When this occurs, contact lenses themselves become viewed as a commodity rather than as a medical device. In this day and age, it is extremely easy for existing and potential contact lens patients, or even non-patients, to obtain contact lenses and contact lens education online. There are many distributors that sell them online without ever requiring a prescription from the buyer.
There also are numerous resources available on popular video hosting sites, some of which unfortunately feature unqualified and untrained persons who erroneously educate the public on how to use contact lenses.
Some of these videos have literally been viewed, at minimum, hundreds of thousands of times, suggesting their wide and far-reaching impact. Although, to be fair, there are some videos that advise properly and emphasize the importance of visits to an eyecare professional. However, in the former, the spread of misinformation is a disservice to the general public.
Interestingly, many of these videos fail to discuss how a contact lens should fit on the eye or what to do if the contact lens does not fit. For practitioners, it may be easy to neglect properly assessing the fit of a soft lens because it is expected to fit well. While parameters are quite easy to choose from, we should not expect that any soft lens can be applied on the eye and perform well. It is a well-known fact that comfort is a major driving force for contact lens success, with discomfort being the primary reason why patients discontinue lens wear.
A soft lens that does not have sufficient coverage may irritate the cornea with its edge. Soft lenses that are poorly fitted may result in decreased ocular surface integrity. Tight-fitting lenses, although appearing to fit more comfortably, 2 , 9 , 10 when excessively tight will reduce tear exchange, leading to a buildup and trapping of metabolic waste and toxins under the lens. For these reasons, the process of contact lens fitting and the assessment of contact lens fit are both vital to healthy contact lens wear.
The fitting of soft contact lenses essentially comes down to selecting a lens with the appropriate sagittal height. It was thought that steeper corneas theoretically exhibited a higher sagittal height and should therefore be fitted with steeper lenses. Likewise, it was thought that flatter corneas with the same given diameter exhibited lower sagittal height and should therefore be fitted with flatter lenses Figure 1.
Figure 1. The idea is similar with contact lenses, where r1 and r2 would be base curves and d would be lens diameter. This led to a belief that central corneal curvature was a major dictator of sagittal height, but this idea was disproven in In fact, that study found that normal variation in population corneal asphericity and corneal diameter separately contribute more variability to sagittal height than does apical corneal radius of curvature.
To complicate matters, the labeled base curve of a lens may not truly reflect the actual radius of curvature of the back of the lens.
There are many reasons why this is the case. Manufacturers often incorporate modifications e. Environmental factors such as dehydration, 12 temperature, 13 , 14 and osmolality 15 can also impact lens shape. Because base curve is a poor predictor of sagittal height and is a poor representation of lens curvature, information from central keratometry does not really contribute anything related to selecting the initial base curve or how well the initial lens will fit.
If central keratometry does not predict the base curve nor the sagittal height of the potential lens, then what does? And what information does a clinician now use to best select a lens? With the advent of corneal topographers and optical coherence tomography OCT imaging, we can gain a complete characterization of the shape of the anterior ocular structures.
It follows, therefore, that sag height might become a new standard in soft contact lens fitting, as it seems to be a much more relevant parameter relative to other variables. Radius and curvature may become obsolete in the future.
Such capabilities are generally possible in this new age of 3D printing. But what should we do as contact lens practitioners? We still seem to live in the last century, relying on trial sets that have huge parameter steps and on contact lenses that offer no clue about their design and shape.
So, is the base curve radius that is printed on the soft lens box completely useless? Does an 8. For the exact same lens design, type, and brand , that is probably true. In other words, within the same lens, if the 8. However, if we compare an 8. An even better idea would be to print the absolute sag value on each lens pack. Almost by definition, soft lenses in-vivo dehydrate. This is especially true with traditional hydrogel materials. For this reason, manufacturers and educators suggest compensating for this by selecting traditional lenses that are around 0.
Low-water-content silicone hydrogel materials and some more water-retaining materials tighten up as well, but to a lesser extent. In theory, this means that these contact lenses can be fitted somewhat steeper. So, for instance, we should select them to be only 0. But it is interesting to note that with only one exception, the base curves listed on silicone hydrogel contact lens packages typically have the same or very similar values as compared to traditional hydrogel lenses even though they could be fitted steeper.
There recently has been a bit of a revival of custom-made soft lenses. Thanks to recent innovations, such custom soft contact lenses can now be manufactured in a silicone hydrogel material. But apart from these outside-of-standard and custom-made soft lenses, it may be a good idea to revisit the fitting of soft contact lenses altogether. In an average contact lens practice, time and energy are invested in acquiring new lens wearers.
But how much do we invest in our current lens wearers to give them the best available lenses today? Perhaps we can reverse the trend that is making soft lens fitting a lost art. Using sag heights in the fitting process and having those available on lens packages and trial lenses would be a big step forward.
As contact lens practitioners, we are limited in what we can do, even if we strive for the best possible lens for our patients. We need better tools. Quite literally—thinking in sag heights. Special thanks to Helmer Schweizer of Euromcontact for his valuable input. For references, please visit www. Eef van der Worp is an educator and researcher. He has served as a head of the contact lens department at the Hogeschool van Utrecht in the Netherlands NL for over eight years.
He is currently affiliated with the University of Maastricht as an associate researcher, a visiting scientist at Manchester University Manchester, U. Hold a ruler in front of the person to be measured and have them look straight ahead. Line up the 0 on the ruler in front of his right eye. This number is usually between 13 and 15 and determines where on your eye the contact lens will sit.
If this measurement isn't correct the contact lens will be uncomfortable and may scratch your eye. For Toric Lenses. CYL: Cylinder value. Usually a. These biweekly contact lenses are the 1 prescribed brand in the U. According to the steepness or flatness of your cornea, your prescription indicates how large your contact lens should be.
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The patented lens material helps to maintain moisture and a smooth lens surface helps to prevent lens deposits and optimize comfort. Just Now Proclear toric XR. We can do the rest for you. Your eye doctor is the only other information needed, including their name and contact …. Confused and becoming irritated. It could be that you need to have the prescription adjusted because it's not quite right yet.
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