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

Cornea donors who have had prior refractive surgery: data from the Eye Bank Association of America

PURPOSE OF REVIEW

Millions of Americans have undergone refractive surgeries, including radial keratotomy, photorefractive keratectomy, and laser-assisted in situ keratomileusis. Eye Bank Association of America medical standards do not permit corneas from patients who have undergone refractive procedures to be used in penetrating keratoplasty, anterior lamellar keratoplasty, or tectonic grafting procedures. Such corneas, can, however, be used for endothelial corneal transplantation. The objective of this article is to provide an update on current trends for the screening and usage of corneas that have undergone refractive surgery.

RECENT FINDINGS

Several case reports have highlighted the difficulty in using postrefractive surgery corneas in penetrating keratoplasty. However, tissue with anterior stromal flaws, including a history of refractive surgery, has been used in endothelial keratoplasty with equivalent outcomes in topography, endothelial cell count, and visual acuity. Many modalities for proper identification of postmortem donor corneas that have undergone refractive surgery have been studied.

SUMMARY

Corneas with a history of refractive surgery have found use in endothelial keratoplasty. Multiple objective methods of tissue identification have been investigated to avoid the use of these corneas in penetrating or anterior keratoplasty surgeries.

Central corneal thickness in southern Egypt

The study aimed to determine mean central corneal thickness (CCT) in a southern Egyptian population according to gender and age using ultrasonic pachymetry and to compare these CCT measurements to different populations. A prospective, observational, consecutive case series of 4,368 non-glaucomatous subjects (emmetropes and myopes) aged 16-70 years was carried out from August 2010 to March 2013 at the outpatient ophthalmology clinic in Sohag University Hospital and the Laser Vision Center in Sohag City, Egypt. Refraction, keratometry, slit-lamp examination, and intraocular pressure (IOP) measurements were obtained for all subjects. CCT was measured by ultrasonic pachymetry. The average CCT was 530.06 ± 38.03 μm. Average CCT was 532.6 ± 33.3 μm in emmetropes, 531.5 ± 31.3 μm in myopes <6 diopters (D), 531.1 ± 31.4 μm in myopes >6 D and 533 ± 33 μm in hyperopes, with no statistically significant difference between the groups. There was a statistically significant difference in CCT between age groups and gender. There was a strong correlation between CCT and IOP among the non-glaucomatous population. CCT was found to be lower in Egyptians than in Caucasian, Hispanic, and Japanese populations but comparable to African and African American populations.

WaveLight FS200 vs Hansatome LASIK: intraoperative determination of flap characteristics and predictability by hand-held bioptigen spectral domain ophthalmic imaging system

PURPOSE

To intraoperatively determine and compare the characteristics and predictability of LASIK flaps made by the WaveLight FS200 femtosecond laser (Alcon Laboratories Inc) and Hansatome (Bausch & Lomb) microkeratome using a hand-held spectral domain ophthalmic imaging system (Bioptigen Inc).

METHODS

Sixty eyes from 30 patients undergoing bilateral LASIK were prospectively evaluated. Patients were divided into two equal groups to undergo flap creation with either 100-microm femtosecond laser flaps (FS flap group) or 120-microm microkeratome flaps (MK flap group). Flap thickness was measured intraoperatively after creation of the flap but prior to lifting using the hand-held probe of the spectral domain imaging system. Geometry of the flap edge and smoothness of the stromal bed after lifting the flap was also evaluated in all cases.

RESULTS

Mean difference between planned and achieved flap thickness in the paracentral region was 2.84 +/- 3.16 mm for the FS flap group and 11.33 +/- 10.27 mm for the MK flap group, whereas in the periphery, it was 5.72 +/- 3.26 mm in the FS flap group and 24.67 +/- 10.35 mm in the MK flap group. The differences between groups were statistically significant (P < 0.001, Kruskal-Wallis test). The edges of the flaps were vertical in the FS flap group and the stromal bed was smoother, whereas in the MK flap group, the edges were more sloping and the stromal bed more irregular.

CONCLUSIONS

The WaveLight FS200 femtosecond laser is able to produce planar flaps with a high degree of predictability between the desired and achieved flap thickness. The ability to study the flap characteristics intraoperatively (when flap edema and stromal bed hydration changes have not yet occurred) with the hand-held probe of the Bioptigen imaging system ensures greater accuracy than measurements done postoperatively using other anterior segment optical coherence tomography prototypes.

Wide tuning range wavelength-swept laser with a single SOA at 1020 nm for ultrahigh resolution Fourier-domain optical coherence tomography

In this study, we demonstrated a wide tuning range wavelength-swept laser with a single semiconductor optical amplifier (SOA) at 1020 nm for ultrahigh resolution, Fourier-domain optical coherence tomography (UHR, FD-OCT). The wavelength-swept laser was constructed with an external line-cavity based on a Littman configuration. An optical wavelength selection filter consisted of a grating, a telescope, and a polygon scanner. Before constructing the optical wavelength selection filter, we observed that the optical power, the spectrum bandwidth, and the center wavelength of the SOA were affected by the temperature of the thermoelectric (TE) cooler in the SOA mount as well as the applied current. Therefore, to obtain a wide wavelength tuning range, we adjusted the temperature of the TE cooler in the SOA mount. When the temperature in the TE cooler was 9 °C, our swept source had a tuning range of 142 nm and a full-width at half-maximum (FWHM) of 121.5 nm at 18 kHz. The measured instantaneous spectral bandwidth (δλ) is 0.085 nm, which was measured by an optical spectrum analyzer with a resolution bandwidth of 0.06 nm. This value corresponds to an imaging depth of 3.1 mm in air. Additionally, the averaged optical power of our swept source was 8.2 mW. In UHR, FD/SS-OCT using our swept laser, the measured axial resolution was 4.0 μm in air corresponding to 2.9 μm in tissue (n = 1.35). The sensitivity was measured to be 93.1 dB at a depth of 100 μm. Finally, we obtained retinal images (macular and optic disk) and a corneal image.

Comparison of laser in situ keratomileusis flaps created by 3 femtosecond lasers and a microkeratome

PURPOSE

To evaluate the thickness and side-cut angle of laser in situ keratomileusis (LASIK) flaps created by 1 of 3 femtosecond lasers or a microkeratome using Fourier-domain optical coherence tomography (OCT).

SETTING

Department of Ophthalmology, Institute of Vision Research, Yonsei University College of Medicine, Seoul, Korea.

DESIGN

Comparative case series.

