FRONT SURFACE GEOMETRY
An Evaluation of Oxygen Delivery in High-Volume Hydrogel Lenses
By Tony Hough, BA, MBA,
Ross Grant, MSC, FCOptom &
Erich Bauman, OD, FAAO
Does lens geometry have an impact on the oxygen delivery of hydrogel contact lenses?
The development of the modern hydrogel contact lens has focused on two principal areas of manufacturing and materials technology. For over a decade, it has been a primary objective of all major manufacturers to produce high volumes at low unit cost. At the same time, clinicians have been acutely aware of the need to deliver more oxygen to the cornea. Often, these fundamental design goals have been in conflict -- the drive for high volumes has imposed limitations on lens design, which has had the effect of reducing oxygen transmission to the cornea.
Other factors also impose important constraints on lens design, the most common factor being that success in the marketplace depends on how well the lens handles during normal use. For example, patients won't continue to wear hydrogel lenses that are difficult to insert and remove, as thin lenses frequently are. The lens design team must therefore understand the nature of the material, the manufacturing process being used and the needs of the market in its attempt to provide a design which is well-behaved throughout the manufacture and supply chain.
In 1984, Holden and Mertz established that in order to avoid edema when wearing contact lenses, it's necessary to have oxygen transmissibility of 24 x 10-9 for daily wear or 87 x 10-9 for extended wear. To obtain the daily wear value, it's necessary to have 38 percent water content lenses with a thickness of 33µm (0.033mm), or 75 percent water content lenses with a thickness of 166µm (0.166mm). In practice, such values of thickness are only possible at the center of minus-powered lenses. Most commercially available lenses are thicker for reasons of production and handling. Before frequent replacement hydrogels, it wasn't just handling that mandated thicker midperipheries, lens durability was also important because unit cost was an order of scale higher than it is now, and contact lenses were expected to survive months of regular handling.
Virtually all commercial hydrogels have had a back surface geometry with either a single radius or a simple bicurve, and front surface geometries have been almost exclusively lenticular. (Sometimes the lenticular front surface is modified with a narrow bevel close to the edge for reasons related to the manufacturing process. We don't count this as a "purposeful" multicurve design.) The typical way to improve handling characteristics, especially in the case of low to medium powers, has been to manufacture lenses with relatively large front optic zone diameters. This increases midperipheral thickness, which means that the oxygen transmissibility away from the center is much less than the values identified as desirable by Holden and Mertz.
This study evaluated the oxygen delivery characteristics of current high-volume hydrogel lenses with respect to lens geometry, which was identified using thickness measurements. The impact of that geometry on oxygen delivery was then examined.
Methods and Materials
We assessed the following representative sample of current commercial hydrogel lenses: Aspect Frequency 55 (CooperVision), Focus Dailies (CIBA Vision), 1-Day Acuvue (Vistakon), Precision UV (Wesley Jessen) and Soflens One Day (Bausch & Lomb). All of these lenses are manufactured by high-volume process. The radial thickness of the sample lenses having nominal powers of -2.00D and -5.00D was measured at controlled intervals across a meridian of the total diameter using a modified low force gauge and an optical/sectioning method based on a high precision measuring microscope. These thickness data were then used to derive the underlying lens geometry and to calculate the oxygen transmissibility across the complete lens profile.
TABLE 1: Permeability Values Derived from the Morgan and Efron Formula
|PRODUCT NAME||WATER CONTENT (%)||PERMEABILITY (DK)|
|Aspect Frequency 55||55||14.83|
|Soflens One Day||73||30.29|
|Note: Dk value calculated using the Morgan-Efron formula.|
Measurement method 1 -- The only ISO approved test method for the thickness measurement of hydrogel lenses is the low force gauge (Figs. 1 and 2). In this method, a mechanical gauge has a sensor that acts against a spring, which exerts a maximum force of 0.015 Newtons when resting on the lens to be measured. The sensor travels through a linear transducer, coupled to a digital readout that displays the value of thickness in microns. To enable the lens to be positioned accurately for measurement, a standard low force gauge was modified so that the measuring anvil could be displaced accurately using a digital micrometer. The displacement mechanism permitted the lens to be displaced linearly.
FIG. 1: The modified low force gauge.
FIG. 2: Detail of the displacement mechanism.
Measurement method 2 -- In addition to the low force gauge, we used a measuring microscope to determine lens thickness. Sample lenses were carefully aligned and a thin section was cut using two scalpel blades locked together. The resulting section was immersed in the lens packing solution at 20C. A video link from a high-precision measuring microscope enabled the lens section to be displayed on a computer screen at 50 times magnification, from which accurate readings were taken automatically by proprietary control software.
Hydrogel Material Permeability
There has been continuous discussion since the Holden and Mertz paper about the values of permeability associated with hydrogel materials of given water content. Leading experts in the field of permeability measurement have frequently challenged the values quoted by lens manufacturers, who have often understandably wished to select the Dk value that would present their particular product in the most positive light (a practice which is still widespread).
