Accuracy of Orbscan II slit-scanning elevation topography

Dual interferometer for dynamic measurement of corneal topography

The cornea is the anterior most surface of the eye and plays a critical role in vision. A thin fluid layer, the tear film, coats the outer surface of the cornea and serves to protect, nourish, and lubricate the cornea. At the same time, the tear film is responsible for creating a smooth continuous surface, where the majority of refraction takes place in the eye. A significant component of vision quality is determined by the shape of the cornea and stability of the tear film. A dual interferometer system for measuring the dynamic corneal topography is designed, built, verified, and qualified by testing on human subjects. The system consists of two coaligned simultaneous phase-shifting polarization-splitting Twyman–Green interferometers. The primary interferometer measures the surface of the tear film while the secondary interferometer tracks the absolute position of the cornea, which provides enough information to reconstruct the absolute shape of the cornea. The results are high-resolution and high-accuracy surface topography measurements of the in vivo tear film and cornea that are captured at standard camera frame rates.

Corneal topography matching by iterative registration

Videokeratography is used for the measurement of corneal topography in overlapping portions (or maps) which must later be joined together to form the overall topography of the cornea. The separate portions are measured from different viewpoints and therefore must be brought together by registration of measurement points in the regions of overlap. The central map is generally the most accurate, but all maps are measured with uncertainty that increases towards the periphery. It becomes the reference (or static) map, and the peripheral (or dynamic) maps must then be transformed by rotation and translation so that the overlapping portions are matched. The process known as registration, of determining the necessary transformation, is a well-understood procedure in image analysis and has been applied in several areas of science and engineering. In this article, direct search optimisation using the Nelder–Mead algorithm and several variants of the iterative closest/corresponding point routine are explained and applied to simulated and real clinical data. The measurement points on the static and dynamic maps are generally different so that it becomes necessary to interpolate, which is done using a truncated series of Zernike polynomials. The point-to-plane iterative closest/corresponding point variant has the advantage of releasing certain optimisation constraints that lead to persistent registration and alignment errors when other approaches are used. The point-to-plane iterative closest/corresponding point routine is found to be robust to measurement noise, insensitive to starting values of the transformation parameters and produces high-quality results when using real clinical data.

A descriptive study of ocular characteristics in Marfan syndrome

PURPOSE

To study the ocular characteristics in patients with Marfan syndrome (MFS).

METHODS

One hundred and two eyes of 51 participants with MFS were included in this descriptive study. Ocular characteristics, visual acuity (VA) and biometric values were studied.

RESULTS

The mean patient age was 39.0 years (range, 12.7-71.6). Seventy-seven eyes were phakic, five aphakic and 20 pseudophakic. Thirty-one eyes had ectopia lentis (EL), 12 eyes cataract, nine eyes strabismus, five eyes glaucoma and four eyes had undergone surgery for a retinal detachment (RD). Ninety-four eyes (92%) had a best spectacle-corrected VA of 0.3 logarithm of the minimum angle of resolution (logMAR) or better. In the 77 phakic eyes, myopia exceeding -3 D was seen in 12/31 eyes (39%) with EL, and in 12/46 eyes (26%) without EL. The mean axial length (AL) was 24.73 mm (range, 21.59-32.75); 39 eyes (51%) had abnormally increased AL (≥24.5 mm). The mean Kmed (mean value of Kmin and Kmax ) was 42.2 D (38.7-46.9 D). No significant difference was found in refraction, AL or corneal curvature between eyes with and without EL. Lens-related astigmatism was seen in 15 (48%) eyes with EL versus four (12%) eyes without EL. Seventeen of the pseudophakic eyes underwent surgery for EL, with history of postoperative RD in one eye.

CONCLUSION

Despite increased AL, high myopia is not as common in MFS as expected because of a flat cornea. Biometry should be considered in suspected cases of MFS when EL is not present.

Scheimpflug imaging for laser refractive surgery

PURPOSE OF REVIEW

To review the principles and clinical applications of Scheimpflug corneal and anterior segment imaging with special relevance for laser refractive surgery.

RECENT FINDINGS

Computerized Scheimpflug imaging has been used for corneal and anterior segment tomography (CASTm) in different commercially available instruments. Such approach computes the three-dimensional image of the cornea and anterior segment, enabling the characterization of elevation and curvature of the front and back surfaces of the cornea, pachymetric mapping, calculation of the total corneal refractive power and anterior segment biometry. CASTm represents a major evolution for corneal and anterior segment analysis, beyond front surface corneal topography and single point central corneal thickness measurements. This approach enhances the diagnostic abilities for screening ectasia risk as well as for planning, evaluating the results, managing complications of refractive procedures, and selecting intraocular lens power, type, and design. In addition, dynamic Scheimpflug imaging has been recently introduced for in-vivo corneal biomechanical measurements and has also been used for anterior segment imaging of femtocataract surgery.

SUMMARY

Scheimpflug imaging has an important role for laser refractive surgery with different applications, which continuously improve due to advances in technology.

Orbscan topography in primary open-angle glaucoma

PURPOSE

To compare anterior and posterior corneal curvatures between eyes with primary open-angle glaucoma (POAG) and healthy eyes.

