Contact lenses for ophthalmic laser treatment

Abstract

Contact lenses are frequently used for the diagnostic examination and laser treatment of the fundus and anterior segment of the eye. The lenses differ in magnification, laser spot size, and field of view. This chapter deals with the optical characteristics of various contact lenses, and how they apply to laser treatment.

Introduction

This chapter presents a short survey of a number of optical systems used in laser therapy in ophthalmology. It is restricted to the most common optical systems used today, and is predominantly oriented toward the application of the laser at the posterior segment.

When visible or near infrared laser light is used for treatment of the fundus of the eye, the light must pass through the transparent ocular media, to reach the fundus of the eye. The ocular media must be considered as part of the optical system, together with the laser instrument and its delivery optics. A thorough review of the optics of ocular laser application has been written by Fankhauser et al., and this chapter relies substantially on that work.1

Contact lenses

Contact lenses are commonly used to direct laser energy to various sites in the eye. They were introduced in 1967 by Fankhauser and Lotmar as coupling elements for photocoagulation of the retina.2,3 The use of the contact lens for this purpose has now become universally accepted. The more recent clinical use of low-power pulsed lasers for photocoagula-

tion in the human eye places even greater demands upon the optical performance of contact lenses as auxiliary coupling elements.

Some general and well-known advantages of using contact lenses for laser treatment, as well as for examination, may be briefly summarized as follows: the cornea is kept moist, the eye is stabilized, and the lid is kept out of the way. Although handling a contact lens implies additional effort for the physician, given the considerable advantages, this minor inconvenience is worthwhile.

For a more thorough understanding of the advantages of using certain contact lenses, it may be helpful to review some basic principles. The laser treatments considered in this chapter will be divided into ‘photocoagulation’ and ‘disruption’.

Photocoagulation

Photocoagulation, which followed the pioneering work of Meyer-Schwickerath and the introduction of the laser, is based in the first approximation upon the ‘linear’ absorption of light for at least as long as the evaporation of water is not reached. The absorbed light energy is converted into thermal energy, which leads to denaturing, coagulation, evaporation and/or carbonization of the tissue, depending upon the interaction time and the amount of energy deposited. All these effects can be seen as ‘thermal effects’, and the laser working in the thermal domain may be defined as a low power laser, although the pulse energy may be high. Typical tissues treated in this way include retina/choroid, trabecular meshwork, iris, ciliary body, and blood vessels. The most common lasers used are the continuous wave (cw) models, such as the argon ion, krypton, dye, diode, and Nd:YAG.

The demands made on low-power lasers and on their handling by the physician are less than those involved in the application of high-power lasers. A focal spot diameter of less than 50 µm is not required, whereas with high-power lasers, the focal spot diameter is often reduced to a tenth of this value.

Good reproducibility of the laser parameters is important in order to avoid excessive and inadequate effects. In addition to stable laser emission, the focal diameter on the target tissue should be kept constant once the laser parameters have been set for the desired therapeutic effect.

Photodisruption

The application of g-switched lasers, which usually work at high power levels, is based on an interaction mechanism between light and tissue, which is completely different from the thermal effects mentioned above. Instead, very high laser intensities are used to cause optical breakdown in the media through which the laser ray passes. The intense shock waves produced in the region of the breakdown cause the cutting or disruptive effects. In order to obtain the high intensities required for this nonlinear effect, the laser energy must be delivered in a very short time interval, i.e., within the ns or ps range (10-9-10-12 seconds), and in a very small volume, which requires a small focal spot.

The confinement of laser energy to a small focal spot is another feature that distinguishes the highpower from the low-power laser. As a consequence of the Lagrange invariant, the size of the focal spot of a laser beam is inversely proportional to the beam convergence anterior and divergence posterior to the focal spot (i.e., the cone angle of the beam is greater for smaller focal spots, and small focal spots are required to get therapeutic effects at small pulse energies).

