L&T: L&T 101

Mar
2008

Eyeglasses AND Contact Lenses

Finding common ground


From top: HILCO 101-M648, hard eyeglasses case, from Hilco; EYE-GO chantilly, contact lens case, from California Accessories
Photographed by Karen Schuld

It’s important for eyecare professionals to be familiar with the basics of vision correction when dispensing either spectacle or contact lenses. Whether the patient has settled on spectacles or would like to try contacts, a repertoire of common skills is needed to evaluate, fit and dispense both lens types. There is a lot to be gained by having a general understanding of spectacles and contacts, not the least of which makes the dispenser a more valuable asset in the eyecare practice or optical retail shop.

Patients
The first thing opticians and contact lens technicians have in common is the patient. Their shared objective, as always, is acceptable health, comfort and visual acuity. Multiple modalities of acceptable correction exist for both contact and glasses wearers. Each patient has a unique visual need based on the lifestyle that they lead. It is the duty of the technician, therefore, to determine not only which correction might be most appropriate but also which, if any, might be contraindicated. A person who works around dust or strong fumes all day, for example, may not be a good candidate for contacts at all. Since the patient cannot be expected to know these things, it is the responsibility of the eyecare provider to always keep them in mind. There may be some compromising involved (though one should never use such a word in front of the patient) to achieve the best outcome. A prime example is achieving the balance of near versus distance vision in multifocal contacts, which is not always a predictable process. Another example lies in sports vision correction. Contacts can be used to eliminate the presence of a frame, reduce magnification and increase field-of-view. Special tints are now available as well, to improve the athlete’s contrast sensitivity and reduce glare out on the field. Furthermore, another patient may benefit from the strong intermediate and near correction in his office progressive, a case where spectacle lenses might do a better job. The concepts are familiar and clearly seen throughout: determining visual needs, identifying solutions, applying the best options and evaluating the results.

Fit
Opticians, contact lens technicians and optometrists are of all part the fitting process. Opticians are aware of the importance of a good fit with spectacle wearers. They routinely troubleshoot complaints and improve vision by observing, adjusting frames, adjusting lens parameters and evaluating the results. Spectacle fit and contact lens fit mean very different things though. Since contacts, as their name indicates, are in direct contact with the eye multiple additional considerations must be made. Now the health of the eye must become the foremost concern as an improperly fit lens can cause damage to the cornea. The well-fitting contact lens has adequate movement, delivers crisp vision and is acceptably comfortable. The contact lens must allow enough oxygen to reach the cornea. One way to accomplish this is tofit it loose enough so that it moves during blinking but not so much as to cause discomfort or compromise vision. It’s important for contacts to show both good centration and coverage. The optical zone of a lens, or the area of correction, must be large enough to cover the entire pupil in any lighting condition. At the same time it must not irritate the lids during blinking as might occur with a lens that is too flat. Fit is crucial with contacts because we are dealing with a device resting directly on the tear film, which means much more attention to detail is required in their fit and care.

Base Curves

Contact and spectacles lenses both come in multiple base curves. This measurement can take two different forms, millimeters
or diopters. It’s easy to convert between the two once you realize that contact lens base curves are generally expressed as radius of curvature and spectacle lens base curves are expressed as the dioptric power produced. Just remember, contacts can be said to get their power from the difference between the front and back curves, just like eyeglass lenses. With contact lens base curves, the length in millimeters of the radius indicates the steepness of curvature. A 7-base is steeper than an 8-base. This is because a 7mm radius represents more curvature in a shorter area. Spectacle lens base curves are a little simpler, with base curves being expressed in diopters. The higher the number the steeper thecurvature resulting in a tighter fit. Using the radius length as an indicator of curvature may be counter intuitive, but once the concept behind each is fully understood there is no trouble transitioning from one to the other. Consider, as an example, the last –2.00 you made on an 8 base, where there was all that curve for a little power. This is a different application of the same principle.
 
