant differences were found in the paracentral (2 mm, P = 0.044) and midperipheral (4 mm, P = 0.002) areas between the preoperative period and 1 month after surgery. However, the elevation of the posterior corneal surface in every position eventually returned to the original levels, and no significant differences were found in the posterior elevation in any position between the preoperative period and 6 months after LASIK (center, P = 0.351; 2 mm, P = 0.485; 4 mm, P = 0.157; 6 mm, P = 0.110). 
Figure 4.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the meridian. The posterior elevation in various optical zones (center and 2, 4, and 6 mm) was the nearly the same in the preoperative period and at 1 and 6 months after surgery, as a function of the meridian.
Figure 4.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the meridian. The posterior elevation in various optical zones (center and 2, 4, and 6 mm) was the nearly the same in the preoperative period and at 1 and 6 months after surgery, as a function of the meridian.
Figure 5.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the optical zone. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Posterior elevation in almost all points protruded slightly 1 month after surgery, with the exception of the 6-mm optical zone, where it exhibited a slight but not significant backward displacement. However, it returned to its original level 6 months after LASIK.
Figure 5.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the optical zone. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Posterior elevation in almost all points protruded slightly 1 month after surgery, with the exception of the 6-mm optical zone, where it exhibited a slight but not significant backward displacement. However, it returned to its original level 6 months after LASIK.
Table 4.
 
Posterior Elevations in Various Optical Zones after LASIK
Table 4.
 
Posterior Elevations in Various Optical Zones after LASIK
District Examination Time Point P
Preop 1 mo 6 mo (Preop-1 mo) (Preop-6 mo)
Center 0.90 ± 2.53 1.92 ± 4.20 0.33 ± 4.48 0.094 0.351
2 mm 1.73 ± 1.60 2.69 ± 3.22 1.40 ± 3.71 0.044* 0.485
4 mm 0.54 ± 1.34 1.39 ± 1.82 0.92 ± 4.47 0.002* 0.157
6 mm −3.70 ± 2.56 −3.86 ± 3.17 −2.91 ± 3.36 0.731 0.110
Epi-LASIK
The posterior surface in various optical zones (center and 2, 4, and 6 mm) was constant as a function of the meridian, even after epi-LASIK (Fig. 6). However, the posterior elevations (Fig. 7, Table 5), as a function of location (center and 2, 4, and 6 mm), showed significant backward displacement of the posterior elevation in the center and paracentral areas between the pre- and postoperative periods (center, P = 0.008, P = 0.000; 2 mm, P = 0.011, P = 0.000). Meanwhile, the level of midperipheral posterior corneal elevation did not change throughout the period of observation (4 mm, P = 0.370, P = 0.076), and the periphery showed a distinct progressive forward shift (6 mm, P = 0.018, P = 0.011). 
Figure 6.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the meridian. The posterior surface in various optical zones (center, and 2, 4, and 6 mm) was constant as a function of the meridian, despite epi-LASIK.
Figure 6.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the meridian. The posterior surface in various optical zones (center, and 2, 4, and 6 mm) was constant as a function of the meridian, despite epi-LASIK.
Figure 7.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the various optical zones. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Significant backward displacement of the posterior elevation was found at the center and 2-mm diameter between the preoperative and postoperative times, whereas the 4-mm diameter posterior corneal elevation did not change throughout the period of observation. The 6-mm diameters showed a progressive forward shift.
Figure 7.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the various optical zones. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Significant backward displacement of the posterior elevation was found at the center and 2-mm diameter between the preoperative and postoperative times, whereas the 4-mm diameter posterior corneal elevation did not change throughout the period of observation. The 6-mm diameters showed a progressive forward shift.
Table 5.
 
Posterior Elevation in Various Optical Zones after Epi-LASIK
Table 5.
 