METHODS

This is a prospective, cross-sectional, observer-masked study. A total of 138 white subjects (one eye per patient) were consecutively recruited; 69 eyes had POAG (study group), and the other 69 comprised a group of healthy control eyes matched for age and central corneal pachymetry with the study ones. Exclusion criteria included any corneal or ocular inflammatory disease, previous ocular surgery, or treatment with carbonic anhydrase inhibitors. The same masked observer performed Goldmann applanation tonometry, ultrasound pachymetry, and Orbscan II topography in all cases. Central corneal thickness, intraocular pressure, and anterior and posterior topographic elevation maps were analyzed and compared between both groups.

RESULTS

Patients with POAG had greater forward shifting of the posterior corneal surface than that in healthy control eyes (p < 0.01). Significant differences in anterior corneal elevation between controls and POAG eyes were also found (p < 0.01).

CONCLUSIONS

Primary open-angle glaucoma eyes have a higher elevation of the posterior corneal surface than that in central corneal thickness-matched nonglaucomatous eyes.

Methodology for the construction and comparison of 3D models of the human cornea

This paper describes a methodology to build and compare 3D models (or atlases) of the cornea for specific populations. Using topography data of the anterior and posterior corneal surfaces, average and statistical variation maps are computed after registration of individual corneas on a reference sphere. With this methodology, a normal population model is constructed and compared with known eye anatomic data. Comparison of left and right eyes is also performed to see their natural symmetry. Our results demonstrate that spatial normalization is an important step for corneal atlas construction and comparison.

Comparison and repeatability of keratometric and corneal power measurements obtained by Orbscan II, Pentacam, and Galilei corneal tomography systems

PURPOSE

To assess the repeatability and comparability of corneal power and central corneal thickness (CCT) measurements obtained using Orbscan II (Bausch & Lomb), Pentacam (Oculus), and Galilei (Ziemer) tomographers.

DESIGN

Prospective, comparative study.

METHODS

setting: Departments of Ophthalmology, University of Auckland and Auckland District Health Board, Auckland, New Zealand. study population: Thirty eyes of 30 healthy participants. observations. CCT and corneal power measured using Orbscan II, Pentacam, and Galilei tomography. main outcome measures: Degree of agreement in and repeatability of CCT and corneal power measures.

RESULTS

Orbscan II measured significantly lower CCT compared with Pentacam (20 μm; P < .0005) and Galilei (18 μm; P < .0005). The Orbscan II had wide limits of agreement when compared with both the Galilei (-11 to 47 μm) and Pentacam (-88 to 47 μm). For each device, the intraclass correlation coefficient for CCT was higher than 0.9. The coefficient of variation ranged from 0.33% to 0.93%. There was no significant difference in mean steep keratometry or mean flat keratometry between instrument pairs. However, there was poor agreement in flat keratometry and steep keratometry obtained by Orbscan II compared with those obtained by the Galilei and Pentacam.

CONCLUSIONS

The keratometry and pachymetry measurements obtained by Orbscan II, Pentcam, and Galilei tomographers were sufficiently disparate that the 3 devices could not be considered equivalent. All 3 devices demonstrated a high level of repeatability, although the Galilei exhibited the best repeatability.

Corneal curvature, pachymetry, and endothelial cell density in Marfan syndrome

PURPOSE

To evaluate corneal curvature, pachymetry, and endothelial cell density (ECD) in Marfan syndrome (MFS).

METHODS

A case-control study in which K values, pachymetry, and ECD were compared in 39 MFS eyes and 40 control eyes matched for age and refraction was conducted. MFS eyes with lens subluxation also were compared with eyes without subluxation.

RESULTS

The mean K(med) value in MFS eyes was lower than in the control eyes, 42.2 ± 1.9 versus 43.4 ± 1.4 dioptres (D), respectively (p = 0.02). Fifteen MFS eyes (38%) and three control eyes (8%) had K(med) values below 41.5 D (p = 0.0012). MFS eyes had generally more corneal astigmatism than control eyes, 1.1 ± 0.9 versus 0.8 ± 0.4 D (p = 0.035), and MFS eyes with lens subluxation had more corneal astigmatism than those without, 1.6 ± 1.1 versus 0.6 ± 0.3 D (p = 0.0002). Nine MFS eyes with corneal astigmatism exceeding 1.5 D also had a subluxated lens. No eyes had keratoconus. The mean pachymetry value was lower in MFS eyes compared to the controls, 485 ± 54.5 versus 514 ± 37.3 μm (p = 0.007); 24 MFS eyes (62%) and 10 control eyes (25%) had measurements below 500 μm (p = 0.01). The mean ECD values were similar in MFS and control eyes, 2815 ± 430 versus 2858 ± 458 cells/mm(2) (p = 0.66). The mean K value, pachymetry, and ECD values did not differ between MFS eyes with and without lens subluxation.

CONCLUSION

Decreased K values and pachymetry could indicate MFS regardless of subluxation. High corneal astigmatism is associated with subluxation in MFS. Subluxation should be identified in MFS eyes with high corneal astigmatism.

Posterior corneal elevation after LASIK with three flap techniques as measured by Pentacam

PURPOSE

To evaluate and compare posterior corneal changes using elevation data obtained from Pentacam (Oculus Optikgeräte GmbH) Scheimpflug imaging in eyes undergoing LASIK with three different modes of flap creation: IntraLase femtosecond laser FS60 (Abbott Medical Optics) (femtosecond group), Amadeus (Ziemer Group AG) mechanical microkeratome (keratome group), or flap formation using 20% alcohol laser epithelial keratomileusis (LASEK) (LASEK group).

