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Optical coherence tomography for evaluation of anatomical changes in the cornea after laser in situ keratomileusis

Ustundag, Can MDa; Bahcecioglu, Halil MDa; Ozdamar, Akif MDa,*; Aras, Cengiz MDa; Yildirim, Rengin MDa; Ozkan, Sehirbay MDa

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Journal of Cataract & Refractive Surgery: October 2000 - Volume 26 - Issue 10 - p 1458-1462
doi: 10.1016/S0886-3350(00)00558-7
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Abstract

Laser in situ keratomileusis (LASIK) has become a popular method for the treatment of myopic refractive errors.1 The complications of LASIK have been reported.2–5 Intraoperative complications include an incomplete pass, thin flap, buttonhole, free cap, and flap displacement. The epithelial ingrowth rate is 9.1%, and 1.8% of these eyes require surgical intervention.6 To appropriately manage the complications of corneal refractive surgery, the exact anatomic cause must be determined. Surface videokeratography provides only a graphic representation of the presence and location of irregularity, not the underlying anatomic basis. Optical coherence tomography (OCT) is a new method for high-resolution, cross-sectional visualization of tissue.7 Like ultrasound, OCT obtains cross-sectional images of reflectivity in tissue that can differentiate internal tissue structure. In this study, we used OCT to evaluate the anatomic changes in the cornea after LASIK and the complications related to the interface and flap.

Patients and Methods

Eleven eyes of 11 patients who had LASIK for the treatment of myopia were included in this retrospective study. Mean age of the 7 men and 4 women was 29.4 years ± 6.9 (SD). Cases analyzed included uneventful LASIK (4 eyes), epithelial ingrowth (5 eyes), and flap striae (2 eyes). The preoperative data and postoperative complications are summarized in Table 1. A full ocular examination including manifest refraction, anterior segment slitlamp photography, pachymetry, and videokeratographic mapping was performed before OCT evaluation. The mean interval between the LASIK procedure and the OCT imaging was 4.09 ± 2.11 months (range 2 to 9 months).

Table 1
Table 1:
Summary of the cases before OCT evaluation.

Surgical Technique

Surgery was performed with topical anesthesia (oxybuprocaine 0.1%). The Hansatome microkeratome (Chiron Vision) with the 180 μm plate was used in each patient to create the corneal flap. Ablations were performed with the Summit Apex Plus excimer laser. Laser parameters included a repetition rate of 10 Hz and a fluence of 180 mJ/cm2. An aspheric multizone pattern with 5.5/6.5 mm was used in the ablation procedure. A pretreatment protocol of 8% of total ametropia with an optical zone of 2.6 mm was used in each patient to prevent central island formation. A drop of tobramycin was placed in the eye at the end of the procedure.

Optical Coherence Tomography

Commercially available OCT equipment (Humphrey Systems) was used with the A4 software derived from the prototype described by Puliafito and coauthors.7 Each patient had single-line scans at the central cornea, at the flap edge, and over the epithelial ingrowth area and flap striae. The scan length varied between 1.12 and 3.80 mm.

The patient was seated on the patient side of the OCT unit. The unit was moved toward the patient slowly, and the unit's elevation was adjusted to match the height of the patient's eye. The patient was directed to fixate on an internal or external target. The pupil was centered on the monitor by using the joystick to slowly move the OCT unit toward the patient until the cornea was seen on the monitor. The image clarity was adjusted by turning the focus knob, which alters the position of front lens. The joystick button was pressed to lock the OCT unit into position.

Because the ablation was centered over the entrance pupil, the measurements were made in the center of the ablation zone. Measurements of tissue thickness were performed with the scan profile display. Computer- software-controlled cursors were manually placed at the peak of the reflectivity spikes corresponding to the anterior corneal surface, the interface, or the posterior corneal surface. Tissue thickness was calculated between peaks from the time delay of reflected light.

Corneal epithelium represented the first highly reflective layer in the OCT tomogram and corneal endothelium, the last reflective layer. The corneal stroma was assumed to represent the minimum reflectivity layer located between the anterior and posterior borders of the cornea. The images were displayed in false color in which bright colors (red to white) corresponded to regions of high relative optical reflectivity and dim colors (blue to black), to areas of minimal or no relative reflectivity.

