Scheimpflug records without distortion--a mythos?

Comparison of intraocular lens tilt after capsular sutured scleral fixation with capsular segments versus uneventful cataract surgery

PURPOSE

To evaluate and compare intraocular lens (IOL) tilt between uneventful phacoemulsification with in-the-bag IOL implantation and sutured scleral fixation (SSF) of the lens bag with a capsular tension segment (type 6 D / Morcher) using a Sheimpflug camera.

SETTING

Clinical Practice, Hospital. Barcelona and A Coruña, Spain.

DESIGN

Retrospective, comparative multicenter study.

METHODS

IOL tilt was compared between patients who underwent sutured scleral fixation with a capsular tension segment in a single eye (SSF group, n = 15) with patients who underwent uneventful IOL implantation (control group, n = 12) that were matched by biometric measurements. Post-operative refractive accuracy of biometric formulas by means of mean absolute error (MAE) was also reported. All patients underwent a general ophthalmic evaluation, anterior segment photography, and postoperative optical biometry (Zeiss IOLMaster® 500). In addition, IOL tilt was measured with a Scheimpflug camera (Pentacam R, Oculus Optikgerate Gmbh).

RESULTS

Mean vertical tilt was similar in both groups (2.20+/-2.47° SSF vs 1.97 +/- 1.79° control; p = 0.836) but mean horizontal tilt tended to higher values in the SSF series (2.09 +/- 2.74° vs 0.94 +/- 1.17°; p = 0.139). Considering post-operative refractive error in diopters by MAE calculations, there was an underestimation of IOL power in the SSF group which was only statistically significant for Barrett Universal II (1.07 vs 0.32; p = 0.028) and Hill-RBF (0.95 vs 0.26; p = 0.024) formulas, but not for SRK/T (0.99 vs 0.42; p = 0.285) and Kane (0.96 vs 0.33; p = 0.083).

CONCLUSION

Sutured scleral fixation of capsular tension segments in the presence of zonular instability does not seem to induce clinically significant IOL tilt compared to uneventful cataract extraction cases.

Comparative Analysis of Two Clinical Diagnostic Methods of the Corneoscleral Geometry

PURPOSE

To compare the sagittal height (SH) measurements and best fit sphere (BFS) scleral curvature obtained with two different diagnostic technologies.

METHODS

Prospective, nonrandomized, and comparative study analyzing 23 healthy eyes of 23 patients (age, 14-52 years) was conducted. A complete eye examination was performed including a corneoscleral topographic analysis with the Scheimpflug camera-based system Pentacam (Oculus GmbH, Wetzlar, Germany) and afterward with the Fourier domain profilometer Eye Surface Profiler (ESP) (Eaglet-Eye, Houten, the Netherlands). Differences between devices in temporal SH (TSH), nasal SH (NSH), and mean SH (MSH) measurements at 13- and 15-mm chord length and in scleral BFS were analyzed.

RESULTS

Statistically significant differences were found in TSH, NSH, and MSH obtained with Pentacam and ESP (P≤0.017), with a range of agreement from 0.21 to 1.28 mm, and a clear trend of the Pentacam system to provide higher SH measurements. Significant differences were found between Pentacam scleral BFS and ESP outer BFS (P<0.001) (range agreement, 3.57 mm). Significant correlations were found between spherical equivalent and differences between devices in 15-mm TSH (r=0.485; P=0.048), as well as between the difference between devices for some SH measurements and the magnitude of such SH values (r≥0.504; P≤0.014).

CONCLUSION

Sagittal height and scleral BFS measurements obtained with the two devices are not interchangeable, with an increased difference in SH measures between devices in eyes with less myopic refractive error and increased SH values.

Changes in Central Corneal Thickness With Air-Puff-Induced Corneal Deformation Using a Method to Correct Scheimpflug and Refractive Distortion

PURPOSE

To establish a method to determine central corneal thickness (CCT) and anterior chamber depth (ACD) of an air-puff-deformed cornea at the highest concavity (HC) state.

