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Intraocular lens damage from Nd:YAG laser pulses focused in the vitreous Part II: Mode-locked lasers
Intraocular lens damage from N d: YAG laser pulses focused in the vitreous Part II: Mode-locked lasers David H. Sliney, Bodo R. Dolch, Anders Rosen, Fred W. DeJacma, Jr.
II:
,
Although the physics of laser photodisruptor-induced focal optical breakdown in ocular tissues and intraocular lenses (IOLs) has been extensively investigated,1-5 it has only recently been noted that prefocal damage to IOLs is possible from repeated exposure to a Q-switched (nanosecond) Nd:YAC laser beam. 6 This report considers studies of such damage induced by a mode-locked Nd:YAC photodisruptor. MATERIALS AND METHODS In the preceding article,6 Capon, Mellerio, and Docchio report on a study which indicated that an IOL could be damaged by a Q-switched (nanosecond) Nd:YAC laser photodisruptor beam focused behind it. As a result of this study, a follow-on series of tests using a mode-locked laser was conducted. The same exposure protocol as that used with the Q-switched laser exposures was followed and identical IOLs were exposed. For this study the tests were performed at the research laboratory of Medical Lasers, Inc., using a commercial mode-locked Nd:YAC laser ph6todisruptor.
The Nd:YAC, 1,064-nm laser beam from an M-TEC 2000 mode-locked laser was focused at a point 4 mm behind each IOL sample. The cone angle of the la ser beam was 12.8 degrees measured at lIe 2 points. The energy was maintained at 6 mJ by the built-in laser energy monitor and the output was calibrated before and after each series of exposures with a Cen Tec model ED-200 joulemeter monitored with a Tektronix model 2213A oscilloscope. To set the mode-locked laser at an energy level of 6 mJ, it was necessary to use internal adjustments not accessible to the clinician , since the maximum setting of this laser was 5 mJ. An energy setting of 1 mJ to 3 mJ would be more typical clinically. More than one exposure site was used in each of the four IOLs used . To identify the relative position of each exposure zone , amark was created by the photodisruptor at the 12 o'clock position in each IOL. Although the IOLs were manufacturer reject lenses from the same batch used in the first study,6 exposure sites without any apparent imperfections could be found . The exposure sites were carefully chosen to avoid overlap of
From the Department of Clinical Ophthalmology, Institute of Ophthalmology , London , England ( Mr. Sliney) and Medical Losers , Inc., Rockville, Maryland , U,S,A . ( Messrs , Dolch , Rosen, DeJacma ). Stephen Rothery, Institute of Ophthalmology, London, performed the microscopy, Reprint requests to David H, Sliney, 406 Streamside Drive, Fallston, Mal'yland 21047. 530
J CATARACT
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previously exposed areas and any imperfections noted under slitlamp examination. The position of the laserinduced optical breakdown (the spark) was viewed by a microscopic viewer mounted orthogonally to the beam axis to assure that the spark remained 4 mm from the posterior surface of the IOL. RESULTS Each of four IOLs was suspended in a transparent polymethylmethacrylate (PMMA) test chamber fill ed with distilled water. In some initial tests , saline water was used , but sediment from optical breakdown in the solution appeared and it was thought that this would provide confounding results ; hence, only distilled water was used for the reported exposures. The laser beam was fired at 1 Hz or 2 Hz repetition rates for up to 1,000 shots. In all cases, the laser beam created bulk damage characterized by a central spot with crevices surrounding it. This pattern clearly differed from the pitting characteristic of IOL damage when the laser is focused directly on the IOL surface or into the IOL. In some cases, damage was detected prior to the conclusion of 1,000 shots . When this occurred, laser exposures were halted and photographs of the damage site were made. A train of 1,000 exposures focused 4 mm behind the IOL always resulted in bulk damage exhibiting a localized cracking characteristic. Damage was always subsurface inside the IOL as found in the study with Q-switched lasers.6 The damage was less extensive at 3.0 m}, and was still present , but difficult to see, at 1.2 mJ E ven after 100 shots at 6.0 m}, damage was noted. It was then decided to move the beam with
Fig. 1.
