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Module 35 |
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Module 35: |
Hard Contact Lenses (Rigid Gas Permeable) |
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Part 1 | ||
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Table of Contents (with bookmarks)
Corneal topography Calculating the initial lens power The base curve/ lens power relationship Lens diameter and the optic zone Evaluation of the contact lens fit
The following evaluation points are continued in Module 36
Flexture Residual astigmatism Comfort Fluorescein pattern Corneal and eyelid integrity |
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Even though the vast majority of contact lenses fit today are soft contact lenses, the hard contact lens is simply another tool available to the contact lens practitioner. Some practitioners never fit a hard lens, others fit a hard lens as their first choice. The soft vs. hard philosophy of the practitioner, I think, depends very much on where he or she went to school, who his instructor may have been, and/or what contact lens publications she reads. As ophthalmic technicians, we simply need to be familiar with the basic characteristics of both.
Hard lenses offer some different modes of correction for astigmatism which have some advantages over soft toric correction. Hard lenses are generally less expensive for the patient (less frequent replacement), and hard lens wearers are less susceptible to corneal diseases than soft lens wearers. Hard lenses are not comfortable initially, requiring a build up of wearing time, and may never be comfortable if not fit properly. |
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| Once upon a time, there was but one predominant hard contact lens material. That was PMMA (polymethylmethacrolate), otherwise known as plexyglass. Yes, I do remember those days, and no, Teddy Roosevelt was not president. Although very durable and optically efficient, PMMA plastic had an oxygen transmission capacity near zero. We know from corneal anatomy and physiology that the cornea usually gets most of its oxygen through the epithelium, from contact with the air. So, how did the cornea get its oxygen when covered with PMMA? Oxygen came from the tear layer, which was continually being "pumped" under the contact lens as the lens moved with each blink. The PMMA lens fit had to be adjusted not only for good vision, but also for good movement and good tear circulation. The practitioner could vary the base curve, the peripheral curves, the edge design, and the diameter to arrive at an acceptable fit. It was not unusual to see small diameters (7 to 8mm) in hard lenses. As you might expect, a small diameter was not conducive to optimum vision. Some practitioners busied themselves thinking up new ways to improve oxygen transmission so that larger diameters could be used. One method was to drill small holes near the edge of the lens (fenestration). | |||
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Then along came silicone acrylate, and later fluorinated silicone acrylate. These materials allowed some oxygen to pass through the lens material to the cornea, enough oxygen so that tear exchange became less important and larger diameter hard lenses became practical. The rigid gas permeable lens (RGP) was born (also known as a hard gas permeable, HGP). These lens materials were not without their problems, such as not wetting well. Some of these problems persist to this day, such as susceptibility to scratching and cracking.
Oxygen transmissibility is expressed in terms of the Dk value, or the diffusion coefficient value. The higher the Dk value is, the higher the oxygen transmission through the material. It is usually given as Dk/L, with L being the center thickness of the lens. This means that, no matter what material is being measured, oxygen transmission will decrease as lens thickness increases. No matter what material is used, thinner is better as far as oxygen transmission is concerned. The down side of ultra thin lenses is undesirable flexing on an astigmatic cornea and decreased durability. |
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The base curve (BC) is also known as the central posterior curve (CPC). As the name suggests, this is the curve in the center of the lens on the posterior surface, the surface that touches the cornea. The base curve is adjusted relative to keratometer readings, which give the curvature of the corneal cap, or apex. |
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With regard to the keratometry readings, "K" usually refers to the flattest of the two readings. For example, with readings of 45.00 x 180 and 42.00 x 90, 42.00 would be the flatter reading and would be the value of K. A contact lens can be fit "on K", which would be a base curve of 42.00 in our example. The lens can be fit steeper than K. An example would be a base curve of 42.50. The lens can be fit flatter than K. An example would be a base curve of 41.50.
The initial base curve is typically selected relative to the amount of corneal cylinder present. The protocol is usually supplied by the manufacturer of the lens, or it can come from another source. More on this when fitting methods are discussed.