METHODS

Flap creation for bilateral LASIK was performed using an IntraLase (femtosecond group 1), VisuMax (femtosecond group 2), or Femto LDV (femtosecond group 3) femtosecond laser or an M2 microkeratome. Flap thickness was determined at 14 points. The side-cut angle was measured in 4 directions at the margin interface. Measurements were taken 2 months postoperatively using an RTVue Fourier-domain OCT device and integrated software.

RESULTS

Femtosecond group 1 comprised 50 eyes; femtosecond group 2, 40 eyes; femtosecond group 3, 64 eyes; and the microkeratome group, 52 eyes. Eyes in femtosecond groups 1 and 2 had relatively even flap configuration. Flaps in femtosecond group 3 and the microkeratome group had a meniscus shape. Flaps in femtosecond group 1 had the least difference between the mean peripheral and the central flap thickness (P<.001). The greatest flap thickness predictability (measured versus intended thickness) was in femtosecond group 3 (P<.001). Flaps in femtosecond group 1 had a side-cut angle closest to 90 degrees (P<.001).

CONCLUSIONS

Flap morphology differed according to the system used. The 3 femtosecond laser systems appeared to be superior to the microkeratome system generally. The 3 femtosecond laser systems also produced different flap configurations depending on their individual mechanisms.

Traumatic Flap Striae 6 Years After LASIK: Case Report and Literature Review

Purpose:

To report a case of traumatic flap striae without flap dislocation 6 years after LASIK and provide a literature review of surgical flap striae, late traumatic flap striae, and their management.

Methods:

A 28-year-old man presented with late traumatic flap striae without concurrent flap dislocation, which closely approximated the longest reported interval between LASIK and the development of flap striae.

Results:

In the absence of flap dislocation, the finding of striae alone was subtle and went undetected initially. The flap was successfully refloated, stretched, and smoothed with recovery of 20/20 vision.

Conclusions:

Traumatic LASIK flap complications may occur many years after the original procedure. This report presents the first case of late traumatic flap striae without concurrent flap dislocation. Proper management can restore good visual function.

Optical coherence tomography anatomy of the corneal endothelial transplantation wound

PURPOSE

The goal of this study was to prospectively assess the deep lamellar endothelial keratoplasty (DLEK) wound anatomy and its evolution during the 12 months after surgery, using optical coherence tomography (OCT).

METHODS

The eyes of 8 patients (1 eye per patient) who consecutively underwent DLEK for Fuchs dystrophy or pseudophakic bullous keratopathy were prospectively studied before and 1, 3, 6, and 12 months after surgery. The Stratus OCT apparatus (Carl Zeiss Meditec, Dublin, CA) was used to acquire central and radial scans perpendicular to the wound at 3-, 6-, 9-, and 12-o'clock positions. The following parameters were analyzed: central total thickness, posterior donor-recipient edges gap, donor-recipient height mismatch, tissue compression, and graft detachment.

RESULTS

A posterior gap was observed in 4 of the 8 DLEK eyes. At 12 months, the mean gap contour, depth, and width were 242 +/- 67, 101 +/- 45, and 87 +/- 29 microm, respectively. A step was documented in all DLEK eyes (average step height 108 +/- 24 microm). A micrograft detachment was observed in one case and tissue compression in another. In all corneas, the mean central corneal thickness returned to normal range and almost normal anatomy with time after surgery.

CONCLUSIONS

OCT was found to be a very useful tool for DLEK corneal wound architecture analysis. It revealed microscopic wound irregularities and allowed their quantitative follow-up with time.

High-resolution optical coherence tomography visualization of LASIK flap displacement

Uneventful myopic laser in situ keratomileusis (LASIK) was performed in both eyes of a 33-year-old woman. Two weeks after LASIK, examination of the left eye revealed flap striae radiating inferonasally from the superior hinge. The flap was relifted and repositioned to remove the irregular astigmatism and reduce the striae. Before the flap was relifted, Fourier-domain optical coherence tomography (FD-OCT) showed the gap at the temporal flap, which had not been detected by biomicroscopy. After the flap was relifted, FD-OCT documented that the gap was closed. High-resolution FD-OCT was helpful in the visualization and management of flap displacement.

Optical coherence tomography of the anterior segment

Perhaps no diagnostic technology has emerged as rapidly in ophthalmology as optical coherence tomography (OCT). A single clinical device for this noninvasive imaging technique was first released in 1996, and now at least ten clinical devices are available. Although the first clinical anterior segment OCT was marketed only 2 years ago, a substantial amount of work has been done using modified retinal imagers or prototype laboratory-based imagers. In this review, we discuss OCT imaging primarily of the cornea. We also highlight previous and current publications on nonclinical and clinical uses of the device to illustrate how anterior segment OCT can be used to understand corneal structure and function in health and disease.

Detection of an abnormally thick LASIK flap with anterior segment OCT imaging prior to planned LASIK retreatment surgery

PURPOSE

To report a case of an abnormally thick flap that was detected with the use of anterior segment optical coherence tomography (OCT) prior to planned LASIK retreatment surgery.

METHODS

A 43-year-old man presented 6 years after bilateral LASIK surgery with uncorrected visual acuity (UCVA) of 20/20 in the right eye and 20/100 in the left eye, and best spectacle-corrected visual acuity (BSCVA) of 20/20 in the right eye and 20/20 in the left eye (refraction -2.25 -1.25 x 131). Anterior segment OCT was performed before LASIK retreatment surgery in the left eye.

RESULTS

The flap was found to be 394 microm and the residual stromal bed was too thin (152 microm) to allow for safe LASIK retreatment surgery. After waiting 4 months to ensure refractive stability, photorefractive keratectomy was performed. Postoperative UCVA was 20/20 plano.

CONCLUSIONS

The use of anterior segment OCT in the preoperative examination for planned LASIK retreatment surgery provided more reliable data.

Corneal imaging with anterior segment optical coherence tomography for lamellar keratoplasty procedures

PURPOSE

To assess and describe the uses of anterior segment optical coherence tomography (OCT) in the evaluation of the cornea before and after lamellar corneal transplantation procedures.

DESIGN

Prospective, noncomparative, observational case series.

METHODS

Seven eyes of seven patients undergoing anterior and posterior lamellar corneal transplantation procedures at the Singapore National Eye Centre were included in the study. High-resolution anterior segment OCT scans of the cornea and anterior segment were performed both before and after lamellar transplantation procedures on the cornea with the Visante anterior segment OCT system (Visante OCT; Carl Zeiss Meditec, Inc, Dublin, California, USA), and the imaging findings were correlated with the clinical picture. Measurements of lamella thickness were performed with the software provided.

