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Posterior Corneal Curvature Assessment after Epi-LASIK for Myopia: Comparison of Orbscan II and Pentacam Imaging

Purpose To compare the changes in posterior corneal curvature using scanning slit topography (Orbscan II) and Scheimpflug imaging (Pentacam) before and after Epi-laser in situ keratomileusis (LASIK) for myopia. Methods In a prospective observational case-series study, 20 myopic patients having undergone Epi-LASIK were examined serially with two different devices, Orbscan II and Pentacam, preoperatively and one month postoperatively. Posterior central elevation (PCE) and posterior maximal elevation (PME) were compared between the two devices, and the changes in parameters after Epi-LASIK were analyzed using a difference map. Results All parameters (preoperative and postoperative PCE and preoperative and postoperative PME) that were measured using the Orbscan II were significantly greater compared to those of the Pentacam (for all p < 0.001). PCE and PME were significantly increased one month postoperatively in the Orbscan II measurements (p < 0.05) but were not significantly increased in the Pentacam measurements. Also, ΔPCE and ΔPME, in the difference map obtained by each serial scanning, were significantly greater in the Orbscan II measurements than with the Pentacam (p = 0.012, p = 0.016). Conclusions The Pentacam measurements displayed significantly reduced values in all parameters related to posterior corneal elevation compared to those of the Orbscan II. The Pentacam showed no significant change in posterior corneal curvature after Epi-LASIK, based on the difference map.

However, most of these studies that mention posterior corneal changes are based on Orbscan II measurements of posterior corneal elevation [1][2][3][4][5][6][7][8][9][10]13]. Recently, reports using the Pentacam demonstrated that the posterior corneal elevation after PRK or LASIK is less common than previously reported with the Orbscan II [15,16]. Because the Orbscan II is based on slit scan beam imaging and uses mathematical calculations to recreate the posterior cornea, this strategy can cause false positive readings of posterior corneal elevation [17]. In contrast, the Pentacam uses a rotating Scheimpflug camera to directly image the posterior cornea and analyzes the posterior corneal elevation without mathematical calculations [18].
The present study measured the posterior central elevation (PCE) and posterior maximum elevation (PME) above the best fit sphere in eyes before and after Epi-LASIK for the first time based on difference maps integrated by serial measurements in individuals and compared these measurements between the Orbscan II and the Pentacam.

Materials and Methods
Forty eyes of 20 patients undergoing Epi-LASIK for myopia were enrolled in this prospective observational case series. All surgeries were performed using the Amadeus II microkeratome (Ziemer, Port, Switzerland) and VISX S4 (AMO, Irvine, CA, USA) at the Ophthalmologic Department of Kangnam St. Mary's Hospital. The epithelial sheet was reflected nasally in all cases. The nature and purpose of the study were explained in detail to all patients, and informed consent was obtained before commencement. The study was approved by the Kangnam St. Mary's Hospital Institutional Review Board. Both Orbscan II (Bausch & Lomb, Rochester, NY, USA) and Pentacam (Oculus, Wetzlar, Germany) images were obtained on the same day, and preoperative and postoperative (1 month after surgery) images were serially measured in both eyes of all enrolled patients.
Measured variables included PCE, and PME above the best fit sphere as a preoperative and postoperative values, and difference maps were obtained by serial imaging. The pre-operative posterior best-fit float sphere was adjusted to the post-operative posterior best-fit float sphere in posterior elevation maps in both machines. PCE and PME values were respectively compared between the two devices using a difference map.
Statistical analysis was performed using SPSS ver. 13.0 (SPSS Inc., Chicago, IL, USA). The paired t-test was employed when comparing pre-and post-operative values and inter-device differences. Results were considered statistically significant if a p-value < 0.05 was obtained.

Results
The mean age of the 14 women and 6 men in this study was 24 ± 2.37 years (range, 20 to 32 years). Table 1 shows demographic data for the 40 eyes that underwent Epi-LASIK. All parameters measured using the Orbscan II were significantly greater, compared to those of the Pentacam ( p < 0.001 for all parameters) ( Table 2). Preoperative PCE and PME were greater when measured using the  Paired t-test. PCE = posterior central elevation; PME = posterior maximal elevation.