The diameter Df of the focal spot of a fundamental (TEM00) Gaussian laser beam is in good approximation proportional to the wavelength λo in vacuum in the medium, and inversely proportional to the refractive index of the medium n as well as to its cone angle α, i.e., the full angle of the laser beam:

Df = 4λ0

Df

π n0α

This means that, by focusing a laser beam from air into a medium with a refractive index greater than one, even without changing the cone angle α, the focal spot diameter is reduced because the refractive index is enlarged. A further reduction of the spot diameter can be obtained by enlarging the cone angle. (The increase in the cone angle, however, is limited mainly by optical aberrations, which tend to increase with increasing cone angle.) This problem can be solved by using appropriately designed optical components such as contact lenses,4-6 and by aiming the laser beam correctly.1,7

Contact lenses: principles and safety

The power and energy density of the laser at its focus are proportional to the inverse size of the laser spot. This is true in photocoagulation as well as in photo- dis-ruption. The high intensities required to achieve optical breakdown depend upon the cone angle of the beam, and this influences the cutting efficiency, the volume of the focus, and the energy deposited in the media in front of and behind the focus. The cone angle of both the observation and the aiming beams is equally important. Increasing the cone angle of the observation beam increases the magnification of the observed image and the optical resolution, although it also results in a reduced depth of focus and therefore enhanced focusing accuracy.

According to the above, two interdependent quantities, namely the size of the focal spot and the cone angle of the laser beam, must be considered when evaluating contact lens performance. The former is responsible for the cutting efficiency and dictates the volume of the tissue affected by laser irradiation, and the latter is responsible for the amount of tissue damaged beyond the focal spot and, hence, for the safety of the preand postfocal media and structures. Therefore, the reduction in pulse energy and the increase in cone angle of the beam act in the same way with regard to cutting efficiency.

When working with a photodisruptive laser, intensities equal to or greater than a certain threshold must be generated in order to achieve optical breakdown. This threshold intensity is a function of the optical and chemical properties of the medium in which the breakdown is generated. For instance, the threshold intensity for bulk water has been measured to be about 1012 W/cm2.8

The focal spot should be kept as small as is necessary to safely meet the energy density requirement of the microsurgical task at hand. By so doing, the intensity and energy density in the prefocal media are minimized, and safety is maximized. The same applies to the postfocal media if we assume that the laws of geometric optics hold for laser light passing through the disturbed media after optical breakdown has been induced.

Optics of the eye with a contact lens

The imaging properties of optical systems such as the eye with a contact lens satisfy the paraxial Gaussian approximation. This means that the rays of an incident light bundle are all focused on the same point, as long as they are relatively near the optical axis. It the rays are not close to the optical axis, they are not longer focused on one single point, and the resulting aberrations, known as optical aberrations, increase with increasing cone angle (numerical aperture). However, there are two exceptions, and it is upon these exceptions that all contact lenses for photodisruptive purposes should be based. The first

exception applies to light bundles which are focused towards the center of curvature of the front refracting surface of the contact lens system. Here, all the rays are normal to the optical surface, and consequently they are not refracted. This means that, by focusing a beam towards the center of curvature of a surface, no spherical aberration occurs. This also means that the cone angle of the incident beam remains constant and is not changed by a spherical surface. There are other aberration free points: a refracting surface has two distinct conjugated points, A and A’, which are completely free from any optical aberration. Therefore, all the rays from a light bundle focused towards the point A are perfectly focused into point A’ after passing through the surface, regardless of their angle to the optical axis. These points are known as aplanatic points, and are defined by the Young-Weierstrass theorem.9

By rotating a converging beam around the center of rotation of a refracting spherical surface, aplanatic surfaces are realized. A beam imaged on such a surface suffers no spherical aberration. One such surface is displayed in Figure 1. For each photodisruptive task (iridotomy, capsulotomy, dissection of vitreous membranes), a specific working distance (or focal length) is required, and appropriate contact lenses must be used in order not to compromise safety.5-7,10

Fig. 1. Definition of aplanatic surfaces. These are realized by rotating the converging beam around C. There are two aplanatic surfaces, S and S’, where S’ is the image of S. A is the focal spot in air, A’ the focal spot in a medium with index n’. All points of S’ are aplanatic. (Reproduced from Born and Wolf9 by courtesy of the publisher.)