Results

Optical coherence tomography resolved corneal flap and residual stromal layers in all cases. It demonstrated the flap thickness profile at the selected area of the cornea (Figure 1). The central flap thicknesses were 131 μm, 124 μm, 162 μm, and 136 μm and residual stromal thicknesses, 310 μm, 366 μm, 321 μm, and 290 μm in cases 1, 2, 3, and 4, respectively. The interface between the corneal flap and residual stroma was shown in all cases. Optical coherence tomography revealed that the eyes with flap striae had flap displacement to the temporal side; this was undetected by biomicroscopy in case 10. Biomicroscopic examination showed only a faint circular gray scar at the nasal periphery of the cornea in case 11 (Figure 2, left). Optical coherence tomography clearly showed bare stroma that was covered by corneal epithelium outside the flap at the nasal peripheral cornea (Figure 2, right). Epithelial ingrowth was shown by OCT as a red-white highly reflective area in cases 5 to 9 (Figure 3, left). The thickness of the epithelial sheet was not homogenous and varied along the interface. The thicknesses were measured as 38 μm, 52 μm, 65 μm, 121 μm, and 34 μm in cases 5, 6, 7, 8 and 9, respectively, with the digital OCT system.

Figure 1.
Figure 1.:
(Ustundag) Profile of the corneal flap, interface (arrows), and corneal stroma following LASIK.
Figure 2.
Figure 2.:
(Ustundag) Left: Slitlamp photograph shows faint double circular gray scar at the nasal periphery of the cornea (Case 11). The line indicates the area scanned by OCT. Right: Optical coherence tomography shows flap displacement to the temporal side. Arrow indicates the end of the flap displaced temporally.
Figure 3.
Figure 3.:
(Ustundag) Epithelial ingrowth following LASIK by OCT (left) and slitlamp biomicroscopy (right); the line indicates the area scanned by OCT.

Discussion

Optical coherence tomography is a new diagnostic technique for performing noncontact, noninvasive, cross-sectional imaging of ocular structures. In contrast to subjective diagnostic techniques, such as slitlamp biomicroscopy, OCT provides direct, quantitative measurements of intraocular dimensions and scattering. It does not require direct contact with the eye or immersion in saline solution, and 10 μm full-width half-maximum longiditunal imaging resolution can be achieved at arbitrary depth in transparent tissue.7 It has been demonstrated that OCT acquires high-quality topographic imaging for measuring corneal thickness and curvature.8 In our study, OCT successfully demonstrated the flap thickness profile and provided valuable information about flap uniformity. It was able to detect the stromal interface despite near optical transparency. The technique was therefore able to resolve the flap from the residual stromal bed.

There has been no reliable way to measure the thickness of individual corneal layers after LASIK. This information is crucial to perform LASIK retreatment safely, as it is generally believed that 250 μm of residual stroma is necessary to preserve corneal tectonic integrity and prevent iatrogenic keratoconus. Simple measurement of corneal thickness before LASIK enhancement could be unsafe.9 Variations in corneal flap thickness despite using the same instrument,10 varying rates of excimer laser ablation due to the quality of the optics in the delivery system and individual corneal changes such as hydration, and significant epithelial thickening after LASIK prevent the estimation of residual corneal thickness after surgery. Intraoperative measurement of flap and bed thickness by ultrasound probe is very inaccurate because of the low frequency of the handheld probes (20 MHz), the location and positioning of the probe, and the immediate changes that occur in stromal hydration as the flap is raised.11,12 Thus, postoperative measurement of individual corneal layers after LASIK provides more reliable data for planning the retreatment procedure. In our study, we were able to measure the thickness of the residual stromal layer.

The other method for measuring residual stromal thickness is the digital 50 MHz high-frequency ultrasound system, which can resolve the individual corneal layers and measure the thickness of the corneal epithelium, corneal flap, and residual stromal bed in keratomileusis with a precision of 1.3 μm.13 Although the system provides accurate measurements, it requires immersion and is not commerciallyavailable.

Flap striae result from misalignment of the corneal flap after replacement or movement of the flap during the first postoperative day. Flap striae that extend through the visual axis cause a decrease in the best corrected visual acuity or induce irregular astigmatism. In our cases with flap striae, misalignment of the corneal flap was demonstrated by OCT. The flap misalignment was directed to the temporal side in cases 10 and 11. Recognizing the direction and amount of flap displacement can help in the treatment of flap striae using various removal techniques.

The incidence of epithelial ingrowth in LASIK varies, but the cumulative mean is 4.3%.14 Although the presence of epithelial ingrowth can be clearly shown by biomicroscopy, the thickness of the epithelial ingrowth in the interface is difficult to evaluate. In our cases, OCT showed epithelial ingrowth in the interface and quantified its thickness. The measurement of epithelial thickness in the interface may help in the follow-up of cases with epithelial ingrowth.

In conclusion, OCT appears to be a promising technique for evaluating anatomical changes after lamellar refractive surgery.

References

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© 2000 by Lippincott Williams & Wilkins, Inc.