Photorefractive Keratectomy (PRK) for Astigmatism Correction Workup

Updated: Apr 07, 2017
  • Author: Manolette R Roque, MD, MBA, FPAO; Chief Editor: Douglas R Lazzaro, MD, FAAO, FACS  more...
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Workup

Imaging Studies

Corneal topography

Corneal topography provides surgeons with effortlessly understood color-coded maps of corneal curvature in addition to quantitative indexes of irregular astigmatism that correlate with potential visual acuity. [38, 39]

Modern instrumentation produces a video keratograph, which generally is in the form of a color-coded contour map.

Different manufacturers use different methods (eg, Placido, 40 scanned slits, combination Placido and 40 scanned slits, phase modified laser holography, raster stereography).

The use of video keratography in preoperative and postoperative evaluations of all refractive surgery patients is valuable.

Dual Scheimpflug analyzers

Scheimpflug imaging differs from conventional techniques in that the object plane, lens plane, and image plane all intersect in a straight line.

This type of imaging allows assessment of anterior and posterior corneal topography, anterior chamber depth, as well as anterior and posterior topography of the lens. [40]

Pachymetry

The pachymeter (optical, ultrasonic) is used to measure corneal thickness.

Accurate determination of corneal thickness preoperatively allows the surgeon to set the depth of incision to two thirds of the measured result.

Orbscan II by Orbtek uses both 40 scan slits and Placido methods to provide anterior and posterior corneal curvature in addition to data on corneal thickness.

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Other Tests

See the list below:

  • Wavefront aberrometry

  • Keratometry

  • Placido ring reflections

  • Refraction (manifest, automated, cycloplegic)

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Histologic Findings

Several authors have performed and reported histologic analysis of treated tissue. The excimer laser ablation of the cornea results in a very smooth ablated surface, no damage to adjacent tissue, and an abrupt transition to the untreated tissue. [35] An estimated 0.25 µm of corneal tissue is removed with each pulse of the laser. Furthermore, with electron microscopy, an extremely narrow zone of damage may be demonstrated next to the treated area.

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