Contact lenses for photocoagulation

Laser light for photocoagulation may be applied anywhere in the fundus, including off-axis regions, and not necessarily near the paraxial space. Contact

lenses for coagulation tasks (Table 1) are not restricted to the Gaussian (aberration-free) space, nor to an aplanatic point in the eye. On the contrary, they may sometimes be directed towards eccentric sites in the eye. The price to be paid may be an aberra- tion-dependent enlargement of the focal spot, which often results in a considerable increase in the laser energy required for the therapeutic effects, according to the square of the focal spot size.

Some such lenses are realized by internal mirrors (such as the classical Goldmann three-mirror lens) for the irradiation of peripheral retinal areas. The characteristic feature of mirror contact lenses is their off-axis, often quite oblique beam path produced in the eye. This oblique path results in aberrations

– mainly at the anterior surfaces of the cornea and at the crystalline lens or implant – such as coma and astigmatism, in addition to the previously-mentioned spherical aberration. This has been shown to be an important performance-degrading factor for laser irradiation of the peripheral retina. For a more intimate understanding of the effect of image degradation on both observation and laser irradiation, the reader is referred to the data derived from schematic eye modelling.11-14

Aberrations in the periphery of the fundus oculi increase with visual angle. They impair both observation and coagulation efficiency. These difficulties can be overcome, to a large extent, by the use of a plano-concave contact lens.11,15 Other attempts to combat peripheral image degradation have not been very successful.16,17

Another problem that arises in biomicroscopy of the eye is the determination of the absolute dimensions of objects and structures at the periphery of the retina. Little is known about the effective focal length of the eye in oblique view, and therefore about its magnification, as a function of the visual angle.

Auxiliary lenses

Illuminated emmetropic eyes emit parallel rays from the fundus. Therefore, the fundus cannot be exam-

Fig. 2. Ray path and symbols as used in the eye-contact lens system with mirrors: α: visual angle without contact lens; γ: angle of acceptance at the retina; dashed line: angle of incidence of the ray with the contact lens. (Reproduced from Fankhauser and Lotmar45 by courtesy of the publisher.)

ined with a microscope which has only a short object distance. In order to use a microscope to view the fundus, an intermediate image of the fundus must be made at the objective plane. This can be achieved using either a concave or a convex auxiliary lens. Concave (negative) lenses create an upright, virtual image of the fundus or vitreous, whereas convex (positive) lenses create an inverted, real image. Prototype concave lenses include the Goldmann contact lens18,19 and the Hruby preset lens,20 and there are a large number of convex preset and contact lenses (Table 1). No essential difference exists between indirect ophthalmoscopy and slit-lamp biomicroscopy using a preset convex lens, except that, in indirect ophthalmoscopy, a real, inverted image in air is inspected by the naked eye instead of having to use a microscope. Figure 3 shows the intermediate image created by some commonly used contact lenses.21

The Goldmann three-mirror contact lens, which is used as a universal lens, is a contact lens with excellent optics. Its field of view is limited to about 30° (full angle) in each direction of observation. Its outstanding optics make it ideal for examination of the vitreous, and the fact that it is a contact lens makes for a somewhat wider field of view. Finally, magnification of the fundus image with the Goldmann lens is not so dependent on the refractive power of the patient’s eye as are other negative lenses. The images seen via the mirrors are laterally reversed, i.e., they are mirror images compared to direct viewing. All negative lenses suffer from the fact that the greater the patient’s myopia, the shorter the working distance, although in practice this is rarely a serious problem.

