Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Mapping a Profile of the Whole Eye

The wavefront sensing device provides a new and objective way of mapping the profile of refraction and of higher order defects in the eye such as coma and spherical aberrations. Whereas corneal topography allows us to map a profile of the corneal surface, wavefront mapping makes it possible to map a profile of the whole eye.

Wavefront analysis is a more sophisticated method of defining aberrations that the surgeon is trying to correct through refractive surgery (Figs. 1, 2, 3). Until the present time the basis for diagnosis has essentially been corneal topography.

Laser Treatment - Pre-op

This conceptual view shows a cornea where Lasik for myopia is indicated, which also has a local aberration. Most of the light passing through the cornea is focused in front (green arrow) of the macula (M), a myopic refraction. Light passing through one extra steep area of the cornea (white arrow) causes light to focus even further forward in the eye (yellow arrow). Minute aberrations as such can also exist within the eye through any tissue or medium between the cornea and macula. (Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 2- Inability to Correct Aberrations with Broad Beam Laser Treatment - Application

The broad beam Excimer laser (L) treats a large area of the cornea without regard to the special requirements for custom treatment to a local aberration (white arrow). For this reason, the aberration is not eliminated, giving a refractive result which is theoretically less than optimal. Reflected corneal flap of the Lasik procedure (F). (Art from Highlights of Ophthalmology collection of medical illustrations).

Development of Wavefront Technology

Different Methods Available

Wavefront technology originated from two main sources more than 100 years ago. A physicist named Hartmann developed principles of subjectively measuring optical aberrations in a reproducible way. This system was later developed into what is called the Hartmann-Shack wavefront analyzing

Figure 3 - Aberrometry Type 1 - Wavefront Sensing - Concept of "Outgoing" Reflective Aberrometry (ShackHartmann Device)

Rather than an average refraction taken across the cornea, wavefront analysis measures refraction at each area of the cornea. This is accomplished by analyzing and recording light that is reflected off the macula and refracts out of the eye through each part of the cornea and lens. First, a small low energy laser beam (1-red) is directed into the eye. The light is then reflected (2-green) off the macula (M), with some directed back out the pupil and out through the cornea as a wavefront. This light reflected off the macula is analyzed for how it refractively emanates through each part of the cornea. In the simplified example shown, a local aberration in the cornea (3) causes this outward reflected light to deviate (yellow rays) in comparison to the light emanating through the rest of the cornea. The light then passes through a series of small lenses (lenslet array - 4) which defines the deviation of focused spots from their ideal position. The wavefront pattern, with denoted deviations from aberrations, is recorded (5 ˆ note area of aberration). This information can be used to treat local areas of the cornea with a small spot laser to give an optimal overall refractive correction. (Art from Highlights of Ophthalmology collection of medical illustrations).

device, which is used by most manufacturers today (Fig. 3). Tscherning, an ophthalmologist working in the late 1800s, devised another method of doing wavefront mapping. Tscherning’s method was fur-

ther developed by Howland and Howland in the 1970’s and more recently, Theo Seiler modified this method for clinical use, as adopted by two German manufacturers (Fig. 4). The Tscherning’s principle is also utilized in a retinal tracing method applied with the Tracey technology.

A third method is used by the group at Emory University in Atlanta, Georgia. Their method

involves a spatially resolved refractometer which evaluates the wavefront profile by soliciting the patient’s subjective response to a series of light rays entering the eye (Fig.5). Still another method of wavefront analysis, which Nidek is using, operates more by retinoscopic principles (Fig. 6).

Figure 4 - Aberrometry Type 2 - Wavefront Sensing - Concept of "Retinal Imaging" Aberrometry (Tscherning Device)

With Retinal Imaging Wavefront sensing, laser light

(L) as a grid (B) passes through an aberroscope lens (A) and the laser pattern is projected on the retina (G). Any deviation from ideal computes the aberration profile by ray tracing. In the simplified example shown, an aberration in the cornea (white arrow) causes misdirection of the refraction of laser light onto the retina. The resulting deviation in the grid pattern can be seen (C), and is recorded. This information can be used to treat local areas of the cornea with small spot laser to give a more optimal overall refractive correction. (Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 5 - Aberrometry Type 3 - Wavefront Sensing - Concept of "Ingoing" Adjustable Aberrometry (Spatially Resolved Refractometer)

Ingoing Adjustable Aberrometry involves recording the ingoing rays of light which are manually steered by the patient to define the wavefront needed to cancel ocular aberrations. In the simplified example shown, the patient steers points of light (A) presented at various locations across the cornea toward their macula (M). In an area of aberration (white arrow), the patient subjectively redirects the point source of light (B), compensating for the aberration, so that the light strikes the macula. Recording these deviations presents a wavefront pattern at the level of the cornea to custom treat each part of the cornea for a more optimal overall result. (Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 6 – Aberrometry Type 4 – Wavefront Sensing – Concept of "Double Pass" Aberrometry (Slit Skioloscopy / OPD Scan Devise)

A slit of light is scanned into the eye along a given meridian. The timing and scan rate of the reflected light can be determined by photodetectors to determine the wave aberrations along that meridian. Multiple meridian are scanned to analyze the full area of entrance pupil.

