Eef van der Worp

Simulated Optical Performance of Custom Wavefront Soft Contact Lenses for Keratoconus

Purpose. Outstanding improvements in vision can theoretically be expected using contact lenses that correct monochromatic aberrations of the eye. Imperfections in such correction inherent to contact lenses are lens flexure, translation, rotation, and tear layer effects. The effects of pupil size and accommodation on ocular aberration may cause further difficulties. The purpose of this study was to evaluate whether nonaxisymmetric soft contact lenses could efficiently compensate for higher-order aberrations induced by keratoconus and to what extent rotation and translation of the lens would degrade this perfect correction.. Methods. Height topography data of nine moderate to severe keratoconus corneas were obtained using the Maastricht Shape Topographer. Three-dimensional ray tracing was applied to each elevation topography to calculate aberrations in the form of a phase error mapping. The effect of a nonaxisymmetric soft contact lens tailored to the corneal aberrations was simulated by adding an opposite phase error mapping that would theoretically compensate all corneal-induced optical aberrations of the keratoconus eyes. Translation (0.25, 0.5, 0.75, and 1.0 mm) and rotation (2.5°, 5.0°, 7.5°, and 10°) mismatches were introduced. The modulation transfer function (MTF) of each eye with each displaced correction and with various pupil sizes (3, 5, and 7 mm) was deduced from the residual phase error mapping. A single performance criterion (mtfA) was calculated as the area under the MTF over a limited spatial frequency range (5 to 15 periods per degree). Finally, the ratio (RmtfA) of corrected mtfA over uncorrected mtfA provided an estimate of the global enhancement in contrast sensitivity with the customized lens. Results. The contrast improvement ratios RmtfA with perfectly located lenses were for an average pupil size of 4.5 mm between 6.5 and 200. For small translation errors (0.25 mm), RmtfA ranged between 2 and 7. The largest lens translation tested (1 mm) often resulted in poorer performance than without correction (RmtfA <1). More than threefold improvements were achieved with any of the angular errors experimented. RmtfA values showed significant variations for pupil diameters between 3 and 7 mm.. Conclusions. Three-dimensional aberration-customized soft contact lenses may drastically improve visual performance in patients with keratoconus. However, such lenses should be well positioned on the cornea. In particular, translation errors should not exceed 0.5 mm. Angular errors appeared to be less critical. It is further questioned whether the visual system is able to adapt to variations in optical performance of the correction in situ due to lens positioning and pupil size.

Demographics of international contact lens prescribing

Knowledge of differences in the demographics of contact lens prescribing between nations, and changes over time, can assist (a) the contact lens industry in developing and promoting various product types in different world regions, and (b) practitioners in understanding their prescribing habits in an international context. Data that we have gathered from annual contact lens fitting surveys conducted in Australia, Canada, Japan, the Netherlands, Norway, the UK and the USA between 2000 and 2008 reveal an ageing demographic, with Japan being the most youthful. The majority of fits are to females, with statistically significant differences between nations, ranging from 62 per cent of fits in Norway to 68 per cent in Japan. The small overall decline in the proportion of new fits, and commensurate increase in refits, over the survey period may indicate a growing rate of conversion of lens wearers to more advanced lens types, such as silicone hydrogels.

International rigid contact lens prescribing.

Rigid lenses have been fitted less since the introduction of soft lenses nearly 40 years ago. Data that we have gathered from annual contact lens fitting surveys conducted in Australia, Canada, Japan, the Netherlands, Norway, the UK and the USA between 2000 and 2008 facilitate an accurate characterization of the pattern of the decline of rigid lens fitting during the first decade of this century. There is a trend for rigid lenses to be utilized primarily for refitting those patients who are already successful rigid lens wearers-most typically older females being refit with higher Dk materials. Rigid lenses are generally fitted on a full-time basis (four or more days of wear per week) without a planned replacement schedule. Orthokeratology is especially popular in the Netherlands, but is seldom prescribed in the other countries surveyed.

Optimising RGP lens fitting in normal eyes using 3D topographic data.

PURPOSE To analyse, retrospectively, the effect of fitting characteristics on comfort of wear and the role that 3D topographic data can play in attaining an optimal lens fit with rigid gas permeable (RGP) contact lenses in normal eyes. METHODS Included were 60 normal myopic eyes (1.00-5.00 D) with astigmatism limited to 2.00 D. Lenses were ordered empirically, based on traditional fitting rules. RESULTS The initial fit based on traditional computation was accepted in 40% of the eyes. To achieve an acceptable fit, 15% of the eyes needed an adaptation of the back optic zone radius, in 28% switching from a multicurve to an aspheric lens design was indicated and in 17% a non-rotational symmetric lens design was favoured. The reason for changing the lens parameters could in 88% be attributed to mid-peripheral differences between corneal shape as found with 3D corneal topography. Average comfort of wear improved statistically significantly (chi(2), P<0.05) from 5.2 initially to 7.7 after 3 months in the group with optimal lens fits, while comfort slightly decreased (not significant) in the group with sub-optimal fits. Switching to a non-rotational symmetric lens design, when indicated, improved comfort significantly (chi(2), P<0.05) from 5.0 to 7.3. CONCLUSION In normal eyes, the measurement of corneal shape, especially in the mid-peripheral regions, is of importance to optimise RGP lens fit. Optimised lens fits increases comfort of wear.

