Eccentricity Changes in GP Lens Design
Eccentricity Changes in GP Lens Design
Knowing how eccentricity affects GP lens design can help improve your GP fitting success.
By Kelly Indovina, OD, & Roxanna Potter, OD
Dr. Indovina is the director of Professional Affairs at Art Optical Contact Lens. She enjoys teaching advanced contact lens fitting and problem solving to Michigan College of Optometry students and residents who spend time at Art Optical as part of their clinical rotations.
Dr. Potter is currently in private practice in Sylvania, Ohio. She has designed and participated in numerous research projects and has given lectures to both students and optometrists on eye-related topics.
Fitting irregular/keratoconic corneas with GP lenses is a challenging process. It requires knowledge of available parameters and designs to create lenses that will provide the best fit and vision for these unique patients.
Newer lens design options allow for the control of the base curve eccentricity value. It's important to understand how you can manipulate this variable to create desired resultant changes in lens fit.
Figure 1. Patient #1 baseline topography maps for OD (left) and OS (right).
The purpose of this analysis is to demonstrate, with photos, the differences that occur with changes in eccentricity when all other parameters remain constant. The first part of this analysis will focus on changes in fit due to various eccentricities with two different optical zone diameters on a normal cornea, and the second part will discuss further uses of these concepts as they apply to an irregular, keratoconic cornea.
Figures 2a (top left) through 2f (bottom right) demonstrate the impact of increasing back surface lens eccentricity on the lens-to-cornea fitting relationship.
Figure 3a (left) shows a lens with a 0.0 e-value and an optical zone diameter of 7.6mm. Figure 3b (center) shows an eccentricity value of 0.0 and an OZD of 8.2mm. Figure 3c (right) shows and e-value of 0.50 and an OZD of 8.2mm.
How Eccentricity Works
Many variables determine how a GP lens will fit. We've chosen to evaluate eccentricity and how it relates to optical zone diameter (OZD). Eccentricity is the rate of flattening of the posterior central curve radius of a lens and is a way to quantify aspheric changes across the diameter of that curve. The higher the eccentricity (e-value), the more quickly the lens flattens from center to periphery. A spherical lens has an e-value of 0 (zero) and a hyperbola has an e-value of 1.
When a spherical and an aspheric lens have the same base curve radius (BCR), the aspheric base curve will fit flatter because of the eccentricity. If two lenses have different optical zone diameters, the lens with the larger OZD will fit steeper due to increased sagittal depth. Knowing how to control these two parameters will help improve success in fitting GP lenses.
Varying Eccentricity and OZD for a Normal Cornea
Table 1 lists the refraction, keratometry and lens parameters for Patient #1, and Figure 1 shows the topography. We empirically fit this patient with standard tri-curve GP lenses of two different optical zone diameters (7.6mm and 8.2mm). Following the initial fit, we placed lenses of increasing e-values for both optical zone diameters on each eye and photographed them in order.
From this analysis, we found four general points of clinical relevance:
1. As e-value increases, lenses tend to ride higher with increased inferior edge lift. Figures 2a through 2f of Patient #1's left eye illustrate the progressive changes in fit due to varied eccentricity. The trends of superior decentration and increasing edge lift continue until excessive flattening causes the lens with the greatest e-value to fall inferiorly.
In lenses fitted with lid attachment, the superior lid will pull the lens higher with each increasing step in e-value (and resultant peripheral flattening). You would traditionally fit a patient who has wide apertures and little or no lid attachment with smaller, steeper lenses to avoid inferior decentration. This was the case with our patient, but we were able to maintain centration with a steeper base curve and larger optical zone and avoid midperipheral seal-off by including eccentricity.
Figure 4a (left) shows a lens with an e-value of 0.75 and an OZD of 8.2mm. Figure 4b (right) shows a lens with an e-value of 0.75 and an OZD of 7.6mm.
2. Increasing eccentricity can decrease midperipheral bearing without the need to change base curve. Figure 3a shows a spherical base curve lens (e-value = 0) with a 7.6mm OZD. You can see that the lens is decentered superiorly. In Figure 3b we increased the OZD to center the lens, but now the lens has excessive midperipheral bearing due to increased sagittal depth and a resultant steeper fit. In Figure 3c we added an eccentricity of 0.50 to this design. The lens is well centered with no midperipheral bearing.
To decrease midperipheral bearing, traditionally you'd need to either flatten the BCR or decrease the OZD, which may induce central touch or lens decentration. Increasing the e-value (in this case from 0 to 0.5) would allow you to maintain the base curve and desired central fit but eliminate any midperipheral bearing.
Figure 5. Patient #2 baseline topography maps for OD (left) and OS (right).
