NEW MATERIALS DEMAND MORE ACCURATE MEASUREMENTS OF PERFORMANCE

BY PATRICK H. BENZ, PH.D, & JOSE A. ORS, PH.D.

JULY 1997

Water balance is a new measurement that may help you identify which soft contact lens materials maintain their initial stability and comfort throughout the wearing cycle.

If contact lens practitioners were to select a soft lens material based solely on Dk and initial comfort, a high water material would be the obvious choice. However, high water lenses lose water more rapidly and at greater percentage than low water lenses, affecting stability, fit, Dk and ultimately, patient comfort.

HIGH WATER, HIGH EXPECTATIONS

High water content lenses can initially feel more comfortable than lower water lenses, but they become steeper during wear. Therefore, they may cause problems for patients with dry eye or marginal dry eye and for patients wearing lenses during intensive-near tasks and in dry environments. To compensate, practitioners fit high water lenses flatter because a tighter, steeper fit can cause discomfort, particularly towards the end of the day. Table 1 shows some of the lens characteristics related to water content.

Dk value can also be misleading. Dk and Dk/L are in vitro values that are always reported at saturation. However, all hydrogels lose water when exposed to ambient conditions. In vivo measurements show that most high water lenses lose water rapidly within the first hour of wear and can reach an equilibrium level of more than 10 percent below saturation within a few hours. Hill (1983) defined this reduced water content level as the ambient hydration zone (Fig. 1). Therefore, high water lenses that lose substantial water can have a much lower Dk than indicated as early as the first hour of wear. A 10 percent loss in water content results in approximately a 2.5 percent decrease in equivalent oxygen percentage (EOP) of the lens. In Dk terms, a 10 percent water loss is about a 25 percent drop in Dk value in a 60 percent water content lens.

NEW MEASURE OF PERFORMANCE

Today, new hydrogel polymers bind water much stronger than most established materials, enabling patients to wear high water lenses with comparable and even steeper base curves than low water (Polymacon 38%) materials.

These new materials require a new measure of performance since water content alone doesn't fully describe it. We call this new measurement the "water balance" value -- the ratio of the time it takes a lens of standard dimensions under standard conditions to dehydrate 10 percent of its water weight to the time it takes the lens to rehydrate to saturation (Figs. 2 & 3). The higher the water balance value, the better the material binds water.

Water balance approximates in-eye water dynamics, dehydration through evaporation and rehydration from tear fluid. As an inherent property, each lens material will exhibit its own water balance value. Figure 4 is a comparative water balance chart based on the values given in Table 2 (p. 46). All values are normalized relative to Polymacon (p-HEMA, Benz 38) as a standard. Traditional hydrogels occupy a relatively narrow band of water balance values in the range of 0.5 to 2.5, but in the p-GMA/HEMA copolymer series, water balance values increase with increasing water content. In practice, lenses made from these materials stay closer to saturation on the eye for a longer period of time during wear, according to research conducted by Urs Businger, O.D., in Switzerland. A 59 percent water content p-GMA/HEMA copolymer lens with a relative water balance of 5 remains within one percent of its saturation point after 12 hours of wear. Also, the Dk value stays close to that at saturation, providing continually higher oxygen permeability throughout the wear cycle.

A soft lens material with a high water balance value promises a complete reversal from the traditional expectations for high water lenses. The water balance measurement enables practitioners to predict the clinical characteristics of a specific lens material, providing insight that may relate to overall patient comfort and satisfaction in the long term. CLS

References are available upon request from the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #26.

Table 1: Lens Characteristics

LOW WATER (38%-50%)	HIGH WATER (50-75%)
more dimensionally stable	less dimensionally stable
lower Dk	higher Dk
typically more durable	usually less durable
more easily handled	less easily handled
comfortable daily wear	greater initial comfort & longer wear time
preferred for dry eye	less indicated for dry eye

Patrick Benz is president of Benz Research and Development, Sarasota, Fla. Jose Ors is vice president of Benz Research and Development.

DETERMINING WATER BALANCE OF HYDROGELS USING A GRAVIMETRIC TECHNIQUE In contrast to the in vivo ratio of imbibition and dehydration suggested by Young and Mandell (1983), we can obtain this in vitro value using the following simple test procedure. It is imperative that ambient conditions for the test be maintained accurately, and that all samples be measured under the same controlled conditions. Test equipment: High precision, calibrated balance (such as Sartorius, Mettler, etc.) with 0.0001 gram capability. The balance should be placed in a controlled temperature and relative humidity environment of 21o ± 2o C and 50% ± 5% relative humidity. Test lens design: For each material, dry, constant thickness lenses that will yield a final wet (even) thickness lens of 0.1mm and a 14.0mm diameter. Constant thickness eliminates design-related factors. Finished dry lenses are cleaned and hydrated overnight in buffered saline solution. Lens dehydration procedure: Remove a clean sample lens from a saline vial. Secure the lens on a wire holder by carefully perforating the lens and placing a fine wire through the hole, then blot the lens gently with a lint-free paper to remove excess surface water. Hang the wire holder on a balance scale and record the weight. Dehydrate the lens, recording the weight and cumulative time every 10 seconds until 10 percent of the saturated water content is lost. After each test cycle is complete, return the lens to the saline flask, allow the lens to rehydrate to saturation, and repeat the drying procedure at least two more times to obtain an average weight loss. Remember, the key to removing excess surface water is a consistent and repeatable blotting technique. Lens rehydration procedure: Remove a clean sample lens from the saline vial, secure the lens on a wire holder and blot gently with a lint-free paper. Hang the wire holder on the balance scale and record the saturated lens weight. Allow the lens to dehydrate 10 percent of its water weight (saturated weight x water content x 10%), and record the weight. Remove the wire holder from the scale and submerge the lens in buffered saline (21o ± 2o C) for 10 seconds. Remove the lens from the saline, blot gently with a lint-free paper and weigh the lens. Record the weight and time hydrated, then re-submerge the lens 10 more seconds. Blot, and record the weight and cumulative time hydrated. Repeat this procedure until the saturated weight of the lens is achieved. Repeat complete procedure three times to obtain a consistent, average weight gain. Water balance value: Divide the time (in minutes) to dehydrate the lens 10 percent by the time (in minutes) to rehydrate the lens back to saturation. This ratio value is reported relative to the p-HEMA control.