For examination of a highly myopic eye in which a larger working distance is required, positive lens systems are preferred.22 Since positive lens systems tend to magnify the entrance pupil of the eye under examination, the iris in first approximation, in contrast to negative lenses, no longer acts as a field stop; a favorable field size is achieved which can even enable examination of the ora serrata. Curvature of field, once considered a problem in early positive lenses, such as the El Bayadi lens22 or the panfundoscope,23 is corrected in modern wide-angle lenses, although peripheral image degradation obviously remains a problem. Illumination of the peripheral fundus is virtually impossible with most slit lamps, and fiberoptic transillumination has been suggested. A greater working distance than with negative lenses is required when positive lens systems are used, and this may be beyond the working range of some slit lamps.

Examination and laser treatment of the peripheral retina

Observation of the lateral periphery can be difficult; the reduced effective aperture of the pupil in oblique

a.

b.

c.

d.

Fig. 3. a. The Goldmann lens has a flat anterior surface and produces an erect, virtual ophthalmoscopic image located near the posterior surface of the crystalline lens. b. The Krieger lens has a concave anterior surface and produces an erect, virtual ophthalmoscopic image located in the anterior vitreous humor. c. The Panfundoscope lens has a biconvex, spherical anterior lens element, and produces an inverted, real image inside the biconvex lens. d. The Mainster lens has a biconvex, aspherical anterior lens element, and produces and inverted, real image anterior to the biconvex lens. (Reproduced from Mainster et al.21 by courtesy of the publisher.)

projection makes it difficult to assemble both objectives, the illumination and the laser beam, into the narrow space available. Figure 4 clearly shows the advantages when using positive lenses or lens systems.1

However, it should not be overlooked that narrowing of the beam associated with positive lenses results in an increase of intensity and radiant exposure in prefocal regions, proportional to the square of the reduction in beam diameter. This should be taken into consideration, particularly when photocoagulation is performed in the presence of opaque media, which have enhanced absorption, since otherwise damage to the cornea, iris and crystalline lens

Fig. 4. Images of the microscopic objectives and illuminating slit together with the laser beam in the pupil of the eye to be examined and irradiated, with negative (top) and positive (bottom) contact lenses. (Reproduced from Fankhauser et al.1 by courtesy of the publisher.)

could result.24 In particular, vignetting of the laser beam by the iris, when using positive optics, may have serious consequences, whereas damage when using negative lenses, due to the larger beam diameter at the pupil and therefore to reduced intensity and fluence, is negligible. Similar problems have been observed in laser irradiation of the fundus by means of indirect ophthalmoloscopy,25-30 which is identical to laser irradiation with positive contact lenses, except that in the former a non-contact method is used and observation is performed with the naked eye. Contact lens-assisted irradiation using positive lenses is much safer than when using non-contact irradiation such as that in indirect ophthalmoscopy, because with rigid slit-lamp delivery systems, movements of the surgeon’s and/or the patient’s head will only slightly affect aiming accuracy. In the best circumstances, using indirect ophthalmoscopy-related retinal irradiation, targeting has been estimated to be approximately ±200 µm,26 although under unstable conditions, this may be significantly greater.

Laser treatment using a contact lens

While generally helpful, the contact lens-eye optical system used for imaging the laser beam may also be a possible threat to safety, if mismanipulations or improper calibration lead to inadvertent exposure of the cornea, iris, and crystalline lens to high-in- tensity laser radiation.

As shown in Figure 5, with a wide-angle positive lens (Volk Quadraspheric for the same spot setting), the size of the retinal spot is twice as great as with the Goldmann lens and Volk 78 D lenses. As a consequence, for the same threshold effect, four times as much power is required with the Volk Quadraspheric lens as with the Goldmann and the Volk 78 D lenses. Since the beam diameter at the cristalline lens is about twice as small, the energy density and power density are 16 times as much with Volk Qua-

spot setting on the slit lamp (microns)

beam diameter in air at the observation plane

Fig. 5. Laser spot diameter on the retina (mm) as a function of the laser spot diameter on the observation plane in air (µm), as indicated on the scale. The laser spot diameter is twice as large with the Volk Quadraspheric contact lens as with the Goldmann three-minor and the Volk 78 D contact lenses. When working with the Volk Quadraspheric lens, four times as much power is necessary. Because the beam waist at the crystalline lens is about twice as small as with the Goldmann and Volk 78 D lenses, the intensity of the beam at the crystalline lens is 16 times as much with the Volk Quadraspheric lens. (Reproduced from Fankhauser et al.1 by courtesy of the publisher.)

draspheric lens compared to the other two auxiliary lenses (Fig. 5).

