New retinal tack designs: an analysis of retention forces in human scleral tissue.

PURPOSE: The study aimed to construct a new retinal tack design with high retention forces to prevent spontaneous disentanglement in cases of complicated retinal surgery. METHODS: Six new forms for the peak of a retinal tack were developed using computer-aided design (CAD); then a prototype was produced for each model. Finally, standardised design testing was conducted using human (ex vivo) sclera by logging 15 consecutive measurements for each model. RESULTS: Seven different models underwent pull-out testing (six new models and the original tack model), but two tack models (Model 4, Model 5) failed to penetrate the human tissue. The highest pull-out forces (median) were measured for Model 3, followed by Model 6, Model 2 and Model 1. The original Heimann tack (Model H) was found to have the lowest retention forces. CONCLUSION: The different tack designs altered the penetration and holding forces. The retention forces of the proposed peak design led to a significant increase in the retention forces that were more than twice as high as those in the original Heimann Model.

PROSPECTIVE RANDOMIZED TRIAL ASSESSING THE IMPACT OF FEEDBACK MECHANISMS ON PATIENT POSITIONING.

PURPOSE: To investigate the use of a head-fixed feedback sensor to improve good positioning times after macular hole or retinal detachment surgery. The instructional methods, macular hole closure rates, and questionnaire responses were also evaluated. METHODS: After randomization, sensor devices with different feedback types (none/acoustic/vibration) were fixed on the patients' heads. Two positioning recommendations (verbal/illustrated) were used. The posturing data were logged every 500 ms for 24 hours. RESULTS: Forty-eight data sets (24 per group) were evaluated. Using sensory feedback, the median time for face-down positioning after macular hole surgery (Group 1) was significantly boosted from 463 minutes (range: 61-1,168) to 1,257 minutes (range: 1,024-1,327). The side positioning time after retinal detachment surgery (Group 2) increased from a median of 1,032 minutes (range: 520-1,165) to 1,284 minutes (range: 1,231-1,437). The night-time alarm records were reduced; however, the instructional methods exhibited no noteworthy effects. The questionnaires indicated positive acceptance of the sensors. CONCLUSION: Sensory feedback may help in cases where face-down or side positioning is recommended. These constant reminders were superior to verbal or written reminders; however, further studies are required to assess the clinical impact of sensory feedback on patient positioning.

New digital methods: remodelling the harms tangent scale setting.

PURPOSE: The aim of this study is to present a feasibility study to evaluate and compare a self-constructed, digital-based tangent scale using the Harms tangent screen test for the examination of patients' strabismus angles. METHODS: The documented strabismus angles of 10 patients were measured at nine gaze-positions and compared using the Harms tangent screen test (Harms Screen) and a self-constructed digital screen test (Digital Screen). The primary outcome is the difference in the measured angles between both methods. Secondary outcomes include the duration of the examinations and the diagnostic conclusion based on the results of the measurements. RESULTS: The datasets from 10 of 13 patients were used for comparisons (median age 52 years; females 5; males 5). All measurements showed a mean of 1.77° (SD 2.95°) in horizontal deviations and 1.11° (SD 2.23°) in vertical deviations. The two methods showed differences (> 5°) in 12 cases (13.3%) for horizontal angle measurements and in four cases (4.4%) for vertical angle measurements. The median examination time was 238 s (range 60-430 s) for the Harms Screen and 150 s (range 120-600 s) for the Digital Screen tests. The diagnostic conclusions were identical for both methods. CONCLUSIONS: The feasibility of the experimental digital setup in principle is shown. Despite some deviations in the measured strabismus angles, the resulting diagnosis of the paresis was identical. The new method showed a reduction of examination time.

A new digitised screen test for strabismus measurement.

