Pattern noise (PANO): a new automated functional glaucoma test.

PURPOSE: To present a newly developed visual field device (pattern noise: PANO) designed to be sensitive to glaucoma defects, cost-effective, material-practical and easy to repair and therefore particularly suited for low-income countries, where glaucoma can be highly prevalent (e.g. sub-Saharan Africa). METHODS: This is primarily a descriptive paper, but it also includes a prospective matched case-control pilot study. Hardware, stimulus, target configuration, testing strategy and result sheet are described. The main outcome measure is the contrast level (range 2-64). Targets are composed of bright/dark pixels flickering with 18 Hz and have a size of 5°. Pixel size is approximated to the hill of vision. Average luminance of targets is constant and equals background luminance.The study was performed in the West Region in Cameroon. Twenty eyes of 20 newly presenting patients with glaucomatous optic disc cupping on funduscopy were compared with 20 eyes of 20 normal patients matched in age and laterality of eye. RESULTS: Mean age was 32.9 ± 18.8 years for glaucoma patients and 32.2 ± 15.6 years for healthy subjects. Mean contrast threshold was significantly higher in eyes with abnormal disc (16.2 ± 14.3 vs. 4.4 ± 0.8, P = 0.002). Correlation of mean contrast thresholds and cup-to-disc ratio was significant (r = 0.59; P = 0.006). Average examination time was significantly longer for glaucoma eyes compared to healthy eyes (8.2 vs. 6.1 min, P < 0.001), whereas error rate did not differ (4.8 ± 2.5% vs. 4.1 ± 1.8%, P = 0.33). CONCLUSION: PANO demonstrated visual field defects in patients with glaucomatous optic disc. Defects correlated significantly with glaucomatous optic nerve head morphological alterations. Healthy eyes obtained normal results. More studies are needed to establish PANO.

Accuracy comparison of tomography devices for ray tracing-based intraocular lens calculation.

PURPOSE: To evaluate the interchangeability of different tomography devices used for ray tracing-based intraocular lens (IOL) calculation. SETTING: Eye clinic, Castrop-Rauxel, Germany. DESIGN: Retrospective analysis. METHOD: Measurements from 3 Placido-Scheimpflug devices and 3 optical coherence tomography (OCT) devices were compared in 83 and 161 other eyes after cataract surgery, respectively. 2-dimensional matrices of anterior local corneal curvature and local corneal thickness are transferred to the ray-tracing software OKULIX. Calculations are performed with the same IOL in the same position of an eye with the same axial length. Differences in spherical equivalent (SE), astigmatism, and spherical aberration are evaluated. Furthermore, the influence of the size of the matrices (optical zone) on the accuracy is quantified. RESULTS: For the Placido-Scheimpflug devices, the deviations from the average of three measurements taken for each eye in SE (mean ± SD) were 0.17 ± 0.24 diopters (D), -0.26 ± 0.29 D, and 0.08 ± 0.39 D ( P < .001, analysis of variance [ANOVA]), for the centroids of the astigmatic differences 0.04 D/173 degrees, 0.14 D/93 degrees, and 0.10 D/7 degrees, and for the median of the absolute values of the vector differences 0.31 D, 0.33 D, and 0.29 D. For OCT devices, the corresponding results were 0.01 ± 0.21 D, -0.03 ± 0.21 D, and 0.02 ± 0.20 D ( P = .005, ANOVA); 0.18 D/120 degrees, 0.07 D/70 degrees, and 0.22 D/4 degrees; and 0.26 D, 0.30 D, and 0.33 D. The accuracy of the calculated spherical aberrations allows for an individual selection of the best fitting IOL model in most cases. CONCLUSIONS: The differences are small enough to make the devices interchangeable regarding astigmatism and spherical aberration. Although there are significant differences in SE between Scheimpflug and OCT devices, the differences between OCT devices are also small enough to make them interchangeable, but the differences between Placido-Scheimpflug devices are too large to make these devices interchangeable.

Distribution and causes of blindness and severe visual impairment in children at a tertiary referral centre in Rwanda.