RESULTS

Anterior segment OCT images were able to provide valuable information on donor apposition, Descemet membrane detachment after deep anterior lamellar keratoplasty (DALK), posterior lamellar dislocation, primary graft failure, and anterior chamber crowding with consequent chamber angle encroachment and pupillary block after Descemet stripping automated endothelial keratoplasty (DSAEK).

CONCLUSIONS

Anterior segment OCT is a valuable imaging tool for assessing the feasibility of lamellar transplantation surgery in the diseased cornea and in the management of surgical complications after such procedures.

A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography

OBJECTIVE

To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Fifty-one eyes of 26 healthy persons undergoing LASIK.

METHODS

The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.

MAIN OUTCOME MEASURES

Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.

RESULTS

The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.

CONCLUSIONS

We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.

Results of Phototherapeutic Keratectomy in the Management of Flap Striae after LASIK before and after Developing a Standardized Protocol

OBJECTIVE

To evaluate the efficacy and safety of phototherapeutic keratectomy (PTK) for treatment of striae in LASIK flaps.

DESIGN

Retrospective consecutive series.

PARTICIPANTS

Forty-seven eyes of 44 patients with flap striae after LASIK with reduced best-corrected visual acuity (VA) or visual symptoms with at least 3 months' follow-up (consecutive cases from January 2001 to May 2004).

INTERVENTION

Transepithelial PTK.

MAIN OUTCOME MEASURES

Uncorrected VA (UCVA), best spectacle-corrected VA (BSCVA), resolution or reduction of preoperative symptoms, and corneal haze.

RESULTS

Mean UCVA and BSCVA improved from 20/43 and 20/29 to 20/33 and 20/23, respectively. Five eyes (10%) developed 1+ haze and 14 eyes (30%) developed trace haze, generally peaking at 1 to 2 months postoperatively. No visual symptoms or loss of acuity were attributable to haze. A mean hyperopic shift of +0.80 diopters (D) occurred after PTK treatment. Development of a standardized protocol significantly reduced the variability of the refractive shift after PTK (P = 0.003). Results were stable in up to 2 years of follow-up.

CONCLUSIONS

Phototherapeutic keratectomy treatment of LASIK flaps is well tolerated, with stable outcomes and minimal complications. A standardized treatment protocol reduced postoperative refractive variability.

Quality of vision and graft thickness in deep anterior lamellar and penetrating corneal allografts

PURPOSE

To compare visual function after deep anterior lamellar keratoplasty (DALK) with visual function after penetrating keratoplasty (PK) for keratoconus and correlate this with corneal thickness.

DESIGN

Retrospective case series.

METHODS

Twenty-three patients (32 eyes) with unilateral or bilateral DALK or PK for keratoconus were analyzed for visual quality after suture removal. Evaluation included measurement of visual acuity, contrast sensitivity, and higher order aberrations (HOAs) (WaveScan; Visx, Santa Clara, California, USA). Readings were performed with both spectacle and rigid contact lens correction of refractive error. Total and residual stromal thickness after DALK was measured using optical coherence tomography (OCT) and correlated to visual quality.

RESULTS

Eyes after PK had better visual acuity than eyes after DALK (P = .018). Subgroup analysis of DALK eyes revealed that the level of visual acuity was related to the thickness of residual recipient corneal stroma. Eyes with a recipient corneal bed thickness of <20 microm had visual acuities similar to eyes with a PK, whereas those with a recipient thickness of >80 microm had a significantly reduced visual acuity (P = .0009). Contrast sensitivity was similar in DALK and PK eyes. There was no significant difference in HOAs between eyes with DALK or PK.

CONCLUSIONS

These data suggest that the main parameter for good visual function after DALK for keratoconus is the thickness of residual recipient stromal bed. An eye with a DALK with a residual bed of <20 microm can achieve a similar visual result as a PK.

High-speed optical coherence tomography for management after laser in situ keratomileusis

PURPOSE

To report applications of optical coherence tomography (OCT) in the management of laser in situ keratomileusis (LASIK) related problems.

SETTING

Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA.

METHODS

Five patients referred for LASIK-related problems were enrolled in a prospective observational study. Clinical examination, ultrasound (US) pachymetry, Placido ring slit-scanning corneal topography (Orbscan II, Bausch & Lomb), and high-speed corneal OCT were performed.

RESULTS

In cases of regression and keratectasia, OCT provided thickness measurements of the cornea, flap, and posterior stromal bed. Locations of tissue loss and flap interface planes were identified in a case with a recut enhancement complication. The information was used to determine whether further laser ablation was safe, confirm keratectasia, and manage complications. Optical coherence tomography measurements of central corneal thickness agreed well with US pachymetry measurements (difference 6.4 microm +/- 11.7 [SD]) (P = .026), while Orbscan significantly underestimated corneal thickness (-67.5 +/- 72.5 microm) (P = .17).

CONCLUSIONS

High-speed OCT provided noncontact imaging and measurement of LASIK anatomy. It was useful in monitoring LASIK results and evaluating complications.

Comparison of Corneal Thickness of Long-Term Contact Lens Wearers for Different Types of Contact Lenses

PURPOSE

To compare the central corneal thickness (CCT) values of long-term wearers of different types of contact lenses with those of each other and a normal control group.

METHODS

Twenty-three healthy eyes with no history of contact lens wear were used for the control group. Forty-eight eyes were evaluated in four different groups according to the type of contact lens worn; each group comprised 12 eyes. Group 1 (SofLens 38) and group 2 (Contact Day 30) had worn daily-wear frequent-replacement soft contact lenses with a 38% and 55% water content, respectively, and they had worn their lenses for an average of 4.28 +/- 1.70 years and 3.71 +/- 1.11 years, respectively. Group 3 (Wöhlk A90) and group 4 (Conflex-air) had worn daily-wear rigid gas-permeable (RGP) contact lenses with a Dk/t value of 68 and 22, respectively, and they had worn their lenses for an average of 3.00 +/- 0.63 years and 3.25 +/- 1.17 years, respectively. CCT values in micrometers were measured by optical coherence tomography.

RESULTS

When comparing the CCT between the control group and the contact lens groups, the CCT was significantly thinner in the group wearing Conflex-air RGP contact lenses with a low Dk/t value (P = 0.010). In comparing the CCT among the four contact lens groups, the CCT was significantly thinner in the group wearing SofLens 38 contact lenses with a water content of 38% than in the group wearing Contact Day 30 contact lenses with a water content of 55% (P = 0.004). There was also significant thinning in the group wearing Conflex-air RGP contact lenses versus the group wearing Contact Day 30 soft contact lenses (P = 0.0005).

CONCLUSIONS

CCT was significantly thinner in patients wearing long-term low-Dk/t RGP contact lenses compared to no contact lens wear and soft contact lenses with a water content of 55%. CCT was also decreased significantly in long-term soft contact lens wear with a water content of 38% compared to 55%.