Posterior Elevation in Various Optical Zones after Epi-LASIK
District Examination Time Point P
Preop 1 mo 6 mo (Preop-1 mo) (Preop-6 mo)
Center 1.32 ± 2.58 −0.33 ± 3.63 −1.27 ± 4.62 0.008* 0.000*
2 mm 1.87 ± 1.49 0.69 ± 2.78 −0.09 ± 3.63 0.011* 0.000*
4 mm 0.15 ± 1.69 0.43 ± 1.82 −0.41 ± 2.16 0.370 0.076
6 mm −3.88 ± 2.65 −2.77 ± 2.82 −2.68 ± 2.93 0.018* 0.011*
Correlations
The Pearson correlation test showed no significant correlation between the changes in the posterior corneal surface data and the MRSE, central corneal thickness (CCT), ablation depth (AD), and residual bed thickness (RBT) at 1 and 6 months after surgery. 
Discussion
In recent attempts to measure the posterior corneal surface, the radius of curvature has been extensively measured on the basis of size and location by using various techniques such as Purkinje imagery, 9,36 pachymetry, 5 and Scheimpflug photography. 37 However, in this study we initially described the posterior corneal shape by using another parameter—elevation—which is independent of the axis or orientation and the reference surface. 13 It is therefore more accurate in corneal surface reconstruction. Moreover, we initially tested our posterior surface separately at various diameters including 0, 2, 4, and 6 mm. From the database of the corneal posterior shapes that we have set up, four characteristics emerged: (1) the shape of the posterior surface exhibited more toricity and complexity than previously assumed, since the elevation was not constant over the corneal surface (neither along the diameters nor the meridians) but instead had some discernible regularity. In fact, Dunne et al. 36 also found that the posterior surface exhibited more toricity than the anterior surface, and Dubbelman et al. 11 recently reported that the radius of curvature for both corneal surfaces could be well fitted by using the cos2 function, which is periodic. (2) The height data appeared to be different between the vertical meridian and the horizontal meridian. This result was not surprising, given that the difference may be related to the higher incidence of with-the-rule astigmatism in young adults in whom an average cornea is mostly steeper in the vertical meridian by approximately 0.5 D compared with the horizontal meridian. In addition, some previous studies have demonstrated a difference in the vertical and horizontal corneal radii of individual subjects. 36,38 Hirji and Larke 39 and Rüfer et al. 40 found that the peripheral cornea was thicker vertically than horizontally, which could explain the discrepancy to some extent. (3) The elevation of the superior hemisphere was significantly lower when compared with that of the inferior hemisphere, a trend that was more obvious in the peripheral cornea. To explain this phenomenon, Kasprzak and Pierscionek 41 proposed a model of the gravitational sag of the cornea and showed that the upper eyelid may have a certain degree of influence. It was intriguing that this geometry characteristic appeared to share similar regional specificity with the inherent cohesive weakness found by Smolek, 42 who noted that the mean cohesive strength in the inferior periphery was significantly less than the strength in the superior periphery. Since the ultimate determinants for corneal shape were the structural elements of the tissue itself and the magnitude and direction of forces distributed within the load-bearing structures, the unequal cohesive strength distribution is likely to have played a role to some extent. (4) There was the same tendency in the posterior corneal surfaces of the right and left eyes with regard to different evaluated diameters and meridians. Bilateral symmetry occurred in almost all the eyes, which may indicate that the Pentacam is a reliable technique for the posterior corneal surface and, consequently, for elevation measurements. 
In the postoperative groups, we compared the mean changes in posterior corneal elevation changes after LASIK and epi-LASIK along four optical zones. Although the slight forward displacements in posterior corneal elevation at the 2- and 4-mm diameters (2 mm, 1.73 ± 1.60 μm; 4 mm, 0.54 ± 1.34 μm) 1 month after LASIK were statistically significant (P = 0.044; P = 0.002), the values were within the manufacturer's reported range of error (±5 μm) for the Pentacam and were therefore below the sensitivity of the instrument. Moreover, the changes after LASIK and epi-LASIK are all within the measurement noise of the device. Ciolino and Belin 26 studied posterior corneal changes after LASIK and photorefractive keratectomy (PRK) with a Pentacam and found no posterior corneal ectasia in either group, which was consistent with our results. 