Eyeblink Frequency and Type in Relation to3- and 9-O’Clock Staining and Gas Permeable Contact Lens Variables

Purpose. The primary goal of this study was to evaluate if there is a difference in frequency and in completeness of eyeblinks (type of eyeblink) in rigid gas permeable (GP) lens wearers experiencing 3- and 9-o’clock staining compared with GP lens wearers without substantial staining and nonlens wearers. Methods. The study involved 26 nonlens wearers and 55 GP contact lens wearers, with and without 3- and 9-o’clock staining. Every eyeblink over a 5-min period was categorized as a complete eyeblink, incomplete eyeblink, or an eyeblink attempt. In addition, type of lens fit (interpalpebral vs. lid attachment), lens-to-cornea fit (flat vs. steep), spherical equivalent of the refraction, and corneal coverage (on-eye lens diameter) were analyzed. Results. No difference in overall eyeblink frequency was found between nonlens wearers (group I, n = 26), GP lens wearers with less than grade 1 corneal staining (group II, n = 25) and GP lens wearers with grade 1 or more corneal staining (group III, n = 30). Fewer complete eyeblinks, more incomplete eyeblinks and more eyeblink attempts were seen in group III than in group I or II (p < 0.01, p = 0.03, p < 0.01, respectively). Fewer complete eyeblinks and more eyeblink attempts (p < 0.01 for both) were found in interpalpebral lens fits than in lid attachment fits. Fewer complete eyeblinks (p = 0.02) and more incomplete eyeblinks (p = 0.03) were found with lenses rated as large or optimal in size than with lenses rated as small. Conclusions. GP lens wearers experiencing 3- and 9-o’clock staining showed a different eyeblink frequency for individual types of eyeblinks, but not for overall eyeblinks. Fewer complete eyeblinks, more incomplete eyeblinks, and more eyeblink attempts were observed in GP wearers with 3- and 9-o’clock staining compared with wearers with minimal staining and nonwearers. In addition, some individual contact lens variables were associated with more incomplete eyeblinks.

Consensus on recording of gas permeable contact lens fit

PURPOSE To develop a new schematic scheme for efficiently recording the key parameters of gas permeable contact lens (GP) fits based on current consensus. METHODS Over 100 established GP fitters and educators met to discuss the parameters proposed in educational material for evaluating GP fit and concluded on the key parameters that should be recorded. The accuracy and variability of evaluating the fluorescein pattern of GP fit was determined by having 35 experienced contact lens practitioners from across the world, grading 5 images of a range of fits and the topographer simulation of the same fits, in random, order using the proposed scheme. The accuracy of the grading was compared to objective image analysis of the fluorescein intensity of the same images. RESULTS The key information to record to adequately describe the fit of an GP was agreed as: the manufacturer, brand and lens parameters; settling time; comfort on a 5 point scale; centration; movement on blink on a ±2 scale; and the Primary Fluorescein Pattern in the central, mid-peripheral and edge regions of the lens averaged along the horizontal and vertical lens axes, on a ±2 scale. On average 50-60% of practitioners selected the median grade when subjectively rating fluorescein intensity and this was correlated to objective quantification (r=0.602, p<0.001). Objective grading suggesting horizontal median fluorescein intensity was generally symmetrical, as was the vertical meridian, but this was not the case for subjective grading. Simulated fluorescein patterns were subjectively and objectively graded as being less intense than real photographs (p<0.01). CONCLUSION GP fit recording can be standardised and simplified to enhance GP practice.

Soft toric contact lens prescribing in different countries.

There have been significant improvements in toric soft contact lens design over the past decade. Data from our international contact lens prescribing survey were mined to assess recent trends in toric soft contact lens fitting. This survey was conducted by sending up to 1000 survey forms to contact lens fitters in Australia, Canada, Japan, the Netherlands, Norway, the UK and the USA each year between 2000 and 2009. Practitioners were asked to record data relating to the first 10 contact lens fits or refits performed after receiving the forms and to return them to us for analysis. The data revealed a gradual increase in the extent of toric soft lens fitting this century. Excluding Japan--which had a consistently low rate of soft toric lens fitting over the survey period--soft toric lenses now represent over 35% of all soft lenses prescribed; it can be assumed that, on average (and again excluding Japan), all cases of astigmatism 0.75 D or less remains uncorrected among contact lens wearers. Toric lenses are fitted more to those who are older, full-time wearers and reusable lens wearers, and less to those wearing silicone hydrogel and extended wear lenses.

