Klaus Ehrmann

Corneal and conjunctival sensitivity to air stimuli.

Aims: To determine the repeatability of ocular surface sensitivity to mechanical stimulation using air stimuli and the effect of contact lens (CL) wear on sensitivity. Methods: Repeatability: 14 subjects (24–39 years) participated. Mechanical sensitivity to warmed (34°C) and ambient (20°C) air was measured for the central cornea (CC), inferior cornea (IC), and inferior conjunctiva (ICON). Measurements were taken on 12 days; six morning and six afternoon measurements. Differences between sites, time of day, and stimulus temperature were evaluated. CL wear: 10 subjects (22–30 years) participated. Measurements were taken at the same time of day, either following no wear, wear of a CL of oxygen permeability [Dk] of 28×10−9 [cm/s][ml O2/ml mm Hg] or wear of a CL of Dk 140×10−9 [cm/s][ml O2/ml mm Hg]. Differences between sites and wear conditions were evaluated. Results: Repeatability: Sensitivity varied between sites (p<0.01), time of day (p<0.05), and stimulus temperatures (p<0.01). There were no significant differences between days. Mean thresholds for eye temperature stimuli were; CC 64.4 (SD 28.6) ml/min; IC 84.6 (40.0) ml/min; ICON 120.6 (40.4) ml/min and for ambient temperature stimuli were CC 53.9 (16.0) ml/min, IC 59.0 (20.0) ml/min; ICON 72.6 (43.7) ml/min. CL wear: Sensitivity varied between sites and wear conditions (p<0.05). Conjunctival sensitivity was increased after wear of highly oxygen permeable CLs but unaffected by wear of low oxygen permeable CLs. Conclusions: The prototype gas aesthesiometer is able to repeatably measure ocular surface sensitivity and measurements are consistent with previously reported techniques.

A review of peripheral refraction techniques.

The recently discovered link between myopia development and peripheral refraction has triggered a stream of clinical and animal investigations to confirm the theory and to understand the underlying mechanisms. For this, precise peripheral refractometry has now gained importance in myopia research. For more than 70 years, many researchers have measured off-axis refraction of the eye in horizontal and sometimes vertical meridians over a range of angles, using several modified refraction techniques. These techniques varied not only with respect to the instrumentation used, but also the modifications performed to enable off-axis refraction. Modifications included either head or eye turn of the participant with respect to the peripheral angle tested or rotation of the instrument itself around the center of the eye. The main focus of this study is to review and compare all refraction techniques for off-axis measurements including necessary modifications made to equipment or procedures. Because a difference in instrumentation and techniques potentially limits the comparability of reported results, it is of particular importance to understand all the details of the particular refractometric technique chosen and any potential problems. Difficulties relating to all the methods are highlighted to provide information on preference and usefulness of certain peripheral refraction techniques for future technology and research work. All refraction techniques exhibited similar drawbacks, such as off-axis fixation, protraction when many peripheral angles were tested, and difficulties to obtain reliable measurements at large peripheral angles. Yet, from all the methods reviewed, the Shin-Nippon NVision K5001 open field autorefractor and the Hartmann-Shack wavefront sensor technique seem to be the most useful commercially available instruments to measure peripheral refraction.

A 3D optical profilometer using a compact disc reading head

The design, construction and performance of a novel optical profilometer based on the laser reading head of a compact disc player is described. The instrument is capable of self-guided scanning along arbitrary contours by utilizing the focus and radial tracking signals from the sensor head. The sensor is mounted on an x, y, motorized stage. The radial tracking signal provides the feedback for the angular position of the sensor, ensuring that the optical axis of the sensor is always perpendicular to the profile preventing the signal loss that occurs in conventional profilometers due to deflection of the light beam. The focus signal has a sensitivity in the nanometre range which makes the precision of the stages used to perform the scanning movements the dominant limiting factor for the measuring accuracy. The sensor head intermittently follows the contour, collecting datapoints at a predetermined lateral resolution. The profile is digitized in the form of x, y, and coordinates for each scanpoint. The accuracy of the instrument was verified by scanning standard measures with flat and circular cross sections. The mechanical structure of the prototype instrument was purpose built for the measurement of rigid corneal contact lenses. Various lenses have been measured and profile graphs are illustrated.

