- Email: [email protected]
Optical coherence tomography for evaluation of anatomical changes in the cornea after laser in situ keratomileusis1
Optical coherence tomography for evaluation of anatomical changes in the cornea after laser in situ keratomileusis Can Ustundag, MD, Halil Bahcecioglu, MD, Akif Ozdamar, MD, Cengiz Aras, MD, Rengin Yildirim, MD, Sehirbay Ozkan, MD ABSTRACT Purpose: To examine the use of optical coherence tomography (OCT) in evaluating anatomical changes after laser in situ keratomileusis (LASIK) and complications related to the interface and corneal flap. Setting: Istanbul University Eye Research Center and Department of Ophthalmology, Cerrahpasa Medical School, Istanbul, Turkey. Methods: Eleven eyes of 11 patients who had myopic LASIK were included in the 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). Corneas were examined by OCT (Humphrey Systems). Results: Optical coherence tomography resolved corneal flap and residual stromal layers in all cases. The mean thickness of the corneal flap and residual stroma was 138.2 ⫾ 16.5 m and 321.7 ⫾ 32.1 m, respectively. Interface between the corneal flap and residual stroma was shown by OCT. Optical coherence tomography revealed that the eye with flap striae had flap displacement undetected by biomicroscopy. Epithelial ingrowth was shown as a highly reflective area. Conclusion: Optical coherence tomography appears to be a promising method for evaluating anatomical changes in the cornea after LASIK. J Cataract Refract Surg 2000; 26: 1458 –1462 © 2000 ASCRS and ESCRS
L
aser 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 Accepted for publication May 15, 2000. Reprint requests to Akif Ozdamar, MD, Vali Konagi Caddesi, Leylak Sokak No:3 D2A, Basinkoy—Istanbul, Turkey. E-mail: aozdamar@ iris.com.tr. © 2000 ASCRS and ESCRS Published by Elsevier Science Inc.
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. 0886-3350/00/$–see front matter PII S0886-3350(00)00558-7
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Patients and Methods
Optical Coherence Tomography
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). Surgical Technique Surgery was performed with topical anesthesia (oxybuprocaine 0.1%). The Hansatome microkeratome (Chiron Vision) with the 180 m 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.
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. Computersoftware-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.
Table 1. Summary of the cases before OCT evaluation.
Case
Preoperative Refraction* (D)
Treatment
Postoperative Time (Months)
1
⫺6.50
LASIK
3
None
2
⫺5.25
LASIK
3
None
3
⫺8.87
LASIK
4
None
4
⫺4.50
LASIK
3
None
5
⫺8.00
LASIK
2
Epithelial ingrowth
6
⫺6.00
LASIK
6
Epithelial ingrowth
7
⫺12.50
LASIK
4
Epithelial ingrowth
8
⫺8.50
LASIK
6
Epithelial ingrowth
9
⫺10.25
LASIK
9
Epithelial ingrowth
10
⫺13.00
LASIK
2
Flap striae
11
⫺4.00
LASIK
3
Flap striae
Postoperative Complication
OCT ⫽ optical coherence tomography; LASIK ⫽ laser in situ keratomileusis *Spherical equivalent refraction
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
1459
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Figure 1. (Ustundag) Profile of the corneal flap, interface (arrows), and corneal stroma following LASIK.
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 m, 124 m, 162 m, and 136 m and residual stromal thicknesses, 310 m, 366 m, 321 m, and 290 m 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 m, 52 m, 65 m, 121 m, and 34 m in cases 5, 6, 7, 8 and 9, respectively, with the digital OCT system.
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 m full-width half-maximum longiditunal imaging resolution can be achieved
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.
1460
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Figure 3. (Ustundag) Epithelial ingrowth following LASIK by OCT (left) and slitlamp biomicroscopy (right); the line indicates the area scanned by OCT.
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 m 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 m.13 Although the system provides accurate measurements, it requires immersion and is not commercially available. 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.
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
1461
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
References 1. Pallikaris IG, Papatzanaki ME, Stathi EZ, et al. Laser in situ keratomileusis. Laser Surg Med 1990; 10:463– 468 2. Pallikaris IG, Siganos DS. Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia. J Refract Corneal Surg 1994; 10:498 –510 3. Salah T, Waring GO III, el Maghraby A, et al. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996; 121: 143–155 4. Gimbel HV, Basti S, Kaye GB, Ferensowicz M. Experience during the learning curve of laser in situ keratomileusis. J Cataract Refract Surg 1996; 22:542–550 5. Knorz MC, Liermann A, Seiberth V, et al. Laser in situ keratomileusis to correct myopia of ⫺6.00 to ⫺29.00 diopters. J Refract Surg 1996; 12:575–584 6. Stulting RD, Carr JD, Thompson KP, et al. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology 1999; 106:13–20 7. Puliafito CA, Hee MR, Schuman JS, Fujimoto JG. Optical Coherence Tomography of Ocular Diseases. Thorofare, NJ, Slack Inc, 1996 8. Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112: 1584 –1589 9. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia
1462
10.
11.
12.
13.