Sagittal height differences of frequent replacement silicone hydrogel contact lenses

PURPOSE This study aims to evaluate the sagittal height differences among a selection of commercially available monthly or two weekly replacement silicone hydrogel soft lenses. METHODS The sagittal height (CL-SAG) of four frequent replacement silicone hydrogel lenses (lotrafilcon B, balafilcon A, comfilcon A and senofilcon A) was measured for all base curve radius manufactured in spherical (-3.00D and +3.00D) and toric (=C-0.75×180°) designs (11 spherical and 8 toric lenses in total). Two different lenses of each lens type were evaluated (the intra CL-SAG) using the SHSOphthalmic omniSpect by Optocraft. RESULTS The intra CL-SAG difference (the difference between two identical lenses from the same batch) was 11±SD 2μm. The CL-SAG of all minus lenses ranged from 3454 to 3765μm (an inter CL-SAG difference of 311μm), while the plus spherical lenses ranged from 3493 to 3757μm (inter CL-SAG difference 264μm). In the toric lens group, the range in toric minus lenses was 3495-3953μm (inter CL-SAG difference 458μm) and 3493-3980μm in the plus group (inter CL-SAG difference 487μm). The inter CL-SAG difference between the spherical and the toric lenses was statistically significant (p=0.03). CONCLUSION Marked differences in sagittal height among different commercially available frequent replacement silicone hydrogel lenses exist. Different lenses with the same package base curve value had marked differences in CL-SAG, with potential clinical significance on-eye in terms of lens behavior. The inter CL-SAG variance in the spherical lens group was smaller than in the toric lens group.

Relationship of placido corneal topography data with scleral lens fitting parameters

PURPOSE To analyze the relationship between corneal sagittal height and asymmetry parameters derived from Placido-videokeratoscopy with the parameters of fitted scleral lenses (ScCLs). METHODS Corneal topographies were measured with MedmontE300 in a total of 126 eyes with irregular and regular corneas before ScCL fitting were analyzed. Measurements of sagittal height (OC-SAG) at steep and flat corneal meridians were obtained for 10 mm and 12 mm chords. Estimated Height (EHChord) parameters were taken for a chord equal to the diameter of the lens that each subject was wearing at different semi-meridians. Corneal asymmetry (difference in OC-SAG between steep and flat corneal meridians) was also assessed. These outcomes were correlated to ScCL parameters that subjects were wearing after 1 month. RESULTS The mean ScCL-SAG was 4696 ± 240 μm, and the mean OC-SAG ranged from 1891 μm (10 mm), 2914 μm (12 mm), and between 4162 μm and 4251 μm for EH0-180º and EH30-210º. Stronger correlations (p < 0.001) between OC-SAG and ScCL-SAG were determined for EH0-180º (r = 0.595) and EH30-210º (r = 0.618). The mean differences between OC-SAG and ScCL-SAG were between 447 ± 290 μm (EH0-180º) and 389 ± 360 μm (EH30-210º). There was no relationship between corneal asymmetry and the need to fit a ScCL with toric haptic design in irregular corneas. Orientation of flat corneal and scleral meridians were similar only in corneas with high regular astigmatism. CONCLUSIONS EHChord attributes were the parameters that best correlated with the ScCL-SAG. The corneal asymmetry was shown to be a poor predictor for the need to fit a ScCL with toricity at landing zone in irregular corneas, but could have some predictive power in regular corneas.

On-eye breakage and recovery of mini-scleral contact lens without compromise for the ocular surface

PURPOSE To report the on-eye breakage of a mini-scleral contact lens in a healthy cornea after being hit by a speeding object, without causing any severe corneal damage. CASE REPORT A 24-year-old Caucasian male involved in a clinical study reported the in situ breakage of a mini-scleral contact lens during motorbike maintenance. The patient reported eye redness and irritation that significantly decreased after all the pieces of the lens were recovered from the eye. Ocular examinations within 48 h showed absence of corneal damage other than superficial punctate keratitis inferiorly and no fragments of the lens were found in the conjunctival sac. The patient was wearing a 15.2 mm mini-scleral lens in a high Dk material. The evolution of rigid materials towards higher Dk values has resulted in a decreased hardness and modulus values, so these materials are more elastic when subjected to mechanical stress, which could be a beneficial aspect in absorbing the energy of an impact before breaking in pieces. CONCLUSION This case report shows that ScCL could have a protective effect to the corneal surface from the direct impact of a high-speed object. Mechanical material properties, wide supporting area and post-lens tear volume acted as protective factors helping to absorb and distribute the kinetic energy of the impacting object.