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Vitreous Changes After Neodymium-YAG Laser Irradiation of the Posterior Lens Capsule or Mid-Vitreous
VOL. 98, NO. 3
LETTERS TO THE JOURNAL
prepared and the donor cornea uncov ered, we discovered that the donor tissue had been "cooked" in this Teflon-metal oven, which caused destruction of corneal clarity and form (Fig. 2). Another donor cornea was obtained to complete the penetrating keratoplasty. In this time of "fast turnover" operat ing rooms with autoclaved instruments, the accidental heat destruction of donor corneal tissue must be considered. Awareness and prevention of this intraop erative complication should completely eliminate this cause of "primary graft failure."
CORRESPONDENCE Correspondence concerning recent articles or other material published in THE JOURNAL should be submitted within six weeks of publi cation. This correspondence must be typed and prepared in the same way as Letters to THE JOURNAL.
Every effort will be made to resolve contro versies between the correspondents and the authors of the article before formal publica tion.
Vitreous Changes After Neodymium-YAG Laser Irradiation of the Posterior Lens Capsule or Mid-Vitreous EDITOR:
The article "Vitreous changes after neodymium-YAG laser irradiation of the posterior lens capsule or mid-vitreous" (Am. J. Ophthalmol. 97:470, April 1984), by S. Lerman, B. Thrasher, and M. Moran, raises potentially important questions concerning intraocular effects of the short-pulsed YAG laser. The in troduction states that the intent of the laser is to induce focal vaporization,
381
with production of shock waves as an undesirable secondary effect. The refer ences cited by the authors, however, clearly state that the desired principal mechanism for intraocular application is production of the shock wave, with va porization as a highly localized and less significant mechanism. 1 We are confused by the methodology and data presented, and would appreci ate clarification by the authors on sever al points: 1. What published studies support the use of rabbits as a model for human vitreous? 2. The clinically relevant concern is the quantitation of changes in the vitre ous in the course of posterior capsulotomy. In the method chosen, posterior capsulotomy is simulated by rupturing the posterior capsule of an intact lens. Did the authors consider the changes in the vitreous that could occur from re lease of liquid protein into the vitreous, which occurs dramatically with rupture of a rabbit lens? 3. Tables 1 and 2 present only control data, apparently designed to show con sistent results between the two eyes of control animals and between different animals of the same weight. A statistical analysis was not given, however. Fur thermore, when one compares the con trol results to the controls in Tables 3 and 4, there is considerable variance between different control animals, in contrast to the consistency presented within each of the first two tables. The difference between the control eyes of treated animals and the control eyes of the untreated animals is greater than the difference between the control and treated eyes. Can the authors explain these disparities and provide a statistical analysis? 4. In Table 2, the T2 values were ex trapolated to three subgroups; in Table 4 only two subgroups are presented.
382
AMERICAN JOURNAL OF OPHTHALMOLOGY
What were the results of the third (T2c) group in Table 4? 5. The viscometry measurements in Table 3 indicate that only one monkey was studied by this method. The results in Table 4 are presented for only three rabbits and no primates. Was nuclear magnetic resonance performed on any primate eyes or any of the other rabbit eyes? Where are the data on the second monkey? Despite the overall excellent clinical experience with the neodymium-YAG laser to date, extending for a six-month follow-up, 2 this study raises interesting questions which we believe deserve careful consideration and investigation. We look forward to the answers to these questions and more complete studies in the future. ROGER F. S T E I N E R T ,
M.D.
CARMEN A. P U L I A F I T O ,
M.D.
Boston,
Massachusetts
REFERENCES 1. Taboada, J.: Interaction of short laser pulses with ocular tissues. In Trokel, S. L. (ed.): YAG Laser Ophthalmic Microsurgery. Norwalk, AppletonCentury-Crofts, 1983, pp. 15-38. 2. Keates, R. H. ; Steinert, R. F., Puliafito, C. A., and Maxwell, S. K.: Long-termfollow-upof Nd-YAG laser posterior capsulotomy. Am. Intra-Ocular Im plant Soc. ]. 10:164, 1984.
