The Dk Project: An Interlaboratory Comparison of Dk/L* Measurements

The Dk Project: An Interlaboratory Comparison of Dk/L* Measurements.

BA Holden, J Newton-Howes, L Winterton, I Fatt, H Hamano, D La Hood, N Brennan and N Efron. Optometry and Vision Science, 67:6;476-481

Oxygen transmissibility (Dk/t) is a measure of the amount of oxygen diffusing through a contact lens at a particular thickness, usually specified as centre thickness, and can be directly measured in the laboratory under standard conditions. Oxygen permeability (Dk) on the other hand measures the amount of oxygen passing through the material from which the lens is made and is a calculated value derived from a knowledge of the measured Dk/t.

In 1986, there were substantial variations in the Dks that were reported by different laboratories for the same contact lens material. This inconsistency in quoted Dk values was confusing to practitioners and not helpful for industry as the question was continuously raised as to how different laboratories could measure the Dk/t of selected materials with such variability. Inevitably calls were made to see if these measurements could be made more comparable and this prompted a gathering of notable researchers to discuss the problem. As a result of this meeting, an agreement was reached to conduct an inter-laboratory investigation as a means of standardizing the relevant measurements. The results of those investigations were published in the journal Optometry and Vision Science in 1990.

The five laboratories and principle personnel involved are listed in the Table below.

Four of the centres measured Dk/t using the polarographic technique and one used the coulometric technique. In the course of the study the results using the Titron polarographic system (Corneal Biophysics Laboratory) proved to be too variable and were not used.

To eliminate some of the possible variables, all measurements at all the laboratories were performed using the same contact lenses and measurement of lens thickness was performed by one centre. Thus the only variations in the measures were caused by the differences in measurement technique. To try to maintain minimal variability between laboratory measurements, the permeability (Dk) was calculated from the measured transmissibilities using a standard graphical technique. This method has the advantage that the different electrolytes used in the laboratories using the polarographic methods, ie CCLRU, Fatt and Hamano, does not affect the gradient of the graph.

Forty-eight contact lenses produced from eight different rigid gas permeable materials were used in this study and all lenses were manufactured with standard parameters.

The results from this collaborative work demonstrated that different laboratories can obtain similar results for materials when the Dk is less than 70. For the technically minded, the conditions needed to achieve this are as follows:

• Current or voltage readings should be converted into flux values cm 3 O 2/(cm 2s)
• Calculation of Dk/t should be based on Fick’s first law
• When a front mask is not used during the measurements, the values obtained should be corrected for edge effects
• Dk is determined as the inverse of the gradient of the slope from the graph produced when t/Dk is plotted against t. This process eliminates constant resistances such as boundary layer effects
• Standard error should always be considered as it indicates the degree of variability of the measures.

When materials with Dk over 70 were considered however, the standard errors encountered were notably larger and it was suggested that alternative strategies would be needed to arrive at suitably consistent values.

Additionally it was demonstrated that the coulometric technique produces negligible boundary layer effects and thus has the advantage that Dk can be determined from the measurement of a single lens.

The Dk project clearly demonstrates that standardised measurement of Dk/t can be attained. However, since the release of high Dk silicone hydrogels, confusion surrounding the variability in measured values of Dk/t has returned. This is in part because of the lack of appropriate methods for measuring such high Dk lens types. When making decisions based on lens Dk/t, practitioners need to be wary of the quoted values of Dk/t for particular lenses and should attempt to understand the basis for how these numbers are derived.

Footnote:

*In 1990 the standard abbreviation for oxygen transmissibility was Dk/L, this review has used the current abbreviation of Dk/t.