Journal of Refractive Surgery

A Comparison of Excimer Laser (308 nm) Ablation of the Human Lens Nucleus in Air and Saline With a Fiber Optic Delivery System

Marvin Martinez, BSc; Ezra Maguen, MD; David Bardenstein, MD; Michael Duffy, MD; Seth Yoser, MD; Thanassis Papaioannou, MSc; Warren Grundfest, MD

Abstract

ABSTRACT

BACKGROUND: Photoablation with excimer lasers has demonstrated precise tissue cutting and minimal thermal damage. Potential ophthalmic applications of these lasers include remodeling of the corneal surface, glaucoma treatment, and phacoablation. Ablation of human lens with a 308 nm XeCI excimer laser light delivered through a fiber has been demonstrated in preliminary experiments. Intraocular delivery of laser light must be done in a fluid medium to preserve the integrity of ocular structures. However, little information is available on the effect of the fluid media on the ablation process. Therefore, a series of experiments was conducted to determine whether the ablation of human lens nucleus at 308 nm via a fiber differs in air and saline media.

METHODS: Ablation of human lens nuclei (n = 30) was conducted with a XeCI excimer laser (308 nm) coupled to a 600 µm core size fiber. Irradiation was performed at 2.8 mJ/cmp 2 energy density and 20 Hz. The fiberoptic was brought to contact with the lens nucleus and remained fixed for the duration of irradiation. Variables consisted of the medium (air or saline) and number of pulses delivered (100 to 10 000). Following establishment of the tissue shrinkage ratio, the depth of each crater and the tissue volume removed were measured histologically. The histological features of nucleus ablation in air and in saline were also examined with both light and scanning electron microscopy.

RESULTS: Light microscopy revealed that the average zone of thermal damage adjacent to the crater is thinner in the presence of saline (60 µm, SD = 6 µm) than it is in air (90 µm, SD = 12 µm). In both media, the thickness of the zone of thermal damage is greater at the surface than it is at its base. Following irradiation in air, deep sharpedged craters with smooth walls are formed. Craters formed by irradiation in saline are characterized by reduced depth and irregular walls. For the same number of pulses applied (500, 1000, and 2000), the mean depth of ablation per pulse in air (8.6 to 2.7 µm/puise) was greater by approximately a factor of two than that in saline (4.10 to 1.30 µm/pulse) at P <.01. However, the mean ablated volume removed per pulse was greater in saline (0.00250 to 0.00150 mmp 3/pulse) than in air (0.00120 to 0.00080 mmp 3/pulse), for the same number of pulses (1000, 2000) at P< .01.

CONCLUSIONS: In comparing the data for the same number of pulses applied in air and in saline, it appears that the depth of crater formed by irradiation in air is deeper than that in fluid. The overall volume ablated is greater in fluid than it is in air at 1000 and 2000 pulses. Additionally, the zone of thermal damage is thinner in the presence of saline than it is in air. Smoother crater shapes were observed following irradiation in air than in saline. These results suggest that under this specific experimental setup, the ablation in saline is different from that in air. Refract Corneal Surg 1992;8:368-374.)

Abstract

ABSTRACT

BACKGROUND: Photoablation with excimer lasers has demonstrated precise tissue cutting and minimal thermal damage. Potential ophthalmic applications of these lasers include remodeling of the corneal surface, glaucoma treatment, and phacoablation. Ablation of human lens with a 308 nm XeCI excimer laser light delivered through a fiber has been demonstrated in preliminary experiments. Intraocular delivery of laser light must be done in a fluid medium to preserve the integrity of ocular structures. However, little information is available on the effect of the fluid media on the ablation process. Therefore, a series of experiments was conducted to determine whether the ablation of human lens nucleus at 308 nm via a fiber differs in air and saline media.

METHODS: Ablation of human lens nuclei (n = 30) was conducted with a XeCI excimer laser (308 nm) coupled to a 600 µm core size fiber. Irradiation was performed at 2.8 mJ/cmp 2 energy density and 20 Hz. The fiberoptic was brought to contact with the lens nucleus and remained fixed for the duration of irradiation. Variables consisted of the medium (air or saline) and number of pulses delivered (100 to 10 000). Following establishment of the tissue shrinkage ratio, the depth of each crater and the tissue volume removed were measured histologically. The histological features of nucleus ablation in air and in saline were also examined with both light and scanning electron microscopy.