PURPOSE: Our study presents a digitised tangent screen test for ocular motility analysis according to the Harms and Hess tests (measurement of the squint angle in all fields of vision). This test uses an image beamer to display the tangent screen, a position sensor to measure the patient's head orientation, and a distance sensor to measure the fixation distance. Digital measurement of head orientation allows for a test procedure that eliminates the conventional requirement for a light pointer in the patient's hand. Thus, the digital screen test is presented, and the uncertainty of the measurement system is evaluated. METHODS: A mathematical relationship was given between the measured squint angles, as well as the angle of diagnostic gaze direction, and the influence quantities on their measurement uncertainty. The individual uncertainties resulted from deviations in the measured values by the position and distance sensors, the calibration of the projection image of the beamer in length units, and the finite image resolution of the beamer. The individual standard uncertainties of the influence quantities were determined. The combined standard measurement uncertainties of the squint and gaze direction angles were given based on the model equation of the error propagation law at the tangent table according to Harms at a test distance of 2.5 m. The patient's uncertainty contribution to the mobility analysis was not considered. RESULTS: The combined standard uncertainty of the measurement system (coverage factor k = 2 for 95% confidence level) for the squint angle is ≤ 0.43° for the angle of diagnostic gaze direction ≤ 3.13° at the test distance of 2.5 m. The individual standard uncertainties of the influence quantities on the angles are (k = 1): 1.55°/1.01° (horizontal/vertical angle of the position sensor), 0.19° (distance sensor), 0.06° (calibration of the projection image of the beamer), and 0.02° (image resolution of the beamer). The maximum valid test distance of the digital screen test is 3.8 m. CONCLUSION: The digital screen test is compact and can be used at different locations. Compared to the traditional test, the time required for examination via the digitised test is less; additionally, its documentation is simplified. The measurement uncertainty of the diagnostic gaze direction angle is dominated by the sensor drift of the position sensor in the horizontal direction (yaw angle) and is due to the sensor technology. However, this drift error does not affect the squint angle measurement result nor its measurement uncertainty because the measurement principle used here is based on the congruence between the position cross and the fixation object and the confusion principle and compensates for the drift error. The measurement uncertainties of the determined measurement system are the lower limits of the uncertainties in the clinical use of the digital screen test if there are no effects due to significant patient deviations.

Extended measuring depth dual-wavelength Fourier domain optical coherence tomography.

In this work an enhanced wide range dual band spectral domain optical coherence tomography technique (SD-OCT) is presented to increase the depth and accuracy of the measurement of optical A-scan biometry. The setup uses a Michelson interferometer with two wide-spectrum Superluminescent Diodes (SLD). The emissions of the SLDs are filtered by a long-pass filter (900 nm) in front of the reference mirror. The light is spectrally decomposed using a single reflective diffraction grating (1,800 lines/mm) and the whole spectrum captured with two CCD line sensors. The capabilities of the system have been validated using a self-made human model eye.

Comparison of spherical and aspherical intraocular lenses with decentration and tilt error using a physical model of human contrast vision and an image quality metric.

PURPOSE: In this study, two intraocular lenses (spherical IOL SA60AT and aspherical IOL SN60WF) are examined in an eye model under conditions of misalignment (defocus, decentration and tilt). The lenses are rated using the contrast sensitivity function (CSF) based on Barten's physical model. The square root integral (SQRI) method is used as a quality criterion comparable to the subjective image quality assessment of the human eye. METHODS: The IOLs to be tested are decentered from 0 to 1mm and tilted from -5 to +5 degrees in the Navarro eye model (optimized for far-point 6m and pupil aperture 3mm). The defocus of the IOLs is ±0.1mm at the anterior chamber depth (ACD). The optical modulation transfer function (MTF) is simulated with a ray tracing program. The SQRI is calculated using this MTF and the Barten CSF model (for in-focus at aperture 3 and 4.5mm and for defocus at 3mm). RESULTS: With increasing decentration, the spherical IOL shows a significantly smaller loss of quality for both apertures compared to the aspherical lens. With an aperture of 4.5mm, the image quality of the aspherical IOL is better for small decentration and tilt. The loss of quality of the spherical IOL increases with increasing tilt in both directions. In contrast, the image quality of the aspherical IOL is reduced under decentration for certain tilt values. For ACD-0.1mm, both IOLs behave similarly to the in-focus situation. For ACD+0.1mm, the influence of tilt without decentration is small for both IOLs. With increasing decentration, the quality loss of the aspherical IOL is similar to that in-focus and greater than that of the spherical lens. CONCLUSION: In general, under the same conditions the spherical SA60AT displays a lower tolerance in loss of quality of subjective vision with lens alignment errors, in comparison to the aspherical SN60WF, limited by certain combinations of decentration and tilt according to this study. This study shows a way to evaluate IOLs based on the subjective visual performance of the eye.

