Overview of Refractive Surgery

Transcription

1 Overview of Refractive Surgery Michael N. Wiggins, MD Assistant Professor, College of Health Related Professions and College of Medicine, Department of Ophthalmology Jones Eye Institute University of Arkansas for Medical Sciences

2 Can we do better than contact lenses or glasses? Surgeons knew from experience that the power of the eye changed with cataract surgery beyond just the new IOL implant Tight wounds could lead to increased astigmatism Cutting sutures or cutting in the cornea parallel to the limbus lead to decreased astigmatism

3 For example, tight sutures at 12 o clock could give 4 diopters of 90 degrees

4 Treatment: Cut the 12 o clock sutures

5 Manifest refraction now = plano

6 Could we do that on any eye?

7 Radial Keratotomy Developed in the 1890 s, but popularized in the 1970 s 1 We learned we could manipulate these cuts to change the overall power of the cornea in addition to changing the astigmatic power

8 RK: The Technique

9 Radial cuts are made

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14 Here it is in cross section

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16 Notice the depth of the cut

17 Why does RK work? Let s assume we have a patient with a K reading of 47 diopters Let s also visualize the structural support of the cornea as concrete pillars holding it up

18 It would look like this in cross section Average K = 47D

19 Removing the central pillar, the central cornea flattens Average K now = 44D

20 Question Why does flattening the cornea reduce it s power?

21 Answer The power is reduced because the cornea is flatter. The cornea can be thought of (and is) a very strong plus lens. The power of a lens depends upon the refractive index of the material and the amount of curvature of the lens. Less curvature = less power.

22 Radial Keratotomy-The Good News Up to -5D could be treated Prospective Evaluation of RK (PERK) study found 53% of patients were 20/20 or better without correction 10 years post-op 2

23 Radial Keratotomy-The Bad News 43% lose 1D or more 6 months to 10 yrs postop 2 Perforation of the cornea Neovascularization into the spokes Hyperopia at high altitudes Glare Halos

24 New Questions Were Asked What can we do to improve this technique? How can we reduce the power of the cornea without weakening it s structural integrity? Can we develop a procedure that can treat hyperopia also?

25 A Solution 1983, Trokel observed extreme precision with the new excimer laser (wavelength = 193nm) when used on cow s eyes 3 Photorefractive keratectomy (PRK) was developed and FDA approved in the USA in

26 PRK: The Technique

27 20% alcohol is applied inside a metal ring on the cornea for seconds

28 The central epithelium is then removed

29 Here is the cross section view after removing the epithelium

30 An excimer laser is then applied

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34 3-5 days later, new epithelium grows over the stromal bed

35 PRK-The Good News Good results up to -6D or up to +7D 3 94% at least 20/40 without correction 3 Does not weaken the cornea as much as RK

36 PRK-The Bad News May take up to 1 year for the best vision to come in May be on steroid drops for up to 6 months Pain for 3-5 days

37 PRK-More Bad News Regression usually occurs, so you overtreat initially and hope the amount of regression stops at emmetropia Too much epithelium growth in the center = steeper cornea = more myopia (regression) Haze can develop as quickly as 1 month 3

38 More Questions Were Asked How can we keep using the excimer laser, but decrease the pain, haze, regression, and delayed vision?

39 Laser-Assisted In situ Keratomileusis LASIK Automated lamellar keratoplasty had been previously developed during which a corneal button was cut, reshaped and put back on 3 LASIK was developed in the 1990 s using the principles of ALK and the precision of the excimer laser

40 LASIK: The Technique

41 A flap is created with a keratome

42 A nasal hinged flap in a right eye

43 Cross sectional view

44 The flap is retracted

45 The excimer laser is then applied

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49 The flap is reposited

50 LASIK-The Good News Almost no pain Near immediate visual recovery Eye drops for 5 days, not 6 months Rare to have any haze Can treat -12D to +4D and up to 8D of astigmatism 3 97% are 20/40 or better post op without correction 3

51 LASIK-The Bad News Flap complications Corneal ectasia (can cause loss of vision and may require a corneal transplant to correct) Infections under the flap Epithelial ingrowth Diffuse lamellar keratitis Can cause dry eyes to become worse Striae in the flap

52 Who should not have LASIK? Any history of poor healing: collagen vascular disease, immunocompromised patients Recurrent erosions: anterior basement membrane dystrophies Unstable refractions: keratoconus, pregnancy, poorly controlled diabetes, cataracts, patients under 18 yrs old Thin corneas Extreme dry eyes

53 How thin can the cornea be? Must retain 250 um of structurally sound tissue after the surgery to prevent corneal ectasia

54 Therefore: If the cornea is 540 um thick and a 160 um flap is created: 540 um 160 um flap X = 250 um X = 130 um of tissue available for removal If the laser takes 15 um per diopter (and this amount varies with the brand of the laser), 130/15 = 8.6 D of correction possible in a cornea 540 um thick

55 A Structural Concern The tissue used in the flap, 160 um in our example, no longer adds to the structural integrity of the cornea. Therefore, it cannot be counted in the 250 um needed after the procedure The advantage of PRK over LASIK is that less structural support is removed because no flap is created For example, if 100 um were removed by the laser in both PRK and in LASIK, the PRK eye would only lose 100 um of structural support. The LASIK eye would lose 100 um um from the flap created, or 260 um total.

56 Terminology PRK is called a surface ablation since tissue is removed starting at Bowman s layer and moving down into the stroma LASIK is called a stromal ablation since tissue is removed starting in the stroma

57 More Questions Is there a way that we could do a surface ablation to preserve more structural integrity, but not have the haze and pain seen in PRK?

58 Laser Subepithelial Keratomileusis LASEK EK LASEK was developed as a possible solution

59 LASEK: The Technique

60 18% alcohol is applied to the cornea for seconds

61 The epithelium is then displaced, but not discarded

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63 The excimer laser is then applied

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67 The epithelium is then reposited

68 LASEK-The Bad News Compared to PRK, LASEK has not been consistently proven to: Reduce post-op pain Speed the recovery of vision or Decrease post-op haze 2

69 Summary Radial keratotomy was the first mainstream refractive surgery, but is now rarely performed PRK was the next generation to evolve and remains a useful refractive technique LASIK takes advantage of the precision of the excimer laser used in PRK, but with a faster healing time and less haze LASEK is an improvement over LASIK in terms of the structural support, but is similar to PRK in terms of post operative issues

70 References 1. American Academy of Ophthalmology, Basic and Clinical Science Course, Cornea, ed., San Francisco, CA. 2. American Academy of Ophthalmology, Basic and Clinical Science Course, Refractive surgery, ed., San Francisco, CA. 3.Yanoff, M and Duker, J, Ophthalmology, 2 nd ed., St Louis, MO, Mosby 2004, p