Designing GPs from Corneal Topography
DESIGNING GP LENSES
Designing GPs from Corneal Topography
Using a corneal topographer for easy or challenging cases can help make your fitting process more efficient.
By Randy Kojima, FAAO, & Patrick Caroline, FAAO
Randy Kojima is a research scientist and clinical instructor, Pacific University College of Optometry and director, Technical Affairs, Precision Technology Services.
Patrick Caroline, FAAO, is an associate professor at the Pacific University College of Optometry. He is also a consultant to Paragon Vision Sciences.
For many practitioners, the topographer has become an indispensable tool for fitting specialty contact lenses. It allows us to appreciate much about the shape of the cornea and its contribution to vision. Topography also plays a major role in determining the correct fitting technique; yet, few practitioners have come to trust the instrument to design GP lenses. The goal of this article is to review the step-by-step process of creating a GP lens from a corneal topographer for more efficient and successful fitting outcomes.
The Topographical Fitting Process
Table 1 provides our "Rules" of GP Lens Fitting. Figure 1 represents the ideal image of all the rules in Table 1. Let's begin the process of building a GP lens from a topographical map. Our patient (Case 1) has a right eye manifest refraction of –3.00 –1.00 × 180. Corneal topography demonstrates a symmetrical, apical, with-the-rule corneal astigmatism (Figure 2) and a visible iris diameter of 11.0mm (Figure 3). Table 2 summarizes the four steps for lens design in this case.
Figure 1. An ideal GP fit with apical clearance, landing at 3 o'clock and 9 o'clock, and a channel of tear layer along the vertical meridian.
|Rules for Designing GP Lenses from Corneal Topography|
The optimum diameter should be large enough to stabilize the lens along the horizontal meridian while smaller than the visible iris diameter to allow for 1mm of movement vertically.
The central base curve should be steep enough to clear the corneal apex (definite apical clearance – approximately 20 microns).
The GP lens should be steep enough to land midperipherally along the horizontal meridian.
VERTICAL CHANNEL OF TEARS
The lens should exhibit unobstructed movement along the vertical meridian.
Figure 2. A topographical image of the patient's right eye with a moderate 1.40D corneal astigmatism. This topography can be described as symmetrical, with-the-rule, apical astigmatism.
|Applying the Design Rules to Case 1|
STEP 1: DIAMETER SELECTION
11.0mm (VID) – 2.0mm = 9.0mm lens diameter
STEP 2: APICAL CLEARANCE
Steepen or flatten the base curve in the topographer fitting software to achieve approximately 20μm central tear layer clearance. In this example a base curve of 42.25D (7.99mm) produces the desired apical clearance (Figure 4).
STEP 3: HORIZONTAL ALIGNMENT
By fine tuning the base curve to produce 20μm of apical clearance, a landing at 3 o'clock and 9 o'clock along the flat horizontal meridian occurs on the with-the-rule corneal astigmatism.
STEP 4: VERTICAL CLEARANCE
A channel of tears and, therefore, free and unobstructed movement is created along the vertical meridian that results due to the steeper curvature along the steep corneal axis.
Step 1: Determining Lens Diameter
You cannot determine the ideal GP lens without knowing the optimum size of the lens for each individual eye. Many corneal topographers provide highly accurate visible iris diameter measurements based on the photo keratoscope image. Some instruments automatically derive pupil size and visible iris diameter (VID) by comparing the pixel density changes between these anatomical features. Other instruments require a manual, user selected measurement of the corneal diameter from white to white (limbus to limbus, Figure 3). Measuring across an oblique axis provides an excellent mean VID value because the cornea is generally longer horizontally and shorter vertically. Regardless of which method or axis used, this is a more accurate way of taking the correct first step in designing your GP lenses.
Figure 3. A corneal topographer can easily and accurately measure the visible iris diameter (VID) to aid in determining the optimal GP lens diameter.
