Erbium:YAG laser vitrectomy

Abstract

In posterior segment surgery, mechanical cutting systems have become standard for fragmenting intravitreal structures and for removing vitreous. However, the mechanical cutting procedure in pars plana vitrectomy needs suction that necessarily produces traction and shear forces acting on the fragile intraocular structures, especially the retina. As a result, undesirable intraoperative complications may occur. The safest vitrectomy requires low suction, high cutting rates, and large ports for optimal removal of the liquefied vitreous. Such high cutting rates can be provided by modern erbium:YAG (Er:YAG) laser systems. This chapter introduces the current status of Er:YAG laser vitrectomy and discusses in detail the basic cutting mechanism, possible side-effects, and current results of clinical trails.

Introduction

Removal of the gel-like vitreous from within the eye while still maintaining normal intraocular pressure, the so-called vitrectomy procedure, is one of the most delicate surgical problems in ophthalmology today. The surgical situation for vitrectomy is shown in Figure 1. The operating site is visualized though the cornea and the lens, with the assistance of an operating microscope, while the implements for surgery (vitreous cutter, optical fibers for illumination, infusion, etc.) are inserted through tight pressure holes in the pars plana, a region in which the retina is both firmly attached and bloodless. The normal internal pressure of the eye is maintained and is required to control the bleeding.

Mechanical cutting systems have become standard in posterior segment surgery for fragmenting the intravitreal structures and for removing vitreous. Most of the currently used devices are based on the guillotine principle developed 30 years ago.1 How-

Microscope

Irrigation

Aspiration

Illumination

Fig. 1. Surgical situation during pars plana vitrectomy. The hole surgery is performed within the vitreous under microscopic control.

ever, the mechanical cutting procedure in pars plana vitrectomy needs suction that necessarily produces traction and shear forces acting on the fragile intraocular structures, especially the retina. As a result, undesirable intraoperative complications may occur.2 Specialists in this field postulate that the safest vitrectomy requires low suction, high cutting rates, and large ports for optimal removal of the liquefied vitreous.3

Unfortunately, standard mechanical cutting systems are limited to a cutting frequency of less than 20 Hz, due to their mechanical nature. Moreover, the cutting velocity is limited to a few milliseconds and, as a result, standard suction forces of more than 200 mmHg are routinely used during surgery. To

overcome such high suction forces, it could be considered that the same aspiration flow can be achieved by increasing the cutting rate and the cutting velocity. At least two of these ‘optimal conditions’ can theoretically be fulfilled by Er:YAG laser vitrectomy: cutting rates of up to 6000 cpm (100 Hz) and low suction (< 50 mmHg).4 When combined with an appropriate handpiece, this technique could offer the possibility of a semicontinuous vitrectomy with minimal tension on the intraocular structures.

Erbium:YAG laser

Mid-infrared lasers emitting near the 3 µm absorption peak of water have become viable tools for vitreous ablation.5 To put this in more detail, the Er:YAG laser emitting at a wavelength of 2.94 µm was proposed to be ideally suited for underwater tissue ablation because the absorption coefficient of water was reported to be about 104 cm–1 at this wavelength.6 Combined with a fiber delivery system, the Er:YAG laser seems to be a good choice for microsurgery procedures. In particular, this mid-infrared laser is useful for various applications in ophthalmology, such as sclerostomy and trabecolectomy in glaucoma treatment, phacoemulsification in cataract surgery, as well as in posterior segment surgery for the removal of vitreous structures.4,6-17

As well as precise tissue ablation, due to the high absorption in water-containing tissue, the microsurgical applications of pulsed Er:YAG lasers require flexible (such as low OH– quartz fibers) and high transparent optical wave guides (such as ZrF4 fibers) that do not impede the surgeon, for efficient light transmission. While standard low OH– quartz fibers are not sufficiently transparent (attenuation 100 dB/ m) to guide 2.94 µm radiation, the ZrF4 fibers with an attenuation of less than 0.1 dB/m seem to be the most suitable. However, the hydroscopic properties of the ZrF4 fibers require protection against contamination by aqueous solutions. Thus, several commercially available Er:YAG lasers systems use ZrF4 fibers to transmit the mid-infrared radiation to an application handpiece and, within the handpiece, the radiation is optically coupled to an interchangeable, low OH– quartz fiber tip a few centimeters long.

