letters to the editor
Materials and Modulus
Thank you for publishing Gregory J. Nixon, OD, FAAO's November 2008 continuing education article, "Contact Lens Materials Update 2008." His contribution is timely and a nice overview. I would like to clarify two important points regarding modulus.
The first regards the statement, "The negative impact of high-modulus silicone hydrogels on some patients provoked two companies to develop additional products with lower silicone content." The article cites CIBA Vision with introducing the lower-modulus Air Optix Aqua (lotrafilcon B, MPa 1.2) in the wake of Night & Day (lotrafilcon A, MPa 1.4). It then cites CooperVision with introducing Avaira (enfilcon A, MPa 0.5) following Biofinity (comfilcon A, MPa 0.75).
The text might infer that CooperVision introduced Avaira to address negative modulus-related issues with Biofinity. That is incorrect. Avaira and Biofinity have comparable silicone content. CooperVision positions its silicone hydrogels for different market segments. Avaira is for two-week replacement and daily wear. Biofinity is for monthly replacement and recently received FDA extended wear approval for overnight wear of up to six nights and seven days. Biofinity has the lowest modulus of the commercially available U.S. silicone hydrogels with a Dk of 110 or more.
The second point is related to the relationship between material stiffness, modulus and thickness. The article states that, "…contact lens modulus or material stiffness reemerged with the introduction of silicone hydrogels." Clinicians should know that lens modulus is not exactly synonymous with stiffness. Stiffness is proportionate to modulus and thickness. As an example, Bausch & Lomb received premarket approval (PMA) in 2008 to market PureVision 2 (balafilcon A, MPa 1.1), which has a thinner center thickness than does PureVision. Both lenses use balafilcon A, and since modulus is a material-intrinsic property, PureVision 2 and PureVision have a modulus of MPa 1.1. However, PureVision is stiffer than PureVision 2 because of its greater lens thickness.
As a parting thought, although oft maligned, high-modulus contact lenses are sometimes appropriate. They may evoke images of superior epithelial arcuate lesions, mucin balls, and contact lens papillary conjunctivitis, but highmodulus lenses can compensate for complex aberrations due to corneal irregularity. When high-modulus takes precedence, GP optics are frequently appropriate.
Brian Chou, OD, San Diego, CA
Dr. Chou is a consultant for CooperVision and SynergEyes.
Dr. Nixon's Response:
I thank Dr. Chou for his interest in the details of the "Contact Lens Materials Update 2008" and for his comment on the two points outlined above. I regret that my statements concerning modulus were misinterpreted as a rebuke on silicone as a material or of any individual lens product within the silicone hydrogel category. To the contrary, as you nicely outline, high-modulus lenses have many attractive features. Likewise, lower-modulus silicone hydrogels also possess many benefits including being "softer" as CooperVision's own marketing materials promote. As Dr. Chou suggest, these facts support that there is a place in the market for a variety of silicone hydrogel lenses with various silicone contents, modulus, wearing times and replacement schedules.
As to his second point, because the focus of the article was on contact lens materials, I made reference to "material stiffness" as it relates to the silicone content. I appreciate your additional insights that both lens design and thickness also contribute to the overall stiffness of a finished lens.
Commenting About KBA
In the December 2008 Contact Lens Case Reports column "Correcting Residual Refractive Error Post-Hyperopic LASIK," Patrick J. Caroline, FAAO, and Mark P. André, FAAO, describe the KBA as their "lens of choice" for post-hyperopic LASIK regression. I agree that fitting the KBA lens via the Medmont CL designer is a very efficient process. However, I am compelled to explain the KBA fitting process for practitioners who don't have access to Medmont topography.
The KBA (keratoconus biaspheric) is a large-diameter GP lens designed for central nipple and oval cones which, much like post-hyperopic LASIK corneas, have high eccentricity values. I agree with Pat and Mark that the major advantage of the KBA lens is the ability to customize the back-surface eccentricity to the patient's corneal eccentricity. However, there are two additional methods for fitters who don't have access to Medmont topographers. All three methods require a diagnostic lens set for precise fitting. The set contains 30 lenses with two eccentricity values (0.98e and 1.3e). It also includes KBA Software, which assists in customizing the final lens. This software is independent of the Medmont topographer and can be installed on any computer.
You can determine the KBA base curve using keratometry or topography. Select the initial diagnostic lens by subtracting 0.6mm from the flat central corneal radius in millimeters via keratometry or the cornea's apical radius in millimeters via topography. The KBA trial eccentricity of 0.98 is recommended when using the keratometry method. With topography, choose the KBA trial eccentricity closest to the measured corneal eccentricity. Then flatten or steepen the base curve until ideal apical clearance (central clearance without bearing) is achieved. This is equivalent to 20-to-30 microns of tear film clearance on the Medmont CL designer. To determine the final KBA lens power, perform an over-refraction over the trial with ideal apical clearance.
Determine the eccentricity by evaluating the peripheral alignment of the above trial lens. If the edge clearance at 3 o'clock and 9 o'clock is insufficient, select a flatter trial until you achieve ideal peripheral alignment. For excessive edge clearance, select a steeper trial for sufficient alignment.
The KBA software calculates the final base curve and eccentricity. Enter the above data (KBA trial with ideal apical clearance + KBA trial with ideal peripheral alignment) into the software to generate a custom KBA base curve and eccentricity. The software also generates a tear layer compensation which is added to the over-refraction to determine the final lens power. It is important to note that increasing the KBA back-surface eccentricity requires steepening the base curve to maintain the desired apical clearance or "sag." The KBA software will calculate the equivalent base curve values for desired changes in eccentricity.
KBA is now manufactured in the United States by Essilor Contact Lens with a turnaround time of about three days. Precision Technology Services continues to manufacture the KBA in Canada with a turnaround time to the United States of about 10 days.
I think Pat and Mark's recommendation to fit a post-hyperopic LASIK patient with the KBA lens is a novel idea. Using the Medmont CL designer is advantageous as it allows you to manipulate the base curve and eccentricity to "virtually" create the ideal KBA lens. However, it is critical to emphasize that the KBA is a keratoconus lens design that can be fit without a Medmont topographer. This affords the opportunity to many more practitioners to take advantage of this technology.
Dianne M. Anderson, OD, FAAO, Aurora, IL
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Contact Lens Spectrum, Issue: March 2009