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Intraocular Lens Power Check: Reply
590
AMERICAN JOURNAL OF OPHTHALMOLOGY
older American Optical instruments—do not use this standardized index and therefore require a slightly different con version factor. 2 As the authors stated, this keratometric method measures only the power of the frontal surface of the intraocular lens and assumes that the back surface of the lens is flat with a power of piano. Unfortunate ly, this assumption leads to errors be cause both lathe-cut and injectionmolded lenses are polished to remove rough edges, sometimes resulting in power on the back surface. All manufac turers, however, measure their intraocu lar lenses on an optical bench with a "through-the-lens" device so that the stated powers include the back surface power error. Finally, only three of the 11 major manufacturers package their intraocular lenses with the front surface of the lens optically accessible. Two of the three manufacturers state that the poor optics of the covering case can create significant measurement errors when this kerato metric technique is used. Olson, Kolodner, and Kaufman 3 have reported greatly improved quality con trol of the power and resolution of intra ocular lenses. However, we agree with Miller and associates that ophthalmic sur geons should be able to confirm this power themselves. We believe that man ufacturers should satisfy this need with an improved packaging design allowing sterile and accurate verification with a through-the-lens technique before sur gery. J A C K T . HOLLADAY, J O H N GOSSEY,
M.D. M.D.
Houston,
Texas
MARC CRUCIGER,
M.D.
San Francisco,
California
REFERENCES 1. Southhall, J. P. C : Introduction to Physiologi cal Optics. New York, Dover, 1961.
OCTOBER, 1981
2. Sampson, W. G.: Applied optical principles. Keratometry. Ophthalmology 86:347, 1979. 3. Olson, R. J., Kolodner, H., and Kaufman, H. E.: The optical quality of currently manufactured intraocular lenses. Am. J. Ophthalmol. 88:548, 1979.
Reply Editor: We are pleased that Dr.Binkhorst agrees that the conversion factor of 0.46 times the reading on the keratometer yields the power of the frontal surface of the intraocular lens. We also agree that measuring the power of the intraocular lens in the patient's eye is a more compli cated problem than implied in our paper. Certainly, the power of the cornea and the distance between the cornea and the intraocular lens make the rays from the mire converge (probably producing a minifying effect). This same combination probably magnifies the image reflected from the frontal surface of the intraocular lens. The total of these effects varies with corneal power and the distance of the intraocular lens to the cornea. Perhaps Dr. Binkhorst can calculate a series of conversion factors that would allow us to measure precisely the power of the intra ocular lens in the patient's eye. For the present, however, we believe that our method is a helpful approximation. We thank Drs. Holladay, Gossey, and Cruciger for pointing out that the true index of refraction is 1.376, not 1.3375. We are pleased that they also agree with us that a conversion factor of 0.46 allows one to use a keratometer to get the re fracting power of the convex surface of the intraocular lens. We also agree that the piano surfaces on some intraocular lenses are not actual ly piano. In our study, we compared the predicted power of the intraocular lens obtained with the keratometer to that obtained with the optical bench method used by the manufacturer. We were sometimes inaccurate by 0.25 to 0.5 diop-
VOL. 92, NO. 4
591
CORRESPONDENCE
ter. This discrepancy is consistent with the comments of Drs. Holladay, Gossey, and Cruciger. Finally, we agree that most intraocular lens packages do not lend themselves to this mode of measurement. We were able to convince the Precision Cosmet Com pany, who assisted us in this study, to package their lenses with a tight, wrinklefree, clear piano covering so that the powers of their intraocular lenses could be checked without removing the sterile packaging. DAVID MILLER,
M.D.
Boston, Massachusetts
bacteria but never of true fungi. (6) A few actinomycetes are acid-fast, a feature not found in true fungi. For all these reasons, therefore, we do not consider actinomycetes to be fungi, and we would not classify D. congolensis as a fungus. KHALIO F. TABBARA, M.D. MASAO OKUMOTO, M.A.
San Francisco, California REFERENCES 1. Buchanan, R. E., and Gibbons, N. E.: Bergey's Manual of Determinative Bacteriology, 8th ed. Baltimore, Williams and Wilkins, 1974. 2. Waksman, S. A.: The Actinomycetes. Balti more, Williams and Wilkins, 1959, vol. 1, p. 53.