METHODS

The Fink method for refractive correction of Scheimpflug images of a convex pre-deformed (PRE) cornea was implemented for 155 eyes of 155 participants imaged with the Corvis ST (Oculus Optikgeräte GmbH). This method was subsequently modified for the HC state of deformation. The tracked edges of each participant's cornea were exported at the PRE and HC states. Ten participants who had a visible crystalline lens in the image were selected to determine ACD in both states. The center points on the corneal tracked edges and lens were used to determine uncorrected CCT and ACD, respectively.

RESULTS

Average undeformed CCTPRE was significantly lower than deformed CCT_HC (584 ± 31 and 626 ± 34 µm, respectively) (P < .0001). No significant difference was found for the corrected ACD between the two states. Corrected CCT and ACD were significantly greater than the corresponding uncorrected values for both deformation states (P < .0001). Percent change in CCT was found to be correlated to change in arc length at HC (P < .0001).

CONCLUSIONS

Distortion in Corvis ST images at the HC state can be corrected using a modified Fink method. CCT was found to increase in the HC state, compared to the PRE state. The CCT change during deformation may be important in the study of the compressive response of the cornea. [J Refract Surg. 2021;37(6):422-428.].

The Effect on Post-Operative Intraocular Lens Centration by Manual Intraoperative Centration versus Auto-Centration

Introduction

To determine if intraoperative manual centration of the intraocular lens (IOL) during cataract surgery results in better early post-operative centration. It is common practice for cataract surgeons to align intraocular lens centration to the visual axis by manual intraoperative manipulation of the intraocular lens. We aim to compare post-operative intraocular lens centration between intraocular lenses that were allowed spontaneous positioning in the capsular bag and IOLs that were manually centred during implantation.

Materials and Methods

One hundred and twenty-five consecutive eyes that underwent either femtosecond laser-assisted cataract surgery (FLACS) or phacoemulsification and IOL (monofocal/toric/multifocal) implantation by a single surgeon were included. Post-operative IOL centration at 4 weeks was assessed and measured on a slit-lamp by a masked observer.

Results

A total of 17 (13.6%) IOLs were off-centered to some extent at 4 weeks post-operatively. All the decentered IOLs were graded as minimal or mild (<0.5mm) decentration. There was no statistically significant difference in the proportion of decentered IOLs between the manual centration group and non-centration group (p = 0.59).

Conclusion

The final position of the IOL is not dependent on manual centration but rather on the design and symmetry of the IOL, as well as the integrity of the capsular bag.

Automated Detection of the Stromal Demarcation Line Using Optical Coherence Tomography in Keratoconus Eyes After Corneal Cross-linking

PURPOSE

To evaluate the role of a novel automated detection software as compared to human operators in assessing the presence and depth of stromal demarcation line on optical coherence tomography (OCT) in keratoconus eyes post cross-linking.

DESIGN

Reliability analysis study.

METHODS

Two independent operators and an automated detection software examined corneal OCTs of 25 eyes of 25 patients post corneal cross-linking using the Dresden protocol, at 3 months postoperatively. Operators evaluated the presence of the demarcation line and measured its depth by looking at OCT images (128 cuts) on 2 separate occasions 1 week apart. The automated software examined all 128 cuts of each OCT measurement.

RESULTS

The mean corneal demarcation line depth was 321.54 ± 47.71, 322.86 ± 45.77, and 309.21 ± 40.98 μm, as computed by the automated detection software and the human operators, respectively. The intraclass correlation coefficients (ICC) between the automated detection software and Observers #1 and #2 were 0.884 and 0.847, respectively (P < .001). The ICC for interoperator reproducibility was 0.890, and for intraoperator repeatability for Operator #1 and Observer #2 were 0.922 and 0.925, respectively. The ICC for intersoftware repeatability was 1. Bland-Altman plots showed a good agreement between both observers and the software, with adequate 95% limits of agreement.

CONCLUSION

Detection of the demarcation line by human operators is repeatable and reproducible, but it can be further optimized and standardized by an ultrafast and accurate automated software detection tool, providing a reliable indicator for treatment success.