respect to the IOL and to delive r 1 ,000 shots over a vertical line from roughly the 7 oclock ' to the 11 o'clock positions. No damage was detected, which indicated that the same volume had to be repeatedly exposed for this cumulative damage to occur. DISCUSSION The mechanism of bulk damage in polymersspecifically PMMA-was studied by Manenkov and coworkers at the Institute of General Physics, Moscow. In their classic studies/,8 they concluded from luminescence and damage-site growth measurements that submicrometer absorbing defects in the polymer matrix evolved to create a thermal instability, which then caused enlargement of the damage site from the propagation of an ionization-inducing absorption wave. In simpler terms, this means that a subvisible defect serves as a damage initiation site at beam irradiances far lower than the normal threshold for immediate damage (e.g. , optical breakdown and bubble form ation in the plastic) at the focal zone of the beam . Only from repeated exposures at the same site can the damage grow-first to a spherical zone of 10 J.Lm to 30 J.Lm in diameter, and then to cracks in the neighborhood of the central zone. O'Connel and coworkers 9 confirmed the studies of Manenkov. The findings of this IOL damage study also support the earlier basic studies of Manenkov. The photographs clearly show the two stages of growth (Figures 1 and 2). CONCLUSIONS There were no uniquely different findings in this study of mode-locked laser induced IOL damage from
(Sliney) Example o fcumulative damage in a PMMA IOL from 500 mode-locked Nd:YAG laser pulses of 6.0 m] average energy focused 4 mm behind the posterior IOL surface. Left: Three sites were exposed to 8, 200, or 500 pulses , but only the site exposed to 500 pulses is clearly visible in viewing the entire IOL . Right: Scanning electron microscopic view of cumulative damage resulting from continued exposure (500 pulses) long after the initial pitting was noted.
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Fig. 2.
(Sliney) Scanning electron micrographic magnified views of minimal type of initial pitting from eight pulses (left), and extended zones created by repeated exposure to hundreds of exposures (right).
those in the previous Q-switched laser study.6 We conclude that the bulk damage reported by Manenkov 7 ,8 and O'Connel 9 with PMMA will occur in IOLs from clinical radiant exposure levels used in both Qswitched and mode-locked photodisruptors if the beam path is fixed in the same spot on the IOL. The clinician must be warned against continually firing from the same position through the IOL. Using minimal pulse energy and limiting the number of pulses will reduce the chance of IOL damage when focusing the beam in the vitreous. Although the damage can be minimal and insignificant to vision if detected as soon as it appears, it could have a serious impact upon entoptic glare levels if repeated exposures are made before any initial damage is detected. Damage will accumulate more rapidly at higher radiant exposure levels. It is not clear whether this effect could happen to the crystalline lens. The organic structure is different; however, once again it would be wise to follow the clinical rule to minimize laser exposure dose levels. REFERENCES 1. Mainster MA, Sliney DH, Belcher CD III, Buzney SM: Laser photodisruptors; damage mechanisms, instrument design and
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J CATARACT REFRACT
safety. Ophthalmology 90:973-991, 1983 2. Puliafito CA, Steinert RF: Short-pulsed Nd:YAG laser microsurgery of the eye: Biophysical considerations. IEEE] Quant Electron QE-20:1442-1448, 1984 3. Vogel A, Hentschel W, Holzfuss J, Lauterborn W: Cavitation bubble dynamics and acoustic transient generation in ocular surgery with pulsed neodymium:YAG lasers. Ophthalmology 93:1259-1269, 1986 4. Docchio F, Sacchi CA, Marshall J: Experimental investigation of optical breakdown thresholds in ocular media under single pulse irradiation with different pulse durations. Lasers Ophthalmol 1:83-93, 1986 5. Davi SK, Gaasterland DE, Cummings CE, Liesegang G: Pulsed laser damage thresholds in vitro for intraocular lenses and membranes. IEEE] Quant Electron QE-20:1458-1465, 1984 6. Capon M, Mellerio J, Docchio F: Intraocular lens damage from Nd:YAG laser focused in the vitreous. Part I: Q-switched lasers. ] Cataract Refract Surg 14:526-529, 1988 7. Manenkov AA, Matyushin GA, Nechitalio VS, Tsaprilov AS: Mechanism of the accumulation effect in laser damage to polymers: Appearance of microdamage due to an ionization absorption wave. Sot) ] Quant Electron 14:568-572, 1984 8. Manenkov AA, Matvushin GA, l'.'echitalio VS, Prokhorov AM, et al: Nature of the ~umulative effect in laser damage to optical materials. Sot) ] Quant Electron 13:1580-1583, 1983 9. O'Connel RM, Deaton TF, Saito TT: Single- and multiple-shot laser-damage properties of commercial grade PMMA. Appl Optics 23:682-688, 1984
SURG-VOL 14, SEPTEMBER 1988
II:
,
Although the physics of laser photodisruptor-induced focal optical breakdown in ocular tissues and intraocular lenses (IOLs) has been extensively investigated,1-5 it has only recently been noted that prefocal damage to IOLs is possible from repeated exposure to a Q-switched (nanosecond) Nd:YAC laser beam. 6 This report considers studies of such damage induced by a mode-locked Nd:YAC photodisruptor. MATERIALS AND METHODS In the preceding article,6 Capon, Mellerio, and Docchio report on a study which indicated that an IOL could be damaged by a Q-switched (nanosecond) Nd:YAC laser photodisruptor beam focused behind it. As a result of this study, a follow-on series of tests using a mode-locked laser was conducted. The same exposure protocol as that used with the Q-switched laser exposures was followed and identical IOLs were exposed. For this study the tests were performed at the research laboratory of Medical Lasers, Inc., using a commercial mode-locked Nd:YAC laser ph6todisruptor.