RESULTS: Light microscopy revealed that the average zone of thermal damage adjacent to the crater is thinner in the presence of saline (60 µm, SD = 6 µm) than it is in air (90 µm, SD = 12 µm). In both media, the thickness of the zone of thermal damage is greater at the surface than it is at its base. Following irradiation in air, deep sharpedged craters with smooth walls are formed. Craters formed by irradiation in saline are characterized by reduced depth and irregular walls. For the same number of pulses applied (500, 1000, and 2000), the mean depth of ablation per pulse in air (8.6 to 2.7 µm/puise) was greater by approximately a factor of two than that in saline (4.10 to 1.30 µm/pulse) at P <.01. However, the mean ablated volume removed per pulse was greater in saline (0.00250 to 0.00150 mmp 3/pulse) than in air (0.00120 to 0.00080 mmp 3/pulse), for the same number of pulses (1000, 2000) at P< .01.

CONCLUSIONS: In comparing the data for the same number of pulses applied in air and in saline, it appears that the depth of crater formed by irradiation in air is deeper than that in fluid. The overall volume ablated is greater in fluid than it is in air at 1000 and 2000 pulses. Additionally, the zone of thermal damage is thinner in the presence of saline than it is in air. Smoother crater shapes were observed following irradiation in air than in saline. These results suggest that under this specific experimental setup, the ablation in saline is different from that in air. Refract Corneal Surg 1992;8:368-374.)

Figure 1: Scanning electron microscopy of a crater formed in lens nucleus following ablation in air with the 308 nm excimer laser (60 ×).Figure 2: Scanning electron microscopy of crater formed in lens nucleus following ablation in normal saline with the 308 nm excimer laser (60 ×).

Figure 1: Scanning electron microscopy of a crater formed in lens nucleus following ablation in air with the 308 nm excimer laser (60 ×).

Figure 2: Scanning electron microscopy of crater formed in lens nucleus following ablation in normal saline with the 308 nm excimer laser (60 ×).

The excimer laser1 has demonstrated precise tissue cutting with minimal thermal damage by photoablation.2 In recent years, the interaction of a variety of ocular tissues3 with excimer laser light wavelengths ranging between 193 nm and 351 nm has been investigated. Potential applications include keratectomy,4 keratotomy,5,6 remodeling of the corneal surface,4 phakoablation,7 and glaucoma treatment.8 The use of the 308 nm excimer laser phakoablation may allow the preservation of the lens capsule, thus enabling accommodation if such lens capsule could be filled with an inert transparent substance. UV grade optical fibers with adequate light transmission at 308 nm are now available. The UV laser energy transmitted through a fiber can, therefore, be delivered to a target organ intraocularly. This modality has, therefore, been investigated for possible use in lens ablation3·" and glaucoma surgery.8 Preliminary experiments have shown that ablation of the human lens is feasible using a XeCl excimer laser (308 nm) with its energy delivered through a fiberoptic.7

Figure 3: Light microscopy, trichome stain of a crater formed in lens nucleus following ablation in air with the 308 nm excimer laser.Figure 4: Light microscopy, trichrome stain of a crater formed in lens nucleus following ablation in normal saline with the 308 nm excimer laser.

Figure 3: Light microscopy, trichome stain of a crater formed in lens nucleus following ablation in air with the 308 nm excimer laser.

Figure 4: Light microscopy, trichrome stain of a crater formed in lens nucleus following ablation in normal saline with the 308 nm excimer laser.

The delivery of laser energy to intraocular structures via a fiber must be done in a fluid medium to preserve the integrity of ocular structures. Little information is available regarding the laser tissue interaction at 308 nm in fluid media. Previous experimentation on vascular tissue9 suggests that excimer laser ablation of most atherosclerotic plaques is considerably less effective when performed in saline solution than in ah-. To determine the difference in ablation of human lens nucleus in saline and in air, we compared the total ablated volume per pulse and the ablated depth per pulse of the crater created during ablation in the two media. In addition, the histological features of lens ablation in air and fluid were compared.

