Imaging Studies
In order to produce accurate arcuate incisions, the use of corneal topographers and/or corneal tomographers is essential when visualizing the corneal landscape. Elevation maps can identify regular versus irregular astigmatism, the magnitude of the astigmatism, and its relationship with the overall contour of the anterior corneal surface. Fortunately, over the past ten years, both topographers and tomographers have become commonplace in ophthalmic practices, allowing for better preoperative and postoperative assessment of corneal contour.
Combined elevation and thickness measurements can be obtained from machines such as the Orbscan and Pentacam. Both technologies offer an added advantage over standard topographers: mapping the posterior corneal curvature, an area whose relevance had been overlooked for many years. Recently, some ophthalmic investigators have attributed improved refractive results when considering the contribution of the posterior corneal curvature to total corneal power. [18]
Since traditional astigmatic keratotomy (AK) nomograms are based on ultrasonic pachymetry measurements, one must correlate corneal thickness values measured with the Orbscan or Pentacam with those obtained from a standard ultrasonic pachymeter. Statistical differences between thickness readings from these machines have been reported. [19]
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Thornton astigmatic keratotomy ruler.
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Close-up view of Thornton astigmatic keratotomy ruler.
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Example 2. Routine astigmatic keratotomy. In this image, a nasal astigmatic keratotomy is shown. This image was taken minutes after performing the incision. Note that the area around the cut stains lightly with fluorescein.
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Example 3. Preoperative topography in a patient about to undergo astigmatic keratotomy (AK) following corneal transplantation. This 30-year-old woman underwent corneal transplantation for keratoconus 6 years ago. She presented with a clear graft but with excessive astigmatism. Based on this pre-AK topography and prior surgical experience, an incision was placed just inside the donor-recipient interface from 250° to 300°. A second incision was placed between 30° and 70°. Incisions were not paired at 180° apart, but were localized based on topography. Refraction improved from +0.75 + 3.00 X 58° to +0.50 + 1.50 X 130°. Best-corrected vision remained 20/20.
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Example 3. Postoperative topography following astigmatic keratotomy for the 30-year-old woman with penetrating keratoplasty for keratoconus.
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Example 4. Topography shows preoperative and postoperative readings with differential analysis in a corneal transplant recipient (performed for Fuchs dystrophy) who underwent astigmatic keratotomy (AK). After all sutures were removed, she had approximately 6.5 D of keratometric astigmatism. The previous sutures seemed to have been tighter temporally than nasally. She underwent AK with a 50° incision temporally and a 30° incision nasally. The alteration in arcuate size values was related to surgeon preference. Following AK, residual astigmatism was slightly less than 1 D. The 5.5 D of astigmatism alteration from an 80° relaxing incision in a normal cornea is not expected. Posttransplant corneas are variable, and individual results can vary widely. Monitor the results as best as possible intraoperatively and be willing to go back and suture an overcorrection if one should occur.