Anterior lens capsule strains during simulated accommodation in porcine eyes

Experimental protocols have been developed to measure the spatial variation of the mechanical strains induced in the lens capsule during ex vivo lens stretching. The paper describes the application of these protocols to porcine lenses. The deformations and mechanical strains developed in the anterior capsule during each experiment were determined using full field digital image correlation techniques, by means of a speckle pattern applied to the lens surface. Several speckling techniques and illumination methods were assessed before a suitable combination was found. Additional data on the cross section shape of the anterior lens surface were obtained by Scheimpflug photography, to provide a means of correcting for lens curvature effects in the determination of the strains developed in the plane of the capsule. The capsule strains in porcine lenses exhibit non-linear behaviour, and hysteresis during loading and unloading. Peripheral regions experience higher magnitude strains than regions near the lens pole. The paper demonstrates the successful application of a procedure to make direct measurements of capsule strains simultaneously with ex vivo radial lens stretching. This experimental technique is applicable to future investigations on the mechanical characteristics of human lenses.

The eye lens: age-related trends and individual variations in refractive index and shape parameters

The eye lens grows throughout life by cell accrual on its surface and can change shape to adjust the focussing power of the eye. Varying concentrations of proteins in successive cell layers create a refractive index gradient. The continued growth of the lens and age-related changes in proteins render it less able to alter shape with loss of capacity by the end of the sixth decade of life. Growth and protein ageing alter the refractive index but as accurate measurement of this parameter is difficult, the nature of such alterations remains uncertain. The most accurate method to date for measuring refractive index in intact lenses has been developed at the SPring-8 synchrotron. The technique, based on Talbot interferometry, has an X-ray source and was used to measure refractive index in sixty-six human lenses, aged from 16 to 91 years. Height and width were measured for forty-five lenses. Refractive index contours show decentration in some older lenses but individual variations mask age-related trends. Refractive index profiles along the optic axis have relatively flat central sections with distinct micro-fluctuations and a steep gradient in the cortex but do not exhibit an age-related trend. The refractive index profiles in the equatorial aspect show statistical significance with age, particularly for lenses below the age of sixty that had capacity to alter shape in vivo. The maximum refractive index in the lens centre decreases slightly with age with considerable scatter in the data and there are age-related variations in sagittal thickness and equatorial height.

Reliability of the corneal thickness measurements with the Pentacam HR imaging system and ultrasound pachymetry

PURPOSE

To obtain the central corneal thickness (CCT) of healthy eyes using the rotating Scheimpflug imaging system and compare these results with conventional ultrasound pachymetry values. Interoperator reliability and repeatability of the different techniques were also determined.

METHODS

CCT was measured on the right eye of 46 healthy subjects with the Pentacam HR (Pentacam High Resolution; Oculus, Wetzlar, Germany) and contact ultrasound pachymetry (AL-2000; Tomey, Tennenlohe, Germany) by 2 independent investigators.

RESULTS

Both observers obtained significantly higher CCT readings with the Pentacam HR imaging system (572 ± 33 and 575 ± 31 μm) than with the ultrasound device (546 ± 27 and 548 ± 28 μm) (P < 0.0001, paired Student t test). Strong and significant correlation was found between the 2 pachymetry techniques regardless of the investigator (r = 0.845 and 0.831, Spearman correlation coefficient) (P < 0.0001). For the Pentacam HR values, statistically significant difference was detected between the observers (P = 0.033). For both pachymetry methods, the degree of agreement between the investigators was reflected by the high interoperator intraclass correlation coefficient value (ICC = 0.96). No significant difference was disclosed between the 3 consecutive readings of either instrument (P > 0.05, analysis of variance).

CONCLUSIONS

Significant difference in CCT values between the high-resolution Pentacam imaging system and the ultrasound pachymeter suggests that the 2 techniques cannot be used interchangeably in clinical practice.

Scheimpflug imaging of corneas after collagen cross-linking

PURPOSE

To compare geometrical shape factors of keratoconus corneas after cross-linking (CXL) by means of Scheimpflug imaging with those of untreated fellow eyes.

SETTING

Institut für Refraktive und Ophthalmo-Chirurgie, Zürich, Switzerland.

