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International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision |
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Good Bye Radius
 Over the last couple of years, 'elevation' in terms of describing the ocular surface and also as an aid in contact lens fitting has received much attention - including in this newsletter. In fact, the title of my second column ever ( July 2009) was 'Elevation,' named after the U2 concert I had just attended back then. Now 5 years later, and with the new U2 album just out, it may be time to take that knowledge and understanding to the next level. Quite literally - to start thinking in height rather than radius of curvature. If I want to understand the shape of an eye - to fit a lens especially - all I have to do is think in microns of height difference: between central and peripheral, between one meridian and another 90 degrees apart, etc. That way I know exactly where I am, and where I want to go. Good old radius may have had its best time - it dates back to the origin of keratometry, now more than 100 years ago. Height is the metric of the future in contact lens practice: In scleral lens fitting, orthokeratology, specialty soft lens fitting and - although probably to a lesser degree in corneal (R)GP lens fitting - height is becoming the driving force of the respective modalities. It's time to say goodbye to radius. Cheers old chap! We had a jolly good time. Now it's time to move on to a higher level. |
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Scleral Curves
 In the July issue of Optometry & Vision Science was a paper by Choi et al on the "Measurement of Anterior Scleral Curvature Using Anterior Segment Optical Coherence Tomography." The paper confirms previous studies published in IOVS, OVS and in CLAE, that the nasal portion of the anterior scleral shape is flatter than the temporal. In this case, they measured radius of curvature (where previously angles or other metrics were used), finding a significantly flatter radius nasally (13.3mm) than temporally (12.3mm). The authors conclude that anterior segment optical coherence tomography is a valuable tool for designing scleral lenses, but also that it can potentially assist in selecting the suction ring during LASIK surgery. Choi et al - Optometry & Vision Science, July 2014
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Understanding Topography
Stephanie Woo in Cornea & Contact Lenses looks at 'Surveyor of the Surface,' and how corneal topographers are great diagnostic tools for contact lens specialists - "if you know how to use them." Some key points regarding understanding corneal topography are highlighted. Axial power maps are very important and very useful diagnostic tools, as they are capable of distinguishing between regular astigmatism and irregular astigmatism, she states. Tangential maps are used to define points of curvature change. This map is particularly useful for your keratoconus patients, as it will allow you to measure the exact size and shape of the cone, according to the article. Elevation maps are incredibly useful resources that, curiously, many practitioners do not take full advantage of, the author states. This map highlights "high" and "low" areas of the cornea. Higher areas of the cornea will appear as warm colors (e.g., red and yellow), while the lower areas appear as cooler colors (e.g., green and blue).Corneal topographers can also establish the exact corneal diameter, and ideally they give information about the entire sagittal height of the anterior ocular surface to aid in soft and scleral lens fitting. A book by Michael Belin, Stephen Khachikian and Renato Ambrósio called "Elevation Based Topography" looks at cross-sectional Scheimpflug analysis to offer significant advantages in treatment, diagnosis and patients' safety, according to the authors. The main focus was on screening patients for refractive surgery - the interpretation of elevation maps, corneal thickness maps and its progression analysis. An instrument that truly can measure ocular surface shape beyond the corneal borders and can analyze sagittal height in a 360-degree fashion is the eye surface profiler (ESP).
Woo - Review of Cornea & Contact Lens, May 2014
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Moving Upfront in Wavefront Technology
 Abdu et al looked at "Visual performance and aberration associated with contact lens wear in patients with keratoconus" in a (full access) paper in Clinical Optometry. Little difference in contrast sensitivity values was detected between (R)GP and silicone hydrogel lenses and both lenses significantly reduced total ocular and higher-order aberrations, but still it could be concluded that (R)GP lenses overall provided better visual acuity and greater reduction of aberrations than spectacles or silicone hydrogel lenses in patients with keratoconus. A paper by Ravikumar et al in the Journal of Vision (full access) concludes that "Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors." As with corneal radius in contact lens practice, wavefront aberrometry may be the next subjective refraction - at least to some degree. It surely is more accurate, although it's obviously missing the subjective component. Further investigation of the predictive ability of these metrics for a given individual will establish whether these metrics can be used as an objective surrogate to predict change in logMAR visual acuity resulting from therapy such as wavefront-guided customized refractive corrections, intraocular lens designs, and novel contact lens designs for the highly aberrated eye. Marsack et al, in Optometry & Vision Science, looked at wavefront-guided scleral lens correction in keratoconus. The paper concluded that wavefront-guided scleral contact lenses are capable of optically compensating for the deleterious effects of higher-order aberration concomitant with the disease and can provide visual image quality equivalent to that seen in normal eyes. Longer-duration studies are needed to assess whether the visual system of the highly aberrated eye wearing these lenses is capable of visual performance levels typical of the normal population.
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One of the more common scleral lens complications is "mid-day fogging," in which particulate matter accumulates in the tear chamber beneath the contact lens, creating fog-like blur. The fluid forces associated with a scleral lens pressure system are likely responsible for the introduction of debris into the tear chamber. The force may be induced by the coup-contracoup movement of a scleral lens that occurs during a blink - as the eyelids close, they apply directional pressure on the lens toward the anterior surface of the eye (retro-pulsion), and as the eye reopens and the pressure is released, the lens rebounds outward and particulate matter is drawn from the peripheral confines of the lens chamber into suspension within the chamber fluid. Over the course of a few hours, the chamber fluid becomes saturated with debris, creating turbidity within the fluid. Perhaps contrary to general belief, our analyses have shown that a lipid substance may be primarily responsible for the post-lens clouding. Considering the hydrophobic nature of the lipids found, it is logical that these molecules would form a precipitate, as they are unable to dissolve in the aqueous solution beneath the lens. To reduce fogging in these individuals, the design of a scleral lens can be manipulated to discourage the movement of particulate matter under the lens. Decreased apical clearance has been shown to reduce the fog in many patients because it thins the layer of fog in general, and it decreases limbal clearance, which narrows the channel for entering debris. Applying the lenses with a high-viscosity/high-solute solution (preservative-free artificial tears) can also be an effective method of managing mid-day fog.
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Upcoming Events
- OSO, Oct 17-19, Gold Coast (AU)
- Scleral Lenses Training - Nov 6-7, Hertford (UK)
- AAO, Nov 12-15, Denver (US)
- Cornea, CL & Cont. Vision Care Symposium, Dec 6-7, Houston (US)
- Global Specialty Lens Symposium, Jan 22-25, Las Vegas (US)
- CLSS, March 14-15, Billund (Dk)
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I-site is an educational newsletter that is distributed on a monthly basis and provides an update on rigid gas permeable related topics (scientific research, case reports and other publications worldwide). I-site is objective and non-political. Its editor Eef van der Worp, optometrist, PhD, FAAO, FBCLA, FIACLE, FSLS is a lecturer for a variety of industry partners, but is not related to any specific company. Please contact us at: i-site@netherlens.com.
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