Corneal Mechanical Thresholds Negatively Associate With Dry Eye and Ocular Pain Symptoms.

Purpose To examine associations between corneal mechanical thresholds and metrics of dry eye. Methods This was a cross-sectional study of individuals seen in the Miami Veterans Affairs eye clinic. The evaluation consisted of questionnaires regarding dry eye symptoms and ocular pain, corneal mechanical detection and pain thresholds, and a comprehensive ocular surface examination. The main outcome measures were correlations between corneal thresholds and signs and symptoms of dry eye and ocular pain. Results A total of 129 subjects participated in the study (mean age 64 ± 10 years). Mechanical detection and pain thresholds on the cornea correlated with age (Spearmans ρ = 0.26, 0.23, respectively; both P < 0.05), implying decreased corneal sensitivity with age. Dry eye symptom severity scores and Neuropathic Pain Symptom Inventory (modified for the eye) scores negatively correlated with corneal detection and pain thresholds (range, r = −0.13 to −0.27, P < 0.05 for values between −0.18 and −0.27), suggesting increased corneal sensitivity in those with more severe ocular complaints. Ocular signs, on the other hand, correlated poorly and nonsignificantly with mechanical detection and pain thresholds on the cornea. A multivariable linear regression model found that both posttraumatic stress disorder (PTSD) score (β = 0.21, SE = 0.03) and corneal pain threshold (β = −0.03, SE = 0.01) were significantly associated with self-reported evoked eye pain (pain to wind, light, temperature) and explained approximately 32% of measurement variability (R = 0.57). Conclusions Mechanical detection and pain thresholds measured on the cornea are correlated with dry eye symptoms and ocular pain. This suggests hypersensitivity within the corneal somatosensory pathways in patients with greater dry eye and ocular pain complaints.

Power profiles of single vision and multifocal soft contact lenses.

PURPOSE The purpose of this study was to investigate the optical zone power profile of the most commonly prescribed soft contact lenses to assess their potential impact on peripheral refractive error and hence myopia progression. METHODS The optical power profiles of six single vision and ten multifocal contact lenses of five manufacturers in the powers -1.00 D, -3.00 D, and -6.00 D were measured using the SHSOphthalmic (Optocraft GmbH, Erlangen, Germany). Instrument repeatability was also investigated. RESULTS Instrument repeatability was dependent on the distance from the optical centre, manifesting unreliable data for the central 1mm of the optic zone. Single vision contact lens measurements of -6.00 D lenses revealed omafilcon A having the most negative spherical aberration, lotrafilcon A having the least. Somofilcon A had the highest minus power and lotrafilcon A the biggest deviation in positive direction, relative to their respective labelled powers. Negative spherical aberration occurred for almost all of the multifocal contact lenses, including the centre-distance designs etafilcon A bifocal and omafilcon A multifocal. Lotrafilcon B and balafilcon A seem to rely predominantly on the spherical aberration component to provide multifocality. CONCLUSIONS Power profiles of single vision soft contact lenses varied greatly, many having a negative spherical aberration profile that would exacerbate myopia. Some lens types and powers are affected by large intra-batch variability or power offsets of more than 0.25 dioptres. Evaluation of power profiles of multifocal lenses was derived that provides helpful information for prescribing lenses for presbyopes and progressing myopes.

Inherent ocular spherical aberration and multifocal contact lens optical performance.

Purpose. The role of inherent spherical aberration (SA) in the optical performance of presbyopic eyes corrected with simultaneous vision multifocal contact lenses was investigated. Methods. Presbyopic schematic eyes were modeled with partial accommodative function to represent 45- and 55-year olds and were further classified into five categories based on their magnitude of inherent SA. Two representative ametropic models of each category were corrected with four multifocal contact lens iterations. High-add designs were used to correct 55-year olds, whereas low-add designs served 45-year ones. The overall performances were gauged in terms of visual Strehl ratio and area under through-focus modulation transfer function. Results. The root mean square error of higher order aberrations of the eye and correcting lens combination were significantly different (p < 0.05) within the five inherent SA models, for all pupils and accommodative states. Area under through-focus modulation transfer function at all three spatial frequencies tested was found to be significantly different (p < 0.05) within the five SA models. Visual Strehl ratio measures were also different but statistically insignificant. Eyes having the same refractive prescriptions but diverse levels of inherent SA perform differently even when corrected with identical multifocal designs, and the performance is dependent on pupil size and level of residual accommodation. Overall, the distinct performances within the five SA models were optically relevant for pupils ∼4 mm and greater. Among the designs investigated, the low-add multizone iteration demonstrated performance relatively independent of the inherent SA because of the favorable interactions of defocus with primary, secondary, and tertiary SA. Conclusions. These findings confirm that the coupling of ocular SA and correcting lens aberrations contributes to the multifocal functionality.

