Some applications of the neodymium:YAG laser operating in the thermal and photodisruptive modes. Vitreolysis

Abstract

The clinical effects of the Nd:YAG laser operating in both the photodisruptive (Q-switched) and thermal (free-running, cw) modes are discussed, and their clinical applications are explored. Moreover, the physical background of the working modes is analyzed. When working in the photodisruptive and fundamental (TEM00) modes, minimally invasive effects are possible with regard to delicate clinical tasks. When the laser is working in the multimode regime, tasks that are highly resistant to photodisruptive laser radiation can be solved. In the thermal mode, photocoagulation can be performed. Nd:YAG laser light (1064 nm) has high optical tissue penetration and good hemostatic properties, particularly when being operated in the frequency-doubled mode (KTP laser).

Introduction

The neodymium:YAG (Nd:YAG) laser can be used for photocoagulation in the thermal (free-running and cw) and, for microsurgery, both in the thermal and the photodisruptive, mode. When working in the thermal mode for photocoagulation, its action is analogous with other thermal lasers, such as the argon ion or diode laser, and involves tissue heating1 due to linear absorption, the thermal damage being described by the Arrhenius integral.2

Moderate heating is associated with the breaking of hydrogen bonds and Van der Waal’s forces, resulting in the loss of biological or structural activity,3 while high temperatures can induce explosive effects due to water evaporation. The Nd:YAG laser emits at 1064 nm, but can also be operated in its frequency-doubled configuration (632 nm: the potassium kalium phosphate (KTP) laser).

Photodisruption with short, intense laser pulses has come into widespread use because it makes

noninvasive, intraocular microsurgery possible. Photodisruption depends on the nonlinear absorption of light at the laser focus, by which plasma reaching a temperature of a few thousand degrees of Kelvin are produced.4,5 This nonlinear absorption process induces a phenomenon known as ‘optical breakdown’, which makes it possible to deposit the laser energy within spatially highly limited regions in transparent ocular structures, as well as in pigmented tissue.6,7 Although photodisruption is possible with other lasers and other wavelengths, only the Nd:YAG laser is now used in commercial apparatus. Most clinical Nd:YAG lasers use the fundamental TEM00 mode, so that the spot size, and consequently the pulse energy required for breakdown, can be minimized. Because they improve optical resolution, contact lenses are essential for focus spot compression. Beam divergence is improved by such lenses, and is of the order of 0.5-3.0 mrad. The minimum pulse energy that allows delicate tasks to be solved is of the order 0.4 mJ, and many types of apparatus have a maximum pulse energy of 30 mJ. The span between minimum and maximum pulse energy available for a given apparatus is the dynamic range.

The action of both the photodisruptive laser at the upper and lower limits of its dynamic range, and of the thermally operated Nd:YAG laser, will be discussed below, using several examples.

Case reports

Photodisruptive mode of operation at the lower end of the dynamic range

Case 1

The effects of laser irradiation upon deposits at the anterior face of an implant are shown in Figure 1. A 92-year-old white male was referred due to pseudophakic glaucoma, with an intraocular pressure (IOP) of 30 mmHg, an iris bombe, and a shallow anterior chamber. The anterior face of the implant was covered by a transparent membrane, in which a dense carpet of deposits was embedded. Following iridectomy with a burst of multimode Q-switched pulses at 12 mJ, the IOP and anterior chamber depth normalized. The membrane and the deposits on the anterior lens capsule had previously been shown to be resistant to intense steroid therapy. However, they reacted when irradiated with low-energy photodis-

ruptive pulses. 120 single pulses at an energy level of 0.4 mJ directed at the implant. Following this, intense local steroid therapy was provided, which resulted in the IOP remaining at normal levels and the anterior segment not being irritated.

Parameters: A CGI contact lens8 was used for both iridectomy and destruction of the deposits, and pulse energies of 0.4 mJ were used with the laser operating in the TEM00 mode (fundamental, transversal mode), while for the iridectomy, a pulse energy of 12 mJ was selected and the laser was operated in the multimode regime.

Cases 2, 3, 4 and 5

These cases (Figs. 2, 3, and 4) included interventions at the anterior face of a normal, clear crystalline lens.10,11 Their irides all had an adherent pupillary margin at the anterior lens capsule. In Case 5 (aphakia) (Fig. 5) the border of the iris was fixed by an inflammatory pupillary membrane. All these adhe-

Fig. 3. (Case 3) A (see previous page): one synechia persists at 7 o’clock. B: The situation following laser synechiolysis. The pupil is entirely free. C: Same as B, showing a free pupil (A: high magnification; B: low magnification). (Reproduced from Kwasniewska et al.9 by courtesy of the publisher.)

