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Visual Optics 2007/2008

Visual Optics 2007/2008. Chapter 5 Astigmatism; The Cornea ; Astigmatic Eyes, Lenses & Image Formation . Which of the following statements is true about a 2.00 D cylinder axis 90?. Power in the 90  meridian is 2.00D Power in all meridians is 2.00D

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Visual Optics 2007/2008

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  1. Visual Optics 2007/2008 Chapter 5 Astigmatism; The Cornea; Astigmatic Eyes, Lenses & Image Formation

  2. Which of the following statements is true about a 2.00 D cylinder axis 90? • Power in the 90 meridian is 2.00D • Power in all meridians is 2.00D • Power in the 180 meridian is 2.00D • Power in the 180 meridian is zero

  3. +5 D How much astigmatism does this lens have (Optic Cross representation)? +2 D • 2 D • 2 D • 3 D • 3 D • 5 D •  5 D

  4. Page 5.14 • Most important ocular refracting component • Power +43 D  carries 2/3 of total ocular power • Reason for large power contribution is the index difference across the anterior corneal surface (1.376  1.000) • Anterior corneal power (+48.83 D) higher than total • Posterior cornea must  have negative power • The majority of ocular astigmatism is due to the anterior cornea • This is due to the large n, NOT due to higher anterior corneal toricity (radius difference between PMs) Section 2 - The Cornea OBJ:

  5. Page 5.14 Schematic eye to compare anterior and posterior corneal contributions to total corneal power?

  6. 1.376 r2 = +6.8 mm r1 = +7.7 mm Exact Eye – Thick Lens r2 1.00 1.336 r1

  7. Page 5.14 r1 = +7.7 mm r2 = +6.8 mm Corneal Surface Powers Fig 5.12 Page 5.15

  8. Page 5.15 F2 = 5.88 D F1 = +48.83 D The Cornea as a Thick Lens  ALMOST NEGLIGIBLE

  9. Page 5.15 F2 = 5.88 D F1 = +48.83 D Corneal Back Vertex Power

  10. Page 5.15 • A good way to demonstrate the importance of a high n across the anterior corneal surface is to change the object medium • Swimming with your eyes submerged in water, the new object medium has an index 1.333. How will this affect your ametropia? The Cornea

  11. A 20 D myope swims underwater without a swimming mask. Her new refractive error while submerged would be approximately: • +20 D hyperopia • +40 D hyperopia • Emmetropia (plano) • 20 D myopia

  12. Page 5.15 • A good way to demonstrate the importance of a high n across the anterior corneal surface is to change the object medium • Swimming with your eyes submerged in water, the new object medium has an index 1.333. Prediction??? New corneal power? The Cornea • With posterior corneal surface power unchanged in water (5.88 D), the new equivalent power of the cornea is ~zero • The emmetropic eye becomes effectively 43 D hyperopic underwater • Wearing a swimming mask restores the normal air/cornea interface

  13. Corneal Surface Curvatures & Powers

  14. OS Cornea OD Cornea Page 5.16 Example 5.6 A patients’ right cornea differs from the Gullstrand Exact Eye cornea in anterior corneal radius only, which is 0.2 mm steeper. The left cornea differs in posterior radius only, which is 0.2 mm flatter. Find differences in total corneal power from the Exact Eye cornea: r2 = +7.0 mm r2 = +6.8 mm r1 = +7.7 mm r1 = +7.5 mm

  15. OD Cornea Page 5.16 (Exact F1 = +48.83 D) (Exact Fe = +43.05 D) r2 = +6.8 mm r1 = +7.5 mm The right cornea is +1.31 D (3.04%) stronger than the Exact Eye cornea. F2 5.88 D F1 +50.13 D

  16. OS Cornea Page 5.16 (Exact F2 = 5.88 D) (Exact Fe = +43.05 D) r2 = +7.0 mm r1 = +7.7 mm The left cornea is 0.17 D (0.39%) stronger than the Exact Eye cornea F2 5.71 D F1 +48.83 D

  17. Page 5.16 Example 5.6 A patients’ right cornea differs from the Gullstrand Exact Eye cornea in anterior corneal radius only, which is 0.2 mm steeper. The left cornea differs in posterior radius only, which is 0.2 mm flatter. Find differences in total corneal power from the Exact Eye cornea: r2 = +7.0 mm r2 = +6.8 mm r1 = +7.7 mm r1 = +7.5 mm F2 5.88 D F2 5.71 D F1 +50.13 D F1 +48.83 D OS Cornea OD Cornea Fe +44.36 D  1.31 D  Fe +43.22 D  0.17 D 