Fungal Growth in Aphakic Soft Contact Lenses Reply Editor: We read with interest the article, "Fungal Editor: growth in soft contact lenses," by R. O. Tabbara and Okumoto have summar Berger and B. W. Streeten (Am. J. Oph- ized well the bacterial characteristics of thalmol. 91:630, 1981). The authors de the actinomycetes which have led taxontected Aspergillus species and Derma- omists to place these atypical organisms, tophilus congolensis, in two aphakic along with Chlamydia and Rickettsia or hydrophilic contact lenses. The authors ganisms, among the bacteria. This change considered D. congolensis, an actinomy- has been slow to be fully implemented. cete, to be a fungus. They also stated that Medical mycology texts continue to carry "actinomycetes have many characteristics extensive sections on the actinomycetes,1 of bacteria but are still classified as fungi mainly because the diseases they cause for most purposes." Actinomycetes are and their mode of growth resemble those not fungi for the following reasons1'2: (1) of fungi more than those of bacteria. The Actinomycetes are fine filamentous struc tendency of Dermatophilis organisms to tures with diameters of less than 1 μ, grow within a contact lens is an excellent whereas the diameters of hyphal ele example of the fungal-like growth charac ments of fungi are usually greater than teristics (not cited by Tabbara and Oku 3 μ. (2) Actinomycetes lack nuclear mem moto) that caused the actinomycetes to branes and mitotic processes and are pro- be classified as fungi originally. Even in caryotes, whereas fungi are eucaryotic. recent ophthalmic texts, the actinomy (3) The cell walls of actinomycetes, like cetes have been considered to be transi those of bacteria, contain muramic acid tional forms between fungi and bacteria and lack the chitin or cellulose that is a by many 23 and are indexed under "fungal constituent of true fungi. (4) Some actino infections." Our statement that the acti mycetes are sensitive to antibiotics that nomycetes were still classified among the affect bacteria and are resistant to anti- fungi "for most purposes" referred to this fungal agents. (5) Some actinomycetes are discrepancy between disease classifica obligate anaerobes, a property of many tion and taxonomy. We should have at-
AMERICAN JOURNAL OF OPHTHALMOLOGY
older American Optical instruments—do not use this standardized index and therefore require a slightly different con version factor. 2 As the authors stated, this keratometric method measures only the power of the frontal surface of the intraocular lens and assumes that the back surface of the lens is flat with a power of piano. Unfortunate ly, this assumption leads to errors be cause both lathe-cut and injectionmolded lenses are polished to remove rough edges, sometimes resulting in power on the back surface. All manufac turers, however, measure their intraocu lar lenses on an optical bench with a "through-the-lens" device so that the stated powers include the back surface power error. Finally, only three of the 11 major manufacturers package their intraocular lenses with the front surface of the lens optically accessible. Two of the three manufacturers state that the poor optics of the covering case can create significant measurement errors when this kerato metric technique is used. Olson, Kolodner, and Kaufman 3 have reported greatly improved quality con trol of the power and resolution of intra ocular lenses. However, we agree with Miller and associates that ophthalmic sur geons should be able to confirm this power themselves. We believe that man ufacturers should satisfy this need with an improved packaging design allowing sterile and accurate verification with a through-the-lens technique before sur gery. J A C K T . HOLLADAY, J O H N GOSSEY,
M.D. M.D.
Houston,
Texas
MARC CRUCIGER,
M.D.
San Francisco,
California
REFERENCES 1. Southhall, J. P. C : Introduction to Physiologi cal Optics. New York, Dover, 1961.
OCTOBER, 1981
2. Sampson, W. G.: Applied optical principles. Keratometry. Ophthalmology 86:347, 1979. 3. Olson, R. J., Kolodner, H., and Kaufman, H. E.: The optical quality of currently manufactured intraocular lenses. Am. J. Ophthalmol. 88:548, 1979.