Real ray tracing simulation versus clinical outcomes of corneal excimer laser surface ablations.

PURPOSE: To investigate the recently reported discrepancy between theoretical expectations and clinical outcomes of corneal ablations after excimer laser corneal refractive surgery. METHODS: Thirty-four eyes of 25 patients who had laser epithelial keratomileusis (LASEK) for myopia (mean -6.26±2.52 diopters [D], full correction diameter 6.71±0.25 mm) without astigmatism correction were assessed. Based on preoperative topographies, corneal ablation was simulated using the Munnerlyn formula--one with homogeneous beam fluence and another with variable beam fluence (including reflection loss and non-normal laser beam incidence). Corneal shape was analyzed pre- and postoperatively. Corneal wavefront aberration was calculated with real ray tracing and reported according to the Optical Society of America standard. RESULTS: Corneal asphericity for a 6.0-mm diameter showed that preoperative measured corneas (-0.21±0.11) and postoperative simulated corneas (homogeneous beam fluence -0.32±0.19, variable beam fluence -0.41±0.22) were prolate, whereas postoperative measured corneas (0.40±0.57) were oblate. Corneal wavefront aberration (higher order aberrations 3rd to 6th/spherical aberration Z[4,0] in microns) for a 6.0-mm diameter, compared to the preoperative state (0.34±0.19/0.15±0.09), did not increase in postoperative simulation (homogeneous beam fluence 0.29±0.15/0.05±0.08, variable beam fluence 0.34±0.14/0.16±0.08), but in contrast increased for postoperative measurements (0.64±0.17/0.49±0.15). CONCLUSIONS: The increase in oblateness, higher order aberrations, and spherical aberration of real patients' postoperative corneas was not seen in simulations using a Munnerlyn ablation profile. The fluence loss of the laser was one important factor, but did not explain all increasing aberrations observed clinically. We hypothesize that corneal wound healing and biomechanics play a role.

The Cosine Similarity Technique: A new method for smart EXCIMER laser control.

PURPOSE: To introduce additional steps towards smart laser control in eye surgery, with the use of the cosine similarity technique to analyze the spectra of organic polymers obtained using non-contact photoacoustic spectroscopy (NCPAS). METHODS: The experiments were performed with two organic polymers: polyethylene and polyamide. A 193 nm excimer laser was used for photoablation at a repetition rate of 200Hz. The resulting acoustic signal of the ablation process was recorded by a capacitor microphone and then preamplified and digitized. For each specimen, four measurements with 1000 single pulses were taken. The cosine similarity technique was then used to compare the spectra of the polymers. The performance of the discrimination technique was evaluated by receiver operating characteristic analysis. RESULTS: It was possible to correctly recognize a material with a probability of approximately 98% using the cosine similarity technique at a laser repetition rate and recording rate of 200 Hz, which represents the acoustic signal of one laser pulse. CONCLUSIONS: The determination of materials with the cosine similarity method (CSM) is a fast, precise and promising approach towards smart laser control. Additional steps could include the design of a database containing generic spectra, using higher repetition rates, and the combination of NCPAS results with the position of the laser beam.

A systematic comparison and evaluation of three different Swept-Source interferometers for eye lengths biometry.

This study reviews the development of Swept-Source interferometers and compares systematically three different Swept-Source interferometer designs for biometric measurements of the eye. Principles characteristics, conveniences and accessibilities of the three developed systems are presented. The main difference between the three Swept-Source systems is the method of tuning the wavelength at the broadband optical amplifier. The implementation of a "quasi-phase-continuous method" (QPC) for wavelength tuning led to longer measuring depth but was more time-consuming. The wavelength tuning using a rotating polygon mirror scanner was faster. The wavelength tuning via Fourier Domain Mode Locking (FDML), where the tuning frequency ft of the filter must be matched to the inverse cavity roundtrip time τ, achieved the widest tuning range combined with a rather better resolution and signal to noise ratio (SNR). The swept sources were compared using a fiber-optic based Michelson interferometer setup. Measurements of a self-made human model eye demonstrate excellent capturing of the biometric data, with all interfaces of eye optical components and their contours being clearly detected.

