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Dynamics of the Ultraviolet Laser Ablation of Corneal Tissue
470
AMERICAN JOURNAL OF OPHTHALMOLOGY
Dynamics of the Ultraviolet Laser Ablation of Corneal Tissue
March, 1987
being absorbed in the tissue (Fig. 1). It followed that bond breaking occurred on a subnanosecond time scale and the ejection of ablation products began within 5 to 10 nsec after the R. Srinivasan, P h . D . , pulse front impinged on the corneal surface. and E. Sutcliffe, P h . D . The new model incorporates the theory of an IBM Thomas J. Watson Research Center. absorbed photon flux threshold below which Inquiries to R. Srinivasan, Ph.D., IBM Thomas]. Watson photofragmentation is negligible. Above this Research Center, Yorktown Heights, NY 10598. threshold level, relaxation back to the ground The discovery that pulsed ultraviolet laser state reaches a steady-state condition and radiation can precisely etch the surface of the photofragmentation may then successfully cornea 12 has led to interest in potential clinical compete with relaxation. Ablation occurs when applications of this phenomenon. Routine use a critical density of photofragments, defined by of this process will be possible only when there the ablation condition, is reached within the is a thorough understanding of the factors that material. With this proposed dynamic model the ablation step takes place in the time span of affect the ablation so that control can be exer cised over the dimensions of the cut and ther the laser pulse. The laser radiation interacts with a fast and permanently mutating sample. mal damage is minimized. To promote such A purely geometric change is the possible dis understanding, we proposed a model of the placement of the interface during a single pulse ultraviolet laser etching process 3 that has been and is a consequence of the rapidity of the applied succcessfully to the etching of organic ablation step. Moreover, the photoproducts ac polymers. In this report, we discuss the appli cumulate below the moving interface within cation of our model to the etching of corneal each laser pulse and also from pulse to pulse. tissue. The ablation condition is therefore reached The key result on which the present model is progressively and possibly only after several based relates to a study of the stress pulse laser shots. The bulk encountered by the etch generated by the ejected material from rabbit beam is thus constantly changing its properties cornea when single laser pulses of 193 nm and according to its photochemical history. Only a 13 nsec halfwidth fall on the surface. 4 This fraction of the very first laser pulse interacts study showed that ablation occurred within the with a virgin bulk sample and is, in this sense, time span of the laser pulse, that is, the materi unique. al was being ejected even as the photons were There are few quantitative data on ultraviolet laser etching of tissue in general, and cornea in 100 | i 1 i i i i 1 i i i " particular. The only systematic data on rabbit cornea are those of Krueger and Trokel.6 Their measurements of the etch depth per pulse at ..-"" 80 / various incident energies (fluences) of the laser are shown to fit the theory (Fig. 2). The two parameters fitting the etch curve at 248 nm can be applied to predict the dynamics of ablation 40 / at 193 nm. Within the fairly large uncertainty in the data at 193 nm, the ablation condition agrees well at the two wavelengths but the flux 20 / / / threshold differs by a factor of two. More data / /' must be obtained at 193 nm before this discrep ' ' • -"'"' i i i i i i : 0 20 40 60 BO 100 ancy can be resolved. Time (%) 100% = 120 ns The possibility of accumulating fragments Fig. 1 (Srinivasan and Sutcliffe). Ultraviolet abla from laser pulse to laser pulse is an important tion of cornea in one pulse (193 nm; 0.4 J/cm2). Solid consequence of the basic assumptions underly line: energy deposited (as a percent of total energy in ing the dynamic model. In the cornea, as many one pulse) as a function of pulse duration. Dashed as 100 laser pulses may be needed at a fluence line: integrated intensity of stress pulse (as a percent just above the threshold to reach a steady-state of total integral per pulse) as a function of pulse condition, where each pulse etches exactly the duration. The delay in the start of the stress pulse is same depth of the tissue. Evidently, this will in part caused by the time taken by the acoustic signal to travel through the sample. 4 place a severe constraint on attempts to etch Q
Vol. 103, No. 3, Part II
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p l o r e d in tissue a b l a t i o n . It m a y b r i n g c o n s i d e r able s a v i n g s in d e l i v e r y s y s t e m s a n d laser costs for clinical a p p l i c a t i o n s .