METHODS

Ninety myopic patients (90 eyes) undergoing refractive surgery were recruited. The change in posterior corneal elevation at 21 predetermined points in the central 5-mm area was measured using exported elevation data from the Pentacam before LASIK and 18 months postoperative and was compared among and within three modes of flap creation.

RESULTS

Mean change in posterior elevation in the central 5-mm area was 5.13±4.16 μm for the femtosecond group, 5.78±4.42 μm for the keratome group, and 6.68±4.72 μm for the LASEK group and was similar among groups (P=.59). Change in posterior elevation before and after LASIK was not significant within any group (P=.342, P=.232, and P=.321 for the femtosecond, keratome, and LASEK groups, respectively). Preoperative spherical equivalent, central corneal thickness, ablation depth, and estimated residual bed thickness did not correlate with change in posterior corneal elevation for the femtosecond, keratome, or LASEK groups (P>.05).

CONCLUSIONS

Using Pentacam elevation data, there were no significant changes in posterior corneal elevation following LASIK among or within the three methods of flap creation. At 18 months after LASIK, the posterior corneal surface is not displaced anteriorly significantly and is equally stable using these three surgical techniques.

Investigation of corneal topography after deep lamellar endothelial keratoplasty

PURPOSE

To analyze the 6-month anterior and posterior topographic changes of the cornea after deep lamellar endothelial keratoplasty.

METHODS

Orbscan topographies of 22 eyes from 21 patients with corneal decompensation were retrospectively analyzed: 11 eyes received a 9-mm scleral access incision (large-incision group) and 11 eyes received a 5-mm scleral access incision (small-incision group). All the preoperative and postoperative corneal raw data were imported into custom software which computed the average composite corneal maps and difference maps for both study groups in order to evaluate the corneal response to the surgery. The software delineated 2 concentric zones of the cornea to characterize the regional response following the surgery: the central and peripheral regions.

RESULTS

There were no significant differences (analysis of variance, <0.35 D, p>0.05) between 6-month postoperative and preoperative average anterior central corneal topographies in either group. At the end of follow-up, the average posterior curvature tangential map did not significantly differ from before surgery in either group (<0.35 D, p>0.05). However, a significant increase (>0.60 D, p<0.01) in the average posterior central astigmatic power of the cornea was found in both groups.

CONCLUSIONS

Deep lamellar endothelial keratoplasty effectively preserves the preoperative keratometric corneal topography, minimizing changes in curvature and astigmatism of the cornea. The great predictability of corneal topography following deep lamellar endothelial keratoplasty is likely to be attributed to the minimal changes that occur in the anterior stroma, the portion of the cornea that appears to be mainly responsible for maintenance of corneal shape.

Performance in specular reflection and slit-imaging corneal topography

PURPOSE

Assessment of the relative performance in measuring corneal shape and corneal aberrations for two specular reflection topographers: Keratron Placido Ring Topographer, VU Topographer, and two slit-lamp imaging instruments: Orbscan II and Topcon SL-45 Scheimpflug.

METHODS

Corneal height maps of the anterior corneal surface were obtained from a group of 34 subjects with all four instruments; posterior corneal surface height maps were only obtained with the two slit-lamp imaging instruments. Corneal surface shapes are calculated in terms of radius of curvature and asphericity fitting an aspheric model. Wave aberrations for the anterior corneal surface and the total cornea are determined up to and including sixth order Zernike convention by means of ray tracing.

RESULTS

Clinical relevant differences were observed for radius of curvature of the anterior corneal surface, where the slit-imaging instruments measure higher values (mean difference = 0.05 mm, p < 0.05) and anterior corneal astigmatism for which the Orbscan II measures higher values than the VU Topographer [mean difference = 0.174 microm (0.134 Equivalent Diopters), p < 0.01]. Small significant differences were observed for asphericity and spherical aberration of the anterior corneal surface; however, these are not clinically relevant. Clinically relevant differences were also observed for posterior radius (difference = 0.135 mm p < 0.001), total corneal astigmatism (difference = 0.207 microm (0.159 Equivalent Diopters), p = 0.001), and central corneal thickness (CCT) (difference = -18.6 microm, p < 0.001). The differences found for total corneal coma and trefoil were not clinical relevant. Furthermore, the precision of the specular reflection topographers is superior to that of the slit-lamp instruments by at least a factor of two.

CONCLUSIONS

For traditional spectacle and contact lens applications, the corneal topographers are interchangeable except for measuring anterior radius of curvature. However, for more modern techniques as customized corneal refractive surgery, the subtle differences (e.g., total corneal astigmatism and CCT) between the instruments are clinically relevant.

Evaluation of corneal topography with Orbscan II in first-degree relatives of patients with keratoconus

PURPOSE

To evaluate the corneal topographic characteristics of first-degree relatives of patients with keratoconus with corneal topography to determine the incidence of clinical keratoconus and topographic abnormalities.

METHODS

Between February and August 2006, Orbscan II analysis was done in 144 eyes of 72 cases who were first-degree relatives of patients diagnosed with clinical keratoconus. The findings were compared with preoperative Orbscan analyses of 52 clinically normal individuals who underwent laser in situ keratomileusis surgery and did not develop corneal ectasia in 3 years of follow-up.