Reply Editor: We are pleased that Dr.Binkhorst agrees that the conversion factor of 0.46 times the reading on the keratometer yields the power of the frontal surface of the intraocular lens. We also agree that measuring the power of the intraocular lens in the patient's eye is a more compli cated problem than implied in our paper. Certainly, the power of the cornea and the distance between the cornea and the intraocular lens make the rays from the mire converge (probably producing a minifying effect). This same combination probably magnifies the image reflected from the frontal surface of the intraocular lens. The total of these effects varies with corneal power and the distance of the intraocular lens to the cornea. Perhaps Dr. Binkhorst can calculate a series of conversion factors that would allow us to measure precisely the power of the intra ocular lens in the patient's eye. For the present, however, we believe that our method is a helpful approximation. We thank Drs. Holladay, Gossey, and Cruciger for pointing out that the true index of refraction is 1.376, not 1.3375. We are pleased that they also agree with us that a conversion factor of 0.46 allows one to use a keratometer to get the re fracting power of the convex surface of the intraocular lens. We also agree that the piano surfaces on some intraocular lenses are not actual ly piano. In our study, we compared the predicted power of the intraocular lens obtained with the keratometer to that obtained with the optical bench method used by the manufacturer. We were sometimes inaccurate by 0.25 to 0.5 diop-
VOL. 92, NO. 4
591
CORRESPONDENCE
ter. This discrepancy is consistent with the comments of Drs. Holladay, Gossey, and Cruciger. Finally, we agree that most intraocular lens packages do not lend themselves to this mode of measurement. We were able to convince the Precision Cosmet Com pany, who assisted us in this study, to package their lenses with a tight, wrinklefree, clear piano covering so that the powers of their intraocular lenses could be checked without removing the sterile packaging. DAVID MILLER,
M.D.
Boston, Massachusetts
bacteria but never of true fungi. (6) A few actinomycetes are acid-fast, a feature not found in true fungi. For all these reasons, therefore, we do not consider actinomycetes to be fungi, and we would not classify D. congolensis as a fungus. KHALIO F. TABBARA, M.D. MASAO OKUMOTO, M.A.
San Francisco, California REFERENCES 1. Buchanan, R. E., and Gibbons, N. E.: Bergey's Manual of Determinative Bacteriology, 8th ed. Baltimore, Williams and Wilkins, 1974. 2. Waksman, S. A.: The Actinomycetes. Balti more, Williams and Wilkins, 1959, vol. 1, p. 53.
Fungal Growth in Aphakic Soft Contact Lenses Reply Editor: We read with interest the article, "Fungal Editor: growth in soft contact lenses," by R. O. Tabbara and Okumoto have summar Berger and B. W. Streeten (Am. J. Oph- ized well the bacterial characteristics of thalmol. 91:630, 1981). The authors de the actinomycetes which have led taxontected Aspergillus species and Derma- omists to place these atypical organisms, tophilus congolensis, in two aphakic along with Chlamydia and Rickettsia or hydrophilic contact lenses. The authors ganisms, among the bacteria. This change considered D. congolensis, an actinomy- has been slow to be fully implemented. cete, to be a fungus. They also stated that Medical mycology texts continue to carry "actinomycetes have many characteristics extensive sections on the actinomycetes,1 of bacteria but are still classified as fungi mainly because the diseases they cause for most purposes." Actinomycetes are and their mode of growth resemble those not fungi for the following reasons1'2: (1) of fungi more than those of bacteria. The Actinomycetes are fine filamentous struc tendency of Dermatophilis organisms to tures with diameters of less than 1 μ, grow within a contact lens is an excellent whereas the diameters of hyphal ele example of the fungal-like growth charac ments of fungi are usually greater than teristics (not cited by Tabbara and Oku 3 μ. (2) Actinomycetes lack nuclear mem moto) that caused the actinomycetes to branes and mitotic processes and are pro- be classified as fungi originally. Even in caryotes, whereas fungi are eucaryotic. recent ophthalmic texts, the actinomy (3) The cell walls of actinomycetes, like cetes have been considered to be transi those of bacteria, contain muramic acid tional forms between fungi and bacteria and lack the chitin or cellulose that is a by many 23 and are indexed under "fungal constituent of true fungi. (4) Some actino infections." Our statement that the acti mycetes are sensitive to antibiotics that nomycetes were still classified among the affect bacteria and are resistant to anti- fungi "for most purposes" referred to this fungal agents. (5) Some actinomycetes are discrepancy between disease classifica obligate anaerobes, a property of many tion and taxonomy. We should have at-