Reply EDITOR:
In reply to Drs. Steinert and Puliafito's comments regarding the produc tion of the shock wave as the desired principal mechanism for intraocular ap plication, I am unable to comment on the first reference they cite. However, I question this assertion because, in order to induce vaporization of the lens cap sule, one requires a power density of approximately 1 or 2 X 1012 W/cm 2 at the focal point (in this case the posterior lens capsule). In essence, one relies on the thermal effect of this power density. I certainly would not rely on the acous
SEPTEMBER, 1984
tic gradient generation to perform a capsulotomy. Furthermore, the refer ence cited in our article does not clearly state the foregoing concerning "shock waves." The paper cited in reference 4 of our article does not state that "the shock wave is the desired principal mechanism." In answer to the five additional points raised: 1. We are all aware that the rabbit eye is not the ideal model and thus that we must not extrapolate such experi ments directly to the human condition. We therefore used monkey eyes as well as rabbit eyes. Because the cost of pri mates is very high, the studies were performed on rabbits to provide a great er data base. The trends in both experi mental animals were similar. 2. The methods chosen (phakic vs aphakic or pseudophakic) are valid. Since the animals in this study were killed immediately after laser exposure, it would be surprising if "the release of liquid protein into the vitreous" could cause the vitreous changes. Further more, identical results were obtained in those animals that received mid-vitreous laser irradiation. (Their lenses were not affected at all.) 3. Tables 1 and 2 are clearly labeled to indicate that they present only con trol data. I see no reason for a statistical analysis on these data since they merely demonstrate the reproduceability of our methods and obviously do not require further analyses. These control studies were performed to evaluate the feasibil ity of utilizing the contralateral eye as the control for the eye exposed to the laser. The data in Tables 1 and 2 show an excellent correlation. Obviously, the data between different animals' eyes will differ much more compared with both eyes from one animal since the animals varied in age (that is, according to their weight); we should expect dif ferences in vitreous viscosity (with age).
LETTERS TO THE JOURNAL
prepared and the donor cornea uncov ered, we discovered that the donor tissue had been "cooked" in this Teflon-metal oven, which caused destruction of corneal clarity and form (Fig. 2). Another donor cornea was obtained to complete the penetrating keratoplasty. In this time of "fast turnover" operat ing rooms with autoclaved instruments, the accidental heat destruction of donor corneal tissue must be considered. Awareness and prevention of this intraop erative complication should completely eliminate this cause of "primary graft failure."
CORRESPONDENCE Correspondence concerning recent articles or other material published in THE JOURNAL should be submitted within six weeks of publi cation. This correspondence must be typed and prepared in the same way as Letters to THE JOURNAL.
Every effort will be made to resolve contro versies between the correspondents and the authors of the article before formal publica tion.
Vitreous Changes After Neodymium-YAG Laser Irradiation of the Posterior Lens Capsule or Mid-Vitreous EDITOR:
The article "Vitreous changes after neodymium-YAG laser irradiation of the posterior lens capsule or mid-vitreous" (Am. J. Ophthalmol. 97:470, April 1984), by S. Lerman, B. Thrasher, and M. Moran, raises potentially important questions concerning intraocular effects of the short-pulsed YAG laser. The in troduction states that the intent of the laser is to induce focal vaporization,
381
with production of shock waves as an undesirable secondary effect. The refer ences cited by the authors, however, clearly state that the desired principal mechanism for intraocular application is production of the shock wave, with va porization as a highly localized and less significant mechanism. 1 We are confused by the methodology and data presented, and would appreci ate clarification by the authors on sever al points: 1. What published studies support the use of rabbits as a model for human vitreous? 2. The clinically relevant concern is the quantitation of changes in the vitre ous in the course of posterior capsulotomy. In the method chosen, posterior capsulotomy is simulated by rupturing the posterior capsule of an intact lens. Did the authors consider the changes in the vitreous that could occur from re lease of liquid protein into the vitreous, which occurs dramatically with rupture of a rabbit lens? 3. Tables 1 and 2 present only control data, apparently designed to show con sistent results between the two eyes of control animals and between different animals of the same weight. A statistical analysis was not given, however. Fur thermore, when one compares the con trol results to the controls in Tables 3 and 4, there is considerable variance between different control animals, in contrast to the consistency presented within each of the first two tables. The difference between the control eyes of treated animals and the control eyes of the untreated animals is greater than the difference between the control and treated eyes. Can the authors explain these disparities and provide a statistical analysis? 4. In Table 2, the T2 values were ex trapolated to three subgroups; in Table 4 only two subgroups are presented.