The Repeatability of Corneal and Corneal Epithelial Thickness Measurements Using Optical Coherence Tomography

PURPOSE

The purpose of this study is to examine within and between session repeatability of clinical optical coherence tomography (OCT) imaging for anterior segment morphometry.

METHODS

Images of the corneal apex of each eye in 18 subjects were obtained using a Humphrey Zeiss OCT imager. Subjects viewed a target positioned to ensure that scans were orthogonal to the ocular surface and each image, consisting of 100 adjacent sagittal scans, analyzed using custom software. Repeatability data were analyzed using intraclass correlation coefficients (ICCs), correlation coefficient of concordance (CCC, perfect test-retest agreement ICC or CCC = 1.0), and coefficients of repeatability (COR, 95% confidence interval of test-retest differences). To account for each eye, the multivariate repeatability statistic Iota was estimated.

RESULTS

Mean central corneal and epithelial thickness of 32 eyes (OD and OS combined) is 536 +/- 26 microm (standard deviation [SD]) and 52 +/- 3 microm (SD) with 5th and 95th percentile thicknesses of 507 and 591 microm for central cornea and 48 and 57 microm for central epithelial. Worst case within session repeatability was defined as repeatability between images with greatest differences in mean thickness within a session. Corneal thickness worst case ICC was 0.95 and COR was +/- 9.98 microm. Epithelium worst case ICC was 0.36, CCC was 0.12, and COR was +/- 11.11 microm. First image between session corneal thickness had an ICC = 0.98 and a COR = 10.83 microm, whereas epithelium ICC = 0.38, CCC = 0.37, and COR was +/- 12.84 microm. When we compared the average of the first three tests with the first three retest images, corneal ICC was 0.98 and COR was +/- 10.64 microm and epithelium ICC = 0.73, CCC = 0.72, and COR was +/- 6.53 microm. Iota (multivariate repeatability, using eye as a factor) for the cornea was at least 0.96 (worst case) and increased to at least 0.98 when within-session image data were averaged. Iota for epithelium measures ranged from 0.29 when first images were compared with 0.57 when within-session image data were averaged.

CONCLUSIONS

There is very good repeatability of corneal thickness measurement using OCT; even the worst case measurements are similar between sessions. On the other hand, this is not the case for epithelium measurements, and if multiple images within a session are acquired, the worst case results demonstrate how important it is to optimize each OCT scan and also average multiple scans to maximize intersession repeatability.

Topographic screening of donor eyes for previous refractive surgery

PURPOSE

To determine whether donor eyes had previous refractive surgery using Orbscan (Bausch & Lomb Surgical) corneal topography.

SETTING

Lions Eye Bank of Oregon, Portland, Oregon, USA, and Maisonneuve-Rosemont, Hospital, Montreal, Quebec, Canada.

METHODS

Orbscan corneal topographies of 50 donor eyes from the Lions Eye Bank of Oregon were obtained; 10 eyes had previous refractive surgery (6 laser in situ keratomileusis, 2 photorefractive keratectomy, 2 radial keratotomy) to correct myopia, and 40 had not had surgery. Algorithms based on corneal anterior and posterior elevations and anterior tangential curvature were developed: The difference in curvature (DC) was based on the difference in the mean anterior tangential curvature between central and midperipheral areas; difference in elevation (DE) represented the difference between the anterior and posterior central elevations. Receiver-operating characteristic (ROC) curves for each algorithm were obtained, and sensitivity values at fixed specificities were calculated.

RESULTS

The mean area under the ROC curve, which corresponds to the probability of correctly identifying the presence of a previous refractive surgery, was 0.853 +/- 0.079 (SE) for DC and 0.933 +/- 0.057 for DE. The DC algorithm resulted in a sensitivity of 80% for a specificity of 87.5%, and DE yielded a sensitivity of 90% for a specificity of 92.5%. There was a strong correlation between the value of the DE and DC algorithms and the amount of previous refractive surgery (DC: r = 0.84, P = .008; DE: r = 0.76, P = .028).

CONCLUSION

The results led to a proposed criteria-based system using Orbscan corneal topography to screen eye-bank eyes for previous refractive surgery.

Corneal pachymetry mapping with high-speed optical coherence tomography

OBJECTIVE

To map corneal thickness before and after LASIK with optical coherence tomography (OCT).

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Forty-two eyes of 21 normal subjects undergoing LASIK.

METHODS

A high-speed (2000 axial scans/second) 1.3-microm-wavelength corneal and anterior segment OCT prototype was used for corneal scanning. The scan pattern consisted of 10-mm radial lines on 8 meridians centered on the vertex reflection. The entire scan pattern of 1024 a-scans was acquired in 0.5 seconds. We developed automated computer processing for 3-dimensional corneal reconstruction and measurement. Corneal thickness was measured normal to the anterior surface and presented as color pachymetry maps and zonal statistics. The maps were divided into a central zone (<2 mm) and 3 annular areas (pericentral, 2-5 mm; transitional, 5-7 mm; peripheral, 7-10 mm), which were further divided into quadrantal zones. The average, minimum, and maximum corneal thicknesses were computed for zones within the 7-mm diameter. Optical coherence tomography and ultrasound pachymetry were measured 3 times at the preoperative and 3-month postoperative visits. Reproducibility was assessed by the pooled standard deviations (SDs) of the repeated measurements.

MAIN OUTCOME MEASURES

Optical coherence tomography pachymetric map and zonal statistic, and ultrasound pachymetry.

RESULTS

Before LASIK, central corneal thicknesses (CCTs) were 546.9+/-29.4 microm (mean +/- SD) for OCT and 553.3+/-33.0 microm for ultrasound. After LASIK, CCTs were 513.7+/-44.5 microm for OCT and 498+/-46.6 microm for ultrasound. Optical coherence tomography and ultrasound CCT were highly correlated (Pearson correlation r = 0.97 before LASIK and 0.98 afterwards). Optical coherence tomography CCT was slightly less than ultrasound CCT before surgery (mean difference, -6.4 microm; 95% limits of agreement, -23.2 to 10.4 microm) but slightly greater after LASIK (15.7 microm; -1.6 to 33 microm). These differences were statistically significant, but no more than the CCT measurement differences between ultrasound pachymeters. The reproducibility of the OCT zonal pachymetry averages was roughly 2 microm.

CONCLUSIONS

High-speed OCT provided noncontact, rapid, reproducible pachymetric mapping over a wide area of the cornea. It is equivalent to ultrasound for CCT measurement before and after LASIK. This technology could be valuable for planning keratorefractive procedures and diagnosis of corneal diseases.

