WAVEFRONT ANALYSIS
Figure 34-9. Optical effects of tear film disruption. The upper row of images was captured immediately after a blink; the bottom row of images was obtained after the subject had held his lids open for about 40 seconds. Left column contains images obtained by retroillumination of the pupil; middle column shows the data images captured by the SH aberrometer; right column shows contour maps of the aberrated wavefront emerging from the eye computed from the SH data image. Contour intervals in the reconstructed wavefront are 1 micron and the wavefront phase at pupil center has been set to zero. Pupil coordinates are in millimeters. (Figure courtesy of the American Academy of Optometry - Thibos LN, Hong X. Clinical applications of the ShackHartmann Aberrometer. Optom Vis Sci 1999;76:817-825.) 7
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
What are the Shortcomings of ShackHartmann Wavefront Analysis?
Tear film abnormalities can significantly affect the quality of wavefront analysis. This can be to such a degree that Thibon and Hong have suggested that wavefront analysis via the Shack-Hartmann method may be useful in future investigations of the tear film and the dry eye syndrome. 7 So, even though the cornea proper and all optical elements behind it in the optical pathway are normal, an irregular tear film will provide data suggesting significant wavefront aberration. See Figure 34-9.
Opaque opacities are also poorly defined by the current Shack-Hartmann like devices. This is
likely do to complete light scatter and the inability of the source testing light to reach the retina and reflect back let alone get back to the CCD video camera.
Eyes with marked aberrations may be virtually impossible to obtain a measurement such as scars or keratoconus. In addition, eyes with relatively miotic pupils may be very hard to measure and require pharmaceutical dilation (See Figures 34-10 & 34-11).
While current devices appear to work extremely well with normal eyes and eyes with mild to marked aberrations, there is room for improvement in measuring eyes with marked aberrations. In addition, Thibon and Hong have recommend that light scatter needs to be incorporated into our optical model of the eye to fully account for the optical imperfections of specific eyes such as keratoconics. 7
LASIK AND BEYOND LASIK 355
Chapter 34
Reproducibility and Effect of Pupil Size
Figure 34-10. The above three contour maps show that wavefront measurements can demonstrate consistent results through time. It is important to understand that the tear film is an ever changing structure and the contour maps will thus change accordingly.
Figure 34-11. The observed map and data can also vary depending on the size of the pupil in which the study is carried out. In the accompanying figure the same examination at varying pupil sizes demonstrates the contour changes within the defined three and seven millimeter apertures.
Are There Limitations for Adaptive Optics and the Improvement in Best Unaided Vision?
There appear to be at least three junctures where the best realization for exquisite unaided acuities may possibly be constrained: visual cortex, retina proper and the spectacle, corneal, or implant level. Will clinical or subclinical amblyopia dilute our results? Will all maculas be able to support 20/10 vision? Do all maculae have the optimal orientation of cone receptors or Stiles-Crawford profile or even sufficient cone density to support “super vision “. Can we as clinicians create the “ perfectly adapted optic “ be it spectacle or corneal or implant surgical technique to neutralize the pre-existing abnormal wavefront?
It is also very important to remember that as we currently understand and work with wavefront sensing technology it may not in most cases define the exact locale of the pathology causing the aberration: hence, clinical examination and other refractive tools, such as corneal topographic mapping, along with sound clinical judgment will be required for proper understanding of the eye and its individual refractive status.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
WAVEFRONT ANALYSIS
Clinical Examples
Case 1. Keratoconus
areas where light travel along the optical pathway is behind or latent from the reference plane (traveling through thicker areas of tissue or longer distance). This patients measured acuity via wavefront is –5.4 - 1.8 x 000. Manual refraction was –4.75 – 1.25 x 170 for 20/40best spectacle corrected acuity. Autorefraction was –5.5 – 2.75 x 175. Manual keratometry was 43.5 @ 175 / 44.75 @ 085 with 1 + mire distortion. Autokeratometry was 43.75 @ 007 / 46.00 @ 097.
Visual acuity map of a 45 year-old keratoconic patient without RGP in place (left slide) and with RGP in place (right slide). Left Acuity Map: Note 46 micron scaling range of the left map. The higher order aberration map does show the area of cone (red, lower right quadrant). The numerous contour lines in this locale describe the intensity of the aberration. Areas of red depict areas where the light travel along the optical pathway is ahead of the reference plane (traveling through thinner areas of tissue or shorter distance) and blue areas depict
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Note that on the right slide most of the higher order aberrations appear to
disappear. In some keratoconics a mea-
Help ?
surement can not be made unless an RGP is placed over the eye as described by Thibon and Hong. 7 In this situation, it is hopeful that future versions of the wavefront device will be able to image even the most irregular surface contours.