METHODS

Scheimpflug imaging of the anterior segments was performed with the Pentacam (Oculus, Wetzlar, Germany) in 21 patients with progressive keratectasia before and after CXL. Only 1 eye per patient was treated with corneal cross-linking using the riboflavin/UV-A approach, the fellow eye serving as control. The following corneal parameters and their postoperative evolution during 1 year after treatment have been evaluated: minimal curvature radius and its location, thickness at the thinnest point, location of the thinnest point, anterior and posterior elevation, conoid asphericity constants of the anterior and posterior surface, and 7 keratoconus indices. Statistical comparison was performed by means of the Wilcoxon test.

RESULTS

None of the treated eyes showed topographic progression in contrast to the untreated group where 8 eyes experienced significant progression. Minimal curvature radius increased significantly after 1 year compared with preoperative (6.14-6.21 mm), whereas in the untreated fellow eye, it significantly decreased (6.94-6.86 mm). Minimal corneal thickness was significantly reduced after treatment (P < 0.002 at 12 months). The cornea showed an evolution toward a more regular shape as indicated by a significant reduction in 4 of 7 keratoconus indices. No complications of CXL occurred in this small study group.

CONCLUSIONS

After cross-linking, the corneal shape undergoes a process of regularization. This process is active during the first year after treatment and may continue. Longer follow-up is warranted to estimate the full amount of regression of the keratectasia after CXL.

Tilt and decentration of spherical and aspheric intraocular lenses: effect on higher-order aberrations

PURPOSE

To determine the impact of spherical and aspheric foldable intraocular lens (IOL) tilt and decentration on optical quality after cataract surgery in an intraindividual comparative study.

SETTING

Department of Ophthalmology, Goethe-University, Frankfurt am Main, Germany.

METHODS

Randomized implantation of a spherical IOL (Sensar AR40e) was performed in 1 eye and of an aspheric IOL with negative spherical aberration (Tecnis Z9000) in the contralateral eye. Three to 4 months postoperatively, the wavefront was measured and higher-order aberrations (HOAs) were calculated for virtual pupil diameters of 3.5 mm and 6.0 mm. Tilt and decentration of the IOLs were measured using Scheimpflug photography. The tilt and decentration, HOAs, and best corrected visual Strehl ratio of the optical transfer function (BCVSOTF) calculated from the wavefront aberration were compared. The effect of tilt and decentration on HOAs and optical quality was assessed using multiple regression analysis.

RESULTS

The mean optic tilt was 2.89 degrees +/- 1.46 (SD) for the spherical IOL and 2.85 +/- 1.36 degrees for the aspheric IOL. The mean optic decentration was 0.19 +/- 0.12 mm and 0.27 +/- 0.16 mm, respectively. No significant intergroup differences in IOL tilt or decentration were found. Tilt and decentration did not significantly affect the BCVSOTF with either IOL.

CONCLUSIONS

The amounts of tilt and decentration of both IOLs were not large enough to cause deterioration of optical quality. Thus, when IOL tilt and decentration are within normal limits, they do not compromise the correction of spherical aberration by the aspheric IOL.

Mild topographic abnormalities that become more suspicious on Scheimpflug imaging

PURPOSE

Although several screening methods exist, postoperative corneal ectasia after refractive surgery is a severe complication. One possibility for this might be the fact that screening methods may fail in detection of preoperative risk factors such as forme fruste keratoconus (FFKC).

METHODS

Retrospective evaluation of four cases that showed only mild changes of FFKC on placido-based topography but revealed indicative findings on Scheimpflug imaging (Pentacam).

RESULTS

While in placido-based topography evaluation of corneal topography did not show a clear FFKC, the evaluation of corneal topography on Scheimpflug imaging together with the data of spatial corneal thickness revealed distinctive FFKC in all cases presented.

CONCLUSIONS

Although both methods bear the risk of not detecting pre-existing FFKC, Scheimpflug imaging seems superior to placido-based corneal topography alone.