Two optical systems for controlling laser spot size are currently in use, namely, parfocal (generic parfocal) and defocusing (generic defocus) systems, which differ in their basic optical principles.1,31

Parfocal systems

With the parfocal system, the core of the laser delivery fiber is imaged on to the slit-lamp focal plane

at varying magnifications. This produces a clean, round image with relatively homogenous laser intensity over the entire spot. The parfocal approach changes the beam divergence as the laser spot size is changed, using a zoom, and this results in a large depth of field, i.e., a long cylindrical beam waist at large spot sizes (Fig. 6). The large depth of focus associated with large laser focal spot sizes ensures that the correct focal spot size is delivered to the retina, even if small aiming errors occur. As the focal spot size at the retina is increased, the beam diameter at the cornea, iris and crystalline lens decreases. With all positive fundus lenses, the situation can arise where the beam diameter is larger at the retina than it is at the pupil of the eye. Indeed, for each combination of laser and lens, a critical point can be found where the corneal or lenticular beam diameter becomes smaller than the retinal beam diameter. This cross-over point has been suggested to be the limit of the largest retinal spot size that is recommended for use with particular combinations of fundus lenses and laser delivery systems. In all laser-lens combinations in which the corneal or lenticular beam diameter becomes smaller than the retinal beam diameter, the fluence at the cross-over point is about 2 J/mm2.32 Therefore, the limit of 2 J/mm2 has been used for the calculation of safe beam diameter limits (Table 2). This limit only applies to transparent media; if opacities, which absorb energy more strongly, are present in the eye, lower limits are required if safety is not to be compromised. The critical point must be computed or must be indicated by the manufacturer of both the auxiliary lens and the biomicroscope.

Defocus systems

In the defocus system, a smaller laser spot (typically 50-200 γm) is shifted to a point beyond the slit-lamp focal plane (Fig. 6). This avoids the major drawback of the parfocal systems, in which the beam diameter at the pupil of the eye is relatively constant. The price paid for the larger beam diameter at the pupil with the defocus system is that the nice sharp image of the fiber core, typical of parfocal systems, gradually fades from high power to low power at the edge (Fig. 7).

The Goldmann contact lens/Varispot32 combination (parfocal/defocus) may be regarded as the one providing maximum safety for the preretinal media (Fig. 8). Here, the retinal irradiance and fluence will always be greater than that in the preretinal media. The Goldmann contact lens/parfocal combination provides the maximum safety and quality performance for laser spot sizes up to about 300 µm, and the Goldmann lens/defocus combination matches this performance even with larger spot sizes (Fig. 8).

In contrast, when using the combination Volk Quadraspheric lens/VariSpot projection system (or any other lens of the same power as the Volk Quadraspheric lens), the energy density and power density with a focal spot size of about 30 µm are almost the same at the retina as at the crystalline lens, when utilizing the parfocal mode (Figs. 8 and 9). With larger retinal spot sizes, both in the defocus and in the parfocal regime, the energy density in the pre-

G

Fig. 7. Schematic parfocal (generic parfocal) and defocus (generic defocus) beam profiles with a 500-µm diameter spot size. Perfect ‘top hat’ and Gaussian profiles are plotted for reference. (Reproduced from Fankhauser et al.1 by courtesy of the publisher.)

retinal media far exceeds the energy density at the target (Fig. 8).

In most modern laser projection systems, mixed parfocal/defocus systems are used, which combine the advantages of both principles.

