Scanning-slit Corneal Topography Research Articles

Contribution of Fourier-domain optical coherence tomography to the diagnosis of keratoconus progression

To determine the anatomic criteria for diagnosing keratoconus progression by corneal optical coherence tomography (OCT). Quinze-Vingts National Ophthalmology Hospital, Paris, France. Prospective case series. Scanning-slit corneal topography (Orbscan II) and Fourier-domain corneal OCT (RTVue) were performed in eyes with mild to moderate keratoconus (progressive or nonprogressive [stable] ectasia) at each examination to assess the keratoconus. Disease progression was defined as an increase of at least 1.0 diopter (D) in the steepest keratometry (K) measurement over 6months. Of the 134 eyes of 134 patients with mild to moderate keratoconus, 98 had had progressive ectasia and 36 nonprogressive ectasia. The mean maximum K increased significantly in the progressive group (2.1 D ± 1.2 [SD], P<.0001) and remained constant in the stable group (-0.03±0.39 D, P=.31). The mean thinnest corneal thickness increased significantly in the progressive group (-7.98±9.3μm,P<.0001) and remained constant in the stable group (-0.52±4.21μm,P=.22). The change in maximum K was significantly correlated with changes in the thinnest corneal thickness (r=-0.61, P<.0001). A cutoff value of -5μm for the change in thinnest corneal thickness was identified on receiver operating characteristic curves as a threshold separating cases of progressive and stable keratoconus (area under the curve, 0.79; sensitivity, 68%; specificity, 89%). Topographic data partly reflected the structural changes occurring during the progression of corneal ectasia. Based on the pachymetric parameters provided by OCT, corneal and epithelial thinning was correlated with corneal deformation. The use of corneal OCT might therefore improve the diagnostic sensitivity for keratoconus progression.

Visual and Topographical Outcomes Following Accelerated Trans-Epithelial Corneal Crosslinking in Progressive Keratoconus.

To assess the visual and topographical outcomes following accelerated trans-epithelial corneal crosslinking (TE-CXL) in progressive keratoconus. Case series. Laser Vision Centre, Karachi, from January 2015 to December 2016. Forty-five eyes of 25 patients affected with progressive keratoconus and treated with accelerated TE-CXL with riboflavin (vitamin B2) and ultra-violet A(UV-A) irradiation were enrolled in this study. The visual outcome was measured by ETDRS chart as improvement in best corrected visual acuity (BCVA) of at least one line or more as compared to pre-CXLBCVA. The topographical outcome was measured as decrease in maximum simulated keratometry values, astigmatism and spherical equivalent (SE) and an increase in central corneal thickness (CCT). K-max was defined as the steepest radius of curvature of the anterior corneal surface. SE was measured by subjective refraction, and K-max, astigmatism and CCTby scanning-slit corneal topography. Patients were followed-up 12 months post-treatment. At the end of follow-ups, mean BCVAshowed improvement of one line from LogMAR 0.58 ±0.067 to LogMAR 0.48 ±0.077. Seven (15.5%) eyes showed two lines of improvement while 3 (6.7%) eyes showed worsening of one line in BCVA. The mean K-max flattened by 0.7D. Mean astigmatism and SE decreased up to -0.5D and -0.4D, respectively. Mean preoperative CCTwas 454.31 ±36.34 µm, whereas mean postoperative CCTwas 456.47 ±35.60 µm with an average increase of 2.15 µm. No postoperative complications were reported. Based on topographical outcomes, accelerated TE-CXL is effective in preventing the progression of keratoconus without any safety concerns with improvement of vision in majority of cases.