RESULTS

In 8 of the 72 first-degree relatives of patients with keratoconus, clinical keratoconus was diagnosed by the topographic pattern in Orbscan and clinical examination (group 1). The remaining 64 subjects (group 2) were compared with the control group (group 3). The central corneal thickness was 523.7 +/- 40.4 microm in group 2, whereas it was 546.3 +/- 33.1 microm in group 3 (P < 0.05). The central corneal thickness, thinnest pachymetric reading, posterior elevation value, distance between the greatest anterior/posterior elevation points, and corneal center, posterior best fit sphere (BFS) values, posterior BFS:anterior BFS ratio, and irregularity values were significantly different between group 2 and group 3 (P < 0.05).

CONCLUSIONS

The keratoconus incidence was found to be 11% in first-degree relatives of patients with keratoconus as opposed to a reported incidence of keratoconus of 0.05% in the general population. In first-degree relatives of patients with keratoconus who did not have a topographic keratoconus pattern, abnormal corneal topographic values were detected. The asymptomatic relatives of patients with keratoconus should undergo a thorough preoperative analysis for subtle topographic abnormalities before any keratorefractive surgery.

Comparison of ultrasonic pachymetry with orbscan in corneal haze

BACKGROUND

To compare the results of ultrasonic and Orbscan pachymetry in corneal haze not related to surgery.

METHODS

An institutionally based, prospective, controlled clinical trial included 40 eyes of 40 consecutive patients with corneal haze not related to surgery. Fifty normal eyes were used as a control group to calculate the customized acoustic factor. Corneal haze was graded with slit-lamp by a single examiner. Ultrasonic and Orbscan II pachymetry measurements were obtained. Paired t- and Kruskal-Wallis tests were used to evaluate the difference between ultrasonic and Orbscan pachymetry measurements. Correlation of the two methods and haze grade were determined with Kendall's tau-b correlation analysis.

RESULTS

Eighty-five percent of patients had moderate haze. With the customized acoustic factor, Orbscan values were significantly smaller than ultrasonic pachymetry measurements (p = 0.001, paired samplet-test). Increased haze grade resulted in smaller Orbscan pachymetry measurements however, this difference was insignificant (p = 0.444, Kruskal-Wallis test). A positive correlation was found between two methods, though insignificant (p = 0.064, Kendall's tau-b correlation). When grouped in respect of diagnoses, differences in pachymetry measurements as well as the correlation were not significant (p > 0.05).

CONCLUSION

An insignificant positive correlation was found between the two methods in the presence of corneal haze. Orbscan measurements were inversely related to haze grading, and were significantly lower than ultrasonic pachymetry. According to our results, we recommend that clinicians should not count on Orbscan topography alone in the presence of corneal haze.

Corneal changes after laser refractive surgery for myopia: comparison of Orbscan II and Pentacam findings

PURPOSE

To determine the agreement between scanning slit topography (Orbscan II, Bausch & Lomb) and Scheimpflug imaging (Pentacam, Oculus) in corneal elevation, corneal curvature, and anterior chamber depth (ACD) measurements before and after laser refractive surgery for myopia and to compare the postoperative changes seen with these devices.

SETTING

Noor Vision Correction Center, Tehran, Iran.

METHODS

In a prospective observational case-series study, 23 consecutive myopic patients having laser refractive surgery were examined with the Orbscan II and Pentacam preoperatively and 6 weeks postoperatively. Readings of ACD, anterior (A-) and posterior (P-) best-fit sphere (BFS) size, central elevation (CE), maximum elevation (ME), axial power in the 3.0 mm zone (AX3) and 5.0 mm zone (AX5), and tangential power in these zones (TG3 and TG5, respectively) were collected and used in the analyses.

RESULTS

Statistically significant interdevice differences were found preoperatively for all parameters except P-TG3 (P = .014) and in operated eyes for ACD, anterior parameters of A-AX3 and A-AX5, and all posterior corneal parameters. In posterior corneal measurements, Orbscan II demonstrated significant postoperative changes in all parameters except P-AX5 (P = .004) and P-TG5 (P = .034), although none of the differences was statistically significant with the Pentacam. The devices measured similar postoperative changes in anterior curvature parameters and in P-AX5 and P-TG5; all other changes were significantly different.

CONCLUSIONS

Compared to the Pentacam, the Orbscan II yielded larger posterior elevation values before and after surgery and significant postoperative changes in P-CE and P-ME. Further studies can determine which device gives more accurate images of the cornea and redefine screening criteria.

Cornea Measurement Comparison with Orbscan II and EyeSys Videokeratoscope

PURPOSE

To compare the performance of the Orbscan II topographer with a videokeratoscope for a set of tilted test buttons with known aspheric surface profile characteristics, and for a series of measurements made on normal human corneas.

METHODS

Measurements of apical radius and p-value were obtained from 12 aspheric test surfaces using the Orbscan II, the EyeSys videokeratoscope, and Form Talysurf analysis. Measurements from the corneas of 18 human subjects were assessed by comparison with data from the EyeSys videokeratoscope. Repeatability of Orbscan measurement was also assessed for both the test surfaces and the human corneas.

RESULTS

Measurement of the aspheric surfaces produced small but significant differences between the three instruments. For human corneas, the Orbscan apical radius measure under-read from 0.020 to 0.070 mm, and the p-value measure under-read from 0.011 to 0.085 compared with the EyeSys results.