Corneal Ectasia After Laser In Situ Keratomileusis in Patients Without Apparent Preoperative Risk Factors

PURPOSE

To evaluate patients who developed ectasia with no apparent preoperative risk factors.

METHODS

Potential cases of patients who developed ectasia without apparent risk factors were identified by contacting participants in the Kera-Net (n = 580), ASCRS-Net (n = 450), and ISRS/AAO ISRS-Net (n = 525) internet bulletin boards from April to October 2003. Cases were included if ectasia developed after laser in situ keratomileusis in the absence of apparent preoperative risk factors. Reported cases were excluded for the following reasons: (1) calculated residual stromal bed less than 250 microm, (2) preoperative central pachymetry less than 500 microm, (3) any keratometry reading greater than 47.2 diopters (D), (4) a calculated inferior-superior value greater than 1.4, (5) more than 2 retreatments, (6) attempted initial correction greater than -12.00 D, (7) an Orbscan II "posterior float" (if obtained) greater than 50 microm, and (8) surgical/flap complications.

RESULTS

A total of 27 eyes of 25 patients were submitted for consideration. Eight eyes (8 patients) met our inclusion criteria. Mean age was 27.7 years (range, 18-41 years). Preoperative manifest refraction spherical equivalent was -4.61 D (range, -2.00 to -8.00 D); steepest keratometric reading was 43.86 D (range, 42.50-46.40 D); keratometric astigmatism was 0.93 D (range, 0.25-1.90 D); and preoperative central pachymetry was 537 microm (range, 505-560 microm). The mean calculated ablation depth was 82.8 microm (range, 21-125.4 microm), and mean calculated residual stromal bed was 299.5 microm (range, 254-373 microm). Mean time to recognition of ectasia onset was 14.2 months (range, 3-27 months) postoperatively. At the time of ectasia diagnosis, the mean manifest refraction spherical equivalent was -1.23 D (range, +0.125 to -3.00) with a mean of 2.72 D (range, 0.75-4.00 D) of astigmatism.

CONCLUSIONS

Ectasia can occur after an otherwise uncomplicated laser in situ keratomileusis procedure, even in the absence of apparent preoperative risk factors.

Filtering blebs imaging by optical coherence tomography

PURPOSE

To assess the ability of standard optical coherence tomography to visualize filtering blebs after glaucoma surgery.

METHODS

A prospective interventional case series was conducted in a private practice. Twenty-nine eyes of 24 patients (21 with good, 2 with fair and 6 with poor intraocular pressure [IOP] control) were investigated. After the focus was manually adjusted on the conjunctiva, blebs were scanned perpendicularly to the limbus.

RESULTS

Hyporeflective fluid-filled spaces were detected in 19 out of the 21 eyes with good IOP. Within this group, blebs were classified into three different categories according to their optical coherence tomography pattern: type A (featuring a thick wall and a single large fluid-filled space), type B (featuring a thin wall and multiple large fluid-filled spaces) and type C (featuring multiple, irregular and flattened fluid-filled spaces). Fluid-filled spaces were not observed in three out of the six eyes with poor IOP control. Trabeculectomy without antimetabolites was associated with type A blebs (P = 0.015, Fisher's exact test), mitomycin-C trabeculectomy with type B blebs (P = 0.0025) and mitomycin-C phacotrabeculectomy with type C blebs (P = 0.0173).

CONCLUSIONS

Although it was not developed to evaluate the anterior segment of the eye, standard optical coherence tomography can visualize filtering blebs and reveal interesting details of their morphology. Clinicians using optical coherence tomography to diagnose glaucoma can take advantage of this ability of the instrument to obtain more information about their patients in the postoperative course of trabeculectomy and phacotrabeculectomy.

Central corneal thickness and its relationship to the patient's origin

OBJECTIVES

To report central corneal thickness (CCT) measurements on patients requesting refractive surgery and to search for possible associations between CCT and patient's origin, age, sex, and preoperative data.

SETTING

Department of Ophthalmology, Soroka University Medical Center, and Enaim Refractive Center, Beer-Sheva, Israel.

METHODS

Subjects were patients undergoing preoperative examination at our refractive centre during 2003. Patient's age, sex, preoperative CCT, refractive status, keratometry, and intraocular pressure (IOP) were obtained together with country of origin of the patients and their parents. Main outcome measures were CCT measurements and relationship between CCT and patient's origin and preoperative data.

RESULTS

A total of 204 patients were included in the study. Patients of North African origin had statistically significantly thinner corneas than patients of other origins (518.9+/-31.5 vs. 545.4+/-30.4 microm in the right eye and 518.4+/-32.1 vs. 546.3+/-29.7 microm in the left eye; P<0.00001). Patients of North African origin were significantly older than patients of other origins (P=0.028). No differences were found when comparing for sex, intraocular pressure, refractive status, and keratometry. When multivariate analysis was performed patient's origin was the only independently associated factor associated with CCT.

CONCLUSIONS

This is the first work in reporting lower CCT in people of North African origin examined in a refractive surgery centre. Studies from similar populations are needed to confirm our results.

Optical coherence tomography of the Ex-PRESS miniature glaucoma implant

PURPOSE

To describe the localisation of the Ex-PRESS miniature glaucoma implant with an experimental setup for optical coherence tomography (OCT) of the anterior segment of the eye.

METHODS

An OCT scanner, central wavelength 1,280 nm, bandwidth 60 nm, resolution of 12 microm, was built onto a slitlamp to scan the anterior segment of the eye. Five ex-vivo porcine eyes received an Ex-PRESS miniature glaucoma implant and were used as a model to visualise the position of the implant in the anterior segment.

RESULTS

In the ex-vivo porcine eyes, the OCT images showed the anatomy of the anterior segment in great detail. The anterior segment OCT was able to visualise the whole outline and position of the implant. The abrupt change in reflectivity going from tissue to the implant resulted in a clear border along the circumference of the whole device.

CONCLUSION

In this paper, we have shown that we were able to outline the Ex-PRESS miniature glaucoma implant in the anterior segment of the ex-vivo porcine eye by using an experimental OCT setup built onto a slitlamp. The acquisition time of 0.8 s is short enough to allow for the scanning of patients, and anterior segment OCT is expected to aid in providing answers to the question regarding which parameters will determine the success or failure of such a device.

Assessment of central corneal thickness using optical coherence tomography

PURPOSE

To demonstrate the capability of model OCT3 optical coherence tomographer to evaluate central corneal thickness (CCT) in normal human corneas in vivo and compare the results with the those of standard ultrasound (US) pachymetry and Orbscan.