Corneal thickness differences between type 2 diabetes and non-diabetes subjects during preoperative laser surgery examination

AimsTo evaluate the differences in corneal thickness between type 2 diabetes subjects with HbA1c under 7.0% and non-diabetes subjects during their preoperative laser surgery examinations. MethodsThe mean of five consecutive corneal thickness measurements at the central and mid-peripheral cornea was obtained by means of noncontact scanning-slit corneal topography (Orbscan Topography System II; Orbscan, Inc., Salt Lake City, UT, USA) in 35 myopic non-insulin dependent type 2 diabetes subjects (17 males and 18 females) and 48 healthy myopic controls (23 males and 25 females). ResultsThe corneal thickness values at the central and mid-peripheral cornea were significantly higher in the diabetic group (p<.001). The diabetic subjects presented the highest thickness value in the superior cornea (n=22; 62.9%) followed by the nasal (n=9; 25.7%) and the temporal (n=4; 11.4%) cornea, but never in the inferior cornea. The control subjects presented the highest thickness value in the superior cornea (n=19; 39.6%) followed by the nasal (n=18; 37.5%), the inferior (n=6; 12.5%), and the temporal (n=3; 6.3%) cornea. The central corneal thickness (CCT) of the diabetes patients was not statistically correlated with their HbA1c (r2=.078; p=.104), body mass index (r2=.007; p=.633), and time from diagnosis of diabetes (r2=.025; p=.363), but it was correlated with their corneal endothelial cell density values (r2=.543; p<.001). ConclusionsDiabetes subjects with HbA1c under 7.0% who are candidates for laser refractive surgery present thicker corneas than their age-matched control subjects. In these patients, there is a correlation between their CCT values and their corneal endothelial cell density values, so when higher CCT values were found, lower corneal endothelial cell density values were observed.

Effect of keratoconus grades on repeatability of keratometry readings: Comparison of 5 devices

To determine the repeatability of keratometry (K) measurements of a Scheimpflug pachymeter (Pentacam), Placido topographer (Eyesys), scanning-slit corneal topographer (Orbscan), partial coherence interferometry (PCI) device (IOLMaster), and Javal manual keratometer with different grades of keratoconus. Noor Eye Hospital, Tehran, Iran. Retrospective case series. Keratometry was performed first with Scheimpflug pachymetry followed, in order, by Placido topography, scanning-slit corneal topography, PCI, and manual keratometry. Repeatability was examined in groups with a maximum K of less than 50.0 diopters (D), 50.0 to 55.0 D, and more than 55.0 D. Seventy-eight eyes of 45 keratoconus patients were assessed. In Group 1, repeatability was highest with Scheimpflug pachymetry and lowest with scanning-slit corneal topography (0.36 to 1.24). In Group 2, the intraclass correlation coefficients (ICCs) for maximum K ranged from 0.823 with the scanning-slit corneal topography to 0.974 with Scheimpflug pachymetry. The repeatability index for minimum K (0.53 to 2.11) and maximum K (0.60 to 1.92) in this group was highest with Scheimpflug pachymetry and with for scanning-slit corneal topography. In Group 3, the ICCs for minimum K and maximum K ranged from 0.890 to 0.990, and the repeatability index for minimum K varied between 1.66 with Scheimpflug pachymetry to 2.98 with Placido topography; for maximum K, the index was from 2.15 with PCI to 2.81 with the manual keratometer. In mild keratoconus, the 5 devices had acceptable repeatability in K readings. In cases with a maximum K reading greater than 55.0 D, all devices had reduced repeatability as a result of measurement errors; thus, measurements might not be so reliable.

Corneal epithelial thickness mapping using Fourier-domain optical coherence tomography for detection of form fruste keratoconus