CONCLUSIONS

The Orbscan appears to under-read slightly for both apical radius and p-value when compared to both the Talysurf and the EyeSys on both the aspheric surfaces and the corneas. The differences are small but statistically significant. The corneal shape factor provided by the Orbscan instrument display is of limited value.

Preoperative Assessment of Corneal and Refractive Stability in Soft Contact Lens Wearing Photorefractive Candidates

PURPOSE

This pilot study compared traditional methodologies; manifest refraction and keratometry, with that of newer technologies; corneal topography, and optical pachymetry, in assessing corneal and refractive stabilization after soft contact lens wear in photorefractive candidates. The timeline differences among these various technologies in determining refractive and corneal stability were investigated.

METHODS

This was a masked prospective observational clinical study of full-time soft contact lens subjects on various wear schedules, all eligible candidates for photorefractive surgery. Subjects discontinued contact lens wear within 30 min of the initial study visit. During each study visit, the same sequence of tests were performed (manifest refraction, keratometry, corneal topography, and optical pachymetry). The timing of the last visit was determined when the four procedures resulted in stable findings when compared with the previous visit.

RESULTS

Fifteen soft contact lens wearers and five noncontact lens wearing controls completed the study. The mean number of days until stability of the 15 test subjects were: 10.7 +/- 10.4 days with manifest refraction, 16.2 +/- 17.5 days with keratometry, 28.1 +/- 17.7 days with topography, and 35.1 +/- 20.8 days with pachymetry. Within the control group, intraclass correlation coefficients for all four methods were > or =0.89, indicating very little variability. Analysis with the randomized block design found statistical differences between traditional and newer technologies in their assessment of stability (p < 0.001). One-way Analysis of variance of the various soft contact lenses modalities suggested extended hydrogel wearers taking the longest time to reach stability after discontinuing full-time contact lens use.

CONCLUSIONS

Corneal curvature and thickness measurements took the longest to achieve consistency. Thus, topography and pachymetry may be better methods to determine ocular stability before photorefractive surgery. In light of this finding, the current protocol in practice of determining the readiness of contact lens candidates for photorefractive surgery may be inadequate.

The Genetic Effect on Refractive Error and Anterior Corneal Aberration: Twin Eye Study

PURPOSE

To investigate the role of heredity in determining refractive variables, anterior corneal curvature, and anterior corneal aberrations.

METHODS

Thirty-three monozygotic and 10 dizygotic twin pairs were enrolled in this study. Corneal curvature, corneal astigmatism, and corneal topography were obtained from computerized videokeratoscope. The CTView program was used to compute anterior corneal aberrations from corneal height data of the videokeratoscope. Correlation analysis was performed to investigate the symmetry of the refractive error, corneal curvature, corneal astigmatism, and anterior corneal aberrations between right and left eyes of each twin pair. Heritability (h2) of these parameters was also calculated.

RESULTS

Positive correlations were noted between right and left eyes for spherical power, total astigmatism, mean corneal curvature, and corneal astigmatism. In monozygotic twins, vertical coma, secondary vertical coma, spherical aberration, and secondary spherical aberration were moderately correlated. In dizygotic twins, vertical coma, secondary horizontal coma, and spherical aberration were moderately correlated. In unrelated controls, secondary vertical coma, secondary horizontal coma, and secondary spherical aberration were moderately correlated. Root-mean-square (RMS) of higher order aberrations (3rd to 6th orders), RMS of spherical aberration, and RMS of coma were moderately correlated between right and left eyes in all three groups. Heritability of spherical aberration, RMS of spherical aberration, and corneal astigmatism (h2 = 0.56, 0.44, and 0.46) were greater than those of refractive power, corneal curvature, and other higher order aberrations.

CONCLUSIONS

These results suggest that corneal astigmatism and spherical aberration possess a greater genetic predisposition than those of other refractive errors and higher order aberrations.

Sectorial and Annular Quantitative Area Pachymetry With the Orbscan II

PURPOSE

To present a new application of quantitative area pachymetry of the cornea.

METHODS

Sectorial quantitative area pachymetry of small corneal zones located at the center and surrounding areas is possible by means of the statistical analysis device and tools menu of Orbscan II (Bausch and Lomb).

RESULTS

Average pachymetry of 1 central zone and 12 small zones around the center may be shown in the pachymetry map by simply formatting the system. Average pachymetry, standard deviation, rate of thickness variation (slope), and volume of analyzed annular corneal zone can also be automatically shown in a data window. The examination results can be printed and kept on record.

CONCLUSIONS

The method is simple and descriptive. This type of preoperative evaluation may guide the manual or laser preparation of channels for intracorneal segments and the corneal trephination for lamellar or penetrating keratoplasty.

Repeatability and Reproducibility of Posterior Corneal Curvature Measurements by Combined Scanning-Slit and Placido-Disc Topography after LASIK

OBJECTIVE

To assess the repeatability and reproducibility of posterior corneal curvature (PCC) measurements made by combined scanning-slit/Placido-disc topography (Orbscan II) after LASIK.

DESIGN

Experimental instrument validation study.

PARTICIPANTS

We recruited 22 consecutive postmyopic LASIK patients for the repeatability study and another 50 consecutive postmyopic LASIK patients for the reproducibility study.

METHODS

To analyze intrasession repeatability, 1 examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in 2 consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. We explored any association between residual stromal bed thickness and measurement variability.

