Dk/L: Into the Ultra-High Zone
Contact lens manufacturers are racing to develop much-needed materials with unprecedented levels of oxygen transmissibility. Here's why.
BY BRADLEY J. SMITH, OD, MS, BARBARA A. FINK, OD, PhD, and RICHARD
M. HILL, OD, PhD
It's obvious that the health of the contact lens-wearing cornea is not ensured by sufficient oxygen permeability (Dk) values alone. The real commodity is the oxygen that is finally delivered to the cornea, the contact lens transmissibility (Dk/L). Put another way, in order for Dk to be effective, L (local or averaged thickness) must remain sufficiently small to allow reasonable oxygen passage through the lens to the corneal surface.
In 1984, Holden and Mertz found that a minimum (Dk/L) of 24 x 10-9 (cm/sec)(ml O2/ml mm Hg) is required for daily wear without inducing corneal edema, assuming no significant dependence is placed on the lid-tear pump. Their minimum Dk/L finding for overnight wear, allowing normal resolution of physiologic edema (the 3% to 4% naturally present on awakening), is substantially higher at 87 x 10-9.
The Dk/L limitations of hydrophilics had been recognized by the late 1970s, however, with practical compromises between transmissibility and durability becoming increasingly evident in contact lenses on the market. Acknowledging that pure water has a Dk of about 80 x 10-11 (cm2/sec)(ml O2/ml mm Hg), we realize that conventional hydrophilic lenses of even 75 percent to 80 percent water content at clinically feasible thicknesses are unlikely to exceed Dk/L levels of 50 x 10-9. In contrast, several current RGP materials at feasible thicknesses have already crossed the "100 barrier" of contact lens transmissibility. Some recent RGP entries are already reaching for the "200 barrier" and for what we'll call the "ultra-high" transmissibility zone.
Looking for the Upper Limits
How practical is it for the contact lens industry to exceed the 200 barrier of transmissibility, or to forge even higher? Figure 1, which shows responses of the cornea (residual hypoxia) associated with Dk/L levels from 0 to 200 x 10-9, offers one perspective on the return for the Dk/L investment. This curve is a smoothed model based on the relativized responses of 10 human corneas to each of seven different contact lens induced Dk/L environments (Smith, Fink & Hill, JAOA 68:478, 1997). A corneal response of 100 hypoxic stress units (HSUs) represents the hypoxia resulting from the static wear of a PMMA lens (i.e., the maximum hypoxic response associated with a 0% oxygen condition). The 0 HSU level represents no hypoxic stress.
As Figure 1 shows, increasing Dk/L from 0 to just 30 x 10-9 reduces corneal hypoxia by some 70 percent, leaving a residual hypoxia of only 30 percent of that caused by an oxygen impermeable PMMA contact lens (Example A). But, reducing that remaining hypoxia just 20 percent further would require a tripling of Dk/L to 90 x10-9 (Example B). Thus, an increasingly greater Dk/L investment is required to gain the same increment of hypoxic relief.
Still Higher Dk/Ls? Seven Other Reasons Why
Is a further 20 percent reduction of hypoxia really worth 60 more units of Dk/L? In addition to the ongoing quest for better and healthier overnight contact lens wear, there are at least seven other considerations which support an affirmative answer to this question.
1Not Every Cornea is "Average" -- While the initial observations of Hill and Fatt in 1963 indicate a normal corneal oxygen demand of 5 �l/cm2/hr, Larke et al., in 1981 reported a natural distribution around that number extending from a low of 1 �l/cm2/hr for some corneas, to a high of 10 �l/cm2/hr for others. Fink, Carney and Hill (1990) have demonstrated differences among corneas under hypoxic conditions as well. Higher Dk/L contact lenses should certainly benefit the more demanding corneas of both of those distributions by allowing them a more tolerable overnight environment as well as a lower level of chronic hypoxic stress during daily wear. Higher Dk/L values hold an immediate and tangible promise for adding those marginally accommodated corneas to today's total wearing pool.
2Reducing Toric Risk -- Higher material Dk means contact lenses can be thicker without exceeding the edema threshold of a cornea. The growing use of cylinder corrections in both hydrophilic and RGP lenses can directly be improved by additional transmissibility through the minimization of corresponding meridional hypoxia. Reducing the risk of "cylinder edema," deriving from the added local thicknesses of toric and bitoric designs, could be particularly beneficial when extended wear is the ultimate fitting objective.
3Safer Orientation Control -- Attempting to gain stabilizing advantages, such as by incorporating a prism ballast for astigmatic axis control, often carries an increased risk of chronic hypoxia in the inferior region of the cornea, where the greater bulk of prism ballast must reside. Using more highly permeable lens materials can directly moderate this risk.
4Presbyopic Advancements -- The two most common strategies for incorporating near power in a contact lens both carry local hypoxic penalties. When curvature is added annularly for a midperipheral reading zone, the penalty is a corresponding increase in L. The same penalty occurs through the addition of an embedded reading segment of higher refractive index but lower Dk, as well as prism ballast for orientation control. At present, higher Dk materials offer the most direct means of reducing those hypoxic penalties.