  18. Page 5.16 Example 5.6 A patients’ right cornea differs from the Gullstrand Exact Eye cornea in anterior corneal radius only, which is 0.2 mm steeper. The left cornea differs in posterior radius only, which is 0.2 mm flatter. Find differences in total corneal power from the Exact Eye cornea: OS Cornea OD Cornea Fe +44.36 D  1.31 D  Fe +43.22 D  0.17 D  For a given change in radius of curvature, the anterior cornea produces an approximately 8 times greater power change than the posterior cornea

  19. Page 5.17 Example 5.7 Using the same numbers as the previous example, but this time to induce 0.2 mm toricity in the corneal surfaces O.D. O.S. 0.17 D astigmatism 1.36 D astigmatism

  20. Page 5.17 Corneal Astigmatism O.D. corneal radii O.S. corneal radii +8.1 mm +6.8 mm +7.7 mm +6.4 mm +6.8 mm +7.7 mm +7.7 mm +6.8 mm AnteriorCornea PosteriorCornea AnteriorCornea PosteriorCornea How much corneal astigmatism: OD vs OS? (A) 2.41 D O.D., 2.41 D O.S. (B) 1.31 D O.D., 0.17 D O.S. (C) 2.41 D O.D., 0.36 D O.S. (D) 0.36 D O.D., 0.36 D O.S. (E) 2.41 D O.D., 1.31 D O.S. (F) 1.31 D O.D., 2.41 D O.S. 

  21. Classification of Astigmatism by Ocular PMs Page 5.18

  22. Classification of Astigmatism by Ocular PMs Page 5.18 Fig. 5.13 - Anterior corneal (a) radius, & (b) power vs. age

  23. Classification of Astigmatism by Ocular PMs Page 5.18 • With-the-Rule (wtr) Astigmatism • Power greater in the vertical meridian (90  15) • In early decades, average astigmatic cornea steeper at 90 wtr corneal astigmatism (highest magnitudes up to age 30) • Because anterior cornea is predominant cause of ocular astigmatism  most young astigmats have wtr total ocular astigmatism

  24. Classification of Astigmatism by Ocular PMs Page 5.18 • Against-the-Rule (atr) Astigmatism • Power greater in the horizontal meridian (180  15) • Difference between average H and V corneal power decreases after 30 • This produces an increase in incidence of atr corneal astigmatism • In 70s, mean horizontal power becomes steeper than vertical, making atr corneal astigmatism more common

  25. Classification of Astigmatism by Ocular PMs Page 5.18 • Overall trends With-the-Rule versus Against-the-Rule Corneal Astigmatism • Over 90% of astigmatic infants have wtr astigmatism • By age 50, this has decreased to 80% • Wtr rapidly decreases to be overtaken by atr from age 50 - 75

  26. Total Ocular Astigmatism vs. Age “Total” Wtr-atr crossover at ~age 45 Fig. 5.14 Page 5.20 Corneal wtr-atr crosses over at a much later age

  27. Corneal vs. Total Ocular Astigmatism Fig. 5.14 Page 5.20 Crossover occurs much earlier for total ocular astigmatism than corneal Difference is due to intraocular astigmatism (physiological astigmatism) Intraocular astigmatism averages 0.50 to 0.75 D atr in non-oblique astigmats This skews the incidence of total ocular astigmatism toward atr, and produces the earlier crossover between wtr/atr vs. corneal astigmatism

  28. Corneal vs. Total Ocular Astigmatism Fig. 5.14 Page 5.20 Oblique astigmatism much less common than wtr or atr Lowest incidence in early life; remains ~constant after age 30 Oblique wtr means greatest power closer to vertical than horizontal

  29. The Tear Film as the Anterior Corneal "Surface" Page 5.22 Fig. 5.15 - The tear film and cornea can be treated as two thick lenses separated by air

  30. The Tear Film as the Anterior Corneal "Surface" Page 5.22 The tear film is very thin and conforms exactly to anterior corneal shape It therefore has negligible effect on anterior corneal power

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