In this study, we showed a systematic and characteristic response of the posterior surface to LASIK that was related to the biomechanical and wound–healing responses as a function of time and space. In most areas, the posterior corneal elevation exhibited forward protrusion 1 month after surgery, but this occurrence tended to be stable and returned to its original level 6 months after surgery. Such time-dependent changes agree with the findings of Pérez-Escudero et al., 30 who detected a change in the radius of curvature and asphericity of the posterior corneal surface at the first day after surgery, but noted that the changes disappeared on a timescale of days to weeks. We suspect that these postsurgical changes may be relevant to the process of wound healing after LASIK, since previous studies have shown that LASIK corneal wounds remain in the active wound-healing stage for up to the first 6 months after surgery and then enter the wound-remodeling stage. 43,44 LASIK produced circumferential severing of corneal lamellae resulting in a forward herniation at a very early stage after surgery, but in the wound-remodeling stage, chronic biomechanical changes and complete fibrotic and primitive wound healing would improve the corneal transparency and wound strength. Thus, changes in the posterior corneal elevation at a region with a 6-mm diameter showed a backward shift at the beginning and then a forward displacement 6 months after surgery. Other studies have accounted for this by demonstrating that fibrotic wound repair after LASIK was restricted to a narrow band peripheral to the corneal flap edge, 45 causing disordered morphology and deflection of the optical paths. LASIK thus represents a particularly complex situation that could lead to time-dependent changes in areas surrounding the cornea as well as in the underlying flap. 
An interesting finding from this study was that epi-LASIK had posterior elevation changes with a progressive backward shift at all follow-up time points in both the central and paracentral regions. However, a 4-mm diameter ring positioned between the central part and the periphery marked the division of the posterior elevation after epi-LASIK into two parts, creating a boundary between the backward shift and the forward shift. The cause of this feature is unclear to us. The phenomenon may be related to the hyperopic shift model, as demonstrated by Dupps and Roberts 46 during photo-therapeutic keratectomy (PTK), in which a depth-dependent ablation flattening was observed in the donor. Because of the swelling and thickening of the peripheral stromal surface immediately after surgery, flattening of the optical surface occurs, resulting in hyperopic shift. On the other hand, there may be some noise in the actual measurement or during the wound-healing responses that may constitute a measurement bias. In particular, keratocyte apoptosis, keratocyte proliferation, and myofibroblast generation over the central cornea in epi-LASIK could all amplify the wound-healing response and possibly account for variations in measurement accuracy. 
Moreover, there was no significant correlation between the elevation changes and residual bed thickness. It is likely that the changes became more obvious when current acceptable standards were not followed (RBT < 250 μm). 17 Our study was limited, in that no patient had an estimated RBT below 270 μm, and the deepest AD was 98 μm in the LASIK group. 
Along with the findings of this study, more experimental data on corneal optical and biometric properties and more accurate models of corneal biomechanics should be studied to provide better explanations of the changes in corneal shape as a response to LASIK and advanced surface ablation procedures. 
Footnotes
 Supported in part by National Key Natural and Science Program Grant 60978068 and Tianjin Natural and Science Program Grant 07JCYBJC09500.
Footnotes
 Disclosure: L. Zhang, None; Y. Wang, None
The authors thank Jie Hou for case collection and patient follow-up. 
References
Dingeldein SA Klyce SD . The topography of normal corneas. Arch Ophthalmol. 1989;107:512–518. [CrossRef] [PubMed]
Carney LG Mainstone JC Henderson BA . Corneal topography and myopia: a cross-sectional study. Invest Ophthalmol Vis Sci. 1997;38:311–320. [PubMed]
Gullstrand A . Procedure of the rays in the eye imagery-law of first order: the optical system of the eye. In: von Helmholtz H Southall JPC eds. Helmholtz's Treatise on Physiological Optics. Rochester, NY: The Optical Society of America; 1924.
Lowe RF Clark BAJ . Posterior corneal curvature; correlations in normal eyes and in eyes involved with primary angle-closure glaucoma. Br J Ophthalmol. 1973;57:464–470. [CrossRef] [PubMed]
Royston JM Dunne MCM Barnes DA . Measurement of posterior corneal surface toricity. Optom Vis Sci. 1990;67:757–763. [CrossRef] [PubMed]
Edmund C . Posterior corneal curvature and its influence on corneal dioptric power. Acta Ophthalmol. 1994;72:715–720. [CrossRef]
Eryildirim A Ozkan T Eryildirim S Kaynak S Cingil G . Improving estimation of corneal refractive power by measuring the posterior curvature of the cornea. J Cataract Refract Surg. 1994;20:129–131. [CrossRef] [PubMed]
Dunne MCM Royston JM Barnes DA . Posterior corneal surface toricity and total corneal astigmatism. Optom Vis Sci. 1991;68:708–710. [CrossRef] [PubMed]
Garner LF Owens H Yap MKH Frith MJ Kinnear RF . Radius of curvature of the posterior surface of the cornea. Optom Vis Sci. 1997;74:496–498. [CrossRef] [PubMed]
Royston JM Dunne MCM Barnes DA . Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques. Ophthalmic Physiol Opt. 1990;10:385–388. [CrossRef] [PubMed]
Dubbelman M Sicam VA Van der Heijde GL . The shape of the anterior and posterior surface of the aging human cornea. Vision Res. 2006;46:993–1001. [CrossRef] [PubMed]
Sicam VA Dubbelman M Van der Heijde RG . Spherical aberration of the anterior and posterior surfaces of the human cornea. J Opt Soc Am A Opt Image Sci Vis. 2006;23:544–549. [CrossRef] [PubMed]
Belin MW Zloty P . Accuracy of the PAR Corneal Topography System with spatial misalignment. CLAO J. 1993;19:64–68. [PubMed]
Szczotka LB Thomas J . Comparison of axial and instantaneous videokeratographic data in keratoconus and utility in contact lens curvature prediction. CLAO J. 1998;24:22–28. [PubMed]