AIM: To determine the prevalence and the causes of severe visual impairment and blindness (SVI/BL) in children at a tertiary referral centre in Rwanda. METHODS: In this retrospective study, files of all patients <18 years presenting during the year 2019 at the Kabgayi Eye Unit in Rwanda with SVI/BL (presenting visual acuity of <6/60 Snellen or lack of preferential looking behaviour) in at least one eye were analysed for age, sex, laterality, province of origin and cause of SVI/BL. Causes were categorised according to WHO standard classification. RESULTS: Out of 3939 children presenting to the clinic, 428 (10.9%) had SVI/BL in at least one eye. 165 (4.2%) patients had bilateral and 263 (6.7%) had unilateral condition. Of patients with BL/SVI, 36.7% were below the age of 6 years. In bilateral BL/SVI, the main causes were cataract (18%), refractive error (18%), keratoconus (13%), congenital eye anomaly (9%), glaucoma (8%), cortical blindness (8%) and retinoblastoma (6%). In unilateral BL/SVI it was trauma (46%), cataract (8%), keratoconus (8%), infectious corneal disease (7%) and retinoblastoma (7%). In preschool children, retinopathy of prematurity accounted for 7% of bilateral BL/SVI. Avoidable BL/SVI accounted for 87% of all cases. CONCLUSION: The high number of avoidable causes for SVI/BL may be reduced through several cost-effective ways.

A ray tracing approach to calculate toric intraocular lenses.

PURPOSE: To quantify the precision of astigmatic correction in routine cataract surgery with toric intraocular lenses (IOLs) and to evaluate the predictability of keratometric and anterior/posterior topographic measurement for the improvement of the overall accuracy. METHODS: Seventy-eight eyes of 56 patients were implanted with toric IOLs. Data acquired by the Lenstar optical biometer (Haag-Streit, Bern, Switzerland) and TMS5 topography (Tomey, Nagoya, Japan) were processed with the ray tracing software Okulix (Tedics, Dortmund, Germany) to predict the residual refraction. Four different inputs were examined: keratometry only, anterior topography, anterior and posterior topography/ tomography, and combination of keratometry only and anterior and posterior topography/tomography. Four weeks postoperatively, the spherical prediction error and the cylindrical prediction error (difference vector between predicted and achieved cylindrical refraction) were determined. RESULTS: Mean absolute error of spherical prediction error was 0.27 diopter (D). Cylindrical prediction errors were 0.57 D (keratometry only), 0.56 D (anterior topography), 0.56 D (anterior and posterior topography/ tomography), and 0.50 D (combination of keratometry only and anterior and posterior topography/tomography). Differences between intraocular lens groups were statistically significant (Friedman test, P < .05). CONCLUSION: The combination of keratometry and anterior and posterior topography/tomography of anterior and posterior surface yielded the best results for toric IOL power calculations.

Accuracy of intraocular lens calculation with ray tracing.

PURPOSE: To quantify the current accuracy limits of ray tracing for intraocular lens (IOL) calculations, compare results for spherical vs aspheric IOLs, and determine the value of using crystalline lens thickness in IOL calculations. METHODS: Of 591 eyes, 363 eyes were implanted with spherical IOLs (320 SA60AT [Alcon Laboratories Inc] and 43 Y-60H [Hoya Corp]) and 228 eyes had aspheric, aberration-correcting IOLs (57 SN60WF [Alcon Laboratories Inc], 112 Tecnis ZCB00 [Abbott Medical Optics], 21 CTAsphina404 [Carl Zeiss Meditec], and 38 iMics1 [Hoya Corp]), all calculated with OKULIX ray tracing (Tedics), based on Lenstar (Haag-Streit) measurements of axial length, corneal radii, and position and thickness of the crystalline lens. The measure of accuracy was the prediction error, ie, the difference between calculated refraction and manifest refraction (spherical equivalent) 1 month after surgery calculated as mean absolute error (MAE). RESULTS: The prediction error with aspheric IOLs was lower than that with spherical IOLs (MAE 0.27 vs 0.36 D) and was lower for patients with corrected distance visual acuity (CDVA) ⩾1.0 compared to CDVA <1.0 (MAE 0.26 vs 0.38 D). For aspheric IOLs and CDVA ⩾1.0, MAE differed by a factor of two compared to spherical IOLs and CDVA <1.0 (MAE 0.21 vs 0.42 D). Taking the crystalline lens position and thickness into account improved the prediction error by ∼9% overall (MAE 0.33 vs 0.36 D) and was most beneficial in patients with aspheric lenses and CDVA ⩾1.0 (MAE improved from 0.26 to 0.21 D). All differences between the investigated subgroups were statistically significant (P<.05). CONCLUSIONS: Ray tracing for IOL calculation is particularly beneficial with aspheric IOLs and in eyes with good (20/20 or better) postoperative visual acuity.