MAIN OUTCOME MEASURES

Orbscan II scanning-slit PCC data, precision, within-subject coefficient of variation (CV(w)), limits of agreement (LoA), and intraclass correlation coefficient (ICC).

RESULTS

For intrasession repeatability, precision was 0.067 mm (best-fit sphere [BFS]), 0.110 diopters (D; power within 5 mm), 0.158 D (power within 3 mm), and 0.46 (eccentricity). Repeatability was high for PCC BFS and power measurements within 3-mm and 5-mm zones (CV(w) ranged from 0.5%-1.2%) but poor for eccentricity data (CV(w), 31.6%). Correspondingly, ICCs ranged from 0.89 to 0.98 for PCC BFS and power, and the ICC was 0.20 for PCC eccentricity values. For intersession reproducibility, on average, no difference in PCC measurements could be found, indicating that when there is variability, it is due to random factors. The width of the 95% LoA between sessions was clinically acceptable for BFS (0.25 mm) and power (0.4 D [within 5 mm] and 0.6 D [within 3 mm]). Similarly, ICCs indicated good intersession reliability for BFS and power (0.98, 0.96, and 0.85 for BFS, power within 5 mm, and power within 3 mm, respectively) but poor reliability for eccentricity (0.59). Repeatability and reproducibility were unrelated to stromal bed thickness.

CONCLUSIONS

Orbscan II provides reliable post-LASIK PCC data for symmetrical parameters (BFS and power), independent of the residual stromal bed thickness, but is unreliable for measurements that are radially asymmetrical (eccentricity). Orbscan II is useful for monitoring the PCC after LASIK once the early postoperative period is over.

Corneal elevation indices in normal and keratoconic eyes

PURPOSE

To determine an index that distinguishes keratoconus and keratoconus-suspect eyes from normal eyes with Orbscan (Bausch & Lomb) corneal elevation maps.

SETTING

Department of Ophthalmology, Tan Tock Seng Hospital, Singapore, and iLaser Centre, Island Hospital, Penang, Malaysia.

METHODS

In this initial phase of this multicenter nonrandomized retrospective study, 1 eye of 166 normal subjects, 15 keratoconus patients, and 11 keratoconus suspects was examined at 1 clinic. The anterior best-fit sphere (BFS), posterior BFS, anterior elevation, posterior elevation, and maximum and minimum keratometries were analyzed. Two indices with the highest sensitivity and specificity classifying these conditions were identified using receiver operating characteristic curves. These 2 indices were evaluated in a subsequent validation study using 1 eye of 221 normal subjects, 43 keratoconus patients, and 23 keratoconus suspects from another clinic.

RESULTS

The anterior elevation and anterior elevation ratio (anterior elevation ratio = anterior elevation/anterior BFS) best classified the different groups. An anterior elevation ratio of 0.5122 mm or less had 99% sensitivity and 95.2% specificity while a ratio 16.5 mum or less had 80.1% sensitivity and 80.8% specificity in discriminating normal eyes from keratoconus and keratoconus suspects. The results were similar in the validation study. In addition, these anterior elevation and anterior elevation ratio cutoff values had high sensitivity and specificity in identifying keratoconus suspects from normal eyes in the validation study.

CONCLUSION

Anterior corneal elevation parameters are clinically relevant measures for detecting keratoconus and suspected keratoconus eyes.

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.

Posterior corneal surface changes after hyperopic laser in situ keratomileusis

PURPOSE

To evaluate posterior corneal surface topographic changes after hyperopic laser in situ keratomileusis (H-LASIK) using Orbscan I (Orbtek, Inc.).

SETTING

Department of Ophthalmology, Nara Medical University, Nara, Japan.

METHODS

In 25 eyes of 15 patients who had H-LASIK, the posterior corneal surface was measured with slit-scanning corneal topography (Orbscan I) preoperatively and 1 year postoperatively. The center as a fit zone and calculated posterior corneal surface changes were taken at 4 points: nasal, temporal, superior, and inferior sides in the 5.0 mm diameter. The posterior corneal topographic changes were analyzed using an analysis of variance. The postoperative:preoperative magnification ratio of the posterior corneal surface was calculated in a theoretical eye model.

RESULTS

When a "+" reading was defined as the forward displacement and "-" was defined as the backward displacement, the mean posterior corneal topographic changes were -2.8 microm +/- 27.9 (SD) at the nasal side, -4.5 +/- 27.8 microm at the temporal side, -3.9 +/- 20.1 microm at the superior side, and -2.3 +/- 20.1 microm at the inferior side. The posterior corneal surface between any 2 examined points showed no significant difference after H-LASIK. In addition, the hypothetical change in the posterior cornea was -8.3 microm after +3.0 diopter H-LASIK, which was approximately closer to the study results. In each side, the amount of the attempted correction was significantly correlated with the posterior corneal topographic change.

CONCLUSIONS

Clinical measurement of the posterior corneal displacement after H-LASIK with Orbscan revealed a backward shift. This change corresponded to the hypothetical artifactual changes with Orbscan; that is, changes in the magnification ratio.