Klein SA Mandell RB . Shape and refractive powers in corneal topography. Invest Ophthalmol Vis Sci. 1995;36:2096–2109. [PubMed]
Belin MW Khachikian SS . An introduction to understanding elevation-based topography: how elevation data are displayed—a review. Clin Exp Ophthalmol. 2009;37:14–29. [CrossRef]
Wang Z Chen J Yang B . Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999;106:406–409. [CrossRef] [PubMed]
Baek TM Lee KH Kagaya F Amano S Oshika T . Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology. 2001;108:317–320. [CrossRef] [PubMed]
Naroo SA Charman WN . Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg. 2000;26:872–878. [CrossRef] [PubMed]
Cairns G McGhee CNJ . Orbscan computerized topography: attributes, applications, and limitations. J Cataract Refract Surg. 2005;31:205–220. [CrossRef] [PubMed]
Cairns G Ormonde SE Gray T . Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures. Clin Exp Ophthalmol. 2005;33:147–152. [CrossRef]
Maldonado MJ Nieto JC Dı̆ez-Cuenca M Piňero DP . Repeatability and reproducibility of posterior corneal curvature measurements by combined scanning-slit and Placido-disc topography after LASIK. Ophthalmology. 2006;113:1918–1926. [CrossRef] [PubMed]
Kopacz D Maciejewicz P Kecik D . Pentacam: the new way for anterior eye segment imaging and mapping (Polish). Klin Oczna. 2005;107:728–731. [PubMed]
Ciolino JB Belin MW . Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg. 2006;32:1426–1431. [CrossRef] [PubMed]
Ciolino JB Khachikian SS Cortese MJ Belin MW . Long-term stability of the posterior cornea after laser in situ keratomileusis. J Cataract Refract Surg. 2007;33:1366–1370. [CrossRef] [PubMed]
Hashemi H Mehravaran S . Corneal changes after laser refractive surgery for myopia: comparison of Orbscan II and Pentacam findings. J Cataract Refract Surg. 2007;33:841–847. [CrossRef] [PubMed]
Nishimura R Negishi K Saiki M . No forward shifting of posterior corneal surface in eyes undergoing LASIK. Ophthalmology. 2007;114:1104–1110. [CrossRef] [PubMed]
Vicente D Clinch TE Kang PC . Changes in posterior corneal elevation after laser in situ keratomileusis enhancement. J Cataract Refract Surg. 2008;34(5):785–788. [CrossRef] [PubMed]
Pérez-Escudero A Dorronsoro C Sawides L Remón L Merayo-Lloves J Marcos S . Minor influence of myopic laser in situ keratomileusis on the posterior corneal surface. Invest Ophthalmol Vis Sci. 2009;50:4146–4154. [CrossRef] [PubMed]
Ha BJ Kim SW Kim SW Kim EK Kim TI . Pentacam and Orbscan II measurements of posterior corneal elevation before and after photorefractive keratectomy. J Refract Surg. 2009;25:290–295. [PubMed]
Sun HJ Park JW Kim SW . Stability of the posterior corneal surface after laser surface ablation for myopia. Cornea. 2009;28:1019–1022. [CrossRef] [PubMed]
Lackner B Schmidinger G Pieh S Funovics M Skorpik C . Repeatability and reproducibility of central corneal thickness measurement with Pentacam, Orbscan and ultrasound. Opt Vis Sci. 2005;82:892–899. [CrossRef]
Jain R Dilraj G Grewal SPS . Repeatability of corneal parameters with Pentacam after laser in situ keratomileusis. Indian J Ophthalmol. 2007;55:341–347. [CrossRef] [PubMed]
Shankar H Taranah D Chandramalar F Santhirathelagan T Pseudovs K . Anterior segment biometry with the Pentacam: comprehensive assessment of repeatability of automated measurements. J Cataract Refract. 2008;34:103–113. [CrossRef]
Atchison DA Markwell EL Kasthurirangan S Pope JM Smith G Swann PG . Age-related changes in optical and biometric characteristics of emmetropic eyes. J Vis. 2008;8:1–20. [CrossRef] [PubMed]
Dunne MC Royston J Barnes DA . Normal variations of the posterior corneal surface. Acta Ophthalmol. 1992;70:255–261. [CrossRef]
Dubbelman M Weeber HA Van der Heijde RGL Völker-Dieben HJ . Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography. Acta Ophthalmol Scand. 2002;80:379–383. [CrossRef] [PubMed]
Patel S Marshall J Fitzke FW . Shape and radius of posterior corneal surface. Refract Corneal Surg. 1993;9:173–181. [PubMed]
Hirji NK Larke JR . Thickness of human cornea measured by topographic pachometry. Am J Optom Physiol Opt. 1978;55:97–100. [CrossRef] [PubMed]
Rüfer F Schröder A Arvani MK Erb C . Central and peripheral corneal pachymetry: standard evaluation with the Pentacam system. Klin Monatsbl Augenheilkd. 2005;222:117–122. [CrossRef] [PubMed]
Kasprzak H Pierscionek BK . Modelling the gravitational sag of the cornea and the subsequent quality of the refracted image. J Mod Opt. 2002;49:2153–2166. [CrossRef]
Smolek MK . Interlamellar cohesive strength in the vertical meridian of human eye bank corneas. Invest Ophthalmol Vis Sci. 1993;34:2962–2969. [PubMed]
Dawson DG Kramer TR Grossniklaus HE Waring GOIII Edelhauser HF . Histologic, ultrastructural, and immunofluorescent evaluation of human laser-assisted in situ keratomileusis corneal wounds. Arch Ophthalmol. 2005;123:741–756. [CrossRef] [PubMed]
Dawson DG Edelhauser HF Grossniklaus HE . Long-term histopathologic findings in human corneal wounds after refractive surgical procedures. Am J Ophthalmol. 2005;139:168–178. [CrossRef] [PubMed]
Ivarsen A Laurberg T Møller-Pedersen T . Characterisation of corneal fibrotic wound repair at the LASIK flap margin. Br J Ophthalmol. 2003;87:1272–1278. [CrossRef] [PubMed]
Dupps WJ Roberts C . Effect of acute biomechanical changes on corneal curvature after photokeratectomy. J Refract Surg. 2001;17:658–669. [PubMed]
Figure 1.
 