While many GP manufacturers utilize a specific lens diameter, clinical experience has shown us that the lens diameter is best calculated based on the measured VID. In a study of 200 left eyes, Caroline and Andre (2000) found that the average corneal diameter was 11.8mm. Clinical experience has shown that the overall diameter of a GP lens is best determined by subtracting 2.0mm from the VID. This formula results in a lens large enough to be stable on-eye yet small enough to allow for appropriate movement and tear exchange. Based on this, for our patient the suggested lens diameter was 9.0mm (11.0mm VID – 2.0mm).
Step 2: Determining the Base Curve Radius
Historically, the base curve of a rigid contact lens was calculated from a fitting nomogram referenced to the central keratometric readings. Today, modern corneal mapping provides us with data across the entire corneal surface. Additionally, one of the principal advantages of corneal topography fitting software is that it allows us to view the tear layer profile along any meridian. Beyond selecting the initial base curve radius to produce an ideal apical tear layer clearance, we can also see how the lens clears and bears over the entire back surface area in every axis.
Most of you are probably familiar with creating a GP fit that has definite apical clearance across the central back optical zone of the lens. We do not typically quantify the tear layer thickness in microns (μm) but rather qualify the central clearance as inadequate, acceptable, desirable, or excessive. An advantage of corneal topography fitting software is that it allows us to design contact lenses to a micron level of tear layer thickness between the lens and cornea. In other words, we can calculate a specific apical clearance rather than "guesstimating" based on the in situ appearance.
The base curve radii (BCR) you choose should create an apical clearance of between 20μm to 30μm (Figures 4 and 5). Apical clearances >40μm may produce apical steepening and spectacle blur, and the pattern will exhibit the appearance of excessive central clearance. In such cases simply flatten the BCR, or steepen the BCR if the apical clearance is <20μm, to produce a central or apical tear film thickness of approximately 20μm to 30μm between the contact lens and cornea.
Figure 4. The topographer's lens fitting software generates a predicted fluorescein pattern with the tear film thickness and scale (left) measured in microns. The graph corresponds to the white line, along the horizontal (3 o'clock and 9 o'clock) meridian and displays the tear profile along that axis. The goal is to steepen or flatten the BCR until an ideal apical clearance of 20μm has been created (blue arrow). On our with-the-rule corneal astigmatism, this results in the lens landing along the horizontal meridian at 3 o'clock and 9 o'clock as shown by the red arrows. The green arrows indicate the lift at the edge of our GP lens of approximately 90μm to 100μm.
Figure 5. The tear layer profile along the flat meridian (axis of lens bearing) indicating approximately 20μm of apical clearance (blue arrow), a soft landing on both sides of the cornea (red arrows) and good edge lift of 80μm to 100μm (green arrows).
Steps 3 (Horizontal Alignment) and 4 (Vertical Clearance)
After selecting the BCR and creating the desired apical clearance, we can expect a principal law of GP lens fitting to surface. The flattest meridian of the cornea, and therefore the axis of lowest sagittal depth, will exhibit the greatest lens bearing in the midperiphery (Figure 5). In other words, we can expect the GP lens to bear along the flattest axis, creating a "fulcrum" of contact with the peripheral cornea as described in a previous paper by Caroline et al (1998). Conversely, the steepest meridian would exhibit clearance in the midperiphery. As you would expect, our with-the-rule corneal astigmat would manifest the fulcrum or midperipheral bearing on the horizontal meridian where the cornea is flattest (lowest sagittal depth) and a channel of tears under the vertical meridian where the cornea is steeper (highest sagittal depth). In against-the-rule corneal astigmats, the reverse is true in which the lens creates bearing in the midperiphery at approximately 12 o'clock and 6 o'clock and clearance across the horizontal meridian at or near 3 o'clock and 9 o'clock.
For good lateral stability of the GP, the fulcrum should exist along the horizontal meridian. Combined with a channel of clearance along the vertical meridian, the lens can freely move with the blink, producing the desired tear exchange, lid attachment, and comfort. With a fulcrum at 12 o'clock and 6 o'clock and clearance along 3 o'clock and 9 o'clock, the lens movement is unobstructed laterally and may not produce acceptable centration, movement, tear pump, lid attachment, or comfort.
Step 5: Determining Lens Power
This is the easiest step in the process and one you will be most familiar with. If you have entered the patient's prescription into your topographer's lens fitting software, the instrument can determine the final power.