Mitomycin-C assisted photorefractive keratectomy in the treatment of buttonholed laser in situ keratomileusis flaps associated with epithelial ingrowth

The prophylactic intraoperative use of mitomycin-C (MMC) to prevent haze and scarring after excimer laser surface ablation (phototherapeutic/photorefractive keratectomy [PTK/PRK]) in an eye with a previous laser in situ keratomileusis (LASIK) flap buttonhole with epithelial ingrowth is described. A well-centered buttonhole measuring 2.0 mm in diameter was cut within a thin LASIK flap in an amblyopic eye. Over the next 8 weeks, corneal haze and progressive epithelial ingrowth formed centrally. An early transepithelial PTK/PRK approach was chosen to manage the buttonhole together with the epithelial ingrowth and to treat ametropia before the onset of scarring. The approach included epithelial removal with PTK, application of MMC 0.02% for 1 minute, irrigation, a short waiting period to allow for diffusion, PRK correction of -4.0 diopters without nomogram adjustment, and bandage contact lens. A regimen of prednisolone acetate 1% and ofloxacin 0.03% 5 times a day for 1 week (steroid tapered) was prescribed. Epithelial ingrowth was removed successfully. Minimal haze formation was visible 2 weeks after the retreatment but did not reduce best spectacle-corrected visual acuity (BSCVA) and resolved within the next few weeks. After 6 weeks, uncorrected visual acuity was equal to BSCVA preoperatively (20/50). There was no evidence of recurrent epithelial ingrowth or central scarring after 24 months. Transepithelial PTK/PRK was effective in managing central epithelial ingrowth in a buttonholed LASIK flap. Prophylactic intraoperative use of MMC may reduce haze formation and corneal scarring in early treatments and may also prevent recurrent epithelial ingrowth. This approach may offer faster visual recovery and no risk for a repeated buttonhole creation compared with the widespread recutting a new flap after a couple of months. The optimal application time and concentration of MMC need to be established.

Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures

PURPOSE

To establish a relationship between the forward protrusion of the cornea (keratectasia), following successful LASIK, and anterior chamber depth.

METHODS

One hundred and fifteen eyes of 59 patients who had undergone Zyoptix LASIK and 39 eyes of 21 patients who had undergone PlanoScan LASIK at the Eye Institute, Centre for Refractive Surgery, Remuera, Auckland, New Zealand, were included in the study. The results of Orbscan II acquisitions, taken before and 8.3 weeks +/- 4.0 (mean +/- standard deviation) following the procedure, were analysed to determine the amount of forward protrusion of the central 2 mm diameter of the posterior surface. The apparent keratectasia was then related to the Orbscan II anterior chamber depth (ACD) measurement in a linear mixed model analysis that included 'right or left eye', 'sex', 'type of LASIK procedure', 'change in pachymetry', 'postoperative pachymetry', 'change in anterior corneal curvature', 'postoperative anterior corneal curvature', 'planned ablation depth' and 'time until follow-up' as factors and covariates.

RESULTS

The mean +/- SD amount of keratectasia determined by Orbscan II was 22.8 +/- 26.8 microm for the Zyoptix group and 16.9 +/- 24.6 microm for the PlanoScan group. The reduction in depth of the anterior chamber was 14.7 +/- 79.3 microm for the Zyoptix group and 18.2 +/- 68.8 microm for the PlanoScan group. The mixed model analysis demonstrated strong relationships with postoperative pachymetry and change in anterior corneal curvature (P < 0.001 and P < 0.001, respectively). Furthermore, the statistical model showed a marked significant difference in the Orbscan II assessment of keratectasia and change in ACD (P < 0.001).

CONCLUSION

Using the Orbscan II device, the forward protrusion of the posterior corneal surface appears to coincide with a paradoxical reduction in depth of the anterior chamber. Although the keratectasia found in this study is accompanied by thin postoperative corneas and larger central pachymetry change, the contradictory decrease in anterior chamber depth creates uncertainty of measurement, and of ectasia as a mechanism of regression.

Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery

PURPOSE

To calculate the apparent posterior corneal changes after keratorefractive surgery and reevaluate corneal ectasia displayed by Orbscan (Orbtek).

SETTING

Department of Ophthalmology, Nara Medical University, Nara, Japan.

METHODS

Postoperative:preoperative magnification ratio of the posterior surface of the cornea was calculated in a theoretical eye model.

RESULTS

Assuming the preoperative corneal thickness is 600.00 microm, the preoperative refractive power of the anterior corneal surface is 48.0 diopters (D), the refractive power of the cornea is 1.376, the ablation diameter is 6.0 mm, the postoperative corneal thickness is 480.00 microm, the postoperative refractive power of the anterior corneal surface is 38.0 D, and the posterior surface of the cornea does not change postoperatively, the apparent image of the posterior surface of the cornea becomes 0.778% smaller postoperatively. If the posterior radius of curvature of the cornea is 6.2 mm, it becomes smaller by 48.24 microm. If this change directly affects the difference map, the posterior surface of the cornea moves forward by 48.24 microm.

CONCLUSION

The results correspond to the amount of ectasia in previous reports. This artifact may explain the apparent ectasia detected by Orbscan.

Orbscan computerized topography: Attributes, applications, and limitations

An extensive electronic search was undertaken in January 2004 to identify all relevant peer-reviewed publications on Orbscan slit-scanning/Placido computerized topography. Ninety-one publications were identified. These address elevation topography and best-fit sphere, accuracy and repeatability of anterior and posterior corneal elevation and keratometric maps, comparison of Orbscan-acquired data and Placido-based computerized videokeratography instruments, pachymetry measurement and correlation with ultrasound, screening eye-bank corneas, detection of keratoconus, identifying corneal ectasia after refractive surgery, and miscellaneous applications. Studies were analyzed and critically compared in relation to attributes, applications, and limitations of Orbscan corneal topography. The review highlights advantages of this technique in assessing the cornea in health and disease and after surgery and identifies specific aspects that require further investigation and clarification.