Changes in refraction by silicone oil or gas in vitreous surgery

As stated at the beginning of this chapter, the contact lens and the eye should be seen as one optical system. Changing the optical characteristics of the

Fig. 8. Beam diameter at the crystalline lens as a function of the selected spot setting for Goldmann three-mirror, Volk 78 D, and Volk Quadraspheric contact lenses, imaged in the generic defocus mode. Spot setting: beam diameter in air at the observation plane. See text. (Reproduced from Fankhauser et al.1 by courtesy of the publisher.)

spot setting on the slit lamp (microns)

beam diameter in air at the observation plane

Fig. 9. Beam diameter at crystalline lens as a function of the selected spot setting for Goldmann three-mirror, Volk 78 D, and Volk Quadraspheric contact lenses, imaged in the generic parfocal mode. See text. (Reproduced from Fankhauser et al.1 by courtesy of the publisher.)

ocular media will affect the optics of the eye. Filling the vitreous cavity with liquid silicone, air or gases, as is frequently done in vitreous surgery, has a profound effect on the total refractive power of the eye, on the visibility of the fundus, and on the refractive state of the eye during and following sur- gery.33-36 More seriously, targeting of the laser may be affected by the modified optical parameters of the eye, and this may alter both the diameter of the laser focus spot and its intensity. Stefánsson and Tiedeman examined this optical situation, and discussed fundus image location and magnification using water, air, or silicone oil as vitreous substitutes in phakic and aphakic eyes, and for various auxiliary contact lenses.37 They also computed the fundus image location and magnification in the eye when using both planoconvex posterior and anterior chamber lens implants (the former positioned 5 mm and the latter 3 mm behind the front surface of the cornea) and water, air or silicone oil in the eye. In addition, they determined when the fundus was visible with the above combinations using

either the operating microscope or slit-lamp biomicroscope.

They stated that exact determination of the location of the fundus image requires complex optical calculations, although it is possible to simplify this issue by concentrating on a few principles. When air or silicone oil is placed in the vitreous cavity, the refractive power of the posterior surface of the crystalline lens is changed. This surface is normally a low-power positive lens, as the refractive index of the lens cortex, 1.386, is higher than the refractive index of the vitreous gel, 1.336. As the refractive index of air is only 1.000, when the cavity is filled with air, the power of the posterior crystalline lens surface is dramatically increased. A highpower negative lens, such as the biconcave corneal lens, is then needed to counteract the effects of this high-power positive lens (Fig. 10).

While the refractive index of vitreous gel is lower than that of the crystalline lens, the refractive index of silicone oil is slightly higher. Thus, when the vitreous cavity is filled with silicone oil, the posterior surface of the crystalline lens changes from a low-power positive to a low-power negative lens. While this change is too small to be a problem during vitreous surgery, it must be corrected postoperatively with spectacle glasses for hyperopia.

If the aphakic eye is filled with silicone oil, the posterior surface of the cornea changes from a lowpower negative lens to a low-power positive lens, as the refracting index of silicone oil is higher than that of the cornea. This conversion of the cornea to a positive lens reduces the hyperopic correction that the aphakic eye would otherwise need. The convex silicone ball in the aphakic eye has a positive refractive power and, therefore, induces a myopic shift, whereas the concave silicone ball in the phakic eye induces a negative lens and, therefore, makes the phakic eye slightly hyperopic (Fig. 11).

In the aphakic eye, the posterior corneal surface is important. When the aphakic eye is filled with air, the posterior surface of the cornea changes from being a low-power to a high-power negative lens that neutralizes the refractive power of the cornea. The latter is reduced to nearly zero, and thus the fundus can be visualized without any additional optical aids (Fig. 12).

In the pseudophakic eye, where the posterior surface of the intraocular lens is flat, there is no change in the refractive power at this surface when vitreous substitution is performed, since a flat surface has no refractive power, regardless of the refractive index of the substances on either side. This has important implications in vitreous surgery, in that it is no longer necessary to use a biconcave corneal lens in a pseudophakic eye which has an air-filled vitreous cavity. It is possible to observe and treat the fundus in these eyes using a flat-faced contact lens or a prism lens.