A NOVEL INSTRUMENT TO QUANTIFY THE TENSION OF UPPER AND LOWER EYELIDS

An instrument was developed for the objective measurement of passive lid tension of the upper and lower eyelid. Eyelid tension is defined as the first derivative of the force over elongation function. Lid tension was determined by gripping the eyelid at the eyelashes and displacing the lid while measuring the force exerted by the eyelid. Two motorised spindles provided motion control of the gripper in anterior/posterior and nasal/temporal directions. Each axis was fitted with a sensitive force transducer to record the force while stretching and relaxing the eyelid. Motion control, data collection and data analysis were computer controlled, with various user-selectable options for measurement speed, direction and end-of-measurement conditions. Calibration of the instrument reduced the maximum error of the recorded forces to +/- 10 mN. Repeatability for on-eye measurements was verified by multiple measurements on the same eye under identical conditions. Preliminary results from a pilot study, investigating differences between Asian and Caucasian lid tension showed no statistically significant difference between the two groups.

Evaluation of porcine crystalline lenses in comparison with molded polymer gel lenses with an improved ex vivo accommodation simulator

The ex vivo accommodation simulator stretches crystallin e lenses radially under controlled conditions, while monitoring the changes in zonular tension and lens di ameter. A dual coaxial laser beam system was added for measuring changes in optical power as the lens is stretched. The EVAS instrument was fully characterized and calibrated. The results from 3 porcine eyes will be presented. Several crystalline lenses, similar in shape and size to porcine lenses, were cast molded in silic one gels of different stiffness. These lenses were evaluated using the EVAS instrument and custom made lens holding fixtures. With their well defined shape and material properties, results from these measurements will be useful for cross-validation of finite element models.

Biomechanical analysis of the accommodative apparatus in primates

Background:  The restoration of natural accommodation in the presbyopic and cataract affected eye is a subject of intense research effort. A new instrument has been developed to test the viability and efficacy of procedures and methods to restore accommodation ex vivo in animal or human eyes.

Lens parameter changes under in vitro and ex vivo conditions and their effect on the conjunctiva

PURPOSE To quantify changes in contact lens parameters induced by lens wear and determine whether these changes are associated with contact lens-induced conjunctival staining (CLICS). METHODS In vitro: Lens diameter, sag, edge shape, base curve of six contact lens brands (balafilcon, comfilcon, etafilcon, lotrafilcon B, omafilcon and senofilcon) measured at 21°C and 35°C (eye temperature). Ex vivo: Diameter of lenses collected from a prospective, randomised, contra-lateral, cross-over clinical trial from 36 subjects wearing all lens types for 1 week daily wear, measured in 35°C PBS after removal. Ocular surface was examined for lens-induced conjunctival staining by masked examiner. RESULTS In vitro: Changes in diameter and base curve outside ISO tolerance were found with etafilcon A and omafilcon A. Ex vivo: Comfilcon A and etafilcon A had greatest shrinkage in diameter (0.18mm) and base curve (0.11mm steeper) with temperature increase from 21°C to 35°C. Senofilcon A, lotrafilcon B and balafilcon A maintained most stable parameters between 21°C and 35°C. Changes in diameter and base curve from lens wear were not correlated with CLICS (p>0.49). Multivariate analysis showed significantly greater levels of lens induced staining were associated with lens modulus (p<0.001) and knife (p<0.001) and chisel (p<0.001) edge shapes. CONCLUSIONS Parameter changes induced by lens wear were associated with increasing temperature, but these changes in lens diameter and base curve did not induce CLICS. Modulus and edge shape were associated with increased CLICS. The susceptibility of etafilcon A and omafilcon A lenses to parameter changes might be related to their high water content.