For comparison sake a Humphrey Atlas Topography has been included (lower figure).
LASIK AND BEYOND LASIK 357
Chapter 34
Case 2. Status of Post Radial Keratotomy
Left eye of 50 year-old female who was 6 years s/p four cut radial keratotomy. She has re-
sidual |
wavefront |
defined refractive error of –0.3 |
– 0.7 |
x 011. |
Her autorefractometer and |
autokeratometer readings were –0.75 – 0.5 x 015 and 42.75 @ 136 / 43.25 @ 046, respectively. Manifest refraction revealed –0.25 – 0.75 x 007. Manual keratometry revealed 42.5 @ 178 / 43.12 @ 088
with no distortion. Note her higher order map displays numerous contour changes centrally describing higher order aberration but the scaling of this map is 3 microns with the bulk of the map from 0 to +1.5 microns centrally. Compare the Humphrey Irregularity / Wavefront Map ( Upper Map, Right Image ) to the Visx 20/10 Map. She had no subjective visual complaints.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
WAVEFRONT ANALYSIS
Case 3. Posterior Subcapsular Cataract and Anterior Cortical Cataract
Sixty –seven year-old female with night glare symptoms from cataract. Best spectacle corrected vision is 20/25 O.S. and Potential Acuity Measures 20/20. Uncorrected distance vision is 20/40. Manifest refraction is +1.25 – 0.5 x 149. Wavefront refraction was + 4.3 –1.6 x 020. Autokeratometry measurements were 44.00 @ 031 / 44.25 @ 121. The SH data image clearly outlines the morphology of the posterior subcapsular cataract in the central region. Significant higher order aberrations are noted on the lower right quadrant of
the higher order aberration map (slide right) consistent with anterior subcapsular / cortical changes. If one looks back at the SH data image there are no light points for these corresponding areas. This incongruity amplifies the importance of comparing the raw data image to the color coded map and making a clinical correlate with direct visualization of the anatomical structure, in this case the posterior crystalline lens (PSC) and anterior crystalline (anterior cortical spoking).
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK AND BEYOND LASIK 359
WAVEFRONT ANALYSIS
Case 4. Unoperated “ Normal “ Eyes (Cont.)
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK AND BEYOND LASIK 361
Chapter 34
Case 5. Status of Post Penetrating Keratoplasty for Keratoconus
Forty-five year-old male who was 19 months s/p PKP for keratoconus. Sutures were removed at 12 months. Wavefront refraction was – 7.5 – 0.4 x 125. Autorefraction and autokeratometry were –8.75 – 0.75 x 127 and 46.00 @ 126 / 46.75 @ 036, respectively. Manifest refraction was – 7.50 –1.00 x 070 with best spectacle corrected vision of
20/20. Manual keratometry was 45.75 @ 100 / 46.5 @ 010 with 1+ mire distortion. Higher order aberrations appear to be present on the right higher order acuity map but the scaling range is 1 micron. With trial frame lenses he stated excellent subjective quality of vision.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
Case 6. Irregular Lasik Ablation
Twenty-six year-old man who complains of blurred vision after myopic lasik of his left eye. It is the surgeon’s (JFD) opinion that fluid masked the ablation in the supero-temporal quadrant. The fluid accumulated unbeknownst to the surgeon at the time of the case and retrospectively is the most likely source of this patient’s irregular pattern. Un-
WAVEFRONT ANALYSIS
corrected vision was 20/15 O.D. and 20/30+ O.S. Autorefraction O.S. was + 0.75 – 0.25 x 054. Manifest refraction O.S. was + 3.25 - 1.50 x 031 for 20/15 best spectacle corrected vision. Wavefront refraction O.S. was –0.7 sphere. Autokeratometry O.S. was 37.25 @ 097 / 38.00 @ 007.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK AND BEYOND LASIK 363
Chapter 34
Case 7. Normal Examination / No Refractive Error
Thirty year-old female with no subjective complaints and no ocular or surgical history. Uncorrected acuity was 20/15 in each eye. Manifest refraction was plano sphere O.D. for 20/15 best spectacle corrected acuity and + 0.25 – 0.25 x 180 for 20/15 best spectacle corrected acuity. Wavefront refraction was –0.2 – 0.6 x 092 O.D.
and + 0.1 – 0.4 x 058. Autokeratometry was 43.25
@099 / 43.5 @ 009 O.D. and 43.5 @ 046 / 43.75
@136 O. S. and manual keratometry was 43.25 @ 177 / 43.5 @ 187 O.D. and 43.37 @ 177 / 43.75 @ 187 O.S. Humphrey topography provided for comparison (See two additional topographies of this case in next page).
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