As discussed in the soft contact lens modules, the shape of the cornea is complex (aspherical to be more precise). Conventional keratometry only characterizes a small portion, the corneal cap. A hard contact lens has to be fit more precisely to the shape of the cornea because it does not "drape" over the surface like the soft lens does. It would be helpful to have an instrument that would more precisely measure the shape and guide you in designing the lens. Although hard lenses have been successfully fit for many years using the keratometer, the corneal topographer is a relatively new tool which will advance the art and science of hard contact lens fitting. |
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Corneal topography
A corneal topographer is an instrument that makes a contour map of the shape of the cornea, much like the topographical maps of the earth's surface. In recent years topographers have become more portable and pricing has come down, so that what was once a research tool has become a more common clinical tool. The keratometer is limited to measuring 4 points of data within the corneal cap (central 4mm). The corneal topographer measures thousands of points over the entire surface of the cornea. |
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| There is more than one method for obtaining the measurement. A common method is based on Placido's disk, which projects an image of concentric rings upon the corneal surface. On the immediate right is a picture of an antique Placido's disk, with a more modern version pictured on the far right. With a computerized topographer, the computer compares the shape of the reflected rings to the shape of the projected rings and a contour map is produced, as pictured above. |
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An example of such an instrument is the Oculus Easygraph (we have no financial interest in this instrument). | ||
| The contour maps are color coded to make evaluation easier, with the hotter colors representing more height/curvature/power and the cooler colors representing the opposite. The Oculus claims that the Easygraph also functions as a keratometer, providing "real" K readings instead of "simulated" K readings. |
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The corneal topographer has most commonly been used to screen patients for keratoconus and irregular astigmatism prior to refractive surgery, and to provide additional data to determine how much corneal tissue should be removed in refractive surgery. It is also used to diagnose and manage diseases and conditions that affect the corneal curvature, such as keratoconus, irregular astigmatism, corneal scars, and corneal transplants. | ||
| The corneal topographer may come with software that aids in the fitting of RGP lenses. Based upon the topographic map, the software will recommend RGP lens parameters for an "optimum" fit, and some software will provide a simulated fluorescein pattern. The parameters can be altered by the practitioner and the software will present a predicted fluorescein pattern based upon the changes. | |||
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Calculating the
initial lens power The initial power of the RGP lens is selected as follows: 1. Convert the manifest refraction or the glasses correction to minus cylinder form. Example: -7.50+1.50x180 transposes to -6.00-1.50x90. 2. Drop the cylinder power and use only the sphere component of the prescription. Example: -6.00-1.50x90 becomes -6.00. 3. Adjust for vertex power. You will need a table or a vertex calculator for this. You are changing the vertex distance from around 12-14 mm to zero (corneal contact). Using the free optics calculator from eyetec.net, a -6.00 lens power at 12 mm changes to a -5.62 lens power. If the lens was +6.00, then the power would adjust to +6.50. This is an important concept to keep in mind. As the vertex distance is reduced, minus lens powers go down, and plus lens powers go up. The initial power for the trial lens in our example will be -5.62 D. Of course, if working from a trial set, you would choose the lens power closest to -5.62 D. But what happened to that -1.50 D cylinder power, you might be asking yourself, how does that get corrected? This is where the "tear lens" comes into effect. Suppose that we fit our lens on K. This of course means that the base curve corresponds to the flattest meridian of the corneal cap. Let's continue with our example of a -5.62-1.50x90 correction. Suppose our K readings are 46.00 x 180 and 44.50 x 90. Since we are fitting on K, our base curve would be 44.50. In the 90 degree meridian, our lens would have the same (approximate) profile as the cornea. In the 180 degree meridian, however, the corneal curvature is steeper than the contact lens curvature (46.00 vs. 44.50). |
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| This creates a gap toward the periphery of the lens. This gap is filled by the tear layer (red layer on the 180 degree image). Optically, this tear layer acts like cylindrical lens power to correct for the astigmatic curvature. Thus, the "tear lens". |
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The tear lens of the RGP contact lens is
more efficient than the cylinder correction in the soft toric lens,
because the tear lens fills the gap perfectly, and rotation of the lens
has no effect on the performance of the correction.