Predictability of intraocular lens power calculation after small-incision lenticule extraction for myopia.

PURPOSE: To evaluate and compare the predictability of intraocular lens (IOL) power calculation after small-incision lenticule extraction (SMILE) for myopia and myopic astigmatism. SETTING: Department of Ophthalmology, Philipps University of Marburg, Marburg, Germany. DESIGN: Retrospective comparative case series. METHODS: Preoperative evaluation included optical biometry using IOLMaster 500 and corneal tomography using Pentacam HR. The corneal tomography measurements were repeated at 3 months postoperatively. The change of spherical equivalent due to SMILE was calculated by the manifest refraction at corneal plane (SMILE-Dif). A theoretical model, involving the virtual implantation of the same IOL before and after SMILE, was used, and the IOL power calculations were performed using ray tracing (OKULIX, version 9.06) and third- (Hoffer Q, Holladay 1, and SRK/T) and fourth-generation (Haigis-L and Haigis) formulas. The difference between the IOL-induced refractive error at corneal plane before and after SMILE (IOL-Dif) was compared with SMILE-Dif. The prediction error (PE) was calculated as the difference between SMILE-Dif-IOL-Dif. RESULTS: The study included 204 eyes that underwent SMILE. The PE with ray tracing was -0.06 ± 0.40 diopter (D); Haigis-L, -0.39 ± 0.62 D; Haigis, 0.70 ± 0.48 D; Hoffer Q, 0.84 ± 0.47 D; Holladay 1, 1.21 ± 0.51 D; and SRK/T, 1.46 ± 0.54 D. The PE with ray tracing was significantly smaller compared with that of all formulas (P ≤ .001). The PE variance with ray tracing was σ2 = 0.159, being significantly more homogenous compared with that of all formulas (P ≤ .011, F ≥ 6.549). Ray tracing resulted in an absolute PE of 0.5 D or lesser in 81.9% of the cases, followed by Haigis-L (53.4%), Haigis (35.3%), Hoffer Q (25.5%), Holladay 1 (6.4%), and SRK/T (2.9%) formulas. CONCLUSIONS: Ray tracing was the most accurate approach for IOL power calculation after myopic SMILE.

Comparison of the Automated Pattern-Noise (PANO) Glaucoma Test with the HFA, an FDT Stimulus, and the Fundus Area Cup-to-disk Ratio.

