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Oxygen Permeability of Contact Lenses
VOL. 89, NO. 6
BOOK REVIEWS
Oxygen Permeability of Contact Lenses Editor: In the article, "The microbial flora in extended-wear contact-lens wearers" (Am. J. Ophthalmol. 88:543, 1979), by G. Smolin, M. Okumoto, and R. A. Nozik, the oxygen permeability of a contact lens was expressed as a percentage. I realize that this manner of expressing the oxygen permeability of contact lenses is often encountered in published reports. However, this terminology is incorrect and should not be used. The oxygen permeability of a contact lens must be expressed by a permeability coefficient (P = D·k) in units that represent the amount of oxygen transported through a lens of unit area in unit time by unit driving force. The custom of using "oxygen percent" to characterize the permeability of contact lenses originated from the work of Hill, Fatt, and others. 1 The technique detects a "short-term oxygen debt" on the cornea by a given lens, and is expressed as the "equivalent oxygen percent" that would produce the same effect on the cornea. Undoubtedly, this oxygen percentage is related to the oxygen permeability coefficient of the contact lens, or, even better, to the oxygen transmissibility of the lens (D·k!L, where L is the thickness of the lens). Incidentally, in my opinion, the short-term oxygen debt produced on the cornea by a lens provides a useful indication of contact lens performance. But this is not the point. When an author wishes to give the oxygen permeability coefficient for a given lens, he should use the proper units. Ifhe wishes to give the short-term equivalent oxygen debt, it should be so designated. MIGUEL F. ROFoJo, D.Sc. Boston, Massachusetts REFERENCE
1. Rubin. M. (ed.). Soft Contact Lenses: Clinical and Applied Technology. New York, John Wiley & Sons, 1978.
881
BOOK REVIEWS Advances in Ophthalmology: Volume 40. Edited by M. J. Roper, H. Sautter, and E. B. Streiff. Basel, Switzerland, S. Karger AG, 1980. Hardcover, 224 pages, introduction, 55 black and white figures. $106.60 The 40th volume of Advances in Ophthalmology contains three monographs. Two of the three will be of great interest to the readers of this journal and one probably of interest only to research workers. The most important is probably the initial article on demography and causes of blindness throughout the world by Dr. Goldstein of Bethesda, Maryland. In the introduction, the author explains the difficulty of gathering reliable data. The definition of blindness varies enormously around the world and the personnel needed to determine the causes of blindness are frequently unavailable in the less developed parts of the world. The best data are probably collected in the United States through a mechanism set up by the author some years ago. He established a model reporting area consisting at first of a few states and more recently of between 14 and 16 states. Further, a strict definition of blindness was established so that reporting might be comparable from state to state. These surveys now include about 30% of the population of the United States. Of great interest to ophthalmologists are two factors: first, that of the population over the age of 65, 0.75% are blind; second, that the most common cause of blindness is "other retinal disease," mainly senile macular degeneration. The author makes a plea for a uniform definition of blindness and better collection of data with reference to incidence rather than just prevalence. The second article is on protein in the aqueous humour and this will be of
BOOK REVIEWS
Oxygen Permeability of Contact Lenses Editor: In the article, "The microbial flora in extended-wear contact-lens wearers" (Am. J. Ophthalmol. 88:543, 1979), by G. Smolin, M. Okumoto, and R. A. Nozik, the oxygen permeability of a contact lens was expressed as a percentage. I realize that this manner of expressing the oxygen permeability of contact lenses is often encountered in published reports. However, this terminology is incorrect and should not be used. The oxygen permeability of a contact lens must be expressed by a permeability coefficient (P = D·k) in units that represent the amount of oxygen transported through a lens of unit area in unit time by unit driving force. The custom of using "oxygen percent" to characterize the permeability of contact lenses originated from the work of Hill, Fatt, and others. 1 The technique detects a "short-term oxygen debt" on the cornea by a given lens, and is expressed as the "equivalent oxygen percent" that would produce the same effect on the cornea. Undoubtedly, this oxygen percentage is related to the oxygen permeability coefficient of the contact lens, or, even better, to the oxygen transmissibility of the lens (D·k!L, where L is the thickness of the lens). Incidentally, in my opinion, the short-term oxygen debt produced on the cornea by a lens provides a useful indication of contact lens performance. But this is not the point. When an author wishes to give the oxygen permeability coefficient for a given lens, he should use the proper units. Ifhe wishes to give the short-term equivalent oxygen debt, it should be so designated. MIGUEL F. ROFoJo, D.Sc. Boston, Massachusetts REFERENCE
1. Rubin. M. (ed.). Soft Contact Lenses: Clinical and Applied Technology. New York, John Wiley & Sons, 1978.
881
BOOK REVIEWS Advances in Ophthalmology: Volume 40. Edited by M. J. Roper, H. Sautter, and E. B. Streiff. Basel, Switzerland, S. Karger AG, 1980. Hardcover, 224 pages, introduction, 55 black and white figures. $106.60 The 40th volume of Advances in Ophthalmology contains three monographs. Two of the three will be of great interest to the readers of this journal and one probably of interest only to research workers. The most important is probably the initial article on demography and causes of blindness throughout the world by Dr. Goldstein of Bethesda, Maryland. In the introduction, the author explains the difficulty of gathering reliable data. The definition of blindness varies enormously around the world and the personnel needed to determine the causes of blindness are frequently unavailable in the less developed parts of the world. The best data are probably collected in the United States through a mechanism set up by the author some years ago. He established a model reporting area consisting at first of a few states and more recently of between 14 and 16 states. Further, a strict definition of blindness was established so that reporting might be comparable from state to state. These surveys now include about 30% of the population of the United States. Of great interest to ophthalmologists are two factors: first, that of the population over the age of 65, 0.75% are blind; second, that the most common cause of blindness is "other retinal disease," mainly senile macular degeneration. The author makes a plea for a uniform definition of blindness and better collection of data with reference to incidence rather than just prevalence. The second article is on protein in the aqueous humour and this will be of