The base curve / lens power relationship As discussed earlier, sometimes a base curve is selected that is steeper or flatter than K. How does this affect the tear lens and lens power? Fitting the base curve steep or flatter than K modifies the tear lens. If the base curve is flatter than K, then the power must be adjusted by the same amount in the plus direction (flatter-add-plus, or FAP). If the base curve used is steeper than K, then the power must be adjusted by the same amount in the minus direction (steeper-add-minus, or SAM). Using our example, suppose that we choose to use a 45.00 base curve. This is .5 D steeper than our flat K of 44.50. We would add .5 D more minus to the -5.62 power to arrive at an adjusted power of approximately -6.00 D. The final lens power should be confirmed by an over-refraction once the trial lens has stabilized on the patient's eye. Lens diameter and the optical zone RGP contact lenses are usually designed with a diameter 2 to 3 mm smaller than the horizontal visible iris diameter (HVID). The visible iris diameter is similar to the corneal diameter, which is also similar to the measurement known as "horizontal white to white".
If you were to measure these anatomical distances with a microscope, you would find a microscopic difference in the numbers. For you or me eyeballing it with a millimeter ruler, there is no difference. For a 12 mm cornea, a typical RGP lens would have a diameter from 9 to 10 mm, depending upon the fitting technique. As it is with soft contact lenses, for a given base curve, a larger diameter lens will fit tighter, and smaller diameter lens will fit looser. When determining the diameter, other factors to consider are the palpebral aperture (the lid opening), the corneal curvature, and the patient's activity level. As will be discussed, the size of the lid opening will influence what type of fit is used, which in turn will affect the diameter. A steeper than average cornea generally requires a smaller diameter lens for a good fit. Patient's involved in sports activities have more stable vision with a larger lens diameter and optical zone diameter. The lens diameter and the optical zone diameter go hand in hand. The optical zone is the area of the central posterior curve (CPC), otherwise known as the base curve.
As pictured above, the optical zone is the lighter colored central area and the peripheral curves are in the darker area at the edge. You can see that the larger the optical zone diameter is, the smaller the area available for the other curves. Generally, the optical zone is 1.0 to 1.5 mm smaller than the overall lens diameter. The optical zone must be large enough to cover the pupil, otherwise the patient will complain if glare, especially when the pupil gets larger in low light situations. The OZD can have a dramatic effect on the fit of the lens. As with the overall diameter, a larger OZD makes the fit tighter, and a smaller OZD makes the lens fit looser. RGP lens manufacturers have standard specifications for the OZD and the other parameters for a given lens diameter. Many practitioners don't vary these values, but a skilled and experienced fitter can manipulate these values to optimize the fit of the lens. In other words, be aware that a change in the optical zone diameter can change the fit of the lens, even if the overall diameter is not changed. Aside from the power, base curve, diameter, and optical zone, the RGP lens has the following design parameters, which are pictured below:
The cornea is aspherical, meaning it is steeper in the center and flatter toward the periphery. Because of this, transitions zones are needed in the contact lens to make the lens fit better. These zones are the "intermediate curve" and the "peripheral curve". Their shapes are progressively flatter than the central "base curve".
These curves are usually determined by a "nomogram", which is a formula based upon other lens characteristics. Skilled contact lens fitters can modify the intermediate and peripheral curves to optimize the fit of the lens. In general, making these curves steeper and narrower will steeping (tighten) the fit of the lens. Making these curves flatter and wider will flatten (loosen) the fit of the lens. The edge of the lens should not be tight against the cornea. It should allow enough clearance for tears to circulate under the lens, but not too much clearance, which may adversely affect the fit of the lens. Edge clearance can be evaluated with fluorescein, as will be discussed. RGP lens fitting techniques generally work best with very thin lenses. Minimal lens thickness should be ordered and should be modified only if excessive lens flexure occurs on the cornea. Lens thickness is expressed as center thickness, which is a function of lens power and lens diameter. A minus lens will be relatively thinner in the center and a plus lens will be relatively thicker in the center. Gas permeable contact lens fitting designs vary with the fitter and are influenced by who the fitter learned from, the fitter's own experience, manufacturer recommendations, what they had for lunch, and other factors. When the fog clears, there are generally two main fitting methodologies: "apical clearance" and "corneal alignment". This method has also been termed "interpalpebral lens design" or "central palpebral design". As the terminology implies, the objective is to get the lens to center on the corneal between the eyelids. This method can be useful for the patient with tight lids and/or a small lid opening and it can also be useful for the patient with a large lid opening that will not support an alignment or lid attachment type of fit. The fit is accomplished with a lens fit steeper than the flat K and with a relatively smaller diameter. The base curve selection is dependent upon the amount of corneal cylinder and the lens diameter. The lens diameter is dependent upon the corneal diameter and the lid opening. Base curve and diameter selection tables vary according to the source, but typical numbers are represented below.