Fig. 4. (Case 4) A: Seclusio pupillae, a sequela of inactive anterior uveitis in a cataractous eye. The pupil is fixed. B: The situation following the first irradiation: the pupillary block is broken. C: The situation following the second irradiation (upper and lower aspect of the pupil). D: Same as C, but centered at the nasal aspect of the pupil. E: The situation following ICCE. The pupil is free. Parameters: fundamental mode operation; 51 single pulses at an average pulse energy of 0.5-1.0 mJ. (Reproduced from Fankhauser et al.10 by courtesy of the publisher.)

Fig. 5. (Case 5, aphakia) A: Fixed pupil due to postinflammatory membrane. B: The pupillary membrane is lysed by a large number of single pulses at a pulse energy of 0.4-0.5 mJ. The pupil has shrunken (A and B: low magnification). C: (high magnification) The pupil is mobile and the shrunken pupillary membrane has fallen to the bottom of the anterior chamber. (Reproduced from Kwasniewska et al.10 by courtesy of the publisher.)

Fig. 6. (Case 6; aphakia) A: (high magnification) An extremely hard hyaline and calcified postcataract membrane following injury before irradiation. B and C: The situation following irradiation. B: The membrane has been shattered into large pieces. C: Reopening of the central part of the pupil by pulverization of fragments situated in the central part of the pupillary area (B,C: low magnification). Parameters: photodisruptive, multimode, burst operation; six pulses per burst; pulse energy: 16 mJ; six bursts per session; two sessions. Contact lens: CGP. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Fig. 7. (Case 7; aphakia) A: An extremely thick pupillary membrane consisting of cortex and semiorganized hematoma (following perforating injury). B: The situation following irradiation. In order to avoid damage to the endothelium, treatment was discontinued as soon as satisfactory vision had been achieved. A portion of the membrane was left behind. Parameters: photodisruptive, multimode, burst operation; pulse energy: 6-8 mJ; two bursts per session; six bursts, two sessions. Contact lens: CGP. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Fig. 8. (Case 8; aphakia) A: An extremely thick hard pupillary membrane consisting of cortex and capsular material following an irrigation-aspiration procedure for a traumatic cataract. B: Repeated irradiations freed the pupil, which was adherent to the postcataract membrane, restoring an optically useful, nearly round pupil. Parameters: photodisruptive, multimode, burst operation; pulse energy: 7 mJ; five bursts per session; four bursts, four sessions. Contact lens: CGP. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Photodisruptive mode in the upper half of the dynamic range

Cases 6, 7, 8, and 9

In these cases (Figs. 6, 7, 8, and 9), the action of photodisruptive pulses in the burst mode, in the upper half of the dynamic range (10-20 mJ), was shown to be necessary in order to shatter, remove, and pulverize hard membranes in aphakia. Pulse energies of this magnitude are a threat to the directly neighboring structures, such as the clear lens, in which the only operations permitted are those close to the lower limits of the dynamic range. This problem obviously does not exist in aphakic eyes.

Rationale

Laser treatment using photodisruptive pulses is an alternative to the invasive removal of dense membranous material obstructing the pupil.

Case 10

Case 11

Incarceration of the iris at the irido-scleral junction following a perforating injury in a phakic eye was lysed with photodisruptive pulses (Fig. 11).

Fig. 11. (Case 11) A: Status following incarceration of the iris root following a perforating injury. B: The incarceration has been relieved by a repetitive burst of four pulses, at a pulse energy of 8 mJ/pulse. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Thermal mode application

Case 12

This patient (Fig. 12) displayed an IOP elevation of 40 mmHg, due to malignant ciliovitreal block glaucoma following trabeculoplasty in aphakia. Following irradiation of six ciliary processes with 10 free-running, thermal Nd:YAG laser pulses at an energy level of 300 mWs, a focus diameter of 50 µm,

and a pulse duration of 20 ms (dose: 3 Ws), the cili- ary-vitreal block was disengaged, the fistula started functioning, and IOP fell to 12 mmHg.

Cases 13 and 14

In cases 13 and 14 (Figs. 13 and 14), the effects of thermal mode operation upon the ciliary body are shown.

