VISION CORRECTION TECHNOLOGY FAQ

1. Recent FDA approval
2. Laser Spot and Profile
3. Tracking System
4. Theatre Environment
5. Wavefront Technology
6. Microkeratome

1. FDA approves the WaveLight ALLEGRETTO WAVE

Recent studies reveal that the WaveLight ALLEGRETTO WAVE excimer laser used by Adelaide Eye & Laser Centre is one of the safest and most effective excimer laser systems ever approved by the FDA.

The US Food and Drug Administration (FDA), widely considered the worlds premier benchmarking organization with regards to medical devices, has recently approved the WaveLight ALLEGRETTO WAVE Excimer Laser System following outstanding results achieved during FDA clinical studies. The studies found the safety and effectiveness outcomes of the excimer laser system currently employed by Adelaide Eye & Laser Centre and in some 30 other countries worldwide, to be outstanding. FDA patient satisfaction surveys completed 3 months after treatment with the WaveLight ALLEGRETTO WAVE reveal, “Patient reports of glare from bright lights, light sensitivity, night driving glare and visual fluctuations all improved after LASIK”.

To read more about the FDA approval of the WaveLight ALLEGRETTO WAVE go to www.fda.gov/cdrh/pdf2/p020050.html and download the Approval Order and for more information about the clinical study download the Summary of Safety and Effectiveness report.

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2. Laser Spot and Profile

The area the laser works over at any one time is called the spot size. The way the energy is distributed over the spot size determines the profile. The Wavelight Allegretto laser has a 0.95-mm treatment spot size. This is the smallest available spot on any commercially available laser and has an area of only 25% of that used by the majority of flying spot lasers employed in Australia. The Allegretto laser spot profile is Gaussian throughout the procedure which, when combined with the small spot size, provides smooth treatment surfaces and advanced transition zones compared to lasers using "Top Hat" profiles (see below) or scanning slit profiles.

Compared with a top hat beam profile used in some flying spot lasers there is a reduced tendency to a copper beaten effect on the treated surface.

The treatment pattern used by the Allegretto employs a superior algorithm using the latest information for correction of refractive errors. Recent research about how the laser interacts with the curved corneal surface at the edges of the treatment zone is also incorporated in the treatment algorithms.

The optical zone edges and transition zones are very important in the quality of night vision and in any tendency for the effect to regress. The spot size of 0.95-mm when compared to 2 mm enables reduced treatment depth over a comparable optical zone.

The Allegretto, therefore, has many advantages arising directly from its' superior technical features including:

· Smooth treatment surfaces.
· Larger treatment zones.
· Compensation of undercorrection tendency in periphery of treatment zones.

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3. Tracking System

Not all lasers have tracking systems and those that do are not necessarily the same. Lasers using a flying spot system, like the Allegretto laser, must employ a reliable tracking system to compensate for the eye movements that occur throughout the treatment. Most tracking systems use an infra-red light reflected off the eye to detect the position of the centre of the pupil and rapidly compare the position of successive images to compute eye movements.

The laser mirrors then adjust the position of the following pulses to ensure correct placement of the pulse. Failure to track properly will result in a significant proportion of the laser spots being misplaced, impacting on the quality and accuracy of the visual results. If the tracking system delays completion of the treatment through intermittent failure to detect the eye, i.e. the system is not robust, then dehydration effects on the cornea will result in reduced predictability of the treatment.

Latency of the tracking system determines the ability of the laser to respond to inevitable eye movements during treatment. The ability to process the images rapidly and move the mirrors determines the latency. Lasers that pulse slower can compensate for increased latency but must remove more tissue per pulse or increase the duration of the laser treatment phase, neither of which is desirable. The Allegretto uses 3 illuminating arrays of infra-red light to ensure optimal illumination under all operating conditions and employs advanced software to detect the pupil and, importantly, the position of the pupil centre. This results in the robust nature of the detection part of the tracking system, which operates at 250 Hz, significantly faster than the 200 Hz-pulse rate of the laser. The response of the Allegretto is typically 4-8 milliseconds. This very fast response is due to the use of high-speed digital image processing and fast treatment mirrors.

Visual distortion with decentration

The combination of these features in the tracking system and the spot characteristics leads to:

• Faster recovery of vision after surgery.
• Better contrast and quality of vision.
• Higher predictability of the outcome of surgery.
• Treatments being centered regardless of the presence of eye movements.
• Enhanced quality of night vision especially where pupils are large.

