Optical coherence tomography (OCT) is a technique that applies low-coherence interferometry and image processing to show the microstructures of optical tissue with noncontact and noninvasive features based on light backscatter from the tissue.
5 9 13 14 15 16 17 Previous studies have demonstrated that it is feasible to measure the thickness of different layers of the cornea, such as the epithelium, corneal flap, and total corneal thickness with a precision within 1 to 5 μm.
6 8 10 This is similar to the precision obtained with very high-frequency ultrasound biomicroscopy
18 19 and confocal microscopy through focusing (CMTF).
20 These three instruments have been used to study the anatomic changes brought about by refractive surgery. However, the latter two methods normally are not used in the very early postoperative period, especially in patients after LASIK procedures, because they require touching the cornea. In this study, we demonstrated that real-time OCT is a useful noncontact tool to evaluate epithelial and flap thickness after LASIK surgery.
Compared with a conventional OCT with its long acquisition time, the real-time OCT used in this study quickly acquired multiple images for data analysis. Briefly, the benefits using this system include real-time imaging and the reduction of misalignment and patient motion artifacts while imaging dynamic ocular events.
9 For this study, custom software was developed to process the raw images and generate mean backscatter profiles
(Fig. 1) , from which dimensional results were directly obtained. The software has been demonstrated to yield repeatable results in previous studies (Fonn D, et al.
IOVS 2000;41:ARVO Abstract 3591).
6 21
Corneal flap thickness was first measured with OCT by Maldonado et al.,
10 who found it a useful tool to perform this task in a noncontact and noninvasive manner, with high precision.
10 However, in Maldonado’s report, the authors did not mention how many longitudinal scans were analyzed by placing the cursors on the peaks and the epithelial layer was not differentiated after surgery. In the present study, multiple longitudinal scans (122 scans) were processed to yield the reflectivity profile and the epithelial thickness was obtained at all visits. Ultrasound subtraction pachymetry (subtracting the intraoperative corneal bed pachymetry measurement from the intraoperative total corneal pachymetry) was often used in previous studies investigating the corneal flap and its impacting factors.
22 23 24 25 26 The major problem with this method is its indirect measurement, which provides an estimate of the bed thickness. The swelling of the corneal bed may cause inaccurate measurements of the corneal flap, which may account for the varying reports of factors that influence flap thickness.
22 23 24 25 26 We found no strong relation between the flap thickness and other measured parameters, using the direct measurement provided by our OCT. Other aspects of the microkeratome and the eye may affect the flap thickness, however, and further studies may be needed.
Flap thickness has been found to have a wide variability (in a range of ±20 μm) with most currently available microkeratomes.
22 23 24 25 26 27 28 29 30 31 32 33 The relation between the flap thickness and other preoperative data or real-time parameters (such as IOP, suction time, and incision angles) has not been well established in LASIK patients.
24 28 34 35 To monitor the flap thickness and the cornea’s changes during healing, it is important to measure the corneal layers as precisely as possible. In this study, 1 day after surgery, flap thickness was different between these two groups with different planned flap thicknesses (160 μm vs. 180 μm), which indicates planned flap thickness was a factor affecting the actual flap thickness. However, great variation in flap thickness in each group was found. Seo et al.
34 found an increase in flap thickness with an increase of suction time during the procedure, with an increase of incision angles in an animal model. Kasetsuwan et al.
24 demonstrated that the difference in flap thickness was not statistically significant in different real-time IOP groups during the procedure and suggested that the suction ring pressure setting may be another factor affecting the flap thickness. Using a microkeratome (Automated Corneal Shaper; Bausch & Lomb, Rochester, NY) in vitro on porcine eyes, no correlation between flap thickness and flap diameter was found by Behrens et al.,
35 who examined corneal flap dimensions and cut quality. To maintain a residual stromal thickness of at least 250 μm to avoid complications, such as iatrogenic keratectasia
36 37 and interstitial keratitis,
38 more predictors of flap thickness are needed. When the major determining factors of actual flap thickness have been identified, cornea flap-cutting devices and procedures can be redesigned to perform more predictably.
Epithelial thickening was evident in this study when measured by noncontact methods. This finding may be due to epithelial hyperplasia and is in agreement with previous studies using such contact methods as confocal microscopy
2 and high-frequency ultrasound.
3 4 Using confocal microscopy in vivo to study myopes who undergo LASIK, Erie et al.
2 found that epithelial thickness was increased by 22% 1 month after LASIK compared with its preoperative baseline and maintained the increased thickness up to 1 year after LASIK. Spadea et al.
4 demonstrated that epithelial thickness increased within the first week after LASIK, with a maximum increase of approximately 6.5 μm by the third month. In this study, epithelial thickness decreased 1 day after surgery, but this was not statistically significant. After that, epithelial thickness was found to increase up to 1 month
(Fig. 2) . Epithelial hyperplasia has been reported to occur after LASIK
4 39 40 and PRK.
41 42 The mechanism of epithelial hyperplasia remains unknown. It may be a part of the wound-healing process or a response to the biomechanical changes of the cornea, such as tissue loss, redistribution of corneal tension,
43 and innervation.
44 45 Linna et al.
44 found a major loss of innervation in the epithelial and superficial stromal layers, except for the area at the flap hinge, in rabbit corneas 3 days after LASIK. These findings were confirmed in human eyes and associated by Linna et al. with a loss of corneal sensitivity.
45
Spadea et al.
4 reported that increased epithelial thickness played a role after LASIK in regression of myopia in patients with high myopia. However, we could not demonstrate a relationship between epithelial hyperplasia and regression of myopia, because there was not a statistically significant regression in our sample
(Table 1) . The reasons may be different study groups (low myopia in this study versus high myopia in Spadea et al.) and follow-up period (1 month in the present study vs. more than 12 months in Spadea et al.). It is also likely that epithelial hyperplasia has little impact in patients with low myopia after LASIK. The factor of bed thinning
(Fig. 5) combined with epithelial hyperplasia should be considered among the range of biomechanical responses of the cornea and the measurement of total cornea thickness.
In summary, real-time OCT was demonstrated to be a valuable tool for noncontact measurements of epithelial and flap thicknesses after LASIK, although flap thickness could not be correlated with other ocular parameters investigated in our study. This method can be used to monitor the changes in specific corneal layers during wound healing after refractive surgery, aiding efforts to understand the biomechanics of LASIK and PRK.
The authors thank Trevor German for the development of the custom software used in this study and Jennifer Anstey for help with editing the manuscript.