To determine whether optical coherence tomography (OCT) epithelial mapping can improve the detection of form fruste keratoconus. French National Eye Hospital, Paris 6 Pierre & Marie Curie University, Paris, France. Retrospective comparative study. Eyes with normal corneas, form fruste keratoconus, moderate keratoconus, or severe keratoconus were assessed using Fourier-domain OCT (RTVue 5.5), scanning-slit corneal topography (Orbscan IIz), and rotating Scheimpflug camera (Pentacam Comprehensive Eye Scanner). Several parameters provided by the software or derived from elevation maps, OCT pachymetric maps, and OCT epithelium parameters were evaluated and compared between the 4 groups. The study involved 145 eyes. There were no significant differences in the keratometry (K) value, inferior-superior value, keratoconus index, central K index, and topographic keratoconus classification indices between the form fruste keratoconus group and the control group (P>.05). Form fruste keratoconic corneas had less epithelial thickness in the thinnest corneal zone than normal corneas, and greater epithelial thickness in the thinnest corneal zone than keratoconic corneas (P<.005). The epithelial thickness in the thinnest corneal zone in form fruste corneas was located inferiorly (P<.005) and corresponded with the zone of minimum epithelial thickness and maximum posterior elevation (P<.005). The receiver operating characteristic curve analysis showedgood overall predictive accuracy of the epithelial thickness in the thinnest corneal zone, with a 52 μm threshold value for discriminating form fruste keratoconic corneas from normal corneas. The epithelial thickness in the thinnest corneal zone and its location provided by the OCT epithelial mapping might be useful for the early diagnosis of form fruste keratoconus. No author has a financial or proprietary interest in any material or method mentioned.

White-to-white corneal diameter, pupil diameter, central corneal thickness and thinnest corneal thickness values of emmetropic subjects

This report assesses white-to-white corneal diameter, pupil diameter, central corneal thickness and thinnest corneal thickness values in a large sample of emmetropic subjects. Three hundred and seventy-nine eyes of 379 young healthy emmetropic subjects were analyzed by means of scanning-slit corneal topography. The age of the subjects ranged from 18 to 53years (mean±SD=29±7). The mean of five consecutive measurements of the central corneal thickness, the thinnest corneal thickness, the white-to-white corneal diameter, and the photopic pupil diameter was recorded. The central corneal thickness ranged from 528 to 588μm; the thinnest corneal thickness ranged from 504 to 574μm; the white-to-white corneal diameter ranged from 11.5 to 12.3mm; and the pupil diameter ranged from 3.0 to 4.7mm. The central and the thinnest corneal thickness were positively correlated (r=0.94, p<0.001), and the pupil diameter was significantly higher in females (p<0.001). This study shows that there are no differences in white-to-white corneal diameter, central corneal thickness, and thinnest corneal thickness between emmetropic females and males. However, pupil diameters are greater in emmetropic females.

Repeatability, reproducibility, and agreement characteristics of rotating Scheimpflug photography and scanning-slit corneal topography for corneal power measurement

To evaluate the repeatability, reproducibility, and agreement in anterior, posterior, and in particular the total corneal power of 2 topography devices, rotating Scheimpflug photography and scanning-slit topography. Department of Ophthalmology and Visual Science, Kitasato University Graduate School of Medical Sciences, Sagamihara, Japan. Seventeen eyes of 17 subjects (mean age 24.7 years +/- 4.1 [SD]) were included in the study. The corneal shapes within the central 3.0 mm were measured with rotating Scheimpflug photography (Pentacam) and scanning-slit corneal topography (Orbscan II). The within-rater repeatability and reproducibility of 2 raters and the overall between-instrument agreement of the measurements were evaluated using intraclass correlation coefficients (ICCs) and the Bland-Altman method. The repeatability of Scheimpflug photography and scanning-slit corneal topography was high (ICC, 0.70 to 0.99). Scheimpflug photography outperformed scanning-slit corneal topography for anterior power, posterior power, and total corneal power. The reproducibility results were similar, with limits of agreement (LoA) consistently narrower for Scheimpflug photography. The between-instrument agreement was moderate, with LoA around the mean value of total corneal power of 0.46 diopter ranging from 0.032 to 0.889. The results suggest that repeatability and reproducibility are higher in Scheimpflug photography than in scanning-slit topography. The agreement between rotating Scheimpflug photography and scanning-slit topography for total corneal power was moderate.

