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

Chapter 3: Fundamentals on Corneal Topography

Figure a14: True Curvature Analysis in Pre/Post PRK

The comparison between preoperative and postoperative true curvature analysis of the same PRK patient shows no variations of the peripheral cornea after surgery.

Figure a15: Paracentral Island

Many are the potential complications of laser refractive surgery. Some may be attributable to the ablative pattern of each model of excimer laser, like central or paracentral islands, although the origin is uncertain. They are defined as any area within the ablation zone surrounded by areas of lesser curvature on more than 50% of its boundaries. They are a topographic pattern in PRK and LASIK patients, not always obvious. Figure displays a paracentral island after myopic PRK: it can be identified down inside the ablation red ring as a yellow-orange spot. Note that only with the calculation method of local powers (true curvature map on the right), this small abnormality is made visible, remaining invisible in the axial map (on the left).

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Section II: Topography

Figure a16: Effect of Suture Removal After

Keratoplasty

Serial topographic exams after a penetrating keratoplasty reveal large configurational changes the first two months, which remain stable until suture removal. Topography is then used to determine the suture to be removed in order to lower suture induced astigmatism and enhance visual recovery. Figure shows a test comparison: left map displays high astigmatism after a penetrating keratoplasty (-6.18 at 173º), right map displays the reduction to 1.33D after suture removal. Note the asymmetry of power between the two hemi-meridians, that improves after suture removal. Observe the red areas of high power (and elevation) near the wound.

Topographer is preferred over keratometer as most changes do occur outside the 3mm area measured by the keratometer.

Figure a17: Software Adjustment of a Decentred Axis

The Keratron™ Corneal Topographer (Optikon 2000® S.p.A, Italy - Europe) offers some interesting features like the possibility of replacing the optical axis when the patient’s fixation is not as desired or corneal centration is not perfect. The system is able to recalculate the optical power values for the whole cornea. Note that values at the optical axis differ from the original map with geometric axis calculations (on the left) and the recalculated map with the new visual axis position (map on the right).

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Chapter 3: Fundamentals on Corneal Topography

Figure a18: Myopic and Hyperopic Keratomileusis (LASIK)

Shown are two "true curvature" maps of both myopic (left) and hyperopic (right) keratomileusis.

To correct myopia, excimer laser removes a central disc of corneal stroma, resulting in central flattening (blue) and the presence of a relative peripheral steepening ring (red). Corneal topographic changes similar to those seen after photorefractive keratectomy (PRK) occur after LASIK for myopia.

To correct hyperopia, the excimer laser does just the opposite: it removes an annulus or ring of tissue from the mid-periphery (blue) to steepen the central cornea (red).

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Section II: Topography

Figure a19: Intrastromal Segmented Graft for the Correction of High Myopias

Figure shows a "true curvature" map of a left eye cornea that received an intrastromal segmented graft for the correction of high myopia. The map is similar to that of a myopic LASIK, but less regular.

Figure a20: Fluorescein Simulation in RGP Contact Lens

Contact lens fitting applications are used to help choosing the best lens for every case, by simulating the fluorescein film pattern and contact lens position of rigid contact lenses (RGP and PMMA). The simulated fluorescein feature is intended to reduce fitting time by viewing the effect of changing lens parameters on a personalised basis, depending on the patient’s corneal exam. Let’s notice that the true "in vivo" result of any computerised fluorescein test may vary due to differences caused by lid action on the lens (aperture and weight).

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Chapter 3: Fundamentals on Corneal Topography

BRIEF ATLAS OF CORNEAL TOPOGRAPHY

SPECIAL CASES

Case 1: A 40 year-old man underwent a myopic LASIK in both eyes, with a B&L Chiron Technolas 217 excimer laser system. The eye moved slightly during the surgery and the eye tracker was switched-off. Note the visually insignificant irregularities of the internal border of the ablation zone, at 6-7, 9 and 12 o’clock). Tangential diopter map, taken with a Dicon™ CT-200 corneal topographer.