References
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J
a. v o 2 0 I 10'
471
Brief Reports
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Fluence / ( m J / c m ) Fig. 2 (Srinivasan and Sutcliffe). Etch depth per pulse as a function of fluence for cornea at 248 nm . uniformly from site to site if t h e fluence is just a d e q u a t e for a b l a t i o n . A p r e d i c t i o n g e n e r a t e d from t h e d y n a m i c m o d e l is t h e i m p o r t a n c e of t h e p u l s e w i d t h o n t h e e t c h i n g p r o p e r t i e s of corneal t i s s u e . This effect of p u l s e c o m p r e s s i o n h a s actually b e e n verified in p o l y m e r s 3 a n d r e m a i n s to b e ex
1. Trokel, S. L., Srinivasan, R., and Braren, R.: Excimer laser surgery of the cornea. Am. J. Ophthalmol. 96:710, 1983. 2. Puliafito, C. A., Steinert, R. F., Deutsch, T. F., Hillenkamp, F., Dehm, E. J., and Adler, C. M.: Ex cimer laser ablation of the cornea and lens. Ophthal mology 92:741, 1985. 3. Sutcliffe, E., and Srinivasan, R.: Dynamics of UV laser ablation of organic polymer surfaces. J. Appl. Physics 58:3315, 1986. 4. Srinivasan, R., Dyer, P. E., and Braren, B.: Far-UV laser ablation of cornea. Photoacoustic stud ies. Lasers Surg. Med. In press. 5. Krueger, R. R., and Trokel, S. L.: Quantitation of corneal ablation by ultraviolet laser light. Arch. Ophthalmol. 103:1741, 1985.
AMERICAN JOURNAL OF OPHTHALMOLOGY
Dynamics of the Ultraviolet Laser Ablation of Corneal Tissue
March, 1987
being absorbed in the tissue (Fig. 1). It followed that bond breaking occurred on a subnanosecond time scale and the ejection of ablation products began within 5 to 10 nsec after the R. Srinivasan, P h . D . , pulse front impinged on the corneal surface. and E. Sutcliffe, P h . D . The new model incorporates the theory of an IBM Thomas J. Watson Research Center. absorbed photon flux threshold below which Inquiries to R. Srinivasan, Ph.D., IBM Thomas]. Watson photofragmentation is negligible. Above this Research Center, Yorktown Heights, NY 10598. threshold level, relaxation back to the ground The discovery that pulsed ultraviolet laser state reaches a steady-state condition and radiation can precisely etch the surface of the photofragmentation may then successfully cornea 12 has led to interest in potential clinical compete with relaxation. Ablation occurs when applications of this phenomenon. Routine use a critical density of photofragments, defined by of this process will be possible only when there the ablation condition, is reached within the is a thorough understanding of the factors that material. With this proposed dynamic model the ablation step takes place in the time span of affect the ablation so that control can be exer cised over the dimensions of the cut and ther the laser pulse. The laser radiation interacts with a fast and permanently mutating sample. mal damage is minimized. To promote such A purely geometric change is the possible dis understanding, we proposed a model of the placement of the interface during a single pulse ultraviolet laser etching process 3 that has been and is a consequence of the rapidity of the applied succcessfully to the etching of organic ablation step. Moreover, the photoproducts ac polymers. In this report, we discuss the appli cumulate below the moving interface within cation of our model to the etching of corneal each laser pulse and also from pulse to pulse. tissue. The ablation condition is therefore reached The key result on which the present model is progressively and possibly only after several based relates to a study of the stress pulse laser shots. The bulk encountered by the etch generated by the ejected material from rabbit beam is thus constantly changing its properties cornea when single laser pulses of 193 nm and according to its photochemical history. Only a 13 nsec halfwidth fall on the surface. 