X Y Axis

The fitting of contacts begins with measuring the cornea to find its amount of curvature. This curvature is read in two meridians at right angles to one another, usually 180 and 90 degrees, as K readings or Ks, the first step in a contact lens exam. (K is short for keratometry, the Latin word for cornea and measure.) Lensometers work in much the same way in reading spectacle lens power by reading curvature in two directions, 90 degrees apart. Opticians know that cylindrical lenses have two curves on the back. Consider for a moment the measurement of the cornea this way. Even spherical contacts are fit based on this system. The difference between the X and Y axis does not automatically indicate astigmatism in the prescription. It’s important not to confuse these concepts. Keratometry is an important first step infitting contacts. But corneal measurements are not the same thing as lens cylinder. CL fitters know that astigmatism may originate at the cornea or it may be due to the shape of the lens within the eye, called lenticular astigmatism. This difference of curvature may run vertical, horizontal or oblique. Astigmatism calls for cylinder on spectacle lenses and toric contact lens designs in many cases. The next time you clock a lens, remember how similar our systems of measuring curvature for correction really are. 

Materials and Lens Design

Contact lenses feature an optical zone and an edge tapered for comfort. The most common material is silicone hydrogel, though both rigid and soft lens designs come in a variety of other material options. Most modern contact lenses offer high oxygen permeability, allowing the cornea to receive adequate oxygen through the lens. Contact lenses were once limited as far as what they could correct. Today though, there are few eyeglass options that can’t be matched suitably by a contact alternative. Everything from astigmatism to Keratoconuous can be addressed. Multifocal contact lenses exist, although they are not as common as their spectacle lens counterparts. These multifocals may be segmented, but more often they utilize a concentric or “target” layout, allowing the wearer to focus on one of two simultaneous images. Others may receive a simpler solution to their presbyopia through their contacts: in the case of monovision contact lenses, prescribers give patients one contact for distance and the other for their near vision needs.

Vertex Distance
Good spectacle dispensers pay attention to vertex distance of eyeglasses, the distance from the back of the lens to the cornea, making sure it’s equal left and right. Too great or too small vertex distance will affect the power through the lenses. The dispensing tech works with the doctor to determine the patient’s working distance, specialized and general needs. Correction and distance of use/visual need have to compute. Changing the distance of the spectacles from the cornea will effectively change the prescription. Workshop, piano, computer monitor, arms-length for short and tall patients—all of these variations of measurement may change the focal length requirement of the lens. A familiar example is a pair of reading glasses worn on the tip of the nose. This increased distance means the effective power of the plus lens increases. The power prescribed may be compensated  to take this into account. It’s another application of the concept that plus lenses gain power further away and minus lenses gain power closer to the eye. This makes vertex distance important and makes compensated prescriptions the key to a successful contact lens fit. In fact, the general rule is that contact lens prescriptions of +4.00 diopters compensate the power because of the fact that the lens goes right on the cornea.

Vision
Benefits to the contact lens wearer include the potential of improved peripheral vision, better sports applications where eyeglasses might not be viable and better fusion of images. Even the best eyeglasses have a little compromise, with correction based a few millimeters from the cornea. Because contact lenses are positioned directly on the eye, they correct with less magnification in the case of plus power eyeglass lenses and less minification in the case of minus. Contact lenses provide an opportunity to help the aniseikonic/anisometropic patient. In the case of anisometropia, there is such a difference between the powers left and right, that the patient often cannot fuse the images. Often the brain suppresses one image. Because contact lenses magnify/minify less than glasses, there is a greater chance of providing images the brain can tolerate without suppressing one image.

The Big Picture
While division of labor is necessary, it’s helpful to step back once in a while and look at the big picture. Patients come into offices for refractions, rechecks and healthy eyes. From these appointments and prescriptions, we have an opportunity to provide the best, most suitable and most comfortable vision correction possible. Sometimes solutions are obvious and other times a little creative. In the course of a routine day, many people come into our offices or dispensaries, often with a vague complaint or frustration. It is routine for us to perform our duties: dispensing, surfacing and edging lenses, fitting eyeglasses and troubleshooting progressives. These things are all the result of several concepts that we understand and utilize every day. The results of our work are a big deal for the wearer. For efficiency, we go about our tasks and we separate into our areas of expertise. But the reality is we are each contributing to part of the whole, and that whole is the vision and comfort that improves the lives of those coming to us for help.


Timothy Coronis is a certified optician based in Keene, N.H. He is also an American Board of Opticianry-certified technical speaker. He can be contacted at t imothycoronis@hotmail.com.

Julie Harp, CPOT, ABO, NCLE, contributed to this article.

 


 

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