Case 2: A 35 year-old man was referred to us for myopic LASIK in both eyes, had used rigid gas permeable contacts lenses for more than 15 years. Note the annular image in the tangential diopter maps of both eyes, simulating a LASIK ablation zone, due to corneal warpage as a result of the contact lenses abuse. Contacts have been discontinued, good ocular lubrication prescribed, and a new topographic exam will be performed in 6-8 months prior to decide refractive surgery. Refractive surgery should only be performed when corneal recovers its original shape and its stability proven. In our experience,

these patients have to wait at least a couple of years before surgery. Axial and tangential diopter maps of both eyes, taken with a Dicon™ CT-200 corneal topographer.

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Section II: Topography

Case 3: A 40 year-old woman with Marfan syndrome underwent Simon’s pectinotomy at birth to treat its congenital glaucoma. A few years later cataract intracapsular extraction (without IOL implantation) was performed to restore the visual acuity. At the age of 25 she needed modified trabeculectomy in both eyes to keep intraocular pressure under control. She is under 3 antiglaucomatous topical medications. Note that as a result of the surgeries and the topical medications she suffers form astigmatism (see axial diopter maps) and peripheral corneal oedema (in red, in tangential diopter maps). Cornea is slightly irregular, and

oedema changes form day to day, but the centre is clear enough to give her a 20/20 corrected visual acuity (aphakic). Axial and tangential diopter maps of both eyes, taken with a Dicon™ CT-200 corneal topographer.

Case 4: A 51 year-old man suffering form a disciform herpetic keratitis. Note the visually imparing corneal irregularities. Local stromal corneal oedema has been reduced with plano therapeutic bandage contact lens and hyperosmotic eye drops, decreasing the suffering form the patient when a corneal bulla breaks. Tangential diopter map of the right eye, taken with a Dicon™ CT200 corneal topographer.

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Chapter 3: Fundamentals on Corneal Topography

Case 5: A 32 year-old woman, biologist, suffered diplopia and head ache while looking through the microscope. Best corrected visual acuity was 10/20 in her left eye. We performed, six years before this topography was taken, a myopic astigmatism LASIK in her left eye (congenital anisometropia). Note the ablation band that corrects her high astigmatism and has facilitated the increase of her best corrected visual acuity that eye, reaching a very satisfactory 15/20. Multiple display maps of her left eye, taken with a Dicon™ CT-200 corneal topographer.

Case 6: Wide parallelepiped topographic view of radial keratotomy scars. Note that the central cornea (blue-violet) has flattened as a result of the radial cuts, correcting the patient’s myopia. Most cases need extra correction with time: despite the fact of having a very high astigmatism (-4,37

D at 11 degrees), the patient is very satisfied. Contact lenses can be used after radial keratotomy, but care has to be taken not to worsen postoperative corneal stability. These patients also need intensive eye lubrication with pre- servative-free solutions, to avoid recurrent painful erosions and vision fluctuations. Multiple display maps of the right eye, taken with a Dicon™ CT-200 corneal topographer.

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Section II: Topography

Case 7: A myopic patient underwent an unsuccessful surgery at the end of 2000. The microkeratome failed to cut the flap (uncompleted flap cut): and surgery was not performed. The patient underwent a new flap cut one month later, and unwillingly a portion of corneal stroma was lost (see arrows). Nobody adverted this fact until corneal topography was performed in the immediate postoperative period: stromal lost can be seen at the slit lamp. The same day, uncomplicated LASIK was performed in the left eye: no explanation is clear for the increase of astigmatism in this eye !. Topographic maps were taken with a Topcon™ 7000P automatic refracto-ker- atometer-topographer. (Case sent by Dr. Esteban Pertejo, Chief Anterior Segment Surgeon, Instituto de la Visión de Castilla y León, Spain).

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Chapter 3: Fundamentals on Corneal Topography

TOPOGRAPHERS CURRENTLY

AVAILABLE

Table 8

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Section II: Topography

ASTRAMAX™ 3-D Stereo Topographer (Lasersight®)

Figure 24: ASTRAMAXTM 3-D stereo topographer from Lasersight ®

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