Table

Table 1Mean Ablated Depth Per Pulse in µm (± SD) for Human Lens Nucleus in Air and in Saline*Table 2Mean Ablated Volume per Pulse in mmp 3 (±SD) for Human Lens Nucleus in Air and in Saline*

Table 1

Mean Ablated Depth Per Pulse in µm (± SD) for Human Lens Nucleus in Air and in Saline*

Table 2

Mean Ablated Volume per Pulse in mmp 3 (±SD) for Human Lens Nucleus in Air and in Saline*

MATERIALS AND METHODS

Laser and Fiber Delivery System

A modified, prototype long pulse XeCl excimer laser (Jet Propulsion Laboratory, Pasadena, Calif) was used, generating pulses of approximately 85 nsec duration.10 The repetition rate could be varied from 1 to 20 Hz. The energy delivery system included a UV-grade quartz spherical lens (2-inch diameter, 100-millimeter focal length) and UV-grade fused silica step index fibers of 600 micron core diameter. The fibers measured 820 µm in outer diameter (including cladding and coating). Fibers with lengths up to 3 meters were used.

The energy output of the fiber was measured with a Laser Precision Corp model RKJ-7200 Energy Ratiometer in combination with a Laser Precision Corp Energy Probe, model RJP-735. The fluence was calculated as the fiber energy output divided by the fiber core area (300 µm radius).

Human Lenses

Thirty human lenses were procured through the regional subsidiaries of Tissue Bank International. The interval between death and lens harvesting was no more than 16 hours and lenses were used within 48 hours of donor expiration. All lenses were procured in whole globes and preserved in moist chambers. In an effort to provide data on an age group in which the incidence of cataracts is highest, the donor age ranged between 66 and 98 years.

Ablation of the Human Lens Nucleus in Air

Fifteen human lenses were used. The cortex and capsule were dissected and the nuclei were placed on a quartz slide. Each nucleus was held in place by a strip of adhesive bandage with a specially devised central opening. For ablation in air, the 600 µm core quartz fiberoptic was brought into contact with the lens nucleus and remained fixed for the duration of irradiation.

Pulses were delivered with a constant operative fluence of 2.8 J/cmp 2 at a constant repetition rate of 20 Hz. The number of pulses was variable. Totals of 100, 250, 500, 1000, and 2000 pulses were delivered to the lens nucleus. Three lenses were used per group of pulses. Each lens was irradiated twice in different locations.

Ablation of the Human Lens Nucleus in Saline

Fifteen human lenses were used. The lens nuclei were dissected away from cortex and lens capsules and fixed on a quartz slide, as previously discussed. The slide was then immersed in a plexiglass petri dish filled with normal saline. The fiber was brought inside the fluid in contact with the surface of the lens and remained fixed while laser energy was delivered. The operative fluence remained constant at 2.8 J/cmp 2. The repetition rate also remained constant at 20 Hz. A series of 500, 1000, 2000, 5000, and 10 000 pulses were delivered. Three lenses were used per group of pulses. Each lens was irradiated twice in different locations.

Histology and Determination of Ablated Volume and Depth

After ablation was completed, the diameter of the opening of the crater formed was measured on fresh tissue with a Zeiss Photoscope and a Zeiss Ocular and Stage Micrometer Calibrator. Following routine formalin fixation and plastic embedding, sections were stained with hematoxylin and eosin, PAS, and the Masson trichrome stain. The diameter of the same crater was measured under a microscope. A ratio between the diameters of the fresh and processed tissue, the shrinkage ratio, was established. It was found to be 36% (SD = 6). Once this ratio was determined for each specimen, the depth and base diameter of the crater in the fixed state could be calculated retroactively after measuring the corresponding dimensions in the deepest histological or widest section. By applying the ratio previously obtained, the depth of the crater in fresh tissue could be established. The diameter of the base of the crater was also measured on the deepest section after processing. After applying the shrinkage ratio, the depth of the crater in fresh tissue could be established.