SETTING

New York Eye & Ear Infirmary Resident Clinic, New York, New York, USA.

METHODS

The CCT in 22 eyes of 11 subjects was determined with the OCT3 (Carl Zeiss Meditec), Orbscan (Bausch & Lomb, Inc.), and US pachymetry (DGH Technology, Inc.). Three central corneal scans of each eye were obtained using the OCT3. First, OCT3 data were processed using the standard OCT software program (OCT3(std)). Second, OCT3 raw data were exported and measurements were repeated using Scion Image for Windows program (OCT3(sci)). The OCT3 and Orbscan results were compared with the mean of 5 US pachymetry measurements in each eye.

RESULTS

The OCT3(std), OCT3(sci), and Orbscan CCT measurements showed high correlations with US pachymetry (r=0.981, r=0.984, and r=0.942, respectively; P<.0001). Bland-Altman analysis showed a high level of agreement between US pachymetry and OCT3 techniques but not Orbscan. High repeatability for OCT3(std) (r(2)=0.05) and OCT3(sci) (r(2)=0.01) was also seen.

CONCLUSION

Results show the OCT3 is an accurate, noninvasive, and reproducible technique for evaluation of CCT.

Pathologic findings in postmortem corneas after successful laser in situ keratomileusis

PURPOSE

To examine the histologic and ultrastructural features of human corneas after successful laser in situ keratomileusis (LASIK).

METHODS

Corneas from 48 eyes of 25 postmortem patients were processed for histology and transmission electron microscopy (TEM). The 25 patients had LASIK between 3 months and 7 years prior to death. Evaluation of all 5 layers of the cornea and the LASIK flap interface region was done using routine histology, periodic acid-Schiff (PAS)-stained specimens, toluidine blue-stained thick sections, and TEM.

RESULTS

In patients for whom visual acuity was known, the first postoperative day uncorrected visual acuity was 20/15 to 20/30. In patients for whom clinical records were available, the postoperative corneal topography was normal and clinical examination showed a semicircular ring of haze at the wound margin of the LASIK flap. Histologically, the LASIK flap measured, on average, 142.7 microm (range, 100-200). A spectrum of abnormal histopathologic and ultrastructural findings was present in all corneas. Findings at the flap surface included elongated basal epithelial cells, epithelial hyperplasia, thickening and undulations of the epithelial basement membrane (EBM), and undulations of Bowman's layer. Findings in or adjacent to the wound included collagen lamellar disarray; activated keratocytes; quiescent keratocytes with small vacuoles; epithelial ingrowth; eosinophilic deposits; PAS-positive, electron-dense granular material interspersed with randomly ordered collagen fibrils; increased spacing between collagen fibrils; and widely spaced banded collagen. There was no observable correlation between postoperative intervals and the severity or type of pathologic change except for the accumulation the electron-dense granular material.

CONCLUSIONS

Permanent pathologic changes were present in all post-LASIK corneas. These changes were most prevalent in the lamellar interface wound. These changes along with other pathologic alterations in post-LASIK corneas may change the functionality of the cornea after LASIK.

Excimer laser phototherapeutic keratectomy in eyes with anterior corneal dystrophies: short-term clinical outcomes with and without an antihyperopia treatment and poor effectiveness of ultrasound biomicroscopic evaluation

PURPOSE

To evaluate the use of high-frequency ultrasound biomicroscopy (UBM) in determining the depth of corneal pathology in eyes undergoing excimer laser phototherapeutic keratectomy (PTK) for primary or recurrent anterior stromal corneal dystrophies. Corneal clarity, visual acuity, and refractive changes in eyes with and without an antihyperopia treatment were also analyzed.

METHODS

Twenty eyes of 14 patients with anterior stromal corneal dystrophies were treated with PTK. Eyes were evaluated pre- and 6-8 weeks postoperatively with slit-lamp biomicroscopy, manifest refraction, keratometry, computerized corneal topography, ultrasound pachymetry, and UBM.

RESULTS

Nineteen of 20 corneas (95%) had greatly improved corneal clarity after PTK. Mean uncorrected Snellen vision improved from 20/102 to 20/69, and best corrected vision improved from 20/62 to 20/38. Nine eyes (45%) improved two or more lines of uncorrected vision, and 13 eyes (65%) improved two or more lines of best corrected vision. Mean change in spherical equivalent was just -0.92 diopters (SD 4.3 diopters); however, the range was large (-13 to +3.88 diopters). UBM measurement of central corneal pathology did not correlate significantly with the actual PTK ablation depth (P = 0.07). The amount of antihyperopia treatment did not correlate with changes in manifest refraction spherical equivalent, keratometry, or computerized corneal topography readings but did correlate with length of time until corneal reepithelialization after PTK (P = 0.003).

CONCLUSIONS

PTK resulted in improvements in corneal clarity and visual acuity in most patients with superficial corneal stromal dystrophies. UBM was not an effective tool to accurately measure the depth of corneal pathology preoperatively. The combined approach of minimizing ablation depth and selective use of an antihyperopia treatment resulted in minimal mean change in spherical equivalent; however, the range was large. PTK is a very good minimally invasive technique to improve vision in eyes with anterior stromal corneal dystrophies.

Monitoring corneal structures with slitlamp-adapted optical coherence tomography in laser in situ keratomileusis

PURPOSE

To monitor corneal structures with slitlamp-adapted optical coherence tomography (OCT) in laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, Germany.

METHODS

In this prospective, nonrandomized, comparative clinical case series of consecutive patients who had LASIK for myopia and myopic astigmatism, the corneal structures were studied with slitlamp-adapted OCT at a wavelength of 1,310 nm. The central corneal thickness (CCT) and epithelial, flap, and residual stromal thicknesses were assessed preoperatively, immediately after surgery, on postoperative day 1, and then, on average, after 8, 35, and 160 days.

RESULTS

Twenty-five eyes of 13 patients were included. The attempted mean spherical equivalent correction was -6.11 diopters (D) +/- 2.16 (SD) with a mean calculated stromal ablation depth of 92 +/- 24 microm. The CCT was 516 +/- 26 microm preoperatively and 453 +/- 40 microm postoperatively (P<.001). The epithelial thickness increased from 57.0 +/- 7.7 microm preoperatively to 61.0 +/- 7.5 microm postoperatively (P =.04). Imaging of the hyperreflective interface was possible in all patients for up to 15 months. The flap and residual stromal thickness was 211 +/- 28 microm and 344 +/- 48 microm, respectively, immediately after LASIK and 164 +/- 21 microm (P<.001) and 284 +/- 32 microm (P<.001), respectively, on postoperative day 1. There were no further significant changes during the follow-up. The overall mean reproducibility was +/-4.50 microm (coefficient of variation [CV] 0.94%) for CCT, +/-4.99 microm (CV 8.57%) for epithelial thickness, +/-6.25 microm (CV 3.55%) for flap thickness, and +/-7.09 microm (CV 2.42%) for residual stromal thickness.