If your instrument does not calculate lens power, you have two options. You can employ the rules of SAM (Steep Add Minus) and FAP (Flat Add Plus), accounting for the tear power that results from the BCR/flat keratometry (K) relationship. Or, the most accurate method would be to place a diagnostic GP of the same BCR on the eye, allow for 20-to-30 minutes of settling, and then over-refract to calculate the required lens power. The diagnostic method reduces any error in the instruments' estimation of shape (flat K or apical radius) and also accounts for any settling or tear power anomalies that might exist. Additionally, diagnostic lenses confirm the accuracy of the entire fit process.
Some other factors to consider when designing GPs from corneal topography include the following.
Accuracy of the Capture For the capture to be accurate, placido disk instruments rely on an even tear film. If there is tear breakup or uneven tear film, the rings may "kiss" or touch (Figure 6), which is known as "ring jam" and causes topographical analysis error (extrapolation error). Considering that the topographer is attempting to determine the sagittal depth of the eye in order for a contact lens to be built, it is critical to avoid analysis error of the eye shape and height. It is recommended to take multiple captures, approximately four, and select the best reading. It is also important to attempt to achieve the largest fissure size possible, avoiding lid blockage and eyelash shadows. Figure 7 shows a poor, small fissure capture while Figure 8 displays a desired photo.
Figure 6. Note the distorting, breaking, and collapsing of rings inside the red circle. This would result in analysis error (extrapolation error). A quality topography capture should exhibit rings that look even and parallel as most appear outside the red circle.
Figure 7. Poor Capture: small fissure.
Figure 8. Perfect Capture: 100 percent corneal coverage with even parallel ring reflection.
Capturing on the Geometric Center Corneal topographers capture a nasally skewed image as a result of our temporally displaced fovea and off-set fixation. In other words, patients look "out" as they look down the axis of the instrument. We can view the evidence of this in the way all topography maps appear to be capturing more nasal than temporal cornea (Figure 9). To adjust for this so that GP lenses are designed based on the geometric rather than the visual axis, have the patient view one-to-two rings toward the nose directly along the horizontal line of sight. In other words, for the patient's right eye, ask him to look one-to-two rings toward 9 o'clock from the fixation target. For the left, the patient should look toward 3 o'clock. This centers the placido disk to the geometric center (Figure 10) and helps prevent error when the rings distort on the limbus or sclera. Considering that we attempt to fit all our lenses to the geometric axis, it's logical that we would build the lenses from the same reference point on the topography.
Figure 9. Typical map of the capture on the visual axis which results in a nasally favored image on this right eye.
Figure 10. Same patient as in Figure 9 but with a capture on the geometric center, which results in less limbal ring distortion and an ideal image to calculate GP lenses from in the topography fitting software.
Surface Area of Bearing A practitioner can "soften" how the GP lens bears on the peripheral cornea by increasing or decreasing the optical zone. Determine the flat axis that will exhibit the greatest bearing of the lens on both sides of the cornea. Observe the tear profile on the graph on this flattest meridian. Note that by changing the optical zone diameter (OZD) width, the surface area of contact with the cornea can increase or decrease. It is desirable to create a lens with more peripheral contact rather than less as narrow areas of bearing could impinge, bind, and produce discomfort and molding. Observe how changes in OZD, smaller (Figure 11) or larger (Figure 12), alter the point of greatest bearing along the horizontal meridian and the volume and depth of tears at the edge.
Figure 11. The correct apical clearance of 20μm (blue arrow) but the OZ is too small: note the point of bearing close to the apex (red arrows) with a wide peripheral lift and edge stand off beyond the scale of the graph (>100μm – green arrows).
Figure 12. An ideal apical clearance of 20μm (blue arrow) but the OZ is too large: note the point of bearing well out on the peripheral cornea, but more concerning would be the small surface area of bearing that comes to an abrupt point of contact on both sides (red arrows). An inadequate edge lift of <50μm could also be problematic (green arrows).