Evaluation of Normal Corneas Using the Scanning-Slit Topography/Pachymetry System

PURPOSE

To obtain anterior and posterior corneal shape, curvature, and thickness of normal human corneas. To provide a semiquantitative analysis of normal topography patterns of the anterior and posterior corneal surfaces.

METHODS

Eighty-eight healthy corneas of 44 normal subjects were analyzed using the scanning-slit topography/pachymetry system. Anterior and posterior elevation and mean power (central and steepest spherical and cylindrical) values and pachymetry data were determined. Right eye values were submitted for primary analysis; however, a comparison between right and left eyes was made in addition to test the possible reliability of the system.

RESULTS

There was no significant difference between right and left corneas in 28 evaluated parameters, except for anterior central cylindrical (P = 0.005) and steepest cylindrical (P = 0.017) mean power. The anterior central spherical mean power correlated inversely with the posterior central spherical mean power (r = -0.27, P = 0.04), and the anterior steepest spherical mean power value showed inverse correlation with the mean posterior steepest spherical value (r = -0.44, P = 0.001). The mean central thickness of the cornea was 593.7 +/- 54.19 microm at the center and was 578 +/- 50.53 microm at the thinnest point, which was localized in 41% (n = 18) of the cases in the inferotemporal quadrant. An oval-shaped pattern was the most characteristic feature of the anterior and posterior elevation, mean power, and pachymetry maps.

CONCLUSION

Orbscan scanning-slit topography seems to be a reliable technique for the evaluation of normal corneas not only for anterior shape and curvature but also for a real pachymetry gradient recording.

Residual bed thickness and corneal forward shift after laser in situ keratomileusis

PURPOSE

To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness.

SETTING

Miyata Eye Hospital, Miyazaki, Japan.

METHODS

Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/- 2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface.

RESULTS

The mean residual corneal bed thickness after laser ablation was 388.0 +/- 35.9 microm (range 308 to 489 microm). After surgery, the posterior corneal surface showed a mean forward shift of 46.4 +/- 27.9 microm, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6 +/- 5.7 microm (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B = 0.736, P<.0001) and the preoperative corneal thickness (B = -0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea.

CONCLUSIONS

Even if a residual corneal bed of 300 microm or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.

A corneal model for slit-scanning elevation topography

Computerised videokeratography is increasing in sophistication and accuracy. The Orbscan II Videokeratographer combines the advantages of Placido derived and slit-scanning derived topography, to acquire shape information on the posterior surface of the cornea. The purpose of this paper is to construct a model of slit-scanning elevation topography and highlight potential sources of variation in this methodology. Ray tracing calculations were performed on a defined, theoretical, spherical model to obtain a pachymetric measurement. These calculations were tested by comparing the pachymetry measurement derived from the model, over a refractive index shift from 1.376 to 1.400 with the pachymetry measurement obtained by Orbscan II, of a human eye, with an identical alteration in refractive index. The two methods of measurement differed by only 0.157 microm, suggesting that the model is a good representation of slit-scanning elevation topography, and that refractive index variations have only a minimal effect on Orbscan II pachymetry measurement.

Analyzing small-incision cataract surgery by Orbscan II fourth-dimensional pachymetry mapping

PURPOSE

To assess the changes in pachymetry after routine corneal and scleral tunnel phacoemulsification.

SETTING

Discipline of Ophthalmology, University of Auckland, Auckland, New Zealand.

METHOD

This prospective study comprised 174 eyes of 174 consecutive patients having uneventful, small-incision, sutureless, phacoemulsification cataract surgery; 124 patients had a clear corneal incision and 50, a superior scleral tunnel incision. Difference pachymetry maps were derived from Orbscan II elevation topography data obtained before and 4 weeks after surgery. Corneal thickness changes at 12 midperipheral areas located in 12 meridians were derived from difference (subtraction) maps. The mean corneal thickness within a single area corresponding to the center of the surgical incision was compared to the mean pachymetry readings of all 12 midperipheral measurements.

RESULTS

The overall mean midperipheral corneal thickness (12 samples per cornea) increased by a mean of 5.89 microm +/- 16.09 (SD) in the clear corneal incision group and 6.89 +/- 14.50 microm in the scleral tunnel group. The mean central corneal thickness increased by 7.28 +/- 20.98 microm and 7.74 +/- 21.34 microm, respectively. The corneal thickness in the meridian closest to the incision was significantly higher than the mean value of the 12 meridians measured in both groups, with means of 14.95 +/- 26.86 microm in the clear corneal group (P =.001) and 16.22 +/- 21.23 microm in the scleral tunnel group (P =.002). There were no statistically significant postoperative differences in the central, midperipheral, or incision-site corneal thickness measurements between the 2 surgical techniques (P =.77).

CONCLUSIONS

Time-dependent (fourth-dimensional) pachymetry subtraction maps are useful for following corneal dynamics over time. This study found that small-incision phacoemulsification techniques--scleral and corneal tunnel--had a minor but similar effect on corneal thickness 4 weeks after surgery.