Differences in ocular dimensions between normal and dry eyes

Assessment of ocular dimensions is essential for ophthalmic surgeons because these values must be determined before scheduling excimer laser refractive and cataract surgeries. Dry eye seems to affect central corneal thickness (CCT) values, but it is not clear if it affects anterior chamber depth (ACD), lens thickness (LT), vitreous chamber depth (VCD) and axial length values. Following on from this, we measured the CCT, ACD, LT, VCD and axial length of 64 healthy eyes (51.20%) and 61 dry eyes (48.80%). CCT was measured with scanning-slit corneal topography (Orbscan Topography System II, Orbscan, Inc., Salt Lake City, UT, USA) and ACD, LT, VCD and axial length with a 10-MHz A-mode ultrasound device (Compuscan; Storz, St. Louis, MO, USA). There were no significant differences in ACD (P=0.588), LT (P=0.739), VCD (P=0.568) and axial length (P=0.199) between normal and dry eyes. Nevertheless, the differences in CCT between normal (549+/-34 microm) and dry eyes (527+/-30 microm) were significant (P<0.001). In sum, it seems that only the CCT values are significantly lower in subjects with dry eye.

Quantitative Anatomical Differences in Central Corneal Thickness Values Determined With Scanning-Slit Corneal Topography and Noncontact Specular Microscopy

This study was designed to analyze the differences in central corneal thickness values determined with noncontact specular microscopy and scanning-slit corneal topography. The measurements were performed on the same eye. We analyzed the central corneal thickness values of 93 patients (n = 93) by means of noncontact specular microscopy (Topcon SP-2000P noncontact specular microscope, Topcon Corp., Tokyo, Japan) and scanning-slit corneal topography (Orbscan Topography System II, Orbscan Inc., Salt Lake City, UT). One experienced physician performed 3 consecutive central corneal thickness measurements with both devices. The central corneal thickness values obtained by means of Orbscan pachymetry were 17 +/- 2.7 (range, 12-24) microm greater. A significant correlation was observed between scanning-slit corneal topography and noncontact specular microscopy (Pearson correlation coefficient, r = 0.976; P < 0.001). Researchers should know of the existence of this difference between noncontact specular microscopy and Orbscan pachymetry when interpreting central corneal thickness values.

Table of Contents

To determine the repeatability of keratometry (K) measurements of a Scheimpflug pachymeter (Pentacam), Placido topographer (Eyesys), scanning-slit corneal topographer (Orbscan), partial coherence interferometry (PCI) device (IOLMaster), and Javal manual keratometer with different grades of keratoconus.Noor Eye Hospital, Tehran, Iran.Retrospective case series.Keratometry was performed first with Scheimpflug pachymetry followed, in order, by Placido topography, scanning-slit corneal topography, PCI, and manual keratometry. Repeatability was examined in groups with a maximum K of less than 50.0 diopters (D), 50.0 to 55.0 D, and more than 55.0 D.Seventy-eight eyes of 45 keratoconus patients were assessed. In Group 1, repeatability was highest with Scheimpflug pachymetry and lowest with scanning-slit corneal topography (0.36 to 1.24). In Group 2, the intraclass correlation coefficients (ICCs) for maximum K ranged from 0.823 with the scanning-slit corneal topography to 0.974 with Scheimpflug pachymetry. The repeatability index for minimum K (0.53 to 2.11) and maximum K (0.60 to 1.92) in this group was highest with Scheimpflug pachymetry and with for scanning-slit corneal topography. In Group 3, the ICCs for minimum K and maximum K ranged from 0.890 to 0.990, and the repeatability index for minimum K varied between 1.66 with Scheimpflug pachymetry to 2.98 with Placido topography; for maximum K, the index was from 2.15 with PCI to 2.81 with the manual keratometer.In mild keratoconus, the 5 devices had acceptable repeatability in K readings. In cases with a maximum K reading greater than 55.0 D, all devices had reduced repeatability as a result of measurement errors; thus, measurements might not be so reliable.No author has a financial or proprietary interest in any material or method mentioned.

