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Module 32 |
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Module 32: |
Soft Contact Lenses: |
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Part 1 | ||
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Table of Contents (with bookmarks)
Indications for contact lens wear cosmetics better vision artificial iris bandage lens why? age occupation medical history expectations diameter/base curve relationship Wear and replacement schedules and terminology daily wear extended wear flexible wear frequent replacement disposable daily disposable |
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Indications for Contact Lens Wear
Most contact lens fittings are performed for one or more of the following reasons: |
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History taking, of course, is an important component of any medical examination. Good history taking is especially important for the contact lens patient. For the vast majority of patients, contact lens wear is optional, and motivation plays a big role in determining success or failure. For the contact lens fitter, "chair time" plays a big role in determining the financial feasibility of a particular contact lens fitting. Good history taking goes a long way toward ensuring reasonable expectations on the part of patient and the practitioner.
Keep in mind that the simplest, most successful contact lens fits are generally on young patients with spherical myopia who do not want to wear glasses. The least successful patients include the presbyope with significant astigmatism who wants to "try" contact lenses. The following list of history taking queries is not all inclusive, but it will give you a good idea of what you should be thinking about in the contact lens history taking process.
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The patient who works in dusty or dirty conditions may not be a good contact lens candidate. |
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No matter how well a contact lens fits the eye, it is still a source of irritation. A good contact lens fitting maximizes visual acuity and minimizes discomfort.
Vision is maximized and discomfort is minimized by fitting a soft contact lens with a diameter that covers the cornea. Most corneas are between 11 and 12mm as approximated by measuring the horizontal visible iris diameter (HVID).
Most soft contact lenses have a diameter between 13.5 and 14.5 mm. The 13.5 diameter lenses are usually only fit on young children. Some practitioners believe in fitting soft lenses with a relatively large diameter. This allows for some lens decentration and it provides more lens stability. Other practitioners believe that a larger than necessary lens diameter may contribute to eyelid irritation. Keep in mind that soft lenses with larger diameters may be more difficult for the patient to insert.
The base curve of a soft contact lens is the posterior apical radius (PAR). This is the curvature of the inside of the contact lens, the surface that touches the cornea.
It is expressed in terms of "radius of curvature". If you were to fit a circle exactly to the curvature of the back side of the soft contact lens, that circle would have a radius of curvature (R) that describes the "base curve" of that lens.
Notice that for a given diameter (the red line in the figure below), as the radius gets shorter, the curve becomes "steeper", and as the radius gets longer, the curve becomes flatter.
The shape of the cornea is aspherical, meaning no one spherical shape describes the shape of the cornea. The corneal "cap", meaning the central 5mm, is spherical. However, the curvature of the cornea then flattens out considerably toward the periphery.
The PAR of the soft contact lens is fit flatter than the flattest corneal meridian as measured with a keratometer. This allows the soft lens to touch the corneal apex and to drape over the peripheral cornea.
A typical spherical soft contact lens model might have a diameter of 14mm with a choice of lenses in 3 base curves: 8.3 mm, 8.6 mm, and 8.9 mm. The 8.6 mm base curve would represent the "medium" base curve, which would fit the majority of patients. The 8.3 mm base curve lens would be "steep" lens, fitting those patients with relatively steep corneas. The 8.9 base curve lens would be the "flat" lens, fitting those patients with a relatively flat cornea. Remember, as the radius lengthens (8.9), the curvature becomes flatter.
The radius of curvature is translatable to diopter values. There is a conversion table in almost every book that deals with contact lenses. The 3 base curves in our example above would convert as follows:
8.3 mm of radius = 40.62 diopters "steeper" 8.6 mm of radius = 39.25 diopters 8.9 mm of radius = 37.87 diopters "flatter"
If 44.00 diopters can be considered to be the keratometry reading on the average corneal apex, then we see that the "medium" (8.6) contact lens in our example is fit about 5 diopters flatter than the "K" reading.
Notice that there is an inverse relationship between the PAR value and the diopter value. Higher diopter values represent steeper curvatures, while higher radius values represent flatter curvatures.
It is not necessary to do this math when fitting soft contact lenses. The manufacturer will give you guidelines pertaining to what lens might fit best for a particular corneal curvature.
For example, this manufacturer may advise that any eye with an average K reading of 43.00 or flatter be fit with the 8.9 lens, and any eye with an average K reading of 45.00 or steeper be fit with the 8.3 lens. These are only guidelines, giving you a place to start. It is best to fit from an inventory of lenses, so that a lens with parameters close to the ideal can be evaluated before lenses are ordered.
Some practitioners prefer to start with the flattest base curve (8.9 in our example) and go to a steeper lens only if necessary after evaluating the fit of the lens. Other practitioners prefer to begin with the medium (8.6 in our example) lens unless the cornea is very steep or very flat. It is best to take the manufacturers recommendations into consideration when you have little or no experience fitting a particular make of contact lenses.
Some manufacturers use a vault system to identify the base curve of the lens. The vault of a contact lens, also known as the sagittal depth, is the distance from a flat surface to the highest point on the lens. You can see from the drawing below, that for a given diameter (red line), the higher the vault is, the steeper the fit of the lens.
Typically, a Vault I lens would be the flatter lens, a Vault II lens would be the medium lens, and a Vault III lens would be the steep lens. Some lenses have been labeled Sag I, Sag II, and Sag III.
The Diameter / Base Curve Relationship
Applications: Suppose you have a very tight fitting lens, you could change to a lens with a flatter base curve and a smaller diameter to significantly loosen the fit of the lens. Suppose you want a larger diameter lens for better corneal coverage but you like the way the lens fits, you could change to a lens with a larger diameter and flatter base curve. The flatter base curve would offset the tightening effect of the increased diameter.