AIM AND OBJECTIVE: To compare the results of a new automated glaucoma test-Pattern-Noise (PANO)-to the Humphrey Visual Field Analyzer-II (HFA), the fundus area cup-to-disk ratio (CDR), and a frequency doubling technology (FDT) stimulus. MATERIALS AND METHODS: This was a prospective study performed in the West-Region of Cameroon. Two hundred and nineteen eyes of 122 adult patients were included with a clinical suspicion of normal-tension or primary open-angle glaucoma and no other major ocular pathology. Eyes were examined with PANO, HFA (24-2 SITA standard), and FDT-stimulus in a randomized order followed by clinical assessment of the CDR. RESULTS: Parametric correlation of the mean contrast threshold of PANO with the mean contrast threshold of FDT-stimulus, total deviation of HFA, and area CDR was 0.94, -0.85, and 0.62, respectively (p < 0.001 for all values). Spatial distribution of sensitivity thresholds is highly correlated (p < 0.001) at all points in the visual field between PANO and HFA. With cut-off values of 3 ± 1 dB for HFA mean deviation and 4 ± 1 for PANO mean contrast threshold and after eliminating borderline cases, PANO's sensitivity was 95% and specificity 60%. The mean patient age was 45.2 ± 15.8 years. Mean thresholds of PANO and FDT-stimulus decreased with increasing age. Mean examination time was 7.1 ± 1.8 minutes for PANO, 5.9 ± 1.3 minutes for HFA, and 4.7 ± 1.3 minutes for FDT-stimulus. The mean percentage of false-positives per examination was 4.95% for PANO, 4.62% (p = 0.025) for FDT-stimulus, and 2.10% for HFA. CONCLUSION: The results showed that PANO was successful in suspecting the presence of glaucoma. Pattern-Noise examination led to findings that were significantly correlated to HFA, FDT stimulus, and area CDR. Some patterns of defect were also correlated. Furthermore, PANO showed a reasonable examination time and error rate. CLINICAL SIGNIFICANCE: Affordable and robust visual field devices are lacking in large parts of the developing world. Comparing them to established methods is a prerequisite to their clinical use. HOW TO CITE THIS ARTICLE: Hannen T, El-Khoury S, Patel R, et al. Comparison of the Automated Pattern-Noise (PANO) Glaucoma Test with the HFA, an FDT Stimulus, and the Fundus Area Cup-to-disk Ratio. J Curr Glaucoma Pract 2021;15(3):132-138.

Controlled cyclophotocoagulation with the 940 nm laser for primary open angle glaucoma in African eyes.

BACKGROUND: "Controlled cyclophotocoagulation" is a real-time dosage control which reduces the complications of transscleral cyclophotocoagulation to a negligible amount in European eyes. Applied to a few African eyes, however, the method failed. Obviously, the physical parameters of the laser procedure need adjustment to African eyes. METHOD AND MATERIAL: After theoretical investigations and tests in African cadaver eyes, 940 nm laser wavelength instead of 810 nm and a different fiber coupling had solved the problem of physical differences between European and African eyes to a large extent. The method was then applied to 272 eyes of 188 patients with primary open-angle glaucoma, of which it was possible to follow 26 eyes of 18 patients for at least 1 year. Median age of the patients was 63.7 years, with the youngest 16.8 years, the oldest 88.8 years. Either 16 or 24 laser spots were applied at random. If both eyes were treated, they were treated in the same session. RESULTS: The average intraocular pressure (IOP) reduction after 1 year was 7.5 mmHg, with average glaucoma drug reduction from 1.5 to 1.2 substances. At least one pop spot occurred in 32% of the eyes. No statistically significant difference between 16 and 24 spots was found. No severe complications such as intraocular bleeding, hypotony <7 mmHg, choroidal detachment or phthisis were observed. CONCLUSION: Controlled cyclophotocoagulation with the 940 nm laser is a safe method which can be applied as the first-choice treatment to African primary open-angle glaucoma eyes. Individual IOP prediction, however, is very difficult.

Glaucoma screening in Western Cameroon.

PURPOSE: To quantify glaucoma-related parameters in a rural African region. MATERIAL AND METHOD: In a population-based investigation, 635 persons in six villages underwent slit-lamp examination including investigation of the optic nerve head with a 90D lens and Goldmann applanation tonometry. The mean age of the persons was 49.4 +/- 19 years, minimum 5, maximum 90, median 52 years. The inferior, superior, nasal and temporal margin width of the optic nerve head (ONH) were estimated as fractions of the total disk diameter, thus allowing the evaluation of the horizontal and vertical cup-disk ratio (CDR), the ratio of the elliptical cup area to the total disk area (area CDR), and violations of the ISNT rule (Inferior>or=Superior>or=Nasal>or=Temporal ONH rim). RESULTS: Area CDR significantly increased with age, on average from 0.1 in the youngest to 0.47 in the oldest person, corresponding to an increase of linear CDR from 0.32 to 0.68. The total fraction of eyes exceeding an area CDR of 0.5 (i.e. linear 0.7) was 13.4%. In addition, the intraocular pressure (IOP) increased on average from 14 mmHg in the youngest to 20 mmHg in the oldest persons, but nevertheless many high CDR values were found in eyes with normal to moderately elevated IOP. Violations of the ISNT rule were found in approximately 25% of the eyes. Application of a combination of glaucoma criteria as commonly used in literature resulted in a total prevalence of 18.7% of the screened persons, corresponding to a prevalence of 8.2% after age correction for the--on average--very young Cameroonian population. CONCLUSION: Compared to Europe, glaucoma prevalence appears to be nearly an order of magnitude higher in this rural African population.