The Nd:YAC, 1,064-nm laser beam from an M-TEC 2000 mode-locked laser was focused at a point 4 mm behind each IOL sample. The cone angle of the la ser beam was 12.8 degrees measured at lIe 2 points. The energy was maintained at 6 mJ by the built-in laser energy monitor and the output was calibrated before and after each series of exposures with a Cen Tec model ED-200 joulemeter monitored with a Tektronix model 2213A oscilloscope. To set the mode-locked laser at an energy level of 6 mJ, it was necessary to use internal adjustments not accessible to the clinician , since the maximum setting of this laser was 5 mJ. An energy setting of 1 mJ to 3 mJ would be more typical clinically. More than one exposure site was used in each of the four IOLs used . To identify the relative position of each exposure zone , amark was created by the photodisruptor at the 12 o'clock position in each IOL. Although the IOLs were manufacturer reject lenses from the same batch used in the first study,6 exposure sites without any apparent imperfections could be found . The exposure sites were carefully chosen to avoid overlap of
From the Department of Clinical Ophthalmology, Institute of Ophthalmology , London , England ( Mr. Sliney) and Medical Losers , Inc., Rockville, Maryland , U,S,A . ( Messrs , Dolch , Rosen, DeJacma ). Stephen Rothery, Institute of Ophthalmology, London, performed the microscopy, Reprint requests to David H, Sliney, 406 Streamside Drive, Fallston, Mal'yland 21047. 530
J CATARACT
REFRACT SURG-VOL 14, SEPTEMBER 1988
previously exposed areas and any imperfections noted under slitlamp examination. The position of the laserinduced optical breakdown (the spark) was viewed by a microscopic viewer mounted orthogonally to the beam axis to assure that the spark remained 4 mm from the posterior surface of the IOL. RESULTS Each of four IOLs was suspended in a transparent polymethylmethacrylate (PMMA) test chamber fill ed with distilled water. In some initial tests , saline water was used , but sediment from optical breakdown in the solution appeared and it was thought that this would provide confounding results ; hence, only distilled water was used for the reported exposures. The laser beam was fired at 1 Hz or 2 Hz repetition rates for up to 1,000 shots. In all cases, the laser beam created bulk damage characterized by a central spot with crevices surrounding it. This pattern clearly differed from the pitting characteristic of IOL damage when the laser is focused directly on the IOL surface or into the IOL. In some cases, damage was detected prior to the conclusion of 1,000 shots . When this occurred, laser exposures were halted and photographs of the damage site were made. A train of 1,000 exposures focused 4 mm behind the IOL always resulted in bulk damage exhibiting a localized cracking characteristic. Damage was always subsurface inside the IOL as found in the study with Q-switched lasers.6 The damage was less extensive at 3.0 m}, and was still present , but difficult to see, at 1.2 mJ E ven after 100 shots at 6.0 m}, damage was noted. It was then decided to move the beam with
Fig. 1.