By utilizing the above numbers, the volume of the ablated tissue could be calculated with the following formula:

V = [(πh)/3] [Dp 2 + dp 2 + dD]

where V = volume of tissue ablated, h = depth of crater, D = radius at the entry site of the crater (depth = 0), and d = radius of the base of the crater (maximal depth).

In addition, the histologic features of the lens tissue surrounding the craters were also studied. Light microscopy sections of all nuclei ablated in air and in fluid were stained with hematoxylin and eosin, PAS, and the Masson trichrome stains which were used to reveal thermal damage.2 Scanning electron microscopy was also performed.

RESULTS

Histological Features of Nucleus Ablation in Air and in Saline

Scanning Electron Microscopy. Figures 1 and 2 demonstrate respectively craters formed in the lens nucleus following ablation in air and in saline. The original magnification of the Figures is identical (6Ox). The laser parameters and the number of pulses delivered were also identical.

Figure 1 demonstrates a relatively small diameter sharp-edged crater following ablation in air. The crater is deep and its walls are smooth. The base of the crater is also smooth.

Figure 2 demonstrates ablation of human lens nucleus in saline. The diameter of the crater is wider. The depth of the crater is much reduced, and the walls are irregular.

Light Microscopy. Figures 3 and 4 demonstrate craters obtained following ablation of the lens in air and in fluid respectively.

The trichrome stain demonstrated a rim of thermal damage with disruption of normal lens fiber following ablation as amorphic lens tissue at the ridges of the crater.

Following ablation in air (Fig 3) the crater walls were smooth with a zone of thermal damage that was wider at the top of the crater than at its base. The average width of the zone of thermal damage was 90µ or less than 10% of the diameter of the crater (range = 60 to 120 µ; SD = 12 µ).

Figure 4 demonstrates the appearance of the lens following ablation in saline. As before, the zone of thermal damage was greater at the top of the crater than at its base. In 80% of the sections studied, there was no evidence of thermal damage at the base of the crater. The average width of the zone of thermal damage was 60 µ or less than 5% of the diameter of the crater (range = 40 to 90 µ; SD = 6 µ). The zone of thermal damage of the crater obtained while the lens was irradiated in saline is, therefore, less wide than that obtained in air (P < .01) under these particular experimental conditions.

Determination of the Mean Ablated Depth per Pulse on Human Lens Nucleus in Air and Saline

The mean depth of ablation per pulse was determined as follows: The maximum crater depth was calculated as described previously and was then divided by the number of pulses delivered to the tissue. Table 1, column 1, shows the mean crater depth per pulse obtained from 100, 250, 500, 1000, and 2000 pulses in air. Column 2 of the same table shows the mean ablated depth per pulse obtained from 500, 1000, 2000, 5000, and 10 000 pulses in saline. These results represent the mean of six values.

From the data presented, it appears that the mean depth per pulse decreases as the number of pulses increases. This occurred both when the energy is applied in air and in saline. In comparing the average depth per pulse obtained for the same number of pulses (500, 1000, 2000) in air and in saline, it appears that depth ablation is less efficient in saline than it is in air (P < .01) under these experimental conditions whereby the fiber remains fixed during irradiation.

Determination of Ablated Volume per Pulse of Human Lens Nucleus in Air and Saline

The ablated volume per pulse was calculated as the ratio of the total ablated volume of a crater to the number of pulses delivered to the tissue.

Table 2, column 1, demonstrates the mean lens volume ablated per pulse in air at fixed fluence and repetition rate (2.8 J/cmp 2, 20 Hz). A variable number of pulses was delivered in each series of the experiments: 100, 250, 500, 1000, and 2000 pulses.

Column 2 of the same table demonstrates the mean ablated volume per pulse of human lens nucleus per pulse in saline with fixed laser parameters (2.8 J/cmp 2, 20 Hz). A series of 500, 1000, 2000, 5000, and 10 000 pulses was delivered. These results demonstrate that in both air and saline, the average volume of tissue removed per pulse decreases with the number of pulses delivered. An increased volume ablation efficiency has been observed when irradiation is performed in saline as compared to ablation in air under the same experimental conditions of 1000 and 2000.