Mixed mode operation

Cases 15, 16, and 17

In these cases (Figs. 15, 16, and 17), the synergistic action of the thermal and photodisruptive operation mode is shown. The usual procedure consists of initial thermal pretreatment of the tissue with thermal pulses, resulting in evaporation and gentle thinning, as well as in hemostatic action on the structure being aimed at. Following this, low energy

Fig. 13. (Case 13) The damaging effects produced by Nd:YAG laser radiation operating in the free-running mode on the ciliary body of an autopsy eye (transscleral operation). Parameters: pulse energy of Nd:YAG radiation delivered transsclerally: 4 J; pulse duration: 20 ms; Negative magnification: x 6. (Reproduced from Van der Zypen21 by courtesy of the publisher.)

Fig. 14. (Case 14) Scanning electron micrograph of a vascular cast, showing widespread atrophy of the ciliary body of a rabbit eye seven months after transscleral irradiation with a Nd:YAG laser, operating in the free running mode. Ramifying vessel sprounts with narrowed proximal ends extend from the parent vessels. E: The effect of laser cyclophotocoagulation on rabbit ciliary body vascularization. Negative magnification x 200. Parameters: pulse energy: 6.7 J; exposure duration: 20 ms. (Reproduced from Van der Zypen21 by courtesy of the publisher.)

Rationale

The Nd:YAG laser working in the thermal mode is a powerful tool for coagulation, due to its large dynamic range and the good penetration depth of its wavelength (1064 nm) it leads to strong coagulation (and later) to necrosis of the irradiated tissue.

Fig. 15. (Case 15; phakic eye) A: Incarceration of the ciliary body following trabeculectomy in a phakic eye. B: Following prophylactic thermal irradiation of the ciliary body for prevention of hemorrhage (discrete whitish discoloration of the ciliary processes: arrow). Photodisruption was used to reopen the internal fistula aperture. Parameters: step 1 (thermal): free-running mode operation; 20 single pulses at a pulse energy of 200 mWs (dose: 4 Ws); step 2 (photodisruptive): multimode, burst operation; pulse energy: 12 mJ; three bursts, four pulses per burst, 10 pulses burst (dose: 360 mJ). (Reproduced from Fankhauser et al.12 by courtesy of the publisher.)

Fig. 16. (Case 16; aphakic eye) A: updrawn, ectopic pupil, resulting from an unsuccessful extracapsular intervention with prolapse of the vitreous. The upper lid is in the normal position.

B:The lid has been lifted, showing incarcerated vitreous strands.

C:The situation after the first step, consisting of thermal pretreatment, forming a ‘dissection groove’ and photodisruptive dissection of the vitreous strands. D: The situation following combined thermal and photodisruptive coreoplasty: a half- moon-shaped piece of the iris has been resected. E: The results of disruption and pulverization of the resected segment of the iris, and final dissection of the vitreous strands adhering to the posterior face of the iris. F: Same as E with lid in normal position. Parameters: step 1: a thermal, free-running mode operation; 45 single pulses at a energy level of 400 mWsec; pulse duration: 20 msec; focus diameter: 50 µm; (dose: 18 Ws) step 2: photodisruptive, multimode operation; burst mode; ten bursts, four pulses per burst; pulse energy: 12 mJ (dose: 480 mJ). (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Fig. 17. (Case 17; phakic eye) A: (low magnification) Incarceration of the iris in the 12 o’clock position, following a failed fistulizing operation: ectopic pupil, almost invisible, phakic eye. Formation of a new pupil by resection of a half- moon-shaped section of the iris with free-running and lowenergy photodisruptive pulses (circles). Arrows indicate traction forces. B: (high magnification) The final result: roundish pupil approximately 4.5 mm in diameter. There is no damage to the clear lens. The smooth edges are due to thermal treatment. Parameters: step 1: thermal, free-running operation; four sessions; a total of 25 single pulses at an energy level of 400 mWs; (dose: 10 Ws), step 2: photodisruptive, fundamental mode operation; 21 single pulses of pulse energy of 1-2 mJ, (dose: 210-420 mJ.) (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

photodisruptive pulses from the lower end of the dynamic range are used for final evaporation of the remaining tissue.

Rationale

The advantages of the combined thermal and photodisruptive action mode can clearly be seen in Cases 15, 16, and 17. Thermal tissue evaporation leaves a thin tissue layer behind, which can be dissected by low photodisruptive energy pulses.