14.
after laser in situ keratomileusis. J Refract Surg 1998; 14:312–317 Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087–1092 Casebeer JC, Slade SG, Dybbs A, Mahanti RL. Intraoperative pachometry during automated lamellar keratoplasty: a preliminary report. J Refract Corneal Surg 1994; 10:41– 44 Waring GO III. A cautionary tale of innovation in refractive surgery. Arch Ophthalmol 1999; 117:1069 – 1073 Reinstein DZ, Silverman RH, Sutton HFS, Coleman DJ. Very high-frequency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery; anatomic diagnosis in lamellar surgery. Ophthalmology 1999; 106:474 – 482 Farah SG, Azar DT, Gurdal C, Wong J. Laser in situ keratomileusis: literature review of a developing technique. J Cataract Refract Surg 1998; 24:989 –1006
From Istanbul University Eye Research Center, Istanbul, Turkey. Presented in part at the XVIIth Congress of the European Society of Cataract & Refractive Surgeons, Vienna, Austria, September 1999. None of the authors has a financial interest in any product mentioned.
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
L
aser 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 Accepted for publication May 15, 2000. Reprint requests to Akif Ozdamar, MD, Vali Konagi Caddesi, Leylak Sokak No:3 D2A, Basinkoy—Istanbul, Turkey. E-mail: aozdamar@ iris.com.tr. © 2000 ASCRS and ESCRS Published by Elsevier Science Inc.
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. 0886-3350/00/$–see front matter PII S0886-3350(00)00558-7
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Patients and Methods
Optical Coherence Tomography
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). Surgical Technique Surgery was performed with topical anesthesia (oxybuprocaine 0.1%). The Hansatome microkeratome (Chiron Vision) with the 180 m 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.
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. Computersoftware-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.
Table 1. Summary of the cases before OCT evaluation.
Case
Preoperative Refraction* (D)
Treatment
Postoperative Time (Months)
1
⫺6.50
LASIK
3
None
2
⫺5.25
LASIK
3
None
3
⫺8.87
LASIK
4
None
4
⫺4.50
LASIK
3
None
5
⫺8.00
LASIK
2
Epithelial ingrowth
6
⫺6.00
LASIK
6
Epithelial ingrowth
7
⫺12.50
LASIK
4
Epithelial ingrowth
8
⫺8.50
LASIK
6
Epithelial ingrowth
9
⫺10.25
LASIK
9
Epithelial ingrowth
10
⫺13.00
LASIK
2
Flap striae
11
⫺4.00
LASIK
3
Flap striae
Postoperative Complication
OCT ⫽ optical coherence tomography; LASIK ⫽ laser in situ keratomileusis *Spherical equivalent refraction
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
1459
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Figure 1. (Ustundag) Profile of the corneal flap, interface (arrows), and corneal stroma following LASIK.
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 m, 124 m, 162 m, and 136 m and residual stromal thicknesses, 310 m, 366 m, 321 m, and 290 m 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 m, 52 m, 65 m, 121 m, and 34 m in cases 5, 6, 7, 8 and 9, respectively, with the digital OCT system.
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 m full-width half-maximum longiditunal imaging resolution can be achieved
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.
1460
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
Figure 3. (Ustundag) Epithelial ingrowth following LASIK by OCT (left) and slitlamp biomicroscopy (right); the line indicates the area scanned by OCT.
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 m 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 m.13 Although the system provides accurate measurements, it requires immersion and is not commercially available. 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.
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000
1461
OCT TO EVALUATE CORNEAL CHANGES AFTER LASIK
References 1. Pallikaris IG, Papatzanaki ME, Stathi EZ, et al. Laser in situ keratomileusis. Laser Surg Med 1990; 10:463– 468 2. Pallikaris IG, Siganos DS. Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia. J Refract Corneal Surg 1994; 10:498 –510 3. Salah T, Waring GO III, el Maghraby A, et al. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996; 121: 143–155 4. Gimbel HV, Basti S, Kaye GB, Ferensowicz M. Experience during the learning curve of laser in situ keratomileusis. J Cataract Refract Surg 1996; 22:542–550 5. Knorz MC, Liermann A, Seiberth V, et al. Laser in situ keratomileusis to correct myopia of ⫺6.00 to ⫺29.00 diopters. J Refract Surg 1996; 12:575–584 6. Stulting RD, Carr JD, Thompson KP, et al. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology 1999; 106:13–20 7. Puliafito CA, Hee MR, Schuman JS, Fujimoto JG. Optical Coherence Tomography of Ocular Diseases. Thorofare, NJ, Slack Inc, 1996 8. Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112: 1584 –1589 9. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia
1462
10.
11.
12.
13.
14.
after laser in situ keratomileusis. J Refract Surg 1998; 14:312–317 Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087–1092 Casebeer JC, Slade SG, Dybbs A, Mahanti RL. Intraoperative pachometry during automated lamellar keratoplasty: a preliminary report. J Refract Corneal Surg 1994; 10:41– 44 Waring GO III. A cautionary tale of innovation in refractive surgery. Arch Ophthalmol 1999; 117:1069 – 1073 Reinstein DZ, Silverman RH, Sutton HFS, Coleman DJ. Very high-frequency ultrasound corneal analysis identifies anatomic correlates of optical complications of lamellar refractive surgery; anatomic diagnosis in lamellar surgery. Ophthalmology 1999; 106:474 – 482 Farah SG, Azar DT, Gurdal C, Wong J. Laser in situ keratomileusis: literature review of a developing technique. J Cataract Refract Surg 1998; 24:989 –1006
From Istanbul University Eye Research Center, Istanbul, Turkey. Presented in part at the XVIIth Congress of the European Society of Cataract & Refractive Surgeons, Vienna, Austria, September 1999. None of the authors has a financial interest in any product mentioned.
J CATARACT REFRACT SURG—VOL 26, OCTOBER 2000