When integrating the entire design together, build your ideal tear layer profile of 20μm to 30μm of apical clearance, a wide area of peripheral alignment near the maximum OZD, and finally an edge lift of 60μm to 100μm (Figure 13).
Figure 13. Ideal apical clearance (blue arrow) and OZ: the profile exhibits a wide, aspheric surface area of contact (red arrow) while producing an appropriate edge lift of approximately 60μm to 100μm.
Astigmatic Eyes When fitting astigmatic eyes (Figure 14), build your lenses along the flat meridian (Figure 15) as noted above, then determine the tear layer profile along the steepest meridian. If the thickness of the tears at the edge of the OZD junction is greater than 40μm on opposing sides, there may be excessive lift, instability of the fit (too much movement), and a typical "dogbone" fluorescein pattern. When the tear layer clearance at the edge of the OZD surpasses the 40μm threshold (Figure 16), it is recommended that you build a toric back surface to better align with the cornea.
Figure 14. A right eye with limbus-to-limbus corneal astigmatism. Patients exhibiting limbus-to-limbus astigmatism typically require toric GP lenses.
Figure 15. The ideal tear layer profile along the horizontal meridian of our Figure 14 cornea.
Figure 16. The vertical (steep) meridian displaying excessive tear film thickness well in excess of the 40μm maximum threshold at the OZ junction (red arrows) and an edge lift well beyond the ideal of 60μm to 100μm (green arrows). Such a profile along the steep meridian would result in an unstable, uncomfortable GP fit. A toric surface would be recommended to better align along this steep meridian.
Asymmetrical Eyes For many reasons, keratoconic, post-surgical, and asymmetrical eyes are best fit with diagnostic lenses to confirm the ideal design parameters and lens power (Figure 17). It is on these eyes that the topographer becomes an invaluable tool for optimizing the initial diagnostic lens selected and, in many cases, the final parameters required.
Figure 17. An asymmetrical right eye of a patient who has a typical inferior oval keratoconus.
You should select the diagnostic kit you believe optimal for the shape of the particular eye. Then build the same trial parameters in the topographer fitting software to create a theoretical lens that produces an ideal or acceptable tear layer profile (Figure 18) and trial fit the actual lens (Figure 19). If the pattern of the theoretical and actual agree, you can manipulate the software-derived fit to produce an improved on-eye outcome in the custom ordered product.
Figures 18 and 19. Theoretical trial lens (Figure 18) determined by the topography fitting software and the same parameters of actual trial lens on-eye (Figure 19). Note the similarity in the patterns and the accuracy and predictability even on our Figure 17 keratoconic eye.
Instrument Accuracy Topographers come in a variety of accuracies, so it's critical to determine whether your instrument has error and to what degree. Monitor your initial fits to gauge whether the topography-design lenses result in a steeper or flatter fitting relationship than desired. If you have lenses that come out 0.50D too steep or flat, then factor this into your design calculation on all future patients. Knowing the error and the tendency of your topographer to measure either steep or flat will greatly aid you on all future fits.
Your topographer is clearly a fantastic tool for understanding corneal shape. But are you using it for all that you could? If your instrument comes with GP fitting software, let it aid you in the initial lens design or trial determination. Or, simply acquire perspective on the likely fluorescein pattern you will observe and possible areas of concern to monitor or adjust for.
Prior to designing your lenses, capture multiple readings of each eye and select the best capture from which to design your lenses. Attempt to capture the geometric center with as large a fissure size as possible, avoiding lid and eyelash hindrances.
Determine the initial diameter by subtracting 2mm from the measured VID. Next steepen or flatten the base curve to produce a lens with a desired apical clearance (approximately 20μm to 30μm) and calculate the final lens power required through the fitting software or by over-refracting on the same parameter diagnostic lenses.
A modern corneal topographer can be an efficient and effective tool for streamlining your GP practice, both for the simplest and most challenging eyes. Be sure to use this instrument to its fullest potential in your practice. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #167.
The authors want to thank Dr. John Mountford for his inspiration and guidance.
Images were supplied employing the Medmont E300 Corneal Topographer from Precision Technology, Vancouver, B.C.
Contact Lens Spectrum, Issue: October 2009