4 This fraction of the very first laser pulse interacts study showed that ablation occurred within the with a virgin bulk sample and is, in this sense, time span of the laser pulse, that is, the materi unique. al was being ejected even as the photons were There are few quantitative data on ultraviolet laser etching of tissue in general, and cornea in 100 | i 1 i i i i 1 i i i " particular. The only systematic data on rabbit cornea are those of Krueger and Trokel.6 Their measurements of the etch depth per pulse at ..-"" 80 / various incident energies (fluences) of the laser are shown to fit the theory (Fig. 2). The two parameters fitting the etch curve at 248 nm can be applied to predict the dynamics of ablation 40 / at 193 nm. Within the fairly large uncertainty in the data at 193 nm, the ablation condition agrees well at the two wavelengths but the flux 20 / / / threshold differs by a factor of two. More data / /' must be obtained at 193 nm before this discrep ' ' • -"'"' i i i i i i : 0 20 40 60 BO 100 ancy can be resolved. Time (%) 100% = 120 ns The possibility of accumulating fragments Fig. 1 (Srinivasan and Sutcliffe). Ultraviolet abla from laser pulse to laser pulse is an important tion of cornea in one pulse (193 nm; 0.4 J/cm2). Solid consequence of the basic assumptions underly line: energy deposited (as a percent of total energy in ing the dynamic model. In the cornea, as many one pulse) as a function of pulse duration. Dashed as 100 laser pulses may be needed at a fluence line: integrated intensity of stress pulse (as a percent just above the threshold to reach a steady-state of total integral per pulse) as a function of pulse condition, where each pulse etches exactly the duration. The delay in the start of the stress pulse is same depth of the tissue. Evidently, this will in part caused by the time taken by the acoustic signal to travel through the sample. 4 place a severe constraint on attempts to etch Q
Vol. 103, No. 3, Part II
8 I
1
1—i—i—|
1
1—i—i |
i
i—i—r-
V
1
CORNEA a t 2 4 8 n m
E
• Experiment - Case I
w
\
4 -
•*
j ** * / /
I I /
o ■=! 63
/L•
i
1—i—i_l 10 2
1
-/* Sri i
i_l 10 s
p l o r e d in tissue a b l a t i o n . It m a y b r i n g c o n s i d e r able s a v i n g s in d e l i v e r y s y s t e m s a n d laser costs for clinical a p p l i c a t i o n s .
References
/
J
a. v o 2 0 I 10'
471
Brief Reports
i 2
i
i
i_ 10*
Fluence / ( m J / c m ) Fig. 2 (Srinivasan and Sutcliffe). Etch depth per pulse as a function of fluence for cornea at 248 nm . uniformly from site to site if t h e fluence is just a d e q u a t e for a b l a t i o n . A p r e d i c t i o n g e n e r a t e d from t h e d y n a m i c m o d e l is t h e i m p o r t a n c e of t h e p u l s e w i d t h o n t h e e t c h i n g p r o p e r t i e s of corneal t i s s u e . This effect of p u l s e c o m p r e s s i o n h a s actually b e e n verified in p o l y m e r s 3 a n d r e m a i n s to b e ex
1. Trokel, S. L., Srinivasan, R., and Braren, R.: Excimer laser surgery of the cornea. Am. J. Ophthalmol. 96:710, 1983. 2. Puliafito, C. A., Steinert, R. F., Deutsch, T. F., Hillenkamp, F., Dehm, E. J., and Adler, C. M.: Ex cimer laser ablation of the cornea and lens. Ophthal mology 92:741, 1985. 3. Sutcliffe, E., and Srinivasan, R.: Dynamics of UV laser ablation of organic polymer surfaces. J. Appl. Physics 58:3315, 1986. 4. Srinivasan, R., Dyer, P. E., and Braren, B.: Far-UV laser ablation of cornea. Photoacoustic stud ies. Lasers Surg. Med. In press. 5. Krueger, R. R., and Trokel, S. L.: Quantitation of corneal ablation by ultraviolet laser light. Arch. Ophthalmol. 103:1741, 1985.