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Fig. 5.6 Diagram showing the spherically curved contact glass used in the Zeiss VisuMax®
created is larger compared to a fully applanated eye. For example, an 8.8 mm VisuMax® flap bed diameter equals a 9.4-mm “Intralase® applanated bed diameter” (7.7 mm corneal curvature radius).
Currently, four femtosecond laser machines are available commercially, the Ziemer LDV system, Zeiss VisuMax®, IntraLase® FS laser, and 20/10 Perfect Vision (Femtec), with the Intralase® FS laser having the most clinical experience. All four systems are based on the same principle of photodisruption in corneal tissue, but differences exist regarding the concept of these devices. Further evaluation will be useful to compare the results of the flaps created by these four devices.
5.3.1.2Technological Advances in Laser Delivery Platforms
LASIK uses excimer laser systems to deliver laser on the corneal stromal bed to ablate the cornea, thereby altering the refractive power of the cornea. There is a wide range of excimer laser delivery systems available to the refractive surgeons today. Continuous improvements in laser delivery systems have improved the outcomes of LASIK.
Over the last 15–20 years, excimer laser platforms have evolved to become highly sophisticated instruments. First-generation excimer laser platforms used broad beam lasers with small optical zones and were only able to correct myopia. Second-generation
excimer laser platforms have a larger optical zone and were able to perform myopic and hyperopic corrections. Some second-generation excimer laser platforms also incorporated a passive eye tracker. The Schwind Keratom F, a third-generation excimer laser platform, used a broad beam laser with fractal mask capabilities and has aspheric ablation profiles. The fourth-generation systems incorporated fractal rotating mask and scanning slit capabilities and are capable of some customised corrections. These systems also have active eye-tracking capabilities. Fifth-generation systems, like the Schwind Esiris (Schwind eye-tech solutions, Kleinostheim, Germany), uses a high-speed, flyingspot laser with high-speed active tracking system (340 Hz) and is capable of customised wavefront driven corrections. The latest, sixth generation excimer laser available today is the Schwind Amaris (Schwind eyetech solutions, Kleinostheim, Germany). This system incorporates high-speed (500 Hz), flying-spot laser with a high-speed (1,050 Hz), 5-dimensional, active eye tracker amongst other new innovations.
The following advancements in technology of the excimer laser delivery system allow refractive surgeons today to achieve predictable and accurate results while at the same time reduce the amount of time and tissue ablated, which is the goal of minimal invasive surgery in LASIK.
1.Faster excimer lasers
2.Reduction of collateral thermal tissue damage
3.Advanced eye trackers
4.Newer ablation profiles
5.3.1.3 Faster Excimer Lasers
The repetition rate of the excimer laser system determines the number of laser pulses applied to the cornea per second and this frequency is expressed in terms of Hertzs (Hz). Naturally, an excimer laser system which can delivery more laser spots per second will be able to ablate more corneal tissue in a given time, and thus result in a faster treatment time. The speed of existing laser platforms varies from 15 to 500 Hz. However, the trend towards faster laser platforms is evident. The speeds of the newer laser platforms are faster than the older platforms and are reaching 400 Hz (Eye-Q, Wavelight) and 500 Hz (Amaris, Schwind Eye-tech and Concerto, Wavelight). Laser platforms with speeds
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LASER system
Alcon LADARVISION 6000
Bausch&Lomb 217 Zyoptix100
Kera IsoBeam
LaserSight Astrascan
Nidek EC5000 CXIII
VISX S4
Wavelight Concerto
Wavelight Allegretto Eye-Q
Wavelight Allegretto Wave
ZEiSS MEL80
SCHWIND ESRIS
SCHWIND AMARIS
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Fig. 5.7 Graph comparing speeds of the various excimer lasers
of 500 Hz require less than 4 seconds per dioptre to ablate a 6.5-mm optical zone compared to 7–10 s per dioptre using conventional laser platforms (Fig. 5.7).
The Amaris (Schwind Eye-tech) laser platform is among the newest and fastest laser platform commercially available today. It has a repetition rate of 500 Hz, making it one of the fastest laser platforms around. Ensuring accuracy in laser spot placement is a key consideration in such a fast laser platform. The Amaris laser platform is able to achieve this by incorporating two fluence levels into its system. A high fluence level is used to speed up the treatment while a low fluence level is used to ensure higher accuracy. High fluence level is used for the initial treatment and low fluence level is used for the last 1–2 dioptres of corneal ablation. On average, for each treatment, 80% of the treatment procedure uses high fluence and low fluence is used for the remaining 20%. Figure 5.8 shows the difference in spot profiles for high and low fluence levels. The high fluence spot profile has a larger spot diameter and ablates more cornea tissue with each spot, which allows faster ablation in a given time, whereas the low fluence spot profile has a smaller spot diameter enabling more precise ablations. This concept results
in minimised ablation time while at the same time, ensuring maximal ablation smoothness.