Section IV: Aberrations and Aberrometer Systems

the wavefront is defined by the foveally reflected laser light going out of the eye. The HartmannShack devices represented by Alcon, Visx, Bausch & Lomb, Meditec and Topcon are all based on this form of wavefront analysis (Fig. 3). The Tscherning device, named after a prominent ophthalmologist from the late 1800s, is based on "retinal imaging" wavefront analysis (Figs. 4). The Tscherning device involves a grid of laser energy shone into the eye. The way the grid deviates as it enters the eye and is imagined on the retina defines the wavefront pattern. This device uses the retina to obtain the wavefront pattern. It has been popularized through the efforts of Dr. Theo Seiler, who introduced the technology to two German companies, Wavelight, and Schwind. The Tracey retinal ray tracing method sequentially delivers one ray at a time which is imaged on the retina in a rapid (<10 mSec) fashion. The third method is an ingoing adjustable way of determining the wavefront pattern (Fig. 5). It measures the light rays coming in, being manually adjusted by the patient to a central retinal focus. The Spatially Resolved Refractometer use this mechanism (Fig. 5). The final method uses a double pass (in and out) method of analyzing the wavefront by slit Skioloscopy, using retinoscopic principles. The Nidek OPD scan uses this mechanism (Fig. 6).

The Mechanisms of Wavefront

Devices

Light passing in and out of the eye has to go through multiple structures like the vitreous, lens and the back and front surfaces of the cornea. Aberrations inside the eye can affect the passage of the light. Ultimately, seeing where the light is emitted from the eye in relation to the cornea allows the ophthalmologist to predict the change in corneal shape needed to give the patient perfect focus (Fig. 3).

Wavefront devices can be categorized into four groups. With "outgoing" wavefront analysis,

Benefits of Wavefront Analysis

Probably the best analogy to the development of wavefront technology relates to the early days of radial keratotomy in refractive surgery. At that point, before the age of corneal topography, all the surgeon needed to know was the keratometry value and certain other numbers about the shape of the cornea. The advent of corneal topography allowed us to map a whole profile of the shape of the cornea, giving us much more information for diagnosis.

Approaching patients with spherocylindrical refraction, we base the laser treatment on the refractive error with sphere, cylinder and axis. But those are only three numbers, just as keratometry is defined

with only a few numbers. Our goal is to get the whole profile of refraction, with an equivalent value at every point within the pupillary aperture. Once this information is obtained, the ophthalmologist can use the laser to create the perfect optical surface.

Linking Diagnostic Information from Wavefront Mapping to Laser Treatment

It is already possible to link diagnostic information obtained from wavefront analysis to the excimer laser treatment (Figs. 7, 8, 9). Several companies are actively doing this form of customized treatment in studies performed in non-US countries.

Alcon is using the technology as part of clinical trials in cooperation with the U.S. Food and Drug Administration (FDA). Autonomous Technologies Corp., which had a very effective scanning spot laser, was purchased by Summit Technologies, which owns many patents in the U.S. Now Summit has been acquired by Alcon. Alcon is now refining their LADARVision excimer laser to be used with the custom cornea wavefront device. The specific aberrations can be used to obtain diagnostic information. Then, with the laser, that diagnostic information can be directly applied to the treatment. This custom cornea platform of Alcon was recently approved in the U.S. for the correction of myopia.

