Quarterly newsletter on Soft Specialty Contact Lens
Research, Developments, Designs and Materials FALL 2014
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The Story of Limbus Demarcation
Limbal Border, Corneal Diameter, New Technology
For contact lens fitting, particularly soft lens fitting, it is paramount to know the diameter of the cornea, as also previously discussed in this newsletter. The question is: how do we define the limbus and know the diameter of the cornea? Not as easy a task as it appears at first glance. The traditional definition of limbus corneae tells us that the annular transition zone between the cornea and sclera manifests itself as a slight furrow mark. Histologically, at the limbus, the epithelium gradually thickens toward the sclera, where it is replaced by conjunctival tissue. Demarcating the limbal borders is difficult in standard-intensity images of the eye because the transition from the cornea (iris) to the sclera is gradual. Measuring the visible iris diameter seems unfit to provide an accurate estimate. Extended light source projection systems have been proposed in the past. Now it seems that new technology such as optical coherence tomography (OCT) and the Eye Surface Profiler (Eaglet Eye, the Netherlands) provide us with new insights to help us to better define the limbal transition zone. We are facing different definitions of corneal limbus; but with more advanced technologies becoming readily available in our practices, one is able to choose a specific definition suitable for his or her particular purposes. Click here for full report.
Robert Iskander PhD DSc
Wroclaw University of Technology, Wroclaw, Poland
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IN THE LITERATURE
Personal-eye-zing
Optimized Vision, Increased Comfort and Better Ocular Health
Robert Davis, Barry Eiden and Jeffrey Sonsino make soft lens fitting 'personal' again in an article in Contact Lens Spectrum. They look at vision improvement, specifically regarding presbyopic and aberration control corrections. They conclude that soft lens patients become legacy patients loyal to the practice. They list the advantages that soft specialty lenses can offer. In their view, it will allow eye care practitioners to:
- Refine lens powers in 0.1D increments for both sphere and cylinder.
- Prescribe cylinder axes in 1º increments.
- Choose from unlimited base curve options to better match the cornea.
- Select a secondary curve that can enhance the cornea-to-lens relationship.
- Specify diameters in 0.1mm increments.
- Choose central zone thickness options that can successfully mask astigmatism and corneal irregularity.
- Select optic zone sizes to reduce aberrations from large pupils during scotopic vision.
- Choose zone sizes to appropriately fit multifocal lens modalities.
- Prescribe custom tinted/opaque cosmetic lenses.
Jerry Legerton, in Review of Cornea & Contact Lenses, turns this discussion up a notch, looking at how high-tech tools may improve contact lens manufacturing efficiency - and patient vision and comfort. A number of factors, such as numerically controlled lathes, auto-loading robotics, low-cost latheable conventional and silicone hydrogel materials and base curve molded buttons, are allowing manufacturers of customized lenses to produce quarterly replacement lenses, he states, and in some cases even monthly replacement lenses on a competitive basis. It is hoped and forecasted that manufacturers will continue to drive down the costs of lens manufacturing. Economies of scale may also result in lower cost of lenses, as the segment of customized soft lenses expands to its potential to serve the percentage of wearers who are expected to need them.
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IN THE LITERATURE
H-eye Definition
Spherical Aberration, Coma, Trefoil, Lower-order Aberrations, Refraction 
'Vision' with regard to specialty soft lens fitting was the topic of discussion in a column in Contact Lens Spectrum by Diana Nguyen and Timothy Edrington. It focuses on 'High Definition Vision with Contact Lenses.' They advise, though, to 'Crawl Before You Walk.' In other words: before correcting higher-order aberrations (spherical aberration, coma, trefoil, etc.), it is imperative to first correct the lower-order aberrations of defocus (myopia and hyperopia) and astigmatism as accurately as possible. 'Because sphere and cylinder account for about 93% of ocular aberrations, most patients can achieve excellent vision with their sphero-cylindrical corrections based on subjective refraction and are not affected by higher-order aberrations,' the authors state.
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IN THE LITERATURE
I've Got the Power!
Multifocal Contact Lenses, Power Profile, Individual Pupil Diameter
More on vision: in Contact Lens & Anterior Eye, an article by Montés-Micó et al from Valencia (Spain) evaluated the power profile of multifocal contact lenses using a new technology based on quantitative deflectometry. The authors found that the relation between patients' pupil diameter and the power profile of the lenses tested has a crucial impact on the final distance correction and near addition that these lenses provide to patients. Therefore, they state, practitioners should know the power profile of the lens they fit and measure the individual pupil diameter under different conditions when they carry out a customized soft lens presbyopic lens fitting.
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IN THE LITERATURE
Toric Optics
Soft Toric Lenses, Marks, Axis, Rotation
Thomas Quinn in Contact Lens Spectrum looks at soft toric lens rotation. 'Some practitioners determine lens rotation by simply observing the toric marking and estimating its position. It's quick and simple, but if inaccurate, more time may be needed in the fitting process to make adjustments to achieve the desired visual outcome' he states. It is further concluded that according to the literature, experienced clinicians can estimate rotation of a toric lens marking within 8 degrees of the true reading 95% of the time. It is clear that 10 degrees of rotation can make a large difference, optically.
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IN THE LITERATURE
Corneal Reshaping with Soft Lenses
Corneal Warpage, Ring Pattern, Peripheral Strain, Mechanical Forces
Whether we like it or not, the shape of the ocular surface is altered during soft lens wear - despite the apparently limited mechanical forces involved. A two-part paper in Global Contact looks at a series of cases of silicone hydrogel lens wear, the resulting topographical changes and why this may occur. The authors observed that corneal topographical changes do occur, irrespective of lens power, lens design and replacement frequency. Sometimes the changes are irregular, and often they appear in a circular fashion - almost like an orthokeratology pattern. This raises questions about the 'peripheral strain' that soft lens may apply to the fragile corneal epithelium. This topic is further discussed and evaluated in the part two section of the paper (see link below). Part one was discussed in the summer edition of this newsletter.
Van der Worp, Molkenboer - Global Contact, 2/2014
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IN PRACTICE
Living in a Material World Indications for Silicone Hydrogels, Pterygium, Surgery |
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