[Corneal topography. Analysis of the corneal surface]

Various diagnostic procedures have been developed for analyzing the corneal surface. Corneal topographical measurements can be performed by classic Placido disc topographers and tomographers creating three-dimensional corneal models from cross-sectional images. Using a slit scanning system or the Scheimpflug technique additional information about the anterior segment can be obtained. New systems give us a wide range of diagnostic tools. Beside the measurement of the corneal anterior and posterior surfaces and point-to-point-pachymetry they enable analysis of corneal aberrations and measurement of the whole anterior segment, including the anterior chamber and the position of implants, pupil diameter, horizontal white-to-white and the kappa angle. Densitometry, statistical programs for the evaluation of corneal pathology and assistance with intraocular lens calculation following excimer laser treatment are further options available. This article is intended to give an overview of the principles and limitations of the various diagnostic systems.

Biometry of phakic intraocular lens using Scheimpflug photography

PURPOSE

To examine lateral and axial positioning of phakic intraocular lenses (IOLs) with iris fixation in the anterior chamber and to examine short-term stability of the IOL position.

SETTING

The Netherlands Opthalmic Research Institute, Amsterdam, the Netherlands.

METHODS

Thirty patients participated in the study. Thirty-one eyes were implanted with the 204 type myopia IOL, 14 eyes with the 206 myopia IOL, and 8 eyes with the 203 hyperopia IOL. Scheimpflug slitlamp photographs were made through the optical axis along 4 meridians of the eyes. Ray tracing was used to obtain the lateral and axial position of the IOLs.

RESULTS

Centration of the IOL with respect to the pupil's center and the tilt angle of the IOL with respect to the optical axis of the eye were measured. Standard deviation of decentration was 0.21 mm vertically and 0.16 mm horizontally. Standard deviation of tilt was 1.30 degrees vertically and 0.90 degrees horizontally. Tilt and decentration are proportional to each other. Vaulting, the distance between the crystalline lens and the IOL, was constant over a period of 24 months, ranging from 0.2 to 0.8 mm, depending primarily on the radius of curvature of the crystalline lens. A geometric model for this dependence was formulated.

CONCLUSION

Phakic IOLs with iris fixation can be positioned in the eye with submillimeter precision. Axial position of iris-fixated phakic IOLs over time is excellent. Axial position and vaulting can be predicted when the radius of curvature of the crystalline lens is known. The IOL behaves as if mounted slightly above a sphere-the anterior surface of the crystalline lens.

Refractive correction method for digital charge-coupled device-recorded Scheimpflug photographs by means of ray tracing

Our purpose is to correct digital CCD-recorded Scheimpflug photographs, imaging both the anterior and posterior corneal surface, the anterior chamber, and the anterior eye lens surface for optical distortions. In a ray-tracing algorithm the imaging of the posterior corneal surface in a given Scheimpflug photograph is corrected by applying Snell's law on parallel incident rays entering through the anterior corneal surface. Once the posterior corneal surface is corrected, the procedure is repeated, again with parallel incident rays entering through both the anterior and now corrected posterior corneal surface, to correct the imaging of the anterior eye lens surface. The refractive indices necessary for Snell's law are taken from Gullstrand's exact schematic eye model. Due to the optical/refractive correction, the digital Scheimpflug photograph decreases in size perpendicular to the direction of the optical axis. As a consequence the curvature radii of both the posterior corneal surface and the anterior lens surface are reduced significantly, as compared to the original digital Scheimpflug photograph. Furthermore, the corneal thickness and the anterior chamber depth are increased. The presented refractive correction method enables us to extract from Scheimpflug photographs the following quantities rather realistically: structure coordinates and curvature radii of both the posterior corneal surface and the anterior lens surface, corneal thickness, and anterior chamber depth. This method can readily be applied to other imaged quantities, such as the posterior eye lens surface, the lens thickness, and the pupillary opening.

Agreement between Scheimpflug photography and A-scan ultrasonography in anterior segment ocular measurements in children

PURPOSE

The Scheimpflug principle, as a method of performing biometry on the anterior segment, has not been widely used in the clinical setting compared with ultrasonography. The purpose of this study was to examine the agreement in anterior chamber length and lens thickness measurements between A-scan ultrasonography and Scheimpflug photography.

