Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Chapter 28

REFERENCES

1.Byer N. Natural history of posterior vitreous detachment with early management as the premier line of defense against retinal detachment. Ophthalmology. 1994;101:1503-1513

2.Byer N. What happens to untreated asymptomatic retinal breaks, and are they affected by posterior vitreous detachment? Ophthalmology. 1998;105:1045-1049

3.Davis M. Natural history of retinal breaks without detachment. Arch Ophthalmol. 1974;2:183-194

4.Alio JL R-MJ, Artola A. Retinal detachment as a potential hazard in surgical correction of severe myopia with phakic anterior chamber lenses. Am J Ophthalmol. 1993;15:145-148

5.Aras C OA, Karacorlu M, Sener B, Bahcecioglu H. Retinal detachment following laser in situ keratomileusis. Ophthalmic Surg Lasers. 2000;31:121-125

6.Arevalo JF A-AO. Retinal detachment in myopic eyes after laser in situ keratomileusis. Am J Ophthalmol. 2000;129:825-826

7.Arevalo JF RE, Suarez E, Antzoulatos G, Torres F, Cortez R, Morales-Stopello J, Ramirez G. Rhegmatogenous retinal detachment after laser-assisted in situ keratomileusis (LASIK) for the correction of myopia. Retina. 2000;20:338-341

8.Farah ME, Hofling-Lima AL, Nascimento E. Early rhegmatogenous retinal detachment following laser in situ keratomileusis for high myopia [In Process Citation]. J Refract Surg. 2000;16:739-743

9.Han HS SJ, Kim HM. Long-term results of laser in situ keratomileusis for high myopia. Korean J Ophthalmol. 2000;14:1-6.

10.Mazur DO HR, Gee W. Related Articles. Retinal detachment in myopic eyes after laser in situ keratomileusis. Am J Ophthalmol. 2000;129:823-824

11.Ruiz-Moreno JM P-SJ, Alio JL. Retinal detachment in myopic eyes after laser in situ keratomileusis. Am J Ophthalmol. 1999;128:588-594

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Providing our patients a corneal ablation customized for each person is the new laser treatment of utmost interest in refractive surgery. This is being accomplished by mapping the profile of refraction of the whole eye through wavefront sensing devices. This very sophisticated method identifies aberrations in the entire optical system and not only the corneal surface. The latter is all we could get from corneal topography. The specific aberrations found can be used to obtain valuable diagnostic information of the whole eye. The present challenge is how to correct these aberrations by providing a distinctive laser treatment plan for each eye, like “matching a finger print” to create a perfect optical surface rather than rely upon the basic laser algorithms or mathematical formulas.

Although we are identifying aberrations in the whole eye, since the cornea is the major refracting surface of the eye, about 80% of the aberrations can be corrected by operating on the cornea itself, whether they are defocused errors of sphere and cylinder or higher order aberrations. We give so much attention to the cornea because all of the light that goes in and out of the eye has to go through the cornea. So a distortion that is present on the lens or possibly even at the retinal level can be corrected by correcting the shape of the cornea.

In the following chapters, very prestigious refractive surgeons present their concepts and experiences on how to link diagnostic information obtained from wavefront analysis and corneal topography to the excimer laser treatment. They also identify the sophisticated equipment being made available for this purpose and discuss the usefulness of the information being provided by these methods. In the end, we seem to be closer to the goal of attaining “bionic vision” or “super vision”.

BENJAMIN F. BOYD, M.D., F.A.C.S.

Contents

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Section 2

Section 3

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Section 7

Subjects Index

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Custom ablation is a very broad term. It can refer to treatment of the cornea that does not depend on recent technological advances: surgeon-oriented customization occurs, for instance, if the surgeon decides to treat a small zone for a central island that has developed following laser vision correction.

The usual meaning of custom ablation today does involve recent advances. Custom ablation can be guided by topography, and more recently by wavefront mapping. Wavefront guided customization will be the most successful method of corneal ablation in the future. Most companies in industry related to ophthalmology are focusing their resources on this technology because it promises to yield all the information needed for doing customized laser treatment.

WAVEFRONT ANALYSIS

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.