Variation in normal posterior corneal elevation in the various optical zones (0-, 2-, 4-, and 6- diameters) as a function of the meridian. The height data on the 2-mm diameter rarely deviated from the BFS, and the superior hemisphere was slightly lower than the inferior hemisphere. At the 4- and 6-mm diameters, the posterior elevation appeared to have two dips lower than the BFS. The first occurred at 90° and the second at 270°. Both were below the BFS along the vertical meridian but were raised off the BFS along the horizontal meridian. The superior hemisphere was significantly lower than the inferior hemisphere.
Figure 1.
 
Variation in normal posterior corneal elevation in the various optical zones (0-, 2-, 4-, and 6- diameters) as a function of the meridian. The height data on the 2-mm diameter rarely deviated from the BFS, and the superior hemisphere was slightly lower than the inferior hemisphere. At the 4- and 6-mm diameters, the posterior elevation appeared to have two dips lower than the BFS. The first occurred at 90° and the second at 270°. Both were below the BFS along the vertical meridian but were raised off the BFS along the horizontal meridian. The superior hemisphere was significantly lower than the inferior hemisphere.
Figure 2.
 
Normal mean corneal posterior elevation and SD as a function of the optical zone (0-, 2-, 4-, and 6-mm diameters). The normal corneal posterior elevation was above the BFS in the central zone, but it was below it in the peripheral region.
Figure 2.
 
