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

Case 8. Status Post Hyperopic Lasik

This forty year-old female underwent uneventful lasik for + 4.75 – 0.75 x 090 which was followed by enhancement for a residual refractive error of + 2.75 – 1.25 x 065. Her best spectacle corrected vision preoperatively was 20/20-0. Manifest refraction at time of wavefront analysis was +0.75 – 0.5 x 105 for 20/30+1 best spectacle corrected vision. Autorefraction was + 3.0 – 0.5 x 100. Wavefront refraction was + 2.7 – 0.3 x

104. Autokeratometry was 46.0 @ 154 / 48.0 @ 064. The patient felt her vision in the left eye was not as “crisp “ as she would desire. Note the eccentricity of the left contour image and its significant large scaling range of 19 microns and the aberration on the higher order map with a scaling range of 5 microns. There is significant higher order aberration for this eye. The Humphrey Atlas topography map is displayed for comparison.

Case 9. Status Post Myopic Lasik

Fifty-one year-old male optometrist who underwent bilateral myopic astigmatic lasik. He has no unwanted subjective visual symptoms and is very happy with his postoperative status monocularly and binocularly. Preoperative refraction of the left eye was – 4.0 – 2.0 x 176 for 20/15 best spectacle corrected vision. Target refraction was –1.87 – 0.25 x 176 for monovision. Uncorrected distance vision in the left eye was 20/80 and near vision was J-1+. Manifest refraction was –1.63 – 0.5 x 160 for 20/15 best spectacle corrected acuity. Autorefraction

was –2.25 – 1.0 x 177 and wavefront refraction was –1.7 – 1.1 x 011. Manual keratometry was 38.25 @ 005 / 39.12 @ 095 and autokeratometry was 38.25 @ 174 / 39.75 @ 084. The left visual acuity map has a scaling range of 28 microns and the higher order map has a scaling range of 3 microns. Note on the left acuity map what appears to be with-the-rule astigmatism.

Thibos and Hong 7 have shown an increase in higher order aberrations specifically spherical aberration after myopic lasik. The following comparison below nicely displays this finding.(Cont. in next page)

Normal eye the day before and the day after myopic lasik. Upper row shows pupil phase maps (contour maps analogous to Visx 20/10 Perfect Vision Acuity Maps), lower row shows the distribution of wavefront error by Zernike order. To emphasize the change in higher order aberrations, the residual spherocylindrical refractive errors were omit-

ted from the analysis. Note the significant increase of 3rd, 4th orders along with increase in 5th –10th orders. (Figure courtesy American Academy of Optometry - Thibos LN, Hong X. Clinical applications of the Shack-Hartmann Aberrometer. Optom Vis Sci 1999;76:817-825.)7

Thirty-one year-old female with no subjective complaints and no ocular or surgical history. She has no contact lens history. Uncorrected acuities were 20/20- O.D. and 20/20 O.S. Manifest refraction was +0.25 – 1.25 x 105 O.D. for 20/15 best spectacle corrected vision and – 1.0 sphere for 20/20+ best spectacle corrected vision O.S. Wavefront refraction was + 0.7 – 1.8 x 101 O.D. and –0.7 sphere O.S. Manual keratometry was

41.37 @ 000 / 42.12 @ 090 O.D. and 41.37 @ 000 / 41.50 @ 090 O.S. and autokeratometry was 41.25 @ 090 / 41.37 @ 180 O.D. and 41.00 @ 022 / 41.50 @ 112 O.S. The acuity map of the right eye shows the againstthe- rule astigmatism pattern nicely corresponding with the plus cylinder manifest steep axis of 015. This can be appreciated by knowing the red color scheme peripherally at 015 (Cont. in next page)

and 195 meridia reflects the wavefront being ahead of the reference plane and the 105 and 285 meridia wavefront shaded in blue lagging behind the reference plane. This accurately describes the wavefront emerging sooner from the steep or recessed horizontal axis peripherally and emerging latter from the flat or protruding vertical axis peripherally. The acuity map of the left eye reveals a relatively spherical eye.

The higher order map has a relatively narrow scaling of 3 microns with the contour changes noted between 0 and – 1.5 microns inferiorly. No clinical correlate could be made to this except for the outside possibility of tear film abnormality at the time of the test. No SH data image was available for assessment. Humphrey topographies provided for comparison.

REFERENCES

1.Liang J, Grimm B, Goelz S, Bille JF. Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor. J Opt Soc Am A 1994;11:1949-57.

2.Optics, Refraction, and Contact Lenses, Basic and

Resolution, and Temporal Charachteristics of the Visual System. In Biomedical Foundation of Ophthalmology, Vol. 2, Chapter 17.

Wave Front Analysis - Clinical Primer

John F. Doane, M.D.1

Scot Morris, O.D.1

Andrea D. Border, O.D. 1

Lon S. EuDaly, O.D.1

James A. Denning, B.A., B.S.1

Louis E. Probst MD2

1Discover Vision Centers

Kansas City, Missouri, U.S.A.