CONCLUSION

Slitlamp-adapted OCT can be used to longitudinally monitor the variable structures of the cornea, epithelium, flap, and residual stroma in LASIK.

Detection of laser in situ keratomileusis in a postmortem eye using optical coherence tomography

With more individuals having laser in situ keratomileusis (LASIK), eye banks are challenged to detect prior refractive surgery in donor tissue. We report the case of a donor who had LASIK 9 months before his death. Slitlamp biomicroscopy, corneal topography, and optical coherence tomography (OCT) were performed to evaluate the corneas. Few changes were detected under slitlamp examination and corneal topography. We demonstrate that OCT is capable of detecting LASIK-induced structural changes in the immediate postmortem evaluation and during the early and late period of organ culture. We recommend OCT screening of potential donor corneas before organ culture and between days 9 and 12 of organ culture.

Exploration du segment antérieur par OCT3

PURPOSE

Posterior pole imaging with OCT3 provides macular scans with a very high resolution. Use of OCT1 has already been described for anterior segment imaging. This work presents the advantages of OCT3 in anterior segment imaging compared to UBM (50 MHz) and 20-MHz ultrasound imaging with long focal distance (patent pending PCT/FR98/02788).

MATERIAL

and methods: We used an OCT3 unit dedicated to the posterior pole. By defocusing the laser beam we obtained high-resolution scans of the cornea, angle, lens and IOLs. Fifty-seven eyes (35 patients) were imaged by OCT3 and high-resolution ultrasound in cases of corneal dystrophy, corneal transplant, LASIK, narrow angle, iris bowing, etc.

RESULTS

In all cases OCT3 provided high-resolution information with a limit of penetration at the posterior part of the iris. This limitation was not observed with ultrasound imaging. Nevertheless, OCT3 is a useful tool for corneal imaging in cases of opacities limiting biomicroscope imaging. OCT3 corneal scans at high resolution make it possible to identify the corneal interface in lamellar transplants or LASIK. With a narrow angle, a dynamic test of angle closure can be done. With iris bowing, OCT3 can differentiate iris cysts and iris tumors. Anterior chamber IOLs can be imaged with their relationships to anterior segment structures.

CONCLUSION

OCT3 provides non-contact anterior segment imaging with high resolution but with a limit of penetration given by posterior part of the iris.

Estimating residual stromal thickness before and after laser in situ keratomileusis

PURPOSE

To determine the factor(s) that influences measurement of residual stromal thickness (RST) after laser in situ keratomileusis (LASIK) surgery.

SETTING

Clinical office-based excimer laser refractive surgery center.

METHODS

In this retrospective comparative interventional case study of 6235 eyes, ultrasonic corneal pachymetry was performed immediately before and after flap creation and immediately after laser ablation in the primary procedure and after 647 enhancements. Differences in the methods for calculating RST were compared statistically.

RESULTS

Using the RST measured at enhancement as the actual RST, measurements of RST immediately after laser ablation underestimated residual thickness due to laser-induced stromal dehydration and microkeratome effects (P<.001). Estimates of RST using a "standard" or estimated flap thickness were less accurate predictors of residual thickness (P<.001) than use of the theoretical laser resection with a measured flap thickness (RST-4) (P =.78) or a modified flap thickness subtracted from the postoperative corneal thickness (RST-8) (P =.98), which provided the best RST estimates.

CONCLUSIONS

Before LASIK, the best means of estimating RST is to subtract the theoretical laser resection obtained from the laser computer and the expected flap thickness normally obtained with a given microkeratome system from the preoperative central corneal thickness. After LASIK, the most accurate means of calculating RST is to subtract the original flap thickness from the postoperative central corneal thickness.

Noncontact Optical Coherence Tomography for Measurement of Corneal Flap and Residual Stromal Bed Thickness After Laser in situ Keratomileusis

PURPOSE

Studies show significant variability in the thickness of laser in situ keratomileusis (LASIK) corneal flaps cut by various microkeratomes. Most studies of corneal flap thickness are based on contact ultrasonic pachymetry measurements taken during the surgical procedure. This study reports a technique to obtain reproducible corneal flap thickness and residual stromal bed thickness measurements using noncontact optical coherence tomography (OCT) following LASIK.

METHODS

The corneal flap thicknesses of 26 eyes of 15 patients were measured following LASIK in which the flap was created using the Amadeus microkeratome: 160-microm head, 9.5-mm ring, 4.0-mm/s translation speed, 8000 oscillations/m, and full vacuum. Zeiss Humphrey OCT-2 line scans were performed on postoperative days 1 and 7. The raw data from three scans for each eye and day were exported to Microsoft Excel for processing, averaging, and analysis.

RESULTS

The OCT corneal flap thickness and residual stromal bed thickness measurements correlated well with ultrasonic pachymetry measurements performed during surgery (R2 = .92). The OCT technique yielded reproducible results, as the variance for repeated scans was only 2.5% of the variance between eyes. In bilateral cases a single blade was used for both eyes. The mean flap thickness of 15 first eyes was significantly greater than that of the 10 second eyes: 181 +/- 31 microm vs. 143 +/- 41 microm (P < .01). A positive correlation was found between the preoperative pachymetry and corneal flap thickness.

CONCLUSIONS

The OCT scan averaging technique is a reproducible, noncontact postoperative method for measuring corneal flap and residual stromal bed thicknesses following LASIK.

Laser in situ keratomileusis flap suturing to treat recalcitrant flap striae

PURPOSE

To describe the management of resistant laser in situ keratomileusis (LASIK) flap striae using interrupted sutures.

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.

METHODS

Interrupted sutures were used to treat striae in the LASIK flaps in 7 eyes of 7 patients. The mean interval from LASIK to the time of suturing was 11.8 months (range 3.0 to 29.0 months). The preoperative best spectacle-corrected visual acuity (BSCVA) was 20/20(-2) to 20/100. All patients had subjective visual complaints with spectacle correction, and all corneas had obvious macrostriae on slitlamp biomicroscopic examination.

RESULTS

The mean patient follow-up was 7.0 months (range 1.5 to 14.0 months). Six patients regained a BSCVA of 20/20 or better with resolution of preoperative irregular astigmatism and subjective visual disturbance. In 1 patient, mild central striae recurred.

CONCLUSION

Flap suturing was an effective means of treating visually significant flap striae.