Intraocular lens calculation accuracy limits in normal eyes.

PURPOSE: To quantify the current accuracy limits, analyze the residual errors, and propose the next steps for prediction accuracy improvements. SETTING: Eye hospitals in Germany, Denmark, and Austria. METHOD: Numerical ray tracing using manufacturer's intraocular lens (IOL) data (vertex radii, central thickness, refractive index) was used for all calculations. Postoperative lens position was predicted by a simple scaling model based on measurements in 1 patient collective. The model was compared with 2 other approaches in 2 patient collectives at 2 hospitals (1121 eyes with 13 IOL models; 936 eyes with 2 models). Axial lengths were measured optically (IOLMaster, Zeiss). No parameter adjustments or individualization of IOL types or of surgeons/localizations were done. The prediction errors and measures of systematic bias for short or long eyes were used to quantify the outcome. RESULTS: The mean prediction errors in the 2 collectives were +0.13 diopter (D) and -0.13 D and the mean absolute errors were 0.44 D and 0.50 D without bias for long or short eyes, but depending on the IOL position model approach. The differences in the mean prediction errors for the IOL types were below the allowed manufacturing tolerances and below human recognition thresholds. CONCLUSIONS: The need to individualize and fudge parameters decreases with better physical models of the pseudophakic eye. Further improvements are possible by individual topography to extract corneal asphericity and measured pupil size to calculate the best focus, by improved position predictions based on individual measurements of the crystalline lens and by smaller tolerances for IOL manufacturing.

Is gaze-dependent tonometry a useful tool in the differential diagnosis of Graves' ophthalmopathy?

BACKGROUND: A rise in intraocular pressure (IOP) in upgaze is regarded as a diagnostic sign in Graves' ophthalmopathy (GO). However, the question of erroneous IOP measurement due to applanation carried out on the peripheral cornea has never been addressed. METHODS: In 22 healthy volunteers, as well as in 51 GO patients, applanation tonometry was performed in the primary position of gaze and at 20 degrees of upgaze. In addition, applanation tonometry was repeated using a flexible chin rest to incline the head and produce 20 degrees upgaze. This enabled applanation on the central cornea. RESULTS: In healthy controls, mean IOP in conventional upgaze showed a significant rise compared to primary position (p < 0.0001). IOP measurements in 20 degrees upgaze/head inclination were significantly lower compared to conventional upgaze tonometry (p < 0.0001) and comparable to mean IOP in primary position (p = 0.7930). Mean IOP in GO patients was also significantly higher in conventional upgaze compared to primary position (p < 0.0001). The upgaze measurements obtained by head inclination were significantly lower than those from conventional upgaze tonometry (p < 0.0001), but showed a statistically significant rise compared to mean IOP in primary position (p < 0.0001). The overlap of IOP readings in upgaze between normal individuals and GO patients was considerable, even in patients with severely impaired ocular motility. CONCLUSION: In both normal volunteers and patients suffering from GO, a rise in IOP was observed in conventional upgaze tonometry. However, this increase in IOP was partially due to applanation on the peripheral cornea. Measurements in upgaze by head inclination on the central cornea led to a significant lowering of the gaze-dependent IOP change. The discriminating power of the IOP difference between upgaze and primary position to diagnose GO was found to be limited. The broad overlap of IOP between normal individuals and GO patients as detected by conventionally performed upgaze tonometry leads us to conclude that this sign may not be of relevant differential diagnostic value in patients with a clinically undetermined diagnosis.