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| Corneal cylinder | BC selection | ||
| 00 to .5 D | .25 D steeper than K | ||
| .75 to 1.25 D | .37 D steeper than K | ||
| 1.5 to 2.0 D | .50 D steeper than K | ||
| 2.25 to 2.75 D | .75 D steeper than K | ||
| 3.0 to 3.5 D | .1.00 D steeper than K | ||
| Corneal diameter | Lens Diameter | ||
| 12 mm plus | 9.1 mm | ||
| 11 to 12 mm | 8.6 | ||
| less than 11 mm | 8.2 | ||
| Remember that a
larger diameter lens will require a flatter base curve to fit the same
as a smaller diameter lens, and vice versa. A smaller diameter may
be required for the lens to "float" between the lids if the lid opening
is small. With this fitting method, plus power lenses might require a slightly steeper fit than minus lenses to improve centering. Remember that plus power lenses are thicker in the center of the lens and minus power lenses are thicker at the edge of the lens. If the lens is thicker in the center, it tends to ride lower due to gravity and the lid pushing downward on the lens. |
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| Above is an animation of an apical clearance (interpalpebral) fitting. The lens diameter is smaller than the diameter of a corneal alignment lens. The lens fits between the lids. As the lid blinks, the lens rides upward with the lid and then sinks downward on the cornea. The lens should not sink to the lower lid margin. It should float between the lids. | |||
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This method is also termed, "upper lid attachment", "lid attachment", "Korb design", "modified Korb design", and "lid interactive". There seems to be endless variations of the basic design. The idea is for a relatively large diameter lens, fit flatter than K, to ride high on the cornea and be supported by an overlying upper lid. The apical clearance fit moves freely on the cornea. The lens fit with corneal alignment moves only when the lid blinks. The vision is supposed to be more stable with less lens awareness and a more natural blink compared to the apical fit. This fit does not work with a high upper lid that covers little or none of the cornea. As with the apical fit, the initial base curve selection depends upon the amount of corneal cylinder. Comparing the apical fit table with this corneal alignment fit table, you will notice that these base curves are generally flatter than K as opposed to the steeper than K base curves of the apical table. |
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| Corneal cylinder | BC selection (for 9.5 diameter lens) | ||
| 00 to .5 D | .75 D flatter than K | ||
| .75 to 1.25 D | .50 D flatter than K | ||
| 1.5 to 2.0 D | .25 D flatter than K | ||
| 2.25 to 2.75 D | .on K | ||
| 3.0 to 3.5 D | .5 D steeper than K | ||
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The diameter of the corneal alignment lens is not so dependent upon the corneal diameter, it just needs to be relatively large. A good place to start is at 9.5 mm. This can be adjusted according to how the lens behaves when on the cornea. Remember, for a given base curve, a larger diameter will fit tighter and a small diameter will fit looser. |
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| Above is an animation of a corneal alignment (lid attachment) fit. The lens has a larger diameter than an interpalpebral lens. The lens is always "attached" to the upper lid and tends to ride high. There is less movement with the blink than there is for an interpalpebral lens. | |||
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Evaluation of the contact lens fit
In my opinion, for the first time RGP contact lens wearer, it is absolutely critical that the lens be inserted the first time with a topical anesthetic drop instilled prior to insertion. This permits the patient's critical first impression to be pleasant and pain free. It also gives you time for a fair evaluation of the lens fit without excessive tearing and squeezed lids. The patient will gradually experience the inevitable foreign body sensation as the anesthetic wears off.
The following discussion of lens evaluation applies to the patient returning for a "yearly exam" as well as for the new fit.
The visual acuity with the contact lenses should be tested before any other testing is done. You do not want the bright light from a slit lamp exam to affect the reading.