The anatomy of the foveola reinvestigated.

OBJECTIVE: In the foveola of the eye, photoreceptors and Müller cells with a unique morphology have been described, but little is known about their 3D structure and orientation. Considering that there is an angle-dependent change in the foveolar photoreceptor response for the same light beam, known as the Stiles Crawford Effect of the first kind (SCE I), which is still not fully understood, a detailed analysis of the anatomy of the foveolar cells might help to clarify this phenomenon. METHODS: Serial semithin and ultrathin sections, and focused ion beam (FIB) tomography were prepared from 32 foveolae from monkeys (Macaca fascicularis) and humans. Foveolae were also analyzed under the electron microscope. Serial sections and FIB analysis were then used to construct 3D models of central Müller and photoreceptor cells. In addition, we measured the transmission of collimated light under the light microscope at different angles after it had passed through human foveae from flat mounted isolated retinae. RESULTS: In monkeys, outer segments of central foveolar cones are twice as long as those from parafoveal cones and do not run completely parallel to the incident light. Unique Müller cells are present in the central foveolae (area of 200 µm in diameter) of humans and monkeys. Light entering the fovea center, which is composed only of cones and Müller cells, at an angle of 0° causes a very bright spot after passing through this area. However, when the angle of the light beam is changed to 10°, less light is measured after transpasssing through the retina, the foveolar center becomes darker and the SCE-like phenomenon is directly visible. Measurements of the intensities of light transmission through the central foveola for the incident angles 0 and 10° resemble the relative luminance efficiency for narrow light bundles as a function of the location where the beam enters the pupil as reported by Stiles and Crawford. The effect persisted after carefully brushing away the outer segments. CONCLUSION: We show that unique cones and Müller cells with light fibre-like properties are present in the center of the fovea. These unique Müller cells cause an angle dependent, SCE-like drop in the intensity of light guided through the foveola. Outer segments from the foveolar cones of monkeys are not straight.

Wavelength-dependent scattering in human eye with cataracts.

The gradual process in which the crystalline lens is cloudy due to the appearance of elements giving rise to variations in the refractive index is known as cataract. Clinical assessment is usually complicated because it considers patient's perception, and individuals with similar development have different visual deficits. This work presents a model which considers the fluctuations in the refractive index as spherical particles produce measurable scatter radial profiles patterns on the retina. Measurements for 2 different wavelengths simultaneously provide information on particle size and a quantitative assessment by measurement of the fluctuations of the refractive index.

Calculating intraocular lens geometry by real ray tracing.

PURPOSE: An implementation of real ray tracing based on Snell's law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs). METHODS: The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae. RESULTS: Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11 +/- 0.43 diopters (D), cylinder -0.18 +/- 0.52 D, and axis 5.13 degrees +/- 30.19 degrees. Pearson's correlation coefficient was sphere r = 0.92, P < .01; cylinder r = 0.79, P < .01; and axis r = 0.91, P < .01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae. CONCLUSIONS: It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history.

First results of automated RAPD-SWIFT method in dynamic pupillometry.