Vitreolysis

The transsection and removal of fibrovascular membranes from the vitreous cavity and the surface of the retina is an important part of vitreoretinal surgical procedures in conditions such as retinal detachment with proliferative vitreoretinopathy, diabetic traction detachment, penetrating trauma, retinopathy of prematurity, epimacular membranes, etc. Despite of the great technical advances in vitreoretinal surgery over the last two decades, surgical approaches are still limited to mechanical methods, such as transsection with scissors, blades, or a vitreous cutter. These maneuvers are completely successful in many cases, yet complications such as breaks and hemorrhages can occur, due to excessive traction, inadequate sharpness of the instruments, and problems regarding tissue engagement and the surgical approach used with these instruments. In contrast, laser vitreolysis emphasizing the advantages offered by photodisruptive lasers could be a valuable alternative.12a Three cases of vitreolysis are presented below (Figs. 18, 19, and 20).

Case 18

In this case, a solid plasto-elastic vitreous band exerting traction on the retina was dissected by means of photodisruptive pulses. Dissection of this band is shown in Fig. 18. Despite of a considerable degree of vitreal turbidity, the task was handled successfully.

Fig. 18. (Case 18) A: Fibrinous plasto-elastic hyaloid band in an eye with chronic posterior uveitis (phakic eye). B: The situation following irradiation. Following dissection, the band started to retract and traction exerted upon the retina was relieved. Parameters: multimode operation, three sessions, five bursts per session, four pulses per burst; pulse energy: 25 mJ, (dose: 375 mJ.) A CGR contact lens was used. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Case 19

In this case (Fig. 19), incarceration of a vitreous strand in the incision wound following ICCE is shown. There was strong vitreous traction at the peripheral retina, therefore the vitreous strand was cut.

Case 20

Status following posterior uveitis with multiple, taught vitreal bands exerting a strong pull at the anterior and posterior retina. These strands were dissected by multiple photodisruptive pulses, and traction at the retina was relieved (Fig. 20).

Laser systems

Dynamic range

Photodisruptive lasers based on Q-switched technology are limited by the extent of their dynamic range, i.e., the span between minimum and maximum pulse energy. While the upper limit of this range is determined by the maximum pulse energy available, the lower limit is determined by the lowest pulse energy of a specific Q-switched laser system, leading to a threshold optical breakdown irradiance of about 1012 W/cm.2,7 This value is constant over a considerable range.13 Therefore, pulse energy, necessary for breakdown, decreases with decreasing pulse duration. The pulse durations used in ophthalmology are typically situated in the lower nsec range. Reducing pulse duration even further would also result in a further reduction of pulse energy,14 but being a constant of the apparatus, it cannot be reduced for the photodisruptive lasers currently in use. Most importantly, pulse energy, and therefore the intensity of the effect, can also be reduced by reducing the spot diameter. Figure 21 demonstrates the dependence of pulse energy from the diameter of the focal spot for photodisruptive tasks.

Fig. 21. The interdependence of the energy required for threshold breakdown and spot size is shown. When ignoring higher order effects, the energy required to produce optical breakdown is inversely proportional to the square of the focus diameter. A breakdown threshold irradiance of 12 W/cm2 and a pulse duration of 12 ns are assumed. (Reproduced from Rol et al.15 by courtesy of the publisher.)

Interaction of variables related to optical and laser factors

As shown in Figure 21, compression of the focal spot will reduce pulse energy and therefore the intensity of the photodisruptive effect. This is important for laser effects at the lower border of the dynamic range. By ignoring spherical aberrations, the spot diameter can be compressed by increasing the cone angle of the laser by means of contact lenses. Increasing the cone angle, preand post-focal irradiance will then fall with the second power of the cone angle.15 Therefore, this configuration is safe. However, the limits of increasing the cone angle are due to the fact that aberrations increase with the increasing cone angle, and are also due to space limitations which are caused by a broad beam that can collide with the structures of the eye. The all-im- portant function of contact lenses is to reduce or eliminate optical aberrations. However, compromises have to be made (e.g., a cone angle of 12° resulting in an aerial spot diameter of 7 µm, and a cone angle of 16° resulting in an aerial spot diameter of 85 µm for the fundamental and the multimode operation of the system Microruptor 2, respectively). This effect of mode configuration upon the spatial distribution of energy within the focus volume is shown in Figure 22.