Figure 7 - Custom Laser Treatment of the Cornea Using Small Spot Laser Coupled with Wavefront Analysis

Using any of the wavefront analysis techniques described, a small spot laser can custom treat each part of the cornea to optimize the overall refractive result. If each part of the cornea is optimally refracting light to strike the macula, the overall refractive result is maximized. In the simplified example shown here, the small spot laser is providing extra treatment (L) to a localized steep portion of the cornea (white arrow) corresponding to the local aberration. Reflected corneal flap of the Lasik procedure (F). (Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 8 - Final Refractive Outcome of non-Custom Lasik Treatment with Broad Beam Laser

This conceptual illustration shows the refractive outcome following broad beam laser treatment without custom treatment to local aberrations of the cornea. Postoperatively, most of the cornea properly refracts light to become focused on the macula (green arrow). However, an area of corneal aberration (white arrow) still causes a deviation in the refraction which is not optimally focused on the macula (yellow rays and yellow arrow). Overall refraction is not optimized. (Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 9 - Final Refractive Outcome of Custom Lasik Treatment Using Small Spot Laser Coupled with Wavefront Analysis

This conceptual illustration shows the refractive outcome following small spot laser application, with custom treatment to local aberrations of the cornea. With this approach optimized, each part of the cornea properly refracts light to become focused on the macula (green arrow). This includes an area of the cornea which preoperatively had an area of aberration (yellow rays) that was treated locally with the laser beam of small spot size. (Art from Highlights of Ophthalmology collection of medical illustrations).

All the companies that have excimer lasers are developing their own unique wavefront devices. Alcon has the LADARWave Wavefront Device. Visx has the WaveScan device, Bausch & Lomb has the Zywave device and Zeiss-Meditec has the Wavefront Sciences Device as part of their WASCA program. Each of these modified Hartmann-Shack devices uses "outgoing optics" to define the wavefront pattern. WaveLight and Schwind have their own wavefront devices based on Tscherning’s design of "retinal imaging" optics. Nidek has the OPD Scan, which is a special device based on slit Skioloscopy, using a modification of retinoscopic principles.

Because there is variation among all these types, it would not be wise to obtain a device from

Section IV: Aberrations and Aberrometer Systems

Alcon and a laser from Nidek, because the two may not correspond to allow for custom ablation. At this point in the development of the technology no one really knows which is the best device, and comparative studies are yet to be done. The best approach is to examine the technology, consider the manufacturers behind the various devices, and try to predict which are likely to be successful.

Wavefront Analysis in Conjunction with Corneal Topography

Ophthalmologists have learned to depend on corneal topography devices to help screen for disease before surgery and to monitor patients after surgery. More and more we will use the wavefront device for diagnostic testing before and after surgery.

Wavefront mapping in conjunction with corneal topography will provide the most complete picture. Although it is uncertain whether corneal topography will continue to be used a decade from now as technology continues to advance, there is definitely a place for it now. Meanwhile, the wavefront device provides even more detailed information about what the patient is likely to see because it measures the light passage into the eye focusing on the retina. Whereas the shape of the cornea is important, it is more important to ensure that the focus on the retina is perfectly sharp.

Personalized LASIK Nomograms

At present there is considerable interest in developing a commercial database system for nomograms. A number of researchers and companies are working on programs tailor-made for collecting data and determining individual nomograms. There may be several ways to achieve this goal. You can obtain your own Excel file. Using this file you can compare the attempted correction to what is achieved, and then assess the difference. Through regression analysis according to different variables, a nomogram can be derived.

Jaime R. Martiz, M.D.

Stephen G. Slade, M.D.

The Bausch & Lomb Zywave II aberrometer is an advanced wave-front sensor based on the Hartmann-Shack principle that provides us with a precise and fast test of the aberrations of the eye. The Zywave II sports a new design that may be combined with the Orbscan IIz Anterior Segment Analyzer, or it can be a standard alone system (Figure 1). The aberrometer is a device that is made with quality precision optical components. The components are extremely sensitive to vapor and deposit of impurities, e.g. the laser radiation can be weakened thereby making the results inaccurate. Another effect of environment impurities is the decrease in the working life of the optical components.

The Bausch & Lomb Zywave II aberrometer uses a low-intensity HeNe infrared light that is focused onto the retina. The laser is turned on for approx. 0.1 seconds. The laser diode used in Zywave works in the near infrared range at 780 nm, the laser intensity used is lower than the tolerated maximum value by approximately a factor of 40, so without any risk to the retina (Figure 2).

The pupil is dilated prior to examination to allow for a measured optical zone of at least 6 mm and to prevent accommodation. After the reflected light rays pass through the eye entire optics, they reach an array of small lenses, a so-called lens let array, and the resulting picture is capture by a CCD camara (Figure 3 and 4). In other words, the reflected light is broken into many individual beams, thereby producing multiple images of the same retinal spot of light.

The positions of the focal points are detected by the Zywave, analyzing the deviation of the points from their ideal position, the wavefront can be reconstructed. For a perfect eye, each of the bright white spots focused by each of the small lenses should have the same intensity and pattern; the reflected plane wave will be focused into a perfect lattice of point images. This would equal a plane wavefront, which means a perfect optical system.