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4.Theatre Environment

The peak health standards body in Australia independently accredits Adelaide Eye and Laser Centre's facility and all procedures are carried out in a theatre level environment. This means that Adelaide Eye & Laser Centre provides;

• Theatre grade sterilisation methods complying with national standards (not bench top models).
• Air-conditioning levels that control temperature and humidity as well as providing filtration of bacterial and particulate matter. (Variations in temperature and humidity are known to significantly decrease the predictability of LASIK and PRK).
• Electrical protection for patient and equipment, together with an uninterrupted power supply for the laser and microkeratome ensuring optimal safety and equipment performance even in the presence of power failures.
• Medical gases and all emergency equipment required for the proper operation of a fully accredited theatre and recovery area.

Only world leading technology is employed at the Adelaide Eye & Laser Centre and Adelaide Eye & Laser Centre is subjected to the rigorous standards of accreditation on an on-going basis. This is done willingly to ensure the best outcomes for you. Adelaide Eye & Laser Centre encourages you to explore these factors when considering laser vision correction as they may significantly influence the final result.

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5.Wavefront Technology

All eyes have refractive errors termed higher order aberrations. These errors have terms such as coma and spherical aberration and cannot be adequately corrected with glasses or contact lenses. Generally these errors contribute only a small amount to the overall error but in many individuals they are significant.

Until recently such optical errors were of more significance to physical optics as found in microscopes but with the advent of wavefront analysers these errors can be detected in the eye. This is done by assessing the optical path of a series of light beams as they pass through the pupil on the way to or from the retina, see figure (a).

Figure (a)

The Output shows a slightly distorted image to the Input (see bottom right of Output image), revealing a slight optical aberration.

By then comparing the output to a known perfect pattern the wavefront and refractive errors of each individual's eye can be determined and a laser treatment customised for that eye. When significant higher order errors are detected a customised treatment pattern is used (figure b), and where insignificant, a treatment based on the spectacle prescription is used. See figure (b).

Figure (b)

The information gained via the WaveFront Analyzer is programmed into the WaveLight Allegretto laser to create a customised Wavefront-guided LASIK treatment.

In order to benefit from wavefront assessment, a laser capable of treating these errors is required. Only lasers with 1 mm or similar spot sizes and having advanced and robust tracking systems can properly treat higher order aberrations. The WaveLight Allegretto laser when combined with the WaveLight aberrometer is ideally suited to both detecting and treating these optical imperfections.

Adelaide Eye and Laser Centre has been investigating and evaluating wavefront technology since mid 2001 and when indicated is able to offer customised treatments at no additional cost. Currently this technology is being reserved for those errors where the treatment algorithms are best developed this being for low to moderate myopia combined with low levels of astigmatism. Long-sighted eyes are currently not suitable for this procedure.

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6.Microkeratome

The microkeratome is the instrument used to create the corneal flap during the LASIK procedure. The Adelaide Eye and Laser Centre uses an Amadeus Microkeratome.

This instrument was chosen because it has the following features:

• It creates a nasal hinge rather than a superior hinge.
• There are separate motors for the blade oscillation and the movement of the cutting head that can be individually controlled.
• There are no external gears and the suction rings are small and have high suction.
• Large flap diameters for wider treatment zones.
• Superior flap edge smoothness.
• No on-eye assembly, the Amadeus is ready to create the flap once the suction is applied.

The cornea gets its nerve supply from two main trunks that enter at the 3 o'clock and 9 o'clock positions. The corneal nerves that are cut recover over time but their absence may result in prolonged dryness of the cornea and consequent visual fluctuation. A nasal hinge preserves one of the nerve trunks and results in faster recovery of corneal sensation, and therefore of visual stability, as opposed to the superior hinge where both nerve trunks are severed.

By having separate motors for the blade and the movement of the microkeratome head, both actions are independent of each other. Therefore translation speed is uniform, as is the cutting force. This results in more even flap depth and surface quality. The ability to individually vary these settings enables the surgeon to tailor the procedure to his or her individual technique.

A common cause of cutting errors with a microkeratome is loss of suction or entrapment of external tissue in the gears. The lack of external gears and the use of small rings and high suction minimise the chance of the procedure being interrupted with consequent deferral of the procedure.

Overall, the Amadeus was selected for its' ability to create reliable corneal flaps with smoother cuts and low error rates and a nasal hinge for faster recovery.

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