Normal mean corneal posterior elevation and SD as a function of the optical zone (0-, 2-, 4-, and 6-mm diameters). The normal corneal posterior elevation was above the BFS in the central zone, but it was below it in the peripheral region.
Figure 3.
 
Boxplots of the normal corneal posterior elevation as a function of the optical zone. Shown is the degree of variation in the normal posterior corneal elevation. The most significant variance occurred in the 6-mm diameter optical zone.
Figure 3.
 
Boxplots of the normal corneal posterior elevation as a function of the optical zone. Shown is the degree of variation in the normal posterior corneal elevation. The most significant variance occurred in the 6-mm diameter optical zone.
Figure 4.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the meridian. The posterior elevation in various optical zones (center and 2, 4, and 6 mm) was the nearly the same in the preoperative period and at 1 and 6 months after surgery, as a function of the meridian.
Figure 4.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the meridian. The posterior elevation in various optical zones (center and 2, 4, and 6 mm) was the nearly the same in the preoperative period and at 1 and 6 months after surgery, as a function of the meridian.
Figure 5.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the optical zone. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Posterior elevation in almost all points protruded slightly 1 month after surgery, with the exception of the 6-mm optical zone, where it exhibited a slight but not significant backward displacement. However, it returned to its original level 6 months after LASIK.
Figure 5.
 
Posterior elevations in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period as well as 1 and 6 months after LASIK, as a function of the optical zone. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Posterior elevation in almost all points protruded slightly 1 month after surgery, with the exception of the 6-mm optical zone, where it exhibited a slight but not significant backward displacement. However, it returned to its original level 6 months after LASIK.
Figure 6.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the meridian. The posterior surface in various optical zones (center, and 2, 4, and 6 mm) was constant as a function of the meridian, despite epi-LASIK.
Figure 6.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the meridian. The posterior surface in various optical zones (center, and 2, 4, and 6 mm) was constant as a function of the meridian, despite epi-LASIK.
Figure 7.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the various optical zones. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Significant backward displacement of the posterior elevation was found at the center and 2-mm diameter between the preoperative and postoperative times, whereas the 4-mm diameter posterior corneal elevation did not change throughout the period of observation. The 6-mm diameters showed a progressive forward shift.
Figure 7.
 
Posterior elevation in the various optical zones (center and 2, 4, and 6 mm) in the preoperative period and 1 and 6 months after epi-LASIK as a function of the various optical zones. Triangle: preoperative value; square: 1 month after surgery; circle: 6 months after surgery. Significant backward displacement of the posterior elevation was found at the center and 2-mm diameter between the preoperative and postoperative times, whereas the 4-mm diameter posterior corneal elevation did not change throughout the period of observation. The 6-mm diameters showed a progressive forward shift.
Table 1.
 
Published Posterior Corneal Elevation Changes after Refractive Surgery with Pentacam*
Table 1.
 