Topography-based intraocular lens power selection.

PURPOSE: To provide mathematical tools for selecting intraocular lens (IOL) power for normal eyes and for "odd" eyes, particularly after corneal refractive surgery. SETTING: Universitats-Augenklinik, Mainz, Germany. METHODS: First, IOL power is selected based on the radii and numerical eccentricity of the cornea, extracted from corneal topography in a consistent numerical model of the cornea. To fine-tune the result, the visual impression is simulated by blurred Landolt rings superimposed on the retinal receptor grid. The calculation uses numerical ray tracing of the whole pseudophakic eye comprising all monochromatic errors. The error contributions of the influencing parameters, such as anterior and posterior corneal shape and corneal thickness, are quantified in detail. The method is verified in IOL power selection for normal eyes and for eyes after corneal refractive surgery. RESULTS: The main difference between normal corneas and corneas after refractive surgery results from different asphericities. Normal corneas are prolate, with typical numerical eccentricities of 0.5, whereas corneas after laser surgery for myopia are oblate. This causes the main difference (hyperopic shift up to 2.0 diopters) in IOL power selection. Shifts in the posterior corneal radius and corneal thickness are of minor importance. CONCLUSION: Intraocular power selection after corneal refractive surgery should be based on all the information corneal topography provides.

Impact of Posterior Corneal Surface on Toric Intraocular Lens (IOL) Calculation.

PURPOSE: To quantify the impact of posterior cornea on toric IOL calculation accuracy using Placido-topography of anterior corneal surface and Scheimpflug measurements of corneal thickness. MATERIALS AND METHODS: Three-hundred seventy-nine non-selected eyes undergoing cataract surgery with non-toric intraocular lens (IOL) implantation were measured with TMS-5 (Tomey, Japan), IOLMaster (Zeiss, Germany) and Lenstar (Haag-Streit, Switzerland). Anterior, posterior and total measured corneal astigmatisms were compared with astigmatisms from postoperative refraction by calculating vector differences. RESULTS: The average absolute vector difference between anterior astigmatism and total astigmatism combining the measurements of anterior and posterior cornea was only 0.3 ± 0.2 D, with a median of only 0.27 D, but a maximum of 1.5 D. Measurements of anterior cornea alone show a systematic difference from refractive cylinder of 0.3-6 D at 90, 0.38 D at 89° and 0.28 D at 91° (IOLMaster, Lenstar and anterior TMS5), whereas the total TMS5 cylinder differs on average by only 0.14D at 81° from the refractive cylinder. With-the-rule (WTR) corneal astigmatism is slightly reduced and against-the-rule (ATR) astigmatism slightly increased on average when posterior corneal surface is taken into account additionally. This could also be confirmed by the calculation of an average pachymetry of all eyes in which the thinnest central part shows an ellipsoidal shape with horizontally long axis. CONCLUSION: Measurements of posterior cornea have on average only a small but significant impact on the outcome of toric IOL calculation, however, they are nevertheless recommended to detect outliers in which corneal irregularities (e.g. beginning keratokonus) may be overlooked.

Simplified mathematics for customized refractive surgery.

PURPOSE: To describe a simple mathematical approach to customized corneal refractive surgery or customized intraocular lens (IOL) design that allows "hypervision" and to investigate the accuracy limits. SETTING: University eye hospital, Mainz, Germany. METHODS: Corneal shape and at least 1 IOL surface are approximated by the well-known Cartesian conic section curves (ellipsoid, paraboloid, or hyperboloid). They are characterized by only 2 parameters, the vertex radius and the numerical eccentricity. Residual refraction errors for this approximation are calculated by numerical ray tracing. These errors can be displayed as a 2-dimensional refraction map across the pupil or by blurring the image of a Landolt ring superimposed on the retinal receptor grid, giving an overall impression of the visual outcome. RESULTS: If the eye is made emmetropic for paraxial rays and if the numerical eccentricities of the cornea and lens are appropriately fitted to each other, the residual refractive errors are small enough to allow hypervision. Visual acuity of at least 2.0 (20/10) appears to be possible, particularly for mesopic pupil diameters. However, customized optics may have limited application due to their sensitivity to misalignment errors such as decentrations or rotations. CONCLUSIONS: The mathematical approach described by Descartes 350 years ago is adequate to calculate hypervision optics for the human eye. The availability of suitable mathematical tools should, however, not be viewed with too much optimism as long as the accuracy of the implementation in surgical procedures is limited.