Quality of vision following clinically successful penetrating keratoplasty

Purpose: To evaluate visual function following clinically successful penetrating keratoplasty (PKP). Setting: Department of Ophthalmology, Ege University, School of Medicine, Izmir, Turkey. Methods: Patient group (PG) included 9 patients (12 eyes) who had clinically successful PKP in our department. The control group (CG) included 12 people (18 eyes) who had no ocular disease other than refractive errors. Those with a visual acuity level less than 20/25 were not included in the study. Contrast sensitivity levels and light threshold values of the central retina were measured; scanning-slit corneal topography–pachymetry and aberrometric analysis were performed. Results: There were no statistical difference in terms of age (32.55 years ± 9.25 (SD) in PG, 36.75 ± 5.85 years in CG; P = .53), cylinder power in plus form (2.60 ± 1.25 diopter (D) in PG, 2.79 D ± 2.51 D in CG; P = .88), and spherical equivalent of refractive errors (−3.66 ± 3.57 D in PG, −5.52 ± 3.37 D in CG; P = .29) between the PG and CG. Cambridge low-contrast grating scores were 96.5 ± 41.1 in grafted eyes and 148 ± 27.7 in CG ( P = .004). Central retinal light sensitivity was measured as 29.91 ± 2.39 db in PG and 33.08 ± 1.56 db in CG ( P = .001). In corneal topographic analysis, mean kappa intercept was 0.69 ± 0.37 mm in PG and 0.55 ± 0.24 mm in CG ( P = .20). Lower-order Zernike root mean squares (RMS) were 7.30 ± 3.89 μm for PG and 8.58 ± 3.46 μm for CG ( P = .37). However, higher-order Zernike RMS were 2.15 ± 0.78 in PG and 0.38 ± 0.10 in CG, which is a statistically significant difference ( P<.001). Conclusions: Even though the clinically successful PKP patients have correctable amount of spherocylindrical refractive errors with spectacle lenses, they still have reduced visual quality because of the significantly high amount of higher- order aberrations when compared with naturally occurring refractive errors.

Changes in the shape of the anterior and posterior corneal surfaces caused by mydriasis and miosis: Detailed analysis

Purpose: To evaluate changes in the anterior and posterior corneal shape, corneal thickness, and anterior chamber depth (ACD) caused by mydriasis or miosis using scanning-slit corneal topography. Setting: Department of Ophthalmology, Iwate Medical University School of Medicine, Iwate, Morioka, Japan. Methods: Twenty-eight eyes of 28 healthy volunteers with refractive errors of −6.00 to +0.25 diopters were studied. One eye of each subject had instillation of tropicamide–phenylephrine hydrochloride (Mydrin P®) to obtain mydriasis and of pilocarpine hydrochloride 2% (Sanpilo®) to obtain miosis. To assess the corneal shape, the best-fit sphere (BFS), axial power, and tangential power were measured for the anterior and posterior corneal surfaces before and after mydriasis and before and after miosis using scanning-slit corneal topography (Orbscan version 3.0, Orbtek, Inc.). The pupil size, corneal thickness, and ACD were also examined before and after mydriasis and before and after miosis. Results: The mean age of the patients was 31.1 years ± 5.6 (SD) (range 20 to 46 years). The anterior BFS changed from a mean of 8.04 ± 0.3 mm at the time of mydriasis to a mean of 8.00 ± 0.3 mm at the time of miosis. The posterior BFS changed from 6.53 ± 0.3 mm to 6.46 ± 0.3 mm, respectively. Thus, the anterior and posterior cornea became significantly steeper after miosis ( P<.01). The ACD was significantly more shallow after miosis than after mydriasis. However, there was no significant difference in corneal thickness after mydriasis or miosis. Conclusions: The anterior and posterior corneal shapes changed as a result of mydriasis and miosis, and the refractive power of the cornea significantly increased after miosis. To date, changes in refractive power from changes in pupil size have been attributed to a change in the refractive power of the crystalline lens; however, it is now thought that changes in corneal refractive power also occur.