Wave-front analysis is a more sophisticated method of defining aberrations that the surgeon is trying to correct through refractive surgery (Figs. 29-1, 29-2, 29-3). Until the present time the basis for diagnosis has essentially been corneal topography.

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 device, which is used by most manufacturers today (Fig. 29-3). Tscherning, an ophthalmologist working in the late 1800s, devised another method of doing wavefront mapping. Tscherning’s method was further developed by Howland and Howland in the 1970’s. Recently, Dr. Theo Seiler became involved with this method, which is the method some of the German manufacturers use.

A third method of wavefront analysis, which Nidek is using, operates more by retinoscopic principles (Figs. 29-4, 29-5). Still another method is used by the group at Emory University in Atlanta, Georgia. Their method involves a spatially resolved re-

Contents

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Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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Chapter 29

Figure 29-2 - 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). (Courtesy of Highlights of Ophthalmology).

Figure 29-1 - Inability to Correct Aberrations with Broad Beam 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.(Courtesy of Highlights of Ophthalmology).

Contents

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Section 7

Subjects Index

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Figure 29-3 - Aberrometry Type 1 - Wavefront Sensing - Concept of "Outgoing" Reflective Aberrometry (Shack-Hartmann 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. (Courtesy of Highlights of Ophthalmology).

Figure 29-4 - Aberrometry Type 2 - 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. (Courtesy of Highlights of Ophthalmology).

Contents

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Section 7

Subjects Index

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Chapter 29

Figure 29-5 - Aberrometry Type 3 - Wavefront Sensing - Concept of "Ingoing" 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.(Courtesy of Highlights of Ophthalmology).

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Subjects Index

fractometer which evaluates the wavefront profile by soliciting the patient’s subjective response to a series of light rays entering the eye (Fig. 29-5).

The Mechanisms of Wavefront Devices

Light passing in and out of the eye has to go through multiple structures like the lens and the back surface of the cornea, ultimately passing through the vitreous. 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. 29-3).

Wavefront devices can be categorized into three groups. With “outgoing” wavefront analysis, the wavefront is defined by the foveal reflection of the laser with light going out of the eye. The

Hartmann-Shack devices represented by Alcon, Visx, Help ? Bausch & Lomb, and Meditec are all based on this

form of wavefront analysis (Fig. 29-3). The Tscherning device, named after a prominent ophthalmologist from the late 1800s, is based on “retinal imaging” wavefront analysis (Figs. 29-4, 29-5). 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 Dr. Theo Seiler, who introduced the technology to two German companies, Wavelight, and Schwind. The last method is an ingoing adjustable way of determining the wavefront pattern (Figs. 29-4, 29-5). It measures the light rays coming in, sometimes individually, sometimes in a retinoscopic fashion. The measured deviation can be either manually adjusted by patients depending upon what they see, or recorded by retinoscopic principles. Nidek OPD and the Spatially Resolved Refractometer use this mechanism (Figs. 29-4, 29-5).

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.

Benefits of Wavefront Analysis

Probably the best analogy to the development of wavefront technology relates to the early days of radial keratometry in refractive surgery. At that point, before the age of corneal topography, the surgeon needed to know the keratometry value and certain other numbers about the shape of the cornea. The advent of corneal topography allowed us to map a

Figure 29-6 - 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).(Courtesy of Highlights of Ophthalmology).

Chapter 29

Figure 29-8 - 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. (Courtesy of Highlights of Ophthalmology).

Figure 29-7 - 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. (Cour-

tesy of Highlights of Ophthalmology).

Contents

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Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

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tomized treatment in studies performed in non-US countries. Alcon-Summit-Autonomous is using the technology as part of the clinical trials in cooperation with the U.S. Food and Drug Administration (FDA). Autonomous, which had a very effective scanning spot laser, was purchased by Summit, 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.

All the companies that have excimer lasers are developing their own unique wavefront devices.

Alcon-Summit-Autonomous, has their Custom Cornea Wavefront System. Visx has the WaveScan device, Bausch & Lomb has the Zioptics device and Meditec has their WOSCA program. Each of these are modified Hartman-Shack devices, using “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

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 pic-

ture. 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. The wavefront device provides a 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