While the mode configuration in a specific commercial apparatus cannot be changed (except in one case: the Microruptor 2), interaction with beam geometry, which reduces the laser spot diameter, is always possible when using contact lenses. Nowadays, most photorefractive apparatuses are equipped with lasers working in the TEM00 (fundamental, transversal) mode. If we did not combat the imagedegrading effects of the ocular media, the spots used by us, in the order of 7 µm (above), would be considerably enlarged and, therefore, the threshold pulse energy and damage effect would increase. Figure 23 shows the beneficial effects of CGP and CGI contact lenses upon the imaging apparatus.

The merits of other laser parameters for photodisruptive tasks, which may enable threshold irradiance to be reduced, e.g., by virtue of a further reduction of emission time, should be considered: in experimental work it has been found that picoand, even more so, femtosecond (ps and fs) pulses gave the best results, if exceptionally fine laser effects were aimed at.16 Here, the threshold energy for optical breakdown is very low (about 0.2 µJ emitted at a pulse duration of 30 ps, by a ps laser at a 16° cone angle). As has been shown in the above case reports, we have been working with our Q-switched system at its lower end of the dynamic range at a pulse energy range of 400-500 µJ. This is about 20003000 times the pulse energy that would have been required by a ps or fs laser. However, it should be considered that (thousands) numerous ps, or even more so fs, pulses would have had to be delivered in order to evaporate enough tissue to be clinically

Fig. 22. Three-dimensional display of focal volumes. A: The fundamental mode. B: The multimode laser beam. Configurations computed for aberration-correcting contact lenses. The focal volume is also related to pulse energy. For discrete photodisruptive laser effects, fundamental mode operation and the use of dedicated contact lenses are imperative. (Reproduced from Fankhauser and Kwasniewska11 by courtesy of the publisher.)

Fig. 23. Laser spot diameters as a function of distance behind the posterior face of the cornea. The position of the natural lens or implant is shown (simplified). It can be seen that the CGI lens is best up to distance of about 3 mm behind the cornea. Both the CGI and CGP lenses are optimal for the anterior surface of a natural lens, an implant, and the iris. The CGP lens is best up to a distance of about 8 mm in the anterior vitreous. It is out of focus for greater distances. P12: Peyman’s contact lens is optimal for irradiation tasks in the mid-vitre- ous. (Reproduced from Rol et al.16 by courtesy of the publisher.)

useful, while with Q-switched (ns) systems, the total number of single pulses would be only in the range of only 10-500. It follows that, in order to be able to work within a useful time frame, ps or fs systems would require a high-frequency system such as that used, for example, by Stern et al.16 Such equipment is not presently available for clinical work, but as we have shown, the much larger pulse energies available with clinical apparatuses using ns pulses, compared to energy levels with ps and fs laser systems, are still adequate for solving some highly delicate tasks, provided other prerequisites have been satisfied.

We still have to evaluate the beneficial thresholdreducing effect that could be due to the photoelec-

tric effect of heating pigmented ocular structures or membranes (Cases 1-11). Here, the estimated pulse energy reduction, (due to the fact that the threshold breakdown irradiance of 1012 W/cm2 is reduced) is about x 5.7. Assuming a breakdown irradiance of 0.5 x 1012 W/cm2 this estimated energy would be reduced by about half (for further discussions, see Docchio et al.17,18).

Other important prerequisites for the precision tasks of photodisruptive lasers in ophthalmology

The threshold for laser damage effects should always be approximated starting from infraliminal energy values. Then, in order not to exceed the breakdown threshold, or in order to remain as close as possible to it, both the size of the energy intervals and the repeatability of pulse energy are most important. In an optimal system, such as that used by us, an energy interval of 20%, and a repeatability of 2%, have been used and have been found to be optimal. For identification and precise focusing, an auxilliary system has been found to be of the greatest value.19 Tilting the contact lens will invariably result in an increase of aberrations. Here, the observer is warned by a distortion in the aiming laser spot, which signals that aberrations are present.

Conclusions

The Nd:YAG laser can be operated in the thermal (free-running and cw), photodisruptive, and mixed modes. Noninvasive microsurgery by means of photodisruptive methods is now universally accepted as an important microsurgical tool. The microsurgical effect has been shown by histological studies mostly to be caused by the expansion and collapse of cavitation bubbles, which are part of the photodisruptive effect.6 Cavitation effects lead to displacement and tearing of the tissue, these effects being strongly energy-dependent. At or near optical breakdown thresholds, the disruptive effects are very small and strongly confined. It has been shown here that minimal anatomical effects can be achieved. The thermal mode of operation induces destructive effects due to coagulation and evaporation, and also entails hemostatic efficiency.20,21

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