Optical coherence tomography for the detection of laser in situ keratomileusis in donor corneas

PURPOSE

In 2001, more than one million laser in situ keratomileusis (LASIK) procedures were performed worldwide. Considering the increasing number of refractive procedures, eye banks will be increasingly confronted with the problem of how to identify those donors with prior refractive surgery. To date, efficient screening methods to identify LASIK surgery in donor eyes have not been established. Therefore, the purpose of the current study was to determine whether optical coherence tomography (OCT) can be used to detect the presence of LASIK-induced changes in human corneas.

METHODS

Laser in situ keratomileusis was performed on 20 organ-cultured human cornea disks. The excimer laser ablation performed ranged from 0 to 12 diopters. The corneas were maintained in culture, and the visibility of flap-stromal interface by OCT was assessed up to 6 months after the LASIK procedure. Additionally, two donor corneas with the history of LASIK treatment before death were screened for structural changes.

RESULTS

Optical coherence tomography scans were able to detect the interface between the corneal flap and the residual stromal tissue in all corneas and at all examined time intervals. There were no differences in signal intensity among the different depths of ablation. The relative signal intensity of the interface compared with the averaged stromal intensity ranged from 2.1 to 6.0. In both donor corneas with suspected prior LASIK surgery, OCT scanning showed the characteristic stromal interface as found in the in vitro model.

CONCLUSIONS

Corneal examination by OCT could be an appropriate technique for eye banks to screen donor corneas for prior LASIK surgery.

Correlational study of central corneal thickness measurements on Hong Kong Chinese using optical coherence tomography, Orbscan and ultrasound pachymetry

BACKGROUND

Central corneal thickness (CCT) of 74 eyes from 39 normal Hong Kong Chinese subjects with ages ranging from 39 to 86 years were studied.

AIM & PURPOSE

To compare the measurements of different devices and to compare the results of ethnic groups in other studies.

METHODS

Non-contact measurements by Orbscan and Optical Coherence Tomography (OCT) were first carried out, followed by contact measurement using Ultrasound Pachymetry. The results of five measurements of Ultrasound Pachymetry and three measurements of OCT and Orbscan were each averaged and compared using correlation, linear regression and one-way analysis of variance methods.

RESULTS

The measurements of three devices were significantly correlated (P < 0.01). The mean CCT in our study group measured by Orbscan (with an acoustic factor set at 0.92), Ultrasound Pachymetry and OCT were 555.96 +/- 32.41, 555.11 +/- 35.30 and 523.2 +/- 33.54 microm respectively. A linear regression model (using ultrasound measurements as standard) was presented.

CONCLUSIONS

When a correction factor of 32 microm was applied to OCT measurements, the means of three devices became significantly equal. The adjusted OCT measurements were less precise within subjects but more accurate than Orbscan when compared with ultrasound pachymetry as a reference standard. The mean CCT measurement of our sample was comparable to some studies on Hong Kong Chinese, Caucasians and Japanese but higher than those on some Europeans, Asian and North Americans of African origin.

Optical coherence tomography for in situ monitoring of laser corneal ablation

OBJECTIVE

To improve the precision of refractive surgery, a new approach for determination of the removed corneal thickness profile in situ with laser ablation by optical coherence tomography (OCT) is developed.

STUDY DESIGN/MATERIALS AND METHODS

The traditional method for precision (less than 10 microm) measurements of intraocular distances is based on the use of the reflected component of probing radiation. This component is characterized by a small range of operating angles between a probing beam and a normal to the surface under study. To enhance this range of operating angles we suggest using a light component backscattered from a biological object. This will enable precision measurements over the entire surface of the cornea without any changes in the orientation between a probing beam and the eye, a necessary condition for in situ monitoring of laser refraction correction in the eye. We suggest a specially developed algorithm of OCT signal processing to measure the corneal thickness by the backscattered light component for a single longitudinal scan (A scan). The corneal thickness profile is obtained by a series of such A scans acquired by successively scanning a probing beam along the corneal surface. The thickness profile of removed layer is determined by changes in the corneal thickness profile in the process of ablation. When the cornea is ablated by a beam with a fixed transverse profile, we propose using integral characteristics of the ablated layer profile, for example, the maximum ablation depth, as criteria of changes in refractive power of the eye. The measurement precision by these characteristics is considerably higher than by a single A scan. Since the cornea is a poorly scattering medium, the Fourier filtering is employed to increase reliability and precision of the method. Model experiments on monitoring the ablation process in a lavsan film and ex vivo human cornea are described. Preliminary experiments on in vivo measurements of human corneal thickness are performed.

RESULTS

In model experiments the precision of measurement of laser ablation depth by one A scan was 5-20 microm, depending on the signal-to-noise ratio (SNR), whereas the precision of measurement of laser ablation depth as the integral characteristic of the ablated layer profile was 0.3-5 microm. The experimental results showed that at small SNR Fourier filtering might considerably increase reliability and precision of measurements. When SNR is high, the measurement precision does not change. The precision of measurements of the corneal thickness in preliminary in vivo experiments was higher than in ex vivo experiments. This factor is very promising for application of the method suggested herein in refractive surgery.

Factors That Affect Corneal Flap Thickness With the Hansatome Microkeratome

PURPOSE

To evaluate factors that influence corneal flap thickness with the Hansatome microkeratome.

METHODS

One hundred thirty-two eyes of 70 patients underwent laser in situ keratomileusis (LASIK). Corneal flap thickness was measured by subtracting the intraoperative corneal bed pachymetry measurement from intraoperative total corneal pachymetry. Variables examined included plate thickness, ring size, blade use, temperature, humidity, barometric pressure, age, average keratometric power, and preoperative corneal thickness.

RESULTS

Mean flap thickness using a 180-microm plate was 143 +/- 19 microm (range 61 to 207 microm). Mean flap thickness using a 160-microm plate was 119 +/- 20 microm (range 83 to 159 microm). The difference was statistically significant (P < .05). Mean flap thickness using a 180-microm plate and the same blade on the right and left eye was 151 +/- 21 microm (range 113 to 200 microm) and 137 +/- 21 microm (range 91 to 191 microm), respectively. The 14-microm difference was statistically significant (P < .001). There was a slight negative correlation of flap thickness with humidity. There was a positive correlation with preoperative corneal thickness (pachymetry).

CONCLUSION

The Hansatome tended to cut thinner flaps than anticipated based on the plate used. Flaps cut on the first eye were thicker than the second eye using the same blade. Thicker corneas tended to lead to thicker flaps. There was no correlation between flap thickness and microkeratome ring size, temperature, barometric pressure, patient age, or average keratometric power.