BACKGROUND: This paper presents preliminary observations on the use of a commercial pupillometric instrument (Albomed PupilX) for the detection and quantification of Relative Afferent Pupillary Defect (RAPD). In this pilot study, video-based pupillometry was used in conjunction with calibrated LED illumination to simulate the effects of the traditional swinging-flashlight test using neutral density filters. METHODS: The results presented in this study follow a method described by Wilhelm et al. (Tübingen SWIFT-test) in which the eyes are illuminated alternately and the response in pupil diameter measured by video pupillometry. Using the PupilX instrument, the LED intensity can be programmed in logarithmic steps starting from a maximum intensity of 1000 lux (lx), with each reduction of 50% in illumination intensity corresponding to a 0.3 log-units increase in filter density. RESULTS: The eyes were stimulated unilaterally with illumination intensities corresponding to a neutral density range of 0.0 to 0.9 log-units. In all normal subjects a symmetrical pupil reaction was seen, independent of which eye was stimulated. In contrast, in a subject with known RAPD, a clear asymmetry in the reaction to stimulation of the left and the right eyes was seen. CONCLUSIONS: These measurements were compared with typical results from the original Tübingen SWIFT study and good qualitative agreement was seen. Furthermore, the method can clearly differentiate between healthy subjects and those with a known RAPD, indicating that the PupilX, programmed with specific stimulus sequences and in conjunction with a suitable analysis software, has the potential for recognition and quantification of RAPD, and prompting the suggestion for further study involving a range of patients including both normal subjects and those with a known and quantified RAPD.

Diode laser thermal keratoplasty for hyperopia and hyperopic astigmatism in patients younger than 40 years.

PURPOSE: We performed a prospective, nonrandomized investigation of contact continuous wave diode laser thermal keratoplasty (DTK) for correction of hyperopia and hyperopic astigmatism. The goal of surgery was not to achieve emmetropia but to investigate the refractive effect in patients younger than 40 years. METHODS: Twenty eyes with spherical hyperopia (Group A) and 15 eyes with hyperopic astigmatism (Group B) were treated with two rings; treatment zone diameter of the inner ring was 6 or 7 mm. Each ring consisted of eight spots for hyperopia correction; for astigmatism correction four additional pairs of spots were applied around the flat corneal meridian. RESULTS: Mean change in manifest spherical equivalent refraction 18 months postoperatively in Group A was 2.00 +/- 0.90 D (Group B, 15 mo, 1.80 +/- 0.60 D). Mean increase in keratometric power was 1.20 +/- 0.60 D (1.30 +/- 0.60 D). Mean refractive astigmatism reduction was 0.10 D (1.70 D). Mean paired differences per month for regression between spherical equivalent manifest refraction/keratometric power was 0.12/0.04 D (0.06/0.05 D) from 9 to 12 months, 0.01/0.04 D (0.06/0.06 D) from 12 to 15 mo, and -0.03/ +/- 0 D from 15 to 18 months. Mean uncorrected visual acuity improved from 20/100 to 20/32 (20/63 to 20/32). No eye lost more than 1 line of BSCVA. The amount of correction showed an age dependency. CONCLUSION: DTK is a minimally invasive, low-risk procedure, and was effective for correction of low hyperopia and low to moderate hyperopic astigmatism in patients less than 40 years.

Length measurement of the eye using a swept-source interferometer.

In this article, a swept-source setup based on a semiconductor optical amplifier at the central wavelength of 1050 nm for measurements of the axial length inside the eye is presented. The large coherence length is achieved using a tunable optical filter, consisting of a reflective diffraction grating, two Littrow prisms, and a scanner. It was possible to achieve a coherence length of 40 mm, which allowed interference measurements in this range to be made. Measurements of the biomedical data of a human model eye are presented. To realize faster measurements, the data acquisition must be increased to more than 2 MS/s. In further studies, the usage of a field-programmable gate array, to achieve faster measurements, will be considered.

TD-PCI system based on an angle modulation for in vivo measurements of the human eye.

In this study, a "time domain" system based on a partial coherence interferometry method is presented. The classic technique of varying the reference arm using a linear motor is replaced by the use of a rotating glass cube. The theoretical definition of the variation of the optical path length and first measurements in a human model eye and a real human eye are presented.

Biometric measurements inside the model eye using a two wavelengths Fourier domain low coherence interferometer.

