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International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision |
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Corneal and Limbal Clear(ance)!
 Creating clearance is probably the number one reason for fitting a scleral lens. It prevents mechanical stress on the cornea, and bypasses the most sensitive part of the anterior ocular surface. When assessing corneal clearance behind a scleral lens (e.g. the tear reservoir thickness), many of us may use the corneal thickness as a reference. We know a lot about central corneal thickness. But what about peripheral corneal thickness? According to a recent study by Usama Fares et al in Contact Lens & Anterior Eye, the central cornea is 550±34 microns (pupil centre), 547±35 microns (thinnest location) and 551±34 microns (corneal apex). Going toward the periphery, at 3mm the thickness increases to 562-581 microns (in different meridians) and to 639-676 microns at 7mm, indicating a gradual increase in thickness from the center to the periphery. Of course, many of the eyes indicated for scleral lenses have pathology, often with corneal ectasia that results in thinner corneas of unknown thickness. Maybe a safer bet would be to use the thickness of the scleral lens as a reference, if known. Using an optical coherence tomographer can make the art of estimating corneal and limbal clearance into a science. Controlling the corneal and limbal clearance is important. First to make sure there is clearance, enough to allow sinking of the lens - and to stay clear of the cornea and limbus. Secondly, to make sure the tear clearance is not too excessive as it appears to serve as an additional filter for oxygen to reach the cornea. In theory at least, it is clear that a smaller clearance may improve the Dk/t of the scleral lens system. Eef van der Worp
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Raising the bar
 Corneal topography has been around for a few years now, so it is surprising that it keeps surprising us. As mentioned before in I-site newsletter, elevation maps may be a valuable tool in understanding true corneal shape, especially in (but not limited to) the irregular cornea. Patrick Caroline and Mark André in Contact Lens Spectrum illustrate this with a case report on a 43-year-old woman with a history of herpes simplex infection. The axial curve map shows the resultant irregular astigmatism, but the elevation map shows much better the true shape of the cornea as high and low areas, rather than steep and flat areas as axial and sagittal maps do. Christine Sindt even proposed in a GPLI Webinar that using the elevation maps would actually be a good way to distinguish keratoconus from pellucid marginal degeneration (PMD). In PMD, the curve maps sometimes show artificial 'red' areas centrally on the cornea, which may be interpreted as elevation, while in reality they represent a drop-off. This is illustrated in the figures in this item (the curve map on the left and the elevation map on the right, courtesy of Christine Sindt). The assumptions the topographer makes regarding the normal cornea do not apply to irregular corneas as in the case of PMD, which may lead to a partly erroneous representation of the actual corneal shape. In other words, the classical 'kissing doves' pattern seen in PMD may be a (partial) artifact and does not necessarily represent the true shape of that cornea.
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A Hole Lot Better?
 Pauline Cho et al report in Optometry & Vision Science an exceptionally useful study on fenestrations in orthokeratology lenses. Twenty-two subjects were fitted with a fenestrated orthokeratology lens in one eye, while the other eye was fitted with a non-fenestrated orthokeratology lens. They concluded that the addition of fenestrations to orthokeratology lenses had no effect on the efficacy or outcome of the treatment, but the severity of lens binding was reduced - so they suggest that fenestrations therefore may be of assistance in cases where lens binding is a problem.
Optometry & Vision Science, April 2012 |
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Cone & Location
A prospective study published in Contact Lens & Anterior Eye from Iran by Mahmood Nejabat et al on 156 eyes of 134 keratoconus patients evaluated the influence of cone location and corneal cylinder on (R)GP corrected visual acuities and residual astigmatism. They found that cone location had no effect on the (R)GP corrected visual acuities and residual astigmatism. In this study they tried to find a simple rule for determining the initial (R)GP lens. For the lens used in this study, and for other lenses with similar optic zone sizes, it was determined with linear regression that the base curve of the (R)GP lens can be calculated by using "10.6 - 0.07 Sim K1," where Sim K1 is the simulated keratometry value of the flat meridian. Initial higher values of corneal cylinder and steeper Sim K values did influence (R)GP corrected visual acuity outcome after lens fit in this study.
In Optometry & Vision Science, an article on aberrations and topography in keratoconus stated that measurement of aberrations can be a good indicator for keratoconus while traditional topography remains an important predictor to identify suspected keratoconus. The article also mentions the high prevalence of keratoconus in countries like India, Israel and Saudi Arabia. A study by Alabdelmoneam provides a retrospective analysis of keratoconus in Saudi Arabia specifically (Clinical Optometry, free access).
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IACLE course: scleral lens fitting
As reported in an earlier edition of I-site, the International Association of Contact Lens Educators (IACLE) has made available several contact lens modules in PowerPoint format. Recently released was the scleral lens module, which provides an excellent overview of scleral lenses and scleral lens fitting. The module, written by Robert Terry, Meredith Reyes and Christopher Snyder, is available to everyone after free registration. It contains fitting procedure, problem solving and lens handling tips.
IACLE - Scleral Lenses Module |
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Seeing the Bigger Picture in 3 and 9 o'clock staining II
The last issue of I-site covered the use of a scleral lens to manage 3 and 9 o'clock corneal staining; in this month's 'Inside the Practice,' Michael Baertschi, Michael Wyss, Simon Bolli and Marc Fankhauser look at using larger diameter corneal lenses to remedy this condition. In the case they present, a 52-year-old patient with a prescription of -13D of myopia and significant 3 and 9 o'clock staining was successfully refitted from a 9.8mm lens to an 11.2mm perilimbal design. The corneal staining vanished completely. The conjunctival hyperemia decreased but did not disappear, and some conjunctival staining persisted as well. But the patient's subjective results were outstanding: the wearing time went up to 16 hours per day and the foreign body sensation and dryness disappeared completely. Click here for the full report.
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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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