In patients undergoing corneal surgery, topography is useful in preoperative planning, intraoperative modification and postoperative monitoring and problem-solving. Myopia is addressed by corneal flattening, hyperopia by corneal steepening and regular astigmatism by differentially addressing perpendicular meridian. These changes may be achieved by reshaping the anterior corneal surface or by altering the mechanical forces across the whole thickness of the cornea.
Preoperatively, topography is valuable in confirming stability of corneal shape over time and excluding subclinical disease, including ectasia. It also quantifies corneal astigmatism prior to cataract surgery. Topographic maps measure a preoperative baseline useful in communicating with patients and healthcare staff and as a medicolegal record.
Intraoperatively the corneal shape is usually assessed qualitatively by keratoscopy using handheld devices with circular rings or illuminated rings mounted in the operating microscope. This can guide the modification of incisions and the tension or placement of sutures.
Postoperatively difference maps can demonstrate the change occurring from preoperatively to postoperatively or during the postoperative period. It can be used to assess the effect of corneal shape on visual function and investigate other postoperative problems.
Radial keratotomy addresses myopia, by deep radial incisions causing bowing of the midperipheral cornea. Higher corrections can result in an oblate cornea with marked central flattening which may exhibit diurnal variation and long-term progression. The topography often shows a polygonal pattern, giving rise to multifocality which can either reduce best-corrected acuity or improve depth of focus.
In astigmatic keratotomy, deep circumferential incisions cause flattening of the affected meridian and steepening of the perpendicular meridian through coupling.
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