We present a setup to measure biometric data of the eye using Fourier domain interferometry. The measuring depth of a Fourier domain system is basically limited owing to the spectral resolution. Combining two spectral domain interferometers with different wavelength ranges creates two measurement sections and allows for a simultaneous biometric measurement in terms of corneal thickness, anterior chamber depth, and axial length. The necessary offset between both sections in the combined setup was calibrated with a known reference object. The setup was tested by measuring a self-constructed model eye. All biometric data of the model eye can be detected simultaneously. This system has a precision of 13 µm (standard deviation) and a trueness of 46 µm. The signal-to-noise ratio was 98 dB for the anterior part and 76 dB for the posterior part. In contrast to time domain interferometry, this setup does not need any mechanically moving parts. Owing to the short time frame of the biometric measurement, potential eye movements should have no influence on the result. In addition to the fast measurement, this setup provides the possibility to adjust the laser power of both sections independently. This could help in the case of dense cataract.

Customized aspheric intraocular lenses calculated with real ray tracing.

PURPOSE: To calculate the exact geometry of custom intraocular lenses (IOLs) for pseudophakic eyes and theoretically predict the residual wavefront error by real ray tracing based on Snell's law. SETTING: Centre for Ophthalmology, University Hospital, Tübingen, Germany. METHODS: Individual computer models were constructed based on measurements, including corneal topography and axial length. The geometry of custom spherical, aspheric, toric, and toric aspheric IOLs was calculated in an optimization process with real ray tracing to provide the minimum root mean square wavefront error. The geometric optical properties in terms of residual wavefront error was simulated and approximated by Zernike polynomials. RESULTS: Data from 45 pseudophakic eyes were used to construct the models. Defocus was almost completely corrected by the spherical IOL and astigmatism, by the toric IOL. The aspheric IOL strongly reduced spherical aberration but only slightly reduced total higher-order aberrations (HOAs); both theoretical predictions corresponded to clinical investigations of wavefront measurements in pseudophakic eyes with a spherical or aspheric IOL. CONCLUSIONS: Real ray tracing calculated the exact geometry of custom IOLs to provide the minimum wavefront error, going beyond simple diopter information. Results show spherical aberration can be significantly reduced with aspheric IOLs. However, the limited possible reduction of total HOAs, even perfectly positioned custom aspheric IOLs, may be a reason for the unclear results in studies assessing the potential benefit to visual performance of currently used aspheric IOLs.

[Measurement of intraocular pressure using the Tono-Pen in comparison with Goldmann applanation tonometry - a clinical study in 100 eyes]. / Messung des Augeninnendrucks mit dem Tono-Pen im Vergleich zum Applanationstonometer nach Goldmann - eine klinische Studie an 100 Augen.

BACKGROUND: In clinical practice ophthalmologists often need a tonometer which is independent of a slit lamp. Such a hand-held device is the Tono-Pen. We compared the precision of two equal Tono-Pens with Goldmann applanation tonometry. MATERIAL AND METHODS: Measurement of intraocular pressure (IOP) was done in 100 eyes of 51 patients (mean age 63 +/- 15 years) suffering from ocular hypertension or glaucoma. According to a random table either the right or left eye was measured using Goldmann tonometer first and the Tono-Pen second. For the other eye the measurement was reversed. One of the two equal Tono-Pens (Solan/USA) was used according to a second random table. Three measurements were obtained with each instrument on both eyes within 15 minutes subsequently. Patients were placed in an upright position for all measurements. RESULTS: Even for well-trained ophthalmologists a learning curve of approximately 10 measurements was observed using the Tono-Pen. The Tono-Pen measured an average IOP of 16.9 mm Hg in all 100 eyes. The Goldmann tonometer measured an average IOP of 17.7 mm Hg. The difference was not statistically significant. The standard deviation for all measurements was better for the Tono-Pen (4.7 mm Hg vs 5.8 mm Hg for Goldmann tonometer). No reduction of the IOP after Tono-Pen measurement was observed (in contrast to the Goldmann tonometer). The reproducibility of the Tono-Pen on the same eye was inferior to the Goldmann tonometer by a factor of 2. There was an almost significant difference in reproducibility between two equal Tono-Pens. CONCLUSIONS: Measurement of IOP with the Tono-Pen is comparable to Goldmann applanation tonometry if an average of 3 measurements is used. The difference between two equal Tono-Pens indicates the need for improvement of the quality check during production.