2Medical Director

TLC The Laser Eye Centers, USA

L. A. Carvalho, MD.

J. C. Castro, MD.

W. Chamon, MD.

P. Schor, MD.

L. A. V. de Carvalho, Ph.D.

Introduction

The technological advances in refractive surgery techniques in the past decade have been overwhelming. For the first time ever there is a plausible chance of using corneal topography and eye aberration data to develop algorithms for optimized excimer laser ablations. The main objective is to obtain the best possible visual acuity. The first excimer (from the word excited dimer) lasers for refractive surgery started to operate at mid 1980’s and could only correct simple cases of myopia. With the evolution of these lasers we can talk today about point by point corneal ablations with "flying spot" lasers and correction of many other corneal abnormalities, such as irregular astigmatism.

Also in the 80’s there was a big "explosion" of computerized instruments for measuring corneal topography. In the beginning, most part of these instruments (generally called Videokeratoscopes (VKS)) had algorithms only for measuring curvature, rendering maps of Axial and Tangential powers. These curvatures were sufficiently precise to make pre and post analyses of corneal power changes, because refractive power is inversely proportional to the radius of curvature. But with the idea of simulat-

ing contact lens adaptation and the fluoresceine test, better algorithms were needed to calculate true elevation maps of the cornea. Because Placido Discs1 became more popular than any other VKS method, theoretical engineers saw them selves with the obligation of developing powerful algorithms for that purpose[2-10]. Today Placido based VKS systems can measure corneal elevation with precision in the order of microns.

The next obvious question that came to mind at the end of this decade is: if we have a precise method for measuring the front surface of the cornea and a laser that can "mold" it into whatever shape desired, what’s missing? Why are we not quite there yet? The answer is that there are still lots of aspects to be considered before refractive surgery gets close to perfection. One of these aspects comes from the current auto-refractors. It’s necessary to know how weak or strong is the optical system of the whole eye, including the lens. Corneal topography by it self doesn’t measure myopia and hyperopia, and in terms of astigmatism, it can determine only the corneal contribution. Axial and Tangential maps are good only for measuring differences in corneal refractive power, but not for determining total eye refraction. Data obtained with actual auto-refractors are incom-

plete because they determine only the best spherocylindrical lens, usually by measuring power in three different meridians[11, 12, 13]. But this is crude information compared to the non-symmetrical aberrations that occur in the eye, and also compared to the precision with which lasers can ablate the cornea and topographers can measure it. These two equipments can act on much more complicated surfaces than simple torics. The conclusion is evident: the actual auto-refractors do not have the required precision; therefore it is necessary to search and develop techniques that can measure refractive error for all points of the entrance pupil.

In 1994, Liang and colleagues [14] from the University of Heidelberg started this search, with an inspiration that came from astronomy. The main principle came from an instrument used in observatories for measuring optical aberrations in images of galaxies and stars, caused by the turbulence of our atmosphere. This instrument, called the HartmannShack (or Shack-Hartmann) sensor was modified during the 1970’s by Shack [15] after experiments of Hartmann [16] in the beginning of the XX century.

This chapter will deal with the evolution of refractive measurement techniques and explain in detail certain aspects of the wave-front technologies evolved. In section (1) we’ll make a brief historical introduction and bibliographical revision about refractive measurement evolution and in section (2) we’ll talk specifically about the Hartmann-Shack device. Section (3) will deal with the present state of the art in refractive surgery technologies and in section (4) we discuss the probable future of refractive surgery and certain aspects that must still be considered.

History

The history of eye aberration measurements started with the German Jesuit philosopher Cristopher Scheiner, professor at the University of Inglostadt, in the year of 1619, when he published his

Section IV: Aberrations and Aberrometer Systems

work "Oculus, sive fundamentum optikum"[17]. Scheiner demonstrated the focusing ability of the eye with a simple instrument nowadays known as the Scheiner Disk (see figure 1).

Figure 1. Scheiner’s disk.

This disk has two small holes on it, one at the center and other at the periphery. For a very far away source of light (in Vision Science this means about 7m or 20 feet), the light rays will hit the disc appoximately parallel to each other (which is called paraxial incidence). If a person that has normal vision (emmetropic) is placed behind the disk in this situation, then he or she will see a single light dot. This is a logical result, since a normal eye should focus light at the retina; therefore both light rays join at the fovea. But if a person with some kind of ametropy is placed behind the same disk, we’ll find a different situation. Because the optics of the eye in this case in imperfect, the two light rays do not come to a focus on the retina, that is, they do not join at the fovea. The simplest cases that we’re all very familiar with are when they join before the retina, characterizing myopia, and after the retina, charactering hyperopia. We’ll see in this chapter that these two anomalies are just a sub-group of many other optical disorders of the eye. At the time of Scheiner, correction of these