A B
Orbscan II compared to the Pentacam ( p < 0.001). Postoperative PCE and PME were also greater when measured using the Orbscan II compared to the Pentacam (p < 0.001).
There was a significant inter-device difference between the Orbscan II and the Pentacam for all parameters. When measured using the Orbscan II, the postoperative PCE and PME values after Epi-LASIK were significantly increased compared to the preoperative values ( p = 0.010, p = 0.048, respectively). When measured using the Pentacam, PCE and PME values after Epi-LASIK were slightly increased compared to the preoperative values. However, the increases were not significant ( p = 0.284, p = 0.43, respectively) (Fig. 1).
The average PCE change (ΔPCE) and PME change (ΔPME) were 18.3 ± 2.63 μm and 22.5 ± 5.32 μm, respectively, when analyzed using the Orbscan II difference map. The average ΔPCE and ΔPME were 1.94 ± 1.32 μm and 2.08 ± 2.29 μm, respectively, when analyzed using the Pentacam difference map. These ΔPCE and ΔPME values in the difference map obtained by two serial scans were significantly greater for the Orbscan II than for the Pentacam ( p = 0.012, p = 0.016, respectively) ( Table 3).

Discussion
Our study demonstrated that significant posterior corneal elevation was not observed in patients after Epi-LASIK when measured using the Pentacam. Recent studies using the Pentacam showed no significant posterior corneal displacement after laser refractive surgery [15,16]. Ciolino and Belin [15] demonstrated that no significant posterior corneal surface displacement was observed using the Pentacam in 103 postoperative LASIK eyes and 18 postoperative PRK eyes. In addition, posterior corneal displacement was not significantly different between the LASIK and PRK patients. However, measurements using the Orbscan II exhibited a significant change in posterior corneal elevation after Epi-LASIK, and the change in posterior corneal elevation was greater using the Orbscan II compared to the Pentacam. Hashemi and Mehravaran [16] reported that the Pentacam yielded no significant postoperative changes in 30 LASIK eyes and 16 PRK eyes compared to the Orbscan II. Ha et al. [19] also demonstrated that the change in posterior corneal elevation after PRK was greater for the Orbscan II compared to the Pentacam.
Our study used a difference map to compare the posterior corneal change between two devices after Epi-LASIK. A difference map is an image that represents the change between serial elevation maps in an individual. While previous studies compared the change after laser refractive surgery by analyzing the subtracted values between preoperative and postoperative measurements [2,4,5,10,15,19], our study has merit in that we compare the change in the posterior corneal elevation (ΔPCE, ΔPME) on a difference map. There were no significant changes in PCE and PME on the difference map obtained by serial measurements in the Pentacam, although significant changes in PCE and PME were observed in measurements from the Orbscan II.
The inter-device difference for posterior corneal elevation may be explained by the difference in the manner of scanning between the two devices. The Orbscan's possible sources of error have been discussed in several studies because many studies using the Orbscan II have determined a much larger value of posterior elevation. Also, some authors reason that keratectasia detected by this device is an artifact [1,4,9,10,13,[19][20][21] and suggested that the Orbscan II might overestimate the posterior corneal elevation due to the "noise" of the measurements and the inaccuracy of system realignment for the second measurement. Additionally, the Orbscan may have an inaccuracy due to the scanning manner itself. The Orbscan II uses only a horizontally moving slit scan beam to produce multiple slit images of the cornea. Alternatively, the Pentacam is a rotating Scheimpflug camera that provides 25 to 50 images during one scan in less than 2 seconds, yielding 500 true elevation points per image. To capture the images of the anterior segment, the rotating wheel produces slit images in two dimensions, and the integration of multiple twodimensional images allows for the generation of a threedimensional model. The slit images are photographed at an angle from 0° to 180° to avoid shadows from the nose. Every picture is a complete image through the cornea at the specific angle, so a true 360° image of the anterior segment of the eye is acquired. Therefore, the Pentacam can obtain more uninterrupted posterior corneal images and may be less influenced by factors such as postoperative subclinical corneal haze. Additionally, the posterior change measured by the Pentacam was more reliable due to the better intrasession and intersession repeatability on posterior best fit sphere, and its intraclass correlation values were 0.99 or above [22,23].
Our study has a limitation in that the sample size was relatively small and the correlations with other parameters, such as ablation size or depth, were not analyzed. Also, one month was too short a time to fully assess the corneal change after Epi-LASIK. According to Miyata's report of the time course of changes in corneal forward shift after excimer laser photorefractive keratectomy, forward corneal shift progressed up to 6 months postoperatively [24]. Further studies including late postoperative data and additional parameters are necessary. The present study found that the Pentacam exhibited significantly reduced values in all parameters related to posterior corneal elevation compared to those of the Orbscan II. Pentacam measurements demonstrated no significant change in posterior elevation between preoperative and postoperative eyes after Epi-LASIK, based on a difference map. In contrast, the Orbscan II measurements displayed a greater posterior corneal elevation after Epi-LASIK. These findings indicate that the prevalence and extent of posterior corneal changes after corneal refractive surgery, including Epi-LASIK, may be overestimated in the Orbscan II. In conclusion, the Pentacam may be a better tool for investigating posterior corneal elevation in eyes after corneal refractive surgery.