5Environmental Challenges -- Patients who are successful contact lens wearers at sea level or within a few thousand feet of that richest oxygen environment may find a stay at higher altitudes less accommodating due to lower oxygen densities. Both the diminished efficiency of contact lens transmissibility and the reduced oxygen levels provided by tear bulk flow (blink exchange) may push the marginally successful fit below its tolerance threshold under such conditions. The same may occur during prolonged periods of time in aircraft cabins which are pressurized at less-than-sea level oxygen tensions.
6Infection Risks -- The chronic hypoxia, the increased warmth and the much reduced, yet negligible, turnover of tears within the closed eye environment may combine to favor mechanical damage, epithelial adhesion and subsequent growth of microorganisms with overnight wear. Higher Dk materials, however, should narrow the difference in corneal oxygen deprivation between the non-lens and the lens wearing cases, resulting in less overnight edema, as well as a more rapid return to normal thickness levels on awakening.
7Corneal Healing -- Mauger and Hill (1992) demonstrated a phase of "hyper-flux" (increased oxygen demand) within the course of corneal healing. Whether due to a minor epithelial disruption, such as from an entrapped dust particle, or a more serious corneal lesion, such as a recurrent erosion, healing must proceed as rapidly as possible to minimize further risk to the eye. Higher lens transmissibilities would be, again, directly favorable, most particularly during bandage lens use.
The Dk/L Frontier
As in the past, expanding the total pool of successful contact lens wearers still depends on improved problem recognition, creativity in physical design and enhancement of material properties. While growth in all three areas continues, advancements in material permeability (Dk), and its clinical manifestation, lens transmissibility (Dk/L), represent the most dynamic and promising of those frontiers.
So can that minimum transmissibility level recommended by Holden and Mertz for overnight wear (87 x 10-9) be practically achieved? Recent RGP materials having Dks in the range of 125 to 150 x 10-11 are already making lens transmissibilities of 90 x 10-9 a clinical reality. But even at that level, as Figure 1 indicates, some 10 percent of the average cornea's hypoxic stress remains. To cut that residual hypoxia in half once again would require an estimated lens transmissibility of 175 x 10-9 or higher. In order to accommodate a full range of refractive errors, such materials would need to have oxygen permeabilities of at least 200 x 10-11.
Using the most permeable material of the day (the "best silicone"), Irving Fatt, Ph.D., estimated some two decades ago that with normal pre- and post-lens tear layers present, the total effective oxygen transmissibility possible is 355 x10-9. But can Fatt's ultimate transmissibility level be reached with materials other than problematic silicones? More advanced RGP materials and newer silicone hydrophilic hybrids are now being explored, and this frontier represents one of the most tantalizing challenges remaining for the contact lens industry today.
Will We Need a Dk Designator?
The need for more oxygen continues -- for overnight wear, environmental challenges (including higher altitudes and partially pressurized airliner cabins), resistance to disease, requirements for healing, and the transmissibility demands of multi-material and multilayered advanced designs. Fortunately, this call is being answered in both the hydrophilic (new hybrids) and rigid gas permeable (ultra-high RGP) realms. With them as well must come new frameworks for scaling their performances. From the international sphere, two examples are: 1) the more benign substitution of "t" for "L" to designate lens or layer thickness, and 2) the more significant substitution of the hectopascal (hPa) for "mm Hg." Benjamin (ICLC, 1996) has pointed out that the coefficient number for lens transmissibility (Dk/L) would be reduced by about 25 percent, (i.e., 80 x 10-9 would become 60 x 10-9) when hectopascals are used. How widely and how soon such international changes might be embraced here in the United States remains at present unclear.
Closer to home, however, within the jurisdiction of the FDA, a shift (uniform or gradual) is now anticipated to properly correct for edge effects inherent to some measurement methods. As material permeability rises, these effects become increasingly accentuated and misleading. Such corrections reduce coefficient numbers by substantial amounts; for example, 82 x 10-9 would become 58 x 10-9 (Benjamin, Contact Lens Spectrum, October 1998).
The limited appeal of such corrections for manufacturing laboratories makes an early and simultaneous adjustment throughout the industry unlikely. Still, corrected scaling should be phased in as new materials are introduced. In order to encourage such current and valid values, however, they would need to be easily distinguishable from their uncorrected counterparts. One strategy might be to use a simple and highly visible corrected value "designator," such as an (*), to assure this distinction (e.g., Dk*, and Dk*/L). Whether this or some other mechanism of transition is elected, it is time we heed Dr. Benjamin's early warnings, and assist all parties through such scaling changes as expediently and painlessly as we possibly can.
Dr. Smith graduated from The Ohio State University and is now in private practice
in Wooster, Ohio.
Dr. Fink is an associate professor at The Ohio State University College of Optometry in Columbus, Ohio.
Dr. Hill is dean and professor emeritus at The Ohio State University College of Optometry in Columbus, Ohio.