In order to provide the fairest comparison of the combined effect of material and lens geometry, we decided to use the value of permeability calculated according to the formula derived by Morgan and Efron, which relates hydrogel material permeability to water content by means of the following equation, where W is the water content percentage:
Dk = 1.67e 0.0397W
This acknowledges the approach put forward by the late Irving Fatt, that in the case of hydrogels, material permeability is essentially a function of water content. The values are shown in Table 1.
The results of the study provide thickness profiles of current high-volume hydrogel contact lenses that we used to calculate transmissibility profiles. The measured thickness profiles are shown in Figs. 3 and 4, with the corresponding transmissibility profiles in Figs. 5 and 6.
FIG. 3: Measured thickness profiles for the -2.00D lenses.
FIG. 4: Measured thickness profiles for the -5.00D lenses.
FIG. 5: The calculated transmissibility profiles for the -2.00D contact lenses.
FIG. 6: The calculated transmissibility profiles for the -5.00D contact lenses.
The results show that in general, the anterior design of hydrogel contact lenses has changed little in the past 15 years, notwithstanding the move to high volume production methods, which strongly suggests that lens designs that have been adapted to high volume technologies have generally been simple variations on the lathe cut designs that were predominant in the early to mid 1980s. The characteristic oxygen transmissibility profiles obtained from the designs discussed by Holden and Mertz is essentially the same as most current contact lenses; the desirable values of transmissibility are only attained within limited zones.
The Thickness Profiles
Most of the products assessed are based on spherical back surface geometries, either single-cut or having a narrow band of a second radius close to the edge. The measured profiles are consistent with front surface geometries having lenticular construction with the optic zone typically eight to 10mm at the -5.00D power, and larger for the -2.00D lens. This construction is clearly evident in the Aspect Frequency, 1-Day Acuvue, Soflens One Day and Precision UV lenses, and less evident in the Focus Dailies lens, which is promoted as having a purposeful tricurve geometry on both the back and front surfaces.
In terms of oxygen transmissibility, the results of the study confirm the benefits of higher water content materials. The results also show that design makes a vital contribution to the overall transmissibility of the lens, with the more complex Focus Dailies product delivering more oxygen to the complete cornea than other high and medium water content lenses. To some extent, this is unsurprising; the Focus Dailies product was designed and developed more recently than the other products assessed and has the benefit of newer materials technology and manufacturing process. It's reasonable to expect that other manufacturers will introduce more complex lens designs in the years ahead. However, due to the heavy momentum of investment in current designs and manufacturing technology, the benefits of these changes to patients will be slow to materialize.
Multicurve Front Surface Geometries
Is it worth considering whether the measured values for the Focus Dailies product are representative of a tricurve construction? Figure 7 shows a calculated hydrogel having a tricurve back and front surface; a more complex thickness characteristic than lenticular contact lenses is evident. Figure 8 shows the measured thickness profile for the Focus Dailies -2.00D lens. Then, remembering that the measurement process in the study was limited to one-half millimeter intervals, which would have the effect of some smoothing in the presented profile, it's clear that the measured profile of the Focus Dailies product for both -2.00D and -5.00D powers is consistent with the manufacturer's claims of using a purposeful tricurve design.
FIG. 7: The calculated thickness profile of a lens having a tricurve back and front surface geometry.
FIG. 8: Measured thickness profile for the Focus Dailies -2.00 contact lens.
Although Holden and Mertz's work has remained definitive, there has been an increasing awareness of the need to deliver more oxygen to the peripheral cornea. However, the commercial imperative within the manufacturing industry has been to deliver high volumes of hydrogel lenses at dramatically reduced unit costs. This has created a climate where the design of hydrogel lenses has been neglected; typical hydrogel design has changed little in the past 15 years, the exception being the relatively recently developed Focus Dailies product. Most medium and high water content hydrogel lenses can now meet the Holden and Mertz criterion for oxygen delivery to the central cornea but few deliver the same levels to the peripheral cornea. As production and materials technology matures, there is an opportunity to offer genuine clinical advantage by introducing more complex lens geometries which can deliver more oxygen to the complete cornea.
Mr. Hough is an established technical expert in the field of contact lenses. He is a partner in the design consultancy CLS based in Cambridgeshire, UK. He is currently honorary meetings secretary of the British Contact Lens Association and a technical consultant to Eurolens Research Limited based at UMIST.
Mr. Grant has worked in general, hospital and specialist contact lens practice and has taught in undergraduate and graduate programs. He now works in a marketing role for the Nordic countries, based in Gothenburg, Sweden.
Dr. Bauman is currently head of CIBA Vision's Bifocal Business Unit in Atlanta. He is a fellow of the American Academy of Optometry and a member of the British Contact Lens Association.