382
AMERICAN JOURNAL OF OPHTHALMOLOGY
What were the results of the third (T2c) group in Table 4? 5. The viscometry measurements in Table 3 indicate that only one monkey was studied by this method. The results in Table 4 are presented for only three rabbits and no primates. Was nuclear magnetic resonance performed on any primate eyes or any of the other rabbit eyes? Where are the data on the second monkey? Despite the overall excellent clinical experience with the neodymium-YAG laser to date, extending for a six-month follow-up, 2 this study raises interesting questions which we believe deserve careful consideration and investigation. We look forward to the answers to these questions and more complete studies in the future. ROGER F. S T E I N E R T ,
M.D.
CARMEN A. P U L I A F I T O ,
M.D.
Boston,
Massachusetts
REFERENCES 1. Taboada, J.: Interaction of short laser pulses with ocular tissues. In Trokel, S. L. (ed.): YAG Laser Ophthalmic Microsurgery. Norwalk, AppletonCentury-Crofts, 1983, pp. 15-38. 2. Keates, R. H. ; Steinert, R. F., Puliafito, C. A., and Maxwell, S. K.: Long-termfollow-upof Nd-YAG laser posterior capsulotomy. Am. Intra-Ocular Im plant Soc. ]. 10:164, 1984.
Reply EDITOR:
In reply to Drs. Steinert and Puliafito's comments regarding the produc tion of the shock wave as the desired principal mechanism for intraocular ap plication, I am unable to comment on the first reference they cite. However, I question this assertion because, in order to induce vaporization of the lens cap sule, one requires a power density of approximately 1 or 2 X 1012 W/cm 2 at the focal point (in this case the posterior lens capsule). In essence, one relies on the thermal effect of this power density. I certainly would not rely on the acous
SEPTEMBER, 1984
tic gradient generation to perform a capsulotomy. Furthermore, the refer ence cited in our article does not clearly state the foregoing concerning "shock waves." The paper cited in reference 4 of our article does not state that "the shock wave is the desired principal mechanism." In answer to the five additional points raised: 1. We are all aware that the rabbit eye is not the ideal model and thus that we must not extrapolate such experi ments directly to the human condition. We therefore used monkey eyes as well as rabbit eyes. Because the cost of pri mates is very high, the studies were performed on rabbits to provide a great er data base. The trends in both experi mental animals were similar. 2. The methods chosen (phakic vs aphakic or pseudophakic) are valid. Since the animals in this study were killed immediately after laser exposure, it would be surprising if "the release of liquid protein into the vitreous" could cause the vitreous changes. Further more, identical results were obtained in those animals that received mid-vitreous laser irradiation. (Their lenses were not affected at all.) 3. Tables 1 and 2 are clearly labeled to indicate that they present only con trol data. I see no reason for a statistical analysis on these data since they merely demonstrate the reproduceability of our methods and obviously do not require further analyses. These control studies were performed to evaluate the feasibil ity of utilizing the contralateral eye as the control for the eye exposed to the laser. The data in Tables 1 and 2 show an excellent correlation. Obviously, the data between different animals' eyes will differ much more compared with both eyes from one animal since the animals varied in age (that is, according to their weight); we should expect dif ferences in vitreous viscosity (with age).