The fit of the RGP contact lens should be evaluated according to the following criteria:
1. Positioning and movement: This depends to upon the fitting method, that is, apical clearance design vs. corneal alignment design. A lens fit for apical clearance should center on the cornea, between the lids. It should move upward slightly with the blink and settle to a centered or slightly below center position. A lens fit for corneal alignment will ride high, under the upper lid. It will tend to move with the lid. In either case, the lens should not drift off to one side or the other, and it should not ride low.
Modification to an initial fit of an apical clearance design generally begins with an adjustment of the base curve of the lens. For a given diameter, a steeper base curve (e.g. changing from 44.00 to 44.50) will fit tighter and a flatter base curve (e.g. changing from 45.00 to 44.50) will fit looser.
Modification to an initial fit of a corneal alignment design generally begins with an adjustment to the diameter of the lens. For a given base curve, a larger diameter with fit tighter and a smaller diameter will fit looser. A larger diameter will also tend to have more coverage by the upper lid, increasing the "lid attachment".
The amount and type of corneal astigmatism will affect the fit of the lens. So called "with-the-rule" astigmatism is created when the vertical corneal curvature is greater than the horizontal corneal curvature (e.g. 44.00 x 90, 42.00 x 180). "Against-the-rule" astigmatism is the opposite. A lens on a WTR cornea will tend to decenter vertically. A lens on an ATR cornea will tend to decenter horizontally. Horizontal decentration is more of a problem than vertical decentration. A steeper than recommended base curve for the ATR cornea will sometimes solve the horizontal decentration problem. I think it is best to try the recommended base curve first, before going to a steeper BC.
These are general guidelines, however. Experienced practitioners have their favored methods for modifying the fit. The diameter of the optical zone, the peripheral curves, and the edge profile can also be modified to affect the fit.
For the patient new to your practice, who is already wearing RGP lenses, it is best not modify the design unless necessary. The old saying, "if it ain't broke, don't fix it", certainly applies to contact lens management.
2. Condition: This has to do with the optical quality of the lens. Is it clear, and does it wet well with the tears? The tear layer should coat the lens evenly so that a continuous, clear optical surface is created, without any dry spots or oily spots. Many times a good cleaning can remove surface deposits and improve a lens that wets poorly. A new lens may need a few cleaning and rinsing cycles before it wets properly. The manufacturer of the lens material usually has specific recommendations as to cleaning and wetting solutions.
A few superficial scratches generally do not cause problems with comfort or acuity. Lens polishing can remove superficial scratches and improve wetting.
3. Visual acuity: The proof of the pudding for most contact lens wearers is their visual acuity. If the vision is good, most of the time they are happy. If their vision is not good, usually they are not happy. The lens positioning and the condition of the lens should always be evaluated before evaluating the vision. (As stated earlier, the visual acuity with the lenses should be checked first thing, before any other testing or examination is performed). If the lens centers poorly or is covered with deposits, you are wasting your time with a vision analysis, because those other problems must be dealt with first. The following discussion assumes that you have a well positioned, clear lens that has a good tear coating.
After examining the lens positioning and movement with the slit lamp, allow a few moments for the patient to recover from the bright light, and then perform a spherical over-refraction with a phoropter, or with loose lenses. If the vision is good and crisp with the over-refraction, and it seems stable with blinking, then you can generally assume that flexure is not a problem and that residual astigmatism is not a problem.
The power of the lens can be adjusted according to the results of the over-refraction. If the over-refraction is +0.50, then you will want to adjust the lens power by .50 D in the plus direction, meaning a +1.00 power becomes a +1.50 power, or a -2.00 power becomes a -1.50 power. In almost all cases, a plus power adjustment should be made to the contact lens power if indicated by over-refraction, but beware of the minus power adjustment. Just because you get an over-refraction of -0.75 D, do not assume that the power adjustment is in the best interest of the patient. Many older (35+) patients may do better with the improved intermediate/near vision of the current situation, rather than give that up for a slight improvement in distance vision. With minus adjustments, make sure your patient understands what is changing.
If the visual acuity with spherical over-refraction is not good, stable, and sharp, then you must test for flexure and residual astigmatism.
Subject matter continued on Module 36 (Modules 35 and 36 each have their own Post-Tests). |
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| Back to top Go to Module 35 Post-test Go to Module 36 | |||