DISCUSSION

Based on the above data, the following conclusions may be drawn:

As can be seen from Tables 1 and 2, both the depth of the ablated crater and its volume increase with increasing number of pulses for both air and saline. However, this increase does not happen in a linear fashion.

In comparing the data for the same number of pulses (ie, 500, 1000, and 2000) applied in air and in saline (Table 1), it appears that the depth of crater formed by irradiation in air is deeper than in fluid. Additionally, as indicated by the histological findings, the crater's opening is greater in fluid than in air under otherwise similar experimental conditions.

The volume of tissue ablated per pulse is greater in fluid than it is in air at 1000 and 2000 pulses delivered to the tissue (Table 2).

The zone of thermal damage adjacent to the crater in lens nucleus is thinner in the presence of saline than it is in air. In both media, the thickness of the zone of thermal damage is greater at the surface of the crater than it is at its base.

A variety of factors may be responsible for the observed decrease of the ablation rate per pulse with increasing number of delivered pulses. A major factor relates to the specific experimental design, in which the fiber was held stationary during ablation. With the use of more pulses and since the fiber is kept fixed throughout the ablation process, the distance between the fiber tip and the tissue increases. Consequently, due to the cone-shaped light output of the fiber, lower energy density (fluence) is delivered to the tissue resulting in a reduced ablation efficiency for both air and saline media.

The observed difference in ablation depth between air and fluid media can be partially attributed to the increased presence of debris between the fiber and the tissue when ablation is performed in fluid. The subsequent absorption of light by the debris reduces the available fluence and, therefore, the ablation efficiency. The presence of fluid also reduces the beam divergence due to its higher index of refraction compared to that of air. Therefore, an increased ablation efficiency at least in the forward direction (depth) may in theory be expected when saline is used instead of air. However, our results indicate that this factor, under the described experimental setup, may not be crucial if compared with the effect of the absorption by tissue debris.

To explain the difference in diameters at the entry sites of the craters as well as the increased ablated volume when ablation performed in saline, one has to assume that mechanical effects due to liquid confinement are responsible. Figure 2 reveals irregular crater edges indicating mechanical damage in addition to photo decomposition. The presence of the fluid could enhance, through confinement, possible photoacoustic mechanisms which can further contribute to tissue removal. Previous observations7 have revealed evidence of mechanical vibrations during lens ablation.

Thermal conduction and/or convection due to the presence of fluid are to be accounted for the decreased thermal damage demonstrated by the histological findings. It has to be stressed that the purpose of these experiments was not to establish ablation rates of the human nucleus at 308 nm but rather to investigate the effect of different media in the ablation process under the specific experimental setup described above. It is expected that constant contact ablation will greatly enhance the ablation efficiency. Additional efficiency might be obtained if the fiber was coupled with an irrigation and aspiration system. This particular system will most probably render the zone of thermal damage even less prominent in fluid because of the cooling effect of the circulating fluid.

REFERENCES

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3. Peyman GA, Kusak JR, Weckstrom K, Mannonen I, Viherkoski E, Auterinen L. Effect of XeCl excimer laser on the eyelid and anterior segment structures. Arch Ophthalmol. 1986;104:118.

4. L'Espérance FA, Taylor DM, Warner JW. Human excimer laser keratectomy: short term histopathology. Journal of Refractive Surgery. 1988;4:118.

5. Tenner A, Neuhann T, Schroder E, SaIz JJ, Maguen E. Excimer laser radial keratotomy in the living human eye: a preliminary report. Journal of Refractive Surgery. 1988;4:5.

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Table 1

Mean Ablated Depth Per Pulse in µm (± SD) for Human Lens Nucleus in Air and in Saline*

Table 2

Mean Ablated Volume per Pulse in mmp 3 (±SD) for Human Lens Nucleus in Air and in Saline*

10.3928/1081-597X-19920901-07

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