The complexity of optical changes with vitreous substitution is huge, and solutions in specific cases

may not always be possible, as mentioned by Launay et al.38 and Docchio et al.39-41 In in vitro experiments, Launay et al. determined the variation in shape of a silicone ball as a function of the volume of fluid injected into the vitreous cavity in emmetropic, phakic or aphakic eyes for different positions of the head. Obviously, the volume, shape, and location of the injected silicone will all strongly and unpredictably influence the optical properties of the eye.

A very ambitious approach was undertaken by Docchio et al.39-41 and Azzolini et al.42 These authors used a ray-tracing model to investigate the refractive properties of the interfaces between different ocular media and vitreous substitutes, with regard to transpupillary laser beam delivery during photocoagulative procedures. The study outlined the role of these interfaces in focusing or defocusing the laser

beam along its path within the eye. This effect is dependent upon the angle of incidence, the number of interfaces, and the change in refractive index across each interface. These studies revealed the main problems to be inadequate power density or nonuniformity of the laser spot, which in turn resulted in non-optimal photocoagulation. Transpupillary photocoagulation through silicone oil demonstrated improved performance compared to perfluoro-n- octane, due to the more favorable sequence of refractive indices encountered by the laser beam. These study highlighted the fact that, in many situations, it is difficult, if not impossible, to correctly focus the beam on the target site, due to the vignetting effect of the iris.39-42 In another approach, Azzolini et al.43 reported the effects of vitreous substitutes in endoocular laser photocoagulation, and they also described

Fig. 11. The refractive correction (spectacle) in diopters in eight aphakic eyes before silicone oil filling, as well as with silicone oil and after removal of intraocular silicone oil. The three measurements in each eye are connected by a line. The two eyes that did not return to the previous degree of hyperopia after removal of silicone oil underwent penetrating keratoplasty at the time of silicone oil removal (dotted lines). (Reproduced from Stefánsson et al.36 by courtesy of the publisher.)

the effect of the air bubbles in the laser beam in endoocular pathology.

So far, our insight into substitution in surgery of the anterior and posterior segments of the eye is almost exclusively based on ray-tracing models and in vitro experiments. Laser irradiation of the silicone oil-filled eye may involve some risk. Huy et al.44 have shown that the in vitro exposure of silicone oil to radiation from Nd:YAG lasers results in the formation of transient breakdown gases, which are mainly composed of methane, ethylene, and traces of ethane, as identified by head-space gas chromatography. However, no clinically significant damage has been reported to result from laser irradiation due to the presence of silicone oil or gas in the eye. Moreover, Azzolini et al.43 performed retinal endophotocoagulation through perfluorodecalin in rabbits after vitrectomy. These studies indicated that no extra care is necessary when endocoagulation is performed through perfluorodecalin, provided circular spots are used and the energy is delivered accurately to the target site.

Fig. 12. Schematic drawing of an aphakic eye. The normal, air-filled, and silicone oil-filled eyes are shown with no contact lens and a flat (planoconcave) corneal contact lens. An object arrow is drawn on the fundus, as well as the image arrow corresponding to it. Each situation where the fundus can be visualized with the slit lamp or operating microscope is marked with a black dot. (Reproduced from Stefánsson and Tiedeman37 by courtesy of the publisher.)

Conclusions

Examination and laser treatment of the eye is aided by the use of contact lenses. A variety of contact lenses is available and, in conjunction with the eye, they form a complex optical system. The laser surgeon must understand the fundamentals of the various optical systems that he or she utilizes, in order to maximize the usefulness and minimize the risks involved in the laser treatment. Several aspects of laser irradiation of the eye have been described which must be considered when setting safety limits. The individual laser surgeon must ascertain whether the safety requirements of each and every therapeutic laser application have been satisfied.

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