Thinner is generally better when talking about contact lenses, but not always. The cells in the outer layers of the cornea get their oxygen through contact with the air, more specifically, through the tear layer. For a given lens material, thinner lenses will permit more oxygen transmission to the cornea.
Thin lenses tend to provide better comfort for the patient, but they are more difficult to insert because they don't hold their shape well when not on the eye. Thin lenses can be fit with a larger diameter than "standard" thickness lenses because more oxygen is reaching the cornea and less movement and less tear circulation is acceptable. The larger diameter provides greater stability and centration.
Standard thickness lenses are easier for the patient to handle than thin lenses. Thicker lenses are usually more durable, and the patient with a low amount of astigmatism may see "crisper" with them because they do not fit every contour of cornea like thin lenses do. Standard thickness lenses are usually more comfortable for the patient with dry eyes than a thin lens would be.
Patient comfort with soft contact lenses depends to some degree on the thickness and the design of the lens edge. This surface can be a source of irritation as the lid moves over the lens when blinking. Relatively thin lenses, of course, will be thinner at the edge. Edge thickness is also a function of the lens power. We know from lens optics that minus lenses are thicker at the edge and plus lenses are thicker in the center.
If a patient with high minus lenses complains of discomfort with a particular lens, investigate the design of the lens edge. You can inspect the edge of the lens using a slit-lamp.
Some lenses have a rounded edge that creates a bump for the lid to pass over. Other lenses have a beveled edge that is usually more comfortable with high minus lenses.
A low water content lens is 38% water or less. A high water content lens is 55% water or greater. Low water content lenses tend to be more durable. High water content lenses transmit more oxygen for a given lens thickness.
Soft contact lenses lose about 10% of their water content after a few hours of wear. Loss of water will steepen the fit of a soft lens and make it fit tighter. The dry eye patient is generally more comfortable with a low water content contact lens, as there is less water to be "sucked out" of the lens by the dry eye.
Most soft contact lenses are made of a hydrogel material. This material is hydrophilic, meaning "water-loving". Discussions of hydrogel lens polymers can get very complicated, but there are general characteristics that are more easily understood. Every manufacturer would like their lens to maximize the following lens characteristics:
Unfortunately, a lens material with higher oxygen transmission characteristics tends to be less deposit resistant and/or less durable, and vice versa. The manufacturer must find a combination of lens polymer (material) and lens thickness that works well relative to the characteristics listed. Some lenses are designed to have greater oxygen transmission at the expense of deposit resistance and durability.
As discussed, good corneal health depends upon adequate oxygen transmission to the cornea. This can be achieved by oxygen passing through the contact lenses and/or through tear exchange under the contact lens. Oxygen transmission is a function of lens material, water content, and lens thickness. Higher water content and thinner lenses are associated with greater oxygen transmission. Oxygen transmission can be measured and is expressed as Dk/L. The higher the Dk/L number, the greater the oxygen transmission.
Wear and Replacement Schedules and Terminology
Daily wear: In the early days of soft contact lenses, every lens was a daily wear lens. This means that the lens is inserted in the morning and it is taken out at night to be cleaned of deposits and disinfected. Oxygen transmission is less important because the cornea gets a "breather" at night. These lenses tend to be more durable, lasting up to one year before replacement.
Extended wear lenses: In the 1970s extended wear lenses became widely used. These lenses were either very thin, or they had a high water content. Extended wear lenses were usually worn for one week before removal for cleaning and disinfection. The same lenses were then re-inserted for another week of extended wear. At one time the FDA had approved extended wear for one month for aphakic (this was before IOLs) patients wearing extended wear lenses.
Shortly after being introduced, problems with soft contact lens extended wear began to surface. The health of the cornea during extended wear of a soft lens depends upon the ability of oxygen to pass through the lens to the cornea. Because of infrequent lens replacement and poor cleaning habits, some of the patients had lenses that became coated with deposits. Oxygen does not pass through lens deposits. The cornea would then show signs of oxygen deprivation, typically corneal edema and limbal injection (redness at the limbus). Some patients developed neovascularization of the cornea (blood vessels growing into the cornea from the limbus). Aside from better cleaning, more frequent lens replacement was an obvious solution to the problem. As manufacturing techniques improved, the cost of an individual soft lens went down, and frequent replacement became practical.
Today extended wear lenses are typically worn for one week. The lenses are then discarded and a new pair is worn for one week. Extended wear is still controversial in the ophthalmic community. Some doctors argue against the possible oxygen deprivation of extended wear, saying that it is not that much trouble to take lenses out at night.
Flexible wear: This is a modification of extended wear. The lenses are worn occasionally overnight, but not for extended periods of time. Disinfection is required and cleaning is recommended. Frequent replacement of the lenses is recommended.
Frequent replacement: Lenses are worn during the day and removed at night. The lenses are replaced at intervals from 1 to 3 months. Disinfection in a multi-purpose solution is required and cleaning is recommended.
Disposable: Lenses are worn during the day and removed at night. The lenses are replaced at 1 to 2 week intervals. Disinfection in a multi-purpose solution is required and cleaning is recommended.
Daily disposable: A new lens is inserted in the morning and disposed of at night. Obviously, no lens care is needed.
Of the schedules listed above, daily disposable is the healthiest for the eyes, but this schedule is also the most expensive. Isn't this over-protection? Why would anyone want to do daily disposable? Some patients develop a hypersensitivity to any kind of lens deposits, and daily disposable may be the only way that they can comfortably wear soft contact lenses. Other patients have a sensitivity to deposits to a lesser degree, and a less frequent replacement schedule may work well for them.
This discussion of this subject matter is continued in Module 33.
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