3. Certain e-values have more clinical significance than others. A change from 0 to 0.5 resulted in beneficial changes in the lens-to-cornea fitting relationship for Patient #1. Above this, trends in peripheral flattening and superior decentration continued, but didn't appear to be clinically useful for this patient. Interestingly, central bearing didn't occur as the eccentricity increased until we used very high e-values (1.2 and 1.3). E-values above 0.9 created undesirable visual aberrations and subjective complaints of halos and blur that may decrease the quality of vision in patients wearing these lenses.
4. Optical zone diameter influence is most significant with e-values less than 0.65. As eccentricity increases, it begins to override OZD as the determinant factor in fit (with a constant base curve).
Figures 4a and 4b show that the lenses (OD) fit similarly superior/temporal with inferior edge lift with both OZDs. As OZD increases, a larger area of the lens is available for the eccentric base curve to continue to flatten. This may explain why the steepening effect a larger optical zone creates isn't observed in higher eccentricities.
Eccentricity can be a valuable parameter option when standard lens parameters fail to create an optimal fit. Next we'll discuss the application of this parameter in irregular corneas, where having increased options in lens design control becomes even more critical.
Varying Eccentricity and OZD for an Irregular Cornea
Table 2 lists the refractive and current GP contact lens specifications for Patient #2. Figure 5 shows the baseline corneal topography maps for this keratoconus patient. We refit him into GP lenses with controllable e-values with two different OZDs. We increased e-values in the same way as for our first patient.
Figure 6. The impact of changing eccentricity from 0.0 (6a, left) to 0.50 (6b, right) in a lens with an OZD of 7.50mm on a patient who has keratoconus.
Figure 7. The impact of changing eccentricity from 0.0 (7a, left) to 0.50 (7b, right) in a lens with an OZD of 8.0mm.
We found three general points of clinical relevance in our analysis of the data:
1. Increasing the eccentricity can eliminate midperipheral bubbles. Figure 6a shows a bubble in the nasal midperiphery (OD) that corresponds to the area visible on the topography map at the transition between the steepened area of the cone apex and the flatter periphery. As Figure 6b demonstrates, increasing the e-value to 0.50 eliminated the midperipheral bubble.
OZD control is also crucial for proper alignment and sometimes for vision. If your patient is experiencing glare or other visual complaints, possibly due to a small OZD, and a good fitting relationship is present, typically you should increase the OZD. This may result in bubbles or midperipheral bearing. You could also control this scenario by increasing eccentricity to maintain the aligned fit. As the OZD increased in the patient's left eye, a bubble appeared, but was eliminated with an increase in eccentricity (Figures 7a and 7b).
Asphericity can be particularly helpful in irregular corneas that have areas of quick and/or large transitions because the eccentricity allows a smoother, junctionless transition between the central base curve radius and the periphery of the lens.
2. Higher eccentricities can create increased amounts of bearing on the apex of the cone. Even though the lenses for the right eye shown in Figures 8a and 8b have different OZDs, they adequately demonstrate the increased bearing related directly to the increase in eccentricity.
Figure 8. The difference in the fitting relationship with a right lens having an e-value of 0.65 and an OZD of 7.50mm (8a, left) versus a lens with an e-value of 0.90 and an OZD of 8.00mm (8b, right).
Figure 9. The difference in the fitting relationship with a right lens having an e-value of 0.90 (9a, left) versus a lens with an e-value of 1.20 (9b, right), both with a constant OZD of 8.00mm.
Increasing e-values can likewise be detrimental if you make no corresponding adjustment to the BCR or other lens parameters. You may be familiar with this concept as it applied to some aspheric multifocal lenses; these lenses are often fit steeper to compensate for aspheric back surfaces.
3. Very high eccentricities can result in lid attachment, even in corneas that have extremely steep inferior areas. Corneas with extremely inferior irregular areas can be some of the most challenging to fit with contact lenses, with the most difficult of all pertaining to pellucid marginal degeneration. Traditional keratoconic lens designs on these patients tend to decenter inferiorly with a corresponding decrease in movement and comfort. Figure 9 shows how a very high e-value causes the upper lid to adhere to and hold the lens up in Patient #2 (OD).
As in Patient #1, however, eccentricities at this level can cause undesired visual side effects such as glare, halos and decreased vision.
Improving GP Fitting Success
Each variable involved in fitting GP lenses contributes to the overall fit. Knowing how to manipulate these variables can increase your success in fitting your patients with GP lenses that provide a good fit as well as good vision. Eccentricity is another tool we can take advantage of when designing GP lenses for both regular and irregular corneas. CLS
Contact Lens Spectrum, Issue: April 2008