respect to the IOL and to delive r 1 ,000 shots over a vertical line from roughly the 7 oclock ' to the 11 o'clock positions. No damage was detected, which indicated that the same volume had to be repeatedly exposed for this cumulative damage to occur. DISCUSSION The mechanism of bulk damage in polymersspecifically PMMA-was studied by Manenkov and coworkers at the Institute of General Physics, Moscow. In their classic studies/,8 they concluded from luminescence and damage-site growth measurements that submicrometer absorbing defects in the polymer matrix evolved to create a thermal instability, which then caused enlargement of the damage site from the propagation of an ionization-inducing absorption wave. In simpler terms, this means that a subvisible defect serves as a damage initiation site at beam irradiances far lower than the normal threshold for immediate damage (e.g. , optical breakdown and bubble form ation in the plastic) at the focal zone of the beam . Only from repeated exposures at the same site can the damage grow-first to a spherical zone of 10 J.Lm to 30 J.Lm in diameter, and then to cracks in the neighborhood of the central zone. O'Connel and coworkers 9 confirmed the studies of Manenkov. The findings of this IOL damage study also support the earlier basic studies of Manenkov. The photographs clearly show the two stages of growth (Figures 1 and 2). CONCLUSIONS There were no uniquely different findings in this study of mode-locked laser induced IOL damage from
(Sliney) Example o fcumulative damage in a PMMA IOL from 500 mode-locked Nd:YAG laser pulses of 6.0 m] average energy focused 4 mm behind the posterior IOL surface. Left: Three sites were exposed to 8, 200, or 500 pulses , but only the site exposed to 500 pulses is clearly visible in viewing the entire IOL . Right: Scanning electron microscopic view of cumulative damage resulting from continued exposure (500 pulses) long after the initial pitting was noted.
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Fig. 2.
(Sliney) Scanning electron micrographic magnified views of minimal type of initial pitting from eight pulses (left), and extended zones created by repeated exposure to hundreds of exposures (right).
those in the previous Q-switched laser study.6 We conclude that the bulk damage reported by Manenkov 7 ,8 and O'Connel 9 with PMMA will occur in IOLs from clinical radiant exposure levels used in both Qswitched and mode-locked photodisruptors if the beam path is fixed in the same spot on the IOL. The clinician must be warned against continually firing from the same position through the IOL. Using minimal pulse energy and limiting the number of pulses will reduce the chance of IOL damage when focusing the beam in the vitreous. Although the damage can be minimal and insignificant to vision if detected as soon as it appears, it could have a serious impact upon entoptic glare levels if repeated exposures are made before any initial damage is detected. Damage will accumulate more rapidly at higher radiant exposure levels. It is not clear whether this effect could happen to the crystalline lens. The organic structure is different; however, once again it would be wise to follow the clinical rule to minimize laser exposure dose levels. REFERENCES 1. Mainster MA, Sliney DH, Belcher CD III, Buzney SM: Laser photodisruptors; damage mechanisms, instrument design and
532
J CATARACT REFRACT
safety. Ophthalmology 90:973-991, 1983 2. Puliafito CA, Steinert RF: Short-pulsed Nd:YAG laser microsurgery of the eye: Biophysical considerations. IEEE] Quant Electron QE-20:1442-1448, 1984 3. Vogel A, Hentschel W, Holzfuss J, Lauterborn W: Cavitation bubble dynamics and acoustic transient generation in ocular surgery with pulsed neodymium:YAG lasers. Ophthalmology 93:1259-1269, 1986 4. Docchio F, Sacchi CA, Marshall J: Experimental investigation of optical breakdown thresholds in ocular media under single pulse irradiation with different pulse durations. Lasers Ophthalmol 1:83-93, 1986 5. Davi SK, Gaasterland DE, Cummings CE, Liesegang G: Pulsed laser damage thresholds in vitro for intraocular lenses and membranes. IEEE] Quant Electron QE-20:1458-1465, 1984 6. Capon M, Mellerio J, Docchio F: Intraocular lens damage from Nd:YAG laser focused in the vitreous. Part I: Q-switched lasers. ] Cataract Refract Surg 14:526-529, 1988 7. Manenkov AA, Matyushin GA, Nechitalio VS, Tsaprilov AS: Mechanism of the accumulation effect in laser damage to polymers: Appearance of microdamage due to an ionization absorption wave. Sot) ] Quant Electron 14:568-572, 1984 8. Manenkov AA, Matvushin GA, l'.'echitalio VS, Prokhorov AM, et al: Nature of the ~umulative effect in laser damage to optical materials. Sot) ] Quant Electron 13:1580-1583, 1983 9. O'Connel RM, Deaton TF, Saito TT: Single- and multiple-shot laser-damage properties of commercial grade PMMA. Appl Optics 23:682-688, 1984
SURG-VOL 14, SEPTEMBER 1988