Photovaporization

The major contribution of underwater tissue cutting (photovaporization) with the Er:YAG laser is the expansion of laser-induced vapor bubbles. In this way, erbium laser radiation is capable of ablating tissue in a non-contact mode under water. The laser radiation is transmitted through a water vapor channel, introduced around the fiber tip by the early part of the laser pulse. This is possible because, in contrast to liquid water, steam has a 1000 times lower density and, in addition, a lower absorption coeffi-

Fig. 2. Scheme of the quartz fiber tips with various polished distal sides. a. plane; b. polished angle; c. half-plane; and d. curvature.

cient.18 The formation and collapse of this vapor bubble, the so-called ‘Moses’ effect, are both accompanied by pressure transients, and demonstrate complex physical dynamics. It has been shown that the formation of vapor bubbles can be effectively controlled by choosing adequate fiber tip and hand piece geometries, as well as optimized laser param- eters.19-21

An additional difficulty regarding the microsurgical applications of the Er:YAG laser in ophthalmology is that the freedom of movement may be limited to a few millimeters or less, and the tissue to be irradiated is not usually in an axial direction from the fiber application. Therefore, the surgeon may use different fiber tip geometries since these may be useful in ophthalmic microsurgery with the Er:YAG laser (Fig. 2).

However, the shape of the fiber tip end influences vapor bubble formation.19 As an example, a sequence of vapor bubbles at different polished angles α is presented in Figure 3. These side-view pictures, obtained during high-speed photography, demonstrate that the radiation only leaves the fiber tip appropriately to the optical pathway of the light at the quartz-vapor boundary. By varying the polished angles α from 0- 70°, two conditions for propagation of the vapor bubbles can be considered: (1) for angles α that are smaller than the angle of total internal reflection αtot = 42.5°, the radiation was refracted according to Snell’s law at the boundary between the quartz tip and the vapor that was formed after the first few microseconds of the laser pulse; (2) for angles α > αtot, the radiation was reflected according to Fresnel laws at the boundary between the quartz tip and the vapor. Surprisingly, splitting of the vapor bubble into three parts can be observed for angles in the range of the angle of total internal reflection αtot, as well as a nearly spherical vapor bubble at an angle of α = 70°. These effects are a result of multiple reflections and refractions within the quartz fiber tip at the quartz-vapor boundary.

An appropriate laser handpiece for pars plana vitrectomy consists of a quartz rod (core diameter, 320 µm) mounted inside an interchangeable micro-

surgical probe (Fig. 4). The microsurgical probe (length, 23 mm) has an outer diameter of 0.9 mm and the inner diameter of the aspiration port is 0.6 mm, similar to the mechanical cutting handpiece (Fig. 5). The handpiece is made of stainless steel and can be sterilized using standard methods. Radiation from the Er:YAG laser can be transmitted by mid-infra- red optical fibers, such as zirconium fluoride, and is optically coupled to the quartz rod.

Side-effects

In order to determine the feasibility of Er:YAG laser vitrectomy, it is necessary to demonstrate safety and

efficacy. Since the principal laser-tissue interaction following absorption of 2.94 µm radiation is the heating of tissue, local temperature increase should be investigated in order to exclude the potential for untoward thermal injury to adjacent structures. Hence, the temperature of a laser vitrectomy handpiece should be well below 45°C in order to prevent the collagen of the ocular structures from denaturation. In addition, with the clinical intraocular use of an instrument that achieves liquefaction of the vitreous by means of photoevaporation, a particular kind of photodisruption raises safety concerns. Pressure transients created during this disruption process may travel through the eye and could potentially damage the tissues localized in the structures, such as

the retinal pigment epithelium at the inner scleral wall, lens epithelium, and corneal endothelium, due to a change in ultrasound impedance.

The risk factors due to the rise in temperature of the handpiece and the stress waves have been investigated under experimental conditions.4,22,23 The rise in temperature at the microprobe was measured in air and under vitrectomy conditions (water and porcine vitreous) without aspiration. The measurements of temperature were performed by means of a NiCr-Ni thermocouple with an accuracy of ± 1 K. In all cases, the measurement was conducted after thermal equilibrium had been obtained (typically within 30 seconds). The maximum temperature in-

crease of the laser handpiece tip was 1 K in water and 10 K in porcine vitreous (averaged laser power, 0.4 W). The temperature increase in air was maximally 110 K at an average laser power of 0.4 W.

The stress wave measurements were performed with a needle hydrophone and an oscilloscope with a bandwidth of 500 mHz. The pressure-sensitive hydrophone was aligned in front of the aspiration hole of the laser handpiece by an x-y-z-microposi- tioner stage. The distance between aspiration hole and hydrophone could be varied in the rage of 0.5- 5 mm. The stress wave measurements yielded in a pressure amplitude below 10 bar, at a distance of 1 mm and a pulse energy of 40 mJ. Due to the low

sensitivity of the hydrophone (1.5 mV/bar), we were not able to measure the pressure amplitude for lower pulse energies. Our long clinical experience with Nd:YAG laser disruption for capsulotomy led to cautious estimates of the damage threshold of such pressure transients. Most probably, pressure amplitudes of up to ten bars are well tolerated by all intraocular structures. In previous studies, we determined that the pressure amplitudes occurring during Er:YAG laser vitrectomy were significantly less than ten bars.

