http://www.revoptom.com/index.asp?Artic ... lesson.htm
Powycinam wam tylko te wazniejsze fakty bo mysle ze nie wszystkim sie bedzie chcialo czytac calosc. Zachecam was jednak bo sie mozna duzo ciekawych rzeczy dowiedziec no ale angielski na dobrym poziomie,s?ownik medyczny i techniczny niestety wymagany)
"There is an emerging realization that oxygen transmissibility is the not the end-all, be-all of ocular health, as was once thought. After a certain point of Dk/t is reached, a law of diminishing returns takes hold, and other characteristics of a lens become just as significant, if not more so"
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The stiffness of a contact lens is determined by the lens modulus and lens thickness. The higher the modulus and the thicker the lens, the stiffer the contact lens. As an example, rigid gas permeable lenses are stiff due to a high modulus value. There is also a general relationship between Dk and modulus. Oxygen permeability tends to be increased in lenses that contain higher levels of silicone. Silicone, in turn, tends to make lenses stiffer. Hence, higher Dk values are inclined to correspond with high modulus (MPa) values.
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For decades, the oxygen performance of lenses has been measured by Dk/t, and scientists have established an international standard for measuring Dk/t in a laboratory. In conventional hydrogel lenses, there has also been a traditional relationship between Dk and the water content of the lens: the higher the oxygen transmissibility, the higher the water content. However, a 1998 investigation determined that in silicone hydrogels, this relationship does not appear to exist.
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Striving for high oxygen transmissibility at the expense of other properties may lead to a range of undesirable performance characteristics.”
This situation has led many in the eye-care community to conclude that in this era of “hyper oxygen” lenses, the measurement of oxygen via Dk/t is dead, or perhaps even worse, it is doing more harm than good. The weaknesses of Dk/t as a measurement are more apparent than ever.
The seeds of this argument were sown a decade ago in a 1996 paper that pointed out Dk/t does not reveal the distribution of oxygen in the cornea, does not allow for direct comparison between the performance of two lenses, and does not allow for comparisons with eyes not wearing lenses
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Part of the problem is Dk/t is not a linear measurement of the amount of oxygen reaching the eye. In other words, doubling the amount of Dk/t does not necessarily double the amount of oxygen supply.
The study points out that Dk/t has a limited ability to measure lens performance because it does not account for oxygen concentration at the interface of the contact lens and cornea.
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Using the oxygen flux model (µl cm-2 h-1) reveals how deceiving Dk/t can be, especially in lenses with high oxygen transmissibility. (See figure 4). Clearly, despite huge jumps in Dk/t values among these lenses, there is little difference in the actual amount of oxygen reaching the eye.
We now face the unavoidable conclusion that corneal hypoxia cannot be the exclusive mechanism responsible for serious ocular morbidity related to contact lens wear