Published Posterior Corneal Elevation Changes after Refractive Surgery with Pentacam*
Source n Surgery Mean ± SD (μm) Range (μm) Follow-up Period
Ciolino and Belin 24 103 LASIK 2.64 ± −4.95 −12 to +14 4–8 weeks
18 PRK 0.88 ± −4.64 −7 to +10 4–8 weeks
Ciolino et al. 25 102 LASIK 0.47 ± −3.48 −7 to +10 13.6 mo (8.8–19.3 mo)
Hashemi and Mehravaran 26 46 LASIK 0.87 ± 6.18 −21 to +13 6 weeks
Vicente et al. 28 10 LASIK 2 ± 4 −9 to +24 11 wk (4–22)
24 Enhancement 5 ± 6 11 wk (4–22)
Pérez-Escudero et al. 29 27 LASIK <8 1.3, 10.8, and 33.4 d
Ha et al. 30 19 PRK 4.60 ± 4.43 1–3 mo
17 PRK 4.55 ± 4.01 3 mo–1 y
Sun et al. 31 30 PRK, LASIK 0.47 ± 4.23 −8 to +9 2.21 ± 1.12 mo
Table 2.
 
The Measured Parameters in the Normal, LASIK, and Epi-LASIK Groups
Table 2.
 
The Measured Parameters in the Normal, LASIK, and Epi-LASIK Groups
Parameters Normal (n = 152) LASIK (n = 79) Epi-LASIK (n = 73)
Range Mean ± SD Range Mean ± SD Range Mean ± SD
SE, D −7.75 to 0.00 −4.49 ± 1.26 −7.75 to −2.25 −4.73 ± 1.17 −6.75 to 0.00 −4.22 ± 1.32
CE D −3.00 to 0.00 −0.69 ± 0.61 −3.00 to 0.00 −0.60 ± 0.63 −3.00 to 0.00 −0.78 ± 0.58
MRSE, D −9.00 to −1.50 −4.84 ± 1.30 −9.00 to −2.38 −5.03 ± 1.30 −7.25 to −1.50 −4.63 ± 1.27
CCT, μm 481 to 621 538.60 ± 29.70 481 to 621 555.56 ± 24.74 481 to 593 520.25 ± 23
AD, μm 22 to 98 68.04 ± 13.67 40 to 98 72.22 ± 13.56 22 to 86 63.52 ± 12.36
RBT, μm 277 to 445 373.34 ± 29.58 403 to 545 456.73 ± 26.17
Table 3.
 
Normal Posterior Corneal Elevation in Various Optical Zones
Table 3.
 
Normal Posterior Corneal Elevation in Various Optical Zones
District Mean ± SD (μm) Range (μm)
Center 1.09 ± 2.56 −4.00–8.00
2 mm 1.79 ± 3.09 −7.00–11.00
4 mm 0.41 ± 8.92 −40.00–22.00
6 mm −3.82 ± 15.44 −55.00–88.00
Table 4.
 
Posterior Elevations in Various Optical Zones after LASIK
Table 4.
 
Posterior Elevations in Various Optical Zones after LASIK
District Examination Time Point P
Preop 1 mo 6 mo (Preop-1 mo) (Preop-6 mo)
Center 0.90 ± 2.53 1.92 ± 4.20 0.33 ± 4.48 0.094 0.351
2 mm 1.73 ± 1.60 2.69 ± 3.22 1.40 ± 3.71 0.044* 0.485
4 mm 0.54 ± 1.34 1.39 ± 1.82 0.92 ± 4.47 0.002* 0.157
6 mm −3.70 ± 2.56 −3.86 ± 3.17 −2.91 ± 3.36 0.731 0.110
Table 5.
 
Posterior Elevation in Various Optical Zones after Epi-LASIK
Table 5.
 
Posterior Elevation in Various Optical Zones after Epi-LASIK
District Examination Time Point P
Preop 1 mo 6 mo (Preop-1 mo) (Preop-6 mo)
Center 1.32 ± 2.58 −0.33 ± 3.63 −1.27 ± 4.62 0.008* 0.000*
2 mm 1.87 ± 1.49 0.69 ± 2.78 −0.09 ± 3.63 0.011* 0.000*
4 mm 0.15 ± 1.69 0.43 ± 1.82 −0.41 ± 2.16 0.370 0.076
6 mm −3.88 ± 2.65 −2.77 ± 2.82 −2.68 ± 2.93 0.018* 0.011*
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