Difference map or single elevation map in the evaluation of corneal forward shift after LASIK

Purpose Forward shift of the cornea after excimer laser refractive surgery has been assessed on a difference map generated from two elevation maps of the scanning-slit corneal topography. The current study was conducted to test whether similar evaluation is possible on a postoperative color-coded elevation map alone. Design Prospective, noncomparative case series. Participants One hundred sixty-three eyes of 86 patients with myopic refractive errors of −1 to −13.50 diopters. Intervention LASIK was performed. Corneal topography of the posterior corneal surface was obtained with the scanning-slit topography system before and 1 month after surgery. Main outcome measures The amount of forward shift of the posterior corneal surface was determined at the center of the difference map generated from preoperative and postoperative elevation maps. For surface alignment in the difference map, the 3-mm wide peripheral annular fit-zone was used. The eyes were classified into two groups depending on the amount of forward shift, using 50 μm as the threshold. Next, on the single postoperative color-coded elevation map, which is drawn relative to the individual best-fit sphere, the eye was judged to be abnormal (with significant forward shift) when more than three colors (discriminant number) were found within the central 3-mm area, and sensitivity and specificity were calculated. By varying the discriminant number from 3 to 9, receiver operator characteristic (ROC) curves were created. Results The ROC curve analyses demonstrated that sufficient true positive ratio (sensitivity) and false-positive ratio (100−specificity [%]) could not be obtained with any discriminant color number when judgments were made on a single color-coded map. There was a weak, but significant, correlation between the amount of corneal forward shift and the radius of curvature of the posterior best-fit sphere (Pearson r = −0.170; P = 0.030), indicating that a cornea with greater forward shift tended to be drawn on a steeper best-fit sphere, and thus the forward protrusion of the posterior surface failed to be depicted. Conclusions Forward shift of the cornea after excimer laser surgery should be evaluated on the difference map generated from two elevation maps, such as preoperative and postoperative maps.

Corneal forward shift after excimer laser keratorefractive surgery

The excimer laser keratorefractive surgery inevitably compromises structural integrity of the cornea by the surgical tissue subtraction and loss of integrity of Bowman’s membrane. Forward shift of the cornea is commonly seen after both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Antero-posterior movement of the cornea is evaluated by measuring the posterior corneal elevation with the scanning-slit corneal topography (Orbscan). Eyes with thinner cornea, higher intraocular pressure, and higher myopia requiring greater laser ablation were more predisposed to forward shift of the cornea. After PRK, there was a trend toward progressive forward shift of the cornea, but the progression stabilized 6 months after surgery. Because progressive thinning and expansion of the cornea were not observed during the one-year observation period after PRK, the forward shift of the cornea does not represent true ectasia. Forward shift of both corneal surfaces would add to the tendency toward myopic regression after excimer laser surgery.

Corneal forward shift after excimer laser keratorefractive surgery

The excimer laser keratorefractive surgery inevitably compromises structural integrity of the cornea by the surgical tissue subtraction and loss of integrity of Bowman’s membrane. Forward shift of the cornea is commonly seen after both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Antero-posterior movement of the cornea is evaluated by measuring the posterior corneal elevation with the scanning-slit corneal topography (Orbscan). Eyes with thinner cornea, higher intraocular pressure, and higher myopia requiring greater laser ablation were more predisposed to forward shift of the cornea. After PRK, there was a trend toward progressive forward shift of the cornea, but the progression stabilized 6 months after surgery. Because progressive thinning and expansion of the cornea were not observed during the one-year observation period after PRK, the forward shift of the cornea does not represent true ectasia. Forward shift of both corneal surfaces would add to the tendency toward myopic regression after excimer laser surgery.