METHODS

A total of 252 children aged between 6 and 12 years participated in this study. All subjects had instillation of cycloplegic eye drops before measurement. Scheimpflug photography and A-scan ultrasonography were performed in this sequence on the same day. The anterior segment length on the Scheimpflug images was taken as the axial corneal thickness added to the anterior chamber depth. A-scan ultrasonography was performed by placing the probe on the corneas after topical anesthesia. Differences were obtained by subtracting the A-scan ultrasonography measurements from the Scheimpflug measurements. Data were analyzed using difference vs. mean plots.

RESULTS

For anterior chamber length, the mean difference between the two methods was -0.03 +/- 0.16 mm, with 95% limits of agreement of -0.35 to +0.30 mm. However, Scheimpflug readings were greater than that of ultrasonography for smaller measurements, and the opposite was true for larger measurements. For lens thickness, the mean difference was -0.2 +/- 0.10 mm, with 95% limits of agreement of -0.40 to +0.00 mm. For the measurement of lens thickness, adding a correction factor of 0.2 mm to Scheimpflug's readings should replace that of A-scan readings reasonably well.

CONCLUSIONS

The readings obtained from these two methods of biometry must be used with the knowledge of the instruments. We discuss the possible reasons for the difference in readings obtained using these two instruments.

Intra-observer and inter-observer repeatability of anterior eye segment analysis system (EAS-1000) in anterior chamber configuration

BACKGROUND

Anterior chamber configuration can be assessed via optical or ultrasonic techniques. Scheimpflug photography is a non-invasive method measuring the anterior segment. The Anterior Eye Segment analysis system, EAS-1000, utilises the Scheimpflug principle and was found to have good repeatability. Previous repeatability studies, however, have had limitations in their design. The current study investigated the intra-observer and interobserver repeatability of the EAS-1000.

METHODS

Twenty-five healthy young subjects were recruited. The anterior chamber angles in different quadrants were measured by two examiners for interobserver analysis. The first examiner repeated the measurement at another session for intra-observer analysis. The 95% limits of agreement and intra-class correlation coefficient (ICC) were calculated. The anterior chamber depth was also measured and compared with ultrasound biometry.

RESULTS

The anterior chamber angle assessment demonstrated good intra-observer (ICC ranging from 0.77 to 0.90 for different quadrants) and interobserver (ICC ranging from 0.68 to 0.81 for different quadrants) repeatability. The 95% intra-observer limits of agreement were within +/-5 degrees. The 95% interobserver limits of agreement were within +/-6 degrees. There was no significant difference between male and female subjects or among angles at different quadrants. The anterior chamber depth was found to be repeatable (ICC > 0.90) with 95% limits of agreement +/-0.1 mm. The anterior chamber depth was shallower than that obtained from ultrasound biometry.

CONCLUSIONS

The EAS-100 is a non-invasive instrument which is repeatable for measuring the anterior chamber angle and depth. It provides quick results and is good for screening purposes. There is an under-estimation of anterior chamber depth, as previously reported.

Anterior chamber depth estimation by Scheimpflug photography

PURPOSE

To obtain a model suitable for evaluation of the anterior chamber depth (ACD) maintaining capacity of viscoelastic substances--a critical parameter in cataract surgery.

METHODS

ACD was estimated using an anterior segment analysis system (EAS-1000, Nidek Co Ltd, Japan). Variation was studied as an effect of batch number and eye within batch, storage, globe fixation, globe position, and repeated analyses of Scheimpflug images.

RESULTS

Considerable variation in ACD was found between eyes, batches, and as a result of prolonged storage (biological factors). Different IOPs before surgery caused significant alterations in ACD after overfilling with a viscoelastic substance. No significant differences were found when the globe position was altered or when the ACD was re-estimated repeatedly from the same photograph.

CONCLUSION

Differences due to biological factors are more important than methodological errors caused by the Scheimpflug instrument. To reduce these variations when viscoelastics are evaluated, all measurements should be calculated as % of ACD before surgery and the anterior chamber should be overfilled with a viscoelastic solution.