1. Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji The authors have no financial interests to disclose

2. Introduction Elevation of the posterior corneal surface after myopic correction with laser in-situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) has been observed in several studies. Early studies investigated the post-PRK or post-LASIK cornea using scanning-slit topography (Orbscan, Bausch & Lomb, Rochester, US), demonstrating a forward protrusion of posterior cornea. (Ophthalmology 2001; 108:317-20; JCRS 2004; 30:1067-72; Arch Ophthalmol 2002; 120:896-900) Later studies utilizing Scheimpflug photography (Pentacam, Oculus, Germany) showed minimal or no changes in posterior corneal elevation, through direct analysis of the posterior corneal surface. (JCRS 2006; 32:1426-31; JCRS 2007; 33:1366-70; JCRS 2008; 34:785-8) This study aimed to longitudinally investigate the changes in posterior corneal elevation over one year after myopic femtosecond assisted-LASIK and PRK using the swept source optical coherence tomography.

3. Methods A total of 49 consecutive subjects (98 eyes) were recruited at Refractive Surgery Clinic of the Chinese University of Hong Kong Eye Centre. Preoperatively, all patients underwent a complete ophthalmic examination and had no ocular abnormality except myopia or myopic astigmatism with a corrected distance visual acuity of 20/20 or better in both eyes. Patients with suspicion of keratoconus on corneal topography, cataract, ocular inflammation, and infection were excluded.

4. Surgical procedures Femtosecond-assisted LASIK LASIK flaps were created using a 150-kHz femtosecond laser (IntraLase, Abott Medical Optics, Chicago, Illinois, USA). All flaps had a superior hinge. The intended thickness and flap diameter were 110 μm and 9.0 mm, respectively. Stromal ablation was performed with Allegretto Wave & Eye-Q 400Hz laser (WaveLight Laser Technologie AG, Germany) using a 6.5 mm optical zone. PRK Alcohol-assisted corneal epithelial removal was performed over a 9.0-mm optical zone centered over the pupil. Stromal ablation was performed with Allegretto Wave & Eye-Q 400Hz laser (WaveLight Laser Technologie AG, Germany) using a 6.5 mm optical zone. After the stromal ablation, a circular cellulose sponge soaked with mitomycin C 0.02% (0.2 mg/mL) was placed on the cornea for 30 to 45 seconds. A bandage contact lens was placed over the cornea at the end of the surgery.

5. Postoperative examination All patients were examined on day 1, week 1, and, months 1, 3, 6 and 12 after surgery. All patients were imaged with a swept source optical coherence tomography (Casia SS-1000, Tomey, Nagoya, Japan) in both eyes before and at each postoperative follow-up (excluding day 1 and week 1). The topographic data of posterior corneal surfaces as well as corneal thickness and keratometry were obtained from the map. The reference best-fit sphere for both preoperative and postoperative maps was identical (8.0 mm) across all examinations. The changes in posterior corneal elevation measured at postoperative month 1, 3, 6 and 12 were designated as B-1, B-3, B-6 and B-12, respectively.

6. Statistical analysis The change in posterior corneal elevation between B-1, B-3, B-6 and B-12 were compared using one-way analysis of variance (ANOVA) models with repeated measures after LASIK and PRK operations. Two-way ANOVA model with repeated measures was adopted to compare the differences in posterior corneal elevation after LASIK or PRK up to different time points (B-1, B-3, B-6 and B-12). Linear mixed effect model was used to evaluate the change in posterior corneal elevations between B-1, B-3, B-6 and B-12, and between LASIK and PRK after adjusting for spherical equivalent, central corneal thickness, thinnest corneal thickness, residual bed thickness and ablation depth. A p-value < 0.05 was considered statistically significant.

7. Results Ninety-eight eyes of 49 patients (62 LASIK, 36 PRK) were included. The mean age of the patients was 35.2 ± 8.5 years. Significant differences were observed in spherical equivalent refraction, central corneal thickness, thinnest corneal thickness and ablation depth between patients under LASIK and PRK groups (p ≤ 0.021)

8. The mean change in posterior corneal elevation values after LASIK was 4.88 ± 0.47, 2.42 ± 0.56, 3.76 ± 0.46 and 2.92 ± 0.46 μm for B-1, B-3, B-6 and B-12, respectively The mean change in posterior corneal elevation values after PRK was 3.67 ± 0.48, 3.00 ± 0.47, 2.76 ± 0.46 and 2.72 ± 0.46 μm for B-1, B-3, B-6 and B- 12, respectively Significance differences were also found in the posterior corneal elevation between post-LASIK and post-PRK eyes up to B-3 and B-12 after adjusting for the preoperative and intraoperative parameters (p ≤ 0.018). The forward displacements of the posterior corneal surface were statistically significant throughout the study period after both LASIK and PRK (p < 0.05).

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10. Discussion In the current study, we observed a forward shift of the posterior corneal surface within the first postoperative year following both femtosecond- assisted LASIK and PRK using swept source optical coherence tomography. Several comparative studies between the scanning-slit topography and Scheimpflug photography have been conducted and showed significant differences in the posterior corneal elevation measured in the same eye after refractive surgery. Compared to Scheimpflug imaging, scanning-slit topography yielded larger posterior elevation values and postoperative changes following LASIK and PRK. (JCRS 2007; 33:841-7; JCRS 2009; 25:290-5)

11. Similar to Scheimpflug photography, swept source optical coherence tomography can measure and compute corneal topography from the posterior corneal surface directly. It is believed that swept source optical coherence tomography is able to image the posterior cornea better than Scheimpflug photography because of its shorter scanning time (0.3 vs. 2 seconds) and longer wavelength of light source (1310 vs. 475 nm). (JCRS 2011;37;1871-8) A shorter scanning time greatly reduces motion artifacts, while a longer wavelength allows better light penetration and less scatter through the LASIK flap interface or post-PRK corneal haze. In our study, the posterior cornea fluctuated during the first postoperative year after LASIK, while it stabilized as early as 3 months after PRK.

12. Conclusion The findings of our study suggested that there was a mild but significant forward protrusion of the posterior cornea after femtosecond laser- assisted LASIK and PRK. The posterior cornea responded differently after the 2 treatments with a more swayed pattern observed in post-LASIK eyes compared to post-PRK eyes.