5.3.1.4Reduction of Collateral Thermal Tissue Damage
Another key consideration in excimer laser technology is the reduction of thermal damage to the cornea. This is especially important in newer, high-speed laser delivery systems as a high repetition rate may result in shorter intervals between laser pulses on the same area on the cornea. This may increase the thermal load on the cornea and result in thermal damage. Conventional laser platforms randomise the laser spot position during treatment to reduce successive overlapping. However, this system does not completely avoid successive overlapping.
The Schwind Amaris laser platform uses an Intelligent Thermal Effect Control to reduce the heating of the cornea significantly. This system ensures that the area around an applied laser spot is blocked for a certain time to let the cornea cool down and this area becomes dynamically smaller as the peripheral areas
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AMARIS ablative spot profiles
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cool down. The cooling-down time for the cornea is dif- |
cornea. In general, the eye tracker should be at least |
ferent during high fluence and low fluence treatments. |
twice as fast as the speed of the laser in order to ensure |
This system ensures that there is no overlapping of suc- |
accurate laser spot placement. Most eye trackers in |
cessive laser spots and minimal thermal load on the cor- |
conventional laser platforms have capturing rates of |
nea, hence reducing the risk of thermal damage. |
between 60 and 330 Hz or are able to detect the pupil |
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position at 4,000 Hz. This results in a response time of |
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up to 36 ms. Clearly, this will not be sufficient for high- |
5.3.1.5 Advanced Eye Trackers |
speed laser platforms of 500 Hz or more. |
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The Schwind Amaris incorporates a high-speed eye |
The introduction of eye trackers in excimer laser plat- |
tracker with an acquisition speed of 1,050 Hz. This eye |
forms have greatly improved the accuracy of the place- |
tracker has a response rate of less than 3 ms and tracks |
ment of the laser spots and minimised the risk of |
both the limbus and pupil. Most existing eye trackers |
decentred ablations. However, improvements in eye- |
also only detect pupil position and do not compensate |
tracker technology of conventional excimer laser plat- |
for pupil size and pupil centre shifts during treatment. |
forms are necessary to meet the demands of laser |
As the pupil centre may shift during treatment as the |
refractive surgery today. High-speed laser platforms |
pupil constricts or dilates, the centre of the pupil may |
and customised treatment ablation require extremely |
change during the treatment process. Hence, the impor- |
accurate laser spot placement to ensure accuracy of the |
tance of eye trackers that are able to track both the |
treatment. The demands on the eye tracker are high |
pupil and the limbus simultaneously to ensure that the |
and multi-fold. |
laser spot placement is accurate with respect to the pre- |
Faster laser platforms correspondingly require |
operative pupil position or the corneal vertex. |
faster eye trackers. Without ultrafast eye trackers, even |
It is evident that eye movements are possible in |
the slight movement of the patient’s eye will result in a |
more than two dimensions (Fig. 5.9). In fact, a total of |
spot placement far from the intended area on the |
five dimensions of eye movements can be recognised. |
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Fig. 5.9 The five dimensions of movement of an eye which is possible during excimer laser refractive surgery
Horizontal and vertical displacements of the eye (X and Y displacements) occur when the patient’s head is moved laterally or vertically. However, the eye may also rotate vertically or horizontally around the centre of the eye ball (X and Y rotation). Lastly, cyclotorsions or rotation around the optical axis may also occur. Cyclotorsion of the eye may occur when the patient is placed in a supine position from a standing position before treatment. Cyclotorsion of the eye may also occur during the treatment procedure. While all laser platforms incorporates an eye tracker, most eye trackers only track horizontal or vertical displacements of the eye and only a few eye trackers are able to track cyclotorsional rotations of the eye. The ability to track all movements of the eye is crucial to enable accurate place of the laser spots with respect to the cornea vertex, especially for customised laser ablations.
Rotational movement of the eye around the centre of the eye ball (X and Y rotation) will result in a shift in the placement of the laser spot with respect to the vertex if the eye tracker is tracking only the horizontal or vertical displacement of the eye (Fig. 5.10). Advanced eye tracker will need to take this into consideration in order to ensure more precise placement of laser spots. The Schwind Amaris incorporates a
Fig. 5.10 Diagram showing the effect of rotational movement of the eye around the centre of the eye ball (X and Y rotation) on the placement of the laser spot with respect to the vertex
rolling compensation by using a Rotation Balance Algorithm to compensate for such rotational movements of the eye.
Cyclotorsion movements of the eye can be classified as either static cyclotorsion movement or dynamic cyclotorsion movements. Static cyclotorsion movement occurs when the patient is moved from an upright to a supine position while dynamic cyclotorsion