Corneal model.

PURPOSE: To describe the optical region of the cornea with as few parameters as possible and to compare this approach to commonly used mathematical models for the cornea. SETTING: University eye hospital, Mainz, Germany. METHODS: Corneal surface is approximated by a simple model (SM) that is defined by 2 perpendicular vertex radii, their angle to the horizontal, and a unique numerical eccentricity. These parameters, together with a parameter quantifying the decentration of the recording, are obtained in a consistent fit of corneal topographic data. The SM is compared to Zernike polynomial approximations of the 4th (Z4 model) and 8th (Z8 model) radial orders. Residual refraction errors for these approximations are calculated by numerical ray tracing, allowing a comparison of the different approaches. The statistical evaluation was carried out in 100 healthy eyes. RESULTS: The model approximation accuracy for the SM was at least as high as the reproducibility of the topographic measurements. For small optical zones up to 4.0 mm in diameter, the SM was on average more accurate than the Z4 model. CONCLUSIONS: The parameters of the SM, which are closely related to conventional parameters of the cornea, provided a highly accurate basis for following refractive interventions (customized corneal or cataract surgery). Zernike polynomials tend to improve peripheral optical quality at the expense of the central quality. Except in cases of technical optics, this is an unwanted effect in the human eye.

Ray tracing for intraocular lens calculation.

PURPOSE: To improve accuracy in intraocular lens (IOL) calculations and clarify the effect of various errors. SETTING: University eye hospitals, Mainz, Germany, and Vienna, Austria. METHODS: A numerical ray-tracing calculation has been developed for the pseudophakic eye. Individual rays are calculated and then undergo refractions on all surfaces of the IOL and cornea. The calculations do not use approximations; ie, the refractions are calculated exactly using Snell's law. Rays can be calculated for any distance from the optical axis and for other parameter variations. The effects of aspheric surfaces can also be investigated. Instead of IOL powers, manufacturers' IOL data (radii, refractive index, thickness) are used in the calculations for different IOL types. The resulting optical quality is visualized by using Landolt rings superimposed on the grid of retinal receptors. RESULTS: Intraocular lens design, corneal asphericity, and specific spherical aberration influence the visual quality of the pseudophakic eye significantly. The IOL refractive power is an ambiguous parameter that cannot characterize the visual outcome sufficiently accurately for an IOL implanted at a given position. The effects can be calculated only in numerical ray tracing, not in Gaussian optics. The accuracy of numerical ray tracing is independent of axial length. Therefore, very long or very short eyes gain the most from the higher accuracy of this approach. For average-size eyes, however, the results are the same as with SRK calculations. CONCLUSION: Calculations in Gaussian optics should be replaced by state-of-the-art numerical methods, which can be run on any standard personal computer.

Predicting postoperative intraocular lens position and refraction.

