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![Page 1: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/1.jpg)
Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after
Refractive Surgery
Dr. Tommy Chung Yan ChanDr. Vishal Jhanji
The authors have no financial interests to disclose
![Page 2: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/2.jpg)
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.
![Page 3: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/3.jpg)
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.
![Page 4: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/4.jpg)
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.
![Page 5: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/5.jpg)
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.
![Page 6: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/6.jpg)
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.
![Page 7: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/7.jpg)
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)
![Page 8: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/8.jpg)
• 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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★★
![Page 10: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/10.jpg)
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)
![Page 11: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/11.jpg)
• 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.
![Page 12: Swept Source Optical Coherence Tomography for Evaluation of Posterior Corneal Changes after Refractive Surgery Dr. Tommy Chung Yan Chan Dr. Vishal Jhanji.](https://reader036.fdocuments.net/reader036/viewer/2022082709/56649d0c5503460f949e0ca9/html5/thumbnails/12.jpg)
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.