In-vitro experiments have demonstrated the high efficiency of vitreous liquefaction and fragmentation.4 However, it has been said that the high cutting rate available with modern Er:YAG laser systems may result in a reduced suction force during vitrectomy. This assumption has been proven by comparing the aspiration flow in vitro as well as the suction forces in vivo during Er:YAG laser and mechanical vitrectomy. The increase in the aspiration flow dependent on the cutting rate was not found to be different for either method (Fig. 6) at a constant suction force of 200 mmHg. However, the aspiration flow through the laser handpiece at a cutting rate of 100 Hz was approximately ten times higher than that through the mechanical system at 10 Hz. From Figure 6 it can clearly be seen that, in case of higher cutting rates, the suction force can be reduced to a much lower level in order to achieve an equal aspiration flow.

Clinical trails

Er:YAG laser vitrectomy is currently under clinical investigation at different clinical sites.17,24-26 Both Binder et al.24 and Petersen et al.17 have reported that no laser-associated complications occurred within an observation time of more than six months when

cutting rates of up to 70 Hz were used. Thus, the Er:YAG laser seems to be an appropriate tool for decreasing the suction forces and the risk of intraoperative complications caused by mechanical stress.

To put this in more detail, Petersen et al. evaluated the clinical usefulness of the Er:YAG laser for vitrectomy, and compared it with a conventional mechanical vitrectomy system with regard to intraoperative parameters.17 They vitrectomized 30 eyes of 30 patients – 15 in each group. For mechanical vitrectomy, a commercially-available vitrectomy unit was used. The operating parameters for cutting rate (7 Hz = 420 cpm), maximum suction force (300 mmHg), and aspiration flow (20 ml/min), were held constant. An Er:YAG laser unit (ADAGIO, WaveLight Inc., Erlangen, Germany) and handpiece were used for laser vitrectomy with predetermined parameters for cutting rate (70 Hz = 4200 cpm), maximal suction force (50 mmHg), and aspiration flow (20 ml/min). Surgery parameters were recorded in real-time, and the operation video was tape-re- corded. The clinical follow-up time was at least three months (average: 6.2 months; range: 3-9 months). Briefly, the surgery time was found to be comparable in both groups, but slightly higher for laser vitrectomy (Fig. 7, right). During Er:YAG laser vitrectomy, the average suction force was significantly reduced (p < 0.001) compared to that during mechanical vitrectomy (Fig. 7, left). The mean-square variation in suction as a measure to quantify the forces acting on intraocular structures during surgery was significantly smaller in the Er:YAG laser vitrectomy group (p << 0.001).

The key finding in Petersen et al.’s study was the clinical feasibility of a semicontinuous laser vitrectomy.17 A high cutting rate offers two options: either to speed up the operation, or to make it smoother by lowering the suction force and, therefore, suction force variation. In their published study, they followed the second strategy because small suction

force variation induces small mechanical forces acting on intraocular structures, thus increasing the safety of the procedure. In this mode, the operation time using the Er:YAG laser was 1.4 times longer than conventional vitrectomy. However, a simple increase of the maximal suction force from 50-70 mmHg would have equalized the operating times. Even then, the suction force and potentially dangerous mechanical forces would have been substantially less compared to the 300 mmHg used during mechanical vitrectomy. The suction force variation being significantly reduced may explain the clinical observation of substantially less motion artifacts during Er:YAG laser vitrectomy. For example, a detached retina did not show pulsatile movements.

Earlier studies using Er:YAG laser vitrectomy focused on the cutting properties of this laser instrument using appropriate handpieces.14-16,27,28 These lasers were used at repetition rates of up to 200 Hz and demonstrated excellent results when cutting vitreous strands, membranes, and even retina. By means of alternative handpieces, for example, sapphire blades, the spectrum of the clinically-used Er:YAG laser system described in this chapter can easily be expanded to such cutting modes. Using the same fiber, handpieces with internal illumination are also currently being designed, as well as handpieces capable of vitrectomy and simultaneous endocoagulation. An additional advantage of a system without mechanically moving elements is the option of having a curved design, which could facilitate removal of the vitreous base in phakic eyes, without the complication of lens touch.

Conclusions

Clinical trials as well as experimental data demonstrate that the use of high repetition Er:YAG lasers for pars planar vitrectomy is feasible for reducing intraocular motion artifacts. Surgeons are able to reduce suction forces during vitrectomy, and this

reduces the mechanical pulsatile forces acting on the intraocular structures, thereby achieving a semicontinous vitrectomy.

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