PURPOSE: To predict the postoperative IOL position and refraction as accurately as possible independent of individualization of the parameters. SETTING: Universitats-Augenklinik, Mainz, Germany, and Vienna, Austria. METHODS: One patient cohort (189 eyes, Vienna) was used to calibrate the prediction method, which was then applied to a second cohort (65 eyes, Mainz). All calculations were based on consistent numerical ray tracing of the pseudophakic eye using the original manufacturer's intraocular lens (IOL) data (radii, thickness, refractive index). A new algorithm to predict IOL position was developed. Ultrasound (US) axial lengths were calibrated relative to partial coherence interferometry (PCI). Corneal radii extracted from topography were checked against radii measured with the IOLMaster (Zeiss) and by Littmann keratometry. RESULTS: Zero mean prediction errors for IOL position and refraction were obtained without adjusting the parameters and with PCI lengths or US lengths calibrated relative to the PCI values. There was no significant loss of accuracy of US data compared to PCI data. Corneal radii extracted from topography were slightly but statistically significantly different from the Littmann values, and they were more accurate than the latter with respect to prediction error. The measured mean central IOL position (distance from posterior corneal surface) for all IOL types was 4.580 mm, a value very close to the mean recalculated from A-constants (4.587 mm). The difference in the individual central IOL position relative to the mean value depended only linearly (ie, no higher orders such as square or cubic are needed) on axial length, with the mean central IOL position as a free parameter. This parameter should be 4.6 +/- 0.2 mm (the same value as independently measured or recalculated) to obtain zero steepness of the prediction error as a function of axial length, producing zero bias for long and short eyes. CONCLUSIONS: Calculation errors from formulas and confusing adjusting parameters can be avoided if calculations and measurements are performed on a clear and simple physical basis. Nevertheless, an individual prediction error, typically 0.5 to 1.0 diopter, seems to be unavoidable.

Determining postoperative anterior chamber depth.

PURPOSE: To compare measured and calculated postoperative anterior chamber depths (ACDs). SETTING: Department of Ophthalmology and Institute of Medical Physics, University of Vienna, Vienna, Austria, and Department of Ophthalmology, University of Mainz, Mainz, Germany. METHODS: The postoperative ACD was measured in 189 pseudophakic eyes using a laboratory prototype of partial coherence interferometry (PCI). In 6 intraocular lens (IOL) groups, the mean ACD was calculated by ray tracing based on the best-known A-constants of the SRK formulas. In addition, for each IOL type, each measured ACD was compared with a value calculated using the individual spherical equivalent of the postoperative refraction. RESULTS: The measured and the calculated ACD values were close and did not show systematic differences. The ACD values obtained in the study, however, differed significantly from the values published by the IOL manufacturers. A comparison of the PCI-assessed ACDs and the calculated values using the postoperative refraction showed more scattered results for the refraction-based data, which was probably the result of higher measurement errors with the autorefractometer than with PCI. CONCLUSIONS: High-precision interferometry measurements and ray-tracing calculations confirmed each other. The resulting mean ACD values should be used instead of the manufacturers' values. The refractive outcome of cataract surgery can be improved by combining preoperative high-precision PCI biometry and numerical ray tracing for IOL power calculations.

Modulatory effect of different riboflavin compositions on the central corneal thickness of African keratoconus corneas during collagen crosslinking.

PURPOSE: A pilot investigation to transfer the established corneal collagen crosslinking (CXL) procedure in European eyes into clinically affected African eyes and to optimize the treatment by adapting the riboflavin composition. MATERIALS AND METHODS: CXL was performed in 15 eyes (11 patients) with advanced stages of keratoconus in the Eye Clinic of Bafoussam in the West Region of Cameroon. The following six riboflavin compositions with different portions of active swelling additives were applied: Solution 1 (0.5% methylhydroxypropylcellulose [MHPC]), solution 2 (1.0% MHPC), solution 3 (1.7% MHPC), solution 4 (5% dextran), solution 5 (10% dextran) and solution 6 (no active swelling ingredient). The central corneal thickness (CCT) was measured by ultrasound pachymetry before and after de-epithelialization and at least every 10 min during CXL. RESULTS: THE APPLICATION OF THE RIBOFLAVIN SOLUTIONS RESULTED IN THE FOLLOWING MEAN FINAL CCT VALUES: 172 ± 15% using solution 1 (60 min/n = 5); 183 ± 8% using solution 2 (60 min/n = 5); 170% using solution 3 (60 min/n = 1); 80% using solution 4 (45 min/n = 1); 99% using solution 5 (45 min/n = 1) and 150 ± 13% using solution 6 (50 min/n = 2). CONCLUSIONS: The combination of riboflavin compositions with swelling and stabilizing effects on the corneal stroma seems necessary in African eyes with advanced keratoconus. Further studies are required to confirm these primary results.