Time course of changes in corneal forward shift after excimer laser photorefractive keratectomy.

Excimer laser refractive surgery has been reported to induce forward shift of the cornea, but its long-term sequelae remain unknown. To prospectively investigate the time course of changes in corneal elevation after excimer laser photorefractive keratectomy (PRK). We performed PRK on 65 eyes of 34 patients with refractive errors of -1.25 to -10.0 diopters. The anterior/posterior corneal elevation and corneal thickness were measured with a scanning-slit corneal topography system before and 1 week and 1, 3, 6, and 12 months after surgery. Twenty eyes of 10 healthy control subjects underwent similar measurements at 3-month intervals. The posterior corneal surface displayed a mean +/- SD forward shift of 36.6 +/- 25.3 microm 1 week after PRK, which gradually increased to 55.1 +/- 46.1 microm at 1 year. All postoperative values were significantly larger than those of healthy controls (2.4 +/- 8.9 microm; P<.001, Mann-Whitney test). The largest forward shift occurred within the first postoperative week. The progression thereafter was most pronounced from 1 to 6 months, and nearly stabilized at 6 months. The variance of postoperative data was statistically significant (P<.001, repeated-measures analysis of variance). Multiple postoperative comparisons demonstrated significant differences between measurements at 1 week and 6 months (P =.002, Tukey Honestly Significant Difference), at 1 week and 1 year (P<.001), at 1 and 6 months (P<.001), and at 1 month and 1 year (P<.001). Progression of forward shift was more prominent in eyes with less preoperative corneal thickness and greater myopia that required larger laser ablation. We observed no progressive thinning and expansion of the cornea during the 1-year follow-up, which refuted the occurrence of true ectasia. A statistically significant correlation was found between the amount of myopic regression and the forward shift of the cornea (Pearson correlation coefficient, r = -0.37; P =.005). Photorefreactive keratectomy induced forward shift of the cornea, which is not true corneal ectasia. The largest forward shift occurred within the first postoperative week. Changes were progressive up to 6 months postoperatively, but became almost stable thereafter. Eyes with thinner cornea and higher myopia, requiring greater photoablation, are more predisposed to progression. Forward shift of both corneal surfaces added to the tendency toward myopic regression after PRK.

Influence of excimer laser photorefractive keratectomy on the posterior corneal surface

Purpose To investigate the influence of excimer laser photorefractive keratectomy on the refraction and geometry of the posterior corneal surface. Setting Miyata Eye Hospital, Miyazaki, Japan. Methods Thirty-seven eyes of 21 patients with refractive errors of −2.00 to −9.75 diopters (D) were treated with the VISX Twenty-Twenty excimer laser system. The refractive and anteroposterior changes in the posterior corneal surface were measured using scanning-slit corneal topography (Orbscan, Orbtek, Inc.) preoperatively and 1 week and 1 and 3 months postoperatively. Results Mean posterior corneal refraction was −6.51 D ± 0.29 (SD) preoperatively; it decreased to −7.00 ± 0.49 D, −7.00 ± 0.55 D, and −6.92 ± 0.42 D at 1 week, 1 month, and 3 months, respectively ( P < .001, Tukey multiple comparison). Mean forward shift of the posterior corneal surface was 29.5 ± 1.9 μm, 34.4 ± 3.4 μm, and 54.3 ± 4.0 μm at 1 week, 1 month, and 3 months, respectively. The amount of posterior corneal refractive change correlated with the degree of forward shift ( r= −0.691, P < .001). The residual corneal thickness correlated with the refractive change ( r = 0.524, P < .001) and the forward shift ( r = −0.851, P < .001) of the posterior corneal surface. Conclusion Photorefractive keratectomy induced significant refractive changes in the posterior corneal surface and forward shift of the cornea, both of which correlated with the thinness of the residual cornea.